Pseudo-ankylosis of the TM Joint

Introduction
Ankylosis of the temporomandibular joint (TMJ) is characterized by a compromised ability to open the mouth. True ankylosis is caused by pathology of the temporomandibular joint itself whereas pseudo-ankylosis or false ankylosis pertains to structures preventing a normal range of motion of the mandible but with an intact temporomandibular joint. The temporomandibular joint is a condylar joint which in the dog allows opening and closing of the mouth and considerable lateral sliding motion. In the cat the temporomanibular only allows an opening and closing motion. The actual joint consists of the condylar process of the mandibular ramus, the mandibular fossa of the temporal bone, a thin articular meniscus, and a joint capsule, which is strengthened with a fibrous lateral ligament in the dog. In cats, the articular meniscus is just a thin fibrous lamella. Muscles in the direct vicinity of the joint are the masseter, temporal, digastric and lateral and medial pterygoid muscles. Bony structures that are located near the temporomandibular joint are the coronoid process of the mandible, the zygomatic arch, the retro-articular process, and the temporal, basisphenoid and pterygoid bones. In cats and dogs, true temporomandibular ankylosis has been attributed to trauma from car accidents and falling from heights, inflammation, and neoplastic disease. True ankylosis is rare in companion animals and reports mainly concern cats with fractures of the condylar process as major finding. In dogs both trauma and tumors of the temporomandibular joint have been reported as causes of true ankylosis. Trauma can be directed to the temporomandibular joint itself but will also has an impact on adjacent structures. Severe trauma to the temporomandibular joint is more likely to result in true ankylosis whereas trauma to structures responsible for normal function of the mandible will results in pseudo-ankylosis. In the dog masticatory muscle myositis can cause pseudo-ankylosis and should be taken into account. As patient with ankylosis or pseudo-ankylosis can present with a severe inability to open the mouth, oral intubation of these animals can be challenging. In severe cases tracheostomy intubation may be necessary. In general, ankylosis or pseudo-ankylosis will not respond to forceful opening of the mouth and stretching will result in additional trauma. Diagnostic imaging including contrast-enhanced CT imaging is essential in evaluating temporomandibular ankylosis and pseudoankylosis. In addition, MR imaging can be useful in evaluating the extent of soft tissue involvement.  [...]