- Researchers used a robotic testing system to compare joint and articular contact kinematics as well as ligament isometry of intact cadaver knees, and the knees implanted with a recently designed single axis radius posterior-substituting knee system
- At increasing degrees of knee flexion, there was significant reduction in the contact area of the medial and lateral epicondyles on the polyethylene insert of the knee system
- The average posterior femoral translation was significantly less in implanted knees than in intact knees at 60°, 90° and 120° of flexion
The FDA approved a new-design posterior-substituting (PS) knee prosthesis designed to fully restore natural knee flexion. In this system a single radial axis in the femoral component is intended to provide ligament isometry during knee flexion. In addition, an extended posterior condylar design is meant to increase the contact area of the femoral component on the polyethylene insert during knee flexion, thereby increasing posterior femoral translation.
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A recent meta-analysis in Knee Surgery and Related Research demonstrated improved patient function with the new system after two to four years. However, an in vitro robotic study by Paul Arauz, PhD, postdoctoral research fellow, and Young-Min Kwon, MD, PhD, program director of the Adult Reconstructive Surgery Fellowship Program and director of the Bioengineering Laboratory in the Department of Orthopaedic Surgery at Massachusetts General Hospital, and colleagues suggests the system does not fully replicate the biomechanics of a healthy knee. The findings are published in The Journal of Knee Surgery.
The researchers used a robotic apparatus to apply external loads to 11 cadaveric knees under various loading conditions. They examined whether the new PS system can restore joint kinematics, articular contact and ligament isometry during knee flexion.
Kinematics of Intact vs. Reconstructed Knees
After measuring the kinematics of intact knees, the researchers reconstructed the knees using the new system. They found:
- The average varus tibial rotation at 60° of flexion was significantly lower in reconstructed knees than in intact knees
- The average posterior femoral translation was significantly less in reconstructed knees than in intact knees at 60°, 90° and 120° of flexion
Articular Contact Kinematics
To measure tibiofemoral articular contact in the reconstructed knees, the researchers placed a thin-film electronic pressure sensor on the articular surface and aligned its posterior edge with the posterior edge of the knee system's polyethylene insert. At increasing degrees of knee flexion, there was a significant reduction in the contact area of the medial and lateral epicondyles.
Lateral and Medial Collateral Ligament Forces
In reconstructed knees, the in situ force of the lateral collateral ligament at 60° of flexion was significantly lower than forces at 30°, 90° and 120°. In situ forces of the medial collateral ligament did not differ significantly across flexion angles.
Although the recently introduced single axis radius PS knee system has the potential to mimic the intact knee kinematics under muscle loading due to its design features, single axis radius PS knee system during in vitro robotic analysis did not fully replicate posterior femoral translation and ligament isometry of the healthy knee during knee flexion.
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