Q: Please review management of temporomandibular joint problems in dogs and cats.
A: Dr. Jennifer Rawlinson gave an excellent lecture titled "Unraveling the Mysteries of TMJ" at the 2008 American College of
Veterinary Internal Medicine Forum in San Antonio. Here are some relevant points:
Problems associated with the temporomandibular joint (TMJ) of dogs and cats are uncommon; therefore, this joint may sometimes
cause uncertainty when an abnormality does arise. Although traumatic dislocations with or without associated condylar fractures
are the most common TMJ abnormality, there are a small number of additional pathologic conditions that affect this joint.
The TMJ is formed by the transversely elongated 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 and rostro-dorsally by a process of the temporal bone in humans termed the articular eminence
(this process remains unnamed in veterinary anatomy).
The articular surfaces of the mandibular condyle and mandibular fossa are enclosed in a joint capsule. A thin fibrocartilagenous
disc, meniscus, lies between the hyaline cartilage-covered articular surfaces. The joint capsule attaches to the entire edge
of the meniscus as it passes between the two bones. Thus, the joint cavity is completely divided into a dorsal compartment,
the meniscotemporal compartment between the disc and the temporal bone; and a ventral compartment, the meniscomandibular compartment
between the disc and mandible. These two compartments do not communicate. A thick band of fibrous tissue on the lateral aspect
of the joint capsule forms the lateral ligament which tightens when the jaw opens.
The TMJ is a condylar joint that can move in flexion, extension and translation. Translation refers to the mandible's ability
to move rostrally and laterally.
The degree of translational ability is related to the dietary habits of the animal. A strictly carnivorous animal (cat) will
have less movement laterally and rostrally than an omnivore (primate).
This difference in TMJ mobility is the result of the fit of the mandibular condyle within the mandibular fossa of the temporal
bone. The more snug the fit, the less translational mobility. The cat has greater congruity of its mandibular condyle and
fossa due to a more prominent retroarticular process and articular eminence; this results in decreased translational movement.
Mandibular movements are in unison mostly, but the synchondrosis at the symphysis of the mandible allows independent movements
of the mandibular rami; this independent movement permits luxations of the TMJ to occur without fracture.
The masseter, temporal and medial pterygoid muscles are responsible for closing the mouth; the digastricus muscle with the
aid of gravity opens the mouth. The lateral pterygoid muscle and the zygomaticomandibularis muscle (suspected deep part of
the masseter muscle) are responsible for lateral movement of the mandible.
The most important question to be answered is: "Does this condition prevent the animal from opening or closing its mouth?"
The vast majority of TMJ pathology and differentials can be placed in one or the other category.
Unable to close the mouth
» TMJ dysplasia with coronoid displacement
» TMJ luxation with or without associated fracture
» Mandibular neurapraxia
» Fracture of the zygomatic arch
» Neurogenic atrophy of muscles of mastication
Unable to open the mouth
» Craniomandibular osteopathy
» TMJ ankylosis
» Fracture of the zygomatic arch
» Osteoarthritis (too painful to open)
» Masticatory muscle myositis
» Retrobulbar abscess or cyst