INTRODUCTION
Medial and lateral soft-tissue releases to correct varus and valgus deformities in primary total knee arthroplasty are achieved by sequential release of contracted soft-tissue constraints or, occasionally, advancement of elongated ligaments1-3. In primary total knee arthroplasty, soft-tissue constraints are typically well-defined anatomical structures, whereas, in revision total knee arthroplasty, the soft-tissue constraints may be thickened and scarred, attenuated, or absent. Soft-tissue balance in revision total knee arthroplasty is achieved by a combination of soft-tissue releases and variation in the bone resection level, implant position, and implant size.

SURGICAL TECHNIQUE
Relationship Between Medial-Lateral and Flexion-Extension Balancing Medial-lateral soft-tissue balance depends on the length of the collateral ligaments and musculotenthat the soft-tissue tension on the medial and lateral sides of the knee, as determined with varus and valgus stress-testing with spacer blocks or trial components in place, is symmetric. This may require release of contracted soft tissues (Fig. 1). Medial-lateral soft-tissue balance may be different in flexion and extension because the posterior capsule and the hamstring tendons contribute to mediallateral stability in full extension whereas they are lax during flexion (Fig. 2). Medial-lateral soft-tissue balance in flexion depends on the integrity of the collateral ligaments. Soft-tissue balance may be achieved in full extension but not in flexion if one of the collateral ligaments is lax. The size of the extension space may be limited by a tight posterior capsule. While the posterior capsule can contribute to medial-lateral stability in extension, it does not contribute to stability in flexion. Release of a tight posterior capsule is necessary to balance the collateral ligaments in both flexion and extension (Fig. 3). Thus, medial and lateral soft-tissue tension should be assessed independently in both flexion and extension. However, medial-lateral softtissue balance in flexion can be difficult to interpret during surgery. At 90° of flexion, varus and valgus stress-testing of the knee causes external and internal rotation, respectively, of the hip. To accurately assess tension of the medial and lateral collateral ligaments at 90° of knee flexion, the hip must be maximally rotated externally and internally, respectively. Alternatively, varus and valgus stress-testing can be performed at 15° to 30° of flexion to relax the posterior capsule and the hamstring tendons while avoiding rotation at the hip joint.