chapter 9:Patellar Subluxation

Natural History

Many patients with simple patellar subluxation without tilt (Type I malalignment) show little or no sign of articular cartilage degeneration. Often, these patients have lax ligaments. When there is tilt associated with subluxation (Type II malalignment), progression of articular cartilage deterioration is more prominent because of the increased lateral facet loading. Type I malalignment may show little or very slow progression of articular cartilage softening and degeneration.

Recurrent subluxation may proceed to a frank dislocation at any time that would then render the patella more susceptible to recurrent dislocation. Even a trivial injury can cause dislocation of the unstable subluxating patella. In a sense, then, the patient with patellar subluxation is in a precarious position‑limited functionally by apprehension and insecurity on the one hand, and risk of dislocation with minor mishaps on the other.

There are three stages of patellar subluxation: Stage I—subluxation alone; Stage II—subluxation with patellofemoral arthrosis; and Stage III—subluxation with both patellofemoral arthrosis and tibial‑femoral arthrosis.

In many cases, the early stage probably passes undetected. A relative equilibrium has been established, and patients are not prone to the giving way and general sense of insecurity characteristic of the recurrent subluxation and dislocation syndromes. Lateral subluxation may not be clinically apparent, and the knee often tracks smoothly in flexion and extension despite lateral placement (Fig. 9.3). Many of these patients have little or no associated tilt.

Stage II presentation is characterized by aching pain, typically anterior, trouble with stairs, giving way, and swelling—in short, all the symptoms of patellofemoral arthrosis. Most of these patients have major recurrent or permanent lateral subluxation. Flexion is frequently limited to the 90‑ to 100‑degree range, and a mild fixed flexion contracture is not uncommon. Quadriceps atrophy is often impressive, particularly the vastus medialis. Difficulty with stairs represents the most frequent chief complaint. The typical patient is a woman, obese, and over 50 years old, although this may occur after disruption of a medial arthrotomy and chronic lateralization of the extensor mechanism. Patients with Stages II and III subluxation have associated tilt.

As contact studies have shown, flexion of more than 90 degrees brings the patella into direct contact with the tibiofemoral weightbearing area of the femur. In Stage III, the degeneration has spread to involve the lateral tibiofemoral compartment. Both our own experience and autopsy studies strongly suggest that patellofemoral disease often precedes tibiofemoral changes (11). Any significant genu valgum is likely to exacerbate the degeneration of both lateral compartments, which explains the predominance of knock‑kneed elderly women in this end‑stage group (Fig. 9.4).

This disorder is very frequently bilateral, although the clinical and radiologic stages are frequently not symmetrical (Fig. 9.5). However, as is characteristic clinically of arthrosis in general, the correlation between symptoms and radiologic changes is not always high. Minor injury may be responsible for pushing the disorder over the clinical threshold.

Although "staging" subluxation may be helpful, it is probably better to describe the specific alignment disorder in each patient, quantitating the relative amounts of sub­luxation and tilt, and grade the amount and location of arthrosis (4). Also, Stage I sub­luxation does not necessarily progress to Stage II, particularly because the specific align­ment patterns may be different (Stage I—subluxation alone; Stage II—subluxation with tilt causing arthrosis). Many individuals with "Stage I" subluxation may go along in­definitely with minor recurrent subluxation.




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