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The composition of the all-important grinding surfaces of a horse’s cheek teeth change with age, researchers have found.
The effective operation of the grinding surfaces are essential to equine health.
Over a horse’s life, their cheek teeth erupt continuously to compensate for grinding wear of 3–4 mm a year.
Parts of the crown become progressively exposed at the occlusal surfaces – that’s the areas of contact between teeth, including the grinding surfaces – over time.
Before now, it has been unclear whether the typical structure of the grinding surfaces, composed of a complex arrangement of enamel, dentine and cementum, remained constant or underwent structural changes with age.
The German study team set out to test their hypothesis that occlusal surface composition did not remain constant.
Lauritz Martin Englisch and his colleagues conducted micro CT scans of 20 upper cheek teeth and 16 lower cheek teeth from 19 domestic horses. The teeth were analysed using imaging and measurement software.
The area for individual dental substances was measured at different levels along the visible areas of the teeth.
The information was analysed to detect changes in the area of individual substances along the length of the teeth.
The area of peripheral cementum – the bonelike connective tissue covering the root of a tooth and assisting in tooth support – was measured separately for levels inside and outside the bony socket for the root of each tooth.
The University of Giessen research team reported that, in both the upper and lower cheek teeth, the enamel area (the hard mineralized surface material) decreased toward the gum, while the dentine (the dense bony tissue forming the bulk of a tooth) increased. They also found there was an increase in peripheral cementum in parts of the teeth as the horses aged.
The scans showed that dentine and enamel were not evenly distributed along the crown. Consequently, the complex composition of the grinding surfaces changed with time – that is, the content of dentine increased while the content of enamel decreased as the teeth erupted with age.
With increasing age, the content of dentine at the grinding surfaces increased while the content of enamel decreased.
“The complex composition of the occlusal surface changes with time,” they concluded.
Discussing their findings, the study team said the absolute and relative enamel area decreased in both the upper and lower check teeth towards the gum.
Thus, with increasing age, as deeper parts of the reserve crown become revealed at the grinding surface, the amount of enamel decreased at these points of contact and dentine increased.
A decrease of enamel should lead to a decrease in wear resistance with age, because less enamel should logically lead to less wear resistance.
However, according to clinical observations, the teeth of young horses appear to be softer and less resistant to floating (that is, corrective dental work using manual rasps or motorized tools) than the cheek teeth of older horses. This suggested there was an increase of wear resistance with age.
This was contradictory to their findings of decreasing enamel and increasing dentine at these grinding surfaces.
“Thus, we assume that the decreasing enamel content at the occlusal surface is compensated by distinct processes which ensure sufficient resistance and therefore sufficient function of the tooth during mastication [chewing].
“Studies of equine incisor teeth documented that secondary dentine was harder on mid tooth levels than on a level near the occlusal surface. Thus, dentine hardness at the occlusal surface increases with age.”
Further studies were needed to evaluate the biomechanical implications – resistance against grinding-related wear – and the clinical consequences (corrective dental procedures) of these findings, they said.
Further work was also needed using a higher number of CT scans to analyse breed influence or tooth position, and to learn more about differences between individual horses, they added.
The study team comprised Englisch, Kathrin Kostrzewa, Klaus Failing and Carsten Staszyk, all from the University of Giessen; and Susan Kopke, from the University of Veterinary Medicine Hannover Foundation.
Englisch LM, Kostrzewa K, Kopke S, Failing K, Staszyk C (2017) Uneven distribution of enamel, dentine and cementum in cheek teeth of domestic horses (Equus caballus): A micro computed tomography study. PLoS ONE 12(8): e0183220. https://doi.org/10.1371/journal.pone.0183220