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The effects of articular, retinacular, or muscular deficiencies on patellofemoral joint stability

A BIOMECHANICAL STUDY IN VITRO

Biomechanics Section, Mechanical Engineering Department, Room 638 Mechanical Engineering Building, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.

A. A. Amis, DSc, Professor

Musculoskeletal Surgery Department, Imperial College London, Charing Cross Hospital 7E, London W6 8RF, UK.

Correspondence should be sent to Professor A. A. Amis at Imperial College London, South Kensington Campus, London SW7 2AZ, UK; e-mail: a.amis{at}imperial.ac.uk

Normal function of the patellofemoral joint is maintained by a complex interaction between soft tissues and articular surfaces. No quantitative data have been found on the relative contributions of these structures to patellar stability. Eight knees were studied using a materials testing machine to displace the patella 10 mm laterally and medially and measure the force required. Patellar stability was tested from 0° to 90° knee flexion with the quadriceps tensed to 175 N. Four conditions were examined: intact, vastus medialis obliquus relaxed, flat lateral condyle, and ruptured medial retinaculae. Abnormal trochlear geometry reduced the lateral stability by 70% at 30° flexion, while relaxation of vastus medialis obliquus caused a 30% reduction. Ruptured medial retinaculae had the largest effect at 0° flexion with 49% reduction. There was no effect on medial stability. There is a complex interaction between these structures, with their contributions to loss of lateral patellar stability varying with knee flexion.

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