vaswyl whatleah tagurt
Lane Frisch
ISO 14242-1: Implants for surgery - wear of total hip prostheses - Part 1: Loading and displacement parameters for wear-testing machines and corresponding environmental conditions for tests
ISO 14242-2: Implants for surgery - wear of total hip prostheses Part 2: Methods of measurement
In total hip arthroplasty (THA), accelerated wear is observed when third-body particles migrate into the bearing surface. Third-body particles can be bone cement, bone chips, debris generated at modular connections or fragments from previous implant failures. All specimens are tested according to ISO 14242-1 using the EndoLab hip joint simulator.
Particle concentration, size and material (PMMA, ceramic, bone chip) is defined in cooperation with the customer and shall correspond to worst case conditions. To maintain the third-body particle suspension in the test fluid, each test chamber can be circulated by individual peristaltic pumps.
As an alternative, roughening of the components to a predefined Ra-value can be performed. This method allows for more consistent wear determination as third body particles do not adhere to the surfaces.
Purpose: Larger glenosphere diameters have been shown to increase prosthesis stability and impingement-free range of motion in reverse total shoulder arthroplasty (rTSA). Higher wear rates increase the potential for polyethylene wear-induced osteolysis and aseptic component loosening. The goal of this study is to evaluate the rate of polyethylene wear for 32 mm and 40 mm glenosphere sizes utilized in rTSA. It is hypothesized that increased polyethylene wear occurs with larger glenosphere diameters.
Methods: Twelve commercially available CoCrMo glenospheres (size 32 mm and 40 mm, n = 6/group) and their respective humeral liners were subjected to 5 million cycles (MC) of a previously established wear simulation protocol, representing 5-7 years of device life. Two motion profiles, abduction-addiction (20-618N) and flexionextension (20-927N), were alternated every 250,000 cycles. Individual stations were enclosed in bowls filled with bovine calf serum as lubrication. Every 250,000 cycles, liners were removed and mass loss was determined gravimetrically according to ISO Standard 14242-2 and converted to volume loss and volumetric wear rate (VWR). At 0, 2.5, and 5 MC, liners were imaged using micro-computer tomography (CT) to determine linear surface deviation of the bearing surface due to wear. Liner volumes were isolated from CT scans, volumes were co-registered, and surface deviations between time points were mapped across the entire bearing surface and by quadrant. White light interferometry was also performed at five locations of the humeral liners at 0, 2.5, and 5 MC to determine changes in micro-scale surface roughness. Wear particles were isolated from test serum from both motion profiles at the beginning and conclusion of testing and imaged via scanning electron microscopy (SEM) for characterization of wear particle morphology
Conclusion: Larger glenospheres result in significantly greater polyethylene volume loss and volumetric wear rates, while smaller glenospheres lead to greater polyethylene surface deviations. These results are consistent with the previously established relationships between femoral head size and volumetric and linear wear rates in total hip arthroplasty. The enhanced stability provided by larger glenospheres must be weighed against the potential for increased polyethylene wear and wear-induced osteolysis to reduce complication rates in rTSA.
The high molecular weight polyethylene remains the gold standard for a number of prostheses and indications. Its controlled wear resistance and its lack of complication under reasonable conditions of use, insure a high rate of satisfaction and survival up to 15 years after implantation. For these reasons, the surgeons in accordance with the health institutions, recommend its systematic use for all patients of more than 70 years candidates to a joint replacement of the hip, the knee and the shoulder.
The shelf-aged polyethylene PEXEL demonstrated a wear reduction of more than 50% in comparison wit a conventional polyethylene (CERAH test according to ISO standards 14242-1 and 14242-2 on shelf aged implants at 5 millions cycles).
Conventional UHMWPE (Ultra High Molecular Weight Polyethylene) is a very tough material, with the highest impact strength of any thermoplastic presently available. It is highly resistant to corrosive chemicals with exception of oxidizing acids; has extremely low moisture absorption and a very low coefficient of friction; is self-lubricating; and is highly resistant to abrasion.
Since the early 2000s, crosslinked variants of the conventionall UHMWPE have been used and validated both in vitro and in vivo. The crosslinking process is generally made by irradiating the material (or by exposing it to electrons) at a level in-between 25 and 100 kGy. The crosslinking modifies deeply the mechanical characteristics of the material and provides an increase in hardness and wear resistance. But this improvement is counterbalanced by a reduction of the tensile strength compared with the natural state of the material and a reduction of the elongation percentage under load.
The development of an othopaedic joint polyethylene is a compromise between the wear resistance and the mechanical characteristics. Also, other manufacturing steps will come over the crosslinking process to maintain the characteristics of the material over time and to limit the oxidation of the material.