Hip resurfacing preserves proximal femoral bone stock, optimises stress transfer to the proximal femur and offers inherent stability and optimal range of movement. The early results of metal–on-polyethylene resurfacing designs were poor and the resurfacing concept was largely abandoned. Modern metal-on-metal articulations enabled the introduction of a new generation of hip resurfacings with encouraging early results. In 1997 two of the authors developed a hip resurfacing system utilizing a metal-on-metal bearing. Our study reports on the clinical and radiological outcomes of the first 200 hips that were treated with the Durom hip resurfacing at an average follow up of 4.6 years (range 3.5–6).

Between May 2001 and December 2003, 200 consecutive hip resurfacings were performed on 189 patients, using the Durom hybrid metal-on-metal system. The average age of the patients was 50 years (range 22.5 – 72.3) and 119 were male. Patients were seen at 6 weeks and at 3, 6 and 12 months and annually thereafter for clinical and radiological evaluation. Clinical results were evaluated using the Harris Hip Score. A subjective assessment of patient satisfaction was obtained and patient activity was assessed using the UCLA activity score.

No patient was lost to follow up. There were no dislocations and no femoral neck fractures. One femoral component was revised due to aseptic loosening 3.9 years postoperatively. There was one late acute haematogenous infection that was successfully treated elsewhere by debridement and retention of the prosthesis. The mean Harris Hip Score improved significantly from 46.7 preoperatively to 94.4 postoperatively. The mean HHS constituents for pain, function and motion all were significantly improved from the preoperative values of 11.9, 25.7 and 4.2 to 41.8, 43.4 and 4.8 respectively following the resurfacing procedure. The mean UCLA activity score was 7.2 indicating a relatively active patient population and 179 hips were rated as excellent by the patients. No cup was considered radiographically loose. Extensive radiologic changes were observed around the femoral stem in 2.5% of the hips, with migration of the femoral component in one case and stem demarcation in 4 cases. All 5 patients maintained excellent function and had no hip pain. Pelvic osteolysis was observed in 2 cases. Neck remodelling changes were observed in 35 hips (17.5%). Kaplan-Mayer survivorship analysis demonstrated the rate of survival of the resurfacing components to be 99.5% (95% confidence interval 98.5 to 100) with revision for any reason as the endpoint.

Early results with the Durom resurfacing system appear encouraging. Although these should be regarded with caution, modern metal-on-metal hip resurfacing potentially offers the ultimate bone preservation and restoration of function in appropriately selected young patients.

Metal-on-metal hip resurfacing arthroplasty is a conservative procedure that is becoming an increasingly popular option for young arthritic patients most likely to undergo a secondary procedure in their lifetime. The stability of the acetabular component is of particular concern in these patients who show an increased risk of failure of the cemented acetabular cups in conventional total hip replacements. The purpose of this study was to examine the initial stability of a cementless interference press-fit acetabular cup used in hip resurfacing arthroplasty and implanted into ‘normal’ versus poor quality bone. Also examined was the effect of the press-fit procedure on the contact mechanics at the cup-bone interface and between the cup and femoral head.

A finite element (FE) model of the DUROM resurfacing (Zimmer GmbH) was created and implanted anatomically into the hip joint, which was loaded physiologically through muscle and subtrochanteric forces.

The FE models included: a line-to-line, 1mm and 2mm interference press-fit cup. Also considered were two FE models based on the 1mm press-fit cups, in which the material properties of the cancellous and cortical bone tissues were reduced by 2 and 4 times, to represent a reduction in bone quality as seen with age or disease.

Increasing the cup-bone interference resulted in a sig-nificant reduction in implant micromotion. All the pressfit models showed predicted cup-bone micromotion below 50 micrometers. This would ensure adequate initial stability and encourage secondary fixation through bone in-growth. The predicted acetabular stresses were found to increase with the amount of press-fit, however, there was no suggestion of a fracture. These stresses would further contribute to securing the cup.

Reducing the bone quality showed an increase in the predicted micromotion and increased bone strain. Micromotion was below 50 micrometers, but the predicted compressive bone stresses, necessary for additional implant fixation, was reduced. This implied that poor quality bone would provide unsuitable support medium for the implant. The bearing surface contact mechanics were little affected by the amount of pressfitting.

Introduction: This study is a prospective series using a porous-coated cobalt-chromium alloy cup augmented with screw fixation for acetabular revisions.

Methods: Between August 1997 and December 2001, eighty-five consecutive cementless acetabular revisions (81 patients) were performed. The mean age at operation was 64 years. Using the AAOS classification of acetabular defects there was one type I defect, 25 type two defects and 59 type III defects. Eighty-three cases were available for review (98%) with an average follow up of 6 years 6 months (range 3 months – 110 months). Clinical outcome was measured using the Charnley Hip Score and radiological assessment by plain radiographic measurement.

Results: The Charnley Hip Score had improved from a pre-operative average of 7.52 (range 4–11) to the latest score of 14.84 (range 8 – 18). Significant cup migration occurred in only one case, which did not require revision. Sixteen cases demonstrated non-progressive radiolucent lines in one or two Delee and Charnley zones but none extended to all three zones. There were no cases of significant osteolysis. There were five dislocations none requiring re-operation; one revision was carried out for deep infection and one liner exchange at time of stem revision for subsidence. The results of Kaplan-Meier survival analysis using revision for all causes as the endpoint was 98.8% (95% confidence limits 0.964 to 1) at seventy-three months.

Discussion: This press fit porous-coated cobalt-chromium alloy cup augmented with screw fixation for acetabular revision surgery produced excellent midterm results. Changes in cup design and material should only be undertaken with consideration of such results.

Introduction: Hip resurfacing is a popular alternative to total hip arthroplasty. It aims to provide more physiological loading of the proximal femur, avoiding stress shielding and associated bone resorption. However, finite element analysis of hip resurfacing systems suggests stress shielding occurs in the femoral neck below the prosthesis and theoretically may lead to femoral neck fracture. Nevertheless, recent DEXA studies indicate femoral bone stock preservation with no evidence of stress shielding. It is not yet known what actually occurs in clinical practice. The aim of this study was to assess whether femoral neck resorption occurs in one particular type of hip resurfacing post operatively and quantify the degree of narrowing.

Methods: Femoral neck resorption of the Cormet 2000 metal-on-metal resurfacing prosthesis (Corin, UK) was assessed by measuring the prosthesis-neck ratio on postoperative radiographs. 39 prostheses (35 patients) were measured on plain AP pelvis radiographs taken immediately and mean 2 years post-operatively. Subsequent follow up radiographs were measured up to maximum 7 years follow up, mean 4.75 years.

Results: There was evidence of neck atrophy in 35/39 hips at an average of 2 years post operation, with the overall average neck narrowing from a ratio of 0.86 to 0.81 at 2 years. Average neck ratios were calculated up to 7 years post resurfacing; 25 hips measured at 3 years average ratio of 0.81, 20 at 4 years ratio 0.8, 25 at 5 years ratio 0.8, 9 at 6 years ratio 0.8, 1 at 7 years ratio 0.81.

Discussion: A degree neck resorption occurs with this resurfacing prosthesis by 2 years, however no further resorption appears to occur beyond this, to a maximum follow up of 7 years. This may indicate that early narrowing is more likely to be due to harmless bone adaption then a progressive absorption jeopardising the femoral neck.

Finite element analysis was used to examine the initial stability after hip resurfacing and the effect of the procedure on the contact mechanics at the articulating surfaces. Models were created with the components positioned anatomically and loaded physiologically through major muscle forces. Total micromovement of less than 10 μm was predicted for the press-fit acetabular components models, much below the 50 μm limit required to encourage osseointegration. Relatively high compressive acetabular and contact stresses were observed in these models. The press-fit procedure showed a moderate influence on the contact mechanics at the bearing surfaces, but produced marked deformation of the acetabular components. No edge contact was predicted for the acetabular components studied.

It is concluded that the frictional compressive stresses generated by the 1 mm to 2 mm interference-fit acetabular components, together with the minimal micromovement, would provide adequate stability for the implant, at least in the immediate post-operative situation.

Introduction: Hip simulator and clinical retrieval studies have shown that metal-on-metal (MOM) hip implants commonly have biphasic wear. An initial high wear or running-in phase is generally followed by a low wear or steady-state phase. A number of hypotheses have been put forward to explain this biphasic phenomenon, including polishing of the metallic bearing surfaces and increasing conformity between the two articulating surfaces. The purpose of the present study was to compare the wear and lubrication of MOM hip implants between the running-in and steady-state periods.

Materials and Methods: A standard 28mm Metasul^TM MOM bearing (Centerpulse Orthopedics, a Zimmer Company, Winterthur, Switzerland) was investigated. The wear testing was carried out using a 6-station AMTI hip simulator in the presence of 33% bovine serum and 67% Ringer solution (PH 7.2). The bearing surfaces of both the femoral and acetabular components were measured using a coordinate measurement machine at different stages of wear testing. The dimensional changes of the bearing surfaces due to wear were directly incorporated into the elastohydrodynamic lubrication analysis using an in-house developed code.

Results: The initial running-in period occurred during the first 1 million cycles, and little wear was observed subsequently up to 5 millions cycles. The maximum total wear depth was measured to be around 13 μm at 1 million cycles. The predicted average lubricant film thickness between the two articulating surfaces was increased from 0.024μm at the beginning, to 0.09μm at the end of the first 1 million cycles. For a given composite surface roughness of 0.03μm often quoted for the metallic bearing surfaces, such an increase in the lubricant film thickness represents a transition from a mixed to a fluid film lubrication regime.

Discussion: A large improvement in lubrication was predicted as a direct result of the running-in wear of the bearing surfaces. This was mainly due to the increased conformity between the two articulating surfaces and the decreased diametrical clearance. It was particularly noted that the improvement in lubrication after 1 million cycles was so significant that continuous fluid film lubrication was possible, leading to extremely low wear for up to 5 million cycles, and only material fatigue and start-up and stopping for wear measurements could cause a further increase in wear.

It is possible in theory to optimise the geometry of the metallic bearing surfaces, based on the worn components, to minimise the running-in wear. However, such an improvement in lubrication cannot be readily achieved because of difficulties in surgical techniques and position of the components.

Introduction: Laboratory simulator and clinical retrieval studies of metal-on-metal (MOM) total hip replacements have shown that the metallic alloy, the femoral head radius, the clearance between the acetabular cup and femoral head and the cup thickness can influence the contact mechanics, the lubrication and the wear of the articulation. MOM hip resurfacing procedures have received significant attention recently. The purpose of the present study was to compare the contact mechanics between a MOM hip resurfacing implant and a MOM total hip replacement under identical conditions.

Materials and Methods: A 50mm diameter DUROM^TM MOM hip resurfacing prosthesis and a 28mm diameter Metasul^TM MOM bearing system (Centerpulse Orthopedics, a Zimmer Company, Winterthur, Switzerland) were investigated. All implants were manufactured from wrought-forged high carbon cobalt chromium alloy (Pro-tasul 21WF^TM). The diameters of the DUROM^TM femoral head and acetabular cup were 50mm and 50.145mm respectively, and the corresponding wall thickness of the acetabular component was around 4mm. The diameters of the Metasul^TM femoral head and acetabular cup were 28mm and 28.12mm. Three-dimensional finite element models were created to simulate the contact between the bearing surfaces of both the femoral head and the acetabular cup fixed to a three dimensional anatomically positioned pelvic and femoral bone consisting of both cortical (with 1mm thickness) and cancellous regions. The load applied to both models was 3200N.

Results: The maximum contact pressure at the bearing surfaces was found to be around 22MPa for the DUROM^TM and the contact area between the femoral and acetabular components was predicted to be 237mm². For the Metasul^TM bearing under identical conditions, the maximum contact pressure and the contact area predicted were approximately 47MPa and 74mm² respectively.

Discussion: A large reduction in the contact pressure, which should improve overall tribological performances, was noted for the DUROM^TM hip resurfacing prosthesis, as compared with the Metasul^TM bearing. The main reasons for this reduction were the large diameter of the articulation and the small acetabular cup thickness of the DUROM^TM system. In contrast, the Metasul^TM bearing has a smaller head diameter, and relies on a polyethylene backing underneath the metallic cup inlay to reduce the contact pressure at the articulating surfaces.

Femoral revision total hip arthroplasty is a technically demanding procedure, especially when there is extensive proximal femoral bone loss. Secure fixation maynot be possible to achieve if the implant relies solely on proximal metaphyseal fit. The use of a long tapered cementless stem having a microporous surface offers a strong anchorage distal to the damaged bony segment. By bypassing the compramised proximal femur, initial implant stability is achieved and a high rate of osseointegration can be expected.

We present our experience with Link MP reconstruction stem which utilises distal fixation and has a modular proximal end.. The prosthesis is made of Tilastan( Ti6AI4V Eli Titanium Alloy) with microporous texture of about 70 microns pore diameter. The distally tapered stem is angled about 3 degrees proximally to enable easy insertion of the stem. In our series there were 53 stem revisions done in 51 patients. Twenty three were males and 28 were female patients. The average age of the male patient was 66 years and that of the female was 74 years. The mean followup was 37 months( range 12–76 months.) Indications for revision were aseptic loosening in 35 cases, infected hip replacements in 8 patients and periprosthetic fracture in ten cases. Revision for infected primary arthroplasty was done in two stages with the Link MP stem inserted at the second stage.

All the proximal femoral defects were classified radiographically as described by Gustillo and Pasternak. There were 29 type 2 defects and 14 type 3 defects. The periprosthetic fractures were classified as per the Vancouver system and all the cases were type b fractures. Clinical scoring as described by d’Aubigne and Postel was used . The mean preoperative score was 3.5 while average score at the time of follow-up was 9.8. The subsidence ranged from none to 6mm. Good radiological evidence of proximal femoral restoration was seen in 44% of the cases in our series. Complications included greater trochanter fracture in 4 cases, perforation of the anterior cortex during reaming using the rasp in 2 cases and dislocation in one patient. In one case there was unacceptable penetration of the implant medially and this was subsequently treated by re-revision with a Huckstep stem. There were two intraoperative fractures which were treated with circlage wire fixation and they went on to union. None of the patients required a re-revision till date.

In conclusion , our experience with Link MP reconstruction stem has been encouraging as shown by the improved d’Aubigne & Postel scores and low rate of complications. We also observed proximal femoral restoration and high implant survivorship in our series.

Since 1977 we did implant all alumina (Al2O3) bearings total hip prostheses. A lot of data were documented concerning tissue reaction, in vivo wear behaviour, fractures, and clinical outcome. Ceramic materials retrieved at revision were analysed. In some cases, wear was as low as a few microns for a 15-year period in use. This is two thousand times less than a regular metal on polyethylene sliding couple. and 100 times less than a metal on metal prosthesis. Fracture mechanism is related to crack propagation into the material. During the first period, the fracture rate was in the range of 2%; it then dropped to less than 0.1 %. Few fractures could not be explained by technical or design mistakes. Clinical outcome: More than 4000 total hips in selected young and /or active patients were implanted. In a recently reported series of consecutive patients operated by P.Boutin during the year 1980, 118 hips in 106 patients were included. Mean age was 62 years. At the twenty-year follow-up evaluation, forty-five patients (fifty-one hips) were still alive and had not been revised, twenty-five patients (twenty-five hips) had undergone revision of either or both components, twenty-seven patients (thirty hips) had died from unrelated causes, and nine patients (twelve hips) were lost to follow-up. The mean Merle d’Aubigné hip score was 16.2 ± 1.8 at the latest follow-up. Survival of the cup at twenty years with revision for any reason as the end-point was 85.6 percent for cementless cups versus 61.2 % for cemented cups, respectively. Survival of the stem at twenty years with revision for any reason as the end-point was 84.9 % for cementless stems versus 87.3% for cemented stems. Wear of the prosthetic components was undetectable on plain radiographs. No fracture of the alumina socket or head was recorded. Another study concerned a more recent design of the socket which consisted in a metal back titanium alloy shell covered with a pure titanium mesh with an alumina liner. The nine year survival rate was 98.4% with revision for aseptic loosening as the end point. Conclusion This alumina on alumina bearing provides interesting results without any physical limitation specially in young and active patients.

Metal-on-metal (MOM) bearings for artificial hip joints have attracted significant attention recently as a way of reducing wear and consequently wear particle induced periprosthetic osteolysis, which is the major cause of failure. One of the most widely used MOM total hip implants is the Metasul system (Zimmer GmbH), in which a thick polyethylene backing is used underneath the metallic inlay. The purpose of this study was to investigate the effect of the polyethylene backing on the transient lubrication under dynamic loading and velocity conditions representative of walking. A 28mm diameter Metasul bearing was analysed, and the predicted lubricant film thickness was compared with that for an all metallic cup. The predicted transient lubricant film thickness in the Metasul system was found to be significantly greater than the corresponding prediction from the all metal cup. Therefore it was concluded that the polyethylene backing may play an important role in the lubrication and overall tribological performance of the Metasul bearing system with a diameter of 28mm.

Introduction: Infection is a serious complication of joint arthroplasty. Detection of low-grade prosthetic infection can be difficult, with major implications on the subsequent treatment, cost and patient morbidity. We evaluated the effectiveness of Polymerase Chain Reaction (PCR) in detecting infection in patients undergoing arthroplasty revision surgery.

Methods: Ninety-one consecutive patients (92 joints) undergoing revision THA or TKA were assessed prospectively. Preoperative assessment included clinical examination, blood tests and plain radiographs. At revision, tissue samples were sent for microbiology and histology. Cultures, using blood culture bottles, and PCR were performed on the synovial fluid. Diagnosis of infection relied on the surgeon’s opinion encompassing the clinical presentation, the results of various investigations and the intraoperative findings. Infected arthroplasties underwent a 2-stage revision. Post-operatively patients were followed up at regular intervals for a minimum of 2 years.

Results: Twelve (13%) joints were infected. Histology was positive for infection in 11 cases, tissue cultures were positive in 12 and PCR was positive in 32 cases. Intraoperative tissue cultures had sensitivity 0.75, specificity 0.96, positive predictive value 0.75 and negative predictive value 0.96; histology had sensitivity 0.92, specificity 1, positive predictive value 1 and negative predictive value 0.99 and PCR had a sensitivity 0.92, specificity 0.74, positive predictive value 0.34 and negative predictive value 0.98. At 2 years no patient showed evidence of infection.

Discussion: PCR is a sensitive method of diagnosing prosthetic infection but has poor specificity. False positive results may be due to contamination in theatre or in the laboratory. Positive results in apparently non-infected cases could be due to the detection of low virulence organisms, a small number of bacteria or a strong host immune response. Bacterial fragments and non-culturable forms of bacteria may also be responsible.

Conclusion: PCR was not helpful as a screening test for prosthetic infection. Cultures and histology combined with the surgeon’s clinical judgment remain the gold standard.

Aims: The purpose of the present study was to investigate the contact mechanics at the articulating surfaces in metal-on-metal hip implants. Methods: A 28mm diameter Metasul (from Sulzer Orthopedics Ltd.) was analysed in the present study. Both the femoral head and the acetabular cup were manufactured from matching cobalt chromium alloy. The cobalt chromium alloy acetabular inlay was thermo-mechanically bonded to an ultra high molecular weight polyethylene (UHMWPE) backing, which was in turn inserted into a titanium shell with a snap-þt for cementless þxation. The radial clearance between the femoral head and the acetabular cup was 60μm. Finite element method (ABAQUS 6.2) was used to model the contact at the articulating surfaces between the femoral head and the acetabular cup, under a load of 3.2kN. Results: The average contact pressure at the bearing surfaces was found to be about 45MPa. This was considerably lower than 63MPa if the UHMWPE backing was replaced by cobalt chromium alloy. It was also interesting to compare the present result with the use of a larger femoral head or a reduced clearance. In order to match the average contact pressure of 45MPa, it was found to be necessary to increase the femoral head radius to 18mm for a given radial clearance of 60μm or to decrease the radial clearance to 35μm for a þxed femoral head radius of 14mm. Conclusions: The use of an UHMWPE backing underneath a cobalt chromium alloy cup signiþcantly reduces the contact stresses experienced at the articulating surfaces in metal-in-metal hip implants.

Introduction: During total hip arthroplasty various femoral stem offsets are available and the femoral stem can be placed in either varus or valgus. The overall effect of this is to increase or decrease the functional offset at the hip joint. Many authors have investigated the effects of the functional offset upon the reconstructed hip joint. To our knowledge no studies have concentrated on the effects, if any, upon the loading and function of the knee joint. The aim of this study was to investigate the effects, if any, of reducing functional offset at the knee.

Materials and methods: To study the effects of alterations in functional offset during hip arthroplasty, a biomechanical computer model was constructed. Normal lower limb anthropometric measurements available in the literature were used within this program. The model thus constructed calculated the effect of different functional offsets upon moments about the knee in stationary standing on one leg. The model also allowed for different varus/valgus placement of the stem.

Results: Reducing prosthetic neck length reduces the moment arm created by the ground reaction force about the knee axis. Placing the stem in valgus reduces the moment arm. This reduction depends upon the length of the neck.

Discussion: In the one legged stance, the line of weight is offset in the coronal plane from the AP axis of the knee joint producing an adducting moment about the knee, balanced by tension in the lateral collateral ligament of the knee and the iliotibial tract. Any reduction in this moment arm would alter the loading of the knee, altering the value of the force on the lateral compartment of the knee. Indeed, it is possible to develop a situation where the knee moment arm creates an abducting moment about the knee, increasing the loading of the lateral compartment of the knee. This may lead to valgus malalignment of the knee.

Background: Revision hip arthroplasty in the presence of extensive proximal femoral bone loss constitutes a major challenge. Several reconstruction alternatives have been introduced to deal with the problem of severe proximal bone loss. One of the techniques widely used is bridging the bone defect with the use of a distally fixing implant. The Link- MP reconstruction prosthesis is such a diaphyseal-fixing stem and we report on the early experience with its use.

Methods: The prosthesis has a modular design that allows restoration of hip biomechanics, femoral anteversion and leg length. It is made of a Titanium alloy with a micro-porous surface. The distal part (the stem) has a conical shape that allows cementless anchoring in the femoral diaphysis and it also has 8 or 10 longitudinal grooves, which provide rotational stability.

We report on the early results of its use, mean follow-up 30 months, (range, 9–54 months). Between 1997 and 2001, 32 hip revisions using the Link MP reconstruction prosthesis were performed in 31 patients. There were 13 females and 18 males with a mean age of 65 years (range 35 – 82). The indication for the revision operation was aseptic loosening in twenty-one cases, septic loosening in six and periprosthetic fracture in five cases. Cancellous bone allografts were used in 25 patients. Patients with proven infection were treated by a two-stage procedure.

Results: The mean Merle d’ Aubigne score increased from 6.8 preoperatively to 14.1 postoperatively and there was radiographic evidence of bone regeneration in the proximal femur in 80% of the patients. There were two intraoperative femoral fractures, which didn’t necessitate more than an extension of the partial weight bearing period and healed without complications. No stem was re-revised.

Conclusions: The early results with the use of the Link MP reconstruction prosthesis are encouraging, as shown by the improved patient functional status, the low incidence of complications and the observed proximal femoral bone regeneration.

The incidence of infection remains 1–2% after primary total joint arthroplasty and even higher after revision procedures in spite of advances in prophylactic antibiotics and clean air operating theatre environment. Detection of low-grade infection in a prosthetic joint can often be very difficult. None of the investigations available so far have 100% sensitivity and specificity. This has huge implications on the subsequent treatment, cost and patient morbidity. Revision of an unrecognized infected arthroplasty may lead to less satisfactory results in a high proportion of cases. We utilized Polymerase Chain Reaction, a molecular biology technique to detect bacterial DNA from the synovial fluid of patients undergoing revision surgery.

We prospectively assessed 70 patients undergoing revision arthroplasty (57 hips and 13 knees). Each patient was pre operatively assessed clinically and radiologically. ESR and CRP results were noted. During revision, synovial fluid and tissue cultures from capsule, bone and bone-cement interface were obtained. None of the patients received pre or intra operative antibiotics till the specimens were taken. Standard microbiology and histology study were done on tissue samples. In addition Polymerase Chain Reaction study was done on the synovial fluid. In this method, DNA is extracted from the bacterial cell, it is polymerized and finally visualized by gel electrophoresis. Post operatively patients were followed up at regular intervals.

Diagnosis of infection included correlation between clinical, radiological and laboratory investigations along with intraoperative findings, tissue culture and histology results and a period of post operative follow up (12 months to 36 months).

Six (8%) of the 70 cases that had revision arthroplasty were clinically infected. Polymerase chain reaction was positive in 25 cases, tissue cultures were positive in 5 cases and histology was positive in 5 cases for infection. PCR showed sensitivity and specificity of 83% and 69% respectively. Tissue culture showed sensitivity and specificity of 83% and 100% respectively. Histology showed sensitivity and specificity of 83% and 100% respectively.

20 out of 25 PCR positive cases did not show any clinical evidence of infection. It is unclear whether this represents contamination during surgery or in the PCR lab. Alternatively this may represent true positive PCR results in cases with low bacterial count that can be detected by ultrasonication of implant and immunofluorescence methods. PCR is more sensitive in detection of bacterial DNA. However it has low specificity and combination of tissue cultures and histology can still provide a reliable diagnosis of infection.

During total hip arthroplasty various femoral stem offsets are available. Additionally, the femoral stem can be placed in either varus or valgus. The overall effect of this is to increase or decrease the functional offset at the hip joint. To our knowledge no studies have concentrated on the effects of these variations in offset, if any, upon the loading and function of the knee joint. The aim of this study was to investigate the effects, if any, of reducing functional offset at the knee.

A computer model was constructed to study the effects of variations in functional offset in different anatomical settings.

Aseptic loosening of orthopaedic implants has a major financial impact on the Health Service. The process is thought to be caused by wear particles that are phagocytosed by macrophages and hence stimulate bone resorption via a cytokine response. Previous work suggests that factors inhibiting or enhancing bone resorption act through regulation of the OPG and RANK-L mechanism. The objective of this study was to identify the role of RANK-L and OPG within the cytokine response leading to orthopaedic implant loosening.

Ten samples of cellular membrane obtained during revision arthroplasty surgery were analysed with basic histological staining, immunohistology and polymerase chain reaction (PCR). In vitro studies were also carried out using explanted cancellous bone, to which PMMA particles were added and bone resorbing osteoclastic cells were identified by their Tartrate-Resistant Acid Phosphatase (TRAP) activity.

PCR identified the presence of OPG in all of the periprosthetic samples, with RANK-L shown in 40% of the specimens. Immunoreactivity was shown for CD3, CD68 and RANK-L. In vitro studies confirm that there is an initial burst of inflammatory cytokine activity that then subsequently plateaus.

A balance of RANK-L and OPG regulates bone resorption at the bone/implant interface of implants by stimulating a significant initial inflammatory response which leads to loosening.

During the last few years there has been renewed interest in hip resurfacing. The advantages of such prostheses include minimal bone resection and more physiological loading of the proximal femur. The purpose of this study was to investigate the stress distribution to the upper femur following a metal-on-metal hip resurfacing and the influence of a short stem on femoral bone loading.

An accurate and validated finite element (FE) model of the proximal femur was utilised. This was created from CT data of cadaveric femurs. The validation process included weighing, modal analysis, strain gauging and ultrasound material testing of the bone. The maximum elastic modulus in the principal direction was 22.9ÊGPa. The elastic moduli of the cement and implant were 1.8 and 200 GPa respectively. The joint force and 4 muscle loads were applied accordingly and adapted to the specific geometry of the bone. The load case represented the 45% position in the gait cycle, corresponding to toe-off. The hip joint force of 2.2kN, approximately 30° superior to the pole of the implant, was applied as a pressure distribution over a 60° spherical segment, modelling the large contact area of the metal-on-metal articulation. Various scenarios with and without an implant were compared.

The distribution of the von Mises stresses in the normal femur without an implant reflected the distribution of the bone’s mechanical properties: the joint load was transferred from the superior surface of the femoral head, through its centre to the dense cortical bone of the calcar and diaphysis. The presence of the resurfacing prosthesis did not significantly affect the stress distribution in the proximal femur, except for a reduction of stresses in the superior region of the femoral head. Varying the length of the stem and its fixation did not significantly affect this stress distribution. A resurfacing prosthesis without a stem resulted in more normal stresses in the superior region of the femoral head.

Compared to the normal femur without an implant the FE analysis of the resurfacing prosthesis demonstrated stress shielding in the superior region of the femoral head. This stress shielding was reduced when a resurfacing component without a stem was used. This advantage must be weighed against the disadvantage that without a stem it is more difficult to accurately position the implant and achieve a uniform cement mantle.

Therapy against Methicillin resistant Staphylococcus aureus (MRSA) infection is mainly restricted to the glycopeptide agents (vancomycin and teicoplanin), which require parenteral administration. At present oral antibiotic therapy against MRSA infection is not available. Linezolid is a recently introduced oxazolidinone synthetic antibiotic which acts by inhibiting the initiation of bacterial protein synthesis. It is effective against MRSA, glycopeptide resistant enterococci and all pneumococci irrespective of their penicillin or macrolide resistance. It has excellent oral bioavailability however, there are no data on the penetration of linezolid into osteoarticular tissues. This aim f this study was to measure the concentration of Linezolid in osteoarticular tissues after oral and intravenous administration.

Ten patients undergoing primary total knee replacements for osteoarthritis or rheumatoid arthritis were included in the study. Linezolid was given orally 600mg BD dose for 2 days prior to operation and a final IV 600mg dose 1h before induction on the day of operation. Intra-operatively at 30min after induction, blood, synovial fluid, synovium, muscle and bone samples were collected, processed and assayed for Linezolid concentration. The assay was performed by High Performance Liquid Chromatography (HPLC) method at Antimicrobial Research Laboratory, Westmead Hospital, Bristol.

High concentrations of Linezolid, above the Minimum Inhibitory Concentration (MIC≤ 4) were obtained all sites. Mean (± SD) concentrations for different tis- were: serum 23.0 (6.5) mg/L, synovial fluid 20.1 .4) mg/L, synovium 18.0 (5.6) mg/kg, muscle 18.5 (6.6) bone 8.5 (3.9) mg/kg

Treatment of Methicillin resistant Staphylococcus (MRSA) infections in bone and native or pros- joints is complex and costly. It requires prolonged parenteral and oral antibiotic combination therapy in addition to aggressive surgical debridement. The MICs Linezolid for staphylococci, pneumococci and streptococci are in a range from 0.5 to 2 mg/L whereas MIC enterococci is constant at 4mg/L. A two to six fold increased bioavailability of Linezolid was observed compared to its desired MIC. This study indicates that Linezolid penetrates osteoarticular tissues in sufficient concentration. Linezolid may prove to be an effective or intravenous therapy for serious bone and joint infections with multi-resistant gram-positive bacteria.