Discussion

Normal teeth:
Dentine is the main component of the equine tooth and following eruption, secondary dentine is continually secreted for the life of the tooth by odontoblasts that reside on the periphery of the pulp. Because of the intimate relationship of dentine and pulp the 2 tissues are sometimes termed the dentino-pulp complex and this emphasises that dentine is a vital living tissue. As much more dentine (up to 100 times more) is laid down subocclusally than on the periphery of the pulps horns, this indicates that there is a specific stimulus for secondary dentine deposition originating from the occlusal aspects of the teeth, that acts on the subocclusal pulp.
Until recently, there was limited information concerning the thickness of cheek teeth subocclusal secondary dentine (SO2D) that is necessary to protect the underlying pulp from exposure to the oral cavity. Becker (1962) suggested that there was 1 cm of secondary dentine above all pulps but recent work has shown much variation in the thickness of SO2D with median values of 10.8 mm and 9 mm reported for mandibular and maxillary cheek teeth, respectively (White and Dixon 2010). Of greater clinical importance is that this work has shown massive variation (from 2 mm to 33 mm) in SO2D thickness even within normal teeth, as well as high variation between different teeth in an individual horse. It has also been shown that contrary to perceived opinion, the thickness of SO2D slightly decreases with age indicating that over time, teeth wear at a slightly faster rate than the rate that dentine is deposited (White and Dixon 2010). The practical significance of these findings is that great care must be made when mechanically floating normal equine teeth to ensure that the pulp is not exposed or thermally damaged by the heat from equipment which is not water cooled, and also explains why pulpar exposure is more common in aged horses.

Overgrown teeth:
As noted, occlusal stimulation is the main driver for the deposition of SO2D. Therefore in the absence of occlusal stimulation, for example on a tooth whose opposite number has been lost, or where there is much reduced occlusal contact, for example in a tooth opposite a fractured, dysplastic or worn tooth (that contains less than a normal amount of enamel) there will be no or reduced stimulus for laying down SO2D. This theoretically should cause the SO2D to become thinner than normal. However this is counterbalanced by the fact that reduced or no attrition (normal wear) is taking place on the surface of the poorly opposed or unopposed tooth.
The combined effect of these 2 opposing factors is that in general, there is a slight net increase in thickness of SO2D in overgrown teeth (Marshall and Dixon 2011). More specifically, there was no significant difference between SO2D thickness in overgrown (mean 11.38 mm) and control (11.41 mm) mandibular CT, but SO2D was significantly thicker in overgrown (mean 12.57 mm) as compared to control maxillary (9.41 mm) CT. However whilst most overgrown cheek teeth have a slightly increased thickness of SO2D, this can vary between horses and even between individual pulp horns in the same overgrown tooth and in fact many overgrown teeth have thinner than normal SO2D. Consequently, if overgrown teeth are reduced to the level of the adjacent normal teeth, this will cause pulp exposure in 58% of teeth (Marshall and Dixon 2011). Even if pulp exposure does not occur during this procedure, the grinding away of a large amount of dentine with un-cooled dental equipment could cause thermal damage to the underlying pulp. If the occlusal aspect of the pulp is thermally injured, it can no longer lay down SO2D and when the existing secondary dentine overlying this pulp is eventually worn away by normal wear, pulpar exposure can then occur and may lead to pulpar infection and even loss of the tooth.

At diastemata:
With recognition of the clinical significance of equine cheek teeth diastemata over the past decade, and the subsequent widespread use of diastema widening to treat this disorder, the relationships of the occlusal aspects of the pulp to the rostral and caudal margins of the teeth has also become very significant, in order to ensure that the pulps are not directly exposed or thermally damaged at these sites during diastemata widening. Recently, Bettiol and Dixon (2011) have shown much variation in the distance between the pulp and the rostral and caudal tooth margins, which can vary from 1.3 to 10.8 mm. In particular, it has been shown that the pulp horns (and thus pulp) at the caudal aspect of cheek teeth are much closer to the interproximal space than are the pulps at the rostral aspect of the teeth. The practical significance of this is that during diastemata widening, just 2 - 3 mm of dentine should be removed from any tooth and most should be removed from the rostral aspect of the tooth caudal to the diastema.