Normal variation of clinical mobility of the mandibular symphysis in dogs

Abstract

IntroductionThe primary objective of this retrospective study was to document the normal variation of clinical mobility of the mandibular symphysis in dogs, and evaluate possible associations with breed, bodyweight, age, sex, and skull morphology. Secondarily, the radiographic appearance of the mandibular symphysis and possible associations with the analyzed data were also evaluated.MethodsMedical records of dogs that underwent anesthetic procedures for maxillofacial, oral and dental evaluation from April 2015 to December 2021 were included.Results567 dogs of 95 different breeds were included, with a total of 695 evaluations. Body weight ranged from 0.8 kg to 79 kg (median 14.4 kg) and age from 3 months to 16 years and 4 months (median 6 years and 9 months). Clinical mobility was evaluated under general anesthesia using a 0 to 3 scale, in lateromedial (LM) and dorsoventral (DV) directions. The symphysis was radiographically classified as being fused or open. The open symphyses were further radiographically divided in having parallel or divergent margins. At the time of the first evaluation DV mobility was 0 in 551 cases (97.2%) and 1 in 16 cases (2.8%). LM mobility was 0 in 401 cases (70.7%), 1 in 148 cases (26.1%) and 2 in 18 cases (3.2%). There was not a significant change in mobility over time for cases examined more than once (P= 0.76). All cases had an intraoral radiographic examination. 83.8% of the radiographs were included in the statistical analysis. Two symphyses (0.4%) were classified as fused and 473 (99.6%) as open, 355 (74.7%) having divergent margins and 118 (24.8%) parallel margins. Logistic regression models exploring factors that affected DV and LM mobility were statistically significant (P < 0.0001; P < 0.0001), with an increase in LM mobility predicting an increase in DV mobility, and vice versa. An increase in age and in bodyweight was associated with a decrease in mobility. There was no statistical difference in clinical mobility across specific breeds or sexes. Increased probability of a divergent symphysis and increased DV mobility was found to be associated with a brachycephalic conformation. The increase in LM mobility was comparatively higher in small brachycephalic breeds compared with larger brachycephalic breed.DiscussionThe majority of the cases showed little to no mobility of the mandibular symphysis and radiographically bony fusion can be rarely seen.

Normal variation of clinical mobility of the mandibular symphysis in cats

“…Despite the inherent differences existing when evaluating live animals versus anatomical specimens, the results of the present study are in agreement with the Scapino studies, as we showed that in feline clinical cases there is greater mobility in LM compared to the DV direction, possibly because of tighter dorsal ligaments and dorsal joint capsule as compared to the ventral structures. These findings also overlap those reported in a dog study from our group ( 26 ). However, the percentage of cats showing mobility of the symphysis in either direction was much higher than in dogs, with 18% of cats showing some DV mobility and 71.7% of cats showing some LM mobility, as compared to 2.8 and 29.3% of dogs, respectively.…”

“…The continuous forces applied over time during mastication on the symphysis may cause degradation of the fibrocartilaginous pad and connective tissues, and favor a compensatory osteogenic production from skeletal and soft tissues, possibly leading to a stiffer symphysis ( 6 , 12–14 ). Indeed, in a recently published canine study by our group it was shown that an increase in age was associated with a decreased mobility ( 26 ). In the present study the results were different, with no statistical difference in clinical mobility between immature and mature cats, and a only statistical association between increasing age and an increase in DV symphyseal mobility in the logistic regression model.…”

“…In the present study, as well as in our canine study ( 26 ), it was initially attempted to transform the Scapino’s four-class anatomical classification ( 5 ) into a similar radiographic classification. However, reaching a common consensus on the degree of radiopacity of the symphyseal space between evaluators was not possible for many of the radiographic images.…”

“…However, in some cases this anatomical landmark seemed to be nearly absent, and the symphyseal plates made a smooth transition with the ventral cortex of the mandibular body, without an obvious radiographic difference in shape or angulation ( Figure 8 ). Interestingly, in dogs the sharply angled outline of the distal symphyseal area was not as frequent as in cats, and was mostly seen in brachycephalic breeds ( 26 ).…”

“…Symphyseal mobility was evaluated in both the lateromedial (LM) and dorsoventral (DV) directions, as previously described ( 26 ). Briefly, during DV mobility evaluation, right and left mandibles were firmly held behind the canine teeth and alternatively pushed in opposite directions (i.e., one mandible in ventral direction and the other one in dorsal direction) ( Figure 1 ).…”