A novel thin-walled direct-to-bone fixation ceramic cup was tested under critical impact loads simulating lateral fall and car crash scenarios. Three sizes of BIOLOX delta ceramic cups (total hip replacement cup with bearing diameter of 32 mm and two hip resurfacing cups with bearing diameters of 40 and 64mm) were implanted into reamed Sawbones blocks representing acetabulae. Three cups of each size were fully supported by the block and three were implanted with 15° of the cup’s outer diameter unsupported by the block. All testing was conducted with the corresponding bearing diameter heads lateralised by approximately 1 mm to represent worst case subluxed hips and all then subjected to test, replicating a lateral fall impact followed by a car crash impact. All cups passed lateral fall conditions without visible damage, although some movement of the cups was measured and damage to the blocks was observed. Five out of six of each cup size survived the car crash impacts with one fracture per size. In all cases, this was in the fully supported condition representing the highest cup inclination angle. The car crash impact force was equivalent to that reported to fracture the acetabulum and in all simulated cases, the Sawbones block showed signs of damage. Survival of five cups per size suggests the pelvis is much more likely to fracture before the cup. The ability of the cups to withstand these critical impact forces indicates they are unlikely to fracture in normal clinical use and should meet the more challenging demands of active patients likely to receive this device.
Ceramics have a history of use in total hip replacement (THR) including early use in hip resurfacing. Improvements in ceramics over the past two decades have resulted in the BIOLOX ® delta material which has a higher fracture resistance, low wear and excellent biocompatibility compared to other orthopaedic biomaterials. Cobalt Chromium Molybdenum (CoCrMo) metal alloy continues to be used in hip resurfacing but is generally limited to use of larger sizes in active males due to a legacy of concerns with metal ion release from discontinued devices and large diameter modular head trunnions. A novel hip resurfacing device (ReCerf ® , MatOrtho Ltd., UK) utilises BIOLOX ® delta, removing CoCrMo metal from the prosthesis. ReCerf ® is based on a clinically successful design from the same manufacturer (ADEPT ® , MatOrtho Ltd., UK) and although BIOLOX ® delta is a clinically proven material in THR, it requires significant testing to prove safety in the clinical environment. Increasing regulations particularly in Europe, has increased the time to market for new devices such as this and whilst the changes are being implemented to ensure the lowest risk for patients, they are also leaving patients unable to access new treatment routes. Ultimately, it is long term clinical outcomes that are required to determine whether the future of resurfacing is in using alternative materials such as ceramic and whether these compare favourably with the established, successful metal designs which have been in clinical use for over 20 years.
Introduction Recent concerns over adverse effects of metal ion release, have led to the development of alternative hip joint replacements. This study reports the performance of new hemispherical MOTIS® (milled pitch-carbon fibre reinforced polyetheretherketone) acetabular cups articulating against Biolox Delta® femoral heads with the aim of producing lower wear and more biologically compatible bearings. Materials and Methods The wear performance of 40mm hemispherical MOTIS® cups articulating against Biolox Delta® heads has been investigated. The diametral clearance was 322±15.3nm (mean ± standard deviation). Wear tests were carried out on the Simplified Mark II Durham Hip Wear Simulator to 8 million cycles. New born bovine calf serum was used as the lubricant, diluted to give a protein content of 17g/l. Friction tests were carried out on the unworn joints and worn joints after 7.5 million cycles using lubricants containing protein (bovine serum based carboxymethyl cellulose (CMC) fluids) and without protein (water based CMC fluids). Temperature measured near every hip joint over a continuous wear testing period of 0.5 million cycles was recorded using PICO TC-08 data logger. One K-type thermocouple was placed carefully and consistently in each wear station and two were used to record the ambient room temperature. After stopping the wear test, the data logger continued recording the temperature for a further ten hours to indicate the cooling period. Additionally surface analyses were undertaken before and after wear testing using a non-contacting profilometer and atomic force (AFM) microscope. Results and Discussion Throughout the wear testing, different degrees of fluid absorption were observed for the load control and soak control MOTIS® cups. After normalising the wear data in the post-processing analysis, the corresponding volumetric wear rates, averaged among five worn cups, were 0.551±0.115 mm 3 /10 6 cycles taking account of the load control and 0.493±0.107 mm 3 /10 6 cycles taking account of the soak control respectively. In contrast there was no difference in the fluid uptake for the ceramic heads between the load control and soak control. Normalised by the mass changes of the load control, the worn heads produced a volumetric wear rate of 0.243±0.031mm 3 /10 6 cycles. Interestingly, varying inclination angles had no effects on the wear of ceramic heads and statistically little effects for the MOTIS® cups. The mean lubricant temperature during wear testing varied from 40°C to 45°C with a mean of 43°C. However, friction testing on worn joints produced a mean friction factor of 0.089 which is relatively low for CFR-PEEK bearings. Further investigation is ongoing. Atomic Force Microscopy showed some partial grain pull-out on the ceramic heads. Compared with the literature, addressing the hard-on-soft hip bearings, the hemispherical MOTIS® cups assessed in the present study have produced the lowest wear [1-4]. Statistically, high inclination angles appeared to have little effect on the wear performance of the hemispherical MOTIS-PEEK-on-ceramic joints.
INTRODUCTION Wear induced osteolysis, material property degradation and oxidation remain a concern in cobalt chrome on polyethylene THR. ECIMA is a cold-irradiated, mechanically annealed, vitamin E blended HXLPE developed to maintain mechanical properties, minimise wear and improve long-term oxidation resistance. This study aimed to compare the in-vitro wear rate and mechanical properties of three different acetabular liners; UHMWPE, HXLPE and ECIMA. METHODS Twelve liners (Corin, UK) underwent a 3 million cycle (mc) hip simulation. Three UHMWPE (GUR1050, O32 mm, γ sterilised), three HXLPE (GUR1020, O40 mm, 75 kGy γ, EtO sterilised) and six ECIMA (0.1 wt% vitamin E GUR1020, O40 mm, 120 kGy γ, mechanically annealed, EtO sterilised) liners articulated against CoCrMo femoral heads (Corin, UK). Wear testing was performed in accordance with ISO 14242 parts 1 and 2, in calf serum, with a maximum force of 3.0 kN and at a frequency of 1 Hz. Volumetric wear rate was determined gravimetrically. ASTM D638 type V specimens were machined from ECIMA material for uniaxial tension testing. Ultimate tensile strength (UTS), yield strength and elongation values were measured. These values were compared to mechanical data available for the other material types. Following completion of the ECIMA wear testing, three of the tested liners were cut in half. One half of each was subject to accelerated ageing in accordance with ASTM F2003-02, while the other half was tested as received. Each liner half was cross-sectioned and a microtome was used to section 200μm thick slices from each cross-section. Oxidation analysis was performed using a Fourier Transform Infra-red technique in accordance with ASTM F2102-01 throughout the thickness of each liner half. Average oxidation indices for each sample were determined. RESULTS The reduction in wear rate for the ECIMA liners compared to the UHMWPE and HXLPE liners was 95 % and a 83 % respectively. There was an increase in UTS, yield strength and percent elongation of 45%, 16% and 32% respectively, for unaged ECIMA compared to HXLPE. Following ageing of the ECIMA samples, there was minimal change in all three mechanical properties. Importantly, the mechanical properties were not substantially degraded and were more comparable to conventional UHMWPE than HXLPE. Further to this, following an aggressive ageing protocol, the ECIMA material maintains the mechanical properties of the unaged condition. All of the oxidation values for the wear tested ECIMA liners, before and after ageing, and the aged, untested ECIMA samples were negative, which shows oxidation levels below the level of detection throughout the thickness of the samples. This indicates a high level of through-thickness oxidation resistance for the ECIMA specimens even after being subject to an aggressive ageing protocol and cyclic loading. DISCUSSION These in-vitro wear results indicate that ECIMA is a very low wearing material with the potential to reduce wear related osteolysis in-vivo . Importantly, the mechanical properties were generally maintained unlike the degradation found in many modified polyethylene materials and were more comparable to UHMWPE than HXLPE. These properties make ECIMA a promising next generation bearing material.
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