Non-optimal clinical alignment of components in total hip replacements (THRs) may lead to edge loading of the acetabular cup liner. This has the potential to cause changes to the liner rim not accounted for in standard wear models. A greater understanding of the material behaviours could be beneficial to design and surgical guidance for THR devices. The aim of this research was to combine finite element (FE) modelling and experimental simulation with microstructural assessment to examine material behaviour changes during edge loading. A dynamic deformable FE model, matching the experimental conditions, was created to simulate the stress strain environment within liners. Five liners were tested for 4Mc (million cycles) of standard loading (ISO14242:1) followed by 3Mc of edge loading with dynamic separation (ISO14242:4) in a hip simulator. Microstructural measurements by Raman spectroscopy were taken at unloaded and highly loaded rim locations informed by FE results. Gravimetric and geometric measurements were taken every 1Mc cycles. Under edge loading, peak Mises stress and plastic deformation occur below the surface of the rim during heel strike. After 7Mc, microstructural analysis determined edge loaded regions had an increased crystalline mass fraction compared to unloaded regions (p<0.05). Gravimetric wear rates of 12.5mm3/Mc and 22.3mm3/Mc were measured for standard and edge loading respectively. A liner penetration of 0.37mm was measured after 7Mc. Edge loading led to an increase in gravimetric wear rate indicating a different wear mechanism is occurring. FE and Raman results suggest that changes to material behaviour at the rim could be possible. These methods will now be used to assess more liners and over a larger number of cycles. They have potential to explore the impact of edge loading on different surgical and patient variables.
Many factors contribute to the occurrence of edge-loading conditions in hip replacement; soft tissue tension, surgical position, patient biomechanical variations and type of activities, hip design, etc. The aim of this study was to determine the effect of different levels of rotational and translational surgical positioning of hip replacement bearings on the occurrence and severity of edge-loading and the resultant wear rates. The Leeds II Hip-Joint Simulator and 36mm diameter alumina matrix composite ceramic bearings (BIOLOX delta, DePuy Synthes, UK) were used in this study. Different levels of mismatch between the reconstructed rotational centres of the head and the cup were considered (2, 3 and 4mm) in the medial-lateral axis. Two cup inclination angles were investigated; an equivalent to 45 and 65 degrees in-vivo, thus six conditions (n=6 for each condition) were studied in total with three million cycles completed for each condition. The wear of the ceramic-on-ceramic bearings were determined using a microbalance (Mettler Toledo, XP205, UK) and the dynamic microseparation displacement was measured using a Liner Variable Differential Transformer.Background
Method
In order to improve the longevity and design of an implant, a wide range of pre-clinical testing conditions should be considered including variations in surgical delivery, and patients' anatomy and biomechanics. The aim of this research study was to determine the effect of the acetabular cup inclination angle with different levels of joint centre mismatch on the magnitude of dynamic microseparation, occurrence and severity of edge loading and the resultant wear rates in a hip joint simulator. The six-station Leeds Mark II Anatomical Physiological Hip Joint Simulator and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta) were used in this study. A standard gait cycle, with a twin-peak loading (2.5kN peak load and approximately 70N swing phase load), extension/flexion 15°/+30° and internal/external ±10° rotations, was applied. Translational mismatch in the medial-lateral axis between the centres of rotation of the head and the cup were considered. In this study, mismatches of 2, 3 and 4 (mm) were applied. Two acetabular cup inclination angles were investigated; equivalent to 45° and 65° in-vivo. These resulted in a total of six conditions [Figure 1] with n=6 for each condition. Three million cycles were completed under each condition. The lubricant used was 25% (v/v) new-born calf serum supplemented with 0.03% (w/v) sodium azide to retard bacterial growth. The wear of the ceramic bearings were determined using a microbalance (XP205, Mettler Toledo, UK) and a coordinate measuring machine (Legex 322, Mitutoyo, UK). The stripe wear was analysed using RedLux software. The dynamic microseparation displacement was measured using a linear variable differential transformer. Mean wear rates and 95% confidence limits were determined and statistical analysis (one way ANOVA) completed with significance taken at p<0.05. Results Increasing the medial-lateral joint centre mismatch from 2 to 3 to 4mm resulted in an increased dynamic microseparation [Figure 2]. A similar trend was observed for the wear. A higher level of medial-lateral mismatch increased the wear rate under both 45° and 65° cup inclination angle conditions [Figure 3]. The mean wear rates obtained under 65° were significantly higher compared to those obtained under the 45° cup inclination angle conditions for a given medial-lateral mismatch in the joint centre (p=0.02 for 2mm mismatch, p=0.02 for 3 mm mismatch, and p<0.01 for 4mm mismatch).Introduction and Aims
Methods
