Recurrent dislocation of the patella is not a common entity. When it does occur, displacement is almost always lateral.
It may be congenital, developmental, or post-traumatic.
In contrast, recurrent subluxation of the patella is quite common. It is more common in females.
A familial tendency has been noted in recurrent subluxation of the patella.
Following causes have been thought of contributing to recurrent dislocation of patella
Laxity of the medial capsule of the knee is a definite factor. In children with diseases that cause ligamentous laxity (e.g. osteogenesis imperfecta, arachnodactyly, or the “Ehlers-Danlos syndrome), lateral dislocation is more common.
Lateral Patellar Soft Tissue Contracture
The lateral patellar retinaculum and patellofemoral ligament are taut. The vastus lateralis may be contracted, hypertrophied, and inserted low.
The iliotibial tract, a thickened strip of fascia lata may abnormally insert on lateral border of the patella. When the knee is flexed, it axis of iloitibial tract passes behind knee and contributes to subluxation of patella.
Atrophy, weakness, or a high oblique insertion of the vastus medialis is a factor in most patients. The vastus medialis is a dynamic medial stabilizer of the patella.
Malalignment of the Lower Limb
Lateral tibiofibular torsion, and genu valgum will displace the insertion of the patellar ligament laterally and cause valgus position of the quadriceps mechanism.
Q angle is the angle formed between the patellar tendon with a vertical line extended distally from the center of the inferior pole of the patella. Its value can provide guide to the rotatory-angular forces.
This is a high riding patella. In this the normal buttressing effect of the lateral femoral condyle, which serves to check the tendency to lateral patellar displacement, will be lost.
A traumatic lateral dislocation inadequately treated will result in stretching and weakening of the medial capsule of the knee and insufficiency of the vastus medialis, predisposing to recurrent lateral subluxation.