chapter 9:Patellar Subluxation
TYPES OF SUBLUXATION (TRANSLATION)
In this case, the patella deviates little from the normal patellar course and is not associated with any gross or perhaps even clinically apparent relocation. These minor forms are often the result of a functional imbalance between the articular surfaces of the patella and the femoral trochlea. Larsen and Lund (1) have noted that patellofemoral incongruence is common after extensor mechanism rupture, and even less dramatic traumatic events may leave residual subluxation.
In this condition, the patella comes nearly astride the lateral trochlear facet and then suddenly returns to the patellofemoral groove with an audible snap. Episodes of near dislocation occur particularly at the beginning of flexion or semiflexion and recur with a variable rhythm. They may occur infrequently, perhaps associated with strenuous activity, and may be associated with patella alta (2). Episodes may occur with each flexion of the knee or intermittently. Most often, they occur at frequent intervals. It is possible that one of these episodes may proceed to a complete dislocation, a possible complication of recurrent subluxation. For our purposes, a patella will not be considered dislocated unless it is completely displaced out of the femoral trochlea. It is likely that many cases of major recurrent subluxation are included in series describing recurrent dislocation, which explains why some authors have reported them in combined series (3).
This form of lateral displacement is stable, in that subluxation is persistent through 90 degrees or more of knee flexion. There is little tendency toward recentering of the patella with progressive flexion, and there is almost always associated tilt (4). This form may progress rapidly to cartilage degeneration, and most examples already show significant degenerative change at the time of clinical presentation.
At the time of initial presentation, degenerative changes are common, but malalignment is usually greater if there has been significant cartilage loss. The mean age in Ficat's experience was 50 years, whereas Merle D'Aubigne and Ramadier (5) reported a mean age of 62 years.
There are several possible mechanisms for the development of a permanent stable form of lateral displacement (1). It is possible that the subluxation is congenital or dysplastic in origin (2). Subluxation may be acquired either through faulty mechanics or surgery, with gradual stretching of medial stabilizers or repetitive minor injury accomplishing the same end (3). It is possible that recurrent dislocation, with its obvious trauma to medial stabilizers, settles into a position of permanent lateral subluxation. Permanent lateral subluxation is frequently accompanied by tilt (Fig. 9.1). The combination of subluxation and tilt, when persistent throughout flexion and extension, usually leads to arthrosis because of abnormal unit loading of articular cartilage.
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