Introduction and Aims. 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. Methods. 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 = ∫. t. t0. F(x) dx + ∫. t. t0. 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). Results. The wear rates of ceramic-on-ceramic bearings increased as the magnitude of dynamic separation (Figure 1), the maximum load at the rim during edge loading (Figure 2), and the severity of edge loading (Figure 3) increased. The magnitude of dynamic separation was found to have the highest correlation to the wear rate under the conditions tested in this study (R. 2. =0.94). Conclusions. A preclinical testing approach has been developed to understand the occurrence and severity of edge loading associated with variation of component positioning. A good correlation was found between the wear rates obtained for ceramic-on-ceramic bearings and the magnitude of parameters obtained under edge loading during a short-term biomechanical study. For figures/tables, please contact authors directly.

Variations in component positioning of total hip replacements can lead to edge loading of the liner, and potentially affect device longevity. These effects are evaluated using ISO 14242:4 edge loading test results in a dynamic system. Mediolateral translation of one of the components during testing is caused by a compressed spring, and therefore the kinematics will depend on the spring stiffness and damping coefficient, and the mass of the translating component and fixture. This study aims to describe the sensitivity of the liner plastic strain to these variables, to better understand how tests using different simulator designs might produce different amounts of liner rim deformation.

A dynamic explicit deformable finite element model with 36mm Pinnacle metal-on-polyethylene bearing geometry (DePuy Synthes, Leeds, UK) was used with material properties for conventional UHMWPE. Setup was 65° clinical inclination, 4mm mismatch, 70N swing phase load, and 100N/mm spring. Fixture mass was varied from 0.5-5kg, spring damping coefficient was varied from 0-2Ns/mm. They were changed independently, and in combination.

Maximum separation values were relatively insensitive to changes in the mass, damping coefficient, or both. The sensitivity of peak plastic strain, to this range of inputs, was similar to changing the swing phase load from 70N to approximately 150N – 200N. Increasing the fixture mass and/or damping coefficient increased the peak plastic strain, with values from 0.15-0.19.

Liner plastic deformation was sensitive to the spring damping and fixture mass, which may explain some of the differences in fatigue and deformation results in UHMWPE liners tested on different machines or with modified fixtures. These values should be described when reporting the results of ISO14242:4 testing.

Acknowledgements

Funded by EPSRC grant EP/N02480X/1; CAD supplied by DePuy Synthes.

Introduction

Edge loading in acetabular hip implants is generally due to mal-orientation or low tissue tension. It is known that edge loading of metal-on-metal THA may lead to higher metal wear and ion release with corresponding adverse body reactions. The inclination angle of the acetabular cup has been positively correlated with the wear rate of explanted components ¹. However, no data published is known about wear rates of edge loaded hard – soft hip bearings.

Introduction: Although clinical results for the Metal-on-Metal (MoM) devices have been excellent, recently some concerns have been raised regarding the occurrence of periprosthetic soft tissue lesions (PSTL) in some patients with MoM devices. Clinical studies and retrieval analyses have shown that devices revised due to groin pain and PSTL generally have significantly higher wear that has been attributed to edge loading of the implants.

Aim: The retrieval study was to investigate the cause of edge-loading of MoM devices in vivo.

Materials and Methods: In this study 13 retrieved Birmingham Hip Resurfacing (BHR) devices were examined. All devices were supplied with radiographs showing the in vivo position of the implant. Linear wear was assessed using a Taylor-Hobson Talyrond 290 roundness machine. Multiple roundness profiles were obtained to locate the area of maximum wear on each component. Edge loaded devices were identified when the maximum linear wear occurred at the edge of the cup. Non-edge loaded pairs showed wear area within the articulating surface of the cup.

The in vivo abduction angle and version angle of the cup were determined by superimposing the BHR models onto the radiographs (ProEngineer Wildfire 4 with ISDX II extension software) using anatomical references and specific features of the BHR.

Results: Linear wear: Among the 13 devices investigated, 11 were edge loaded with the maximum linear wear occurred at the edge of the cup. The remaining 2 pairs were non-edge loaded. The average joint linear wear rate of the edge loaded devices was 49.9 μm per year, and that for the two non-edge loaded devices was 2.4 μm per year. Edge loaded pairs had far greater linear wear than non-edge loaded components.

Cup orientation: The abduction angles of the two non-edge loaded cups were 31° and 39°, and their version angles were 12 and 16° respectively. These angles were within recommended orientation for the BHR. In contrast, the adduction angles and/or version angles of all edge loaded devices were outside the recommended orientation. Their abduction angle varied from 40° to 66° and version angle from 5° to 46°.

The edge loaded devices with higher inclination angles and/or higher version angels generally had higher linear wear. There is strong correlation between the cup orientation and the linear wear of the implant.

Conclusion: Mal-orientated devices in this study showed clear signs of edge loading which in turn resulted in significant increase in wear compared to the well orientated/non-edge loaded devices.

Introduction

All hip replacements depend upon good orientation and positioning to ensure that implants function well in vivo. Mal-orientated devices can lead to poor patient gait, poor range of motion, impingement, edge loading and high wear, which in turn may result in the premature failure of the implants.

Introduction

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.

Introduction: The purpose of this study was to determine the effects of edge loading on in vivo wear of hip resurfacings from retrievals.

Methods: The wear of retrieved BHR heads and cups was assessed using a Taylor-Hobson Talyrond 290 roundness machine. The maximum deviation of the profile from an ideal circle was taken as the maximum linear wear. Edge loaded devices (Figure 1a) were classified as cups which showed the maximum area of wear crossing over the edge of the cup. For all non-edge loaded pairs (Figure 1b), the wear area on the cup was within the sphere of the cup. In this study 50 pairs (diameter size 38 mm to 54 mm) were analysed.

Results: 28 pairs were classified as edge loaded, and 22 were not. Edge loaded pairs display greater linear wear than non-edge loaded components (Table 1). Edge loaded components showed no correlation between time in vivo and linear wear.

Discussion: Edge loaded pairs have a far greater range of linear wear which may be due to the variation of the angles of the components in vivo. Edge loading may be caused by an open cup, impingement and/or high combined anteversion angle of both the head and cup. The success of a hip resurfacing depends strongly upon articulation occurring within the sphere of the cup, which is reliant upon good component orientation.

Background

In vivo fluoroscopic studies have proven that femoral head sliding and separation from within the acetabular cup during gait frequently occur for subjects implanted with a total hip arthroplasty. It is hypothesized that these atypical kinematic patterns are due to component malalignments that yield uncharacteristically higher forces on the hip joint that are not present in the native hip. This in vivo joint instability can lead to edge loading, increased stresses, and premature wear on the acetabular component.

Introduction and Aims

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.

SUMMARY

The relationship between component position, wear rate and edge loading was investigated for 115 explanted current generation Metal-on-Metal (MoM) hips. Edge wear was detected in: 63% of all hips; and 48% of those with cups positioned within Lewinnek's box.

Introduction and Aims

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.

Introduction and Aims: The extremely low wear rates of third generation alumina-alumina bearings in traditional hip simulators are not reflected in vivo. Separation of the bearing during swing phase and edge loading with heel strike is reported to account for this discrepancy.

Method: We have had the opportunity to visually inspect 21 bearings at re-operation from a group of 1588 hip arthroplasties with third generation alumina ceramic-ceramic bearings. Re-operations were for heterotopic ossification (one), loosening (three), femoral fracture (six), psoas tendonitis (six), sepsis (three) and dislocation (two). There were no re-operations for bearing failure. Sixteen of these 21 bearings (16 heads and 12 inserts) were retrieved and analysed. We mapped the location and we measured the volume of the wear and we performed microscopy and measured roughness of worn and unworn areas.

Results: Eleven bearings had visual evidence of edge loading wear, making an incidence of 52% in the 21 patients having re-operations. These 11 bearings and five visually undamaged bearings were analysed. The wear on the insert was always located at the rim indicating edge loading. The location and orientation of the stripe on the head was not consistent with subluxation during normal gait but was consistent with subluxation and edge loading with the hip flexed at 90 degrees. The average wear volume was 0.7mm3 per year (heads plus liners). Longer service bearings had signs under SEM of repolishing of the wear area suggesting that the process of edge loading wear will be self-limiting. The heads without a wear scar showed very little damage: under SEM, a slight relief polishing of individual grains and minor pitting was noted.

Conclusion: The subluxation causing the stripe wear in these patients did not occur during normal walking gait. It probably occurred with rising from a chair. Simulator testing of third generation alumina-alumina components must include edge loading if it is to give a realistic indication of in vivo performance. Alumina-alumina bearings remain an excellent option for total hip arthroplasty, however more work is required to understand the clinical consequences.

Introduction

Two types of ceramic materials currently used in total hip replacements are third generation hot isostatic pressed (HIPed) alumina ceramic (commercially known as BIOLOX®forte, CeramTec) and fourth generation alumina matrix composite ceramic consisting of 75% alumina, 24% zirconia, and 1% mixed oxides (BIOLOX®delta, CeramTec). Delta ceramic hip components are being used worldwide, but very few studies have analyzed retrieved delta bearings. The aim of this study is to compare edge loading ‘stripe’ wear on retrieved femoral heads from delta-on-delta, delta-on-forte and forte-on-forte ceramic bearings revised within 2 years in vivo.

Introduction

Total ankle replacement (TAR) is less successful than other joint replacements with a 77% survivorship at 10 years. Predominant indications for revision include: Insert dislocation, soft tissue impingement and pain/stiffness. Insert edge-loading may be both a product and cause of these indications and was reported to affect 22% of patients with the, now withdrawn from market, Ankle Evolutive System (AES) TAR (Transysteme, Nimes, France). Compressive forces up to seven times body weight over a relatively small contact area (∼6.0 to 9.2 cm²), in combination with multi-directional motion potentially causes significant polyethylene wear and deformation in mobile-bearing TAR designs. Direct methods of measuring component volume (e.g. pycnometer) use Archimedes' principle but cannot identify spatial changes in volume or form indicative of wear/deformation. Quantitative methods for surface analysis bridge this limitation and may advance methods for analysing the edge loading phenomena in TAR.

Background

It is not always clear why some patients experience recurrent dislocation following total hip arthroplasty (THA). In order to plan appropriate revision surgery for such patients, however, it is important to understand the specific biomechanical basis for the dislocation. We have developed a novel method to analyse the biomechanical profile of the THA, specifically to identify edge loading and prosthetic impingement, taking into account spinopelvic mobility. In this study we compare the results of this analysis in THA patients with and without recurrent dislocation.

Purpose of the study: Implantation of the acetabular socket with high inclination generates increased contract stress, wear and revision rate for total hip arthroplasty (THA). Study of ceramic-on-ceramic THA explants has revealed a high wear rate in bands, suggesting a microseparation effect generating edge loading. There have not been any studies examining the influence of the cup inclination on the contact pressures in ceramic-on-ceramic THA exposed to microseparation between the head and the cup.

Material and methods: A finite elements model of a ceramic-on-ceramic hip prosthesis was developed with ABAQUS in order to predict the surface contact and the distribution of the contract pressures, first during ideal centred function then under conditions of microseparation. A 32mm head and a radial clearance head (30μm) were used. The cup was positioned in zero anteversion and 45, 65, 70, and 90° anteversion. Progressive microseparation (0 to 500 μm) was imposed. A 2500N loading force was applied to the centre of the head.

Results: For 45° inclination, edge loading appeared for mediolateral separation greater than 30 μm and became complete for 60 μm separation. When edge loading appeared, the contact surface was elliptic. The length of the lesser axis converged towards 0.96mm; the greater axis towards 8.15mm, respectively in the anteroposterior and mediolateral directions. For inclinations of 45°, the contact pressure was 66 Mpa for the centred force. As the mediolateral separation increased, the maximal contact pressure increased, converging towards an asymptotic value of 205 MPa. Increasing the inclination angle of the cup generated an increase in the maximal contact pressure. However, this increase in contact pressure generated by the increasing inclination angle was negligible if the microseparation increased.

Discussion: Cup inclination and mediolateral laxity increase stress forces of ceramic-on-ceramic THA and should be avoided. However, the influence of the cup inclination becomes negligible beyond a separation value of 240 μm, the stress forces already having reached their asymptotic value.

Purpose of the study: Ceramic-on-ceramic THA explants exhibit a higher wear rate than that predicted by classical simulators. This appears to be related to edge loading, which could perhaps be reproducible in vitro by creating a microseparation between the two components. The purpose of this study was to evaluate this coefficient of friction for ceramic-on-ceramic THA with edge loading. This should enable prediction of wear in the event of microseparation.

Material and methods: Three 32mm alumina inserts (Biolox Forte Ceramtec^®) were tested on a friction simulatior (Prosim^®). The cup was positioned vertically (75° inclination) to reproduce edge loading. The metal-back and the acetabular insert were sectioned to avoid impingement between the neck and cup. Contact was imposed along the border of the cup, then perpendicularly to it. The tests were performed under lubrication conditions (25% bovine serum). In order to simulate severe contact pressures, the tests were also conducted with a third body inserted between the head and the edge of the cup. To obtain reference values of the centred regimen, tests were first run with identical components positioned horizontally.

Results: Edge loading was achieved for cups inclined at 75°. The coefficient of friction was 0.02±0.001 under centred conditions. For edge loading conditions, the coefficient of friction was significantly increased, to a mean 0.09±0.00 for movement along the acetabular border and 0.034±0.001 for movement perpendicular to the border. Squeaking occurred for 15 s when the third body was introduced, corresponding to a coefficient of friction 15-fold higher (0.32±0.003) than under ideal conditions.

Discussion: For the first time, the coefficient of friction of edge loading was determined under conditions of lubrication. The friction coefficient of ceramic-on-ceramic THA was greater for a very vertical cup, but remained (0.1) equivalent to the metal-on-metal coefficient under optimal conditions. When a third body was introduced, transient squeaking occurred with a very high coefficient of friction.

Conclusion: Implantation of cups with a high abduction angle induces edge loading and an increased coefficient of friction, and should be avoided.

Steep angles (> 55°) reduce femoral head coverage decreasing contact area and can subject the acetabular rim to excessive stresses. In the case of metal-metal implants it has been shown that at steep angles there is no bedding-in of the implants and run-away wear occurs. The dual mobility bearing concept mates a metal femoral head with a polyethylene liner that is free to articulate inside a polished metal shell. Previous work has shown acetabular wear can be minimized with this design, possibly through reduction of total amount of cross-shear motion in the joint. An additional potential benefit may exist through the maintenance of conforming contact and head coverage even under high inclination angle. This study evaluates the influence of inclination angle on the wear performance of three hip bearing designs. Four sets of dual mobility implants, three sets of metal-on-metal hip implants, and five sets of fixed hip implants were evaluated per inclination angle. All polyethylene components were made of GUR 1020 UHMWPE that was sequentially crosslinked and annealed three times (X3). The MoM components were fabricated from high carbon cast CoCr as per ASTM F75 (no heat treatment). A hip joint simulator was used for testing for a total of 2.5 million cycles with the cups oriented at either 35° or 65° of abduction. Testing was run at 1Hz following Paul curve physiologic loading and statistical analysis was performed using the Student’s t-test (p< 0.05). results for the 35 degrees of inclination angle condition show no statistical difference between any of the testing combinations with X3 polyethylene showing immeasurable wear. At this angle wear of the MoM devices was similar, although ion levels were not measured. results for the 65 degree condition showed an increase for the fixed PE and MoM systems. The increase in fixed PE bearing wear is consistent with previous findings and still within noise level values. The increase in MoM wear was substantial with both heads and cups showing scratches and abrasion damage related to edge contact. There is a statistically significant wear rate reduction (p< 0.05) of over 94% for both the dual mobility and fixed bearing PE constructs when compared to MoM. When comparing wear rates of the dual mobility system to the standard fixed acetabular bearing, the dual mobility device exhibited an 85% (p< 0.05) reduction in wear rate. The results of this study support our hypothesis that acetabular wear at high angles can be diminished through design. This is likely due to maintenance of the nature of the primary inner bearing contact regardless of shell positioning. Based on these results this dual mobility construct can be expected to outperform a conventional fixed construct and a metal-on-metal construct in terms of wear at high inclination angles, without any of the metal ion release concerns.

Introduction

Stripe wear, observed on retrieved ceramic hip replacements, has only been replicated in vitro under translational mal-positioning conditions where the centres of rotation of the head and the cup are mismatched^1,2; an in vitro condition termed “microseparation”.

The aim of this study was to compare the edge loading mechanisms observed under microseparation conditions due to translational mal-positioning conditions simulated on two different hip joint simulators.

Introduction

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.