Pre-operative frontal plane malalignment predicts the extension gap asymmetry in knee osteoarthritis

In contrast to bone cuts, soft-tissue releases cannot be planned sufficiently prior to total knee arthroplasty (TKA). Intra-operative evaluation of the extension gap after tibial or femoral resection may result in an unnecessarily excessive bone resection. The present study examines whether extension gap asymmetry can be calculated from the pre-operative long standing X-ray. In this retrospective study, 32 patients with navigated unconstrained TKA were included. The pre-operative malalignment was measured on long standing X-rays and compared with the extension gap asymmetry that was documented by the navigation system intra-operatively. The mean pre-operative malalignment was 9.8A degrees (20A degrees valgus to 14.8A degrees varus). The intra-operatively measured extension gap showed a mean asymmetry of 3.4 A +/- 2.4 mm. Both correlated following the equation: asymmetry [mm] = 0.35 * malalignment [A degrees] (R = 0.855, P < 0.001). We recommend planning the intra-operative release and tibial bone cut using long standing X-rays.