The lateral is the most common variety—the medial being rare. Either may be complete or incomplete. Sometimes the bone is rotated so that its edge rests on the front of the femur—vertical dislocation; and in a few cases it has been completely turned round, so that the articular surface is directed forwards.
Clinical Features.—The joint is fixed, usually in a position of slight flexion, and the displaced patella can readily be palpated. The deformity is a striking one, and at first sight suggests a much more serious injury. Although easily reduced, the dislocation is liable to recur.
To effect reduction, the quadriceps must be thoroughly relaxed by extending the leg upon the thigh and flexing the thigh upon the pelvis; the patella is then tilted by making firm pressure on that edge which lies farthest from the middle of the joint, and at the same time pushing towards the middle line. The limb is placed on a posterior splint, and firm elastic pressure made on the joint to prevent or diminish effusion. Massage and movement are carried out from the first.
As the displacement is liable to recur, the patient should wear a firm elastic bandage or a strong knee-cap.
Permanent and recurrent dislocation of the patella will be described later.
Fracture of the Bones of the Leg
The bones of the leg may be broken together or separately.
Fracture of both Bones.—The features of this injury depend to a large extent upon the nature of the violence producing it. In fracture by direct violence, such as the passage of a wheel over the limb or a severe blow, the bones give way at the point of impact, and the line of fracture tends to be transverse, both bones being broken at the same level ([Fig. 89]). There is little or no displacement, and such as there is is angular, and is determined by the direction of the fracturing force.
| Fig. 89.—Radiogram of Transverse Fracture of both Bones of Leg by direct violence. | Fig. 90.—Radiogram of Oblique Fracture of both Bones of Leg by indirect violence. |
When the violence is indirect, as from a fall on the feet, or a twist of the leg, the tibia usually gives way at the junction of its lower and middle thirds, and the fibula at a higher level ([Fig. 90]). Torsion of the tibia is probably the most important factor in the production of the fracture, the distal fragment being fixed by the pressure of the foot upon the ground, while the proximal fragment is rotated by the impetus of the body. Both fractures are usually oblique—that in the tibia running from above downward, forward, and medially, and it is generally found that the obliquity of the fibular fracture corresponds with that in the tibia.