Abstract
Introduction
Stress shielding and wear induced aseptic loosening cause failure in total joint arthroplasty. To improve long-term outcomes in total knee arthroplasty (TKA), the use of a low modulus, low wearing biomaterial may be a suitable alternative to cobalt chromium (CoCr) femoral components. Based on its favorable mechanical properties and observed clinical success especially in spinal surgery, polyetheretherketone (PEEK) is investigated as a candidate material for a metal free TKA. An all polymer TKA has several theoretical advantages, these include a more physiological stress in the distal femur, elimination of biological reaction to metal, better radiographic visualisation of the bone implant interface especially with CT and MRI. In addition, polymers afford a cheaper option for the manufacture of prostheses.
Aims and Hypothesis
This study investigated the wear performance of PEEK and carbon reinforced PEEK (CFR-PEEK) as bearing materials in an all polymer TKA using a unidirectional pin on plate test. Our hypothesis was that reduced wear is generated from PEEK or CFR-PEEK bearings when compared with metal on polyethylene (MoP) bearings and that this combination may provide a suitable alternative in TKA.
Methods
A validated modification of ASTM F7321 was used as test protocol. Twenty millimeter diameter spherically ended pins with a radius of 25mm were articulated against 40mm diameter plates. A load of 1000N was applied to generate an initial contact stress of ∼70MPa similar to high contact stresses previously reported in non congruent knee designs2. Ten material combinations were tested as shown in Table 1.
Table 1: Tribological couples tested (Pin vs. Plate) UHMWPE – ultrahigh molecular weight polyethylene, XLPE – highly cross-linked polyethylene)
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 cycle length of 20 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 rates to metal on polyethylene (MoP)bearings (Fig 1).
Conclusion
At high stresses representative of non conforming knee designs, PEEK pins articulated against XLPE plates generated volumetric wear similar to that noted in MoP bearings. From these results, it may be possible to replace CoCr in TKR with PEEK which may be beneficial because of the low elastic modulus and elimination of biological activity to metal alloy.
