Cervical spinal arthrodesis is the standard of care for the treatment of spinal diseases induced neck pain. However, adjacent segment disease (ASD) is the primary postoperative complication, which draws great concerns. At present, controversy still exists for the etiology of ASD. Knowledge of cervical spinal loading pattern after cervical spinal arthrodesis is proposed to be the key to answer these questions. Musculoskeletal (MSK) multi-body dynamics (MBD) models have an opportunity to obtain spinal loading that is very difficult to directly measure in vivo.

In present study, a previously validated cervical spine MSK MBD model was developed for simulating cervical spine after single-level anterior arthrodesis at C5-C6 disc level. In this cervical spine model, postoperative sagittal alignment and spine rhythms of each disc level, different from normal healthy subject, were both taken into account. Moreover, the biomechanical properties of facet joints of adjacent levels after anterior arthrodesis were modified according to the experimental results. Dynamic full range of motion (ROM) flexion/extension simulation was performed, where the motion data after arthrodesis was derived from published in-vivo kinematic observations. Meanwhile, the full ROM flexion/extension of normal subject was also simulated by the generic cervical spine model for comparative purpose. The intervertebral compressive and shear forces and loading-sharing distribution (the proportions of intervertebral compressive and shear force and facet joint force) at adjacent levels (C3-C4, C4-C5 and C6-C7 disc levels) were then predicted.

By comparison, arthrodesis led to a significant increase of adjacent intervertebral compressive force during the head extension movement. Postoperative intervertebral compressive forces at adjacent levels increased by approximate 20% at the later stage of the head extension movement. However, there was no obvious alteration in adjacent intervertebral compressive force, during the head flexion movement. For the intervertebral shear forces in the anterior-posterior direction, no significant differences were found between the arthrodesis subject and normal subject, during the head flexion/extension movement. Meanwhile, cervical spinal loading-sharing distribution after anterior arthrodesis was altered compared with the normal subject's distribution, during the head extension movement. In the postoperative loading-sharing distribution, the percentage of intervertebral disc forces was further increased as the motion angle increased, compared with normal subject.

In conclusion, cervical spinal loading after anterior arthrodesis was significantly increased at adjacent levels, during the head extension movement. Cervical spine musculoskeletal MBD model provides an attempt to comprehend postoperative ASD after anterior arthrodesis from a biomechanical perspective.

INTRODUCTION

Avascular necrosis (AVN) of the femoral head (FH) initiates from biological disruptions in the bone and may progress to mechanical failure of the hip. Mechanical and structural properties of AVN bone have not been widely reported, however such understanding is important when designing therapies for AVN. Brown et al.[1] assessed mechanical properties of different regions of AVN FH bone and reported 52% reduction in yield strength and 72% reduction in elastic modulus of necrotic regions when compared to non-necrotic bone. This study aimed to characterise structural and mechanical properties of FH bone with AVN and understand the relationship between lesion volume and associated mechanical properties.

Introduction:

Backside wear has been previously reported through in-vitro and in-vivo to have a significant contribution to the total wear in rotating bearing TKRs.

The present study investigated the contribution of backside wear to the total wear in the PFC Sigma rotating platform mobile bearing TKR. In addition, the wear results were compared to the computed wear rates of the PFC Sigma fixed bearing TKR, with two different bearing materials.

There are a number of periprosthetic femoral fracture (PFF) fixation failures. In several cases the effect of fracture configuration on the performance of the chosen fixation method has been underestimated. As a result, fracture movement within the window that seems to promote callus formation has not been achieved and fixations ultimately failed.

This study tested the hypothesis that: PFF configuration and the choice of plate fixation method can be detrimental to healing.

A series of computational models were developed, corroborated against measurements from a series of instrumented laboratory models and in vivo case studies. The models were used to investigate the fixation of different fracture configurations and plate fixation parameters. Surface strain and fracture movement were compared between the constructs.

A strong correlation between the computational and experimental models was found. Computational models showed that unstable fracture configurations increase the stress on the plate fixation. It was found that bridging length plays a pivotal role in the fracture movement. Rigid fixations, where there is clinical evidence of failure, showed low fracture movement in the models (<0.05mm); this could be increased with different screw and plate configurations to promote healing.

In summary our results highlighted the role of fracture configuration in PFF fixations and showed that rigid fixations that suppress fracture movement could be detrimental to healing.

Introduction

Patella femoral joint bearings in total knee replacements have shown low wear (3.1 mm³/MC) under standard gait simulator conditions¹. However, the wear in retrieval studies have shown large variations between 1.3 to 45.2 mm³/year². Previous in vitro studies on the tibial femoral joint have shown wear is dependent on design, materials and kinematics³.

The aim of this study was to investigate the influence of the design (geometry) and shape on the wear rate of patella femoral joints in total knee replacements.

Tribology and wear of articular cartilage is associated with the mechanical properties, which are governed by the extracellular matrix (ECM). The ECM adapts to resist the loads and motions applied to the tissue. Most investigations take cartilage samples from quadrupeds, where the loading and motions are different to human. However, very few studies have investigated the differences between human and animal femoral head geometry and the mechanical properties of cartilage.

This study assessed the differences between human, porcine, ovine and bovine cartilage from the femoral head; in terms of anatomical geometry, thickness, equilibrium elastic modulus and permeability.

Diameter of porcine (3-6 months old), bovine (18-24 months old), ovine (4 years old) and human femoral heads were measured (n=6). Plugs taken out of the superior region of each femoral head and creep indentation was performed. The human femoral heads were obtained from surgery due to femoral neck fracture. Cartilage thickness was measured by monitoring the resistive force change as a needle traversed the cartilage and bone at a constant feed rate using a mechanical testing machine. The percentage deformation over time was determined by dividing deformation by thickness. A biphasic finite element model was used to obtain the intrinsic material properties of each plug. Data is presented as the mean ± 95% confidence limits. One-way ANOVA was used to test for significant differences (p < or = 0.05).

Significant differences in average femoral head diameter were observed between all animals, where bovine showed the largest femoral head. Human cartilage was found to be significantly thicker than cartilage from all quadrupedal hips. Human cartilage had a significantly larger equilibrium elastic modulus compared to porcine and bovine cartilage. Porcine articular cartilage was measured to be the most permeable which was significantly larger than all the other species. No significant difference in permeability was observed between human and the other two animals: bovine and ovine (Table 1).

The current study has shown that articular cartilage mechanical properties, thickness and geometry of the femoral heads differ significantly between different species. Therefore, it is necessary to consider these variations when choosing animal tissue to represent human.

Periprosthetic femoral fractures can occur as a complication of total hip arthroplasty and are often challenging to treat as the mechanical scenario is influenced by the presence of the metal prosthesis within the bone. This research focuses on finding the optimum fixation for transverse, Vancouver type B1 periprosthetic fractures, stabilised using locking plates and secured using screws. The aim of this study was to experimentally validate a computer model of a human femur, develop that model to represent a periprosthetic femoral fracture fixation and show how the model could be used to indicate differences between plating techniques.

In the first development stage, both a laboratory model and a finite element model were developed to evaluate the mechanical behaviour of an intact composite femur under axial loading. Axial strains were recorded along the medial length of the femur in both cases and compared to provide validation for the computational model predications. The computational intact femur model was then modified to include a cemented total hip replacement, and further adapted to include a periprosthetic fracture stabilised using a locking plate, with unicortical screws above, and bicortical screws below the transverse fracture.

For the intact femur case, the experimental and computational strain patterns correlated well with an average difference of 16%. Following the inclusion of the stem, there was a reduction in the strain in the region of the prosthesis reducing by an average of 45%. There was also a large increase in bulk stiffness with the introduction of the prosthesis. When the fracture and plate fixation were included, there was little difference in the proximal strain where the stem dominated, and the strains in the distal region were found to be highly sensitive to the distribution of the screws.

The results of this study indicate that screw configuration is an important factor in periprosthetic fracture fixation. A laboratory model of the periprosthetic facture case is now under development to further validate the computational models and the two approaches will then be used to determine optimum fixation methods for a range of clinical scenarios.

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.

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.

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: 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.

Total meniscectomy has been shown to induce osteoarthritic changes in the underlying articular cartilage(AC) and bone in the natural knee (Fairbank 1948; McDermott 2006). This indicates the meniscus plays an important protective role, providing joint congruity and distributing contact forces, hence reducing contact stress. However, no friction and wear studies have been performed on meniscectomy. The aim of this study was to study the tribological response of the medial compartmental natural knee with and without the intact meniscus, under physiological dynamic loading and motion. The effect of normal and reduced loading was investigated.

Eighteen month old bovine medial compartmental knees were used. A pendulum friction simulator (Simulation Solutions, UK) was used to apply a dynamic axial loads with peak loads of 1000N (normal) and 260N (reduced). Flexion-extension of amplitude 23degrees was applied and the experiments ran for 3600 cycles at 1Hz. Lubricant was 25% bovine serum in saline. A 9.4 Tesla MRI (Bruker) scanner and Analyze software (Mayo Clinic, US) were used to calculate wear volumes. A surface profilometer (Talysurf, Taylor-Hobson, UK) was used to measure the surface roughness of the specimen before and after the test.

Coefficient of friction was found to increase with increased loading, with and without meniscus. With meniscus intact, no wear was found on AC and contact stresses were 4.9MPa and 2.8MPa, for normal and reduced loading respectively. On removal of meniscus, friction was higher at both loading conditions and surface fibrillation found on some of the AC surfaces. Contact stresses rose to 17.2MPa and 8.6MPa for normal and reduced loading.

This study has shown for the first time, the direct elevation of the coefficient of friction, immediate surface fibrillation and biomechanical wear of AC upon removal of the meniscus. On removal of meniscus, peak stresses rose and surface damage occurred on AC surfaces. The removal of the meniscus means forces act across smaller areas and contact stresses are increased. Wear is increased due to the subsequent increase in direct solid-solid contact and loss of fluid support due to the unique biphasic nature of AC. This further supports retaining meniscus whenever possible in knee joint surgery.

Metal-on-metal hip resurfacing has been introduced recently, due to its potential advantages of biomechanics and biotribology. However, a number of problems have been identified from clinical retrievals, including significant elevation of wear when the implant is mal-positioned. Our hypothesis is that implant mal-position and micro-lateralisation can result in edge contact, leading to increases in wear. The aim of this study was to investigate the combined effect of cup position and micro-lateralisation on the contact mechanics of metal-on-metal hip resurfacing prosthesis, in particularly to identify conditions which resulted in edge contact.

Finite element (FE) method was used. A generic metal-on-metal hip resurfacing prosthesis was modelled. The bearing diameters of the femoral head and acetabular cup components were 54.49mm and 54.6mm respectively, with a diametral clearance between the head and the cup of 0.11mm. The resurfacing components were implanted into a hemi-pelvic hip joint bone model and all the materials in the FE model were assumed to be homogenous, isotropic and linear elastic (Udofia et al 2007). The FE models consisted of approximately 80,000 elements, which were meshed in I-DEAS (Version 11, EDS, USA) and solved using ABAQUS (Version 6.7-1, Dassault Systèmes). For this study, the femoral component was fixed with an inclination angle of 45° and an anteversion angle of 10°. The orientation of the acetabular cup was varied, using inclination angles of 35° and 65°, and anteversion angles between −10° to 30°. Contact at the bearing surface between the cup and femoral head was modelled using frictionless surface-based elements, simulating a fully lubricated situation, as coefficients of friction less than 0.1 would not have appreciable effects on the predicted contact mechanics. The femoral component was fixed into the femur (except the guide pin) using PMMA cement with an average thickness of approximately 1mm. The other contact interfaces in the FE model (cup/acetabulum, cement/bone and cement/femoral component) were all assumed to be rigidly bonded. The hip joint model was loaded through a fixed resultant hip joint contact force of 3200N, and was applied through medial, anterior muscle forces and subtrochanteric forces to simulate the mid-to-terminal stance phase (approximately 30% – 50%) of the gait cycle (Bergmann et al., 1993). Micro-lateralisation was modelled through displacing the femoral head laterally, up to 0.5mm, relative the centre of the cup.

Edge contact was detected once the inclination angle became greater than 65°. The effect of ante-version was to further shift the contact area towards the edge of the cup, nevertheless no edge contact was found for ante-version angles up to 25° and inclination angles below 55°. However, when the micro-lateralisation was introduced, edge contact was detected at a much smaller inclination angle. For example, even with a micro-lateralisation of 0.5 mm, edge contact occurred at an inclination angle of 45°. This study highlights the importance of surgical techniques on the contact mechanics and tribology of metal-on-metal hip resurfacing and potential outcome of these devices.

Introduction: The use of hard-on-hard hip prostheses has highlighted specific problems like the “stripe-wear” and the squeaking. Many authors have related these phenomena to a micro-separation between the cup and the head. The goal of the study was to model the hip kinematics under micro-separation regime in order to develop a computational simulator for total hip prosthesis including a joint laxity, and to use it to perform a sound analysis.

Method: A three-dimensional model of the Leeds II hip simulator was developed on ADAMS^® software. A spring was used to introduce a controlled micro-separation (less than 500 microns) during the swing phase of the walking cycle. The increase of the load during the stance phase induced a relocation of the head in the cup. Values of the medial-lateral separation predicted from the model were compared to experimental data measured using a LVDT of less than 5 microns precision. Theoretical wear path predicted from the model was compared to the literature data. The frequencies of the vibratory phenomena were determined, using the Fourier transformation.

Results: There was an excellent correlation between the theoretical prediction and the experimental measurement of the medial-lateral separation during the walking cycle (0.92). Edge-loading contact occurred during 57% of the cycle according to the model and 47% according to the experimental data. Velocity and acceleration were increased during the relocation phase in a chaotic manner, leading to vibration. The contact force according to the model had also a chaotic variation during the micro-separation phase, suggesting a chattering movement. Fourier transformation showed many frequencies in the audible area.

Discussion: A three-dimensional computational model of the kinematics of the hip after total replacement was developed and validated with an excellent precision under microseparation. It highlighted possible explanations for the squeaking that may occur during either relocation phase or edge loading.

Introduction: The goal of the study was to compare the squeaking frequencies of Ceramic-on-Ceramic THR in-vitro and in-vivo among patients who underwent THR.

Method: 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 2 alumina ceramic (Biolox Forte Ceramtec^®) 32 mm diameter (Ceramconcept^®) components were tested using a PROSIM^® hip friction simulator. The cup was positioned with a 70° abduction angle in order to achieve edge loading conditions and the head was articulated ± 10° at 1 Hz with a load of 2.5kN for a duration of 300 cycles. Tests were conducted under lubricated conditions with 25% bovine serum and with the addition of a 3rd body alumina ceramic particle (200 μm thickness and 2 mm length).

Results: In-vivo, recordings had a dominant frequency ranging between 2.2 and 2.4 kHz. 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).

Discussion and Conclusion: Squeaking noises of a similar frequency were recorded in-vitro and in-vivo. In-vitro noises followed edge loading and 3rd body particles and despite, the severe conditions, squeaking was intermittent and difficult to reproduce. The lower frequency of squeaking recorded in-vivo, demonstrates a potential damping effect of the soft tissues. No damage was observed on the components, however, the test duration was very short. Squeaking may be related to third body particles that could be generated by wear or impingement between the femoral neck and the metal back. Cup design seems to be of particular importance in noisy hip, leading to a high variability of squeaking rate according to the implants.

The use of hard-on-hard hip prostheses has highlighted specific problems like the “stripe-wear” and the squeaking. Many authors have related these phenomena to a micro-separation between the cup and the head. The goal of the study was to model the hip kinematics under micro-separation regime in order to develop a computational simulator for total hip prosthesis including a joint laxity, and to use it to perform a sound analysis.

A three-dimensional model of the Leeds II hip simulator was developed on ADAMS® software. A spring was used to introduce a controlled micro-separation (less than 500 microns) during the swing phase of the walking cycle. The increase of the load during the stance phase induced a relocation of the head in the cup. Values of the medial-lateral separation predicted from the model were compared to experimental data measured using a LVDT of less than 5 microns precision. Theoretical wear path predicted from the model was compared to the literature data. The frequencies of the vibratory phenomena were determined, using the Fourier transformation.

There was an excellent correlation between the theoretical prediction and the experimental measurement of the medial-lateral separation during the walking cycle (0.92). Edge-loading contact occurred during 57% of the cycle according to the model and 47% according to the experimental data. Velocity and acceleration were increased during the relocation phase in a chaotic manner, leading to vibration. The contact force according to the model had also a chaotic variation during the micro-separation phase, suggesting a chattering movement. Fourier transformation showed many frequencies in the audible area.

A three-dimensional computational model of the kinematics of the hip after total replacement was developed and validated with an excellent precision under micro separation. It highlighted possible explanations for the squeaking that may occur during either relocation phase or edge loading.

Large diameter metal-on-metal (MOM) bearings are becoming increasingly popular for young, active patients. Clearance is a particularly important consideration for designing MOM implants, considering historical experience of equatorial contact and high frictional torque. Lubrication theory predicts increasing the clearance will result in diminished lubrication, resulting in increased friction and wear. Clinical cases of transient squeaking in patients with resurfacing bearings have been noted in recent years, with some reporting an incidence of up to 10% between 6 months and 2 years post-implantation. This study aimed to investigate the impact of increasing clearance on the lubrication, friction and squeaking of a large diameter metal-on-metal resurfacing bearing through frictional studies.

Clinical-grade MOM implants of 55mm diameter and 100μm diametric clearance, and custom-made, 55mm bearings with diametric clearances of approximately 50μm and 200μm (DePuy International Ltd) were tested in a friction simulator. Components were inverted with a flexion-extension of ±25o applied to the head and lubricated with 25% and 100% newborn bovine serum. A peak load of 2kN, with swing-phase loads of 25N, 100N and 300N were applied.

Sound data was recorded during each friction test using a MP3 recorder and pre-amplifier. A microphone was set up at a distance of 50mm from the implant, and data recorded over a minimum of 10 seconds where sound was generated. Sound data was assessed through narrow band analysis on Frequency Master software (Cirrus Research, UK).

Lubrication was assessed by directly measuring the separation between the head and cup during the test cycle by ultrasonic methods (Tribosonics, UK). An ultrasound sensor was bonded to the back of the cup and reflection measurements were taken during the friction tests with a sampling rate of 100Hz. Using equations which related reflection coefficient to lubricant properties and thickness, values for the film thickness were calculated.

The surface replacement with the largest clearance yielded the highest friction factor for each test condition. The difference between the large clearance bearing and the smaller clearance samples was statistically significant in 25% bovine serum, the more clinically relevant lubricant (ANOVA, p< 0.05). The 50μm clearance group yielded similar results to the 100μm clearance bearing, although a slight increase in friction was observed.

Squeaking occurred during every test in the large clearance group. There was a reduced incidence of squeaking in the smaller clearances, with the lowest incidence observed in the 100μm clearance group.

The smallest separation of the head and cup was observed within the large clearance bearings. The best lubrication condition measured ultrasonically was observed within the 100μm clearance bearing. There appeared to be good correlation between friction, lubrication and the incidence of squeaking. This study suggests a large diametric clearance results in reduced lubrication, increased friction and an increased incidence of squeaking. However, there is a minimum diametric clearance that can be tolerated, as clearance must accommodate the manufacturing tolerance.