Laxity and function in knee replacements. A comparative study of three prosthetic designs.

This study was designed to determine the influence of laxity on clinical function after knee replacement. Using a recently developed computerized method, anteroposterior laxities were measured in 29 Insall-Burstein posterior stabilized, 25 Kinemax Condylar (posterior cruciate retaining), and 10 Oxford bicompartmental (anterior and posterior cruciate retaining) knee replacements. Laxities of less than 5 min, irrespective of implant design, was associated with an impaired range of passive motion and an increased likelihood of incurring a flexion deformity in excess of 4 degrees. Clinical function, as reflected by the American Knee Society Clinical Rating system, demonstrated a particular sensitivity in the Oxford knee to anteroposterior laxity. It was concluded that anteroposterior laxity in excess of 5 mm in prosthetic knees is desirable for unimpaired joint function, although an upper limit of acceptable anteroposterior laxity could not be identified. These findings, which emphasize the need for attention to ligamentous tensions at the time of surgery, also indicate the need for appropriate weighting of anteroposterior laxity in the continuing evolution of clinical rating systems.