CT characterisation and classification of feline temporomandibular joint trauma: a case series of 79 cats

Objectives The aim of this study was to characterise and describe patterns of temporomandibular joint (TMJ) injuries occurring in cats using CT. Methods A cross-sectional study was carried out in adherence with the STROBE guidelines. Among the medical and CT records of 79 cats, 158 TMJs were reviewed in a collaborative study between six institutions. Results TMJ injuries were most commonly unilateral, representing 70.9% of cases. The mandibular condyle was fractured in 88 cases (55.7%) of the 158 TMJs observed. Of those, 84.0% were intra-articular condyle fractures, with the medial half of the mandibular condyle over-represented. Luxations occurred in 32.9% of cases, which was 19.0% of all evaluated TMJs. Rostrodorsal luxations were most common representing 87.0% of all luxations. Temporal bone fractures were observed in 30.4% of all cases, which was 18.4% of TMJs. The majority of fractures were of an unknown cause. When the cause was determined, road traffic accident (RTA) was the most frequent, followed by animal interaction, other external forces (sharp or blunt force) and high-rise trauma. Bilateral injuries were 13.1 times more likely to occur in high-rise trauma (P = 0.01) and temporal bone fracture was significantly associated with RTAs (P = 0.016). No other significant associations were observed between cause of injury and the resulting TMJ injury pattern. Conclusions and relevance Various TMJ injury patterns can occur in cats as a result of trauma. Intra-articular fractures of the medial half of the mandibular condyle occur most commonly. Although unilateral injuries are more frequent, high-rise trauma tends to present with bilateral lesions. Further studies with a larger sample size should be performed to better understand TMJ patterns of injury and how they relate to possible causes.


Introduction
Fractures involving the temporomandibular joint (TMJ) account for between 50% and 56% of all maxillofacial fractures. [1][2][3] The most frequent cause depends on the studies reported, and include road traffic accident (RTA) 2,3 and high-rise trauma. 1 Although these studies described mandibular injuries in some detail, TMJ injuries were only assessed using plain film radiography. [1][2][3] Considering the reported 10% prevalence of ankylosis after TMJ trauma 1 there is a need to further understand, in more detail, which TMJ injuries might lead to ankylosis, in order to guide the preoperative decision-making. In human oral and maxillofacial surgery, CT is the preferred method of imaging complex fractures as it allows the high-quality assessment of the cortical bone. 4 TMJ fracture patterns in cats have not been previously characterised by CT despite several publications reporting maxillofacial trauma in this species. 2,3,5 Therefore, the aim of this study was to use CT to classify TMJ fracture configurations in cats and to understand how that relates to cause.

Materials and methods
A cross-sectional study was carried out in adherence to the STROBE checklist. 6 CT scans and the clinical records of cats presenting with a TMJ injury were retrospectively reviewed from six international university veterinary hospitals. Cats had to have presented with suspected or witnessed trauma and have no history of prior trauma or documented skeletal abnormalities.
Records Clinical variables including age and weight and cause of the injury were analysed.
CT images of 79 cases were reviewed by three authors and a consensus was reached. A total of 158 TMJs were evaluated individually. TMJ injury classification was adapted from Arzi and Lantz 7 and from Neff et al, 8 and classified according to the following criteria: presence of subluxation; luxation (rostrodorsal vs ventrocaudal); or fracture. Fractures were classified as extra-articular or intra-articular. Intra-articular fractures were classified into type A = lateral 1/2, type B = medial 1/2, type C = comminuted, type D = simple with mandibular fossa (MF) fracture and type E = comminuted with MF fracture ( Figure 1). Extra-articular fractures were classified into without override, with lateral override, with medial override or comminuted ( Figure 2). Fracture of the temporal bone that makes up the mandibular fossa was also recorded ( Figure 3). Finally, unilateral or bilateral occurrence was recorded.
Results were evaluated using commercial software packages for the calculation of descriptive (Microsoft Excel for Mac version 16.49) and inferential statistics (IBM SPSS Statistics for Mac version 26). Categorical variables were expressed as frequencies. χ 2 tests or Fisher's exact tests were used to assess the association between injury patterns. To determine the effects of cause on injury patterns, univariate logistic regression was used. A P value <0.05 was considered significant for a 95% confidence interval.

Results
CT scan slice thickness and pitch for all examinations ranged from 0.3 to 1.3 mm and from 0.5 to 1 mm, respectively.

Demographics and classification of injuries
Of the 79 clinical cases included in the study, 49 were male (62%) and 30 were female (37%). One case did not have a recorded age. Mean ± SD age and weight were 5.02 ± 3.96 years and 4.48 ± 1.39 kg, respectively.
The mandibular condyle was fractured in 88 cases (55.7%) of the 158 TMJs observed. In the 88 mandibular condyle fractures, a total of 105 injury types were observed. Of those, 76 were intra-articular and 29 were extra-articular. In 15 mandibular condyles intra-and extra-articular fractures were simultaneously observed. The relative probability of mandibular condyle fracture patterns is illustrated in Figure 4. With regard to intra-articular fracture type, medial intra-articular fractures were the most common (30/105 fractures [28.6%]). With regard to extraarticular fractures, comminuted fractures were the most common and represented 20/105 fractures (19.0%) ( Figure  4). Temporal bone fractures (including or not glenoid fossa) were observed in 31/158 TMJs (19.6%). Fractures beyond the condylar neck were registered as coexisting mandibular fractures, observed in 32 cases (40.5%).
No significant associations were found between the following variables: luxation, fracture of the pars squamosa of the temporal bone, and extra-articular and intra-articular fractures. There was also no significant association with the presence of coexisting mandibular fractures. However, intra-articular fractures were significantly associated with the presence of temporal bone fracture (odds ratio [OR] 3.2, 95% confidence interval [CI] 1.35-7.54; P = 0.008).  Table 2 in the supplementary material).

Cause of injury
No significant associations were found between the cause of injury (RTA, high-rise trauma, animal interaction or other external force) and its pattern (luxation, subluxation, the presence of temporal bone fracture, intraarticular fracture, extra-articular fracture and coexisting mandibular fractures). However, bilateral injury was significantly associated with high-rise trauma (OR 13.13, 95% CI 1.85-92.96; P = 0.01) ( Table 1).
Distribution of the causes of TMJ injuries are illustrated in Figure 5. Mandibular condyle fractures were the most frequent TMJ injury for all causes. Mandibular condyle fractures of the medial half were the most frequently registered lesion secondary to all causes, especially highrise trauma. Temporal bone fractures and coexisting mandibular injuries were observed in all causes except in high-rise trauma. In RTA cases, the most frequently seen lesion was temporal bone fracture. Of all temporal bone fractures where cause was established (21 in total), 69.6% were due to RTAs; this association was significant (P = 0.016).

Discussion
This study is the first to report and characterise TMJ injury patterns with regard to cause, using CT. Prior to this, these injuries had only been reviewed using plain film radiography. 1 CT allowed a more detailed evaluation of the different injury configurations that were missed or underestimated in previous studies. CT is considered superior to plain radiography for evaluation of the TMJ as it allows a three-dimensional assessment of lesions, overcoming the superimposition of anatomical structures. 5,9,10 The cases were collected by six separate institutions, all of which are referral practices. The prevalence of the injuries reported here cannot reflect the true prevalence of these injuries in first opinion practices, albeit traumatic maxillofacial injuries are common in cats. 1 RTA was the most frequently reported cause of TMJ injury, followed by animal interaction, external force and high-rise trauma. While RTA was the most frequently reported cause of maxillofacial trauma in an English study, 3 a study from Turkey reported high-rise trauma as the most common cause. 1 These differences are likely to be the result of regional differences in the home environment and lifestyle. For example, urban areas with multilevel buildings can increase the risk of high-rise trauma. In our case series all the high-rise trauma cases were reported from the same institution, located within an urban area, which supports this speculation (see Table  2 in the supplementary material).
Trends were observed between the cause of trauma and injury pattern. In RTA cases, most of the defined types of TMJ injuries were observed. The same was true  for blunt trauma and animal interaction. The variability of TMJ injuries is likely to be the result of the direction of force, velocity and severity of the impact. Conversely, we reported less variability in TMJ injuries in high-rise trauma, which were significantly more likely to be bilateral. In these cases, intra-articular fractures (comminuted or of the medial half) and extra-articular fractures (comminuted or with lateral override) were mostly observed. This lower injury variability is most likely due to the specificity in force direction, which is always in the rostrocaudal direction. In one previous study, the author also observed fewer TMJ injuries per cat in high-rise trauma, compared with RTA. 1 Interestingly, the same author reports a significantly higher number of isolated and bilateral TMJ injuries, namely caudal luxation or condylar process fractures, in high-rise trauma. 1 In our study, ventrocaudal luxations were not observed in highrise trauma. On the contrary, all cases of luxation were rostrodorsal and in a lower proportion compared with other causes. Luxations were more frequently observed in RTAs and animal interactions. The classification adopted was able to accommodate all TMJ injury patterns encountered in this case series. Differences between fractures of the mandibular head and condyle can be difficult to distinguish in cats because the condyle neck is very short. Therefore, TMJ injury classification was adapted from the Arbeitsgemeinschaft für Osteosynthesefragen -Craniomaxillofacial (AOCMF), 8 considering the recommendations by Arzi and Lantz using intra-and extra-articular fractures instead of intraand extracapsular. 7 Additionally, the term 'luxation' rather than 'dislocation' was used.
Intra-articular fractures occurred more frequently than extra-articular fractures. The medial half the mandibular condyle was the most frequent intra-articular fracture.
Comminuted fractures were more often extra-articular. These observations suggest that the medial half of the mandibular condyle and the neck, albeit very short, are less resistant to load. The mandibular condyle has thinner cross-sectional areas at the level of the neck and, consequently, can be less resistant to direct or indirect force. As the TMJ functions as a shock absorber that dissipates transmitted forces, 11 it seems logical that this area might be a weak anatomical point. The observation of this fracture type as an isolated injury in 11 cases also supports this theory.
Temporal bone fractures were the least frequent TMJ injury (18.3% of 158 TMJs). The squamous portion of the temporal bone appears to have higher resistance to fracture. The force vector and direction of the trauma are also relevant in the occurrence of fracture or luxation. Temporal bone fracture was significantly associated with RTAs, being the most frequently seen injury, compared with the complete absence of this kind of injury in high-rise trauma. These observations suggest that the squamous portion of the temporal bone seems to be more resistant to a perpendicular vector, which occurs in highrise trauma, and less resistant to more oblique vectors of force in RTAs. The impact of the perpendicular/frontal vectors of force seen in high-rise trauma is felt simultaneously in the two temporal bones, potentiating the resistance to fracture, and the predisposition to bilateral injuries, which are also observed in these cases. In comparison, the force vectors in RTAs are more variable and more oblique in direction.
In approximately one-third of the cases evaluated in this study, TMJ luxation or subluxation was observed. Of these, the great majority were rostrodorsal luxations, similar to a previous study. 1 Furthermore, all caudoventral luxations were unilateral and bilateral luxations were all rostrodorsal. This fact reinforces rostral luxation as the most likely to occur and this may be explained by the joint angle and congruence, as well as the mandibular fossa, offering resistance to the caudal dislocation of the mandibular condyle.
Limitations of this study were a direct consequence of the number of cases and the intrinsic variability associated with the multi-institutional concept and retrospective data analysis. Variations of CT algorithms (slice and pitch) between institutions were also a factor and in three cases led to small anatomical gaps that could have inhibited a proper assessment. More standardised CT algorithms would have negated this limitation.

Conclusions
Unilateral TMJ injury patterns, especially intra-articular fractures of the medial half of the mandibular condyle, occur most frequently in cats following trauma. In cases of high-rise trauma these injuries tend to be bilateral.
Further studies using a larger number of cases should be performed in order to better understand TMJ patterns of injury.