Three-dimensional anatomy of equine incisors: tooth length, enamel cover and age related changes

Abstract

Background: Equine incisors are subjected to continuous occlusal wear causing multiple, age related changes of the extragingival crown. It is assumed that the occlusal wear is compensated by continued tooth elongation at the apical ends of the teeth. In this study, μCT-datasets offered the opportunity to analyze the three-dimensional appearance of the extra- and intraalveolar parts of the enamel containing dental crown as well as of the enamel-free dental root. Multiple morphometric measurements elucidated age related, morphological changes within the intraalveolar part of the incisors.

Results: Equine incisors possess a unique enamel cover displaying large indentations on the mesial and distal sides. After eruption tooth elongation at the apical end outbalances occlusal wear for two to four years resulting in increasing incisor length in this period of time. Remarkably, this maximum length is maintained for about ten years, up to a tooth age of 13 to 15 years post eruption. Variances in the total length of individual teeth are related to different Triadan positions (central-, middle- and corner incisors) as well as to the upper and lower arcades.

Conclusion: Equine incisors are able to fully compensate occlusal wear for a limited period of time. However, after this ability ceases, it is expected that a diminished intraalveolar tooth length will cause massive changes in periodontal biomechanics. The time point of these morphodynamic and biomechanical changes (13 to 15 years post eruption) occurs in coincidence with the onset of a recently described destructive disease of equine incisor (equine odontoclastic tooth resorption and hypercementosis) in aged horses. However, further biomechanical, cell biological and microbiological investigations are needed to elucidate a correlation between age related changes of incisor morphology and this disease.

Figure 1

3D-models and μCT-scans of equine incisors. a: 3D model of an upper left first incisor (Triadan 201). Numbers indicate reference points (rp). 1 = Most occlusal point of the distal part of the occlusal table; 2 = Most occlusal point of the labial part of the occlusal table; 3 = Most occlusal point of the mesial part of the occlusal table; 4 = Most occlusal point of the palatal part of the occlusal table; 5 = Most apical point of the palatal enamel cover; 6 = Most apical point of the labial enamel cover. b: 3D model of an upper left first incisor (Triadan 201). Characters indicate reference planes. OP = Occlusal plane, calculated from rp1-rp4; SP = Sagittal plane, calculated from rp2, rp4, rp5, rp6. c: Sagittal μCT - cross section (according to the described sagittal plane SP) of an upper left first incisor (Triadan 201). Characters indicate reference planes. OP = Occlusal plane; SP = Sagittal plane; Numbers indicate reference points (rp). 2 = Most occlusal point of the labial part of the occlusal table; 4 = Most occlusal point of the palatal part of the occlusal table; 5 = Most apical point of the palatal enamel cover; 6 = Most apical point of the labial enamel cover.; 7 = Most apical point of the tooth, labial side; 9 = Most apical point of the infundibular enamel, labial side; 8 = Most apical point of the infundibular enamel, palatal side; 10 = Most apical point of the tooth, palatal side. d: Sagittal μCT - cross section of an upper left first incisor (Triadan 201). Lower-case characters and solid orange lines indicate measure-lines; Lower-case characters and turquoise dotted lines indicate calculated lines. a = Labial length of the tooth, a’ = Palatal length of the tooth; b = Labial length of the enamel cover; b’ = Palatal length of the enamel cover; c = Labial length of the infundibular enamel; c’ = Palatal length of the infundibular enamel; w = Labial length of the root; w’ = Palatal length of the root; x = Distance between the apical infundibular limit and the most apical point of the labial enamel cover; y = Distance between the infundibular limit and the most apical point of the palatal enamel cover; z = Mean length of the infundibulum. e: 3D model of an upper right first incisor (Triadan 101), view on the palatal side of the tooth. Red points and numbers indicate reference points (rp). 7 = Most apical point of the tooth, labial side; 10 = Most apical point of the tooth, palatal side. Characters indicate reference planes, SP = Sagittal plane. Lower-case character and orange line indicates measure-line. a’ = Palatal length of the tooth. Distances between rps, which are not aligned on the sagittal plane (rp 7 and 10) were measured on the 3D surface, following a virtual plane, bisecting the apex sagittally.

Figure 2

Relative tooth length of equine incisors. a: upper jaw incisors. b: lower jaw incisors; symbols indicate individual measurements, line indicates trend line and dashed lines indicate upper and lower 95% confidence limit.

Figure 3

Boxplots representing the tooth lengths of incisors in different Triadan positions. a: upper jaw incisors. b: lower jaw incisor. Triadan position assigned to the tooth length. Box: interquartile range (25%, 75%), horizontal line: median, center point: mean, whiskers: range (5%, 95%), stars: minimum and maximum. Lower-case characters (a, b) indicate a significant difference. (p-values upper jaw incisors: Triadan 01 against 02: p = 0.038, Triadan 01 against 03: p = 0.54, Triadan 02 against 03: p = 0.0019; p-values lower jaw incisors: Triadan 01 against 02: p = 0.16, Triadan 01 against 03: p = 0.007, Triadan 02 against 03: p = 0.32).

Figure 4

3D model of incisors, showing the not uniform tube like enamel cover. a, b, c Triadan 201 (please also see Additional video file 1) with symmetric indentations (black arrows) and most apical points of the enamel cover (black and white arrow heads) located in the sagittal plane of the tooth. d, e, f Triadan 103 (please also see Additional video file 2) with the mesial incision (d, black arrow) reaching further occlusal than the distal one (f, black arrow) and the most apical points of the enamel cover located parasagittal (black and white arrow heads). White = enamel cover, transparent = dental root composed of dentin and cementum, red = pulp cavity, black arrows = mesial and distal incision of the enamel cover, black arrow head = most apical point of the palatal enamel cover, white arrow head = most apical point of the labial enamel cover.

Figure 5

Boxplots representing the increase of the dental root lentgh in time. a: upper jaw incisors; b: lower jaw incisors 01 = center, 02 = middle, 03 = corner incisors; symbols indicate individual measurements.

Figure 6

Boxplots of the dental root length of erupted upper and lower jaw incisors. Tooth age assigned to the root length. Box: interquartile range (25%, 75%), horizontal line: median, center point: mean, whiskers: range (5%, 95%), stars: minimum and maximum. The dental root length increases in time. Teeth older than 15 years p.e. show a significant longer root than younger teeth (p = 5.4 × 10^-13 upper jaw, p = 2.4 × 10^-12 lower jaw).