Abstract
Background
Stress shielding and wear induced aseptic loosening cause failure in arthroplasty surgery. To improve survivorship, the use of a low modulus, low wearing biomaterial may be a suitable alternative to hard bearing prostheses, such as cobalt chromium (CoCr). There has been considerable research interest in the use of polyetheretherketone (PEEK) based on observed clinical success especially in spinal surgery. This study investigated the wear performance of PEEK, carbon reinforced PEEK (CFR-PEEK) and acetal as bearing materials in an all polymer total knee arthroplasty (TKA) using a unidirectional pin on plate test.
Methods
The following material combinations were tested: PEEK vs. UHMWPE, CFR-PEEK vs. UHMWPE, PEEK vs. PEEK, CFR-PEEK vs. PEEK, CoCr vs. UHMWPE, PEEK vs. XLPE, CFR-PEEK vs. CFR-PEEK, PEEK vs. Acetal, Acetal vs. XLPE and CoCr vs. XLPE.Tribological couples tested (Pin vs. Plate) Using a previously validated modification of ASTM F732, 20mm diameter spherically ended pins with a radius of 25mm were articulated against 40mm diameter plates. A load of 1000N was applied to generate a contact stress of about 70MPa similar to contact stresses previously reported in the knee. The lubricant used was 25% newborn calf serum containing 0.3% sodium azide to retard bacteria growth and 20mM EDTA to prevent calcium deposition. Three repeats of pin on plate combinations (including 2 passive soak controls) were tested for 2 million cycles at a cycle frequency of 1Hz and a stroke length of 10 mm. Gravimetric wear was analysed every 250,000 cycles and results converted to volumetric wear using material density.
Results
All CFR-PEEK articulations were stopped due to excessive wear of the counter-surfaces. Results showed a linear wear rate of UHMWPE and XLPE plates over the test period. PEEK vs. XLPE showed similar wear rate to metal on polyethylene (MoP) bearings.
Conclusion
At stresses representative of the knee, PEEK pins when articulated against XLPE plates generated volumetric wear similar to that noted in MoP bearings.
