Overstuffing the Knee Decreases Flexion: But Does It Matter?A good fit between bone and implant is needed to maintain knee range of motion after total knee replacement (TKR). The shape of the bone and thickness of the edges called flanges of the implant are factors affecting motion, too.
Pain and problems after a TKR can be caused by poor alignment of the implant to bone. The wrong size femoral component can change the way the knee works and moves. Any mismatch between the bone and implant can alter the joint biomechanics.
In this study researchers used cadavers models to create a situation called overstuffing. Then they looked to see if overstuffing would decrease knee flexion. Overstuffing occurs when the shape and size of the femoral component of the implant is too large or the wrong shape (often too "boxy" or square).
They found that a small difference where the size of the femoral component (front to back) was only four millimeters larger than the real knee caused a four degree loss of passive knee flexion. This difference may not affect the live human. It's possible the joint capsule will stretch to make up the difference. Or the muscles around the knee may adjust over time to change the tension on the joint.
The authors say it makes sense to use the correct size implant and place it accurately in the joint even if it does not affect motion. Over time an imbalance in the slope or shape of the bone in more than one place can add up. The result may be uneven wear, pain with movement, and even patellar fracture.
William Mihalko, MD, PhD, et al. Patellofemoral Overstuff and Its Relationship to Flexion After Total Knee Arthroplasty. In Clinical Orthopaedics and Related Research. August 2006. No. 449. Pp. 283-287.
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