Stereo Radiography of TKA Patella Mechanics

Successful function and outcome following total knee arthroplasty (TKA) is partly determined by the restoration of patellofemoral (PF) function and recovery of the quadriceps mechanism of the knee. Patients with TKA often exhibit adaptations in their movement patterns during functional activity that have been attributed to quadriceps strength reduction and activation deficits that may be linked to the mechanics of the reconstructed knee. Changes in the articular geometry with patella resurfacing also affect patellofemoral kinematics, contact mechanics and loading on the patellar bone. While movement adaptations persist in patients with TKA long after knee repair, measurement of the 6 degree-of-freedom (DOF) kinematics of the patellofemoral joint are difficult with traditional motion capture methods. High-speed stereo radiography (HSSR), an advancement of biplane fluoroscopy, enables measurement of bone and implant motion during activity with sub-mm accuracy. The ability to measure and model the kinematics of the knee with this precision can improve understanding of implanted knee mechanics. What's more, acquiring HSSR in synchrony with whole-body measurements (body segment video motion capture, muscle electromyography (EMG), and ground forces) establishes connections between joint-level and whole-body function, and enables creation of subject-specific models of the knee and lower extremity. Integration with modeling is important because it reveals the biomechanical consequences of differences in patellofemoral kinematics.

Previous measurement and modeling comparisons of PF kinematics between dome and anatomic resurfacing designs have revealed differences in patellar sagittal plane flexion that have implications for quadriceps efficiency and patient satisfaction. Quadriceps efficiency is a measure of the effective moment arm of the quadriceps muscles; greater efficiency allows the quadriceps to produce knee extensor torque with less force. Patellar flexion can alter quadriceps efficiency by changing the moment arm of the patellar tendon and the way the patella transmits quadriceps force due to changes in patellofemoral contact. Patient satisfaction may be affected by changes in quadriceps efficiency through increased extensor mechanism strain and changes in load transfer that may be linked to anterior knee pain. While HSSR measurements can elucidate differences in dome and anatomic kinematics, computational modeling is required to determine how kinematic differences affect joint and muscle forces and overall limb functionality.

Aim 1: Comparison of patellofemoral kinematics between dome and anatomic implant designs using stereo radiography and correlation with patient function as measured by Knee injury and Osteoarthritis Outcome score, Knee Society score, and strength. Laboratory measurements will be collected from 22 TKA patients with medialized dome and 22 TKA patients with medialized anatomic patellae. Equal numbers of men and women will be recruited for the study. The angle of the patellar component relative to the femoral component will be measured and compared to reported values in healthy subjects. Correlations will be made with patient outcome scores.

Aim 2: Comparison of dome and anatomic moment arm, quadriceps efficiency, patellar contact mechanics and loading through the use of subject-specific simulation. Moment arm will be measured in millimeters as the distance from the patellar tendon to the knee joint center of rotation, and compared to values reported for the healthy knee. Quadriceps efficiency will be calculated as the ratio of quadriceps force to patellar tendon force. Patellar contact mechanics are defined as the force and location of force between the patellar and femoral components.