Introduction: The use of highly cross-linked polyethylene (HXLPE) is now commonplace for total hip arthroplasty, however there is no long-term data to support its use. Hip simulator studies suggest that the wear rate of some types of HXLPE is ten times less than conventional polyethylene (UHMWPE). The outcomes of hip simulator studies are not always reproduced in vivo and there is some evidence that HXLPE wear may increase between 5 and 7 years.

Method: A prospective double blind randomised control trial was conducted using Radiostereometric Analysis (RSA). Fifty-four subjects were randomised to receive hip replacements with either UHMWPE liners or HXLPE liners. All subjects received a cemented CPT stem and uncemented Trilogy acetabular component (Zimmer, Warsaw, IN, USA). The 3D penetration of the head into the socket was determined to a minimum of 7 years.

Results: The total liner penetration was significantly different at 7 years (p=0.005) with values of 0.33 mm (SE 0.05 mm) for the HXLPE group and 0.55 mm (SE 0.05 mm) for the UHMWPE group. The steady state wear rate from 1 year onwards was significantly lower for HXLPE (0.005 mm/yr, SE 0.007 mm/yr) than for UHMWPE (0.037 mm/yr, SE 0.009 mm/yr) (p=0.007). The direction of wear was supero-lateral.

Discussion: We have previously demonstrated that the penetration in the first year is creep-dominated, from one year onwards the majority of penetration is probably due to wear. This study confirms the predictions from hip simulator studies which suggest that the wear rate of this HXLPE approaches that of metal-on-metal and ceramic-on-ceramic articulations. HXLPE may have the potential to reduce the need of revision surgery, due to wear debris induced osteolysis. It may also enable surgeons to use larger couples, thus reducing the risk of impingement and dislocation.

Introduction: Metal on metal hip resurfacing arthroplasty(MoMHRA) is an alternative option to THR in the treatment of young adults with OA. A recognised MoMHRA complication is the development of an inflammatory pseudotumour(IP). Diagnosis is made with the aid of US and/or MRI. To-date, no radiographic indication of the presence of IP has been identified. Neck thinning is a recognised phenomenon in MoMHRA hips not associated with any adverse clinical events. Its pathogenesis is considered multi-factorial. Our aim was to establish whether excessive neck narrowing is associated with the presence of a pseudotumour.

Methods: Twenty-seven hips (26 patients) with IP confirmed clinically, radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with an asymptomatic MoMHRA cohort (Control n=60). For all patients, prosthesis-neck-ratio(PNR) was measured on plain AP pelvic radiographs post-operatively and at follow-up as previously described and validated.

Results: All IP patients (4M:23F) and all (12M:48F) but two controls had a posterior approach at the time of MoMHRA. Post-operatively, there was no difference in the PNR between the two groups (p=0.19). At an average follow up of 3.5 years (range:0.7–8.3), IP patients(mean 1.26, 1.10–1.79) had a significantly higher (p< 0.0001) PNR in comparison to their controls(mean 1.14, 1.03–1.35). Greater neck narrowing occurred in both genders. IP necks had narrowed by an average of 8% (range:3–23). The degree of neck narrowing was correlated with length of survival of implant (p=0.001).

Discussion: This study shows a strong association between IP and neck narrowing. Processes such as impingement and increased wear are considered to be involved in the pathogenesis of both IP and neck narrowing. Furthermore, the presence of an IP, could lead to altered vascularity via a mass effect and further contribute to neck narrowing. Neck narrowing in symptomatic MOMHRA patients should alert surgeons of the possible presence of IP.

Although resurfacing hip replacement (RHR) is associated with a more demanding patient cohort, it has achieved survivorship approaching that of total hip replacement. Occasional failures from femoral neck fracture, or migration and loosening of the femoral head prosthesis have been observed, the causes of which are multifactorial, but predominately biomechanical in nature. Current surgical technique recommends valgus implant orientation and reduction of the femoral offset, reducing joint contact force and the femoral neck fracture risk. Radiographic changes including femoral neck narrowing and ‘pedestal lines’ around the implant stem are present in well performing hips, but more common in failing joints indicating that loosening may involve remodelling. The importance of prosthesis positioning on the biomechanics of the resurfaced joint was investigated using finite element analysis (FEA).

Seven FE models were generated from a CT scan of a male patient: the femur in its intact state, and the resurfaced femur with either a 50mm or 52mm prosthesis head in

neutral orientation,

The fracture risk during trauma was investigated for stumbling and a sideways fall onto the greater trochanter, by calculating the volume of yielding bone. Remodelling was quantified for normal gait, as the percentage volume of head and neck bone with over 75% post-operative change in strain energy density for an older patient, and 50% for a younger patient.

Resurfacing with the smaller, 50mm prosthesis reduced the femoral offset by 3.0mm, 4.3mm and 5.1mm in varus, neutral and valgus orientations. When the 52mm head was used, the natural joint centre could be recreated rrespective of orientation, without notching the femoral neck. The 50mm head reduced the volume of yielding femoral neck bone relative to the intact femur in a linear correlation with femoral offset. When the natural femoral offset was recreated with the 52mm prosthesis, the predicted neck fracture load in stumbling was decreased by 9% and 20% in neutral and varus orientations, but remained in line with the intact bone when implanted with valgus orientation. This agrees with clinical experience and justifies currently recommended techniques. In oblique falling, the neck fracture load was again improved slightly when the femoral offset was reduced, and never fell below 97% of the natural case for the larger implant in all orientations.

Predicted patterns of remodelling stimulus were consistent with radiographic clinical evidence. Stress shielding increased slightly from varus to valgus orientation, but was restricted to the superior femoral head in the older patient. Bone densification around the stem was predicted, indicating load transfer. Stress shielding only extended into the femoral neck in the young patient and where the femoral offset was reduced with the 50mm prosthesis. The increase in remodelling correlated with valgus orientation, or reduced femoral offset. The trend would become more marked if this were to reduce the joint contact force, but there was no such correlation for the 52mm prosthesis, when the natural femoral offset was recreated. Only in extreme cases would remodelling alone be sufficient to cause visible femoral neck narrowing, i.e. patients with a high metabolism and considerably reduced femoral offset, implying that other factors including damage from surgery or impingement, inflammatory response or retinacular blood supply interruption may also be involved in femoral neck adaptation.

The results of this FEA biomechanical study justify current surgical techniques, indicating improved femoral neck fracture strength in stumbling with valgus position. Fracture risk under oblique falling was less sensitive to resurfacing. Furthermore, the results imply that reduced femoral offset could be linked to narrowing of the femoral neck; however the effects of positioning alone on bone remodelling may be insufficient to account for this. The study suggests that surgical technique should attempt to recreate the natural head centre, but still aim primarily for valgus positioning of the prosthesis, to reduce the femoral neck fracture risk.

The Acoustic Emission (AE) technique has been described as possessing ‘many of the qualities of an ideal damage-monitoring technique’, and the technique has been used successfully in recent years to aid understanding of failure mechanisms and damage accumulation in bone cement during de-bonding of the cement-metal interface fatigue loading, pre-load cracking during polymerisation and to describe and locate damage within an entire stem construct. However, most investigations to date have been restricted to in-vitro testing using surface mounted sensors. Since acoustic signals are attenuated as they travel through a material and across interfaces, it is arguable that mounting the sensors on the bone surface to investigate damage mechanisms occurring within the bone cement layer is not ideal. However, since direct access to the bone cement layer is not readily available, the bone surface is often the only practical option for sensor positioning.

This study has investigated the potential for directly embedding AE sensors within the femoral stem itself. This enables a permanent bond between the sensor and structure of interest, allows closer proximity of the sensor to the region of interest, and eliminates potential complications and variability associated with fixing the sensor to the sample. Data is collected during in-vitro testing of nominal implanted constructs, and information from both embedded and externally mounted AE sensors are compared and corroborated by microComputed Tomography (micro-CT) images taken both before and after testing.

The use of multiple AE sensors permitted the location as well as the chronology of damage events to be obtained in real time and analysed without the need for test interruption or serial sectioning of the test samples. Parametric analysis of the AE signal characteristics enabled those events likely to be associated with cracking as opposed to interfacial rubbing or de-bonding to be differentiated and it was shown that the embedded sensors gave a closer corroboration to observed damage using micro-CT and were less affected by unwanted sources of noise.

The results of this study have significant implications for the use of AE in assessing the state of total hip replacement (THR) constructs both in-vitro and potentially in-vivo. Incorporating the sensors into the femoral stem during in-vitro testing allows for greater repeatability between tests since the sensors themselves do not need to be removed and re-attached to the specimen. To date, all in-vivo studies attempting to use the AE technique to monitor the condition of any replacement arthroplasty device have used externally mounted sensors and suffered from the attenuation of acoustic information through flesh and skin. It is hypothesised that the use of directly embedded AE sensors may provide the first steps towards an in-vivo, cost effective, user friendly, non-destructive system capable of continuously monitoring the condition of the implanted construct and locating the earliest incidences of damage initiation.

Metal on metal hip resurfacing (MMHR) is a popular procedure for the treatment of osteoarthritis in young patients. Several centres have observed masses, arising from around these devices, we call these inflammatory pseudotumours. They are locally invasive and may cause massive soft tissue destruction. The aim of this study was to determine the incidence and risk factors for pseudotumours that are serious enough to require revision surgery.

In out unit, 1,419 MMHRs were performed between June 1999 and November 2008. All revisions were identified, including all cases revised for pseudotumour. Pseudotumour diagnosis was made by histological examination of samples from revision. A Kaplan-Meier survival analysis was performed, Cox regression analysis was used to estimate the independent effects of different factors.

The revision rate for pseudotumour increased with time and was 4% (95% CI: 2.2% to 5.8%) at eight years. Female gender was a strong risk factor: at eight years the revision rate for pseudotumours in men was 0.5% (95% CI 0% to 1.1%), in women over 40 it was 6% (95% CI 2.3% to 10.1%) and in women under 40 it was 25% (95% CI 7.3% to 42.9%) (p< 0.001). Other factors associated with an increase in revision rate were, small components (p=0.003) and dysplasia (p=0.019), whereas implant type was not (p=0.156).

We recommend that resurfacings are undertaken with caution in women, especially those younger than 40 years of age, but they remain a good option in men. Further work is required to understand the patho-aetiology of pseudotumours so that this severe complication can be avoided.

Aims: Reverse oblique intertrochanteric fractures (OTA/ AO 31-A3) have unique biomechanical properties that confer difficulties in obtaining stable fixation with the conventional sliding dynamic condylar screw. Recent studies have recommended the use of cephalocondylic intramedullary devices for these unstable fractures. Both the Proximal Femoral Nail (PFN) and the Gamma Nail (GN) have shown good outcome results but the results of treatment with the IMHS have not been reported in the literature.

Methods: Between 1999–2008 6724 consecutive hip fractures were treated at our institute. There were 2586 extracapsular fractures and 307 subtrochanteric fractures. 115 of the extracapsular fractures had a reverse oblique pattern and 63 of these were treated with the IMHS. We retrospectively reviewed clinical and radiological records for the reverse oblique intertrochanteric fractures treated with the IMHS. Follow-up duration ranged from 8 months to 6 years.

Results: Among the 63 patients treated with the IMHS, 56 (88%) fractures were reduced satisfactorily with only one poorly positioned hip screw in the femoral head. There were no cases of femoral shaft fracture, screw cut-out or collapse at the fracture site. The orthopaedic complications were two cases of mal-rotation, two cases of non-union, two cases of distal locking bolts backing out, and one cracked nail. 30 day mortality was 6.5%.

Conclusion: Cephalo-medullary nailing devices have been recommended for the treatment of reverse oblique intertrochanteric femoral fractures. Our clinical and radiological outcomes with the IMHS compare favourably to the results in reports where other cephalo-medullary devices have been used. Therefore we consider the IMHS a good option for the treatment of these unstable fractures.

Introduction: Metal-on-metal hip resurfacing arthroplasty (MoMHRA) has gained popularity as an alternative to THR for younger patients with osteoarthritis. A growing concern has been the association of MoMHRA with the development of inflammatory pseudotumours (IP), especially in women. These have been linked to metal-on-metal wear, which can be related to metal ion concentrations. Elevated metal wear debris levels may result from impingement, rim contact and edge loading. Head-neck ratio (HNR) is a predetermining factor for range of movement and impingement. Neck thinning is a recognised phenomenon post-MoMHRA and we have found an association of IP with increased neck thinning based on a case control study. Our aims were to identify HNR changes a hip undergoes when resurfaced and at follow up; and whether greater neck thinning at follow-up could be associated with the presence of elevated metal ions.

Methods: A cohort of 91 patients (57M:34F) with unilateral MoMHRAs were included in this study. Blood tests were obtained at a mean follow up of 3.9 years (range 1.7–7 years) and serum (Co:Cr) ion levels were measured (ICPMS). High metal ion concentrations were defined as Co> 4.1ppb and Cr> 5.2ppb. For all patients, head-neck ratio (HNR) was measured on plain anterio-posterior pelvic radiographs pre-operatively, immediately post-operatively and at follow-up.

Results: Female patients had significantly bigger HNR pre-op (mean=1.35, range:1.22–1.64) compared to males(mean=1.22, range:1.05–1.38) (p< 0.01). Immediately post-op, female HNRs (mean: 1.26, range: 1.14–1.34) were not different to male patients(mean=1.24, range=1.11–1.38) (p=0.11). At follow-up HNR was once again significantly bigger (p< 0.01) in females (mean=1.35, range: 1.21–1.49), compare to males (mean=1.27, range:1.11–1.38). HNR alterations with operation (p=0.00) and at follow-up (p< 0.01) were significantly bigger in female patients. Furthermore, there was a significant correlation between high ion levels and HNR change at follow-up for both Co (p=0.02) and Cr (p< 0.01).

Conclusion: This study identified gender-specific changes in HNR that resurfaced hips undergo, not previously documented. Female hips have greater HNR pre-operatively, compared to male hips, and appear to be biomechanically disadvantaged when resurfaced. A decrease in HNR with resurfacing could result in impingement and lead to processes, known to be more prevalent in females, such as neck thinning, increased wear and IP development. In addition, we highlight a correlation between high ion levels and greater neck thinning at follow-up. Increased neck thinning in symptomatic MoMHRA hips could be secondary to increased wear and should be investigated further radiologically for the presence of IP.

Introduction: Traditionally Short arm plaster casts have been used to treat distal radius fractures. Judging adequacy of immobilisation has never been defined. A significant proportion of these fractures loose reduction due to inadequate immobilisation. A new non-invasive external fixator technology has been introduced to address the shortcomings of plaster casts. Aim: Is the new non invasive fixator better at reducing skin device interface movement, than conventional plasters.

Materials and Methods: A prospective healthy volunteer study involving application of Short arm plaster of Paris cast, fibreglass cast and a new device Cambfix non-invasive wrist fixator with 15 forearms in each group, was undertaken. IRB approval and informed consent obtained from the volunteers. Colle’s type cast configuration was used. Displacement at the skin-cast and skin-new device interface was measured at proximal and distal ends. Maximal displacements noted immediately after application and after a specified intervals. Casts were windowed at the end of experiment and Cast index and Gap index were measured as ratios at the time of removal of casts. Statistical analysis was done using T-test and SPSS.

Results: The non-invasive Cambfix fixator showed less mean displacement at both the proximal and distal parts compared to plaster and fibreglass casts (p< 0.01). The mean gap index for the Cambfix device was 0.09, which was statistically significantly less than 0.15 and 0.14 for Plaster of Paris and fibreglass casts respectively (p< 0.01). Casts with higher gap index showed increased displacement, however cast index was less predictive of skin-cast displacement.

Conclusions: Skin-device interface movement was significantly better reduced with the Cambfix non-invasive fixator as compared with Short arm plaster of Paris and fibreglass casts. Lesser gap index is known to provide less interface movement. The Cambfix non-invasive fixator appears to achieve a better gap index more consistently. Limitations include healthy volunteer group, and relatively small numbers.

Introduction: Despite the satisfactory short-term implant survivorship of MoM hip resurfacing arthroplasty, symptomatic abnormal periprosthetic soft-tissue masses relating to the hip joint, ‘pseudotumours’, are being increasingly reported. These were found be locally destructive, requiring revision surgery in 75% of patients. Asymptomatic pseudotumours have not been previously investigated.

Methods: The aims were: (1) to investigate the prevalence of asymptomatic pseudotumours; and (2) to investigate their potential association with the level of metal ions. A total of 160 hips in 123 patients with a mean age 56 years (range 33–73) were evaluated at a mean follow-up of 61 months (range 13–88). Radiographs and OHS were assessed. Patients with a cystic or solid mass detected on the ultrasound/MRI had an aspiration or biopsy performed. Cobalt and chromium levels were analysed using Inductively-Coupled Plasma Spectrometer.

Results: Pseudotumours were found in 6 patients (5F: 1M). In 80% of bilateral cases, it was found in both sides. Histological examination showed extensive necrosis and diffuse lymphocyte infiltration. The presence of pseudotumour was associated with higher serum cobalt (9.2 μg/L vs. 1.9μg/L, p< 0.001) and chromium levels (12.0μg/L vs. 2.1μg/L, p< 0.001); higher hip aspirate cobalt (1182 μg/L vs. 86.2μg/L, p=0.003) and chromium levels (883μg/L vs. 114.8μg/L, p=0.006); and with inferior OHS (23 vs. 14 p=0.08).

Discussion: The prevalence of asymptomatic pseudotumour (5%) was higher than previously reported for the symptomatic pseudotumours (1%). There was a sixfold elevation of serum and a twelve-fold elevation of hip aspirate levels of cobalt and chromium in patients with pseudotumours. This suggests that pseudotumours may be a biological consequence of the large amount of metal debris generated in vivo. The association between pseudotumour and elevated metal ion levels might theoretically be explained by either systemic hypersensitivity responses to metal ions or local cytotoxic effects due to a high level of metal ions.

Introduction: Total hip replacement (THR) and total knee replacement (TKR) are widely accepted as effective surgical procedures to alleviate chronic joint pain and improve functional ability. Clinical evidence suggests that joint replacement results in excellent outcomes. Traditionally, reporting of outcomes has been focused on implant survivorship and surgeon based assessment of objective outcomes, such as range of motion, knee stability and radiographic results. However, because there is a discrepancy between patient and clinician ratings of health, patient-reported outcome measures have been validated to allow patients to rate their own health, thereby placing them at the centre of outcome assessment. The aim of this study was to compare the mid-term functional outcomes of TKR and THR using validated patient-reported outcome measures.

Methods: A cross-sectional postal audit survey of all consecutive patients who had a primary, unilateral THR or TKR at the Avon Orthopaedic Centre 5–8 years previously was conducted. Participants completed an Oxford hip score (OHS) or Oxford knee score (OKS). The Oxford questionnaires are self-report joint-specific measures that assess functional ability and pain from the patient’s perspective. They consist of 12 questions about pain and physical limitations experienced over the past four weeks because of the hip or knee.

Results: 1112 THR patients and 613 TKR patients returned a completed questionnaire, giving a response rate of 72%. The median OKS of 26 was significantly worse than the median OHS of 19 (p< 0.001). TKR patients experienced a poorer functional outcome than THR patients on all domains assessed by the Oxford questionnaire, independent of age. The percentage of patients reporting moderate-severe pain was two-fold greater for TKR than THR patients (26% vs 13%, respectively).

Conclusion: This survey found that TKR patients report more pain and functional limitations than THR patients at 5–8 years post-operatively, independent of age. The finding that over a quarter of TKR patients reported moderate-severe pain at 5–8 years post-operative indicates that a large proportion of people are undergoing major knee surgery that is failing to achieve its primary aim of pain relief. This raises questions about whether patient selection for TKR is appropriate. To improve patient selection, it may be necessary to have a preoperative screening protocol to identify patient factors predictive of a poor outcome after TKR. Currently, no such protocol exists and this is an area of orthopaedics requiring further research.

Introduction: Although THR can provide excellent pain relief and restore functional ability for most patients, there is a proportion of patients who experience a poor functional outcome after THR. One factor that could contribute to a poor outcome after THR is leg length discrepancy (LLD). Restoration of leg length is important in optimising hip biomechanics and LLD has several consequences for the patient, including back pain and a limp. Assessment of LLD using radiographs is time consuming and labour intensive, and therefore limits large scale studies of LLD. However, patients self-report of perceived LLD may be a useful tool to study LLD on a large scale. Therefore, the aim of this postal audit survey was to determine the prevalence of patient-perceived LLD after primary THR and its impact on mid-term functional outcomes.

Methods: A cross-sectional postal audit survey of all consecutive patients who had a primary, unilateral THR at the Avon Orthopaedic Centre 5–8 years previously was conducted. Several questions about LLD were included on the questionnaire. Firstly, patients were asked if they thought that their legs were the same length. For those who thought their legs were different lengths, they were asked if the difference bothered them, whether the difference in length leg was enough to comment upon, and whether they used a shoe raise. Participants also completed an Oxford hip score (OHS), which is a self-report measure that assesses functional ability and pain after THR, including limping

Results: 1,114 THR patients returned a completed questionnaire, giving a response rate of 73%. 329 patients (30%) reported that they thought their legs were different lengths. The median OHS for patients with a perceived LLD was 22, which was significantly worse than the OHS of 18 for patients who thought their legs were the same length (p< 0.001). Of the 329 patients with a perceived LLD, 161 patients (51%) were bothered by the difference, 65 patients (20%) thought the discrepancy was sufficient to comment upon and 101 patients (31%) used a shoe raise. 31% of patients with LLD limped most or all of the time compared to only 9% of patients without LLD.

Conclusion: In conclusion, this study found that the prevalence of perceived LLD at 5–8 years after THR was 30%. Of the patients with LLD, over 50% were bothered by the LLD and over a third used a shoe raise to equalise leg lengths. Patients with perceived LLD have a significantly poorer self-report functional outcome than those patients without LLD. It is therefore important that patients are informed pre-operatively of the high risk of LLD after THR and the associated negative impact this may have on their outcome.

Retrieval analysis offers a direct insight into in vivo wear mechanisms. However, the 3D measurement of wear patch characteristics on spherical highly reflective bearings has been difficult. An instrument based on an optical technique has been developed over the past 3 years. It is capable of scanning metallic head and cup in a single measurement, within minutes, at a resolution of 20 nm. From the cloud of 3D points obtained during scanning (typically 35,000 To 1,000,000), a 3D image of the measured part can be obtained. The associated computer program allows for sphericity, roughness, radius and local radius to be calculated, and surface maps of the 3D model can easily be plotted.

Both head and cup of two failed MoM resurfacing devices, a wear simulator test couple and intact components were analysed using the new technique. A successful McKee Farrar head (20 years in vivo) was also scanned. Results were compared with traces obtained on a Mitutoyo RA 300 roundness machine (resolution 0.01 microns).

3D maps of the bearing surfaces of MoM devices were obtained. The maximum linear wear values on heads were 2.5 microns, 99 microns 53.5 microns and 298 microns for the simulator sample, the McKee Farrar head and the two failed resurfacing devices respectively. The corresponding maximum linear wear values on cups were 11 microns, 529 microns and 645 microns for the simulator sample and the two failed resurfacing devices respectively. These results were in good agreement with results obtained on the Mitutoyo machine. Contrary to other worn samples, the two latter cups showed that the cup had worn on the edge of the bearing surface. This resulted in an oval shaped wear patch on the head. For the McKee Farrar device and the simulator device, the wear patch was away from the edge and the outline of the wear patch was circular in shape.

This novel technique has allowed for high resolution 3D mapping of the full bearing surfaces on successful McKee Farrar device and on more recent resurfacing devices. Further studies are required. However, the results suggest that component positioning is paramount to wear performance of metal on metal devices.

Aim: Lower friction in metal on metal (MOM) hip joints can reduce the wear, production of metal ions and loosening of acetabular cups. The effect of the fluid viscosity on the friction, especially in the relation to the physiological range, is still not fully investigated. The aim of this paper was to study the frictional behaviour of MOM hip joint within the human physiological fluid viscosities.

Materials and Methods: Friction measurement was carried out using a friction simulator on CoCrMo hip prostheses of 50 mm diameter with 100 and 200 micron diametral clearances. The samples were taken from a wear simulator test at 1.4, 2.3 and 3.2 million cycles. A dynamic loading of 100–2000N was applied to the femoral head with a movement of 1 Hz and +/−23 degree amplitude. Tests were performed using 25% new born calf serum which consisted of different ratios of serum and carboxymethyl cellulose with viscosities ranged from 0.0011 to 1.1 Pa s determined by a viscometer at a shear rate of 300/s.

Results: The friction of both clearances reduced with the progress of wear. In the lower range of viscosity, the friction of 100 micron clearance was lower than that of 200 micron clearance. However, when the viscosity reached the range of 0.01–0.06 Pa s, the friction of 100 micron clearance surpassed that of 200 micron clearance and this difference became wider with the increase of viscosity. During one measurement, the curves of 100 micron clearance started from low friction (≈0.05), but increased rapidly for all viscosities except the lowest of 0.001 Pa s. For the 200 micron clearance, the curves were stabilized when the viscosity was below 0.36 Pa s and low friction was observed as the viscosity increased. The friction started to increase only from 0.36 Pa s, but the gradient was less stiff compared to that of 100 micron clearance.

Discussion: Normal synovial fluid is non-Newtonian in nature with shear-rate dependent viscosity. Researchers have reported 300/s shear rate viscosities for normal, osteoarthritis, and inflammatory synovial fluids as 0.010.4, 0.0025–0.2 and 0.001–0.07 Pa s respectively. In this study, the 200 micron clearance had lower friction than that of 100 micron clearance in the majority of physiological viscosity range. Theoretical studies have suggested that smaller clearance and higher viscosity can benefit the lubrication in MOM bearings. However, this theory is valid if continuous and complete lubrication film is achieved. Small clearance and high viscosity may prevent the recovery of lubricant between cycles and cause depletion of lubricant, which can lead to direct contact of bearings and increase of friction. It is therefore concluded that the selection of clearance for MOM components should consider the human physiological fluid viscosities so that an optimal tribological performance can be achieved.

Acetabular cup orientation in hip arthroplasty is critical to prevent edge loading and impingement. Aerial alignment guides position the cup at a specified angle to the orthogonal planes, but only if the pelvis is in strict lateral-decubitus. Computer navigation can also be used to position the acetabular cup, but there are limitations associated with defining the pelvic reference plane. It can also be postulated that a fixed angle of inclination and anteversion is not suitable for every patient and every cup design. This paper describes the development and testing of instrumentation that allows patient specific acetabular cup placement without knowing the exact pelvic orientation.

Stage 1 determines the cup position during a trial reduction. A Judd nail retractor is left in the pelvis during the trial reduction. A single-use laser pointer is attached to the top of this nail, is free to move and can be locked in position. The trial acetabular cup has a handle protruding at a fixed angle from the face of the cup. At the end of this handle is another single-use laser pointer that projects a laser beam parallel to the axis of the cup onto the wall/ceiling. Keeping the handle parallel to the medio-lateral axis to control inclination angle, the leg is moved through a range of motion (ROM). The anteversion of the trial cup is adjusted until a position is found where flexion extension ROM is possible without impingement and satisfactory abduction-adduction is achieved with stability. Once this position is found, the Judd nail laser (fixed to the pelvis) is adjusted until its projected point, on the wall/ceiling, coincides with that from the trial handle. The Judd nail laser is then fixed in position, the hip dislocated and trial components removed.

Stage 2 aligns the definitive acetabular cup. The introducer has a laser pointer pointing parallel to its axis (away from the patient) and is attached to the definitive cup. The definitive cup is placed in the acetabulum and the introducer adjusted until its projected laser coincides with that from the Judd nail. The cup is then in the same orientation as determined during the trial reduction and can be impacted.

To demonstrate the accuracy of the laser alignment method, the position of the definitive cup was compared to that of the trial cup in polyurethane foam models. With the laser points projected onto an object > 2m away, the accuracy was ±2°.

To compare the laser guided instrumentation with the conventional aerial device, the ROM of the definitive cup was assessed in Sawbones resurfaced pelvis/femur models. The pelvis orientation was rotated by ±10° about the medio-lateral axis and the superio-inferior axis to investigate the effect of the pelvis being unknowingly out of lateral-decubitus. In the worst case of pelvis position, the aerial halved the required flexion and allowed double the required extension. The laser guided instrumentation maintained the physiological range of flexion/extension regardless of pelvis position and is therefore considered an improvement on current technology and a viable alternative to computer navigation.

Charcot arthropathy is a non-infective destructive process manifesting as dislocation, periarticular fracture or a combination of both, in patients with peripheral neuropathy

The classification and staging of charcot arthropathy assists us in decision making and appropriate management of these patients. Clinical presentation and diagnosis will be discussed as well as management techniques for the different stages of arthropathy. Neuropathic ulcer management, including new ideas, will be briefly covered.

Variability in femoral head preparation and high cement pressures may be associated with failure to seat femoral components during hip resurfacing. Furthermore, excessive pressures may lead to over penetration of bone by cement with resulting necrosis of the underlying bone. We designed an experimental model to test the hypothesis that partial-length pressure-relief slots made longitudinally in the proximal bone of the femoral head, without extending to the head neck junction, would allow controlled leakage of cement during initial insertion of a femoral head resurfacing component, but would then become sealed during final insertion to prevent excessive loss of cement while still allowing accurate seating of the component.

Thirty-one resurfacing femoral components were cemented onto foam femoral head models. The clearance between foam model and implant was measured to determine the minimum space available for cement. Eleven components were inserted using hand pressure alone, 20 were hammered. Pressure relief slots were prepared in 10 femoral heads. The slots, 4mm deep grooves, were made in the proximal bone only, without extending to the head-neck junction. Cement pressure inside the component was measured during insertion. Implants were sectioned after implantation in order to determine whether they had been fully seated or not. The clinical relevance of the measures taken was tested by measuring the diameter of prepared femoral heads during 20 hip resurfacing operations in order to determine the extent of variability in intra-operative femoral head preparation.

Mean intraoperative clearance between bone and implant was −0.19mm (0.11 to −0.93mm). Mean clearance between foam model and implant was −0.30mm (0.35 to −0.94mm). Full seating was obtained in 22/31 components. Of those not fully seated, all had clearance less than −0.74mm. Full seating with a clearance of less than −0.35mm was only possible when pressure relief slots had been made in the femur. The use of a pressure relief slot longer than half the femoral head length allowed full seating in 9/9 cases, compared to 13/22 without. Cement pressure obtained with a hand pressure technique was less than half that observed with hammering (20.8vs56.0psi, p=0.0009) but was not associated with failure to seat the implant if a slot was used.

Variability of the actual diameter of the femoral head prepared may be associated with difficulty in fully seating resurfacing components. The same degree of variability in the space available for cement was observed in both intra-operative and test specimens. The use of a pressure-relief slot allows full seating of resurfacing implants with hand pressure alone, thereby halving cement pressure, in an experimental model, even when clearance between implant and bone is less than optimal.

The clearance between the femoral head and the acetabular cup can significantly affect the lubrication, the wear and the lifetime of metal on metal (MOM) hip joints. The objective of this study was to compare the frictional behaviour of MOM joints with different clearance.

Two CoCrMo MOM 50mm diameter hip joints, with a small diametral clearance of 17 microns and a big diametral clearance of 212 microns, were used in this study. The friction measurement was carried on the wear patches of MOM bearings during a long-term wear simulator test. A dynamic trapezoidal-form loading cycle was applied to the femoral head with a minimum load of 100N during the swing phase and a maximum load of 2000N throughout the stance phase. A simple harmonic motion of amplitude +/−24 degree was applied to the femoral head in the flexion-extension plane with a frequency of 1 Hz. The friction torque was measured at 0, 0.8, 1.3, 1.9, 4 and 5.5 million cycles using 6 different viscosities of 25% new born calf serum.

The results show that the friction factors (f) of small clearance were generally higher than those of big clearance and this difference became wider with the progress of wear. The lower f of big clearance, especially in the lower range of Sommerfeld number (z) after 5.5 million cycles, is significant and will affect the ultimate performance of prostheses as this range has closer rheological properties to synovial fluid and represents long term wear conditions. At the same time, the friction factors were always higher every time when measured from high z to low z, although this difference became slightly smaller with the progress of wear, which indicates that there is still direct contact between the bearings. The lower friction factor when increasing z, is due to the wear and bedding-in with the progress of the measurement. It is concluded that large clearance has lower friction factor than small clearance, and full fluid film lubrication is unlikely to have developed between the MOM bearings in this study, even with a small clearance and high viscosity.

The use of navigation systems to aid in the performance of total knee replacement has become an accepted method of treatment. Previous studies have shown that by using computer aided navigation the components can be implanted with more reproducible accuracy.

We present the results of a prospective randomised trial, with ethical committee approval that was performed to compare the use of a new, two-pin system with the original three-pin system to fix the tracker to bone. There were 37 patients in the two-pin group and 31 patients in the three-pin group. Pre-operative demographic data was similar. Patients were assessed pre and post operatively clinically using the Knee Society Score, WOMAC and SF36. Radiographic assessment was performed using “The Perth CT Protocol.”

At one year there was no significant difference clinically between either group. Radiographic assessment showed no significant difference in the results of all the prosthesis variables as measured by the Perth CT Protocol, except in the femoral prosthesis absolute varus/ valgus position, with the two-pin group being more accurate. The upgraded tracker fixing system in the computer navigation system allows as accurate implantation as the earlier system with less patient morbidity.

During conventional hip arthroplasties, the diseased femur is rigidified using a metallic stem. The insertion of the stem induces a change in the stress distribution in the surrounding femur, and the bone remodels; this stress distribution is a direct result of the stem stiffness characteristics. Healthy healing of the femur requires that the bone be loaded as naturally as possible. If the bone is not loaded appropriately, it can resorb which may result in stem loosening and revision. Although current rigid metallic femoral stems are very successful, a poor stress distribution may become a critical problem for younger patients as the stem/femoral bone construct will be subjected to higher loads for longer times, and since remodelling is faster, loosening can occur earlier. Reduced stiffness stems have therefore been investigated, but early failures have been reported due to increased movements, poor initial stability and the low proximal stiffness of the stem. A novel biocompatible carbon fibre reinforced plastic (CFRP) stem has been developed in light of these past experiences1. Using a series of analytical models and experimental validation tests1, the fibre type and architecture have been tailored along and across the stem to achieve healthy bone remodelling and proximal strength of the construct. In addition, a biocompatible hydroxyapatite coating was specifically designed to enhance interface strength and stability2. The present study describes the mechanical behaviour of this novel stem with particular emphasis on the stem/bone interface. 4 static and 29 fatigue tests were performed according to ISO7206; these tests were complemented by acoustic emission monitoring to identify failure mechanisms3. A stress versus number of cycles to failure (SN) curve was obtained to describe the fatigue behaviour (i) under constant amplitude cycling at various load levels and (ii) incorporating rest periods and overloads. In addition, a mechanical test was designed to characterise the motions between the bone and the stem during sinusoidal fatigue loading (5000 cycles, 0.2–2kN, 1Hz). Two linear variable differential transformers measured the vertical and horizontal displacements at the stem/ bone interface in the proximal region. 3 tests were performed on CFRP stems and 3 on a metallic stem. The CFRP stem exceeded the standard requirements. The SN curve showed good repeatability across the loading spectrum. The inclusion of overloads/static loads during fatigue had a beneficial effect on the stem endurance. This is attributed to the development of microcracks, which dissipate the load, and to creep of the resin. The amplitude of recoverable motion observed at the interface during each load cycle was similar for both types of stem (20mm and 4mm in the horizontal and vertical directions respectively) and remained below the recommended limit4. Composite materials offer high design flexibility. This has been exploited in the development of a compliant, mechanically tailored biocompatible hip stem for femoral reconstruction, and could provide an answer to hip replacement for younger, more active patients.

The concept of a congruent and incongruent joint will be introduced and the results of various distal osteotomies will be presented. The differential diagnosis of pain around the 1st metatarsophalangeal joint will also be discussed.

Although not exclusively a complication of distal osteotomies, the correction of acquired hallux varus will also be presented.