Introduction: Reduction of ultra high molecular weight polyethylene (UHMWPE) surface wear in total knee replacement (TKR) bearings may delay the onset of osteolysis and subsequent loosening of components. The aim of this study was to compare the effect of bearing material on UHMWPE wear using a physiological knee simulator.

Methods: LCS Rotating Platform (RP) mobile bearing TKRs (DePuy) were investigated with standard and custom insert materials (Table 1). Testing was completed on a six-station force/displacement controlled knee simulator (frequency 1 Hz). Kinematic inputs consisted of 0 – 58° extension-flexion [1], maximum 2600 N axial force [1], -262 to 110 N anterior-posterior force [1] and ± 5° internal-external rotation [2]. The test lubricant was 25% (v/v) bovine serum with 0.1% (m/v) sodium azide solution in sterile water. Six components of each material were tested for up to five million cycles. The mean wear rates of the inserts were determined gravimetrically after every million cycles.

Results and Discussion: The higher molecular weight 1050 GP exhibited a higher wear rate than 1020 NI but the difference was not statistically significant (p > 0.05) (Fig. 1). The medium level of crosslinking in the Marathon GP inserts significantly reduced wear in comparison to the uncrosslinked 1050 GP material (p < 0.05) and moderate crosslinking in the 1020 GVF also decreased wear compared with the 1020 NI base material although this was not statistically significant. However, these differences would not be considered to be clinically significant. In addition, further work should be completed to assess the biological activity of the crosslinked materials as increased biological response may negate the benefit of decreased volumetric wear. All RP materials exhibited significantly reduced wear rates (p < 0.05) in comparison to fixed bearing TKR tested under equivalent high kinematic conditions [3]. The RP translates complex motions into more unidirectional motions, benefiting from reduced wear due to decreased cross-shear on the UHMWPE compared with more multidirectional fixed bearing TKR. Therefore, TKR design is an important factor for reduction of UHMWPE wear.

Introduction: Reduction of ultra high molecular weight polyethylene (UHMWPE) wear in total knee replacement (TKR) bearings may delay the onset of osteolysis and subsequent loosening of components. This study used finite element (FE) modelling and in vitro simulator testing to investigate the effect of wear path geometry on UHMWPE surface wear.

Methods: The wear of PFC Sigma fixed bearing TKRs (DePuy) was investigated using a six-station force/ displacement controlled knee simulator (frequency 1 Hz) using previously developed methods [1]. High, intermediate and low kinematic inputs were simulated for up to five million cycles (Table 1) with identical flexion-extension and axial loading for all components. This kinematic data was also applied to a FE model of the PFC Sigma TKR and run using PAM-CRASH-SAFE software. The anterior-posterior (AP), medial-lateral (ML) and inferior-superior data were recorded and the resulting wear paths generated by selecting nodes from the contacting surface of the polyethylene relative to the femoral.

Results and Discussion: The mean wear rates with 95% confidence limits on the simulator when subjected to high, intermediate and low kinematics were 22.75 ± 5.95, 9.85 ± 3.7 and 5.2 ± 3.77 mm3 per million cycles, respectively. All FE models exhibited looped wear paths. An example wear path for the first 60% of the gait cycle for a lateral node is displayed in Figure I. The high kinematics model generated the greatest ML displacement and similar AP displacement to the intermediate kinematics model. The low kinematics model showed least ML and AP displacements. The AP displacements for medial wear paths differed little when subjected to the different kinematics. A looped wear path on the surface of UHMWPE results in greater cross shear transverse to the principal direction of motion, which is parallel to AP displacement in TKR and is the axis along which strain hardening occurs. This study revealed that increased AP displacement and tibial rotation kinematics generate more looped wear paths, increase ML and AP displacements on the surface of fixed bearing TKR and result in greater cross shear which ultimately increases UHMWPE surface wear.

Reduction of ultra high molecular weight polyethylene (UHMWPE) surface wear in total knee replacements (TKR) may delay the onset of osteolysis and loosening of components. This study examined the wear of fixed bearing and rotating platform (RP) mobile bearing TKR with two different bearing materials.

Testing was completed on a Leeds ProSim six-station knee simulator under ‘high’ kinematics [1]. PFC Sigma fixed bearing and LCS RP mobile bearing knee designs were tested (DePuy). Non-crosslinked (non-irradiated (NI) or gas plasma (GP) sterilised) and moderately cross-linked (4.0 MRad gamma irradiation sterilisation under vacuum (GVF)) GUR1020 UHMWPE bearings were investigated for each TKR design. Components were tested in 25 % bovine serum solution for up to five million cycles (frequency = 1 Hz). Volumetric wear was determined from gravimetric measurements of the inserts.

The 1020 GVF fixed bearings exhibited a wear rate of 16.4 ± 4 mm3 per million cycles (MC). This was significantly greater (p < 0.05) than wear of the same bearing material in the rotating platform mobile bearing TKR (10.85 ± 2.39 mm3/MC). Similarly, when uncross-linked 1020 UHMWPE was introduced as the bearing material, a significant (p < 0.05) reduction in wear was observed between the fixed bearing (16 ± 7 mm3/MC) and the RP knee designs (5.85 ± 2.05 mm3/MC).

The RP design decouples the motions between the femoral-insert and tray-insert articulating surfaces. This translates complex knee motions into more unidirectional motions at two interfaces, thus reducing wear under high kinematics compared with fixed bearing TKR. This significant reduction in wear was observed with uncross-linked and moderately cross-linked bearing materials. Design of TKR is an important factor that influences UHMWPE surface wear and may affect long-term success of knee replacements in highly active patients.