The patella is a protective bone located in front of the knee joint, within the patellofemoral groove and is attached to the quadriceps muscle through the quadriceps tendon (Figure 1). The patella facilitates the function of the quadriceps to straighten the knee .
The patellofemoral groove allows the patella to slide vertically with normal movement of the knee joint (Figure 2). Dislocation of the patella occurs from the patella sliding laterally out of its position on the knee. It may occur from a blow to the knee, acute trauma, or severe twisting of the knee. In a majority of the cases, dislocations are associated with pain, swelling, and a lump on the lateral side (outer side) of the knee. The patella will usually relocate into the normal position by itself as the patient straightens the knee.
The x-ray will show lateral dislocation of the patella as seen in an AP view and Sunrise View (Figure 3). The x-ray should also be carefully evaluated for the presence of osteochondral fragments displaced within the knee joint.
When the medial patellofemoral ligament fails, the patella will move laterally and cause a direct impaction injury to both the medial facet of the patella and lateral femoral condyle (Figure 4).
Patellar Dislocations are caused due to weak VMO (vastus medialis obliqus) muscles, large Q angles (Figure 5), excessive foot pronation, patella alta or higher riding patella, and genu valgum (knock knees).
When the first episode occurs, it is usually treated nonoperatively unless there is a loose fragment inside of the joint, in which case will need to be removed surgically. The patient will need to use a knee immobilizer or a brace for 2-3 weeks to allow medial structures and ligaments to heal. The patient will also need to perform quadriceps strengthening exercises. A high percentage of patients treated with conservative methods could experience recurring episodes of instability within five years and some patients will require surgery. A common complication in treating patellar dislocations is chondromalacia which is softening of the patellar cartilage.
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