DISLOCATIONS OF THE PATELLA.—The patella may be dislocated outward or in ward or rotated around its long axis, or the two forms may be combined. Dis placement upward or downward is purely secondary to rupture of the ligamentum patella or the quadriceps tendon, and need not be here considered.
Outward dislocation is complete or complete, and accompanied by various degrees of rotation (Fig. 15: 1, 2, and 3). patella is readily felt in its new tion, though it may be difficult to mine whether tbe outer or the inner border is directed forward. Muscular tion or direct violence are the causes of the dislocation, and hydrarthrosis and ligamentous weakness are predisposing causes. The fibrous expansion of the vastus internus is ruptured, and the mus cle itself may be more or less torn. Re duction is made by direct pressure dur ing extension of the knee and flexion of the hips.
Incomplete dislocations are those in which, during extension or flexion, the patella moves outward on to the external condyle.
Outward, Edgewise, or Vertical Dislo cations (by Rotation).—In these the pa tella is moved outward and its inner edge backward into the intereondylar groove; so that its articular surface looks outward and more or less forward, or completely forward (Fig. 15: 4 to 7). The causes and treatment are the same as for out ward dislocations.
Inward dislocations present the same features, mutatis mutandis, as the out ward, but they are much less frequent.
Habitual dislocations are usually the result of some deformity, such as genu valgum. They are controlled by correct ing the original deformity or by appa ratus, or by tightening up the loose lat eral ligaments (by operation).