Retrieval investigations have shown that cracking or rim failure of polyethylene hip liners may occur at the superior aspect of the liner, in the area that engages the locking ring of the shell1. Failure could occur due to acetabular liner/stem impingement and/or improper cup position. Other contributing factors may include high body mass index, patient activity and design characteristics such as polyethylene material properties, thin liner rim geometry and cup rim design. Currently no standard multi-axis simulator methodology exists for high angle rim fatigue testing, although tests have been developed using static uniaxial load frames2. The purpose of this study was to develop a technique to create a clinically relevant rim crack/fracture event on a 4-axis hip simulator, and to understand the contribution of component design and loading and motion parameters. A method for creating rim fracture Introduction
Method
Multiple sources have consistently reported oxidation indices less than 0.1 with Marathon® inserts implanted up to 10 years. Understanding effects of oxidation level on UHMWPE wear GUR 1050 UHMWPE acetabular inserts, re-melted and cross-linked at 5.0Mrad (Marathon®, DePuy Synthes Joint Reconstruction, Warsaw, IN), were artificially aged per ASTM F-2003 in a stainless steel chamber at 5 atm. oxygen pressure and 70°C. Samples were maintained at temperature for 9, 10.4 and 11 weeks. After aging was completed, Fourier Transform Infra-Red (FTIR) spectroscopy was employed on one insert from each time point to evaluate the induced oxidation as a result of artificial aging. Resulting induced BOI values measured by FTIR were 0.195, 0.528 and 1.184. UHMWPE inserts had an inner diameter of 28mm and an outer diameter of 48mm and were articulated against 28mm diameter M-Spec® metal femoral heads (DePuy Synthes Joint Reconstruction, Warsaw, IN). Testing was conducted on a 12-station AMTI ADL hip simulator (AMTI, Watertown, MA) with load soak controls per ISO 14242-1:2014(E) in bovine serum (18mg/mL total protein concentration) supplemented with 0.056% sodium azide (preservative) and 5.56mM EDTA (calcium stabilizer). The UHMWPE inserts were removed from the machine, cleaned, and gravimetric wear determined per ISO 14242-2:2000(E) every 0.5 million cycles (MCyc) for 4.0 MCyc total. A two-tailed student's t-test was used (variance determined by F-test results) to analyze differences in wear rates between the three test groups.INTRODUCTION
METHODS
The wear rate of Ultra High Molecular Weight Polyethylene (UHMWPE) in joint replacements has been correlated to both contact area and contact stress in the literature, [1], [2]. In both publications and our experiment, UHMWPE articulated with a polished surface of cobalt-chromium alloy was evaluated using a Pin-On-Disk (POD) apparatus (AMTI) implementing bi-directional movement. In publication [1], volumetric wear was independent of normal load and dependent upon increasing contact area. The results demonstrated that increasing contact stress In our experiment, the contact area was dependent on textured POD Pin 390 (T390) which had low wear [3]. T390 reduced the normal POD contact area from 71 mm2 to 8.26 mm2. Hydroxylapatite (HA) particles were introduced to the serum to simulate third body wear debris. We hypothesized that the normal POD Pins would have greater wear rates than the textured POD Pins. A measurement of 0.14 mg HA particles per 250 mL of serum was used for each test 0.33 million cycles. The GUR 1020 resin XLK POD Pins were gamma irradiated to 50 kGy in a vacuum package and then remelted. Three (3) T390 POD pins and nine (9) untextured XLK POD Pins were used. Three untextured XLK POD Pins were tested against three T390 POD pins. The other six (6) untextured XLK POD Pins were used as soak controls. Each pin articulated against a polished, high carbon wrought CoCr metal alloy counterface (ASTM F1537; diameter = 38.1 mm; thickness = 12.7 mm). Wear rate tests were for 1.98 million cycles. In order to perform the t-test analysis, the wear rates for each pin were given by the slope of the linear regression line through the individual data points (cycle count, cumulative wear), excluding the (0, 0) point.Statement of Purpose:
Methods:
