Equine odontoclastic tooth resorption and hypercementosis

Most domestic animals have brachydont teeth, which are short crowned and fully erupted by maturity (Figure 1a). Because the equine diet is predominately tough forage, their dentition is adapted to account for this (Muylle, 2011). Thus, equine teeth are described as hypsodont, meaning they have long crowns which continuously erupt as the surface of the tooth is ground away during mastication (Dixon and du Toit, 2011). The occlusal surface is continuously changing, so the appearance may be used to estimate a horse's age (Figure 1b) (Muylle, 2011). Equine oral health can correlate with behavioural changes and in cases of equine odontoclastic tooth resorption and hypercementosis, these can include avoidance of human contact around the mouth, excess salivation and mouthing of a water bucket (Pearson et al, 2013). Research into the incisor and canine teeth of horses is less extensive compared to that of the premolars and molars (Casey, 2013); however, in 2008, a previously unknown disease of the incisors and canines was clinically recognised for the first time (Staszyk et al, 2008). Staszyk et al (2008) termed the disease equine odontoclastic tooth resorption and hypercementosis as a result of the apparent loss of mineral content and build up of cementum along the reserve crown (Staszyk et al, 2008; Earley and Rawlinson, 2013). Equine odontoclastic tooth resorption and hypercementosis is a relatively common, progressive disease of equine teeth, with 62.0–72.2% of horses around 20 years old displaying moderate to severe radiographical changes consistent with the disease (Rehrl et al, 2018; Tretow et al, 2023). Equine odontoclastic tooth resorption and hypercementosis is most commonly present in the incisors and canines of horses older than 15 years (Staszyk et al, 2008). However, occasionally premolars and molars can be affected (Moore et al, 2016) and less severe forms have been found in horses from 10 years old (Rehrl et al, 2018; Tretow et al, 2023). A study looking at microcomputed tomography images in incisor and canine teeth of 115 horses found that 55.7% of the study population had at least one incisor affected, compared to 43.3% that showed the disease on canine teeth (Bearth et al, 2023). It also found that canines were less likely to undergo hypercementosis compared to incisors, but resorption of the crown, widening of the periodontal space at the root and lamina dura lysis were more likely (Bearth et al, 2023). These findings suggest that canines are affected slightly less frequently and in a slightly different way but further research is needed to confirm this. While its aetiology is unknown, it has been suggested that there is a male predilection (Smedley et al, 2015) and further epidemiological studies are required to determine whether there are other predispositions. Furthermore, it appears that warmbloods and thoroughbreds are more commonly diagnosed (Sykora et al, 2014; Smedley et al, 2015; Lorello et al, 2016; Earley et al, 2017), although others state there are no apparent breed associations (James, 2022; Tretow et al, 2023).