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Reducing the Lateral Force Acting on the Patella Does Not Consistently Decrease Patellofemoral Pressures

John J. Elias, PhD^*,, Jennifer A. Cech, David M. Weinstein, MD and Andrew J. Cosgrea, MD

From the Medical Education and Research Institute of Colorado, Colorado Springs, Colorado, and Johns Hopkins University, Baltimore, Maryland

^* Address correspondence to John J. Elias, Medical Education and Research Institute of Colorado, 3920 North Union Blvd, Suite 210, Colorado Springs, CO 80907 (e-mail: elias{at}meric.info).

Background: Extensor mechanism procedures that decrease the lateral component of the patellar tendon or quadriceps force acting on the patella do not consistently reduce pain.

Hypothesis: Patellofemoral treatments do not consistently decrease patellofemoral pressures because of variations in the moments acting on the patella.

Study Design: Computer simulation study.

Methods: Computational models of 4 knees were constructed to characterize the patellofemoral pressure distribution during simulated squatting from 40°to 90°. The knees were given an initial Q angle of 25°. Patellofemoral treatments were simulated by increasing the percentage of the quadriceps force applied by the vastus medialis by 50% and by medializing the tibial tuberosity to decrease the Q angle to 15°.

Results: Decreasing the Q angle caused a larger decrease in the lateral component of the force applied by the quadriceps and patellar tendon than did increasing the force applied by the vastus medialis and, therefore, was more effective at decreasing patellofemoral pressures and the force needed to resist lateral subluxation. Both treatments also decreased the moments acting to rotate the distal patella laterally and tilt the patella laterally during flexion. Variations in these moments increased patellofemoral pressures for some knees.

Conclusions: Treatments that reduce patellofemoral subluxation can have an unexpected influence on patellofemoral pressures because of the moments acting on the patella.

Clinical Relevance: Extensor mechanism procedures that restore patellofemoral stability may not provide pain relief.

Key Words: patella • cartilage • pressure • pain

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