Abstract
Answering the question of what the patient can teach us about the future of joint replacement starts with a look to the past. The modern era of total joint replacement began in the late 1950's with the pioneering work of John Charnley that established the fundamental structure of a total joint replacement with a metal component bearing against polyethylene and provided many disabled patients with a substantial improvement in function. As the application of joint replacement expanded to a broader patient population it became apparent that a better understanding of the mechanics of patient function was needed to provide more rigorous design criteria and objective assessment of design changes. This presentation will examine how improvements in total knee replacement has been aided by objective measures of ambulatory function and the potential for future improvements in joint replacement that can be based on information from testing patients.
Specifically, from a historical viewpoint one of the major problems limiting the use of total knee replacement in the 1970's was tibial component loosening. The problem of tibial component loosening could be related to the load imbalance between the medial and lateral surface of the tibia. The load asymmetry at the knee resulting from the adduction moment during gait provided a strong rationale for maintaining proper limb alignment following total knee arthroplasty. The analysis clearly showed that knees with a varus alignment of the mechanical axis were more likely to have a substantial load imbalance creating the type of stresses that would eventually lead to tibial component loosening. When the information from gait studies was combined with both clinical and biomechanical studies, tibial component designs were modified using metal backing of the polyethylene articulating surface and instrumentation was modified to allow for proper alignment of the mechanical axis and avoid residual varus deformity following total knee replacement.
Similarly, knee kinematics and moments have been used to differentiate the functional characteristics of different types of designs during stair climbing. Patients with cruciate-sacrificing knee replacements had a tendency to reduce the moment sustained by the quadriceps by leaning forward during the portion of the support phase of ascending stairs when the quadriceps moment would reach a peak value, while patients with a posterior cruciate retaining design were able to sustain normal quadriceps function. The functional differences between the PCL-retaining and sacrificing designs were associated with the normal posterior movement of the femur on the tibia (rollback), with flexion. This finding indicated that TKR design must permit rollback in the early phases of knee flexion to sustain normal stair climbing.
This presentation will conclude with a review of the functional performance of patients with an anterior cruciate deficient knee as a basis for addressing the future needs of a knee replacement to permit natural knee movement. Specifically the role of the anterior cruciate ligament will be discussed in the context of the interaction of the curvature of the articulating surfaces in maintaining a functional envelope of movement that is consistent with retaining both cruciate ligaments.
