Polyethylene wear is a significant factor limiting survivorship of total knee arthroplasty (TKR). Crosslinking of polyethylene has been shown to significantly reduce wear in hip arthroplasty but has not been reported for TKR. This study measured wear in polyethylene cross-linked to two levels in a knee wear simulator. Six polyethylene knee inserts were tested in a knee wear simulator. Inserts were manufactured from polyethylene crosslinked to two different levels: 2.5 Mrad (Low-X) and 10.5 Mrad (High-X). Each implant was enclosed in a closed lubricant (50% alpha fraction calf serum) recirculation chamber, maintained at 37°C and changed every 500,000 cycles. Physiologic levels of load and motion were applied at 1 Hz for a total of 6,000,000 cycles. Wear was measured by the gravimetric method before wear testing and at every 500,000 cycles. Semi-quantitative wear assessment was performed by imaging the insert surfaces at 10x magnification. The Low-X inserts demonstrated significantly higher wear rates (mean 4.66 mg/million cycles) than the High-X inserts (mean 1.55 mg/million cycles, p <
0.001). Wear scars on the Low-X inserts were irregular and visibly deeper than those on the High-X inserts. The machining marks on the surface of the insert were also better preserved in the High-X insert wear scars. These results suggest that crosslinked PE can significantly reduce wear in TKR under physiologic conditions. This can result in reduced lysis and increased survivorship. Localized damage can cause catastrophic failure in polyethylene knee inserts. Therefore, further studies are necessary to evaluate wear under these conditions.
Polyethylene contact stresses have been shown to correlate with wear in total hip arthroplasty (THA). Several liner designs have been introduced in an attempt to increase stability or reduce impingement and increase range of motion. This study analyzed the effect of liner design on range of motion (ROM) and PE contact stresses in a finite element model (FEM). FEMs of four liner designs were generated: Generic was modelled as a simple hemisphere, Chamfer had a wide chamfer on the inner edge of the liner to increase ROM, Highwall had an extended lip to increase stability, and Anteverted created a 20° anteversion with lat-eralisation of the centre of rotation. With the liners in varying positions of abduction and anteversion, physiologic loads were applied through the femoral head. Hip ROM was measured by rotating the head and neck in different directions until prosthetic impingement. Significant differences in ROM were seen relative to the Generic liner. Chamfer increased ROM by mean 16%. Highwall reduced ROM by mean 12%. Anteverted increased flexion by 17% but decreased extension, abduction, and external rotation. Contact stresses were also significantly affected by liner design and acetabular orientation. Overall for the same acetabular position, contact stresses were higher for Chamfer and lower for Highwall and Anteverted. These results underline the complex interaction between cup design, hip stability, range of motion and contact stresses. Design features that increase stability tend to reduce contact stresses and ROM, while those features that increase ROM, tend to increase contact stresses. This data can help the surgeon match liner design to specific patient requirements.
This study measured polyethylene wear and correlated it with design features such as tibiofemoral conformity and contact areas. Two femoral component designs were tested in a knee wear simulator. The femoral condyles of design A were flat-on-flat in the coronal plane, while those of design B were curved-on-curved. These femoral components were tested with two inserts. Insert PLI had a posterior lip, while insert C had a more curved sagital geometry, to improve stability in the anteroposterior direction. All components were tested for up to five million cycles in bovine serum lubricant. Triaxial forces were monitored to ensure that loading conditions were similar in all combinations tested. Gravimetric wear measurements were made at 500 000 cycle intervals. Contact stresses were measured using pressure sensitive film and dynamic finite element analysis. Contact stresses were 22% higher for inserts tested with design A compared to design B. Sliding distance, sliding velocity, and patterns of crossing motion were found to be comparable between the two femoral designs. Inserts tested with design A wore significantly more (mean 10.9 mg/million cycles) than design B (mean 5.71 mg/million cycles, p <
0.001). No appreciable differences were found between wear rates of insert PLI and insert C. Component design can have a significant impact on polyethylene wear rate. Careful control of kinematic and loading conditions allowed for comparison between specific design features. Increase in tibio-femoral contact area led to reduction of contact stresses, which was reflected in the reduced wear rate.