The contribution of the medial retinaculum and quadriceps muscles to patellar lateral stability--an in-vitro study.

Patellofemoral joint stability is a result of the restraining effects of the quadriceps muscles, the retinacular structures, and engagement of the bones. The role and significance of these mechanisms in restraining patellar lateral displacement was investigated in this study by measuring the force needed to cause 5 mm lateral displacement (i.e. the mechanical stability, or 'stabilising force') of the patella. Six cadaver knees had 175 N quadriceps load distributed among three muscle groups. With a force ratio matching the muscles physiological cross sectional areas, no significant change occurred in the patellar stabilising force between 0 and 60 degrees knee flexion, but a significant increase occurred between 60 and 90 degrees, presumably reflecting the contribution of the femoral groove. Variation of the quadriceps force distribution changed the stability significantly. Relaxing the vastus lateralis increased the patellar lateral stabilising force 52+/-8%, while relaxing vastus medialis reduced the stabilising force 47+/-9%. The minimum stabilising force was at 30 degrees knee flexion. Transection of the medial retinaculum reduced the lateral stabilising force 34% in the extended knee. This effect disappeared by 45 degrees knee flexion. It was concluded that the quadriceps muscles had a significant and consistent effect across the whole range of knee flexion, but the contribution of the medial retinaculum was restricted to extended knee postures.