Managing instability in total knee arthroplasty with constrained and linked implants.

Instability is a relatively common cause of failure of total knee arthroplasty. In the management of the unstable total knee, the need for constraint must be anticipated. Careful preoperative planning including history and physical examination of the ligamentous support of the knee is mandatory. A thorough study of current and previous radiographs is important to determine wear, component migration, and bone loss. As a general rule, it is recommended that the minimum amount of constraint necessary to achieve stability should be used. Most of the time, stability can be achieved using a posterior-stabilized or a nonlinked implant, such as a varus-valgus constrained or constrained-condylar knee implant. Occasionally, a hinge prosthesis may be indicated, particularly in a patient with an absent and nonreconstructable medial collateral ligament. Increasing constraint is not without its problems, however, as forces across the knee may be transmitted to the stem-bone interface, resulting in radiographic loosening of stemmed components. Fortunately, these observations of radiographic loosening around stems of revision components have not so far correlated with reports of clinical failure.