Congenital complete absence of the lower jaw has occurred ; also partial defects and incomplete development, the jaw remaining of small size. Fracture occurs most commonly at or near the mental foramen, the bone being weakened by the foramen and the fossa foi the canine tooth. It is practically always compound, owing to the close adherence of the mucous membrane to the bone, but the displacement is generally slight, the mylo-hyoid, which is attached to both fragments, modifying the displacement. As a rule, therefore, the inferior dental nerve, running in the interior of the bone through the inferior dental canal, escapes severe injury at the time of the accident, though it may become involved in callus later. The tendency toward displacement, however, is for the anterior fragment to be drawn downwards and backwards by the digastric, mylo- and genio-hyoid, and genio-hyo-glossus, while the posterior fragment is raised by the masseter, internal pterygoid, and temporal. Fracture of the neck of the condyle occurs occasionally from blows on the chin. The glenoid fossa, situated in front of the osseous meatus and behind the eminentia articularis of the zygoma, which separates it from the zygomatic fossa, is divided into an anterior articular and posterior non-articular portion by the Glaserian fissure. The articular portion is separated from the middle fossa of the skull by a very thin plate of bone, and cases have occurred in which the condyle of the jaw has been driven up through this plate into the cranial cavity. The condyle of the jaw, whose long axis is directed inwards and backwards, is separated from the glenoid by an interarticular fibro-cartilage, on either surface of which is a synovial cavity.
The cartilage is attached to the capsule and to the condyle, with which it moves forwards and backwards on opening and shutting the mouth. It sometimes becomes displaced forwards, especially in delicate women, causing a subluxation of the jaw. The capsule is thin, especially in front, but is strengthened externally by the external lateral ligament, which is directed downwards and backwards from the zygoma to the condyle. The accessory ligaments consist of : (a) The spheno-maxillary (internal lateral), from the spine of the sphenoid to the ligula and ascending ramus of the jaw. Between it and the neck of the jaw the internal maxillary artery and vein, the inferior dental vessels and nerve, the auriculotemporal nerve, and external pterygoid muscle, are situated. (b) The stylo-maxillary (deep parotid fascia), from styloid process to angle of the jaw. (c) The pterygo-maxillary, from hamular process of sphenoid to base of ligula. Dislocation of the jaw is nearly always forwards through the weak anterior portion of the capsule, and is said to be more frequently bilateral. Dislocation in other directions is generally associated with fracture. The forward variety occurs when the mouth is widely open, and is said to be largely due to the action of the external pterygoid muscle, which draws the condyle forward beyond the articular eminence into the zygomatic fossa (the articular cartilage remaining behind), where it is pulled up by the internal pterygoid, temporal, and masseter muscles. In reduction, therefore, it is first necessary to depress the condyle before pushing it backwards. Suppurative conditions may extend from the ear or parotid gland and involve the joint. Abscesses usually point at the front of the joint, and suppurative processes not infrequently lead to anchylosis. The joint is frequently affected in rheumatoid arthritis, giving rise to ' creaking ' during mastication in slight cases and excessive movement or complete anchylosis in severe ones. In such cases an excision of the condyle through a curved incision along the posterior portion of the upper border of the zygoma may be necessary. Fixation of the jaw may also arise from spasms of the muscles (trismus), as in tetanus (risus sardonicus), and sometimes from reflex irritation from the lower teeth (caries or cutting a wisdom tooth), as the third division of the fifth nerve, by its motor root, supplies the muscles of mastication. Dental caries has also been known to cause torticollis, strabismus, areas of hyperesthesia, patches of grey hair, etc., through reflex action. Tumours of the lower jaw frequently arise in connection with the teeth (dentigerous cysts, etc.) Of the tumours involving the jaw itself, fibroma, osteoma, and enchondroma are met with, and also sarcoma. The latter may spring from the periosteum, often about the dental margin (malignant epulis), when it is generally of round or spindle-cell type ; or it may be of the myeloid type and originate in the medulla of the bone, the bone becoming gradually expanded, while the patient experiences progressive toothache as the various dental nerves become involved. In such cases excision of half of the lower jaw may be necessary. This is generally performed through an incision extending through the lip to the chin, and then carried along the lower border of the jaw and superficially up along the posterior margin of the ascending ramus to the lobule of the ear. Unless the sarcoma be periosteal, a subperiosteal excision of the half jaw is next performed, the jaw being cut through near the symphysis and then forcibly depressed, when the insertion of the temporal muscle into the coronoid process is cut and the condyle is disarticulated by a twisting movement, the capsule and external pterygoid muscles being cut. Reformation of the bone may follow such an operation.
The blood-supply of the lower jaw is derived from the facial (which sends a submental branch along the inferior border of the jaw to anastomose with the mental artery) and one of the terminal branches of the external carotid, the internal maxillary, which sends the inferior dental artery to accompany the nerve of that name, and the masseteric to the masseter muscle. The nerve-supply is through the inferior maxillary.