Peripheral caries can affect all of the dental calcified tissues (cementum, enamel and dentine), and so the terms peripheral caries or peripheral dental caries are preferable to the previously used term of peripheral cemental caries (Dixon et al., 2010). This type of caries has some similarities to smooth surface caries in brachydont teeth. The prevalence of equine peripheral caries appears to be increasing in Europe (Gere and Dixon, 2010). Wafa (1988) described a peripheral caries prevalence of 0.3% in 355 horse skulls in a post mortem study in Ireland and a 0.9% prevalence was found in dental surveys in Swedish horses by Lundström and Pettersen (1988, 1990). A more recent post mortem study on Swedish horses reported a prevalence of 6.1% (31/510) peripheral caries (Gere and Dixon, 2010). A recent clinical survey of 800 donkeys from the Spanish-Portuguese border showed a similar peripheral caries prevalence of 5.9% (Rodrigues et al., 2013).
A post mortem study of 22 equine skulls showed dental plaque overlying the interdental tooth surfaces in most mandibular teeth that were examined (Cox et al. 2012). Erridge et al. (2012) found that 67% of (peripheral) equine cheek teeth caries lesions were covered by plaque, that was usually 10-1000 µm thick, sometimes with food adherent to the plaque or teeth. The clinical crown surfaces of all control teeth contained a pellicle (<10 µm in thickness) but no food material was histologically present in this pellicle. On gross examination, Gere and Dixon (2010) found food firmly attached to the sides of some cheek teeth affected by peripheral caries, that may further have contributed to maintaining a localized caries-inducing environment on the surface of these teeth.
Older equids are more commonly affected by peripheral caries and additionally, caries lesions appear to be more severe in older animals (Dacre, et al., 2008; Gere and Dixon, 2010; Dixon et al., 2010). Peripheral caries most commonly affects the caudal cheek teeth (Triadan 09-11) (Gere and Dixon, 2010). The main equine salivary ducts drain rostrally in the mouth, therefore the buffering effect of saliva may be less in the caudal aspect of the equine oral cavity (Gere and Dixon, 2010).
The prevalence of diastemata in a Swedish post-mortem study was significantly higher (64.5%) in horses with, than without peripheral caries (45.7%) and the three caudal cheek teeth were more commonly affected by both diastemata and peripheral caries (Gere and Dixon, 2010). This is in contrast with Ramzan and Palmar’s (2010) clinical study of 108 horses where diastemata were predominantly observed within the mandibular cheek teeth quadrants and affected all interdental spaces, with no significant association found between the presence of diastemata and peripheral caries.
Periodontal disease can sometimes be found adjacent to areas affected by peripheral caries (Gere and Dixon, 2010). However, Cox et al. (2012) showed that dental plaque often covered cemental “erosions”, but no statistically significant relationship could be found between the amount of plaque or degree of peripheral cemental erosions present and the presence and severity of periodontal disease. Rodrigues et al. (2013) found that periodontal disease was present in only 3.9 % of peripheral caries cases. Concurrent infundibular caries was found in 13% (Erridge et al., 2012) and 32 % (Gere and Dixon, 2010) of teeth affected by peripheral caries.
Grade of Caries
Normal tooth, i.e. no macroscopic peripheral caries visible. Discoloration, (without pitting) of peripheral cement, possibly of dietary origin, is present in some normal teeth
Only cementum is affected: lesions appear as superficial or focal pitting lesions or even as extensive cemental loss, although some cementum remains
Grade 1. Class 2
Only cementum affected: more severe peripheral caries with cementum completely lost in some areas, exposing the underlying discoloured (but grossly unaffected) enamel
Cementum and underlying enamel are affected
Cementum, enamel and dentine are affected
Dental integrity is affected (i.e. secondary dental fracture present)
Fig 4:Grading of peripheral caries according to the modified Honma system (from Dacre, 2005).
Peripheral caries lesions that were macroscopically graded as grade 1.1 showed two distinct histological patterns (Erridge et al., 2012). In one type, layers of peripheral cementum became underrun by plaque and flaked off, as also occurs in human root (cemental) caries. In the second type, flask-like carious lesions filled with plaque were present. Grade 1.2 lesions showed extensive histological loss of peripheral cementum and thus the underlying enamel of the clinical crown was exposed. Although enamel exposure was often observed around the entire circumference of the tooth, the enamel was unaffected, possibly because a much lower pH (5.5) is required for enamel to develop caries in contrast to cementum (pH 6.7). Because enamel is dissolved during the histological decalcification process, it histologically appears as an empty space between cementum and dentine. Whilst there was still a very small layer of intact cementum covering the enamel in grade 1.2 lesions, in grade 2 lesions, plaque was found within the enamel space indicating that the full cemental layer and the underlying enamel were now affected.
Although Grade 3 lesions were not macroscopically found in Erridge et al’s study, Gram staining showed bacteria within the dentinal tubules in 63% of sections with peripheral caries, demonstrating the involvement of dentine, thus showing that macroscopic grading of carious lesions underestimates their severity as compared to histopathological examination (Erridge et al. 2012). Additionally, Gram or Picrosirius Red staining revealed even higher grades of caries than was found with hematoxylin and eosin (H&E) staining. It was remarkable that even teeth which macroscopically and microscopically (with H&E staining) appeared to have grade 1.1 lesions, turned out in fact to have grade 3 lesions following Gram staining. This suggests that in equine (peripheral) cemental caries, just as occurs in human cemental (root) caries and donkey infundibular cemental caries, a simultaneous bacterial colonisation and demineralisation occurred (Frank, 1990; du Toit et al., 2008), whereas in human enamel caries, demineralisation appeared to precede bacterial invasion (Frank, 1990).