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Effects of component placement on patellar kinematics and loading in intraoperative and postoperative loading configurations Brimacombe, Jill Maureen


Postoperative patellofemoral complications and anterior knee pain contribute to suboptimal clinical outcomes following total knee replacement (TKR). Pain and complications are thought to be related to patellar tracking and loading, and the placement of the TKR implants can influence these biomechanical parameters. Surgeons aim to improve postoperative performance of the knee by optimizing intraoperative implant placement; however, they can only judge patellofemoral mechanics intraoperatively by observing passive, unloaded knee flexion. The main purpose of our study was to compare the patellar kinematics (tilt, shift and spin) and peak patellofemoral contact forces for cadaveric specimens in testing configurations designed to simulate both passive intraoperative and loaded postoperative flexion. The goal was to rank the effects of varying the placement of the TKR implants on kinematics and forces in both testing configurations. Zimmer NexGen posterior-stabilized implants were implanted in 8 fresh-frozen adult anatomic specimen knees. The knees were cycled dynamically through flexion and extension in rigs designed to simulate intraoperative and postoperative flexion. The femoral, tibial and patellar implants were modified to allow for the following changes in placement: 1) external and internal rotation of the femoral implant 2) external and internal rotation of the tibial implant 3) 3 patellar resection angles 4) 2 mediolateral patellar positions 5) additional patellar thickness. Changes in patellar tilt and shift were moderately- to well-correlated between the intraoperative and postoperative simulations (r = 0.70 to 0.85 for tilt, r = 0.54 to 0.77 for shift). The correlation indicates that changes made intraoperatively to optimize tracking will likely result in similar changes in the postoperative knee. Spin was not correlated between simulations. No changes in peak contact force were statistically significant in either testing rig and peak contact forces were not well-correlated between simulations. It is hoped that the results and protocol of this study will be used to guide a future clinical study comparing optimal intraoperative patellar tracking and loading to optimal postoperative mechanics. We also intend to use the results of this study to assist in the development of a computer assisted surgical technique for resurfacing the patella.

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