Flexion Instability After Total Knee Arthroplasty

Flexion instability after total knee arthroplasty (TKA) is caused by an increased flexion gap as compared to extension gap. Patients present with recurrent effusions, subjective instability (especially going downstairs), quadriceps weakness, and diffuse peri-retinacular pain. Manual testing for laxity in flexion is commonly done to confirm a diagnosis, although testing positions and laxity grades are inconsistent. Non-operative treatment includes quadriceps strengthening and bracing. The mainstays to operative management of femoral instability involve increasing the posterior condylar offset, decreasing tibial slope, raising the joint line in combination with a thicker polyethylene insert, and ensuring appropriate rotation of components. Patient outcomes after revision TKA for flexion instability show the least amount of improvement when compared to revisions for other TKA failure etiologies. Future work is needed to unify reproducible diagnostic criteria. Advancements in biomechanical analysis with motion detection, isokinetic quadriceps strength testing, and computational modeling are needed to advance the collective understanding of this underappreciated failure mechanism.