The initial stability and contact mechanics of a press-fit resurfacing arthroplasty of the hip.

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 mum was predicted for the press-fit acetabular components models, much below the 50 mum 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.

Innervation of vastus lateralis muscle.

The lateral surgical approach to the proximal femur potentially damages the nerve supply to the vastus lateralis (VL) muscle. This study describes the detailed anatomy of the nerve supply to the VL muscle based on dissection of ten cadaveric lower limbs. In all specimens, a single nerve trunk arose from the femoral nerve, which is most subsequently divided into two main divisions. These divisions gave two branches each. These branches coursed from anteriorly and proximally to posteriorly and distally within the muscle. When the muscle was reflected anteriorly from its attachment to the linea aspera, there was no damage to its innervation. Splitting of the VL in the midlateral line of the femur, however, resulted in denervation of the posterior half of the muscle. Precise knowledge of the nerve supply to the VL will help avoid iatrogenic denervation of the muscle in surgical procedures at the proximal femur through the lateral approach.

Importance of head diameter, clearance, and cup wall thickness in elastohydrodynamic lubrication analysis of metal-on-metal hip resurfacing prostheses.

The main design features of metal-on-metal (MOM) hip resurfacing prostheses in promoting elastohydrodynamic lubrication were investigated in the present study, including the femoral head diameter, the clearance, and the cup wall thickness. Simplified conceptual models were developed, based on equivalent uniform wall thicknesses for both the cup and the head as well as the support materials representing bone and cement, and subsequently used for elastohydrodynamic lubrication analysis. Both typical first- and second-generation MOM hip resurfacing prostheses with different clearances and cup wall thicknesses were considered with a fixed large bearing diameter of 50 mm, as well as a 28 mm diameter MOM total hip replacement bearing for the purpose of comparison. The importance of the head diameter and the clearance in promoting elastohydrodynamic lubrication was confirmed. Furthermore, it was also predicted that a relatively thin acetabular cup in the more recently introduced second-generation MOM hip resurfacing prostheses would be capable of improving elastohydrodynamic lubrication even further.

Hip resurfacing arthroplasty: the evolution of contemporary designs.

Metal-on-metal hip resurfacing is considered by many as the most significant recent development in hip arthroplasty. It preserves proximal femoral bone stock, optimizes stress transfer to the proximal femur, and offers inherent stability and optimal range of movement. The early results of hip resurfacing in the 1970s and 1980s were poor and the procedure was largely abandoned by the mid-1980s. The expectation that these prostheses would be easy to revise was not often fulfilled. The large diameter of the articulation combined with thin polyethylene cups or liners resulted in accelerated wear and the production of large volumes of biologically active particulate debris, leading to bone loss and implant loosening. Failure has been attributed to other factors, mainly avascular necrosis of the femoral head. However, this concern has not been confirmed by retrieval studies. The failure of early hip resurfacings was essentially a consequence of the use of inappropriate materials, poor implant design, inadequate instrumentation, and crude surgical technique. It was not an inherent problem with the procedure itself. The renaissance of metal-on-metal articulations for total hip arthroplasty enabled the introduction of new hip resurfacings and most of the major implant manufacturers have already introduced such systems. Early results are encouraging and complications commonly seen in the 1970s and 1980s, such as early implant loosening and femoral neck fracture, now appear to be rare. Whilst early results 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.

Transient elastohydrodynamic lubrication analysis of metal-on-metal hip implant under simulated walking conditions.

The transient elastohydrodynamic lubrication (EHL) analysis was performed in this study for a typical metal-on-metal bearing employing a polyethylene backing underneath a metallic cup inlay under dynamic operating conditions of load and speed representative of normal walking. A ball-in-socket configuration was adopted to represent the articulation between the femoral head and the acetabular cup. The governing Reynolds and elasticity equations were solved simultaneously by using both finite difference and finite element methods. The predicted transient film thickness from the present study was compared with the estimation based on the quasi-static analysis. It was found that the polyethylene backing employed in the typical metal-on-metal hip bearing, combined with dynamic squeeze-film action, significantly improved the transient lubricant film thickness under cyclic walking and consequently a fluid film lubrication regime was possible for smooth bearing surfaces with an average roughness less than 0.005 microm.

The development of the DuromTM metal-on-metal hip resurfacing.

The poor results of conventional total hip replacement in young and active patients have led to a renewed interest in alternative interventions and bearing material combinations. Con-temporary metal-on-metal resurfacing of the hip has emerged as an attractive option. This procedure utilises a low wear producing bearing which minimises the risk of periprosthetic osteolysis, preserves femoral bone stock should subsequent procedures become necessary, and, due to the increased stability and range of motion, allows the rapid resumption of an active lifestyle. We describe the development of the DuromTM metal-on-metal hip resurfacing and report the preliminary results of the first 100 consecutive cases.

Effect of wear of bearing surfaces on elastohydrodynamic lubrication of metal-on-metal hip implants.

The effect of geometry change of the bearing surfaces owing to wear on the elastohydrodynamic lubrication (EHL) of metal-on-metal (MOM) hip bearings has been investigated theoretically in the present study. A particular MOM Metasul bearing (Zimmer GmbH) was considered, and was tested in a hip simulator using diluted bovine serum. The geometry of the worn bearing surface was measured using a coordinate measuring machine (CMM) and was modelled theoretically on the assumption of spherical geometries determined from the maximum linear wear depth and the angle of the worn region. Both the CMM measurement and the theoretical calculation were directly incorporated into the elastohydrodynamic lubrication analysis. It was found that the geometry of the original machined bearing surfaces, particularly of the femoral head with its out-of-roundness, could lead to a large reduction in the predicted lubricant film thickness and an increase in pressure. However, these non-spherical deviations can be expected to be smoothed out quickly during the initial running-in period. For a given worn bearing surface, the predicted lubricant film thickness and pressure distribution, based on CMM measurement, were found to be in good overall agreement with those obtained with the theoretical model based on the maximum linear wear depth and the angle of the worn region. The gradual increase in linear wear during the running-in period resulted in an improvement in the conformity and consequently an increase in the predicted lubricant film thickness and a decrease in the pressure. For the Metasul bearing tested in an AMTI hip simulator, a maximum total linear wear depth of approximately 13 microm was measured after 1 million cycles and remained unchanged up to 5 million cycles. This resulted in a threefold increase in the predicted average lubricant film thickness. Consequently, it was possible for the Metasul bearing to achieve a fluid film lubrication regime during this period, and this was consistent with the minimal wear observed between 1 and 5 million cycles. However, under adverse in vivo conditions associated with start-up and stopping and depleted lubrication, wear of the bearing surfaces can still occur. An increase in the wear depth beyond a certain limit was shown to lead to the constriction of the lubricant film around the edge of the contact conjunction and consequently to a decrease in the lubricant film thickness. Continuous cycles of a running-in wear period followed by a steady state wear period may be inevitable in MOM hip implants. This highlights the importance of minimizing the wear in these devices during the initial running-in period, particularly from design and manufacturing points of view.

Steady-state elastohydrodynamic lubrication analysis of a metal-on-metal hip implant employing a metallic cup with an ultra-high molecular weight polyethylene backing.

The elastohydrodynamic lubrication (EHL) analysis was carried out in this study for a 28 mm diameter metal-on-metal hip prosthesis employing a metallic cup with an ultra-high molecular weight polyethylene (UHMWPE) backing under a simple steady state rotation representing the flexion/extension during walking. Both Reynolds and elasticity equations were coupled and solved numerically by the finite difference method. The elastic deformation was determined by means of the fast Fourier transform (FFT) technique using the displacement coefficients obtained from the finite element method. Excellent agreement of the predicted elastic deformation was obtained between the FFT technique and the conventional direct summation method. The number of grid points used in the lubrication analysis was found to be important in predicting accurate film thicknesses, particularly at low viscosities representative of physiological lubricants. The effect of the clearance between the femoral head and the acetabular cup on the predicted lubricant film thickness was shown to be significant, while the effect of load was found to be negligible. Overall, the UHMWPE backing was found not only to reduce the contact pressure as identified in a previous study by the authors (Liu et al., 2003) but also significantly to increase the lubricant film thickness for the 28 mm diameter metal-on-metal hip implant, as compared with a metallic mono-block cup.

Compartment syndrome--current trends in Scottish practice.

The results are reported of a postal survey into current trends in the management of compartment syndrome and the use of compartment pressure monitoring (CPM) within Scottish trauma units. The majority of consultants in the study felt that all patients, especially the obtunded, with suspected compartment syndrome should be diagnosed using a combination of clinical review and CPM. 73% had CPM devices available representing an increase of 27% compared with previously published UK data. 43% improvised a device using a standard CVP/Arterial-line, transducer and monitor. Marked variation in threshold pressure was noted with the majority recommending perfusion pressure (PP) of diastolic blood pressure (DBP)--intracompartmental pressure (ICP) < 30 mmHg for intervention. We have found no published evidence to suggest that CPM in itself is harmful. Although a marked variation in intervention threshold exists in the literature, we would support a perfusion pressure of < 30 mmHg as being a safe, familiar and conservative intervention threshold, particularly when used in conjunction with clinical assessment.

Contact mechanics of metal-on-metal hip implants employing a metallic cup with a UHMWPE backing.

The contact mechanics in metal-on-metal hip implants employing a cobalt chromium acetabular cup with an ultra-high molecular weight polyethylene (UHMWPE) backing were analysed in the present study using the finite element method. A general modelling methodology was developed to examine the effects of the interfacial boundary conditions between the UHMWPE backing and a titanium shell for cementless fixation, the coefficient of friction and the loading angle on the predicted contact pressure distribution at the articulating surfaces. It was found that the contact mechanics at the bearing surfaces were significantly affected by the UHMWPE backing. Consequently, a relatively constant pressure distribution was predicted within the contact conjunction, and the maximum contact pressure occurred towards the edge of the contact. On the other hand, the interfacial boundary condition between the UHMWPE backing and the titanium shell, the coefficient of friction and the loading angle were found to have a negligible effect on the contact mechanics at the bearing surfaces. Overall, the magnitude of the contact pressure was significantly reduced, compared with a similar cup without the UHMWPE backing. The importance of the UHMWPE backing on the tribological performance of metal-on-metal hip implants is discussed.

Linezolid penetration into osteo-articular tissues.

Penetration of linezolid into osteo-articular tissue and fluid was studied in 10 patients undergoing primary total knee replacement. Linezolid 600 mg 12 hourly was given orally over the 48 h before operation and intravenously 1 h before induction of anaesthesia. Mean concentrations of linezolid at 90 min after the final dose, in serum, synovial fluid, synovium, muscle and cancellous bone, assayed by HPLC, were at least twice the MIC(90) for staphylococci and streptococci. The concentrations obtained indicate good penetration of this antibiotic and support its use in the management of multidrug-resistant Gram-positive bone, joint and deep-seated soft-tissue infections.

Failure of an uncemented press-fit acetabular cup in primary total hip arthroplasty.

We reviewed fifteen primary total hip replacements performed using the uncemented, non-porous coated press-fit AcSys Shearer Cup. A modular titanium straight femoral stem with a 32mm head was used in all cases. The mean age of the group containing five males and 10 females was 66 years. Eight cups have been revised for aseptic loosening at a mean of seven years and the remaining four cups are radiologically loose. At revision surgery none of the cups showed any evidence of bony ingrowth. Our 10-year results with this cup indicate an unacceptably high failure rate of 80%. Absence of bony ingrowth and the lack of a secure locking mechanism between the polyethylene liner and the metal shell are the most important causes of failure. The use of a 32-mm diameter head and implantation of the cup in an open position contributed to this failure. Even though this cup is no longer manufactured, our experience suggests that all non-porous coated cups should be closely followed up. (Hip International 2002; 4: 371-7).

Sarcoma arising adjacent to a total hip arthroplasty.

We present the case of a 79-year-old woman who developed a high grade spindle cell sarcoma adjacent to total hip arthroplasty 13 years after the index operation. MRI scan appeared to show a direct communication between the tumour and intra medullary canal through a breach in the femoral cortex.

Candida glabrata infection after total hip arthroplasty.

Only two incidents of periprosthetic infection due to Candida glabrata have been reported in the literature. We report successful treatment of a total hip replacement infected with this rare organism by a two-stage revision and synergistic combination antifungal therapy with Amphotericin B and 5-Flucytosine.

The effect of stem geometry on stresses within the distal cement mantle in total hip replacement.

Cement-stem debonding is one of the most common reasons for failure in Total Hip Replacement (THR). Four similar THR prostheses design configurations were investigated with reference to the influence of mechanical stress occurring in a cement mantle of differing thicknesses and potentially affecting clinical performance. Non-linear finite element analysis was performed on constant cement mantle thicknesses of 1, 2, and 4 millimetres. The results obtained indicate stress levels within the cement mantle decrease with increasing cement mantle thickness. The prosthesis distal tip is shown to have particular significance. Truncation of the distal tip hemisphere to a flat profile for the fixation of a centralizer increases the cement stresses.

Prosthetic reconstruction of the femur for primary bone sarcoma.

The survival of patients and implants, complications and functional outcome were reviewed in 25 consecutive femoral endoprosthetic reconstructions for treatment of primary bone sarcoma. The diagnosis was chondrosarcoma in 11, osteosarcoma in 10, MFH in 3 and Ewing's tumour in 1. Median follow up was 64 months (34 to 219) and median age at operation was 29 years (10 to 70). Twelve remained disease free at review. One had amputation for local recurrence and another was alive with metastases. 11 patients died at a median of 13 months (5 to 128); 8 from metastatic disease and 3 from other causes. Four implants were revised, at a median of 95 months (53 to 136); two for fractures of the stem and two for aseptic loosening. Three implants had radiological evidence of loosening at a median of 43 months (34 to 49). Fourteen patients had significant complications. The median functional score using the Musculoskeletal Tumour Society system was 68%. In our experience, prosthetic reconstruction of the femur does not compromise survival, although there is a significant complication rate.