The success of total knee arthroplasty (TKA) depends on many factors, but malrotation of the prosthetic components, in particular, is a major cause of patellofemoral complications and can lead to revision surgery. Significant variability can be associated with femoral and tibial component rotational alignment, but how this variability translates into functional outcome remains unknown. The purpose of this thesis was to determine the biomechanical effects of variability in femoral and tibial component rotational alignment in TKA using a forward-dynamic computer model of an Oxford Rig, which simulates flexed-knee stance, such as occurs when riding a bicycle, rising from a chair, or climbing stairs.
To become familiar with the computer modeling environment, we performed a study of hip kinematics in OpenSim, an open-source software package that was developed for the purpose of creating and analyzing musculoskeletal models and dynamic simulations of movement. The simulations commonly use motion capture data as input, but frequently parameters such as the degrees of freedom (DOF) of certain joints is chosen by the user and may not match the same DOF used by the motion-capture software. OpenSim computes kinematics using a least squares approach to minimize the difference between experimental marker location and virtual markers on the model while maintaining joint constraints. We used a simple model, looking only at the motion of the hip, to investigate how marker weights and choice of model degrees of freedom affect kinematics and to compare the simulated kinematics with the same results from a common motion capture analysis technique. We found that high pelvis marker weights and a 6 degree-of-freedom hip model resulted in kinematics that most closely matched the results from motion capture, but large translations of the femoral head were present.
We then used a forward-dynamic model of an Oxford Rig to perform a parametric study on the effects of variations in component rotational alignment in TKA. The femoral component rotational alignment was varied from 15° internal rotation to 15° external rotation in 5° increments and the tibial component rotational alignment was varied from 20° internal rotation to 20° external rotation in 5° increments. The effects of component rotational alignment on knee kinematics, quadriceps muscle force, ligament forces, and contact forces were analyzed for the cruciate-retaining and posterior-substituting versions of the Scorpio implant from Stryker Orthopaedics. We found that femoral component alignment, in general, had a much greater effect on our variables of interest than tibial component alignment or choice of implant design. Internal rotation of the femoral component led to a reversal of the natural screw-home motion of the knee (internal rotation of the tibia with respect to the femur during flexion) as well as high MCL force, quadriceps muscle force, and contact forces between the femoral and tibial components.
Our findings suggest variability in component rotational alignment, especially internal femoral component alignment, may impact post-operative performance and further emphasize the need to accurately establish rotational alignment.