A pin-on-disc tribometer test with a rotating disc and a sector-wise loaded pin was used to determine friction coefficients for different material pairings. The four pin materials porcine cartilage, subchondral bone of the porcine cartilage, UHMWPE, vitamin E enhanced, crosslinked UHMWPE (VEPE) in combination with the three-disc materials zirconia toughened alumina ceramic (ZTA), CoCr, carbon-fibre-reinforced carbon (CrC) were tested. Stepwise loading was employed with the forces 10 N, 5 N, 2 N and 1 N. Test duration was 1 h. Diluted calf serum according ISO 14242-1 was used to determine the friction coefficients. The surface topography of all pins was examined using optical profilometry before and after the rotation tribometer tests. - No wear related modifications of the surface roughness parameters could be found. The coefficients of friction (COF) were lowest for the cartilage pins against all three-disc materials, with steady-state values between 0.01 and 0.02 for the highest applied load (10 N). Friction of subchondral bone yielded COF in the range 0.2 … 0.6, depending on the counterpart material. The two polyethylene materials behaved similar in this friction test with COF of about 0.1. The Ra roughness values of the different pins reflect the COF results: Ra of subchondral bone was one order of magnitude higher than Ra of the cartilage. This is in-line with the COF-values of bone being one order of magnitude higher than those of cartilage. These results will be discussed in view of the use of the disc materials as orthopaedic hemi-prostheses.
When reversing the hard-soft articulation in inverse shoulder replacement, i.e. hard inlay and soft glenosphere, the tribological behaviour of such a pairing has to be tested thoroughly. Therefore, two hard materials for the inlay, CoCr alloy and alumina toughened zirconia ceramic (ATZ) articulating on two soft materials, conventional UHMWPE and vitamin E stabilised, highly cross-linked PE (E-XLPE) were tested. The simulator tests were performed analogue to standardised gravimetric wear tests for hip prosthesis (ISO 14242-1) with load and motion curves adapted to the shoulder. The test parameters differing from the standard were the maximum force (1.0 kN) and the range of motion. A servo-hydraulic six station joint simulator (EndoLab, Rosenheim) was used to run the tests up to 5 times 106 cycles with diluted calf serum at 37° C as lubricant.Background
Methods
Simply stated, carbon reinforced carbon (C/C) may be considered as fibre reinforced pyrocarbon. Pyrocarbon is used e.g. in finger joints and artificial heart valves. Aim of the present study was to evaluate if C/C could broaden the field of orthopaedic applications compared to pyrocarbon. Technically, C/C is used e.g. for brakes of F-1 race cars. The mechanical strength of the C/C material was characterised by a biaxial flexural bending test according ISO 6474-1. Three C/C shoulder heads articulating against vitamin E stabilised, highly cross-linked UHMWPE (E-XLPE) underwent a shoulder simulator study up to 106 cycles. The Coefficient of Friction (CoF) of C/C disks (Ra: 0.045 μm) against cartilage was analysed by a reciprocal cartilage wear tester. The test was conducted in cell culture medium for 4 h and 12 h using bovine cartilage. All test data is compared to the corresponding test results with Al2O3 ceramic.Background
Methods
When reversing the hard-soft articulation in inverse shoulder replacement, i.e. hard inlay and soft glenosphere (cf. Figure 1), the tribological behaviour of such a pairing has to be tested thoroughly. Therefore, two hard materials for the inlay, CoCr alloy and alumina toughened zirconia ceramic (ceramys®) articulating on two soft materials, conventional UHMWPE and vitamin E stabilised, highly cross-linked PE (vitamys®) were tested in a joint simulator. The simulator tests were performed at Endolab GmbH, Rosenheim, Germany, analogue to standardised gravimetric wear tests for hip prosthesis (ISO 14242-1) with load and motion curves adapted to the shoulder. The test parameters differing from the standard were the maximum force (1.0 kN) and the range of motion. A servo-hydraulic six station joint simulator (EndoLab) was used to run the tests up to 5*106 cycles with diluted calf serum at 37° C as lubricant. Visual inspection and mass measurements were done at 0.1, 0.5, 1, 2, 3, 4 and 5 million cycles using a high precision scale and a stereo microscope, respectivly.Background
Methods
Malpositioning still occurs in total hip arthroplasty (THA). As a result of mal-orientation, THA bearing can be subjected to edge loading. The main objective of the study was to assess if the wear rate of ceramic-on-ceramic and metal-on-polyethylene increases under edge loading conditions and to determine which of the most commonly used hip bearings is the most forgiving to implant mal-orientation. Two different polyethylenes (UHMWPE and vitamin E blended HXLPE) and ceramics (pure aluminum PAL and alumina-toughened zirconia ATZ) were tested with a hip simulator and compared to metal-on-metal results. The inclination angle was selected at 45°, 65° and 80°. In addition, the ceramic-on-ceramic barings were tested at conditions that produced microseparation. Contrary to metal-on-metal that is highly susceptible to edge loading, the wear rate of ceramic-on-ceramic and metal-on-polyethylene articulations does not increase with increasing cup inclination. In fact, the polyethylenes showed a contra-intuitive behaviour as its wear rate decreased slightly but significantly with increasing inclination angle. This behaviour can be explained when looking closely at the contact stresses and areas. (Figure 1 shows the wear area of the vitamin E blended HXLPE at 45° and figure 2 at 80° cup inclination). The newest biomaterials, vitamin E blended HXLPE and ATZ, showed markedly lower wear rates compared to their conventional counterparts, UHMWPE and PAL. The ATZ ceramic-on-ceramic articulation showed the lowest wear rate (even when microseparation is included) of all tested pairings, but the new vitamin-doped HXLPE seems to be the most forgiving materials when it comes to implant mal-orientation. It shows low wear rate even at an extremely high cup inclination angle. Therefore, a surgeon that discovers a mal-positioned polyethylene cup at the first post-op X-ray will not need to worry unduly about increased wear (but “only” about a potential dislocation).Materials and methods
Results
In the present hip simulator studies, bearings with the newest generation of HXLPE, stabilised with vitamin E, did not show increased wear under severe conditions, such as accelerated ageing, component mal-orientation and third body wear. Unfortunately, acetabular hip components cannot always be implanted in optimal condition. Therefore, we performed hip simulator studies with cups made from highly cross-linked, vitamin E stabilised UHMWPE in Summary Statement
Introduction
In the most recent type of highly cross-linked UHMWPE, stabilised by vitamin E, the majority of this anti-oxidant cannot be leached out. Even more, the vitamin E molecules are grafted to the UHMWPE polymer backbone by an ether bond. Today, highly cross-linked, vitamin E stabilised UHMWPE is clinically accepted as bearing material in joint replacements. Little is known about the chemistry of this antioxidant in the polymer after irradiation. The present investigation presents a model for the chemical nature of the trapping of vitamin E in PE.Summary Statement
Introduction
Edge loading in acetabular hip implants is generally due to mal-orientation or low tissue tension. It is known that edge loading of metal-on-metal THA may lead to higher metal wear and ion release with corresponding adverse body reactions. The inclination angle of the acetabular cup has been positively correlated with the wear rate of explanted components 1. However, no data published is known about wear rates of edge loaded hard – soft hip bearings. For the hip simulator study, seleXys cup inlays, size 28/EE, (Mathys Ltd Bettlach, Switzerland) were used. Standard PE parts and vitamys® inlays (highly cross-linked, vitamin E stabilised UHMWPE) were tested in the same run. PE inlays were machined out of sintered GUR 1020 slabs, packaged and gamma-sterilised in inert atmosphere at 30 kGy. The vitamys® material was made in-house by adding 0.1 wt.-% of vitamin E to GUR 1020 powder from Ticona GmbH, Kelsterbach/Germany. Cross-linking used 100 (±10) kGy gamma-irradiation and the final sterilisation was gas plasma. Cup inclination was varied: besides the protocol of ISO 14242-1 with an inclination angle corresponding to 45 ° in the medial-lateral plane, a steep cup position corresponding to 75 ° was tested, too. To our knowledge, this is the highest inclination angle ever tested in a hip simulator. The testing was conducted in a servo-hydraulic six-station hip simulator (Endolab, Thansau/Rosenheim, Germany) at a temperature of 37±1°C. Tests were run at the RMS Foundation (Bettlach / Switzerland) for five million cycles. The test fluid was based on bovine serum diluted to a protein concentration of 30 g/l and stabilised with sodium azide and EDTA. At lubricant change interval of 500,000 cycles, the inlays were measured gravimetrically with an accuracy of 0.01 mg.Introduction
Methods
