Bone resection for mechanically aligned total knee arthroplasty creates frequent gap modifications and imbalances.

Purpose: The objective of this study was to calculate bone resection thicknesses and resulting gap sizes, simulating a measured resection mechanical alignment (MA) technique for total knee arthroplasty (TKA).

Methods: MA bone resections were simulated on 1000 consecutive lower limb CT scans from patients undergoing TKA. Femoral rotation was aligned with either the surgical trans-epicondylar axis (TEA) or with 3° of external rotation to the posterior condyles (PC). Imbalances in the extension space, flexion space, medial compartment and lateral compartment were calculated.

Results: Extension space imbalances (≥ 3 mm) occurred in 25% of varus and 54% of valgus knees and severe imbalances (≥ 5 mm) were present in up to 8% of varus and 19% of valgus knees. Higher flexion space imbalance rates were created with TEA versus PC (p < 0.001). Using TEA, only 49% of varus and 18% of valgus knees had < 3 mm of imbalance throughout the extension and flexion spaces, and medial and lateral compartments.

Conclusion: A systematic use of the simulated measured resection MA technique for TKA leads to many cases with imbalance. Some imbalances may not be correctable surgically and may result in TKA instability. Modified versions of the MA technique or other alignment methods that better reproduce knee anatomies should be explored.

Level Of Evidence: 2.