Dysplasia of the quadriceps mechanism: hypoplasia of the vastus medialis muscle as related to the hypermobile patella syndrome.

Disturbance of the patellofemoral mechanism is the most common cause of dysfunction in the knee, especially in athletes of all age groups. This is primarily a congenital familial abnormality due to poor development in the vastus medialis. The patella develops in the line of maximum pull of this muscle group; in this congenital condition it develops in line with the stronger antagonist to the vastus medialis, the vastus lateralis muscle. Therefore, the position in the extremity that the patella assumes is the result of abnormal muscle development. This brings about other changes in the lower extremity that increase the tendency of the patella to migrate laterally. This condition, hypoplasia of the vastus medialis muscle, exists in a large number of people (about 40% of my patient population) in an asymptomatic form that predisposes them to degenerative changes of the patellofemoral articulation later in life, even without symptoms related to dysfunction of the patellofemoral mechanism. These individuals are also predisposed to disturbances of the patellofemoral mechanism which may cause symptoms, that is, the painful knee for which they seek medical attention. The symptomatic types of dysplasia of the quadriceps mechanism are usually brought on by injury to the medical quadriceps retinaculum that stretches or tears it, allowing the lateral migration to increase and producing pain in the inner aspect of the knee. Disuse atrophy of the vastus medialis accentuates the clinical picture as the condition worsens. As the asymptomatic type may progress into the midly symptomatic variant and eacy may worsen into a more serious disabling variety as reinjury occurs, it is important to recognize each type and to treat them properly.