The displacement of the fragments will depend on the line of fracture; and the line of fracture may be determined by the direction and character of the fracturing force. The line of fracture is oblique. It may be oblique from above down or from without in. An examination of the muscles attached to the mandible will show that the elevators of the jaw are attached to it posteriorly and its depressors anteriorly. On this account, when the fracture runs obliquely down and forward there is little or no displacement, because the depressors and elevators tend to press the fragments together. When the fracture runs downward and backward (see Fig. 80), the depressors and elevators tend to separate the fragments. The depression of the anterior fragment is particularly marked when the fracture is double, involving both sides of the jaw. The muscles which tend to depress the anterior fragment are the geniohyoglossus, geniohyoid, mylohyoid (anterior portion), digastric, and platysma. The muscles which elevate the posterior fragment are the temporal, masseter, buccinator, and internal pterygoid.

The displacement may not only be up and down, but may also be lateral. The line of fracture may run from the outside either inward and backward or inward and forward. The jaw is held in place by its own rigidity when intact; when broken, the smaller fragment is liable to be pulled inward by the muscles passing from it toward the median line. These muscles are the internal pterygoid and the mylohyoid. The influence of the former is more marked than of the latter, because the fracture frequently divides the mylohyoid, leaving a part of it attached to each fragment. When the fracture passes from without inward and backward, then there will be little or no displacement, because the internal pterygoid and mylohyoid draw the fragments together. (See Fig. 81.) When the line of fracture is from without inward and forward, the internal pterygoid of the injured side and the mylohyoid draw the posterior fragment inward, while the internal pterygoid of the opposite side draws the anterior fragment outward (Fig. 82).

From a consideration of the foregoing facts, we see that when there is displacement it is because the fracture runs from above downward and backward, and from without inward and forward. The anterior fragment is displaced downward and the posterior fragment is displaced inward.

Fractures through the region of the molar teeth are not particularly uncommon, and this is likewise the case with fractures obliquely downward and outward through the angle of the jaw. In these injuries the firm attachment of the masseter on the external surface of the jaw and the internal pterygoid on its inner prevent displacement.

Fractures of the coronoid process are exceedingly rare. In them displacement is prevented by the attachment of the temporal muscle, which passes much farther down on the inside than on the outside.

Fractures of the neck of the jaw are particularly serious. Inserted into the condyle and neck of the jaw is the external pterygoid muscle. When a fracture of the neck occurs, this muscle pulls the upper fragment anteriorly and tends to tilt its inferior surface forward. This displacement is so marked that an excessive amount of callus is thrown out and ankylosis may result. This so seriously interferes with the use of the jaws as to justify an operation to remove or replace the upper fragment in proper position. The injury is liable to be overlooked in children, and as they grow up the deformity shown in Fig. 83 develops.

Fig. 80.   Fracture of the lower jaw, showing the line of fracture proceeding downward and backward, favoring displacement.

Fig. 80. - Fracture of the lower jaw, showing the line of fracture proceeding downward and backward, favoring displacement.

Fig. 81.   Fracture of the jaw, showing absence of displacement when the line of fracture runs from the inside forward and outward.

Fig. 81. - Fracture of the jaw, showing absence of displacement when the line of fracture runs from the inside forward and outward.

Fig. 82.   Fracture of the jaw, showing the action of the internal pterygoid and mylohyoid muscles in producing displacement when the line of fracture runs from the outside forward and inward.

Fig. 82. - Fracture of the jaw, showing the action of the internal pterygoid and mylohyoid muscles in producing displacement when the line of fracture runs from the outside forward and inward.

Fig. 83.   Deformity of the face following ankylosis due to fracture of the neck of the lower jaw in infancy (from an original sketch).

Fig. 83. - Deformity of the face following ankylosis due to fracture of the neck of the lower jaw in infancy (from an original sketch).