Application
Total knee replacements generally consist of three major parts: a tibial component, which is connected to the shinbone, a femoral component, which is connected to the thighbone, and a meniscal bearing component, which is located between the other two components and allows them to slide over each other. The tibial and femoral components are usually made of metal or a metal alloy while the bearing component is made of a synthetic plastic, such as polyethylene. Early designs of prosthetic knees fixed the bearing component to the tibial component, but newer designs allow the bearing component to float to some extent, allowing increased freedom of movement within the knee. However, all knee prostheses incur some risk of dislocation or spinout, especially if the ligaments fail to provide adequate support. Ideally, a prosthetic knee should allow for some flexion and tension of the knee joint while lowering the risk of dislocation and bearing spinout.
Solution Design
An inventor at Howmedica Osteonics Corporation in Mahwah, NJ, was awarded a patent for his design of a limited motion tibial implant.1 The design calls for the addition of an elastic gasket between the tibial component and the bearing component of a prosthetic knee. Master Bond EP42HT-2Med is cited in the patent as a suitable adhesive for mounting the gasket on a plate. The gasket-plate combination is designed to fit (without affixing) into an undercut rim on the top surface, or baseplate, of the tibial component. A lip around the top perimeter of the gasket is designed to couple with a groove in the bearing component.
Results
When the three parts are sandwiched together, the gasket fills the space between the inner wall of the tibial component and the outer wall of the bearing component. The bearing component is free to move along the tibial baseplate, and as it moves, it compresses the gasket. The spring constant and thickness of the gasket restrict the movement of the bearing component along the tibial baseplate. Thus, the design provides for limited motion with enhanced stability over designs with floating bearing components.