EQUINE VETERINARY EDUCATION / AE / MARCH 2016
131
Clinical Commentary
Equine odontoclastic tooth resorption and hypercementosis (EOTRH): What do we know?
R. Baratt* Salem Valley Equine Clinic, Salem, Connecticut, USA. *Corresponding author email:
rbaratt1dvm@gmail.com
Equine incisor tooth resorption and periodontal disease was initially recognised by equine practitioners in private practice and presented to colleagues at professional meetings (Gregory et al. 2006; Baratt 2007; Caldwell 2007). These practitioners described the clinical and radiographic features of a syndrome not previously identified in the horse. Subsequently, the histopathological features of this syndrome were described and the syndrome given a more descriptive name: equine odontoclastic tooth resorption and hypercementosis (EOTRH) (Staszyk et al. 2008). At meetings and in veterinary journals, equine practitioners subsequently reported similar cases worldwide (Lorello et al. 2015). Surprisingly, this newly described syndrome, although uncommon, was not rare. Examination of archived specimens would likely reveal that this ‘new’ disease was present but unrecognised prior to 2006 although, to this author’s knowledge, this has not previously been documented in the literature. Similarly, tooth resorption affecting the equine cheek teeth has now been reported and may be documented by equine practitioners worldwide. While the embryology of the equine tooth closely follows
tooth development in other species, the horse has unique anatomical and physiological adaptations in its radicular hypsodont dentition not present in man or other mammalian species (Staszyk et al. 2015). However, it is informative to examine tooth resorption in other species, including laboratory animals, in an effort to better understand the pathophysiology in the horse and to guide further research efforts. In this issue, Moore et al. (2016), in addition to proposing that EOTRH affects the cheek teeth as well as the incisors and canines, have reviewed the EOTRH literature. It is hoped that this wider, cross-species literature review of tooth resorption is of interest to the reader. Tooth resorption is actually a normal process required for
the exfoliation of mammalian deciduous incisor, canine and premolar teeth (Nanci 2013). Resorption of root cementum and dentin is also a normal feature of permanent tooth roots in man. It is commonly found in the apical portion of the root and as it is normally accompanied by reparative processes, is not clinically recognised (Andreasen 1985). In the horse, it has been demonstrated that the production of coronal cementum involves the surface resorption of enamel by odontoclastic cells, followed by deposition of cementum by cementoblasts (Jones and Boyde 1974; Sahara 2014). Cementum formation in the developing tooth is still incompletely understood. It has been hypothesised that disruption of Hertwig’s epithelial root sheath (HERS) and contact of the surrounding dental follicle with root dentin results in reciprocal induction of cementoblasts and production of root cementum. There is also evidence supporting the epithelial-mesenchymal transformation of HERS into cementoblasts (Nanci 2013). The disintegration of HERS leaves clusters of epithelial cells, epithelial cell rests of
Malassez, which are incorporated into the developing periodontal ligament (PDL) which is of mesenchymal (dental follicle) origin. There is a growing body of work that would indicate that rather than ‘resting’, the cell rests of Malassez play an active role in the maintenance and regeneration of the PDL (Xiong et al. 2013). Significantly, the absence of the epithelial cell rest of Malassez may prevent normal repair of damaged PDL, resulting in tooth resorption. In man, endodontic disease, periodontal disease and
pressure-induced frontal resorption in orthodontic treatment all result in the presence of inflammatory cells and cytokines that stimulate the differentiation of and activation of osteoclasts. This appears to be regulated, in part, by signalling molecules in the tumour necrosis family (Ne et al. 1999; Darcey and Qualtrough 2013; Nanci 2013). The role of the tumour necrosis family of cytokine-like proteins (RANK, RANKL, osteoprotegrin [OPG]) in the pathogenesis of feline tooth resorption has been previously investigated (Senn et al. 2010). In this study, immunohistochemical staining with anti-RANK, anti-RANKL and anti-OPG antibodies at the sites of active tooth resorption was noted. Similar studies have not yet been reported in the horse. This form of tooth resorption, inflammatory tooth resorption, has a similar radiographic appearance in man, domestic animals and the horse. Initially, there are superficial (surface) areas of root resorption which on histological examination reveal large clastic cells on the surface of Howship’s resorption lacunae. Resorption of dentin progresses but the mantel layer of dentin appears to be resistant to resorption and the pulp is protected until secondary bacterial infection occurs. In man, when tooth resorption is due to pulpal necrosis, early root canal therapy can be curative. There are no reports presently that would indicate that endodontic disease is the underlying cause of EOTRH. As pulpitis in multiple incisor teeth would be highly improbable, this aetiology for EOTRH is unlikely. While odontoclastic tooth resorption is quite prevalent in
the domestic cat population and has been studied extensively, the aetiology remains unclear. In the cat, early resorptive lesions were identified on all aspects of the root surface but appeared to fail to undergo repair at the cemento-enamel junction (DeLaurier et al. 2009). Interestingly, early resorptive lesions in the incisors of horses tend to be located at the apical extent of the peripheral enamel, which would be the equivalent of the cemento-enamel junction in brachydont species. While the cemento-enamel junction may have unique properties that predispose this site to resorption in the cat and man, the unique anatomy of the horse makes comparison between these species difficult. Tooth resorption localised to the cervical area in man also
occurs, albeit rarely and this invasive cervical tooth resorption appears to have distinct histological and clinical features from
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