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second case also displayed mild gingivitis and periodontitis associated with some of the CT. The described cases suggest that there is a difference
between incisor and CT EOTRH in terms of the presence of visible pathognomonic clinical signs. Incisor EOTRH is mostly associated with painful periodontal disorders and/or positional changes within the incisor arcades (Baratt 2007; Caldwell 2007; Kreutzer et al. 2007; Staszyk et al. 2008). In contrast CT EOTRH seems to feature only moderate periodontal changes and characteristic changes in clinical crown cementum might remain undetected. Although these assumptions need to be confirmed by further investigations in more horses with CT EOTRH, there is reason to speculate whether CT EOTRH has a much lower prevalence than incisor EOTRH or whether the prevalence of CT EOTRH is underestimated at this time. In summary, there were two key indicators suggestive of CT EOTRH in these cases:
• Wider than usual CT (can be subtle to detect) with the spatial layered hypercementosis pattern previously described
• Unusual mobility of CT detected when the grinding burr was placed against the teeth.
Aetiological considerations There is a notable spatial pattern in EOTRH affecting the incisors that suggests the existence of a biomechanical factor contributing to the aetiology of this disease. It has been shown that periodontal areas affected by high biomechanical loads (palatal/lingual aspects near the root tip) coincide with the position of initial resorptive lesions in incisors (Schrock et al. 2013). In both cases presented here, a clear special pattern of additional deposits of irregular cementum was also noted in the CT, i.e. palatal side of maxillary CT and buccal side of mandibular CT- Exactly these sides were found to be highly affected by high biomechanical stresses during the power stroke of the masticatory cycle, especially in older horses (Cordes et al. 2012) which further supports a biomechanical involvement in the pathogenesis of EORTH. The biomechanical stresses that occur in the CT of aged horses were estimated to be high enough to cause at least two harmful processes within different areas of the periodontium. First, it is assumed that gingival damage occurs, leading to periodontal pocketing and microbial infection of the periodontal ligament (Cordes et al. 2012). Second, focal overload within the depth of the alveolus causes periodontal micronecrosis and initiates cytokine release. Subsequently, clastic cells are attracted and start the process of odontoclastic tooth resorption (Brudvik and Rygh 1994). However, odontoclastic tooth resorption is often a temporary process and after a while the resorbed areas become repaired by regular cementum, resulting in reattachment of the tooth to the periodontal tissues (Langford and Sims 1982; Reiter et al. 2005; DeLaurier et al. 2009). In EOTRH it seems that the process of tooth resorption is
maintained and that concomitantly the process of cemental repair becomes deregulated and unlimited. A possible explanation for this condition comes from the finding that incisor EOTRH is associated with the presence of different species of Treponema and Tannerella (Sykora et al. 2014). The persistent infection with these microorganisms might
represent the major factor that preserves the stimulus for the continued recruitment of odontoclastic cells and subsequently deposition of high amounts of irregular cementum. Thus, EOTRH is assumed to be caused by at least two factors, i.e. first, focal periodontal damage initiating odontoclastic tooth resorption and second, persistent infection with distinct microorganisms preserving an inflammatory environment with continued recruitment of clastic cells. Clinical observations in incisor EOTRH suggest that the
uls et al. 2012). This is reflected by the presence of resorptive lesions and hypercementotic areas to a different extent. Therefore, it has been proposed to define at least three different types of EOTRH according to the predominant histological/radiological changes, i.e. a hypercementotic, resorptive and a mixed type (H€
ratio between the predominant pathological process, i.e. odontoclastic tooth resorption and deposition of irregular cementum, varies greatly between horses (Staszyk et al. 2008; H€
uls et al. 2012). The resorptive
type is assumed to feature mainly inflammatory processes that lead to odontoclastic resorption. In contrast, the hypercementotic type mainly shows reparative processes in terms of deposition of cementum. However, it is emphasised that in all defined types, all of the typical features of EOTRH were found concomitantly, i.e. odontoclastic resorption and hypercementosis (Staszyk et al. 2008; H€
uls et al. 2012). Further
studies are required to confirm whether these clinical and histopathological observations are actually correlated with the presence of distinct microorganism as suggested (Sykora et al. 2014).
Nevertheless, the presented cases of cheek tooth EOTRH
do not contradict the current concepts of the possible aetiology of EOTRH. Incisor EOTRH and cheek tooth EOTRH share many features in terms of radiological findings, macroscopic appearance of extracted teeth and even histological findings. Although some differences in clinical signs are present between the two conditions, all the findings with CT EOTRH fall within a range of features that are also found with incisor EOTRH.
Clinical considerations
At the time of writing, no treatment other than exodontia is currently available for severe cases of EOTRH (Earley and Rawlinson 2013; Lorello et al. 2015). However, preliminary results of a blinded study on the effects of an additive feedstuff made of a fungus mycelium (Equident) on EOTRH suggests that there seems to be a positive effect on the clinical signs of EOTRH (Simhofer and Kowelka 2012). Final results of this study will show if this may be a palliative treatment option for less severe cases. Anecdotally, daily cleaning of the incisors using a tooth brush and chlorhexidine or a similar disinfectant solution in combination with shortening of the incisors also seems to have a positive effect on inflammatory signs.
Conclusion
Until recently, EOTRH was predominantly reported to occur in the incisors and canines. This report confirms that the condition may also affect the cheek teeth and that the clinical presentation may cover a wide spectrum in terms of both severity and clinical signs. Dental radiographs may be
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