Eight knees in 5 patients are described with a type of habitual subluxation of the patella that differs from the classical description. The unique feature of these patients is that the patella subluxates laterally each time the knee is actively extended, with spontaneous reduction on flexion.
Radiographic evaluation of the patellofemoral joint in these 8 knees showed abnormalities of the sulcus angle, congruence angle, lateral patellofemoral angle and patellofemoral tilting angle. CT scanning of the patella revealed that the patella was subluxated laterally when the knee was fully extended. Moreover, when the quadriceps was contracted in the extended position of the knee, the patella was found to subluxate more laterally.
Although the primary cause of this type of habitual subluxation of the patella is still unknown, we suggest that abnormalities of the vastus medialis attachment and dysplasia of the vastus medialis may play an important role in the subluxation in addition to the malalignment of the knee joint.
