The unstable total knee arthroplasty: causes and cures.

Surgery for the unstable total knee arthroplasty requires a deep understanding of the causes and a plan that specifically addresses them. Isolated ligament reconstructions and polyethylene insert exchanges generally do not work. Patients may experience "buckling" from pain, flexion contracture, recurvatum, or patellar problems. True mechanical instability may result from loosening, bone loss, prosthetic breakage, component size or position, fracture, wear, or collateral ligament failure. Only the last one typically requires a constrained implant. The possible modes (directions) of instability are the following: varus-valgus, recurvatum, flexion, and global. Revision surgery must eliminate deforming forces, most frequently frontal plane alignment. Prosthetic implants, no matter how well engineered, are not a substitute for diagnosis and surgical technique.