THE EFFECTS OF AXIAL AND MULTI-PLANE LOADING OF THE EXTENSOR MECHANISM ON THE PATELLOFEMORAL JOINT

Objective. To compare the effects of axial loading, and anatomically b ased multi-plane loading of the extensor mechanism on the patellofemor al joint. Design. Repeated measures design using an in-vitro cadaver m odel. Background. Since the extensor mechanism is the primary contribu tor to the patellofemoral joint reaction force and can affect patellar kinematics, it is essential that the forces produced by this musculat ure be accurately represented in a simulation model. Methods. Patellar kinematics (magnetic tracking device), contact pressures and areas (p ressure sensitive film) were measured from 6 cadaver knees under two d ifferent loading conditions: 1) axial (rectus femoris loaded in the fr ontal plane), and 2) multiplane (individual components of the quadrice ps loaded along their respective fiber directions in both the frontal and sagittal planes). Specimens were mounted in a custom knee jig, wit h muscle forces being simulated using a pulley system and weights. Dat a were collected at 0 degrees, 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees and 90 degrees of knee flexion. Results. Compared to the axial loading condition, multi-plane loading of the vasti resul ted in significantly greater contact pressure at 0 degrees and signifi cantly less contact pressure at 90 degrees of knee flexion. Furthermor e, the multi-plane loading condition resulted in greater lateral patel lar rotation from 0-75 degrees of knee flexion, and greater lateral gl ide at 30 degrees of knee flexion. Greater patellar flexion was observ ed with the axial loading condition. Conclusions. These findings indic ate that axial loading of the extensor mechanism underestimates contac t pressure at 0 degrees and overestimates contact pressure at 90 degre es of knee flexion when compared to multi-plane loading. Additionally, loading of the individual vasti appears to have an effect on patellar kinematics. Relevance The results of this study indicate that anatomi cally based, multi-plane loading of the vasti will yield subtle yet si gnificant differences in patellofemoral joint mechanics when compared to the more traditional axial lending approach. These differences may have implications for the study of both normal and pathological patell ofemoral joint mechanics, as well as evaluation of surgical techniques and prosthetic implants. (C) 1998 Elsevier Science Ltd. All rights re served.