The objective of this study was to examine the spatial arrangement of the fiber apparatus of the equine periodontium which is supposed to meet two contrary requirements: (1) to attach the tooth firmly and elastically to the alveolar bone; and, to be appropriately remodeled and reconstructed in order to facilitate the prolonged eruption of the tooth. Specimens of periodontal ligament were obtained from the buccal and lingual/palatal aspects of the first molars from the maxilla and mandible of 12 horses. The animals were assigned to three age groups. Histological sections were prepared from three specific horizontal levels of the periodontal ligament and examined with conventional and polarized-light microscopy. At the gingival level, collagen fascicles (diameter > 200 microm) were densely packed. Their spatial alignment was the same in all age groups. The architecture of the collagen fiber apparatus differed at the middle and apical levels in the three age groups. There was a clear distinction between fibers, bundles, and fascicles. Bundling of collagen fibers, density of the fiber arrangement, and collagen fascicles with an alveolo-cemental orientation increased with age. The collagen fiber apparatus of the equine periodontal ligament is highly adaptive, responding continuously to the dynamic changes in the periodontal environment. Site-specific arrangements and age-dependent structural variations are assumed to maintain tooth support as the reserve crown gradually decreases in length with progressive dental attrition. Most of the age-dependent changes to the periodontal ligament in teeth examined in this study occurred at the apical level. The apical region of young teeth had no periodontal attachment, while the roots of older teeth were firmly attached to the alveolus. When evaluating periodontal ligament development, the individual tooth's 'dental age' should be considered rather than the animal's age to account for individual tooth eruption times.
The equine cheek tooth battery is part of a very dynamic system. The aim of this study was to investigate whether the curvature and position of the teeth are also involved in such dynamical processes. The alveolar crest was labelled with a radiodense marker (48 cadaver heads, 15 skulls) and laterolateral radiographs were taken. Then a geometrical method was elaborated to determine a cheek tooth's curvature and its position by means of specific angles. This method respects the remarkable changes of the equine dentition throughout life by considering two items: (1) the alveolar crest was taken as a constant landmark, (2) the central axis of the curved dental crown was determined by calculation of a linear regression equation. This equation considered several geometrically determined points on the curved dental crown which had been marked in the radiographs. Our study yielded the following results: Mandibular cheek teeth became more curved with age, but their positions (represented by the so-called mesio-occlusal angle between tooth and alveolar crest) did not change significantly. In maxillary cheek teeth, however, the mesio-occlusal angle became larger with age (indication of change of dental position), while their curvature did not change. Even though changes of the dental position were not always statistically significant, they are discussed as being biologically/functionally relevant. The mandibular anticlinal tooth, i.e. the tooth positioned at a mesio-occlusal angle of about 908, was not in contact with the maxillary anticlinal tooth. Interestingly, the maxillary anticlinal tooth is known to cause most clinical dental problems.
The Curve of Spee is well known, yet hardly understood in the horse, as there is a striking lack of biometrically related data which could help to explain its biomechanical significance. Older horses display drastic alterations of the Curve of Spee in routine clinical examinations. Since the Curve of Spee is known to be biomechanically related to the masticatory forces in humans, the same may apply to the equine, and masticatory deficits can be expected in the geriatric horse. Quantitative data of the Curve of Spee cannot be obtained directly by means of measurements in the horse's long and narrow oral cavity. Instead, the radiographs of 24 heads and 6 skulls of horses of different age were used to develop a geometrical procedure facilitating a quantitative analysis of the height of the Curve of Spee. The geometrical values gave clear evidence that the height of the Curve of Spee was smaller in groups of older horses compared with those of younger horses. This finding was in accordance with biomechanical studies which reported decreased masticatory forces in older horses.
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