Total Hip Resurfacing Arthroplasty Research Articles

Heterotopic Bone Formation Following Resurfacing Total Hip Arthroplasty

Heterotopic bone (HO), a rare association with total hip arthroplasty (THA), has recently been shown to be more of a problem with resurfacing hip arthroplasty (RHA). It has been speculated to be the result of greater soft tissue dissection required for this procedure. HO most commonly develops in males and patients with bilateral disease. To better understand if this problem does occur in RHA, groups of patients with RHA on one side and conventional THA on the other were evaluated. We retrospectively identified 45 patients that had RHA on one side and conventional cemented THA on the other. Follow-up has been up to 25years. HO was graded at every clinical visit using the Brooker Classification. In the RHA group, there were 32 hips without evidence of HO, ten with grade 1, and three with grades 2 or 3. In the THA group, there were 36 hips without any HO, eight with grade 1, and one with grade 2. There was no statistical difference between either types of hip arthroplasty in HO formation overall or in the development of more severe grades. Motion was not significantly affected with the more severe grades. Our data obtained from a bilateral patient model suggests that RHA does not predispose to a greater development of HO. In doing a RHA, one should be aware that there might be an increased likelihood of HO in males with bilateral disease.

Bone mineral density of the femoral neck in resurfacing hip arthroplasty

Background and purpose Resurfacing total hip arthroplasty (RTHA) may preserve the femoral neck bone stock postoperatively. Bone mineral density (BMD) may be affected by the hip position, which might bias longitudinal studies. We investigated the dependency of BMD precision on type of ROI and hip position.Method We DXA-scanned the femoral neck of 15 resurfacing patients twice with the hip in 3 different rotations: 15° internal, neutral, and 15° external. For each position, BMD was analyzed with 3 surface area models. One model measured BMD in the total femoral neck, the second model divided the neck in two, and the third model had 6 divisions.Results When all hip positions were pooled, average coefficients of variation (CVs) of 3.1%, 3.6%, and 4.6% were found in the 1-, 2-, and 6-region models, respectively. The externally rotated hip position was less reproducible. When rotating in increments of 15° or 30°, the average CVs rose to 7.2%, 7.3%, and 12% in the 3 models. Rotation affected the precision most in the model that divided the neck in 6 subregions, predominantly in the lateral and distal regions. For larger-region models, some rotation could be allowed without compromising the precision.Interpretation If hip rotation is strictly controlled, DXA can reliably provide detailed topographical information about the BMD changes around an RTHA. As rotation strongly affects the precision of the BMD measurements in small regions, we suggest that a less detailed model should be used for analysis in studies where the leg position has not been firmly controlled.

Treatment of osteoarthritis secondary to developmental dysplasia of the hip by total hip resurfacing arthroplasty.

Objective To investigate the clinical effect of total hip resurfacing arthroplasty (THRA) for treatment of osteoarthritis secondary to the developmental dysplasia of the hip (DDH). Methods From March 2005 to June 2006, 26 THRA were performed in 25 patients to treat osteoarthritis secondary to DDH. According to the Hartofilakidis classification, 22 hips were classified as dysplasia; 4 as low dislocation. Clin-ical outcomes were evaluated according to Harris score. Central-edge angle, abduction angle, stem-shaft an-gle, hip center, ratio of offset, cover rate of cup were measured radiologically. Bone mineral density was measured before and after THRA. Results The mean follow-up time of the 25 patients was 3.5 years. Using loosening or revision as the end point, the survival rate was 100%. The mean Harris hip score had improved from (46.1±6.57) points preoperative to (92.1±2.59) points at the final follow-up. Radiological measurement showed that the position of cup restored more anatomically after operation. At the sixth month after operation, the BMD ratio reduction occurred in ROI1-ROI 6, which then increased during 6 to 36 months. The BMD ratios of ROI7, ROIL, ROIM had returned to baseline value at the sixth month postoperatively and surpassed the reference by 36 months. Conclusion The clinical results of THRA for treatment of osteoarthritis sec-ondary to DDH are encouraging. The bone stock of proxiaml femur can be reserved and restored effectively. Key words: Arthroplasty,replacement,hip; Hip dislocation,congenital; Bone density

Are Radiographic Parameters and Prosthesis Size Associated with Hip Resurfacing Failure?

Metal-on-metal total hip resurfacing arthroplasty is a technically difficult procedure. Recent studies have suggested that there is a learning curve associated with this procedure with an increased risk for failures such as femoral neck fractures early in this curve. Other studies have assessed radiographic parameters and suggested that implant positioning is also predictive of prosthesis survival. The purpose of this study was 2-fold: (1) to assess whether implant positioning changed with surgeon experience; and (2) to further evaluate the role of this positioning and component sizing on implant longevity.

Does commitment to rehabilitation influence clinical outcome of total hip resurfacing arthroplasty?

BackgroundThe purpose of this study was to evaluate whether compliance and rehabilitative efforts were predictors of early clinical outcome of total hip resurfacing arthroplasty.MethodsA cross-sectional survey was utilized to collect information from 147 resurfacing patients, who were operated on by a single surgeon, regarding their level of commitment to rehabilitation following surgery. Patients were followed for a mean of 52 months (range, 24 to 90 months). Clinical outcomes and functional capabilities were assessed utilizing the Harris hip objective rating system, the SF-12 Health Survey, and an eleven-point satisfaction score. A linear regression analysis was used to determine whether there was any correlation between the rehabilitation commitment scores and any of the outcome measures, and a multivariate regression model was used to control for potentially confounding factors.ResultsOverall, an increased level of commitment to rehabilitation was positively correlated with each of the following outcome measures: SF-12 Mental Component Score, SF-12 Physical Component Score, Harris Hip score, and satisfaction scores. These correlations remained statistically significant in the multivariate regression model.ConclusionsPatients who were more committed to their therapy after hip resurfacing returned to higher levels of functionality and were more satisfied following their surgery.

Femoral component loosening after hip resurfacing arthroplasty

Before the re-introduction of the current generation of total hip resurfacing arthroplasty, component loosening and osteolysis were of great concern to the orthopaedic community. Early, mid- and long-term clinical results are encouraging, but component loosening still exists. Macroscopic, contact radiographic and histopathological analyses after undecalcified preparation of bone tissue specimens were performed. To investigate the frequency and morphological patterns of the loosening of the femoral component, we analysed a series of 190 retrieved femoral remnants that were revised for aseptic failures. Thirty-five (18.4%) hips were revised for clinical and/or radiographic loosening of the femoral component. Pseudoarthrosis (n = 17; median in situ time: 16 weeks, interquartile range [IQR]: 9 to 34), collapsed osteonecrosis (n = 5; median in situ time: 79 weeks, IQR: 63 to 97), cement-socket debonding (n = 3; median in situ time: 89 weeks, IQR: 54 to 97) and at later follow-up bone-cement loosening (n = 10; median in situ time: 175 weeks; IQR 112 to 198; p =0.005) were distinct patterns of the femoral remnant-implant loosening. Fibrocartilaginous metaplasia of interface bone trabeculae (n = 38; median in situ time: 61 weeks, IQR: 32 to 138) was strongly associated with femoral component loosening (p = 0.009). Both the trabecular hyperosteoidosis (n = 32; median in situ time: 71 weeks, IQR 50 to 129) and excessive intraosseous lymphocyte infiltration (n = 12; median in situ time: 75 weeks, IQR 51 to 98) at the bone-cement interface correlated strongly with fibrocartilaginous metaplasia (p = 0.001 and p = 0.016 respectively) and all three lesions were associated with the female gender (p = 0.021, p = 0.009, and p = 0.051). Femoral component loosening at early follow-up was mostly caused by pathological changes of the femoral remnant bone tissue: pseudoarthrosis and collapsed osteonecrosis. Fibrocartilaginous metaplasia was frequently observed in hips with femoral component loosening. The potential role of interface hyperosteoidosis as a precursor lesion of bone-cement loosening and its possible association with delayed type hypersensitivity reaction are discussed.

Top Twelve Technical Pearls for Hip Resurfacing

Total hip resurfacing arthroplasty has emerged as an alternative treatment for patients with osteoarthritis, and the use of this procedure has increased in frequency during the past 5 years. Results from high-volume centers at midterm follow-up have been similar to outcomes of conventional total hip arthroplasty. However, there are concerns about complications, such as component loosening and femoral neck fractures, particularly early in a surgeon's learning curve. The purpose of this report is to describe patient selection criteria and 12 specific surgical techniques that may improve clinical outcomes, based on the experience of a high-volume surgeon and supported by published evidence. These techniques include preoperative radiographic evaluation, adequate joint exposure, determination of acetabular component size, acetabular exposure, sufficient seating of the acetabular component, removal of osteophytes, guide pin placement as well as assessment, progressive reaming, determination of the femoral component seating level, proper cementing, and evaluation of final alignment. The understanding and implementation of these indications, contraindications, and techniques may substantially reduce the complication rates of hip resurfacing.

Resurfacing is Comparable to Total Hip Arthroplasty at Short-term Followup

Metal-on-metal total hip resurfacing arthroplasty has had excellent reported results at early to midterm followup, and some studies suggest that outcomes are comparable to conventional THA. We compared the clinical and radiographic outcomes of two closely matched groups of 54 patients who underwent resurfacing and conventional THA, respectively. Each group consisted of 36 men and 18 women who had a mean age of 52 years and a mean body mass index of 29 kg/m(2). At a minimum followup of 24 months (mean, 40 months; range, 24-60 months), the mean Harris hip scores increased similarly in both groups (from 52 to 90 points and from 50 to 91 points for the resurfacing and conventional groups, respectively). Radiographic outcomes, revision rates, complications, pain scores, and satisfaction ratings of the two groups were similar. The patients who underwent resurfacing had higher postoperative weighted activity scores than the patients who underwent conventional THA, although they had higher preoperative weighted activity scores as well. The early outcomes of resurfacing are comparable to those of conventional THA. Level III, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

Total Hip Resurfacing in Patients Who Are Sixty Years of Age or Older

Metal-on-metal total hip resurfacing arthroplasty has been associated with excellent early results in patients who are younger than sixty years of age, but it remains controversial whether this procedure is appropriate in older patients. The purpose of the present study was to compare the clinical and radiographic outcomes after hip resurfacing in two cohorts of patients: those who were sixty years of age or older, and those who were younger than sixty years of age. Between November 2002 and August 2005, thirty-five patients (forty hips) who were sixty years of age or older were managed with metal-on-metal total hip resurfacing arthroplasty. The outcomes of these patients were compared with those of 130 patients (153 hips) who were younger than sixty years of age but otherwise had similar preoperative parameters and who had resurfacing arthroplasty performed during the same time interval and by the same surgeon. We evaluated Harris hip scores, Short Form-12 scores, and complications as well as radiographic alignment and radiolucencies. At a mean follow-up time of thirty-six months, the mean Harris hip scores improved from 52 points to 94 points in the older patient cohort and from 53 points to 92 points in the younger patient cohort. The final Short Form-12 scores of the two groups were also similar. Two patients who were sixty years or older and five of the younger patients required conversion to a conventional total hip arthroplasty. Femoral neck fracture was the reason for one conversion in each group. There were no impending radiographic failures in either cohort. Although national registries indicate that the risk of femoral neck fracture is higher in older patients, the present study found that these patients had excellent clinical outcomes that were similar to those of patients who were younger. We await longer follow-up results to determine further outcomes in these patients.

Characterization of the Running-in Period in Total Hip Resurfacing Arthroplasty: An in Vivo and in Vitro Metal Ion Analysis

Metal-on-metal total hip resurfacing arthroplasty is increasingly being performed in young and active patients. Preclinical in vitro testing of implants is usually performed with use of hip simulators, and the serum metal ion concentration is determined for the purpose of monitoring the patients. The goal of this study was to characterize the early running-in period in vivo and in vitro by characterizing metal ion levels. A well-functioning total hip resurfacing prosthesis was implanted in fifteen consecutive patients, and the serum metal ion concentrations in these patients were then determined preoperatively and at intervals during the first postoperative year (at one, six, twelve, twenty-four, and fifty-two weeks). The number of walking cycles was measured with use of a computerized accelerometer in order to compare walking cycles to hip simulator cycles. In vitro, five similar components were investigated for 3 million cycles with use of a hip simulator. Serum samples were obtained at different time points, and wear was measured by quantifying wear particles and ions in the samples. All patient and simulation serum samples were analyzed with use of inductively coupled plasma-mass spectrometry. One simulator implant was investigated with use of scanning electron microscopy. The serum chromium and cobalt levels of the patients continuously increased during the first six months and showed an insignificant decrease thereafter. The molybdenum concentration was unchanged compared with preoperative values. In contrast, the simulator measurements showed a different wear pattern with a high-wear running-in period and a low-wear steady-state phase. The running-in period was delayed by 300,000 cycles and lasted up to 1 million cycles. Scanning electron microscopic analysis showed a carbon-rich protein film predominantly in the early phases of simulation. Scratches were detected originating from pits filled with aluminum oxide and silicon oxide and from pulled-out carbides that were causing third-body wear. The simulator study allowed an exact characterization of the running-in period and showed a delayed onset of running-in wear. In contrast, the clinical data showed a slow increase in measured ion concentrations. These different wear patterns are probably due to the effects of distribution, accumulation, and excretion of particles and ions in vivo.

Results of hip resurfacing for developmental dysplasia of the hip of Crowe type I and II

Recently, the new generation of metal-on-metal total hip resurfacing arthroplasty is well known for preserving the proximal femoral bone stock, minimizing the risk of postoperative dislocation using large femoral heads, and expecting low wear of metal-on-metal articulation for longer prosthesis survival. It also has the advantage in biomechanical loading in the proximal femur. The osteoarthritis secondary to developmental dysplasia of the hip (DDH) has been the most common reason for total hip arthroplasty. Most of the patients are young and active, who require improved range of motion of the hip besides relief of the pain, even expect to resume the ability to run and jump after the joint arthroplasty, thus to be allowed an active lifestyle. The objective of the current study was to evaluate the early outcome of resurfacing arthroplasty for the mild DDH cases (Crowe type I and II). Between September 2005 and May 2007, twenty-one consecutive patients (twenty-six hips) with the diagnosis of osteoarthritis secondary to DDH underwent metal-on-metal resurfacing arthroplasty. The average age at the time of surgery was 46.5 years (range, 37-59 years). Six patients (28.6%) were male and fifteen (71.4%) were female. Clinical and radiographic results were observed. The follow-up was performed at 6 weeks, 3, 6, 9 months and then yearly. All patients were followed for a mean of 18 months (9-29 months). During the follow-up period no complications, such as dislocation of hip joints, infection or femoral neck fracture occurred. The clinical outcomes, as rated with the Harris hip score, improved significantly compared with the preoperative ratings. The mean postoperative Harris hip score was 90.7, compared to 35.5 preoperatively. The radiographic analysis showed that all prostheses were fixed with no radiolucencies. All of the patients who had equal limb lengths preoperatively had equal lengths postoperatively. Of the nine patients with preoperative limb-length discrepancy of 0.8 to 1.2 cm, all regained equal limb length postoperatively. In addition the pain was nearly completely relieved, the range of motion was remarkably improved and no restriction was needed after operation regarding early exercise. The new generation of metal-on-metal resurfacing arthroplasty may be a reasonable option for DDH of the Crowe types I and II.

Influence of bone density on total hip resurfacing arthroplasty in patients with osteonecrosis of the femoral head – A radiological analysis

Osteonecrosis of the femoral head is a more progressive disease for which many patients will eventually require total hip arthroplasty. With the advent of improved metal-on-metal prostheses, total hip resurfacing arthroplasty has emerged as a viable treatment option. However, it remains controversial whether this procedure should be used in patients with osteonecrosis when the femoral resurfacing component is implanted onto non-vital bone. In this study the bone mineral density (BMD) of osteonecrotic femoral heads were compared with the BMD of osteoarthrotic femoral heads. The purpose of this study was to analyse the differences between these two groups to gather information that may be useful in predicting the outcome of total hip resurfacing arthroplasty in cases of severe osteonecrosis of the femoral head. The femoral heads were classified according to the ARCO classification using MRI. For DEXA analysis the femoral heads were subdivided into three regions: the cranial femoral head (R1), the caudal femoral head (R2), and the proximal femoral neck (R3). In R3, BMD in osteonecrotic femoral heads was significantly lower than in those with osteoarthrosis and in R2 it was significantly higher. It can be assumed that a higher BMD in the caudal femoral head (R2) is caused by a "reactive interface" that occurred in this region especially in ARCO IV femoral heads. This fact, the lower BMD in the proximal femoral neck (R3) and the individual expansion of the osteonecrotic region should be considered when planning resurfacing arthroplasty for femoral head necrosis.

Surgical and Molecular Advances in Osteonecrosis: Editorial Comment

Surgical and Molecular Advances in Osteonecrosis: Editorial Comment

Painful Impingement of the Hip Joint After Total Hip Resurfacing: A Report of Two Cases

Many patients undergoing total hip resurfacing arthroplasty present with reduced anterior femoral neck concavity as a part of the degenerative process. Painful hip impingement may develop or persist after hip resurfacing when the retained femoral neck abuts against the metallic acetabular component or the anterior acetabular bony wall. We report on two cases of painful postoperative hip impingement after hip resurfacing, in one of whom anterior neck-contouring osteoplasty restored pain-free range of motion (ROM). To restore natural hip ROM, surgeons performing hip total hip resurfacing arthroplasty should aim to reproduce the normal femoral head-neck offset ratio. Femoral head-neck offset restoration can be achieved by proper femoral component positioning (especially optimal translation), by femoral neck osteoplasty or by increasing femoral component head size appropriately.

Changes of the bone mineral density in proximal femur following total hip resurfacing arthroplasty in osteonecrosis of femoral head

Total hip resurfacing arthroplasty (THRA) is being performed with increasing frequency for osteonecrosis of femoral head (ONFH). To evaluate femoral bone remodeling in ONFH after THRA and determine the impact of stem-neck angle (SNA) of inserted femoral component on bone remodeling, we monitored the changes in BMD in proximal femur in 23 patients with ONFH after surgery. Patients were divided into group A (SNA >or= 5 degrees ) and group B (SNA < 5 degrees ). The BMD was measured in seven Gruen zones and two neck zones using dual-energy X-ray absorptiometry preoperatively, then at 3, 6, 12, and 24 months after surgery. At all ROIs, the BMD decreased significantly by 3 months postoperatively. The BMD ceased to decrease and reversed by 6 months. The BMD in neck increased significantly in group A, compared with group B at 24 months. The BMD increased 2% at ROI1 at 24 months in both groups, and at ROI7, the BMD in group A reversed to baseline value by 6 months and increased 5.81% at 24 months. These findings implied that the bone stock of proximal femur in ONFH can be well reserved after total hip resurfacing arthroplasty with valgus positioning of the femoral component.

One- or two-stage bilateral metal-on-metal hip resurfacing arthroplasty

Patients considered suitable for total hip resurfacing arthroplasty often have bilateral disease. The peri-operative complications, transfusion requirements, hospital stay, outcome and costs in patients undergoing one-stage bilateral total hip resurfacing were compared with a group of patients undergoing a two-stage procedure. A total of 92 patients were included in the study, of which 37 (40%) had a one-stage and 55 (60%) had a two-stage resurfacing. There were no significant differences in age, gender, or American Society of Anaesthesiologists grade between the groups (p = 0.31, p = 0.23, p = 0.13, respectively). There were three systemic complications in the one-stage group (8.1%) and one in the two-stage group (1.8% of patients; 0.9% of procedures). There was no significant difference in the complication rate (p = 0.72) or the transfusion requirements (p = 0.32) between the two groups. The one-stage group had a reduced total hospital stay of five days (95% confidence interval 4.0 to 6.9; p < 0.001), reduced length of time to completion of all surgery of five months (95% confidence interval 2.6 to 8.3; p < 0.001), and the reduced cost was 35% less than that of a two-stage procedure. However, the total anaesthetic time was significantly longer for the one-stage group (p < 0.001; 95% confidence interval 31 to 52). This study demonstrates that consideration should be given to one-stage surgery for patients with bilateral symptomatic disease suitable for metal-on-metal hip resurfacing. A one-stage procedure appears to have benefits for both the patient and the hospital without additional complications.

Use of Metal-on-Metal Total Hip Resurfacing for the Treatment of Osteonecrosis of the Femoral Head

Recently, with the advent of improved metal-on-metal prostheses, total hip resurfacing has emerged as a viable arthroplasty option. However, it remains controversial whether this procedure should be used in patients with osteonecrosis when the femoral resurfacing component is cemented onto dead bone. The purpose of this study was to analyze the clinical and radiographic outcomes of metal-on-metal total hip resurfacing arthroplasty in patients with osteonecrosis of the femoral head. In addition, this group was compared with a matched group of patients who were diagnosed as having osteoarthritis. Forty-two osteonecrotic hips that were treated with a metal-on-metal total hip resurfacing arthroplasty were studied. They were matched by gender, age, prosthesis, surgeon, and surgical approach to forty-two osteoarthritic hips that were treated with the same metal-on-metal prosthesis. In the osteonecrosis group, there were twenty-five men and eleven women, and in the osteoarthritis group, there were twenty-eight men and thirteen women. The mean age at the time of surgery was forty-two years. Patients were followed both clinically and radiographically for a mean of forty-one months. The clinical outcomes were similar for both groups, with a good or excellent outcome in thirty-nine hips (93%) with osteonecrosis and a good or excellent outcome in forty-one hips (98%) with osteoarthritis. In each of the two groups, there were two failures that required conversion to a standard total hip arthroplasty. Survivorship curves were similar for the two patient groups. The short-term results for metal-on-metal total hip resurfacing for this challenging patient population with osteonecrosis were excellent and comparable with those seen in patients with osteoarthritis. We await long-term results to see if these early results are maintained. Prognostic Level II. See Instructions to Authors on jbjs.org for a complete description of levels of evidence.

USE OF METAL-ON-METAL TOTAL HIP RESURFACING FOR THE TREATMENT OF OSTEONECROSIS OF THE FEMORAL HEAD

Background: Recently, with the advent of improved metal-on-metal prostheses, total hip resurfacing has emerged as a viable arthroplasty option. However, it remains controversial whether this procedure should be used in patients with osteonecrosis when the femoral resurfacing component is cemented onto dead bone. The purpose of this study was to analyze the clinical and radiographic outcomes of metal-on-metal total hip resurfacing arthroplasty in patients with osteonecrosis of the femoral head. In addition, this group was compared with a matched group of patients who were diagnosed as having osteoarthritis. Methods: Forty-two osteonecrotic hips that were treated with a metal-on-metal total hip resurfacing arthroplasty were studied. They were matched by gender, age, prosthesis, surgeon, and surgical approach to forty-two osteoarthritic hips that were treated with the same metal-on-metal prosthesis. In the osteonecrosis group, there were twenty-five men and eleven women, and in the osteoarthritis group, there were twenty-eight men and thirteen women. The mean age at the time of surgery was forty-two years. Patients were followed both clinically and radiographically for a mean of forty-one months. Results: The clinical outcomes were similar for both groups, with a good or excellent outcome in thirty-nine hips (93%) with osteonecrosis and a good or excellent outcome in forty-one hips (98%) with osteoarthritis. In each of the two groups, there were two failures that required conversion to a standard total hip arthroplasty. Survivorship curves were similar for the two patient groups. Conclusions: The short-term results for metal-on-metal total hip resurfacing for this challenging patient population with osteonecrosis were excellent and comparable with those seen in patients with osteoarthritis. We await long-term results to see if these early results are maintained. Level of Evidence: Prognostic Level II. See Instructions to Authors on jbjs.org for a complete description of levels of evidence.

Exposure to chromium, cobalt and molybdenum from metal-on-metal total hip replacement and hip resurfacing arthroplasty

Background All metal implants—and metal-on-metal bearings in particular—corrode and cause a release of metal ions. Because cobalt and chromium have been shown to be carcinogenic and mutagenic in human and animal models, systemic toxicity and cancer risk are considered to be possible disadvantages of the metal-on-metal articulation.This study was designed to investigate the serum concentration profiles of chromium, cobalt and molybdenum after implantation of a Birmingham hip resurfacing arthroplasty (BHR) and a cementless total hip replacement with a 28-mm Metasul articulation (MTHR), over the first 2 years after implantation.Methods We analyzed profiles of metal ion serum levels in 111 patients implanted with a BHR, in 74 patients implanted with an MTHR, and in 130 implant-free probands control subjects using atomic absorption spectrophotometry.Results Chromium and cobalt concentrations (in μg/L) of all BHR and MTHR patients differed significantly from those of control subjects (chromium: < 0.25; cobalt: 0.25). The median chromium and cobalt concentrations in BHR patients had increased to 5.1 and 4.3 μg/ L 2 years after surgery. Concentrations in BHR patient exceeded those in the unilateral MTHR patients. Molybdenum serum concentrations hardly changed over time in either group and were not significantly different from the concentrations seen in the control subjects.Interpretation During the first 2 years after surgery, the Birmingham hip resurfacing arthroplasty leads to a significantly greater increase in serum chromium and cobalt levels than the 28-mm metal-on-metal MTHR. Observation of patients over a longer period will be necessary in order to evaluate any chronic adverse effects to the system due to elevated chromium and cobalt serum concentrations.