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
Moderately crosslinked, thermally treated ultrahigh molecular weight polyethylene (UHMWPE) has to date demonstrated a good balance of wear resistance and mechanical properties. MARATHON™ Polyethylene (DePuySynthes Joint Reconstruction, Warsaw, IN) is made from polyethylene resin GUR 1050, gamma-irradiated at a dose of 5.0 Mrads to create crosslinking of polyethylene, and followed by a remelting process to eliminate free radicals for oxidative stability. 10-year clinical study [1] and laboratory wear simulation tests [2–3] have reported excellent wear performance of the MARATHON poly. There continues to be demand for improved head-to-shell ratio acetabular systems because larger head sizes have the benefits of increased stability and range of motion. The increased head-to-shell ratio is often times achieved by using a reduced liner thickness. One of the clinical concerns of thinner poly liners is the potential for rim fracture, particularly in the occurrences of rim loading or impingement at high cup angles [4–7]. This study investigated the performance of thinner poly liners to the challenge of high angle rim loading and neck-to-liner impingement. Three groups of ETO sterilized MARATHON polyethylene liners (ID/OD: 28/44, 32/48, and 36/52 mm) were paired with matching CoCrMo heads (n=6 each group). To simulate rim loading, liners were assembled in the metal shells tilted at 64° (Figure 1) with sinusoidal loading (0 to 5000N at 3Hz) in a 37°C water bath for 5-million cycles or until component failure, whichever occurred first. For neck-liner impingement testing, metal shells were potted at 54º (in the abduction/adduction plane with a ±10° of motion per ISO 14242–1 [8]) on a hip simulator (n=4 each group) using a physiological loading (max 3000N at 1Hz) for 3-million cycles (Figure 2). The impingement occurred at 64º during the simulated gait cycle (Figure 3). The liners were inspected every million cycles, using a high intensity light to search for signs of crack initiation and/or fractures. Both test methods were validated to be able to replicate liner fractures.Introduction
Materials and Methods
Moderately crosslinked polyethylene maintains a balance of wear resistance and mechanical properties. The GVF poly was manufactured from GUR1020 UHMWPE bars, sealed in vacuumed foil package, and gamma sterilized at 4 Mrads. The MARATHON® polyethylene inserts were manufactured from GUR1050 UHMWPE bars, crosslinked by gamma irradiation at 5 Mrad, and followed by a remelting process that eliminates free radicals. The final sterilization method is gas plasma (GP) or ethylene oxide (EtO). Both methods will not introduce free radicals. Previous studies have shown MARATHON polyethylene (GP sterilized) with 83% lower wear than conventional polyethylene in a simulation test [1], compared to a 10-year clinical study that showed 77% wear reduction [2]. There is no study to compare the wear performance of MARATHON (EtO sterilized) and conventional poly. Four groups of polyethylene inserts (Table 1) were paired with matching femoral heads that were manufactured from CoCrMo (ASTM F1537) with diameters of 28, 32, and 36 mm. The inserts were chosen to have similar thickness at the dome for MARATHON, while for GVF it was the largest head size available. Wear testing was performed on an AMTI Hip Simulator per the ISO 14242-1 standard [3] at 1 Hz using the described inputs (Table 2), which provide a larger range of motion than the ISO standard. The cups were mounted in accordance with ISO 14242-1 using custom fixturing and secured with cement while the femoral heads were mounted on a vertical taper support. Testing was performed in 25% bovine calf serum at 37 ± 2°C. Wear of the inserts was determined gravimetrically. Finally, wear rates were calculated by linear regression and then compared between the groups using ANOVA analysis (α = 0.05).Introduction:
Materials and Methods:
