Preparation of the flexion gap affects the extension gap in total knee arthroplasty.

Changes in the extension gap caused by the preparation of the flexion gap were measured in 50 varus osteoarthritic knees undergoing total knee arthroplasty. The extension gap was first prepared and then measured before and after preparation of the flexion gap. The extension gap increased significantly on both the medial and lateral sides (19.3 +/- 0.3 to 22.0 +/- 0.3 and 21.9 +/- 0.3 to 24.0 +/- 0.3 mm, respectively, mean +/- SE, both P < .0001). The increase was greater medially (P = .0014), and in the knees with more varus deformities (> or = 10 degrees, P = .0075). Thus, preparation of the flexion gap resulted in significant widening of the extension gap, especially on the medial side. The medial release should be stepwise and conservative to achieve adequate soft tissue balance.