Habitual Lateral Dislocation and Subluxation of the Patella on Extension of the Knee

We experienced two cases of subluxation and dislocation of the patella which occurred laterally with the knee extended and were reduced with crepitation when the knee was flexed to 20 to 30 degrees. We suspect that the etiology of this disease is abnormality of the knee rotation alignment, genu valgum, bayonet deformity, genu recurvatum, and patella hypermobility and so on.We think that functional abnormality of the quadriceps femoris muscle is a main difference between dislocations of the patella with the knee flexed and extended.