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Abstract
Harmful immunological consequences from ultra-high molecular weight polyethylene (UHMWPE) wear debris in knee prostheses can lead to osteolysis, aseptic loosening of the prosthesis and revision surgery. Following revision surgery, polyethylene debris particles are commonly identified in numerous locations. In order to more clearly understand the process of particle generation and their associated pathways to ultimate locations of osteolysis in the surrounding joint space, UHMWPE was labelled with radioisotope tracers. In a broader sense, the ultimate goal is to work toward the minimisation of wear debris produced in the knee joint, thereby resulting in a longer prosthetic lifetime.
The wear mechanisms and their relevant debris generation were investigated in vitro using the characteristic gamma-rays emitted from radioisotopes attached to the polyethylene. Three separate methods of introduction have been explored in this work: direct ion implantation at low energy, recoil implantation, and diffusion. The direct ion implantation of 111In to a depth of 200 nm in UHMWPE has been successfully implemented, providing effectively a surface labelling of the polymer sample. The recoil implantation of 97Ru, 100Pd, 100Rh, and 101mRh into UHMWPE has been demonstrated. This introduction technique labels the polyethylene with an essentially uniform profile to a depth of 4.4 μm. The feasibility of both radioisotope labelling methods based on ion implantation was verified for two model systems.
The wear-in phase of the uni-directional sliding system has been found to exhibit a process of debris transfer between the metal and polymer surfaces. During this transfer process, the theoretical description developed alludes to a stochastic dropout of debris into the lubricant. Fluid dynamics calculations suggest that the cyclic fluid motion may periodically re-introduce these removed particles as 3rd-body wear, resulting in an exacerbated rate of wear. Computation Fluid Dynamic simulations have confirmed these cyclic debris pathways continually passing through the wear region. Results from the bi-directional model system suggest a decrease in the rate of debris transfer to the metal surface possibly due to the different motion, and an increase in dispersion of debris to the deionised water lubricant. A load dependence has been observed, and an increase in the surface roughness of the steel counterface corresponded with an increased wear rate and volume of debris transferred between the two bearing surfaces.
The diffusion of the antioxidant Vitamin E into UHMWPE was successfully reproduced, thus motivating the use of this substance as a potential radioisotope carrier. 111In radioisotopes were subsequently mixed with Vitamin E and diffused into UHMWPE. The resulting diffusion profile maximum depth attained is of the order of micrometres.
Localised tracing of wear was successfully demonstrated on UHMWPE tibial inserts. UHMWPE plugs were recoil-implanted, following synthesis of the 97Ru, 100Pd, 100Rh, and 101mRh radioisotopes. The plugs were inserted into the superior and inferior wear surfaces of tibial inserts and worn in a constant load actuator and a state-of-the-art ProSim knee simulator. For the constant load actuator, the results from the superior side plug revealed an increased propensity for debris dispersion into the lubricant, as opposed to a transfer to the femoral component. Using the ProSim knee motion simulator, the dependence of wear rate on position was studied for the backsides of tibial inserts and particle transport to different prosthesis locations was searched for. The extrapolated backside wear rate was 0.8 mm3/Mc, which compares well with the range of published values. Debris was not shown to transfer in significant amounts to the femoral component from the backside of the tibial insert, which provides evidence for osteolytic regions formed from locally produced debris. Hopefully the results of this work will aid toward the understanding of debris production and their subsequent pathways in the knee joint so that future prostheses can be designed to minimise the adverse biological response to these particles.