Spinal total disc replacement (TDR) designs rely heavily on total hip replacement (THR) technology and it is therefore prudent to check that typical TDR devices have acceptable friction and torque behaviour. For spherical devices friction factor (f) is used in place of friction coefficient (mju). The range of loading for the lumbar spinal discs is estimated at perhaps 3 times body weight (BW) for normal activity rising to up to 6 times BW for strenuous activity^[1]. For walking this equates to around 2000 N, which is the maximum load required by the ISO standard for TDR wear testing^[2].

Three Prodisc-L TDR devices (Synthes Spine) were tested in a single station friction simulator. Bovine serum diluted to 25% was used as a lubricating medium. Flexion-extension was ±5 deg for all experiments with constant axial loading of 500, 2000 and 3000 N. The cycle run length was limited to 100 and the f and torque (T) values recorded around the maximum velocity of the cycle point and averaged over multiple cycles.

Preliminary results shows that the 500 N loading produced the largest f of 0.05 ± 0.004. The 2000 N load, which approximates daily activity, gave f = 0.036 ± 0.05 and the 3000 N load gave f = 0.013 ± 0.003. The trend was for lower f with increasing loads.

A lumbar TDR friction factor of 0.036 for a 2000N load and the reduction in f for increasing loads is comparable to the lower end of the range of values reported for THR in similar simulator studies using metal-on-polyethylene bearing materials^[3]. The 3000 N result showing that increasing the load above that expected in daily activity does not raise the f could be important when considering rotational stability and anchorage in a TDR device because frictional torque at the bearing surfaces is proportional to the product of load, device radius and f.

Background

High cup abduction angles generate increased contact stresses, higher wear rates and increased revision rates. However, there is no reported study about the influence of cup abduction on stresses under head lateralisation conditions for ceramic-on-Ceramic THA.

INTRODUCTION

Squeaking after total hip replacement has been reported in up to 10% of patients. Some authors proposed that sound emissions from squeaking hips result from resonance of one or other or both of the metal parts and not the bearing surfaces. There is no reported in vitro study about the squeaking frequencies under lubricated regime. The goal of the study was to reproduce the squeaking in vitro under lubricated conditions, and to compare the in vitro frequencies to in vivo frequencies determined in a group of squeaking patients. The frequencies may help determining the responsible part of the noise.

Introduction

Articular hyaline cartilage has a unique structural composition that allows it to endure high load, distribute load to bone and enables low friction movement in joints. A novel acellular xenogenic graft is proposed as a biological cartilage replacement, for repair of osteochondral defects. Acellular porcine cartilage has been produced using repeated freeze thaw cycles and washing using hypotonic buffers and sodium dodecyl sulphate solution (SDS; Keir, 2008). DNA content of the acellular matrix was reduced by 93.3% compared to native cartilage as measured by nanodrop spectrophotometry of extracted DNA, with a corresponding reduction in glycosaminoglycan (GAG) content.

Introduction

It is believed that wear of replacement joints vivo in is strongly dependent on input motions (kinematics) and loading. There is difficulty in accurately measuring total disc replacement (TDR) kinematics in vivo. It is therefore desirable to ascertain the sensitivity of implant wear in vitro to perturbations of the standard testing parameters. An anterior-posterior (AP) shear force input is not currently included in the present ISO and ASTM testing standards for lumbar TDRs but is known to exist in in vivo. Other joint-replacement wear tests have shown that the phasing of input motions influences the ‘cross-shear’ process of polyethylene wear. Polyethylene bearing materials do not behave linearly to axial loading changes and so the effect on wear rate is difficult to predict. The study aim was to assess the effects on wear of a ProDisc-L TDR under the following conditions: ISO 18191-1 standard inputs; an additional input AP shear; input kinematics phasing changes; axial loading changes.

Introduction

The biological response to UHMWPE particles generated by total joint replacements is one of the key causes of osteolysis, which leads to late failure of implants. Particles ranging from 0.1-1.0μm have been shown to be the most biologically active, in terms of osteolytic cytokine release from macrophages [1]. Current designs of lumbar total disc replacements (TDR) contain UHMWPE as a bearing surface and the first reports of osteolysis around TDR in vivo have appeared recently in the literature [2]. The current wear testing standard (ISO18192-1) for TDR specifies only four degrees of freedom (4DOF), i.e. axial load, flexion-extension, lateral bend and axial rotation. However, Callaghan et al. [3] described a fifth DOF, anterior-posterior (AP) shear. The aim of this study was to investigate the effect that this additional AP shear load component had on the size and morphology of the wear particles generated by ProDisc-L TDR devices over five million cycles in a spine simulator.

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.

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.

Dislocation remains one of the most common complications after total hip arthroplasty.

Precise cup position appears to be a main factor as significant variations occur for frontal and sagittal acetabular tilt and anteversion according to sitting or standing positions.

An innovative dual mobility ceramic-on-ceramic joint has been developed to solve these problems.

The dual mobility ceramic-on-ceramic joint allows to move the rotation center much deeper inside the insert in order to increase the joint stability without negative impact on the ROM. This device revealed higher torques against subluxation in comparison to the classical Al-Al systems, even with 36mm head diameters, or 41 mm metal on metal bearings.

The additional outer-bearing surface motion creates a second “adjustable acetabulum” due to the eccentration between the rotation center of the ball head and the rotation center of the bipolar head. This offset creates a resultant force that rotates the bipolar component.

Using two bearing ceramic surfaces, the intermediate component acts as a “self adjusting cup”, dealing with the variations of pelvic orientation and acetabulum anteversion.

The use of the dual mobility ceramic-on-ceramic joint seems an interesting alternative when facing difficult or unexpected situations for cup adjustment and cases with hip instability In a hip simulator in micro separation condition, the wear of the dual mobility ceramic-on-ceramic was less than 0.01 mm3/million cycles, the detection limit for wear measurement. There was no change in the surface roughness of the inserts.

The design of the joint with the mobile ceramic head prevented edge loading of the head on the edge of the cup. No stripe wear was observed.

Since 2006 more than 2000 dual mobility ceramic-on-ceramic systems have been implanted in Europe and clinical studies are conducted. The aim is to demonstrate the resistance to dislocation in primary total hip arthroplasty. Previous results over 125 patients in a prospective multicentric study show a Harris and Womac score equivalent to a standard hip prosthesis. No dislocations have been reported. No ceramic breakage or “squeaking” phenomenon appears.

Dislocation and microseparation are major causes of failure for ceramic-ceramic hip prosthesis. When no ideal solution has been found for acetabular implantation, the dual mobility ceramic-on-ceramic device is a real alternative. The exclusive design of the bipolar head give the high resistance to wear and stripe wear to the dual mobility ceramic-on-ceramic joint. Reducing the risk of dislocation and reducing wear drastically are two advantages that can place the dual mobility ceramic-on-ceramic joint as the best choice in primary Total Hip Arthroplasty. Obviously this choice applies to recurrent dislocation also.

Bioengineering reasons for increased wear and failure of metal-on-metal (MoM) bearings in hip prostheses have been described. Low wear occurs in MoM hips when the centre of the femoral head is concentric with the centre of the acetabular component and the implants are correctly positioned. Translational or rotational malpositioning of the components can lead to the contact-patch of the femoral component being displaced to the rim of the acetabular component, resulting in a ten- to 100-fold increase in wear and metal ion levels. This may cause adverse tissue reactions, loosening of components and failure of the prosthesis.

Introduction: UHMWPE wear particles induce osteolysis and loosening of total joint replacements. Much effort has been directed at reducing the wear volume of UHMWPE, such as crosslinking treatments [1]. Recently, interest in UHMWPE with vitamin E (VE) has increased due to its improved wear resistance in knee prostheses [2], as well as improved mechanical properties. The aim of this study was to culture human peripheral blood mononuclear cells (PBMNCs) with known volumes of clinically relevant wear debris from UHMWPE with and without VE in order to quantify and compare their respective biological activities.

Methods: For UHMWPE with VE, GUR1050 UHMWPE powder was mixed with VE at 0.3% (w/w) and 3% (w/w) using a screw cone mixer. The wear rates were evaluated using a six-station multidirectional pin on plate wear simulator against a smooth CoCr plate (Ra 0.01–0.03 micrometres), in 25% bovine serum, under a load of 160N and a frequency of 1 Hz. Endotoxin-free clinically relevant wear debris was generated aseptically for cell culture studies, using a single-station multidirectional pin on plate wear rig housed in a class II safety cabinet. PBMNCs were isolated from blood collected from three healthy donors then cultured with debris at particle volume (μm3) to cell number ratios of 100:1 using the agarose gel technique [3]. Cells without particles were used as the negative control, and LPS at 200 ng/ml was the positive control. Cell viability was assessed by ATP-Lite assay, and TNF-alpha, interleukin (IL)-1beta, IL-6 and IL-8 were measured by ELISA at 12 and 24 h.

Results: The 3% VE UHMWPE was found to have a higher wear rate than both the Virgin and the 0.3% VE UHMWPE, although there were no significant differences. Particle size and volume distributions were similar for all materials, with the mode of the frequency distributions being in the 0.1–1 micron size range. Cell viability was not adversely affected by any of the treatments. Cells cultured with virgin UHMWPE debris secreted significantly higher quantities (P< 0.05) of TNF-alpha compared to debris from both the 0.3% and the 3% VE UHMWPE, which released comparable levels of TNF-alpha to the cell only control group. The results for the other cytokines, IL-1beta, IL-6 and IL-8, and for the two additional donors showed similar trends as the results for TNF-alpha.

Discussion: The biological response to wear particles is strongly influenced by particle size and volume [3]. Cells cultured with wear debris of UHMWPE containing VE released very low levels of cytokines in comparison with virgin UHMWPE, even there were no significant differences in particle size. Differences in the chemical composition of the particles or different rates of protein adsorption may explain these differences. VE has anti-inflammatory properties, which may act by free radical scavenging. VE has been shown to reduce production of reactive oxygen species and pro-inflammatory cytokines such as TNF-alpha and IL-1beta from monocytes [4]. The anti-inflammatory effects of UHMWPE particles containing VE are currently being investigated.

Ceramic-on-metal (ceramic head and metal liner, COM) hip replacements have shown reduced wear in comparison to metal-on-metal (MOM) bearings. This has been attributed to reduced corrosive and adhesive wear, and differential hardness. The study assessed the performance of ceramic and metal bearings in different configurations under adverse conditions, ceramic heads on metal liners (COM) were compared to metal heads on ceramic inserts (MOC), with head on cup rim loading under micro-separation hip joint simulation.

Components used were made of zirconia-platelet toughened alumina (Biolox Delta) and CoCrMo alloy. Hip simulator testing applied a twin-peak loading cycle and walking motions with the prosthesis in the anatomical position. Testing was conducted in calf-serum for 2-million cycles. A standard simulator cycle was adapted, the head sub-luxed in the swing-phase forcing the head onto the cup rim at heel strike.

The overall mean wear rate for the MOC bearings (0.71±0.30mm3/Mc) was significantly higher than the wear rate for the COM bearings (0.09±0.025mm3/Mc). The contact of the head against the rim of the cup caused deep stripe wear on the metallic heads of the MOC bearings. This region on the head is exposed to high stress conditions and susceptible to damage in edge contact, the effect of this is increased when the cup is a harder material than the head. The wear of a metal-on-metal (MOM) couple under similar conditions was almost two-fold greater than the MOC couple (1.58mm3/Mc, Williams et al., 2006) providing further evidence of the reduced wear with COM in comparison to MOM.

The COM concept allows thin metal shells to be used with larger ceramic heads and protects against ceramic liner chipping. COM bearings are undergoing clinical trials, early data suggests reduced metal ion release in patients compared to metal-on-metal.

Ceramic-on-ceramic total hip replacements (THRs) have shown low wear volumes in standard gait hip simulator studies1. However clinical reports have indicated a variation in wear rates and formation of stripe wear on the ceramic femoral heads2. The aim of this study was to investigate the influence of different clinical conditions such as cup inclination angle and microseparation (head offset deficiency) on the wear of ceramic-on-ceramic THRs. The six station Leeds II hip joint simulator was used to investigate the wear of size 28mm ceramic-on-ceramic bearing couples. The alumina matrix composite ceramic material (AMC, Biolox Delta, CeramTec AG, Germany) was used in this study. The lubricant used was 25% bovine serum. The study was carried out for a total of five million cycles; the first two million cycles under standard gait conditions and a further three million cycles under microseparation conditions. During microseparation, a lateral movement of 0.5mm was applied to the cup relative to the head during the swing phase of the gait cycle3. Three of the cups were mounted to provide a clinical angle of 55°, which is referred to as the ‘standard’ condition; and the other three cups were mounted to provide a clinical angle of 65°, which is referred to as the ‘steep angle’ condition. These combinations provided four different testing conditions: standard, steep cup angle, microseparation, and combination of steep cup angle and microseparation conditions. Volumetric wear was determined gravimetrically and statistical analysis was performed using One Way ANOVA (significance at p< 0.05). Increasing the cup inclination angle from 55° to 65° had no significant effect on the wear rate in Biolox Delta ceramic-on-ceramic THRs under both standard (p> 0.42) and microseparation (p> 0.55) conditions. Under standard gait conditions, the mean wear rate for both cup inclination angles was very low at 0.05 mm3/million cycles. The introduction of microseparation to the standard gait cycle significantly increased the mean wear rates (p< 0.01) to 0.13 mm3/ million cycles for the ‘standard’ cup inclination angle of 55° and 0.11 mm3/million cycles for the ‘steep’ cup inclination angle of 65°. A stripe of wear on the head also formed, with corresponding superior rim wear on the cup. For comparison, the steady state wear rate of HIPed third generation alumina ceramic (Biolox Forte) under microseparation conditions was 1.3 mm3/million cycles [4]. In conclusion, increasing the cup inclination angle by 10° had no influence on the wear rate of Biolox Delta ceramic-on-ceramic bearings. The introduction of microseparation conditions significantly increased the wear rate and resulted in stripe-like wear on the femoral head, which has previously been observed on retrieved ceramic prosthesis. However, these wear rates were still low, and were ten times lower than those previously reported for Biolox Forte.

Introduction: The aim of this study was to develop a technique to decellularise a porcine cartilagebone construct with a view to using this as a biological scaffold for transplantation into human osteochondral defect as a cartilage substitute.

Methods: Decellularisation was based on a modification of the technique of Booth et al (2002). Cartilage bone matrix (n=9) were decellularised by exposing the tissue to 2 cycles of dry freeze-thaw followed 2 more cycles with the addition of hypotonic (10mM tris-HCl, pH8.0) buffer. Samples were then cycled through hypotonic buffer, followed by ionic detergent (0.1% [w/v] sodium dodecyl sulphate [SDS]) in the presence of protease inhibitors (aprotinin 10 KIU/ml) and 0.1% (w/v) ethylene diamine tetraacetic acid (EDTA). This was followed by washes in PBS with aprotinin and incubation in nuclease solution containing DNase (50U/ml) and RNase (1U/ml). Decontamination using 0.1% (v/v) peracetic acid in PBS was then incorporated to achieve disinfection of the tissue samples. Finally, samples were washed in PBS. Three decellularisation protocols were used depending on the number of hypotonic/SDS cycles: this was either done once, three or six times referred to as DC1, DC3 and DC6 respectively. Fresh & decellularised cartilage were compared histologically using haematoxylin and eosin staining, to visualize cellular content, sirius red, to visualise collagen fibres & alcian blue, to visualise glycosaminoglycans (GAG). Immunohistochemistry staining for galactose-α-1,3-galactose (α-gal), collagen I, II & VI was performed for fresh and decellularised samples. DNA assay: Genomic DNA was extracted using a DNA isolation kit for tissues (Roche Applied Sciences). Collagen and DMB sulphated sugar assay, as described by Stapleton et al. (2008), were performed to measure collagen and GAG content. The biphasic property of fresh and decellularised cartilage was determined using a pin on plate indentation test.

Results: H& E staining revealed the absence of visible whole cells. Sirius red stain gave evidence of the retention of collagen following decellularisation. In contrast, the acellular matrix showed evidence of loss of GAGs. There was no evidence of the expression of α-gal in the acellular scaffold. DNA analysis revealed the absence of genomic DNA in comparison to fresh tissues (ANOVA, p< 0.05). The decellularisation process had minimal effect on the collagen content of the cartilage. Nevertheless there was a significant difference in the sulphated sugar content of the fresh tissue when compared to the decellularised tissue (ANOVA, p< 0.05), indicating loss of 92% GAG. Biomechanical testing of decellularised tissues showed a significant change (ANOVA, p< 0.05) in comparison to the fresh cartilage.

Discussion: In conclusion this study has generated data on the production of an acellular cartilage bone matrix scaffold for use in osteochondral defect repair. To our knowledge, this is the first study that has successfully removed whole cells and α-gal from xenogeneic cartilage and bone tissue. Future studies are required to investigate methods to recellularise the acellular matrix using an appropriate cell type and mechanical conditioning and to investigate replenishing GAG loss following decellularisation.

Introduction: Nanometre sized UHMWPE particles have recently been isolated from periprosthetic tissues and hip simulator lubricants [1,2]. The biological response to UHMWPE particles of 0.1 μm and above has been well characterised, with particles in the 0.1–1.0 μm size range having the highest biological activity [3]. The purpose of the study was to determine the biological activity of nanometre-sized particles in terms of osteolytic cytokine release from primary human monocytes.

Methods: Monocytes were isolated from peripheral blood from 5 healthy donors by density gradient centrifugation over Lymphoprep. Cells were cultured using the agarose gel technique [3] at particle volume (μm3):cell number ratios of 10:1 and 100:1. The particles used were:

1 Polystyrene FITC-conjugated FluoSpheres (FS; Invitrogen) in 20 nm, 40 nm, 0.2 μm and 1.0 μm sizes.

All particles were tested for the presence of endotoxin prior to culture with cells. Cells without particles were used as a negative control and 200 ng/ml LPS was used as a positive control. Cell viability was assessed using the ATP Lite assay (Perkin Elmer) and ELISA was used to determine TNF-alpha, IL-1beta, IL-6 and IL-8 release at 3, 6, 12 and 24 h.

Results: FluoSpheres and CD had no effect on cell viability at 10 or 100:1. Clinically relevant UHMWPE particles had no effect on cell viability at 10:1, however, at 100:1 significant differences (P< 0.05) were seen at 3, 12 and 24 h for Donors 1 and 3. The 40 nm, 0.2μm and 1.0 μm FS caused significant elevation of TNF-α release at the 12 and 24 h time points at 100:1. There was no significant increase in TNF-α release for the 20 nm FS (3/5 donors). Particle volume and particle size showed correlation with cellular response, with the 20 nm FS showing the lowest biological activity. Clinically relevant UHMWPE particles and nanometre sized CD produced significantly higher quantities of TNF-alpha at 100:1. Release of interleukins IL-1beta, IL-6 and IL-8 followed a similar trend to TNF-alpha release.

Discussion: This study found that all nanometre-sized particles had the potential to provoke inflammatory cytokine release from macrophages. Particle volume and particle size played critical roles in initiating cellular responses. There was a lower particle size limit, with the 20 nm FS showing the lowest activity. Nanometre-sized polyethylene particles (CD) caused elevated TNF-α release, and since it has been shown that nanometre-sized UHMWPE particles are produced in large numbers in vivo [2], the relative contribution of these particles to osteolysis should be considered. The biological response to nanometre-sized clinically relevant UHMWPE particles is currently under investigation.

Ceramic-on-metal (COM) bearings have shown reduced wear and friction compared with metal-on-metal (MOM) bearings in-vitro. Lower wear has been attributed to a reduction in corrosive wear, smoother surfaces, improved lubrication and differential hardness reducing adhesive wear. Clinical studies have also shown reduced metal ion levels in-vivo compared with MOM bearings. The aim of this study was to examine two explanted COM bearings (one head and cup, one head only), and to assess the effect of in-vivo changes on the wear performance of the COM bearings by comparing the wear of the explanted bearings with three new COM implants in a hip wear simulator.

Two 28mm diameter COM bearings were provided for analysis. These were visually examined and surface profilometry was performed using a 2-D contacting profilometer (Form Talysurf, Taylor Hobson, UK). Scanning electron microscopy was used to image the regions of transfer on the ceramic heads, and EDX to assess the transfer composition (Philips XL30 ESEM).

Hip simulator testing was conducted for 2 million cycles (Mc) comparing the explanted bearings with three new 28mm COM bearings. Tests were performed in a Prosim simulator (SimSol, UK), 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 in the anatomical position. The lubricant was 25% (v/v) calf serum supplemented with 0.03% (w/v) sodium azide and was changed approximately every 0.33Mc. Wear was measured gravimetrically at 0.5, 1 and 2 Mc.

Regions of material transfer, identified on both ceramic explant heads, were shown to be CoCr material by EDX analysis, suggesting metallic transfer from the metal cup. Profilometry traces across metallic transfer showed comparable surface roughness measurements compared to unworn material.

The overall mean wear rate for the new COM bearings at 2Mc was 0.047 ± 0.06mm3/Mc. The mean wear rate for the explanted head articulated with a new cup was slightly lower at 0.034mm3/Mc. The mean wear rate for the explanted head and cup was highest at 0.15mm3/Mc. It was noted that the explanted head/cup had higher bedding in wear compared with the other bearings, but still significantly less than a new MOM bearing (mean bedding-in wear rate 2.03 ± 2.59 mm3/Mc). The steady-state wear was comparable with the new bearings. As the orientation of these implants in-vivo was unknown, it is proposed that the elevated wear during bedding-in of the explanted head/cup bearing may be due to the alignment of the components. The wear rates of the explanted ceramic head against a new cup were comparable with the new bearings, suggesting that the presence of metallic transfer on the ceramic head does not adversely affect the wear behaviour of COM bearings.

Introduction: Squeaking after total hip replacement has been reported in up to 10% of patients. Some authors proposed that sound emissions from squeaking hips result from resonance of one or other or both of the metal parts and not the bearing surfaces. There is no reported in vitro study about the squeaking frequencies under lubricated regime. The goal of the study was to reproduce the squeaking in vitro under lubricated conditions, and to compare the in vitro frequencies to in vivo frequencies determined in a group of squeaking patients. The frequencies may help determining the responsible part of the noise.

Methods: Four patients, who underwent THR with a Ceramic-on-Ceramic THR (Trident^®, Stryker^®) presented a squeaking noise. The noise was recorded and analysed with acoustic software (FMaster^®). In-vitro 3 alumina ceramic (Biolox Forte Ceramtec^®) 32 mm diameter (Ceramconcept^®) components were tested using a PROSIM^® hip friction simulator. The cup was positioned with a 75° abduction angle in order to achieve edge loading conditions. The backing and the cup liner were cut with a diamond saw, in order to avoid neck-head impingement and dislocation in case of high cup abduction angles. The head was articulated ± 10° at 1 Hz with a load of 2.5kN for a duration of 300 cycles. The motion was along the edge. Tests were conducted under lubricated conditions with 25% bovine serum without and with the addition of a 3rd body alumina ceramic particle (200 μm thickness and 2 mm length).

Results: Edge loading was obtained incompletely. In-vitro, no squeaking occurred under edge loading conditions. However, with the addition of an alumina ceramic 3rd body particle in the contact region, squeaking was obtained at the beginning of the tests and stopped after ~20 seconds (dominant frequency 2.6 kHz). In-vivo, recordings had a dominant frequency ranging between 2.2 and 2.4 kHz.

Discussion: For the first time, squeaking was reproduced in vitro under lubricated conditions. In-vitro noises followed edge loading and 3rd body particles and despite, the severe conditions, squeaking was intermittent and difficult to reproduce. However, squeaking is probably more difficult to reproduce because the cup was cut and the head was fixed in the simulator, preventing vibration to occur

Squeaking noises of a similar frequency were recorded in-vitro and in-vivo. The lower frequency of squeaking recorded in-vivo, demonstrates a potential damping effect of the soft tissues. Therefore, the squeaking in the patients was probably related to the bearing surfaces and modified lubrication conditions that may be due to edge loading. The determined values of frequencies may help to analyze the squeaking patients in order to determine the mechanism generating the sound.

The combination of a ceramic head articulating against a metal acetabular liner (CoM) has shown reduced metal ion levels compared with a metal-on-metal bearing (MoM) in hip simulator studies. A randomized prospective clinical trial was undertaken using CoM and MoM bearings in an otherwise identical total hip procedure. The initial clinical results were encouraging. This report comprises a further review of metal ion data.

Patients received identical components with the exception of the bearing surface material but all were 28mm diameter. All components were supplied by DePuy International Ltd. Patients were assessed pre-operatively, 3m, 12m and > 24m (median 32m). Whole blood samples were collected at regular follow-ups, frozen and analysed in batches using high resolution Inductively Coupled Plasma – Mass Spectrometry (ICP-MS). All recruited patients are included irrespective of outcome. However some patients failed to attend specific follow-ups and some contaminated samples had to be discarded. Statistical significance was analyzed using a non-parametric comparison (Mann-Whitney test). After 3m and 12m implantation there were between 21 and 24 patients available for analysis in both the CoM and MoM cohort and after > 24m point 10 and 9 respectively.

There were four outliers (either Cr or Co > 10ug/l) in both the CoM and MoM groups. In common with previous studies (with the exception of two marginal outliers), these were related to component position. They were implanted with either a cup abduction angle of > 55°, an anteversion angle of > 30° or both. Other studies with the same design of component have reported no significant outliers.

The median Cr and the Co levels are lower with the CoM bearing compared with the MoM at all measurements points following implantation. The median background (pre-operative) levels for the combined CoM and MoM group were Cr: 0.22ug/l and Co: 0.49ug/l. These were significantly different (p=0.006).

In the CoM group, the median 12m Cr and Co values were 0.43ug/l and 0.72ug/l respectively. The comparable values for MoM are 0.68ug/l and 0.83ug/l. Increases in metal ion levels from pre-operative levels are used as the primary ion level outcome in this study because the background level will comprise of the order of 30–50% of the overall value. The increase in Cr for CoM and MoM from pre-op levels to 12m significantly different for Cr (p=0.015). It has a lower significance for combined metal ion levels (p=0.029). This difference in not significant for Co (p=0.195).

In agreement with predictions from hip simulator studies, CoM bearings in this study produced lower levels of metal ions than comparable MoM bearings at all time points. However the difference is less than that predicted in the laboratory and is much more pronounced with Cr than with Co.

Introduction: There is increasing interest in the use of ceramic on ceramic bearings for hip replacement, due to recognition of their extremely low wear and biocompatibility of the wear debris [1].

The aim of this study was to investigate the influence of cup inclination angle and head position on the wear of ceramic-on-ceramic total hip replacements.

Methods: The wear of Biolox Delta alumina matrix composite ceramic (CeramTec AG, Germany) was investigated using the six station Leeds II Physiological Anatomical hip joint simulator, using 25% bovine serum as a lubricant. Three ceramic-on-ceramic bearings were mounted with the cup providing a clinical angle of 55o (representing the standard condition) and three were mounted to provide a clinical angle of 65o (representing the steep cup angle condition). Simulator studies were carried out under standard gait conditions for 2 million cycles, and under micro-separation conditions for a further 3 million cycles. Micro-separation and dynamic lateralisation of the position of the head replicate head/cup rim contact at heel strike and simulate stripe wear on a ceramic femoral head as found on ceramic-on-ceramic retrievals [2]. Volumetric wear was determined gravimetrically and statistical analysis was performed using One Way ANOVA.

Results: There was no difference in the wear rates under standard gait conditions for the standard and steep cup angles, with a wear rate of 0.05 mm3/million cycles. Under micro-separation conditions the wear rates increased significantly to 0.13 and 0.11 mm3/million cycles for the standard and steep cup angles respectively. However, there was no significant difference between the standard and steep cup angle groups.

Discussion: Micro-separation and dynamic lateralisation of the position of the head during gait simulation significantly increased wear. However, the inclination of the cup in ceramic-on-ceramic THRs did not have a significant effect on the wear under either standard gait or micro-separation conditions.