Abstract
Objective
Clinical wear depends on several factors such as implant specific factors (material, design, and sterilization), surgical factors/techniques, and patient-specific factors (weights and activities). The load magnitude for wear testing in the standard protocols (i.e., 2 kN as per ASTM F1714 or 3 kN as per ISO 14243-3) represent an average patient weight and does not address the other “what-if”’ scenarios (i.e., wear vs. patient weights, activities, duration, etc.,). The results from in-vitro testing report the data in wear (mg) or wear rate (mg/Mc) and are only applicable to the parameters (i.e., loads, bearing diameter, thickness, etc.,) used for the testing and not suitable to the variations seen in clinical scenarios. Therefore, it is essential to present the wear summary that can normalize the parameters and which is relevant in both in-vitro and in-vivo conditions. The goal of the current study is an attempt to present wear as a parameter (i.e., wear factor that combines the wear test data and established- theoretical relationship) and is thus applicable in both in-vivo and in-vitro scenarios.
Methods
Wear factor was first evaluated using actual wear testing conducted on metal on cross-linked polyethylene bearings along with well-established Dowson's wall bridge equation.
As per Dowson-Wallbridge, volumetric wear is V=2.376·KNWR+C or K=V/(2.376·NWR) where V is the volumetric wear in mm3, K is the wear factor in mm3/Nmm, N is the number of cycles, W is the load in Newtons, R is the bearing radius in mm, and C is the creep (assumed to be negligible, i.e., C=0 in this model.
28 mm simulator wear was first used to evaluate wear factor, but since simulator wear presented as a mass loss, these results were converted to volumetric wear using the equation
(m is the wear in mg and r is the density of XLPE in mg/mm3 (=0.923).
The Dowson-Wallbridge equation was then validated for predictive accuracy against actual wear testing on the predecessor THR system. The wear factor thus obtained was used to compute the theoretical-wear for other sizes (i.e., 42 and 46 mm bearings). The theoretical-wear was then compared to simulator wear for predictive accuracy.
Results & Discussion
Figure1 below shows the verification of the predictive capability of the Dowson-Wallbridge equation against historical wear data. The theoretical-wear (for 42 and 46 mm bearings) evaluated using wear factor was in good agreement with the simulator wear The results show Dowson's Wallbridge equation was verified and thus can be used to assess the wear factor. The results show that the wear factor for XLPE system is 1.79 × 10−10 mm3/N-mm. Elfick et al. evaluated the clinical wear factor for 47 retrieved acetabular components with varying diameters, patients, and liner thickness ranging from 1.8 mm (thinnest) to 11.0 mm thick liners using the Dowson-Wallbridge equation and reported the mean wear factor as 1.93 × 10−9 mm3/N-m. The results of the current evaluation are also in good agreement with clinical studies.
