Clinically many factors such as variations in surgical positioning, and patients' anatomy and biomechanics can affect the occurrence and severity of edge loading which may have detrimental effect on the wear and durability of the implant. Assessing wear of hundreds of combinations of conditions would be impractical, so a preclinical testing approach was followed where the occurrence and severity of edge loading can be determined using short biomechanical tests. Then, selected conditions can be chosen under which the wear can be determined. If a wear correlation with the magnitude of dynamic separation or the severity of edge loading can be shown, then an informed decision can be made based upon the biomechanical results to only select important variables under which the tribological performance of the implant can be assessed. The aim of this study was to determine the relationship between the wear of ceramic-on-ceramic bearings and the (1) magnitude of dynamic separation, (2) the maximum force reached during edge loading and (3) the severity of edge loading resulting from component translational mismatch between the head and cup centres. The Leeds II hip joint simulator with a standard walking cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta, DePuy Synthes Joint Reconstruction, Leeds, UK.) were used. The study was in two parts. Part one: a biomechanical study where the dynamic separation, the maximum load during edge loading, and the duration of edge loading alongside the magnitude of forces under edge loading (severity of edge loading) were assessed. Part two; a wear study where the wear rates of the bearing surfaces were assessed under a series of input conditions. These input testing conditions included inclining the acetabular cups at 45° and 65° cup inclination angle (in-vivo equivalent), with 2, 3, and 4mm medial-lateral component mismatch between the centres of the head and the cup. This equated to six conditions being assessed, each with three repeats for the biomechanical test, and six repeats completed for the wear study. The severity of edge loading was assessed as described in Equation 1. Severity of Edge Loading = ∫tt0 F(x) dx + ∫tt0 F(y) dy … Equation 1, where F(x) is the axial load, F(y) is the medial-lateral load and t-t0 is the duration of edge loading. The wear of the ceramic bearings were determined using gravimetric analysis (XP205, Mettler Toledo, UK).Introduction and Aims
Methods
There are many variables that can affect the occurrence and severity of edge loading in hip replacement. A translational mismatch between the centres of rotation of the head and cup may lead to dynamic separation, causing edge loading and increased wear. Combining a steep inclination angle with such translational mismatch in the medial-lateral axis caused a larger magnitude of separation and increased severity of edge loading. Previous studies have shown variation in the hip Swing Phase Load (SPL) during gait between different patients. The aim of this study was to apply a translational mismatch and determine the effect of varying the SPL on the occurrence and severity of edge loading under different cup inclination angles in a hip joint simulator. The Leeds II hip joint simulator with a standard gait cycle and 36mm diameter ceramic-on-ceramic bearings (BIOLOX® delta) were used in this study. The study was in two stages; [1] a biomechanical study where the magnitude of dynamic separation, the duration of edge loading and the magnitude of force under edge loading (severity) were assessed under variations in component positioning and SPLs. [2] A wear study to assess edge loading with selected input conditions. For the biomechanical study, a combination of four mismatches, three cup inclination angles, and eight SPLs (Table 1) were investigated. For the wear study, three SPL conditions were selected with one cup angle and one mismatch (Table 1). Three million cycles were completed under each condition. Mean wear rates and 95% confidence limits were determined and statistical analysis (one way ANOVA) completed (significance taken at p<0.05). Table 1: Study matrixIntroduction and Aims
Methods
Variations in component position can lead to dynamic separation and edge loading conditions. In vitro methods have been developed to simulate edge loading conditions and replicate stripe wear, increased wear rate, and bimodal wear debris size distribution, as observed clinically [1, 2]. The aim of this study was to determine the effects of translational and rotational positioning on the occurrence of dynamic separation and severity of edge loading, and then investigate the wear rates under the most severe separation and edge loading conditions on an electromechanical hip joint simulator. A hip joint simulator (ProSim EM13, Simulation Solutions, UK) was set up with 36mm diameter ceramic-on-ceramic (BIOLOX®delta, PINNACLE®, DePuy Synthes, UK) hip replacements. Three axes of rotation conditions (ISO 14242-1 [3]) was applied to the femoral head. This study was in two parts. I) A biomechanical test was carried out at 45° (n=3) and 65° (n=3) cup inclination angles with 1, 2, 3 and 4 (mm) medial-lateral translational mismatch between the centres of the head and cup. The amount of dynamic separation displacement between the head and cup was measured using a position sensor. The severity of edge loading was determined from the area under the axial force and medial-lateral force outputs during the time of separation [4]. II) A wear test was carried out at 45° (n=6) and 65° (n=6) cup inclination angles for three million cycles with translational mismatch of 4mm between the head and cup. The lubricant used was diluted new-born calf serum (25% v/v). Volumetric wear measurements were undertaken at one million cycle intervals and mean wear rates were calculated with 95% confidence limits. Statistical analysis was carried out using ANOVA and a t-test with significance levels taken at p<0.05.Introduction
Materials and Methods
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
Increased wear rates [1, 2] and acetabular rim fracture [3] of hip replacement bearings reported clinically have been associated with edge loading, which could occur due to rotational and/or translational mal-positioning [4]. Surgical mal-positioning can lead to dynamic microseparation mechanisms resulting in edge loading conditions. In vitro microseparation conditions have replicated stripe wear and the bi-modal wear debris distribution observed clinically [5, 6]. The aim of this study was to investigate the effect of steep cup inclination, representing rotational mal-positioning, on the magnitude of dynamic microseparation, severity of edge loading, and the resulting wear rate of a ceramic-on-ceramic bearing, under surgical translational mal-positioning conditions. Ceramic-on-ceramic bearings where the ceramic liner was inserted into a titanium alloy cup (BIOLOX® delta and Pinnacle® respectively, DePuy Synthes, UK) were tested on the six-station Leeds II hip simulator. The first test was run with the cups inclined at an angle equivalent, clinically, to 45° (n=6) and the second test was run with the cups inclined at an angle equivalent, clinically, to 65° (n=6). A standard gait cycle was run. A fixed surgical translational mal-positioning of 4mm between the centres of rotations of the head and the cup in the medial/lateral axis was applied on all stations. Both tests ran for three million cycles each. The lubricant used was 25% new-born calf serum. Wear was assessed gravimetrically using a microbalance (XP205, Mettler Toledo, UK) and geometrically using a coordinate measuring machine (CMM, Legex 322, Mitutoyo, UK). Statistical analysis was done using one way ANOVA with significance taken at p<0.05.Introduction
Materials and Methods
Concerns regarding UHMWPE wear particle induced osteolysis in total hip replacement (THR, [1]) have led to alternative materials to be sought. Carbon-fibre reinforced poly-ether-ether-ketone (CFR-PEEK) has shown reduced wear in hip and knee configurations compared with conventional polyethylene [2-4]. The aim of this study was to investigate the wear performance of a ceramic-on-CFR PEEK THR through a simulator study. Five 36mm diameter Biolox Delta heads were paired with extruded CFR-PEEK cups and tested in a hip wear simulator (Simulator Solutions, UK) for 10 million cycles (Mc). Tests were performed in a Prosim hip simulator, which applied a twin peak loading cycle, with a peak load of 3kN. Flexion-extension of −15 to +30 degrees was applied to the head and internal-external rotation of +/− 10 degrees was applied to the cup, components were mounted anatomically. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (w/v) sodium azide. Wear was assessed gravimetrically at several intervals adjusted for moisture uptake using loaded and unloaded soak controls.Introduction
Methodology
A metal ion study was undertaken in patients who had received an articular surface replacement. The design of these components is optimised in line with lubrication theory and produces low levels of wear in hip joint simulators. Patients were recruited in four centres. Whole blood samples were analysed for metal ion levels using high resolution ICP-MS (inductively coupled plasma mass spectrophotometry). A total of 75 patients was enrolled into the study and 65 and 47 patients were assessed after 12 and 24 months implantation respectively. Results are included irrespective of clinical outcome.Introduction
Methods
The search for the ideal bearing surface in Total Hip Replacements continues. The current ‘best’ materials are felt to be combinations of metal, ceramics and cross-linked polyethylene. Laboratory studies suggest that ceramic-on-metal articulations may provide distinct advantages. This study aims to identify the best bearing surface combination with the lowest adverse side effect profile. Between February 2004 and September 2007, 164 hips were replaced in 142 patients. 39% were male and 69% were female. The average age at surgery was 53 years (17-72 years). Follow-up assessment included radiographs, the Harris Hip Score and whole blood samples for metal ion levels. Complications to date included 3 hips which needed femoral revision because of surgery related factors, and 3 cases of sepsis of which 1 settled and 2 needed revision. One hip needed revision of head and liner to a larger bearing size for recurrent dislocations, and is no longer being followed up for blood metal ions. Post-operative whole blood metal ion levels were compared to pre-operative levels to determine the increase or decrease in metal ion levels. There were no changes in those patients with ceramic-on-ceramic and ceramic-on-polyethylene articulations. Moderately raised whole blood metal ion levels were noted at 3 months in the ceramic-on-metal group, while the metal-on-metal group show the greatest increase. This study agrees with laboratory bearing surface wear studies demonstrating lower wear rates in the ceramic-on-metal group compared to the metal-on-metal group. With concerns related to high blood metal ion levels in metal-on-metal articulations, ceramic-on-metal bearing surfaces may well become a bearing surface of choice in the future, but progress needs to be monitored in the longer term.
