Modular knee bearings typically consist of a femoral component, a tibial base-plate and a polyethylene insert, which is located in the tibial base-plate using some sort of locking mechanism. Although modular knee bearings offer many advantages there is the potential for micro-motion between the tibial insert and the base-plate.

Tests were performed on six large Kinemax Plus knee bearings (snap-fit design) to evaluate the amount of movement between the tibial inserts and the tibial base-plates. The knee bearings were tested up to one million cycles on the Durham Six-Station Knee Wear Simulator which subjected the bearings to similar motion and loading profiles that would be experienced by the natural knee during walking. The movement of the tibial inserts was measured with dial gauges (accuracy of ±0.01 mm) before and after the bearings were tested on the simulator, when unloaded, and throughout the tests whilst the bearings were being dynamically loaded in the simulator. Movement occurred between the tibial insert and the tibial base-plate after initial assembly due to the snap-fit mechanism used to locate the tibial insert within the tibial base-plate. However, this decreased appreciably when the bearings were loaded in the simulator. The amount of movement did not change with time when the bearings were continuously loaded in the simulator. However, after each test the amount of movement of the tibial inserts, when unloaded, was less than before the test. This was thought to be due to creep of the UHMWPE inserts. The movement between the tibial insert and tibial base-plate in-situ is likely to be much less than that observed by a surgeon at the time of assembly due to loading of the knee bearing in the body. However, the amount of movement when the tibial inserts are loaded may still be great enough to produce a second interface where wear of the tibial insert may take place.