Hinged External Fixation of the Knee: Intrinsic Factors Influencing Passive Joint Motion

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Abstract

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°. Constraining the knee to rotation around an on-axis aligned hinge significantly reduced the ROM by 35% to 79°. The 5-mm posterior translated hinge was the only alignment to show on average a slightly larger ROM (86°) 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.

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