Abstract
Introduction
First-generation annealed HXLPE has been clinically successful at reducing both clinical wear rates and the incidence of osteolysis in total hip arthroplasty. However, studies have observed oxidative and mechanical degradation occurring in annealed HXLPE. Thus, it is unclear whether the favorable clinical performance of 1st generation HXLPE is due to the preservation of bearing surface tribological properties or, at least partially, to the reduction in patient activity. The purpose of this study was to evaluate the in vitro wear performance (assessed using multidirectional pin-on-disk (POD) testing) of 1st-generation annealed HXLPE with respect to in vivo duration, clinical wear rates, oxidation, and mechanical properties.
Materials and Methods
103 1st-generation annealed HXLPE liners were collected at revision surgery. 39 annealed HXLPE liners were selected based on their implantation time and assigned to three equally sized cohorts (n=13 per group); short-term (1.4–2.7y), intermediate term (5.2–8.0y) and long-term (8.3–12.5y). From each retrieved liner, two 9-mm cores were obtained (one from the superior region and one from the inferior region). Sixteen cores were fabricated from unimplanted HXLPE liners that were removed from their packaging and six pins from unirradiated GUR 1050 resin served as positive controls. Multidirectional POD wear testing was conducted against wrought CoCr disks in a physiologically relevant lubricant (20 g/L protein concentration) using a 100-station SuperPOD (Phoenix Tribology, UK). Each pin had its own chamber with 15mL lubricant maintained at 37±1°C. An elliptical wear pattern with a static contact stress of 2.0 MPa was employed. Testing was carried out to 1.75 million cycles at 1.0 Hz and wear was assessed gravimetrically. POD wear rates were calculated using a linear regression of volumetric losses. In vivo penetration was measured directly using a calibrated micrometer. Oxidation was assessed on thin films obtained from superior and inferior regions of the liners (ASTM 2102). Mechanical properties were assessed using the small punch test (ASTM 2183).
Results
In vitro wear rates from the SuperPOD were positively correlated with implantation time (Rho=0.27; p=0.015) and average oxidation (Rho=0.36; p=0.004) at the bearing surface of the retrieved HXLPE liners. All retrieved HXLPE cohorts had lower in vitro wear rates than uncrosslinked positive control (p≤0.03) and higher wear rates than the never-implanted HXLPE cohort (p <0.001). POD wear rates were negatively correlated with small punch ultimate load (p<0.01). However, the in vitro wear rates were not correlated with clinical penetration rates (p=0.71).
Discussion
This study investigated the effects of in vivo degradation on 1st-generation annealed HXLPE liners. The data in this study suggest that the tribological properties degrade due to in vivo oxidation as the liner is exposed to the in vivo environment. The clinical implications of these findings, however, are not clear as the clinical penetration rates were not correlated with the in vitro POD wear rates. This may be partially due, to decreasing patient activity as they age. These findings will be useful for comparison for evaluating the in vitro wear properties of other HXLPEs, including 2nd generation HXLPE.
Acknowledgements
This study was supported by the NIH(NIAMS) R01AR47904.
