Temporomandibular Joint and Masticatory Muscle Disorders

Introduction

The temporomandibular joint (TMJ) is formed by the condylar process of the mandible and the mandibular fossa of the temporal bone. Bony support for the mandibular condyle within the mandibular fossa is provided caudoventrally by the retroarticular process of the temporal bone. A fibrocartilagenous disc lies between the hyaline cartilage-covered articular surfaces. The joint capsule is reinforced laterally by a thick ligament which tightens when the jaw opens. The masseter, temporal, and medial pterygoid muscles close the mouth. The digastricus muscle opens the mouth. The lateral pterygoid muscles are responsible for lateral movement of the mandible. The masticatory muscles (except the digastricus) are predominantly composed of myofibers that possess a unique myosin isoform and unique heavy and light chains.

Clinical Presentation

Presenting clinical signs may include inability or reluctance to open or close the mouth, pain on prehension and mastication of food, pain on palpation of the TMJ, ear canal, zygomatic arch, ocular structures and masticatory muscles, swelling of the TMJ area and along the zygomatic arch, crepitus upon opening or closing the mouth, enophthalmos and exophthalmos, swelling or atrophy of masticatory muscles, and malocclusion.

Diagnostic Imaging

Imaging of the TMJ is traditionally performed with standard radiographic film and extra-oral technique. In cats and other small mammals, the largest dental radiographic films (size 4) may also be utilized. Dorsoventral (or ventrodorsal) and right and left lateral oblique are standard views, but a rostrocaudal (open-mouth) view may be added to the series. When radiographs do not provide enough detail of the TMJ, computed tomography (CT) and magnetic resonance imaging (MRI) may be necessary. Computed tomography was found to be a useful adjunct in the diagnosis of masticatory myositis in dogs, including selection of sites for diagnostic muscle biopsy and ruling out other conditions with similar signs.

Luxation of the TMJ 

Trauma is usually the cause of TMJ luxation. In rostrodorsal TMJ luxation, the mandibular condyle moves rostrodorsally. Consequently, the lower jaw shifts laterorostrally to the contralateral side. Malocclusion results in inability to close the mouth fully due to tooth-to-tooth contact (between maxillary and mandibular canine and cheek teeth on the contralateral side). The well developed retroarticular process resists caudal displacement of the mandibular condyle. Therefore, fracture of this process may be mandatory for caudal luxation to occur. Reduction of rostrodorsal TMJ luxation is achieved by placing a wood dowel (or a hexagonal pencil in smaller animals) between the maxillary fourth premolar and mandibular first molar teeth on the affected side only (dowel acts as a fulcrum) and closing the lower jaw against the dowel while simultaneously easing the jaw caudally. Chronic luxation is treated by means of unilateral condylectomy. [...]