Ligament Balancing and Constraint in Revision Total Knee Arthroplasty.

Ligament balancing in revision knee arthroplasty is crucial to the success of the procedure. The medial collateral ligament and lateral ligament complex are the primary ligamentous structures that provide stability. Revisions can be performed with nonconstrained cruciate-retaining, posterior cruciate substituting, or anterior-stabilized/ultracongruent inserts when there are symmetrical flexion/extension gaps and intact collateral ligaments. When the collateral ligaments are insufficient either due to attenuation or incompetence from bone loss, a more constrained knee system is needed. Constrained condylar knees provide increased stability to both varus/valgus and rotation forces with a nonlinked construct. This increased constraint, however, does lead to increased stress at the implant-bone interface which requires more robust metaphyseal fixation. In cases of significant soft tissue disruption, severe flexion/extension gap mismatch or extensor mechanism disruption, a rotating hinge knee is needed to restore stability. Advances in revision implant design have led to improved outcomes and longer survivorship then earlier iterations of these implants. Surgeons should always strive to use the least constraint needed to achieve stability but must have a low threshold to increase constraint when ligament integrity is compromised.