Abstract
Background
Medical advances and an ageing population mean that more people than ever rely on artificial joints. In the past years, shoulder joint replacement has developed rapidly and the numbers of shoulder prostheses implanted increased dramatically. Wear is one of the main contributors to the failure of shoulder implants. It is therefore important to measure the wear properties of the articulating surfaces within the joint in vitro. Investigation of wear characteristics through a comprehensive range of motion using a sophisticated shoulder simulator would reveal the durability of the material, the performance of component design and the safety analyses of prostheses. The purpose of the work was to develop and validate a multi-station shoulder simulator, which could accurately simulate physiological gleno-humeral forces and displacements during activities of daily living.
Materials and Methods
Imperial shoulder simulator was designed with six articulating stations and one loaded soak control station for anatomical shoulder system wear simulation. It gives an adduction-abduction (AA) range of-15° to 55°, flexion-extension (FE) range of −90° to 90° and internal external rotation (IER) range of 15° to −90°. The rotations are applied simultaneously to the humeral implants by using stepper motors with integral position encoders. Axial and shear loadings to each glenoid implant were applied using pneumatic cylinders. Force controlled translations were recorded using load cells and LVDTs, and a data acquisition system. Pneumatic cylinders were also installed to work to counterbalance weights during the motion of adduction-abduction. All bearing pairs are within isolated and sealed test chambers to prevent loss of fluid through evaporation, and cross contamination of third body wear (as recommended in F1714-96). The simulator is controlled by LabVIEW program allowing to reproduce shoulder activities of daily living.
Results
A commissioning trial was conducted when shoulder implants were subject to rotational and translational motions and loading to replicate the ‘combing’ activity of daily living. The motion ranges were typically 5° to 15° in AA, 15° to 80° in FE, and −30° to −20° in IER with applied loads from 60 to 440 N, principally along the medio-lateral direction. The waveform was at frequency of 1 Hz. The activity was loaded at 250,000 cycles around 3 full days, when test and control specimens should be cleaned, measured and then re-installed into the simulator. The results from kinematic and kinetic inputs and outputs were obtained from the trial study.
Discussion
A multi-station shoulder simulator was successfully developed, which is capable of reproducing typical activities of daily living by applying physiological patterns of motion and load. The performance of the simulator was validated in the commissioning trial, which leads to evaluation of novel implant designs.
