The Articulations of the Jaws

Although it is customary to describe two temporomandibular joints, these may be regarded as the widely separated halves of a single condylar joint (p. 19). Clearly, movement at one side must be accompanied by a movement, not necessarily identical, at the other side.

The joint is between the condylar process of the mandible and the mandibular fossa of the skull, a facet mainly formed by the squamous temporal bone. The forms of the two surfaces reflect the feeding habits, and in species such as the dog, in which hingelike movements of the lower jaw predominate, the head takes the form of a transverse condyle to which the fossa provides a corresponding gutter. Backward dislocation of the jaw is opposed by the prominent retroarticular process placed directly behind the mandibular fossa. A peculiarity of the joint is the presence of a fibrous or fibrocartilaginous articular disk that divides the cavity into upper and lower compartments. Although the phylogenetic origin of this structure is disputed, it may resolve the complex movements of the joint into simpler components; a hinge movement occurs between the mandible and the disk, whereas gross sliding movements (translations) of the mandible relative to the skull occur at the upper level. The disk is rather thin and poorly developed in the dog. In species in which lateral grinding movements predominate, the mandibular head is larger, the surface more plateau-like, and the disk thicker, although the details differ considerably.

FIG. 3.20 Permanent dentition of cattle. (A) Upper and (B) lower jaws. I, incisor; C, canine; P, premolar; M, molar. Numbers indicate position of a tooth, with count beginning at the front.

In most species the halves of the mandible are firmly fused together. In the dog (and in ruminants) the symphysis joint allows small movements that may be important in securing more precise adjustment of the upper and lower tooth rows and therefore a more effective cutting or crushing mechanism. Two types of movements appear to be possible: a spreading movement, altering the angle between the halves of the mandible, and one in which each half rotates about its own long axis so that the tooth cusps alter their inclination to the vertical. The dog appears to make use of these possibilities when adjusting the position of a bone between the teeth before attempting to crack it.

The temporomandibular joint or ramus of the mandible may need to be accessed to repair fractures. Repairs can be done from the ventral border of the zygomatic arch through incisions of the platysma and the masseter muscles with care taken to protect the dorsal and ventral buccal branches of the facial nerve and the parotid gland along with its duct.