Constraint in primary total knee arthroplasty.

Instability is an important cause of failure following total knee arthroplasty. Increasing component constraint may reduce instability, but doing so also can cause increased forces to be transmitted to fixation and implant interfaces, which may lead to premature aseptic loosening. Constraint is defined as the effect of the elements of knee implant design that provides the stability needed to counteract forces about the knee after arthroplasty in the presence of a deficient soft-tissue envelope. Determining the amount of constraint necessary can be challenging. Most primary total knee arthroplasties are performed for knees without substantial deformity or the need for difficult ligament balancing; in these cases, either a posterior-stabilized or a posterior cruciate-retaining design is appropriate. In certain situations, such as patients with prior patellectomies, rheumatoid arthritis, or substantial preoperative deformities, a posterior-stabilized knee may be favored. With their large posts, varus-valgus constrained implants typically are reserved for patients with substantial coronal plane instability, which is difficult to balance with a posterior-stabilized or cruciate-retaining implant alone. Rotating-hinge knee implants usually are recommended for patients with severe deformity or instability that cannot be managed with a varus-valgus implant.