Specimen-specific, three-dimensional knee joint mechanics: normal and reconstructed

Three-dimensional data was obtained from three cadaver specimens for both the normal, intact knee and specimens with a posterior cruciate ligament (PCL) retaining total knee replacement (TKR). The moment arm of the patellar tendon and the relative displacements of the tibia and patella were measured during leg-extension. The data obtained for the total knee replacement was compared to the normal, intact knee. Specimen-specific, three-dimensional mathematical models of the cadaver specimens were created for both the normal, intact knee and those with a total knee replacement to evaluate ligament and joint function for leg extension. The ligament origins and insertions and articular geometry were obtained from MRI and CT data obtained for each specific cadaver. Twelve elastic elements described the geometry and mechanical properties of the cruciate and collateral ligaments and the posterior capsule of each knee. The values of the ligament reference lengths and stiffness’ were adjusted to match in vitro anterior-posterior and rotational laxity data for the specimen-specific knees. Interpenetration of the femur and tibia was taken into account by modeling cartilage as a linearly-elastic homogeneous material mounted on rigid bone. The contacting surfaces of the patella and femur were assumed to be rigid. The models were used to simulate active knee extension, where a known force was applied to the quadriceps tendon. Quantitative comparisons of each model with the experimental data indicate that specimen-specific models reproduce the relative movements of the tibia and patella more accurately than average knee models due to the care taken in accurately representing the bone geometry and mechanical properties of the ligaments. The model calculations of the patellofemoral joint-reaction force suggest that PCL-retaining TKR’s are capable of reproducing the normal state of patellofemoral mechanics during knee extension. The calculations for the TKR models show that the articular geometry of the TKR components contributes to an increase in PCL force as compared with the intact models.