There are 7 bone cuts in a total knee replacement. The posterior condylar cut determines the flexion gap. Flexion instability in PS knees arises because of an enlarged flexion gap (excessive posterior condylar resection, or increased tibial slope), allowing anterior tibial translation, which is pathognomonic. There will not be posterior subluxation because of the cam-post design. Symptoms include the sensation of instability without giving way, especially with stair climbing, recurrent knee effusions, and diffuse knee pain. Signs include anterior tibial translation at 90° flexion, tenderness at multiple sites (including pes anserinus, peripatellar, posterior hamstrings), and effusion. Revision surgery is indicated for symptomatic patients.
