260 CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Number 402, pp. 260–269 © 2002 Lippincott Williams & Wilkins, Inc. Patellofemoral pain is associated with patellar malalignment and quadriceps weakness which are seen more commonly in women. The objec- tive of the current study was to determine the ef- fects of gender, vastus medialis strength, and tibial rotation on patellofemoral joint biome- chanics. Twelve fresh-frozen knees from cadav- ers were tested using a custom knee jig. Anatomic multiplane loading of the extensor mechanism was used with varying vastus medialis loads. Patellofemoral contact area and pressure were measured using pressure sensitive film at knee flexion angles of 0, 30, 60, and 90 with the tibia in neutral and 15 internal and external tibial rotation. Patellofemoral joint contact ar- eas in specimens from men were larger at knee flexion angles greater than 30. A significant in- crease in mean patellofemoral contact pressures was seen for specimens from women when com- pared with specimens from men at 0 and 30 knee flexion. The knees from women also showed a greater change in contact pressures to varying vastus medialis load at knee flexion angles of 0, 30, and 60. The results of the current study indicate that there are gender differences in patellofemoral contact areas and pressures. These differences may help explain the increased incidence of patellofemoral disorders in women. Patellofemoral disorders are among the most common problems seen by orthopaedists. 21 These disorders may range from anterior knee pain to frank patellar dislocation. Many terms have been used to describe patellofemoral pain disorders including anterior knee pain, exten- sor mechanism dysfunction, medial facet syn- drome, lateral facet syndrome, lateral com- pression syndrome, and patellar malalignment syndrome. 21,23 The pathophysiology of ante- rior knee pain is not well-understood. 4,19 Var- ious causes of patellofemoral pain have been suggested including: vastus medialis weak- ness or atrophy, increased quadriceps (Q) an- gle, genu valgum, femoral anteversion, exter- nal tibial torsion, tight lateral retinaculum, abnormalities of the shape of the patella, fem- oral groove morphologic features, and fore- foot pronation. 23 Patellofemoral pain also has been attributed to excessive stresses associ- ated with abnormal patellofemoral joint me- chanics. 20 Others have concluded that chronic Gender Differences in Patellofemoral Joint Biomechanics Rick P. Csintalan, MD; Michele M. Schulz, MD; Jonathan Woo; Patrick J. McMahon, MD; and Thay Q. Lee, PhD From Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System, Long Beach, CA and the De- partment of Orthopaedic Surgery, University of Califor- nia, Irvine, CA. Funding for the current study was provided by a grant from the Department of Veterans Affairs Rehab R&D Service, and from the California Orthopaedic Research Institute and the John C. Griswold Foundation. Reprint requests to Thay Q. Lee, PhD, Orthopaedic Bio- mechanics Laboratory, VA Long Beach Healthcare Sys- tem (09/151), 5901 East 7th St., Long Beach, CA 90822. Received: November 3, 2000. Revised: September 21, 2001; November 26, 2001. Accepted: December 13, 2001.