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 based multi-plane loading of the extensor mechanism on the patellofemoral joint. DESIGN: Repeated measures design using an in-vitro cadaver model. BACKGROUND: Since the extensor mechanism is the primary contributor to the patellofemoral joint reaction force and can affect patellar kinematics, it is essential that the forces produced by this musculature be accurately represented in a simulation model. METHODS: Patellar kinematics (magnetic tracking device), contact pressures and areas (pressure sensitive film) were measured from 6 cadaver knees under two different loading conditions: 1) axial (rectus femoris loaded in the frontal plane), and 2) multiplane (individual components of the quadriceps loaded along their respective fiber directions in both the frontal and sagittal planes). Specimens were mounted in a custom knee jig, with muscle forces being simulated using a pulley system and weight. Data 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 resulted in significantly greater contact pressure at 0 degrees and significantly less contact pressure at 90 degrees of knee flexion. Furthermore, the multi-plane loading condition resulted in greater lateral patellar rotation from 0-75 degrees of knee flexion, and greater lateral glide at 30 degrees of knee flexion. Greater patellar flexion was observed with the axial loading condition. CONCLUSIONS: These findings indicate that axial loading of the extensor mechanism underestimates contact pressure at 0 degrees and overestimates contact pressure at 90 degrees 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 anatomically based, multi-plane loading of the vasti will yield subtle yet significant differences in patellofemoral joint mechanics when compared to the more traditional axial loading approach. These differences may have implications for the study of both normal and pathological patellofemoral joint mechanics, as well as evaluation of surgical techniques and prosthetic implants.