THE BIOMECHANICS OF THE HUMAN PATELLA DURING PASSIVE KNEE FLEXION

The fundamental objectives of patello-femoral joint biomechanics inclu de the determination of its kinematics and of its' dynamics, as a func tion of given control parameters like knee flexion or applied muscle f orces. On the one hand, patellar tracking provides quantitative inform ation about the joint's stability under given loading conditions, wher eas patellar force analyses can typically indicate pathological stress distributions associated for instance with abnormal tracking. The det ermination of this information becomes especially relevant when facing the problem of evaluating surgical procedures in terms of standard (i .e. non-pathological) knee functionality. Classical examples of such p rocedures include total knee replacement (TKR) and elevation of the ti bial tubercle (Maquet's procedure). Following this perspective, the cu rrent study was oriented toward an accurate and reliable determination of the human patella biomechanics during passive knee flexion. To thi s end, a comprehensive three-dimensional computer model, based on the finite element method, was developed for analyzing articular biomechan ics. Unlike previously published studies on patello-femoral biomechani cs, this model simultaneously computed the joint's kinematics, associa ted tendinous and ligamentous forces, articular contact pressures and stresses occurring in the joint during its motion. The components cons tituting the joint (i.e. bone, cartilage, tendons) were modeled using objective forms of non-linear elastic materials laws. A unilateral con tact law allowing for large slip between the patella and the femur was implemented using an augmented Lagrangian formulation. Patellar kinem atics computed for two knee specimens were close to equivalent experim ental ones (average deviations below 0.5 degrees for the rotations and below 0.5 mm for the translations) and provided validation of the mod el on a specimen by specimen basis. The ratio between the quadriceps p ulling force and the patellar tendon force was less than unity through out the considered knee flexion range (30-150 degrees), with a minimum near 90 degrees of flexion for both specimens. The contact patterns e volved from the distal part of the retropatellar articular surface to the proximal pole during progressive flexion. The lateral facet bore m ore pressure than the medial one, with corresponding higher stresses ( hydrostatic) in the lateral compartment of the patella. The forces act ing on the patella were part of the problem unknowns, thus leading to more realistic loadings for the stress analysis, which was especially important when considering the wide range of variations of the contact pressure acting on the patella during knee flexion.