Paper 82: Regulation of Joint Contact Area is an Essential Mechanism for Maintaining Physiological Patellofemoral Joint Mechanics

Abstract

Elevated pressure, reduced contact area and increased patellar tilt are thought to play a role in the onset of patellofemoral pain and their quantification in an experimental as well as clinical setting is of increasing importance. Nevertheless, these parameters have only been examined under simplified loading conditions, and under considerably lower forces than those occurring during activities of daily living. The purpose of this study was to investigate the healthy patellofemoral joint over the complete range of loading from unloaded up to physiological levels during walking and stair climbing. We hypothesized that the muscle loads significantly influence these biomechanical parameters. Six intact cadaveric knees with physiological trochleas were loaded with muscle forces simulating critical instances of walking (5 load cases) and stair climbing (4 load cases) with knee flexion ranging from 12° to 57°. The set up allowed for individual muscle load control of three quadriceps components from a passive joint state up to physiological load levels (max. 3509 N). Optical markers were attached to the femur, patella and tibia for kinematic measurements, and a pressure sensitive film was inserted into the joint for pressure and contact area measurements. The application of physiological force magnitudes resulted in a maximal patellofemoral pressure of 5.7 ± 0.8 MPa (mean ± SD). Load magnitudes below physiological levels resulted in a significant underestimation of contact area of up to 2.0 cm2. The patellar tilt was also underestimated (up to 2.0° underestimation), although only at initial knee flexion angles. Whilst pressure was linearly dependent upon the applied muscle loads, contact area and tilt increased non-linearly. Specifically, contact area increased logarithmically with load, reaching a plateau at 700 N. For the first time, physiological-like muscle loading (force magnitudes and directions) has been successfully applied to the knee in a multi-planar loading manner, simulating both walking and stair climbing. This study has demonstrated that physiological loading conditions are essential for a clinically relevant assessment of patellofemoral biomechanics. Since the knees were evaluated under physiological loads, this study provides an understanding of the in vivo mechanics and therefore provides the basis for assessing pathologies and surgical outcome.