Abstract
Introduction:
Backside wear has been previously reported through in-vitro and in-vivo to have a significant contribution to the total wear in rotating bearing TKRs.
The present study investigated the contribution of backside wear to the total wear in the PFC Sigma rotating platform mobile bearing TKR. In addition, the wear results were compared to the computed wear rates of the PFC Sigma fixed bearing TKR, with two different bearing materials.
Materials and Methods:
The commercially available PFC Sigma rotating platform mobile bearing and PFC Sigma fixed bearing total knee replacements, size 3 (DePuy, UK) were tested, with either conventional or moderately cross-linked (5 MRad) GUR1020 UHMWPE bearing materials. The computational wear model for the knee implants was based on the contact area and an independent experimentally determined non-dimensional wear coefficient [1,2,3].
The experimental wear test for the mobile bearing was force controlled using the ISO anterior-posterior force (ISO14243-1-2009). However, due to time limitation of the explicit simulation required to run the force controlled model, the simulation was run using the AP displacements taken from the experimental knee simulator which was run under the ISO AP force. The Sigma fixed bearing TKR was run under high level of anterior-posterior displacements (maximum of 10 mm).
Results and Discussion:
The rotating platform bearing showed lower wear rates, compared to that of the PFC Sigma fixed bearing, for both conventional and moderately cross-linked UHMWPE bearing materials (Fig. 1). Moreover, the results showed a high contribution of backside wear to the total wear, approximately 1 mm3/million cycles (∼30% of the total wear). The computational wear predictions were in good agreements with the clinical and experimental measurements [4,5].
Contrasting the effect of bearing material on wear prediction, introducing the moderately cross-linked UHMWPE as a bearing material reduced the predicted wear rates by approximately 1 mm3/million cycles in rotating platform bearing, compared to more than 5 mm3/million cycles in PFC fixed bearing TKR. This reduced effect of cross-linking on wear in mobile bearing was mainly attributed to the lower cross-shear ratios in these bearings, compared to fixed bearings, and the less dependency of wear in moderately cross-linked UHMWPE on the degree of cross-shear, compared to conventional UHMWPE. Decreasing the degree of cross-shear from higher values (Sigma curved insert, high kinematic) to lower ones (rotating platform bearing) changed the predicted wear rates from 8.7 to 3.3 and from 3.4 to 2.4 (mm3/million cycles), for conventional and moderately cross-linked UHMWPE materials respectively (Fig. 2).
Conclusion:
The modelling confirmed the previous experimental observations of very low wear with the rotating platform knee. The models also determined the level of wear from the backside of the rotating platform knee which was approximately 1 mm3/million cycles. The fixed bearing knee with moderately cross linked polyethylene also showed low wear at approximately 3 mm3/million cycles. These low wear rates were determined under high kinematic walking cycles conditions. Future work will consider additional conditions.
