Joint Stability in Total Knee Arthroplasty: What Is the Target for a Stable Knee?

Instability remains a common cause of failure in total knee arthroplasty. Although approaches for surgical treatment of instability exist, the target for initial stability remains elusive, increasing the likelihood that failures will persist because adequate stability is not restored when performing the primary arthroplasty. Although the mechanisms that stabilize the knee joint-contact between the articular surfaces, ligamentous constraints, and muscle forces-are well-defined, their relative importance and the interplay among them throughout functions of daily living are poorly understood. The problem is exacerbated by the complex multiplanar motions that occur across the joint and the large variations in these motions across the population, suggesting that stability targets may need to be patient-specific.