Medial unicompartmental knee arthroplasty in ACL-deficient knees is a viable treatment option: in vivo kinematic evaluation using a moving fluoroscope.

Purpose: Unicompartmental knee arthroplasty (UKA), resulting in similar kinematics to native knees, is functionally superior to total knee arthroplasty (TKA). However, ACL deficiency is generally considered to be a contraindication. The main purpose of this study was to investigate if UKA in ACL-deficient knees would result in similar kinematics to conventional UKA with an intact ACL.

Methods: Ten conventional UKA patients were compared to eight ACL-deficient patients with a reduced tibial slope to compensate for instability, resulting from the deficient ACL. Knee kinematics was evaluated with a moving fluoroscope, tracking the knee joint during daily activities. In a standing position (baseline), posterior shift of the femur was observed for ACL-deficient UKA patients, compared to conventional UKA patients.

Results: A significant posterior femoral shift in the ACL-deficient group was observed during the first 25% (near extension) of deep knee bend, while there was no difference in kinematic waveforms for all other activities. No significant range of motion differences across different activities between the two UKA groups were detected, except for an increase of medial AP translation in the ACL-deficient group, during deep knee bend and stair descent.

Conclusion: Despite the posterior femoral shift due to ACL deficiency, both UKA groups showed similar kinematic waveforms, indicating that posterior tibial slope reduction can partially compensate for ACL function. This supported our hypothesis that fixed bearing UKA can be a viable treatment option for selected ACL-deficient patients, allowing patient-specific kinematics. While anteroposterior laxity can be compensated, rotational stability was a prerequisite for this approach.

Level Of Evidence: III.