Abstract
Introduction
Ion analysis has been used as one of the key indicators to assess the performance of MoM devices in patients. Modular devices, in particular having larger overall surface area (the stem and sleeve), and locking interfaces (head – bore, sleeve- taper and sleeve-bore, stem-taper surfaces) than other MoM devices are expected to release greater number of ions. Concerns have been expressed that the ion release at the taper junction might be a potential cause leading to the failure of the implant [Garbuz et al, 2010].
The aim of this study was to look into the wear and the associated ion release from the taper junction and the articulating surface of modular devices.
Method
For the first time a novel design has been used to isolate the taper junction on modular devices on the hip simulators in order to compare the wear at the taper junction and articulating surface. The taper junction has been isolated in a small gaiter, while the head and cup were contained in a large gaiter. CoCrMo sleeves having an offset of +8 mm have been used on 50 mm modular heads along with Ti6Al4V stems. The acetabular components were standard BHR cups. Three devices (Smith & Nephew, UK) have been tested with newborn calf serum as a lubricant (in the large gaiter) and also as the medium containing the taper junction (in the small gaiter). The serum samples from the articulating surface and taper junction were analysed using HR-ICPMS. The locking interfaces at the taper junction have been left intact throughout the duration of the test. Both the head and the cup have been tested under anatomical conditions using the standard implant development centre's (IDC) profile for 2 million cycles (Mc). The lubricant was newborn calf serum with 0.2% sodium azide diluted with de-ionised water to achieve protein concentration of 20 mg/ml. The flexion/extension was 30°/15° and the internal/external rotation was ±10°. The force was Paul-type stance phase loading with a maximum load of 3 kN and a standard ISO swing phase load of 0.3 kN. The frequency was 1 Hz, with an 8 hour stop after every 16 hours of testing.
Results and discussion
The ion analysis results from the articulating surface expressed in cumulative volume showed a biphasic wear trend at 0.96 ± 0.15 mm3 (0.5 Mc) and 1.05 ± 0.13 mm3 (2 Mc). The ion analysis results obtained for the samples from the small gaiter are (1.83 ± 0.35) x 10−3 mm3 at 0.5 Mc and (4.52 ±0.83) x 10−3 mm3 (2 Mc). On comparison, the ion release from the taper junction is 100 fold lower than that from the articulating surface. It should be noted that the ion release from the taper junction can be influenced by the material, tolerance, surface finish and design of the taper joint.
Conclusion
The ion release associated with the taper junction is extremely low when compared with the ion release from the articulating surface.
