Kauflächenwinkel equiner Schneide- und Backenzähne: Computergestützte Messungen an detailgetreuen, anatomischen 3D-Modell en INAUGURAL – DISSERTATION zur Erlangung des Grades eines Dr. med. vet. beim Fachbereich Veterinärmedizin der Justus-Liebig-Universität Gießen Laura Schröter geb. Listmann Aus dem Institut für Veterinär-Anatomie, -Histologie und -Embryologie Betreuer: Prof. Dr. Carsten Staszyk Kauflächenwinkel equiner Schneide- und Backenzähne: Computergestützte Messungen an detailgetreuen, anatomischen 3D-Modell en INAUGURAL – DISSERTATION zur Erlangung des Grades eines Dr. med. vet. beim Fachbereich Veterinärmedizin der Justus-Liebig-Universität Gießen Eingereicht von Laura Schröter geb. Listmann Tierärztin aus Mainz Gießen, 2017 Mit Genehmigung des Fachbereichs Veterinärmedizin der Justus-Liebig-Universität Gießen Dekan: Prof. Dr. Dr. h.c. M. Kramer Klinik für Kleintiere, Chirurgie Justus-Liebig-Universität, Gießen 1. Gutachter: Prof. Dr. med. vet. C. Staszyk Institut für Veterinär -Anatomie, -Histologie und -Embryologie Justus-Liebig-Universität, Gießen 2. Gutachter: Prof.‘in Dr. med. vet. S.Krämer Klinikum Veterinärmedizin Justus-Liebig-Universität, Gießen Tag der Disputation: 07.05.2018 Meiner Familie In Liebe und Dankbarkeit Inhaltsverzeichnis 1 Einleitung ...................................... ......................................................................... 1 2 Publikation I ................................... ........................................................................ 5 3 Publikation II .................................. ...................................................................... 25 4 Übergreifende Diskussion ........................ .......................................................... 53 4.1 Entwicklung einer Messmethode ................. ................................................ 53 4.2 Winkelmessungen ............................... .......................................................... 55 4.2.1 Winkelung der equinen Backenzähne ........... ........................................ 55 4.2.2 Winkelung der equinen Schneidezähne ......... ....................................... 57 4.3. Schlussfolgerung ............................. ............................................................ 60 5. Zusammenfassung ................................ ............................................................. 62 6 Summary ......................................... ..................................................................... 65 7 Literaturverzeichnis............................. ................................................................ 69 8 Puplikationsverzeichnis ......................... ............................................................. 73 9 Ehrenwörtliche Erklärung ........................ ........................................................... 74 10 Danksagungen ................................... ................................................................ 75 1 1 Einleitung Die routinemäßig durchgeführte Zahnbehandlung gehört zur Standardbehandlung beim Pferd. Der Nutzen der Routinezahnbehandlung, um die Entstehung schwerwiegender Zahnanomalien zu verhindern, ist weithin anerkannt (z.B. Ralston et al., 2001). Bei dieser Behandlung werden unerwünschte Formveränderungen von Backen- und Schneidezähnen, wie z.B. scharfe Zahnkanten, Rampen, Haken und abnorme Kauflächenwinkel, bearbeitet. Infolge dieser Zahnbehandlungen werden zwangsläufig Kauflächenwinkel und Kauflächengeometrien verändert. Ein physiologischer Kauflächenwinkel ist eine wesentliche Voraussetzung für die Okklusion und damit für einen funktionierenden Kauvorgang sowie eine optimale Futterverwertung beim Pferd (Ralston et al., 2001). Ziel einer jeden Zahnbehandlung ist es deshalb die Normokklusion wiederherzustellen, um einen physiologischen Kauvorgang zu gewährleisten (Klugh, 2010). Für Pathologien im Backen- und Schneidezahnbereich, wie die unregelmäßige Abnutzung, Haken- und Rampenbildung sowie für veränderte Kauflächenwinkel, werden zwei wesentliche Ursachen vermutet. Entweder wird der physiologische Kauvorgang durch Schmerzen oder mechanische Blockaden verändert oder dem Pferd steht nicht genügend abrasives Material zur Abnutzung der Kauflächen zur Verfügung (Dixon et al., 2000; Bonin et al., 2007). Beide Situationen führen zu einem abnormen Kauvorgang und damit zu einem abnormen Abrieb der Kauflächen. Therapeutisch muss nach dem Beheben der Ursache eine Normokklusion der Zähne wiederhergestellt werden. Dies geschieht heutzutage durch Schleiftherapie, meist mit Hilfe elektrischer Zahnraspeln, wodurch Zahnhartsubstanz iatrogen entfernt wird. Bei dieser Behandlung ist es entscheidend einen physiologischen Kauflächenwinkel beizubehalten oder wieder herzustellen (Castell und Vogt, 2011). Der physiologische Kauflächenwinkel, besonders im Bereich der Backenzähne, war Ziel verschiedener Studien der letzten Jahrzehnte. Die Untersuchung der Backenzahnwinkel erfolgte teilweise für einzelne Zähne, aber auch für die gesamte Backenzahnarkade (Ralston et al., 2001; Carmalt, 2004; Rucker, 2004; Carmalt et al., 2005; Brown, 2008). Die Kauflächenwinkelmessung im Bereich der Schneidezähne bezog sich immer auf die gesamte Schneidezahnreihe (Rucker, 2004; Allen, 2008). 2 In Untersuchungen von Ralston et al. (2001) und Carmalt et al. (2005) wurden die Kauflächenwinkel der Backenzähne 307* bzw. 406 bestimmt. In beiden Studien wurde eine steife, faltbare Metallplatte gegen die linguale und okklusale Oberfläche der Backenzähne gelegt und der okklusolinguale Winkel gemessen. Kauflächenwinkel des Zahns 307 reichten von 3° bis 20° im Verhältnis zur horizontalen Achse (Ralston et al., 2001). Als idealen Winkel suggerieren Ralston et al. (2001) einen bukkolingualen Kauflächenwinkel von 10° bis 18°. Die Winkelung des Zahns 406 war ähnlich, mit einem mittleren Kauflächenwinkel von 10,61° (Carmalt et al., 2005). Rucker (2004), Carmalt (2004) und Carmalt et al. (2005) setzten eine indirekte Methode zur Messung und Kalkulation des Kauflächenwinkels ein. Diese beruht auf einer Seitwärtsverschiebung der Schneidezähne und wird als Schneidezahnseperationstechnik bezeichnet (Carmalt, 2004). Die für die gesamte Backenzahnarkade kalkulierten Winkel reichen durchschnittlich von 10° bis 15° (Rucker, 2004) bzw. 6,3° bis 19° (Carmalt, 2004; Carmalt et al., 2005). Brown et al. (2008) formten mit einem biegsamen Draht die Kaufläche und die linguale Seite der Backenzähne nach und übertrugen die Umrisse des Drahtes auf grafisches Papier. Diese Methode erlaubte eine individuelle Kauflächenwinkelmessung für jeden einzelnen Zahn in der Backenzahnarkade. Bemerkenswert war ein signifikanter Anstieg der gemessenen Winkel von mesial nach distal, in jeder Backenzahnarkade. Im Oberkiefer wurden Winkel von 11,8° (06) bis 19,1° (11) verzeichnet, im Unterkiefer wurden steilere Winkel von 18,4° (06) bis 31,5° (11) gemessen (Brown et al., 2008). Die große Spannweite der Kauflächenwinkel der Premolaren und Molaren wird als normal angesehen, Winkel über 45° allerdings werden als pathologisches Scherengebiss bezeichnet (Dixon et al., 2000). Die Ausrichtung der Kaufläche equiner Schneidezähne kann aus 2 Perspektiven betrachtet werden. Vor dem Pferd stehend betrachtet man die Schneidezahnlinie in der Transversalebene, auf der Seite stehend betrachtet man die * Die Benennung einzelner Zähne erfolgt gemäß einem modifizierten Triadan-System (Floyd, 1991; Triadan, 1972) 3 Kauflächenausrichtung in der Sagittalebene. Deshalb wird im Folgenden der Schneidezahnwinkel in der Transversalebene als Transversalwinkel und der Schneidezahnwinkel in der Sagittalebene als Sagittalwinkel bezeichnet. Es ist allgemein anerkannt, dass die okklusolabiale Kante der Schneidezähne bei Betrachtung von vorne eine horizontale Linie bildet (Easley, 2011). Der Transversalwinkel der Schneidezähne bildet dann im Idealfall einen 90° Winkel im Verhältnis zur Medianebene bzw. einen 0° Winkel im Verhältnis zu einer Horizontalebene. Anomalien der Schneidezahnarkade verursachen Veränderungen dieser horizontalen Linie. Aus Frontalansicht kann die Schneidezahnlinie dann folglich als ‚Smile‘, ‚Frown‘, stufig, uneben oder schief (diagonal) bezeichnet werden. Über die Ausrichtung des Sagittalwinkels der Schneidezähne gibt es sehr kontroverse Meinungen. Ursache dafür könnte sein, dass es für die Definition und die Messung des Sagittalwinkels verschiedene Vorschläge gibt. Allen (2008) verwendet einen Protraktor für die Bestimmung der sagittalen Schneidezahnwinkel. Gemessen wird die Winkelung zwischen den Kauflächen der Schneidezähne und den Unterkieferladen. Die gemessenen Winkel von 10° bis 15° gelten, nach Allen (2008), für alle Schneidezähne in Ober- und Unterkiefer. Auch Rucker (2004) geht von einem sogenannten rostrokaudalen Kauflächenwinkel (Sagittalwinkel) von 15° für alle Ober- und Unterkieferschneidezähne aus. Rucker (2004) suggeriert, als weiteren anatomischen Referenzpunkt, um die Schneidezahnwinkelung zu bestimmen, die Crista facialis (Rucker, 2004). Bei Betrachtung aus der Sagittalebene soll sich der Winkel der Schneidezähne an den Winkel der Christa facialis angleichen. Andere Autoren haben den Schneidezahnwinkel nicht explizit gemessen, sprechen aber von einem Zusammenhang zwischen dem Winkel der Schneidezahnokklusionsfläche und verschiedenen anatomischen Referenzpunkten (Klugh, 2010; Ros, 2011; Castell und Vogt, 2011). Nach Ros (2011) verläuft bei unverändertem physiologischem Schneidezahnokklusionswinkel eine verlängerte Ebene der Schneidezahnkaufläche durch das Kiefergelenk. Nach Klugh (2010) verläuft die gleiche verlängerte Kauflächenebene durch die Orbita oder den Ohrgrund. Aus den Vorschlägen für diese topografischen Beziehungen geht nicht hervor, ob diese Winkelangaben für alle Schneidezähne gleich gelten. 4 Castell und Vogt (2011) empfehlen für die Schneidezahnbehandlung die 03er Incisivi in eine horizontale Kauflächenausrichtung zu bringen. Die sehr heterogenen und teilweise gegensätzlichen Informationen über die physiologische Winkelung der Schneidezahnkauflächen überraschen noch mehr, wenn man bedenkt, dass die Korrektur der Schneidezahnkauflächenwinkelung während der Routinezahnbehandlung empfohlen wird. Alle vorliegenden Studien wurden an lebenden Pferden oder anatomischen Präparaten durchgeführt (Ralston et al., 2001; Carmalt, 2004; Rucker, 2004; Carmalt et al., 2005; Brown, 2008). Allerdings wird die Messung der Kauflächenwinkel am lebenden Pferd und am anatomischen Präparat durch die komplexe räumliche Gestalt des Schädels, durch Bewegung des Kopfes, auch an sedierten Tieren, und die enorme Herausforderung akkurat reproduzierbare Referenzebenen festzulegen, behindert. All diese methodischen Schwierigkeiten können die Genauigkeit der Messungen beeinflussen. Deshalb war das Ziel dieser Studie eine Methode zu entwickeln, um die obengenannten Schwierigkeiten zu vermeiden und eine Kauflächenwinkelmessung von hoher Genauigkeit von jedem einzelnen Zahn im Kiefer zu ermöglichen. Zusätzlich sollten anatomische Referenzpunkte identifiziert werden, welche ein Abschätzen des Kauflächenwinkels unter klinischen Bedingungen möglich machen. 5 2 Publikation I Occlusal angles of equine cheek teeth Laura Listmann, Patricia Schrock, Klaus Failing, Carsten Staszyk Livestock Science Volume 186, April 2016, Pages 78-84 DOI: 10.1016/j.livsci.2015.04.023 6 Occlusal angles of equine cheek teeth Laura Listmann1, Patricia Schrock2, Klaus Failing3, Carsten Staszyk1* 1 Institute of Veterinary Anatomy, -Histology and -Embryology, Faculty of Veterinary Medicine, Justus-Liebig-University Giessen, Frankfurter Str. 98, D-35392 Giessen, Germany Laura.Listmann@vetmed.uni-giessen.de 2 Institute for General Radiology and Medical Physics, University of Veterinary Medicine Hannover, Bischofsholer Damm 15, D-30173 Hannover, Germany Patricia.Schrock@tiho-hannover.de 3 Unit for Biomathematics and Data processing, Faculty of Veterinary Medicine, Justus-Liebig-University Giessen, Frankfurter Str. 95, D-35392 Giessen, Germany Klaus.Failing@vetmed.uni-giessen.de * Corresponding author Tel.:+49 6419938112 Carsten.Staszyk@vetmed.uni-giessen.de 7 Abstract The aim of this work was to determine occlusal cheek tooth angles in horses. The complex spatial shape of the equine skull hampers exact measurements of the occlusal cheek tooth angles in anatomical specimens and as well in living horses. Therefore a method was developed to perform measurements by using detailed 3D- reconstructions of equine skulls. 3D-models were constructed from CT-datasets by manual identification of relevant anatomical structures and by use of a computer software. Within 3D-skulls anatomical landmarks were identified and reference lines and planes were determined. Subsequently, occlusal angles of check teeth were measured in relation to the median plane. Results for mean values of cheek teeth angles ranged from 15.1° to 20.2°. Angles increased stepwise from rostral to caudal. Considering the total amount of data (20 horses with permanent dentition), there were no significant differences between the jaw quadrants when teeth in same Triadan positions were compared. However, in individuals there was a significant difference between the left and right side of the jaw. Angles of antagonistic teeth correlate with each other. The results are expected to provide a substantial basis for preventive and therapeutic treatments as well as for further biomechanical studies about equine mastication. Highlights Occlusal cheek tooth angles were determined in 3D-reconstructions of equine skulls. Identification of exact anatomical reference planes allowed reliable measurements. Angles ranged from 15.1° to 20.2° with staggered increase in caudal direction. No statistical differences between the jaw quadrants were present. Keywords Horse, Dentistry, Occlusal surface angle, 3D-reconstructions, Mastication 8 Introduction A physiological cheek tooth occlusal angle is essential for a functional mastication and related forage utilization in the horse. Therefore, the target of correction of each type of malocclusion is to return to normal occlusion enabling a physiological mastication (Klugh, 2010). Certain pathological conditions of the occlusal surface geometry require rasping and therefore iatrogenic reduction of dental hard substances. Typical occlusal changes that are often corrected by use of motorized rasping equipment are conditions of partial dental overgrowth, i.e. sharp enamel points, hooks and ramps (Dixon et al., 1999). As a main reason for these conditions the lack of abrasive forage material in combination with a reduced time of mastication has been hypothesized (Dixon et al., 2000). By treating dental overgrowth with electric rasps, it is not only necessary to correct the overgrown parts of the teeth, it is moreover mandatory to maintain or restore a normal and functional occlusal angle (Castell and Vogt, 2011). Accordingly, the determination of normal cheek tooth angels has been subject of several studies during the last decade. Ralston et al. (2001) and Carmalt et al. (2005) measured the cheek tooth occlusal angle of tooth 307 or 406, respectively. In both studies a stiffened hinge metal plate was placed against the lingual and occlusal surfaces of the teeth and the occlusal angles were measured. Occlusal angles for tooth 307 ranged from 3° to 20° (Ralston et al., 2001). The angulation of tooth 406 was similar with a mean occlusal angle of 10.61° ± 7.1° (Carmalt et al., 2005). Rucker (2004), Carmalt (2004) and Carmalt et al. (2005) applied an indirect method for measuring and calculating the occlusal angles by making a side excursion of the incisors, termed incisor separation angle technique (Carmalt, 2004). The average calculated angles, considered for the entire cheek tooth arcades, were 10° to 15° (Rucker, 2004) and 6.3° to 19° (Carmalt et al., 2005), respectively. Brown et al. (2008) placed a stiff but malleable wire on the occlusal surfaces of the cheek teeth and subsequently transferred the outlines of the wires to graph paper. This method allowed to measure occlusal angels for each individual tooth of the cheek tooth arcades. Remarkably, a significant increase of the angulation was measured from rostral to caudal within each arcade. In the upper jaw angles of 11.8° (06s) to 19.1° (11s) were recorded, in the lower jaw even steeper angles, 18.4° (06s) to 31.5° (11s ) were measured (Brown et al., 2008). 9 The wide ranges of premolar and molar occlusal angles were generally considered normal but angulations of more than 45° were recognized as pathological ‘shear- mouth’ (Dixon et al., 2000). Taken together, the results of these studies suggest a wide range of the occlusal angles with marked differences between teeth in different Triadan positions and with marked differences between antagonistic teeth of the upper and lower jaws. All these studies were performed in living horses or on anatomical specimens. However, measuring the correct cheek tooth angle in living horses – or in anatomical specimens – is hampered by the complex spatial shape of the skull, the movement of the head, even in sedated horses, and the enormous challenge to define – and reproducible use – a reference plane of repetitious accuracy. All of these methodological difficulties may influence the accuracy of the measurements. Therefore, the aim of this study was to develop a method which avoids the aforementioned difficulties and guarantees occlusal angle measurements of high accuracy in all teeth within a skull. Material and methods Creating 3D-models Twenty equine skulls with permanent dentition, from warmblood and thoroughbred horses (no pony breeds were included) aged between 5 and 26 years (6 female and 14 male) were examined. Cheek teeth were termed according to the Triadan system using numbers from 06 (second premolar) to 11 (third molar). Cadaveric heads were scanned by a CT system (BrillianceTM CT - Big Bore Oncology Scanner, Philips Medical Systems, Best, Netherlands). Datasets were provided by the Clinic for Horses of the University of Veterinary Medicine Hannover, Germany (for details, see Brinkschulte et al. 2013 and Brinkschulte et al. 2014). DICOM datasets (Digital Imaging and Communications in Medicine) for each head contained between 450 and 500 2D-slices. By using the computer program Amira (version 5.4.2, Visage Imaging GmbH, Berlin, Germany) in each 2D-slice contrasts were adjusted to optimal visualization of mineralized hard substances. Subsequently, the outlines of the bony structures of the skull as well as the outlines of the teeth were identified and marked. Special attention was paid to a most accurate reproduction of the dental occlusal surfaces. Finally, datasets were converted to detailed 3D-models featuring the skull bones and the dentition. 10 For the purpose of measuring the occlusal angles, two sets of geometrical constructions were necessary, i.e. reference planes within the skull and lines /planes indicating the angulation of the occlusal surface of each tooth. Therefore, exact midline planes for the upper skull and for the mandible were created. Subsequently, lines and planes were determined on the occlusal surface of each tooth (Fig. 1). Median planes Each 3D-model was checked for the accurate representation of the following reference points / structures which are placed in the median plane of the skull: Suture line of the ossa nasalia Canalis interincisivus Suture line of the Processūs palatinae of the ossa maxillaria Crista sagittalis externa Crista galli These reference points were marked within the 3D-models and subsequently a median plane for the upper skull was calculated and visualized (Fig. 1). A similar procedure was applied to calculate a median plane for the mandible. As the corpora of the mandible diverge, reference points placed in the median plane were rare and only present at the Sutura intermandibularis. Additional reference points within the median plane were created by use of bilateral anatomical structures of the mandible, i.e.: The midpoint of a line between the medial edges of the Capita mandibulae and the midpoint of a line between the medial edges of the Processūs coronoidei. Additionally, a line according to the bisecting angle of the diverging corpora of the mandible was calculated and visualized. Several points on this line were marked. Finally, all points served as a basis for the calculation and visualization of a median plane of the mandible (Fig. 1). Occlusal surface - lines and planes The enamel ridges of maxillary and mandibular cheek teeth feature a complex but very constant pattern. On maxillary as well as on mandibular cheek teeth, three well definied antomical points on the buccal edge and another three anatomical points at the lingual /palatal edge were marked (Fig. 2). Pairs of reference points (one buccal, one lingual /palatal) were used to create three occlusal lines with an buccal - 11 lingual /palatal orientation. Additionally, all six points marked on each occlusal surface were used to calculate an occlusal surface plane (Fig. 2). Measurements Only teeth without fractures or other pathological changes, e.g. dental overgrowth, were used for measurement. Two different sets of data were recorded. First, occlusal angles were determined by use of occlusal surface lines. The angulation of each surface line in relation to the midline was measured by means of an automated measuring function incorporated in the software Amira. For each tooth, three surface line angles were measured and subsequently summarized to one mean value. Second, the angulation between the occlusal surface plane and the midline plane was measured (Fig. 1). Statistical analyzes Statistical analyzes were performed using the commercial available software packages BMDP (Dixon, W. J. (chief editor), 1993. BMDP Statistical Software Manual, Volume 1 and 2. University of California Press, Berkeley, Los Angeles, London) and Graph Pad Prism 4.0 (Graph Pad Software Inc., San Diego, California). For general data description mean values (ẋ), standard deviations (s), minima (xmin) and maxima (xmax) were tabulated. Statistical significance of possible jaw and side effect were assessed by two way ANOVA with repeated measures to determine differences between the jaw quadrants for each tooth. Using a three way ANOVA with repeated measures (jaw by side by tooth position) differences between teeth in one jaw quadrant were analyzed, additionally. Intraindividual differences between corresponding teeth in the left and right sides of the head were determined by variance decomposition using linear mixed model analysis. Correlations between antagonistic teeth were analyzed by an adequate correlation analysis. The significance level was chosen at α = 0.05. So p-values of less than or equal to 0.05 were assumed to indicate statistical significance. Results The results for the surface line angles and those for the surface plane angles were statistically compared and no significant differences were obtained. Therefore, only results from the surface plane measurements are presented in the following. 12 The mean values for the occlusal surface angles of all check teeth ranged from 15.1° to 20.2°, with a minimal value of -0.5° (206, 20 year old horse) and a maximum value of 38.5° (308, 14 year old horse). Comparing the mean occlusal angles of corresponding teeth (same Triadan positions), no significant difference between the left and right maxilla and the left and right mandible was found. Further, there was no significant difference between the upper and lower jaw on one side of the head. Within one jaw quadrant, teeth featured statistically different means of occlusal angles (p-value < 0.05) and specific patterns were shown for the mandibular and maxillary arcades. The maxillary and mandibular arcades feature an arrangement of three batteries. The 06s show the shallowest angles with 15°, 07s feature angles of 18° and the teeth 08 to 11 show similar occlusal angles of 19° to 20° (Fig. 3). Furthermore, there is a statistically significant correlation between antagonistic teeth (all p-values < 0.05), i.e. steeper angles in maxillary cheek teeth result in steeper angles in mandibular cheek teeth (Fig. 4). Even though there are no significant differences comparing the jaw quadrants within the pool of the examined horses, there are differences in every individual horse in terms of a left-right asymmetry. Teeth in same Triadan positions of the left and right jaws differ in angulation for 3° to 5°. Discussion The occlusal surface of equine cheek teeth features a complex geometrical shape, composed of enamel ridges and dentin basins, rather than a flat plane. This particular shape is a product of continued attrition and abrasion and facilitates forage disruption (Kaiser et al., 2013). During the masticatory power stroke, the occlusal surface of a mandibular cheek tooth performs a transversal, linear course on the occlusal surface of antagonistic maxillary teeth (Kaiser et al., 2013). Enamel ridges serve as shearing edges and in dentin basins the forage becomes compressed and further disrupted (Kaiser et al., 2010). This complex and functional occlusal topography complicates the definition of a simple occlusal plane and thus complicates the measurement of an occlusal angle. For this study, two methodical approaches were applied to define occlusal angles with respect to the anatomical structures of the occlusal surfaces. First, the definition of three surface lines in each tooth according to constant enamel 13 landmarks allowed to measure three different angulations with respect to the marked topography of the occlusal surface. Second, the computer assisted calculation of a geometrically defined occlusal plane allowed to measure a mean occlusal angle. Remarkably, there was no statistically difference found when surface line measurements were compared with the occlusal plane measurements. Thus, the calculated surface planes seem to reflect the anatomical details of the occlusal surface sufficiently in this context and were therefore chosen to be presented here. Moreover, a mean occlusal surface plane can be understood by manual palpation of the occlusal surface and might therefore be of clinical use. However, further analysis of the performed line measurements might generate interesting and useful data supplementing studies in equine dental tribology research. In contrast to several other studies (Ralston et al., 2001; Carmalt, 2004; Carmalt et al., 2005) we carried out measurements for every single cheek tooth in every jaw quadrant. This methodical approach appears mandatory, because it has been clearly shown that marked differences between individual teeth within a jaw quadrant are present (Brown et al., 2008). Accordingly, data derived from examinations in only single teeth (Ralston et al., 2001; Carmalt et al., 2005) are of limited significance for the estimation of occlusal angles within the entire dentition. The same applies for studies that calculated one uniform angle for the entire cheek tooth arcades by indirect measurements using the shift of incisors (Rucker, 2004; Carmalt, 2004). By using CT-datasets and a special software to generate exact 3D-models it was possible to perform measurements of a high accuracy. Similar approaches were previously used for craniofacial and maxillofacial analyzes of human skulls and had been proven to generate most accurate and reliable data (Olszewski et al., 2007; Lopes et al., 2008). In accordance with Brown et al. (2008) the mean occlusal angles in equine cheek teeth were found to increase from rostral to caudal within the arcades. However, while Brown et al. (2008) reported a continuous increase of the angulation, our results showed a staggered increase resulting in three batteries of teeth with similar angulations. This arrangement of batteries corresponds to a remarkable characteristic of the equine chewing cycle. Recorded movements of the mandible clearly showed that the mediodorsally directed power stroke does not feature a continuous movement but instead has at least two phases with a marked step in 14 between (Bonin et al., 2006). Such a two phased power stroke had not been confirmed by Simhofer et al. (2010), however, this study was performed by use of less markers to track the mandibular movement and might have therefore missed slight nuances of the temporospatial pattern of the chewing cycle. The range of the mean occlusal angles was less pronounced in our study (15.1° to 20.2°) compared to the values (11.8° to 31.5°) reported by Brown et al. (2008). Moreover, Brown et al. (2008) emphasized a marked asymmetry of the angulation between mandibular (steep angulation) and antagonistic maxillary teeth (shallow angulation). In contrast, the mean occlusal angles measured in our study did not show statistical differences between antagonistic cheek teeth. Instead, our results showed that there is a correlation between antagonistic teeth, i.e. angles between antagonistic teeth tend to become adjusted to each other. This observation has been previously reported in healthy equine dentition (Bonin et al., 2007) as well as for diseased dentitions (Dixon, 2010). However, it remains unclear which tooth (maxillary or mandibular) modulates the angulation of the other. The contradictory results obtained from the different studies might be due to the fact that very different reference planes were used to determine occlusal cheek tooth angles. In previous approaches, the occlusal angle was determined in relation to the lingual /palatal and /or buccal side of the tooth (Ralston et al., 2001; Carmalt et al., 2005; Brown et al., 2008). However, this methodical approach does not consider a possible tilting of the tooth within its alveolus which would clearly influence the occlusal angle in relation to other structures of the skull. Therefore we developed a method suitable to determine the occlusal angles in relation to the upper skull and the mandible, which are the relevant structures considering the masticatory movements. The obtained data might be of significance for further studies to elucidate details of the complex equine chewing cycle and the related movements within the temporomandibular joint. It should be noted that intraindividual variations were found in every investigated skull. Every horse showed an asymmetry between the left and right side of the head with occlusal angles differing between 3° and 5°. Such a left-right asymmetry is also reflected by the fact that horses have a preferred chewing side (Bonin et al., 2006; Baker, 2005). 15 Conclusion Equine occlusal cheek tooth angles were determined by a highly reliable method. Statistical analysis of the obtained data showed a uniform pattern of the occlusal angles within the arcades with a staggered increase of angulation form rostral to caudal. However, intraindividual left right asymmetries should be considered when performing dental corrections. Acknowledgement The authors thank Prof. Dr. Bernhard Ohnesorge, Dr. Astrid Bienert-Zeit, Dr. Maren Hellige and Dr. Markus Brinkschulte (Clinic for Horses of the University of Veterinary Medicine Hannover, Germany) for providing CT-Datasets. References Baker, G., 2005. Dental Physiology, In: Baker, G., Easley, J. (Eds.), Equine dentistry. 2nd ed. Elsevier Saunders, Edinburgh, pp: 49-54. Bonin, S.J., Clayton, H.M., Lanovaz, J.L., Johnston, T., 2006. Kinematics of the equine temporomandibular joint. Am. J. Vet. Res. 67, 423-428. Bonin, S.J., Clayton, H.M., Lanovaz, J.L., Johnston, T., 2007. Comparison of mandibular motion in horses chewing hay and pellets. Equine Vet. J. 39, 258-262. Brinkschulte, M., Bienert-Zeit, A., Lüpke, M., Hellige, M., Staszyk, C., Ohnesorge, B., 2013. Using semi-automated segmentation of computed tomography datasets for three-dimensional visualization and volume measurements of equine paranasal sinuses.Vet. Radiol. Ultrasound. 54, 582-590. Brinkschulte, M., Bienert-Zeit, A., Lüpke, M., Hellige, M., Ohnesorge, B., Staszyk, C., 2014. The sinonasal communication in the horse: examinations using computerized three-dimensional reformatted renderings of computed-tomography datasets. BMC Vet. Res. 10, 72. Brown, S.L., Arkins, S., Shaw, D.J., Dixon, P.M., 2008. Occlusal angles of check teeth in normal horses and horses with dental disease. Vet. Rec. 162, 807-810. 16 Carmalt, J.L., 2004. Observations of the cheek tooth occlusal angle in the horse. J. Vet. Dent.21, 70-75. Carmalt, J.L., Cymbaluk, N.F., Townsend, H.G.G., 2005. Effect of premolar and molar occlusal angle and feed digestibility, water balance, and fecal particle size in horses. JAVMA 227, 110-113. Castell, J., Vogt, C. 2011. Morphologische Veränderungen am Pferdegebiss und ihre Bearbeitung. In: Vogt, C. (Ed.), Lehrbuch der Zahnheilkunde beim Pferd. 1st ed. Schattauer, Stuttgart, pp. 125-182. Dixon, P.M., Tremaine, W.H., Pickles, K., Kuhns, L., Hawe, C., McCann, J., McGorum, B.C., Railton, D.I., Brammer, S., 1999. Equine dental diseases Part 2: a long-term study of 400 cases: disorders of development and eruption and variations in position of the cheek teeth. Equine Vet. J. 31, 519-528. Dixon, P.M.,Tremaine, W.H., Pickles, K., Kuhns, L., Hawe, C., McCann, J., McGorum, B.C., Railton, D.I., Brammer, S., 2000. Equine dental diseases Part 3: a long-term study of 400 cases: disorders of wear, traumatic damage and idiopathic fractures, tumours and miscellaneous disorders of the cheek teeth. Equine Vet. J. 32, 9-18. Dixon, P.M. 2010. Dental anatomy. In: Easley, J., Dixon, P.M., Schumacher, J. (Eds.), Equine dentistry, 3rd ed. Elsevier Saunders, Edinburgh, pp. 51-76. Easley, J., 2016. Abnormal dental wear – a paradigm shift. Equine vet. educ. 28, 20- 22. Kaiser, T. M., Fickel, J., Streich, W. J., Hummel, J., Clauss, M., 2010. Enamel ridge alignment in upper molars of ruminants in relation to their natural diet. J. Zool. 281, 12-25. 17 Kaiser, T.M., Müller, W.H., Fortelius, M., Schulz, E., Codron, D., Clauss, M., 2013. Hypsodonty and tooth facet development in relation to diet and habitat in herbivorous ungulates: implications for understanding tooth wear. Mammal Rev. 43, 34-46. Klugh, D.O., 2010. Principles of mastication biomechanics, In: Klugh. D.O. (Ed.), Principles of equine dentistry. 1st ed. Manson Publishing, London, pp. 61-68. Lopes, P.M.L., Moreira, C.R., Perrella, A., Antunes, J.L., Cavalcanti, G.P., Pessoa, J., 2008. 3-D volume rendering maxillofacial aequinalysis of angular measurements by multislice CT. Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod. 105, 224- 230. Moore, N.T., 2016. Clinical findings and treatment of shear mouth in two horses associated with ipsilateral painful dental disease. Equine vet. educ. 28, 13-19. Olszewski, R., Zech, F., Cosnard, G., Nicolas, V., Macq, B., Reychler, H., 2007. Three-dimensional computed tomography cephalometric craniofacial analysis: experimental validation in vitro. Int. J. Oral Maxillofac. Surg. 36, 828-833. Ralston, S.L., Foster, D.L., Divers, T., Hintz, H.F., 2001. Effect of correction on feed digestibility in horses. Equine Vet. J. 33, 390-393. Rucker, B.A., 2004. Incisor and molar occlusion: Normal ranges and indications for incisor reduction. In: Proceedings of the 50th annual convention of the American Association of Equine Practitioners, Denver, Colorado (USA), 7-12. Simhofer, H., Niederl, M., Anen, C., Rijkenhuizen, A., Peham, C., 2010. Kinematic analysis of equine masticatory movements: comparison before and after routine dental treatment. Vet. J. 190, 49-54. 18 Table 1 Tooth Horse# 106 406 107 407 108 408 109 409 110 410 111 411 1 8.2 11.3 10.3 15.6 13.9 12.4 17.5 26.9 18.6 19 17.5 23.7 2 13.1 11.3 19.3 21.9 21.6 19.5 19 22.8 18.3 22.5 19.1 19.7 3 14,5 20,9 17.8 20.4 21 21.6 19.7 24.6 21.3 24.8 20.2 25.3 4 18,9 20,4 22.3 19.6 23.8 19.4 19.6 19.2 17.7 16.2 21.3 22.9 5 11 13,9 13.5 17.5 14.6 14 13.6 18.4 13.8 18.2 13.9 14.1 6 14,8 22,3 15.5 16.2 16.5 17 18.3 23.6 17 18.6 16.6 19.7 7 12,6 16,1 19.4 20.4 21.6 22.7 17.9 20.4 18.3 18 21.3 15.8 8 19,8 21,5 22.3 21.8 21.7 23 21.4 23.3 24.9 24.6 20.6 31.2 9 19,8 15,7 23.9 21.9 18.6 20.9 23.2 25.3 24 25.6 23.7 32.7 10 15 13,5 16.6 8.5 20 13.1 20.2 8.7 13.6 11.4 18.7 14.8 11 15,9 2 15 14 19 17 13 12 15 16 10 14 12 16,2 18,2 16.9 21.3 18.3 23.1 16 18.7 17.4 20.4 18.1 23.9 13 12,6 15,6 16.5 18.4 19.8 22.8 19.3 22.8 18.8 25.2 19.1 21.1 14 14,8 16,1 18 19.2 20.6 20.1 22.4 21.3 19 20.1 19.5 22.6 15 15,1 16,6 12.3 18.5 15.4 19.1 17.6 20.9 16.4 24.1 20.5 15 16 11,1 6,4 12.5 10.3 14.1 15 13.9 15.9 13.4 17.4 18 18.9 17 13 12 13 16.1 16.2 17.9 14.1 17.7 14.6 15.3 15.3 20.6 18 15,2 15,6 17.9 16.7 16.5 18.5 14.5 17.1 14.9 18.1 15 25.8 Table 1. Occlusal angles arranged to display the correlation between antagonistic teeth of the right side of the head. Calculated correlation diagrams are shown in figure 4. 19 Table 2. Tooth Horse# 206 306 207 307 208 308 209 309 210 310 211 311 1 16.4 17.7 18.1 18.9 20.9 19.3 24.9 29 24.9 26.9 27.2 26.3 2 16.9 13.7 24.1 24.6 25.1 25.9 23.5 27.4 22.2 23.3 23.3 23.4 3 13.5 11.6 17.5 18.8 21.3 18.1 20.2 17.2 19.5 20.7 17.2 17.7 4 19.6 18.3 19.3 18.2 21.9 20.8 19.8 22 18.6 14.5 22 14.9 5 10.8 10.9 15.2 14 16.1 16.6 14.5 13.8 16.5 16.6 16.7 16.9 6 14.7 16.3 12.1 15.3 13.4 14.9 14.8 16.5 14.6 15.3 12.7 21 7 11.3 7.8 18.1 17.1 20.2 20.1 15.2 20.8 16.3 21.1 21.6 18.2 8 0,5 21.7 17.8 16.8 19.4 12.8 17.7 17.7 21.7 18.2 21.2 19.7 9 14.4 7,5 22.7 18.1 23 12.7 24.5 24.9 24 20 20.7 22.7 10 12.9 13.3 15.7 20.5 15.6 18.7 14.8 15.3 10.5 16.4 16.2 8.7 11 18 9 17 18 18 16 19 18 17 18 18 15 12 21.1 15.5 20.7 18.4 22.8 21.9 21.7 19.2 23.3 16.3 27.5 21.1 13 20.1 16.8 22.3 19.5 22 18.1 21.5 20.6 24 21.6 23.2 20.4 14 20.1 19.6 22.5 21.9 26.5 22.9 23.9 21 20.4 20.8 21.7 25.3 15 21.4 16.6 22.8 16.4 17.9 18.3 18.2 19.9 17.9 12.7 17.9 17.1 16 15.4 32.5 15.3 36.1 16.8 38.5 19.7 25.2 12.6 36.2 20.2 30.1 17 18 16.1 20.3 19.4 20.4 16.5 20.2 20.4 21.1 17.2 18.5 12.8 18 19 20.5 18 18.5 19.4 19.8 17.3 15.3 18.2 17.2 15.6 16.6 Table 2. Occlusal angles arranged to display the correlation between antagonistic teeth of the left side of the head. Calculated correlation diagrams are shown in figure 4. 20 Figure 1. Digital 3D-models of an equine upper skull and corresponding mandible. a, b) Anatomical reference points (yellow) were identified and marked. c, d) Median planes (purple) were calculated and visualized. e, f) Occlusal surface planes (green) were calculated for each tooth (in e, the occlusal surface plane for tooth 207 is shown, in f, the occlusal surface plane for tooth 407 is shown) The angulation between the median planes and the occlusal surface planes were measured (α) and the occlusal angles were determined according to the formulas: 90° - α (maxillary cheek teeth) and α - 90° (mandibular cheek teeth). 21 Figure 2. Occlusal surface of a maxillary cheek tooth (a) and a mandibular cheek tooth (b). Anatomical reference points (numbers 1 to 6) were identified used to create surface lines (dotted lines) and to calculate mean surface planes (as shown in Fig. 1). Reference points identified on maxillary cheek teeth: 1. paracone 2. mesostyle 3. metacone 4. hypocone 5. protocone 6. protoloph 22 Reference points identified on mandibular cheek teeth: a) mandibulary cheek tooth 1. entoconid 2. metastylid 3. metaconid 4. protoconid 5. ectoflexid 6. hypoconid 23 Figure 3. Mean values and standard deviations of cheek tooth angulation calculated for maxillary cheek teeth (a) and mandibular cheek teeth (b). Black numbers indicate Triadan positions. Teeth with similar angulations are assembled in batteries and are marked by the same coloration. 24 Figure 4. Correlation diagrams showing the association between antagonistic cheek teeth. Correlations are displayed separately for each side of the head (blue dots: right side, red dots: left side). Correlation coefficients were calculated for each Triadan position: a) Triadan 06, 0,486; b) Triadan 07, 0.695; c) Triadan 08, 0.718; d) Triadan 09, 0.559, e) Triadan 10, 0.742, f) Triadan 11, 0.462. Correlations were significant with p-values < 0.05. 25 3 Publikation II Occlusal angles of equine incisors Laura Listmann, Patricia Schrock, Klaus Failing, Carsten Staszyk Journal of Veterinary Dentistry Volume 34, December 2017, Pages 259-267 DOI: 10.1177/0898756417739465 26 Occlusal angles of equine incisors Laura Listmann1, Patricia Schrock1, Klaus Failing2, Carsten Staszyk1* 1 Institute of Veterinary Anatomy, -Histology and -Embryology, Faculty of Veterinary Medicine, Justus-Liebig-University Giessen, Frankfurter Str. 98, D-35392 Giessen, Germany Laura.Listmann@vetmed.uni-giessen.de P.Schrock@gmx.de 2 Unit for Biomathematics and Data processing, Faculty of Veterinary Medicine, Justus-Liebig-University Giessen, Frankfurter Str. 95, D-35392 Giessen, Germany Klaus.Failing@vetmed.uni-giessen.de * Corresponding author Tel.:+49 6419938112 Carsten.Staszyk@vetmed.uni-giessen.de 27 Abstract The angulation of equine incisors is frequently used as a parameter for dental corrections. However, the term incisor angle is only vaguely defined and no studies exist presenting a series of reliable measurements in individual incisors of multiple horses. The aim of this study was to establish an exact method to determine incisor angles and to test whether clinically accessible landmarks (facial crest, bars) are suitable to estimate incisor angles. Eighteen horses were used to create 3D-reconstructions of the skulls from CT- datasets. Reference planes (median and transversal plane) were calculated using defined anatomical landmarks. Subsequently, occlusal planes for incisors and for incisor quadrants were calculated. Occlusal table angles were measured in relation to the reference planes. For each incisor a sagittal and a transversal angle was measured. Mean values of individual incisor angles ranged from 3.5° to 6.8° (transversal angle) and from 32.6° to 44.9° ( sagittal angle). No significant differences in mean between the left and right side was detected when teeth in same Triadan positions were compared. However, in individual horses marked differences between the left and right side of the jaws occurred. Lower jaw incisors showed significant steeper sagittal angles than upper jaw incisors. Furthermore angles of opposing incisors were correlated with each other. The facial crest and the upper jaw bars featured a curved shape and are therefore of limited use to estimate the angulation of the upper incisors. In contrast, the lower jaw bars were suitable to determine the angulations of lower incisors. Highlights Occlusal incisor angles were determined in 3D-reconstructions of equine skulls. Mean values of individual incisor angles ranged from 3.5° to 6.8° for the transversal angle and from 32.6° to 44.9° for the sagittal angle. Lower jaw incisors showed significant steeper sagittal angles than upper jaw incisors. Angles of opposing incisors were correlated with each other. Lower jaw bars were appeared to be most suitable as a reference line under clinical conditions. 28 Keywords Horse, Dentistry, Incisor, Occlusal surface angle, Mastication 29 Introduction The orientation of the occlusal surface of equine incisors can be inspected from two perspectives; first, from a position in front of the horse to assess its orientation in a transversal plane, second, from a lateral position to assess its orientation in a sagittal plane. In the following, the incisor angulation viewed in a transversal plane will be referred to as the transversal angle; the angulation viewed in a sagittal plane will be referred to as the sagittal angle – previously described as the incisor table angle (See Fig. 1). It is generally accepted that in unchanged incisor arcades the occlusolabial edges of the mandibular and maxillary incisors form a horizontal line (Easley, 2011). Malformation of the incisor arcades cause a bending or torsion of this incisor line. Viewed from a position in front of the horse (transverse plane) the incisor lines are then described as smile, frown, stepped or uneven and slanted (Easley, 2011). Consequently, it is widely assumed that unchanged transversal angles of incisors show a 90° angulation in relation to the median plane (or a 0° angulation in relation to a frontal plane). Concerning the angulation of the occlusal tables when viewed from the lateral side (sagittal angles), controversial opinions exist. This might be due to different proposals that were made to define and measure this angulation. Allen (2008) used a protractor and measured the angulation between the occlusal surfaces and the lower jaw bars. The determined angles of 10 to 15° were suggested to be valid for all upper and lower incisors (Allen, 2008). Rucker (2004) also assumed a so- called rostrocaudal table angle of 15° for all upper and lower incisors. However, Rucker (2004) suggested a different anatomical reference to assess the angulation of the incisor occlusal tables, i.e. the facial crest. It has been stated, that the incisor angulation approximates the slope of the facial crest (Rucker, 2004). Other authors did not quantify the incisor angulation but proposed a constant relation between the incisor table orientation and anatomical landmarks (Klugh, 2010; Ros, 2011; Castell and Vogt, 2011). According to Ros (2011), an unchanged sagittal angle of equine incisors is best described by an extended plane of the occlusal surfaces which should cross the temporomandibular joint (TMJ). Klugh (2010) also suggested extending an occlusal surface plane, but such a plane should cross the eye or the ear ground. However, the question remains open whether this topographical relation is valid for all upper and lower incisors. Castell and Vogt (2011) suggested a horizontal 30 alignment of the corner incisors (03rd) and recommended to use this orientation for the purpose of incisor treatments. The very heterogeneous and partly contradictory information about the normal angulation of incisor occlusal surfaces is even more surprising when one considers that the correction of incisor tables is widely recommended during dental treatment. The objective of this study was first to determine the incisor table angles in a representative group of horses using a reliable and exact measuring method. Second to identify morphological landmarks (facial crest, bars) which are suitable to estimate incisor occlusal table angles under clinical conditions. Material and methods Creating 3D-models Eighteen equine skulls with a complete permanent dentition were examined. Only large breeds (warmblood and thoroughbred horses, 6 female, 12 male, aged between 5 and 26 years) were included; pony breeds and horses displaying malformations (e.g. overjet, overbite etc.) were excluded from this study. Only teeth without fractures or other pathological changes, e.g. dental overgrowth and abnormal occlusion, were used for measurements. Incisors were termed according to the modified Triadansystem (Triadan, 1972; Floyd, 1991) using numbers from 01 to 03. Cadaveric heads were scanned by a CT system (Brilliance TM CT - Big Bore Oncology Scanner, Philips Medical Systems, Best, Netherlands). Datasets were provided by the Clinic for Horses of the University of Veterinary Medicine Hannover, Germany (for details, see Brinkschulte et al. 2003 and Brinkschulte et al. 2014). DICOM datasets (Digital Imaging and Communications in Medicine) for each head contained between 450 and 500 2D-slices. By using the computer program Amira (version 5.4.2, Visage Imaging GmbH, Berlin, Germany) in each 2D-slice contrasts were adjusted to optimal visualization of mineralized hard substances. Subsequently, the outlines of the bony structures of the skull as well as the outlines of the teeth were identified and marked. Special attention was paid to a most accurate reproduction of the dental occlusal surfaces. Finally, datasets were converted to detailed 3D-models featuring the skull bones and the dentition. 31 For the purpose of measuring the occlusal angles, two sets of geometrical constructions were necessary, i.e. reference planes within the skull and planes indicating the orientation of the occlusal surface of each incisor. Reference planes For each upper jaw as well as for each mandible a median plane and a transversal plane(See Fig.2) was created using reliable anatomical landmarks as described elsewhere (Listmann et al., 2016). Occlusal surface planes Six points, equispaced to each other, on the peripheral enamel line of each incisor were marked to create an occlusal surface plane. In addition combined occlusal surface planes for all incisors in each of the four quadrants and for all incisors in each jaw (upper and lower) were calculated. Supplementary planes Additional planes were created for the bars of the upper skull, the bars of the mandible and the facial crests. To calculate virtual planes for these structures, several marker points, equispaced to each other, on their bony surfaces were identified. For the bars and for the facial crests 4 points on each side (left and right), were marked. The first marker point of the facial crests was its most rostral extension; the last marker point was defined as the most ventral point underneath the orbita. Measurements Two different occlusal surface angles were determined; first, the transversal angle, given by the occlusal surface plane and the median plane, second, the sagittal angle, given by the occlusal surface plane and the transversal plane (Fig. 2). Furthermore, the angulation of the upper and lower jaw bar as well as the angulation of the facial crest was determined in relation to the transversal plane. 32 Subsequently, the angulations between the bars / facial crest planes and all individual incisor planes were calculated. Additionally, an extension of the combined occlusal surface plane of the maxillary incisors was created and the topographical relation of this plane and the TMJ as well as to the orbita was assessed. The position of the TMJ was defined as follows; the most lateral point of the mandibular head was marked and a circle with a diameter of 20 mm was drawn (Fig. 3). The position of the jaw plane was classified as crossing the TMJ/the orbita when the plane crossed the TMJ circle/orbita; otherwise its position was classified as dorsal or ventral to the TMJ/orbita. Statistical analyses Statistical analyses were performed using the commercial available software packages BMDP (Dixon, 1993) and graphical presentation by the program Graph Pad Prism 4.0 (Graph Pad Software Inc., San Diego, California). For general data description mean values (ξ), standard deviations (SD), minima (xmin) and maxima (xmax) were tabulated. Additional, one-dimensional data was presented by box-and-whisker plots and two-dimensional data by correlations diagrams. Statistical significance of possible jaw and side effect were assessed by two way ANOVA with repeated measures on both factors (program BMDP2V) to determine differences between the jaw quadrants for each tooth. Using a three way ANOVA with repeated measures (jaw by side by tooth position) differences between teeth in one jaw quadrant were analyzed, additionally, to detect possible interactions with the factor tooth position (program BMDP2V, too). Intraindividual differences between corresponding teeth in the left and right sides of the head were determined by variance decomposition using linear mixed model analysis. Correlations between opposing teeth were analyzed by an adequate correlation analysis (program BMDP6D). For each tested hypothesis the significance level was chosen at α = 0.05. So p- values of less than or equal to 0.05 were assumed to indicate statistical significance. 33 Results In all investigated incisors the occlusal surfaces were inclined in a rostroventral direction when viewed in a sagittal plane. Thus, the orientation of the sagittal angle was the same in all incisors. In contrast, viewed from a frontal position, the occlusal surfaces were inclined either in laterodorsal or lateroventral direction, thus the orientation of the transversal angle was inconsistent. For the purpose of data processing only the angular degree for the transversal angle was recorded, irrespective of the orientation of the inclination. Individual incisors Mean values of all individual incisors for transverse angles ranged from 3.5° (tooth 101) to 6.8° (tooth 203)for sagittal angles from 32.7°(tooth 101) to 44.9° (tooth 302).The minimum transverse angle was 0° (102, 7-year-old Arabian gelding); the maximum transverse angle was 22.5° (303, 7-year-old Haflinger mare). The minimum sagittal angle was 0.2° (102, 14-year-old warm blood mare); the maximum sagittal angle was 72.7° (303, 19-year-old warm blood gelding) (See Fig. 4, 5). Comparing the corresponding teeth (same Triadan position) of the left and right arcades no significant differences for the transverse angles and for the sagittal angles were present. Even though there is no statistical significant difference between the incisors of the left and right side of the jaw, an asymmetry in every individual horse was observed. In the upper jaw, there is an intraindividual difference between left and right side of 3.8° (SD 1.2°) for the transversal angle, and of 5.1° (SD 1.3°) for the sagittal angle. Similar intraindividual left-right differences were detected in the lower jaw, i.e.3.8° (SD 1.0°) for the transversal angle and 5.9° (SD 1.8°) for the sagittal angle. Comparing opposing incisors of the upper and lower jaw there is a significant difference in the sagittal angle in which the angles of the lower incisors (38.0 to 44.9°) display steeper angles than the incisors of the upper jaw (32.7° to 35.6°). For the transversal angle no significant difference between opposing incisors exists. Furthermore there is a positive correlation between opposing incisors. This positive correlation is shown for the transversal angle for the first and second incisors with a correlation coefficients of 0.41and p-value of 0.013 for Triadan position 01 and 0.40and p-value of 0.016 for Triadan position 02. Triadan position 03had a correlation coefficient of 0.31and a not significant p-value of 0.862. 34 In terms of the sagittal angle all three incisor positions show a positive correlation between the opposing teeth. Correlation coefficients of Triadan position 01 was0.663, of Triadan position 02 was 0.625 and of Triadan position 03 was 0.728. All Triadan positions had a p-value of <0.001 (See Fig. 6). Increasing angles in one quadrant are causing an increase of the angulations in the opposing quadrant. Within the upper quadrants the sagittal angles ranged between 32.7° and 35.6°. Within the lower arcades the incisors showed a range between 38.0° and 44.9°. Differences between Triadan positions appeared statistically significant according to the three way ANOVA with repeated measures; however the pair-wise comparison by the Student-Newman-Keuls-Test failed to confirm a statistical significance. Quadrants Mean values for transversal angles of the quadrants ranged from 4.0° to 6.6°, for the sagittal angle from 34.2° to 41.3°. The minimum transversal angle in a quadrant was 0.1° (quadrant 300, 24-year-old Knabstrupper mare); the maximum transversal angle was 16.4° (quadrant 100, 19-year-old warm blood gelding). The minimum sagittal angle was 5.9° (quadrant 100, 14-year-old warm blood mare), the maximum sagittal angle was 64.8° (quadrant 200, 9-year-old warm blood gelding) (See Fig. 7). Comparing upper and lower jaw there is a positive correlation between the occlusal surface angles for the transversal angle as well as for the sagittal angle. Increasing angles in one quadrant are causing an increase of the angulations in the opposing quadrant. Planes for the upper and lower bars as well as for the facial crest were constantly inclined in rostroventral direction when viewed from the side. The upper jaw bars and the facial crests had a slight curved shape. Therefore the virtual planes calculated by the computer software did not match very well with the actual shape of these structures. In contrast, the bars of the mandible featured a largely straight outline and the calculated virtual planes matched well. The mean sagittal angle of the lower jaw bars was17.3° (SD 4.2°) and 25.4° (SD 4.2°) for the upper jaw bars. The average sagittal angle of the facial crest was measured with 30.0° (SD 3.5°) (See Fig. 8). 35 Mean sagittal incisor angles calculated in relation to the bars ranged from 21.2° to 25.7° in the lower jaw and from 7.1° to 9.3° in the upper jaw. Mean sagittal incisor table angles calculated in relation to the facial crest ranged from 0.6° to 5.2° in the upper jaw (See Fig. 9). Extended occlusal surface planes of the incisors in the upper jaw crossed the temporomandibular joint in 55 % of the cases. In 24% of the cases the occlusal surface plane crossed the bulb of the eye and in another 16% the bridge. In 5% of the cases the occlusal surface plane runs ventral to the facial crest. Discussion Incisor occlusal tables –functional implications The equine incisor dentition is subjected to remarkable age-dependent changes concerning the position of the teeth within the jaws and concerning the length of the teeth. Once the permanent incisors have been erupted, the incisors of the opposing jaws form a straight line (angulation of ± 180°), when viewed in profile. With increasing age the incisors rotate in their alveoli and become aligned in more and more acute angles (Muylle et al., 1996). After eruption into the oral cavity, equine incisors grow in length and reach their maximum size at a dental age of approximately 4 years (Schrock et al. 2013a). Although incisors are subjected to continuous wear, the tooth length is maintained up to an age of 13 to 15 years post eruption due to constant production of dental substances at the apical end. After that period of time, tooth wear exceeds compensatory production of dental substances and the incisor length decreases (Schrock et al 2013a, Schrock et al. 2013b). It shall be emphasized, that these well-known features of the equine incisor dentition do not necessarily affect the incisor table angles. Although the tooth length as well as the position of the tooth within its alveolus changes with age, the angulation of the occlusal surface is assumed to remain constant within a certain range (Allen, 2008; Rucker, 2004; Klugh, 2010). This assumption is in a line with the results obtained in this study as no statistically significant age-related changes were detected. However due to the limited number of horses from different age-groups further, preferentially longitudinal studies are required to confirm this observation. It seems obvious, that the position of the tooth within its alveolus is triggered by different factors than the alignment of the occlusal surfaces. It is most likely that a constant pattern of tooth wear – and therefore a constant pattern of masticatory 36 movements– is the most contributing factor to a constant alignment of the incisor occlusal surfaces. Consequently, asymmetries in the normal masticatory chewing cycle should become reflected in the alignment of the incisor tables. This assumption is widely accepted and it has been stated that changes of incisor occlusal angles are often secondary to disorders of the cheek teeth and resultant abnormal masticatory action (DuToit and Rucker, 2011). Recently, Moore (2016) documented two cases of severe unilateral cheek tooth disease which resulted in shear mouth due to asymmetric grinding of the cheek teeth arcades. Subsequently, a marked diagonal malocclusion of the incisors occurred. Vice versa, a primary (congenital) misalignment of the incisor tables is known to cause, gradually, abnormal conditions in the cheek tooth dentition (Easley, 2016).The above described conditions are mainly focused on the incisor occlusal table alignment viewed from a frontal position, i.e. the transversal angle. However, also the sagittal angle of the incisor tables is shaped by the chewing cycle. It is considered to reflect the normal rostrocaudal movement of the mandible during normal masticatory movements (Bonin et al., 2006). Consequently, it has been recommended to maintain (or even restore) a normal sagittal angle during incisor treatment (Klugh, 2010; Rucker, 2004; Easley, 2011). However, others suggest that optimizing the cheek tooth occlusion is sufficient to restore a normal and functional occlusal table angle of the incisors (Earley, 2011). Incisor occlusal tables - clinical implications Based on these functional considerations it has been generally accepted that the proper inspection of the angulation of the incisor tables is of high diagnostic value (Baratt, 2010; Easley, 2011; Rucker, 2004). Transversal angles The transversal angles are quite easy to assess from a position in front of the horse. It has been suggested by several investigators that normal transversal angles are at approximately 90° in relation to a virtual median plane (Easley, 2011; Rucker, 2004). However, the data presented in this study showed a relative wide range of the transversal angles, with mean angle deviations of up to 6.8° from an ideal alignment in a horizontal plane. This contra dictionary result might be explained by the methodological approach used in this study. Under clinical conditions, actually not the occlusal surface is assessed, but a line presented by the labioocclusal edges of the 37 incisors. In contrast, for this study each occlusal incisor plane was defined using the entire outline of the occlusal surface of each incisor. Additionally, the measurements were performed using a very accurate computerized procedure, determining angles at an accuracy of one tenth of a degree. It is assumed that the observed deviations of up to 6.8° in single incisors represent a normal range and are in accordance with the general observation of a horizontal alignment – determined under clinical conditions. Sagittal angles In contrast to the transversal angles, the sagittal angles of the equine incisors were assessed in very heterogeneous ways leading to varying data of the normal sagittal angulation. All previous investigations suggest a uniform sagittal angle for all upper and lower incisors (Rucker, 2004; Allen, 2008; Ros, 2010; Klugh, 2010). However, the measurements reported here clearly show that lower incisors constantly feature steeper angels compared to their upper jaw antagonists. These contradictory results may be due to the specific measurements which were elaborated for the present study. For the first time, the sagittal angle for every individual incisor was determined. At first glance, the existence of different sagittal angles in opposing teeth seems to be implausible, since their occlusal surfaces have to be pressed firmly on each other during food intake. However, this contradiction becomes resolved upon recognizing that individual reference planes for of the upper and for the lower jaw were used. This methodical approach was urgently necessary to eliminate inaccuracy of measurements due to positional changes of the mandible. The methodical approach we used (computerized 3D-models), allowed repeating sagittal angle measurements with different reference planes. Thus, the reference lines previously suggested by Allen (2008), i.e. the lower jaw bar, and Rucker (2004), i.e. the facial crest, were additionally used to determine alternative sagittal incisors angles. A comparison between the sagittal angles given in the literature and the sagittal angles measured here became possible. In contrast to Rucker (2004) and Allen (2008) who determined a uniform angulation of 10° to 15°, we determined significant steeper sagittal angles (21.2° to 25.7°)for lower incisors (in relation to the lower jaw bars). The sagittal angulation for the upper incisors in relation to the upper jaw bars (7.1° to 9.3°) was lower than the suggested value of Allen (2008) and 38 Rucker (2004) i.e. 10 to 15°. Rucker (2004) stated that the facial crest has the same angulation as the incisor occlusal angle. However, our results document different angulations of the incisor occlusal tables and the facial crest at up to 5.2°. Due to the fact that the facial crest and the upper jaw bars feature a curved shape their use as a reliable reference line is limited. The lower jaw bars feature an easy accessible straight line and appear therefore suitable to be used as a reference line under clinical conditions. In relation to the lower bars, occlusal table angles of 21° to 26° of individual mandibular incisors should be considered normal. Other methods to determine and adjust the incisor occlusal angle by use of reference points (TMJ according to Ros (2010), Eyeball according to Klugh (2010)) which should be crossed by the occlusal surface plane appeared to be vague and are therefore not recommended for use. Conclusion Occlusal surface angles of equine incisors were determined by use of a precise measurement method in computerized 3D-models of equine skulls. Under clinical conditions the lower jaw bars appeared to be most suitable as a reference line to first determine and adjust mandibular incisors and subsequently upper incisors. Although mean angulations for normal occlusal surface angles were calculated, a wide range of occlusal surface angles in individual horses exists. References Allen, T., 2008. Examination. In: Allen, T. (Ed.), Manual of Equine Dentistry. Mosby, St. Louis, pp. 67-88. Baratt, R., 2010. How to recognize and clinically manage class 1 malocclusions in the horse. In: Proceedings of the 56th annual convention of the American Association of Equine Practitioners, Baltimore, Maryland (USA), 458-464. 39 Bonin, S.J., Clayton, H.M., Lanovaz, J.L., Johnston, T., 2006. Kinematics of the equine temporomandibular joint. Am. J. Vet. Res. 67, 423-428. Brinkschulte, M., Bienert-Zeit, A., Lüpke, M., Hellige, M., Staszyk, C., Ohnesorge, B., 2013. Using semi-automated segmentation of computed tomography datasets for three-dimensional visualization and volume measurements of equine paranasal sinuses.Vet. Radiol. Ultrasound. 54, 582-590. Brinkschulte, M., Bienert-Zeit, A., Lüpke, M., Hellige, M., Ohnesorge, B., Staszyk, C., 2014. The sinonasal communication in the horse: examinations using computerized three-dimensional reformatted renderings of computed-tomography datasets. BMC Vet. Res. 10, 72. Castell, J., Vogt, C. 2011. Morphologische Veränderungen am Pferdegebiss und ihre Bearbeitung. In: Vogt, C. (Ed.), Lehrbuch der Zahnheilkunde beim Pferd. 1st ed. Schattauer, Stuttgart, pp. 125-182. Dixon, W. J. (chief editor), 1993.BMDP Statistical Software Manual, Volume 1 and 2.University of California Press, Berkeley, Los Angeles, London. DuToit, N., Rucker, B., 2011. Geriatric dentistry. In: Easley, J., Dixon, P.M., Schumacher, J. (Eds.), Equine dentistry. 3rded. Saunders Elsevier, pp. 279-288. Earley, E., 2011. Skeletal abnormalities in the equine skull associated with diagonal incisor malocclusion. American Association of Equine Practitioners Focus Proceedings 2011, 131-133. Easley, J., 2011. Corrective dental procedures. In: Easley, J., Dixon, P., Schumacher, J., Equine dentistry. 3rd ed. Saunders Elsevier, pp. 261-277. Easley, J., 2016. Abnormal dental wear – a paradigm shift. Equine vet. educ. 28, 20- 22. 40 Floyd, M.R., 1991. The modified Triadan system: nomenclature for veterinary dentistry. Journal of Veterinary Dentistry 8, 18–19. Klugh, D.O., 2010. Anatomical characteristics of equine dentition. In: Klugh, D.O. (Ed.), Principles of equine dentistry. 1st ed. Manson Publishing, London, pp. 27-48. Klugh, D.O., 2010. Principles of occlusal equilibration. In: Klugh, D.O. (Ed.), Principles of equine dentistry. 1st ed. Manson Publishing, London, pp.69-78. Listmann, L., Schrock, P., Failing, K., Staszyk, C., 2016. Occlusal angles of equine cheek teeth. Livestock science 186, 78-84. Moore, N.T., 2016. Clinical findings and treatment of shear mouth in two horses associated with ipsilateral painful dental disease. Equine vet. educ. 28, 13-19. Muylle, S., Simoens, P., Lauwers, H., 1996. Ageing horses by an examination of their incisor teeth: an (im)possible task? The Veterinary Record 138, 295-301. Ros, K., 2011. Biomechanik. In: Vogt, C.(Ed.), Lehrbuch der Zahnheilkunde beim Pferd. 1st ed. Schattauer, Stuttgart, pp. 31-48. Rucker, B.A., 2004.Incisor and molar occlusion: Normal ranges and indications for incisor reduction. In: Proceedings of the 50th annual convention of the American Association of Equine Practitioners, Denver, Colorado (USA), 7-12. Schrock, P., Lüpke, M., Seifert, H., Staszyk, C., 2013a. Three-dimensional anatomy of equine incisors: tooth length, enamel cover and age related changes. BMC Veterinary Research 9, 249. Schrock, P., Lüpke, M., Seifert, H., Staszyk, C., 2013b.Finite element analysis of equine incisor teeth. Part 2: Investigation of stresses and strain energy densities in the periodontal ligament and surrounding bone during tooth movement. The Veterinary Journal 198, 590-598. 41 Triadan, H., 1972. Tierzahnheilkunde: Zahnerhaltung (Füllungstherapie mit ‘Composite materials’ und Endodontie) bei Affen und Raubtieren. Schweizer Archiv für Tierheilkunde 114, 292–316. 42 Figure 1: Three dimensional model of an equine skull. a) Maxillary incisors sectioned by a virtual transversal plane. b) The transversal angle (green) indicates the angulation of the maxillary incisor occlusal plan (red) in relation to the skull’s horizontal plane (light blue). Yellow: Skull median plane. c) Upper incisors sectioned by a virtual upper skull sagittal plane. 43 d) The sagittal angle (green) indicates the angulation of the maxillary incisor occlusal plane (red) in relation to the Skull’s horizontal plane (light blue). Dark blue: upper skull transverse plane. e) Green angle: Angulation of the mandibular incisor occlusal plane (red) in relation to the mandibular horizontal plane (light blue). Yellow angles: Angulations of the lower and upper bar (orange) in relation to the upper skull/mandibular horizontal plane (light blue). Note the curved shape of the upper bar. Dark blue: upper skull/mandibular transverse plane. 44 Figure 2: Occlusal surface angulation measurements exemplarily shown for tooth 101. a) The angulation (α) between the upper skull median plane (ump) and the occlusal surface plane (red) was measured and the transversal angle was calculated according to the formula: α – 90° b) The angulation (α) between the upper skull transversal plane (utp) and the occlusal surface plane (red) was measured and the sagittal angle was calculated according to the formula: 90°– α 45 Figure 3: Upper incisor occlusal surface plane (grey) in relation to the TMJ (red circle). 46 a) 5-year-old warmblood. The occlusal surface plane crosses the TMJ as indicated by the red arrow. b) 7-year-old Arabian. The occlusal surface plane crosses the orbita as indicated by the red arrow. c) 20-year-old warmblood. The occlusal surface plane is placed ventral to the TMJ as indicated by the red arrow. TMJ indicate temporomandibular joint. 47 Fig. 4: Box-and-whisker plot shows the transversal angles of upper and lower incisors measured in relation to the upper skull median plane and the mandible median plane, respectively. Boxes represent the lower and upper quartiles, lines inside the boxes are medians and hashes represent mean values. Whiskers represent the upper and lower 25% except minimal and maximal values (black dots). 48 Fig. 5: Box-and-whisker plot shows the sagittal angles of upper and lower incisors measured in relation to the upper skull transversal plane and the mandible transversal plane, respectively. Lower jaw incisors feature statistically significant steeper occlusal table angles than upper incisors (all p-values<0.05). Boxes represent the lower and upper quartiles, lines inside the boxes are medians and hashes represent mean values. Whiskers represent the upper and lower 25% except minimal and maximal values (black dots). 49 Fig. 6: Correlation diagram shows the association between antagonistic incisor table angles. a, b, c) Correlation diagram shows the association between antagonistic incisor sagittal angles. Correlations are displayed separately for each side of the head (blue dots: right side, red dots: left side). Correlation coefficients were calculated for each Triadan position: a) Triadan 01: 0.663, b) Triadan 02: 0.625, c) Triadan 03: 0.728. Correlations were significant with p-values < 0.05. d, e, f) Correlation diagram shows the association between antagonistic incisor transversal angles. Correlations are displayed separately for each side of the head (blue dots: right side, red dots: left side). Correlation coefficients were calculated for each Triadan position: d) Triadan 01: 0.41; e) Triadan 02: 0.40; f) Triadan 03: 0.31. Correlations were significant with p-values < 0.05. 50 Figure 7: Box-and-whisker plot shows the angulation of the complete upper and lower incisor arcades. a) Incisor arcade transversal table angles measured in relation to the upper skull and mandibular median plane, respectively. b) Incisor arcade sagittal table angles measured in relation to the upper skull and mandibular transversal plane, respectively. Boxes represent the lower and upper quartiles, lines inside the boxes are medians and hashes represent mean values. Whiskers represent the upper and lower 25% except minimal and maximal values (black dots). 51 Fig. 8: Box-and-whisker plot shows the angulation of the bars of the upper jaw (a) and the angulation of the facial crest (b) in relation to the upper skull transversal plane and the upper jaw sagittal incisor table angles in relation to the upper jaw bars (a) and the facial crest (b). Boxes represent the lower and upper quartiles, lines inside the boxes are medians and hashes represent mean values. Whiskers represent the upper and lower 25% except minimal and maximal values (black dots). 52 Fig. 9: Box-and-whisker plot shows the angulation of the bars of the lower jaw in relation to the mandibular transversal plane and the lower jaw sagittal incisor table angles in relation to the lower jaw bars. Boxes represent the lower and upper quartiles, lines inside the boxes are medians and hashes represent mean values. Whiskers represent the upper and lower 25% except minimal and maximal values (black dots). 53 4 Übergreifende Diskussion 4.1 Entwicklung einer Messmethode Die komplexe räumliche Gestalt des Pferdeschädels und die pferdetypische Gestalt der Okklusalfläche der Schneide- und Backenzähne, in Form von erhabenen Schmelzleisten und Dentinbecken, stellten große Herausforderungen bei der Messung der Kauflächenwinkel equiner Zähne dar. Bisherige Messungen wurden am lebenden sedierten Pferd oder am anatomischen Präparat durchgeführt. Schwierigkeiten die sich hierbei ergaben, waren Messungenauigkeiten durch Tierbewegungen und eine schlechte Reproduzierbarkeit von Tier zu Tier, durch den Mangel an festen anatomischen Referenzpunkten. Bei den Messungen von Ralston et al. (2001), Carmalt et al. (2005) und Brown et al. (2008) wurde der Kauflächenwinkel mit einem biegsamen Draht oder einer Metallplatte gemessen und von Hand auf Papier übertragen. Dieser Arbeitsschritt birgt ein zusätzliches Risiko für Ungenauigkeiten bei der Winkelmessung. Zudem wurden in einem Großteil der bisherigen Messungen nur einzelne Backenzähne gemessen (Ralston et al., 2001; Carmalt et al., 2005; Brown et al., 2008) oder die Winkelung für die gesamte Backenzahnarkade berechnet (Rucker, 2004; Carmalt, 2004; Carmalt et al, 2005). Winkelmessungen für jeden einzelnen Backenzahn wurden nur von Brown et al. (2008) durchgeführt. Verlässliche Werte für die Winkelung einzelner Schneidezähne gibt es bisher nicht. Für die hier durchgeführten Messungen wurden 20 Pferdeköpfe mit permanentem Gebiss ausgewählt. Fallzahlen in anderen Studien über die Kauflächenwinkelung beim Pferd (Ralston et al. 2001; Carmalt et al., 2005; Brown et al., 2008) lagen zwischen 8 und 40 Versuchstieren. Lediglich für die Untersuchungen von Rucker (2004) wurde eine deutlich höhere Fallzahl von 730 Pferden verwendet, wobei die Untersuchung in diesem Fall während der Routinezahnbehandlung durchgeführt werden konnte und technisch nicht so umfangreich und aufwendig war, wie in den anderen Studien. Die hier verwendeten Pferdeköpfe stammten von Warmblütern und Vollblütern im Alter zwischen 5 und 26 Jahren. Alle Tiere wurden an der „Klinik für Pferde der Stiftung Tierärztliche Hochschule Hannover“ wegen anderer Erkrankungen als Zahnerkrankungen eingeschläfert. Die zufällig ausgewählten Schädel und Gebisse wiesen keine offensichtlichen pathologischen Veränderungen oder Anomalitäten des 54 Kopfes auf. Vermessen wurden nur Zähne ohne Frakturen oder deutliche pathologische Veränderungen. Die Geschlechterverteilung war 6 weibliche und 14 männliche Tiere. Die Zähne wurden nach dem Triadan System mit den Nummern 01 (erster Schneidezahn) bis 11 (dritter molarer Backenzahn) benannt (Triadan, 1972; Floyd, 1991). Um den oben genannten Problemen gerecht zu werden wurden CT-Schnittbilder der ausgewählten Pferdeköpfe erstellt und dann mit Hilfe des Bildbearbeitungsprogramms Amira (Version 5.4.2, Visage Imaging GmbH, Berlin, Germany) zu 3-dimensionalen Modellen konstruiert. Diese 3D-Modelle gaben die geometrische Form des Schädels und der Zähne millimetergenau wieder. Dadurch konnte die Winkelung der Kauflächen der Schneide- und Backenzähne, mit Hilfe fester Referenzpunkte und Referenzebenen, bis auf wenige Grad Abweichung, sehr exakt bestimmt werden. Die so entstandenen Messungen waren folglich von sehr hoher Präzision und von wenigen Fehlerquellen, wie dem Übertragen auf Papier, betroffen. Nach einer ähnlichen Vorgehensweise wurden bereits kraniofaciale und maxillofaciale Analysen an humanen Schädeln durchgeführt und brachten sehr akkurate und zuverlässige Daten hervor (Olszewski et al., 2007; Lopes et al., 2008). Nachteil dieser Messmethode ist der hohe finanzielle und zeitliche Aufwand zur Erstellung der CT-Schnittbilder, des 3D-Modells und der Winkelvermessung jeden einzelnen Zahns. Dadurch sind die so durchgeführten Messungen in der alltäglichen klinischen Praxis nicht umzusetzen. Für die hier durchgeführten Messungen wurden verschiedene Referenzebenen und Referenzlinien am Schädel und am Zahn konstruiert, die später zur Messung der Kauflächenwinkel verwendet wurden. Diese Referenzebenen wurden, mithilfe fester anatomischer Referenzpunkte am Pferdeschädel und an den Zähnen, erstellt. Dadurch wurde die Messung auf alle Schädel reproduzierbar und die vermessenen Kauflächenwinkel untereinander leicht vergleichbar. Die für die Messung ausgewählten anatomischen Referenzpunkte am Schädel und am Zahn zeichneten sich durch geringe individuelle Schwankungen, wie zum Bespiel durch den Zug von Muskelansätzen, aus und konnten dadurch exakt auf alle Schädel übertragen werden. Auch in der Humanmedizin gibt es Referenzebenen und Referenzlinien am Schädel und dem Gebiss, die anhand fester anatomischer Referenzpunkte konstruiert werden 55 (Pittschieler et al., 2016). So werden zum Beispiel die Campersche Ebene, die Frankfurter Horizontale und die Achs-Orbital-Ebene als Referenz bei der Beurteilung von Röntgenbildern des Schädels, zur Konstruktion von Mittelwertartikulatoren und für die Prothetik und Restauration in der Zahnmedizin herangezogen. In bisherigen Messungen am equinen Gebiss wurde die Winkelung der okklusalen Oberfläche der Backenzähne entweder an einer gedachten Horizontalen (Ralston et al., 2001; Carmalt et al., 2005), durch die indirekte Schneidezahnseperationstechnik (Rucker, 2004; Carmalt, 2004; Carmalt et al., 2005) oder im Verhältnis zur lingualen Oberfläche des Backenzahns (Brown et al., 2008) gemessen. Diese Methoden allerdings berücksichtigen nicht eine mögliche Neigung des Zahns innerhalb der Alveole, welche die Kauflächenwinkelung in Relation zu anderen Strukturen des Schädels sicher beeinflussen würde. Bei der hier vorgestellten Messung wurden nun erstmals Referenzpunkte am knöchernen Schädel verwendet und damit ein Zusammenhang mit relevanten Strukturen der Kaubewegung und dem Kaumechanismus hergestellt. Die Ergebnisse dieser Studie könnten deshalb zukünftig bei der weiteren Erforschung des Kauvorgangs Verwendung finden. 4.2 Winkelmessungen 4.2.1 Winkelung der equinen Backenzähne Die equinen Zähne werden durch das Muster der Kaubewegung abgeschliffen. Das wird durch Kaiser et al. (2013) bestätigt, die beschreiben, dass die Gestalt der Kaufläche der Backenzähne, in Form von Dentinbecken und Schmelzkämmen, ein Resultat stetiger Attrition und Abrasion während des Kauvorgangs ist. Während des ‚Power Strokes‘ führt die okklusale Oberfläche eines mandibulären Backenzahns eine transversale lineare Bewegung auf der Kaufläche des antagonisierenden maxillären Backenzahns aus (Kaiser et al., 2013). Schmelzkämme dienen hierbei als Schleifkanten und in den Dentinbecken wird das Futter gepresst und zerkleinert (Kaiser et al., 2010). Durch diese, für die Futterzerkleinerung, essentielle Topografie der equinen Okklusalflächen der Backenzähne ist die Bestimmung einer okklusalen Referenzebene für die Winkelmessung deutlich erschwert. 56 Um den unterschiedlichen Winkelungen einer Kaufläche, durch die Erhöhungen und Vertiefungen, gerecht zu werden, wurden die Kauflächenwinkel an jedem Backenzahn mit Hilfe von drei bukkolingual bzw. bukkopalatal verlaufenden Linien gemessen. Jede Linie ergab sich aus jeweils zwei anatomischen, zuvor festgelegten Referenzpunkten auf der okklusalen Oberfläche. Auch diese Referenzpunkte wurden so ausgewählt, dass individuelle Schwankungen möglichst gering waren. Durch die gute Reproduzierbarkeit von Zahn zu Zahn, wurden exakte und vergleichbare Winkelmessungen gewährleistet. Da sich die Kaufläche in der praktischen Arbeit palpatorisch besser als Fläche erfassen lässt, wurde zusätzlich für jeden Zahn eine Kauflächenebene zur Winkelmessung, anhand der festgelegten anatomischen Referenzpunkte, erstellt. Bei der nachfolgenden statistischen Auswertung stellte sich heraus, dass die gemittelten Werte der drei Kauflächenlinien eines jeden Zahns, im Vergleich zur Kauflächenebene des jeweiligen Zahns, keinen signifikanten Unterschied ergaben. Folglich wurden nur die Ergebnisse der Kauflächenebenenmessung zur weiteren Analyse herangezogen. Im Gegensatz zu anderen Studien (Ralston et al., 2001; Carmalt, 2004; Carmalt et al., 2005), wurden für diese Arbeit Messungen an jedem einzelnen Backenzahn in allen Kieferquadranten durchgeführt. Diese Vorgehensweise scheint notwendig, da bereits klar gezeigt wurde, dass es wesentliche Unterschiede zwischen den einzelnen Zähnen innerhalb eines Quadranten gibt (Brown et al., 2008). Dementsprechend sind Daten aus Untersuchungen an nur einem einzelnen Zahn (Ralston et al., 2001; Carmalt et al., 2005) von geringer Relevanz für die Beurteilung des Kauflächenwinkels der gesamten Zahnreihe. Das Gleiche gilt für Studien, die einen einheitlichen Winkel für die gesamte Backenzahnarkade anhand einer indirekten Messung der Verschiebbarkeit der Schneidezähne errechnet haben (Rucker, 2004; Carmalt, 2004). In Übereinstimmung mit Brown et al. (2008) steigt der Kauflächenwinkel equiner Backenzähne, innerhalb einer Backenzahnarkade, von rostral nach kaudal an. Jedoch konnten wir im Gegensatz zu Brown et al. (2008), der von einem kontinuierlichen Anstieg ausgeht, zeigen, dass der Anstieg stufenweise in 3 Blöcken stattfindet. Die Anordnung dieser Kaublöcke korrespondiert mit einer bemerkenswerten Charakteristik des equinen Kauzyklus. Aufgezeichnete 57 Bewegungen des Unterkiefers zeigten, dass der mediodorsal gerichtete ‚power stroke‘ keine kontinuierliche Bewegung ist, sondern aus mindestens 2 Phasen besteht (Bonin et al., 2006). Im Gegensatz zu den ermittelten Werten von Brown et al. (2008), die eine Bandbreite von 11,8° bis 31,5° aufwiesen, lagen die Mittelwerte der Kauflächenwinkel der Backenzähne in dieser Studie nur zwischen 15,1° und 20,2°. Außerdem beschreiben Brown et al. (2008) eine bedeutende Asymmetrie zwischen Unterkiefer- (steilerer Winkel) und antagonisierenden Oberkieferbackenzähnen (flacherer Winkel). Einen statistisch signifikanten Unterschied zwischen der Winkelung von Ober- und Unterkieferbackenzähnen konnten wir allerdings nicht feststellen. Die Unterschiede zu den Messergebnissen von Brown et al. (2008) könnten auch auf die unterschiedliche Messtechnik zurückzuführen sein. In jedem untersuchten Schädel wurden zusätzlich intraindividuelle Variationen gefunden. Jedes Pferd zeigte eine Asymmetrie zwischen rechter und linker Schädelhälfte, mit einem abweichenden Kauflächenwinkel von 3° bis 5°. Diese Rechts-Links-Asymmetrie wird bestätigt durch die Tatsache, dass Pferde eine bevorzugte Seite zum Kauen haben (Bonin et al., 2006; Baker, 2005). 4.2.2 Winkelung der equinen Schneidezähne Für die Winkelmessung der Schneidezähne, wurden auf jedem Schneidezahn sechs Punkte auf der peripheren Schmelzlinie der Kaufläche markiert und zu einer Okklusalebene verbunden. Da die okklusalen Oberflächen der einzelnen Schneidezähne beim Kauvorgang eher gemeinsam als Kaublock funktionieren und dieser in der klinischen Praxis palpatorisch leichter zu erfassen ist, wurde zusätzlich auf die gleiche Weise eine Okklusalebene aller Schneidezähne eines jeweiligen Quadranten und des gesamten Ober- bzw. Unterkiefers konstruiert. Für jeden Zahn wurden zwei Winkel bestimmt. Der Transversalwinkel wurde gemessen zwischen der Okklusalebene der Schneidezähne und der Medianebene. Der Sagittalwinkel wurde gemessen zwischen der Okklusalebene der Schneidezähne und der Transversalebene. Besonders die Schneidezähne unterliegen im Laufe der Zeit einer stetigen Veränderung. Während sie mit einem beinahe senkrechten Winkel durchbrechen, ändert sich ihre Stellung im Kiefer altersabhängig und die Winkelung vom Oberkiefer- 58 zum Unterkieferschneidezahn wird immer steiler (Muylle et al., 1996). Davon weitgehend unverändert bleibt der Kauflächenwinkel gemessen an der Transversal- und Sagittalebene. Auch in diesem Zusammenhang zeigt sich, dass ein konstantes Muster der Zahnabnutzung- und damit ein konstantes Muster der Kaubewegung- zur Formung der okklusalen Oberfläche der Schneidezähne führt. Anders als bei den Backenzähnen, bei denen hauptsächlich die bukkolinguale bzw. bukkopalatale Winkelung Beachtung findet, stehen im Bereich der Schneidezähne zwei Winkel im Focus, der Transversalwinkel und der Sagittalwinkel. Es ist allgemein anerkannt, dass der Transversalwinkel, im Normalzustand, einen annähernd 90°- Winkel zu einer visualisierten Medianebene bildet. In diesen Messungen wurden Abweichungen der okklusalen Oberfläche der Schneidezähne von durchschnittlich 6,8° von der Horizontalen festgestellt. Es wird davon ausgegangen, dass diese Abweichung auf die sehr genaue computerisierte Messmethode zurückzuführen ist, und eine Abweichung von 6,8° eine physiologische Spannbreite der einzelnen Schneidezähne darstellt. Sie stimmt trotzdem mit der generellen Beobachtung einer horizontalen Ausrichtung der okklusalen Oberfläche der Schneidezähne überein. Im Gegensatz zum Transversalwinkel wird der Sagittalwinkel sehr kontrovers diskutiert, wodurch eine große Variation von Daten über eine physiologische sagittale Kauflächenwinkelung entstanden ist. Frühere Untersuchungen beschreiben einen einheitlichen Sagittalwinkel für alle Ober- und Unterkieferschneidezähne (Rucker, 2004; Allen, 2008; Ros, 2011; Klugh, 2010). Die hier durchgeführten Messungen zeigen aber klar, dass die Unterkieferschneidezähne konstant einen statistisch signifikant steileren Sagittalwinkel hatten als ihre Antagonisten im Oberkiefer. Dieser Gegensatz könnte eine Folge der spezifischen Messmethode sein, die für diese Studie erarbeitet wurde. Zum ersten Mal wurde der sagittale Winkel für jeden Schneidezahn individuell gemessen. Auf den ersten Blick scheinen unterschiedliche Sagittalwinkel in antagonistischen Schneidezähnen nicht plausibel, da ihre Okklusionsflächen während der Futteraufnahme fest aufeinander gepresst werden. Dieser Widerspruch erklärt sich allerdings dadurch, dass unterschiedliche Referenzebenen für die Messung in Ober- und Unterkiefer verwendet wurden. Diese Vorgehensweise war wegen der Beweglichkeit des Unterkiefers gegen den Oberkiefer notwendig, um inakkurate Ergebnisse durch Positionsveränderungen während der Messung zu verhindern. Die hier angewendete Messmethode, an 59 computerisierten 3D-Modellen, erlaubte wiederholte Messungen des sagittalen Winkels mit verschiedenen Referenzebenen. Bisherige Messungen der Schneidezahnwinkel wurden von Allen (2008) im Verhältnis zur Unterkieferlade durchgeführt. Außerdem entspricht der sagittale Winkel der Schneidezähne, nach Rucker (2004), annähernd dem Verlauf der Crista facialis. Die vorgeschlagenen Referenzlinien von Allen (2008) und Rucker (2004) wurden deshalb zusätzlich genutzt, um einen alternativen sagittalen Winkel zu bestimmen. Ein Vergleich zwischen den sagittalen Winkeln, die in der Literatur beschrieben sind und denen die hier ermittelt wurden, wurde so möglich. Im Gegensatz zu Rucker (2004) und Allen (2008), die einen einheitlichen Winkel von 10° bis 15° gemessen haben, konnten wir einen statistisch signifikant steileren Sagittalwinkel von 21,2° bis 25,7° im Unterkiefer in Relation zur Unterkieferlade messen. Die sagittale Winkelung der Oberkieferschneidezähne, in Relation zur Oberkieferlade, war mit 7,1° bis 9,3° flacher als die vorgeschlagenen Werte von 10° bis 15° von Allen (2008) und Rucker (2004). Außerdem konnten wir zwischen dem Verlauf der Crista facialis und der Winkelung der Schneidezahnokklusionsflächen im Oberkiefer Abweichungen von bis zu 5,2° nachweisen. Da die Crista facialis und die Oberkieferlade eine leicht gebogene Form haben, sind sie als verlässliche Referenz klinisch nur bedingt einsetzbar. Die Unterkieferlade zeigt eine leicht zugängliche gerade Linie und ist deshalb als Referenzlinie unter klinischen Bedingungen gut geeignet. Ein Sagittalwinkel an den Unterkieferschneidezähnen von 21° bis 26°, in Relation zur Unterkieferlade, sollte als normal angesehen werden. Andere Methoden um den okklusalen Winkel der Schneidezähnen zu beurteilen, wie die Verwendung des Kiefergelenks als Referenzpunkt, wie bei Ros (2011) oder der Augapfel (Klugh, 2010), welche von der Kauflächenebene gekreuzt werden sollen, scheinen sehr vage zu sein und können deshalb nicht empfohlen werden. 60 4.3. Schlussfolgerung Bei den hier durchgeführten Messungen zeigte sich, dass sich die Winkelungen der antagonisierenden Ober- und Unterkieferzähne im Schneidezahn- sowie im Backenzahnbereich einander anpassen. Das bedeutet, steilere Winkel im Oberkiefer erzeugten steilere Winkel im Unterkiefer und umgekehrt. Es konnte allerdings nicht genau geklärt werden, welcher Zahn von welchem modelliert wird. Angaben von Baker (2002), dass die Winkelung der Backenzahnkaufläche aus der 15° Winkelung des Kiefergelenks entsteht sind eher unwahrscheinlich. Nach Bonin et al. (2006) ist es umgekehrt, dass nämlich die Bewegung des Kiefergelenks vom Zahnabrieb und der Okklusion bestimmt wird. Einen großen Einfluss auf die Winkelung hat unbestritten der Kauvorgang. Diese Beobachtung wurde von Bonin et al. (2007) auch für gesunde Pferdezähne bestätigt. Weiterhin gibt es Beschreibungen, dass einseitiges Kauen, z.B. durch schmerzhafte Erkrankungen der Backenzähne oder des Kiefers auf einer Seite, zu massiven Veränderungen der Kauflächenwinkelung der Backenzähne auf der gegenüberliegenden Seite führen können, bis hin zum einseitigen Scherengebiss (Dixon, 2010). Bemerkenswert ist dabei, dass die veränderte Kauflächenwinkelung, wie das Scherengebiss, immer auf der erkrankten, schmerzhaften Seite entsteht, auf der nicht gekaut wird (Baker, 2002). Es ist bestätigt, dass sich dieser einseitige Kauvorgang unter Umständen auch im Bereich der Schneidezähne, in Form einer diagonalen Malokklusion, wiederspiegeln kann (Du Toit und Rucker, 2011; Moore, 2016). Außerdem gab es Beobachtungen, dass es durch Veränderungen im Bereich der Schneidezähne, durch z.B. eine kongenitale Missbildung, zu Winkelveränderungen der Kauflächen der Backenzähne kam (Easley, 2016). Ausgeprägtes Kauen führt dementsprechend vermutlich zu einem flacheren Kauflächenwinkel. Diese Vermutung wird von Dixon (2002) unterstützt, der beschreibt, dass Pferde die mit Futtermitteln mit hohem Raufaseranteil gefüttert werden einen flacheren Kauflächenwinkel haben, durch die größere Lateralverschiebung des Unterkiefers beim Kauen (Bonin et al., 2007). Die Fütterung mit energiereichen, faserarmen Futtermitteln führt dementsprechend zu steileren Kauflächenwinkeln (Dixon, 2002). Somit hat auch die Struktur der Futtermittel einen Einfluss auf den Kauflächenwinkel. Zusätzlich kann davon ausgegangen werden, dass alle schmerzhaften Zahnerkrankungen und Abweichungen von der normalen Zahnstellung, sowohl im Schneide- als auch im Backenzahnbereich, durch ihren Einfluss auf den 61 Kauvorgang, Auswirkungen auf die Kauflächenwinkelung aller Zähne haben (Easley, 2016). In Untersuchungen von Smyth et al. (2016) konnte eine Veränderung des Kauzyklus bzw. ein Wechsel der bevorzugten Kauseite nach einer provozierten Entzündung des Kiefergelenks festgestellt werden. Ursachen für einen pathologischen Kauflächenwinkel können folglich eine kongenitale oder erworbene Malformation des Schädels, schmerzhafte Erkrankungen an den Zähnen oder dem Kiefergelenk mit daraus resultierendem fehlerhaften Kauvorgang oder Fehlabnutzung, insbesondere der Schneidezähne, auch zum Beispiel durch Krippenbeißen und Koppen, sein (Dixon, 2010; Easley, 2016; Johnson und Porter 2006; Moore, 2016; Viranta und Mannermaa, 2017). Nach der Therapie und Behebung der schmerzhaften Ursache ist eine Behandlung der veränderten Kauflächenwinkel sinnvoll. Dabei sollte die Rückführung zur Normokklusion langsam und in mehreren Schritten durchgeführt werden. Möglicherweise kann ein wiederhergestellter schmerzfreier und physiologischer Kauvorgang auch zur Korrektur leichter Winkelveränderungen beitragen und bei der Therapie eingeplant und genutzt werden. Eine gute Okklusion der Schneide- und Backenzähne ist nicht nur essentiell für einen funktionierenden Kauvorgang. Aus der Humanmedizin ist bekannt, dass eine schlechte Okklusion begünstigend sein kann bei der Entstehung von Kiefergelenkserkrankungen (Dodic et al., 2009). Auch beim Pferd ist bekannt, dass eine veränderte Okklusion oder Malokklusion nicht nur zu Problemen beim Kauvorgang führt, sondern Auswirkungen auf die Gesundheit des Kiefergelenks und auf de