Overview
The patella (kneecap) sits within a groove at the end of the femur called the trochlear groove. I think of the underside of the patella like the bottom of a boat riding within a channel. This groove guides the patella and keeps it centered as the knee bends and straightens. Besides arthritis, the other major problem involving the patella is instability, where the patella partially or completely slips out of its groove. Nearly all patellar dislocations occur outward, toward the outside and away from the other knee (laterally). The medial patellofemoral ligament (MPFL) is a ligament on the inside of the patella that connects the femur to the patella and acts like a leash, limiting its movement toward the outside.
Causes
Patellar instability generally develops in one of two ways. A traumatic first-time dislocation usually follows a significant twisting injury, awkward landing, or direct blow to the inside of the patella. Many patients recover without recurrent instability, although cartilage injury or loose bodies may occur as a result of the patella striking the femur as it dislocates. Other patients have underlying anatomical factors such as trochlear dysplasia, generalized ligamentous laxity, or other inherited anatomical variations, making recurrent dislocations much more likely.
Diagnosis
Evaluation includes a history, physical examination, X-rays, and often an MRI to evaluate cartilage injury, loose bodies, and injury to the medial patellofemoral ligament (MPFL).
Conservative Treatment
Most first-time dislocations without significant cartilage injury are treated without surgery. Treatment may include aspiration of a large effusion, which usually results from bleeding into the knee, temporary bracing, early weight-bearing, and physical therapy. Therapy focuses on reducing swelling, restoring motion, and strengthening the VMO, which is the inner portion of the quadriceps near the knee. Most patients return to full activities within approximately 2–3 months.
Surgical Treatment
Early surgical treatment is recommended when significant cartilage injury, unstable cartilage fragments, or symptomatic loose bodies are suspected following the dislocation. If patients continue to experience recurrent instability or sustain additional dislocations despite appropriate rehabilitation, surgery may also be indicated. The most common procedure is MPFL reconstruction. The procedure uses donor tendon tissue and is performed as an outpatient procedure. It is followed by protected weight-bearing with progressive rehabilitation and return to sports in approximately 3–4 months.
Patients with significant trochlear dysplasia or other major anatomical abnormalities may require additional reconstructive procedures beyond an isolated MPFL reconstruction. These patients have a significantly higher failure rate if all underlying pathology is not adequately addressed.
Results
MPFL reconstruction has an excellent success rate in restoring patellar stability and preventing recurrent dislocations. Most patients return to their desired activities following rehabilitation.
Long-Term Considerations
Repeated dislocations can progressively damage the cartilage behind the kneecap and lead to post-traumatic patellofemoral arthritis. One of the primary goals of treatment is to restore stability before repeated dislocations produce permanent cartilage damage.