Hinged external fixation of the knee: intrinsic factors influencing passive joint motion.

OBJECTIVE To measure changes in knee kinematics after the application of articulated external fixators along a previously described knee flexion/extension axis and 16 specific "off-axis" fixator hinge configurations. DESIGN Cadaver, biomechanical study. SETTING Biomechanics laboratory. PARTICIPANTS Nine fresh cadaver knee specimens. INTERVENTION Each specimen was mounted on a custom-built frame that constrained the knee to move about a fixed flexion/extension axis. Passive knee motion was induced, and the resulting flexion moment was measured. Data were collected for the on-axis fixator position and 16 distinct rotational and translational off-axis positions. In addition, effects of tibial translation and rotation were investigated. MAIN OUTCOME Range of motion (ROM) attainable within a moment envelope of +/-1 N-m and average energy required to impart movement. RESULTS The average ROM for unconstrained knees was 122 degree. Constraining the knee to rotation around an on-axis aligned hinge significantly reduced the ROM by 35% to 79 degree. The 5-mm posterior translated hinge was the only alignment to show on average a slightly larger ROM (86 degree) than the on-axis hinge. All other hinge alignments showed decreased average ROM compared with the on-axis position. Tibiofemoral alignments significantly affected the obtainable ROM for the on-axis aligned hinge. CONCLUSION It was not possible to replicate precisely the complex kinematics of the knee using a single axis fixator over the entire ROM. Using the axis of rotation previously defined in the literature, however, it was possible to obtain a limited ROM of the knee without placing excessive forces on the periarticular structures.