Midflexion instability in revision total knee arthroplasty.

Midflexion instability is a new concept associated with revision total knee arthroplasty. It appears as rotational instability with combined external rotation and valgus stress in a knee flexed between 45 degrees and 90 degrees. Three main factors contribute to this instability: anterior medial collateral ligament attenuation, femoral-tibial articular geometry, and tibial post-femoral box geometry. Rotational stress should be included when assessing stability intraoperatively to identify those patients who may require additional constraint.