Optimal acetabular orientation for hip resurfacing

Advances in imaging for pre-surgical planning in hip resurfacing arthroplasty

BACKGROUND

Accurate preoperative templating is essential for the success of hip resurfacing arthroplasty (HRA). While digital radiograph is currently considered the gold standard, stereoradiograph and CT converted 3D methods have shown promising results. However, there is no consensus in the literature regarding the preferred modality for HRA templating, and angular measurements are often overlooked. Thus, this study aimed to: (1) compare the performances of different modality in implant sizing and angle measurements, (2) evaluate the measurement reproducibility, (3) assess the impact of severe osteoarthritis on femoral head sizing, and (4) based on the analysis above, explore the optimal imaging and planning strategy for HRA.

HYPOTHESIS

An optimal imaging modality exists for HRA planning regarding implant sizing and angular measurements.

MATERIALS AND METHODS

Preoperative imaging data from seventy-seven HRA surgeries were collected. Three raters performed templating using digital radiograph, stereoradiograph, and CT converted 3D models. Measurements for femoral head size, neck-shaft angle, and calcar-shaft angle were obtained. The femoral head sizing was compared to the intraoperative clinical decision. The reproducibility of measurements was assessed using the intraclass correlation coefficient (ICC). Correlations were examined between sizing disagreement and osteoarthritis grade (Tonnis Classification).

RESULTS

Digital radiograph, stereoradiograph, and 3D techniques predicted one size off target in 27/77 (35%), 49/70 (70%), and 75/77 (97%) of cases, respectively, corresponding to 1.8±1.6 (0 to 5.67), 0.9±0.7 (0 to 2.67), and 0.4±0.4 (0 to 1.67) sizes off target, indicating statistically significant differences among all three modalities, with p-values all below 0.01. There were no statistically significant differences among the different modalities for angular measurements. Measurements showed moderate to excellent reproducibility (ICC=0.628-0.955). High-grade osteoarthritis did not impact image sizing in any modality (r=0.08-0.22, all p>0.05).

DISCUSSION

CT converted 3D models were more accurate for implant sizing in HRA, but did not significantly outperform other modalities in angular measurements. Given the high costs and increased radiation exposure associated with CT, the study recommended using CT scans selectively, particularly for precise femoral head sizing, while alternative imaging methods can be effectively used for angular measurements.

LEVEL OF EVIDENCE

III; retrospective comparative diagnostic study.

Long-Term Outcomes of Birmingham Hip Resurfacing Arthroplasty: A Systematic Review of Independent Series with At Least 10 Years of Follow-up

Background

Hip resurfacing arthroplasty (HRA) provides an attractive alternative to total hip arthroplasty (THA) for the management of osteoarthritis in younger, more active patients; however, concerns persist over complications specific to HRA. The aims of this systematic review were to assess the documented long-term survival rates of the metal-on-metal BIRMINGHAM HIP Resurfacing System at a follow-up of at least 10 years and to analyze the functional outcomes and cause of failures.

Methods

A systematic review was undertaken of all published cohort studies available in the MEDLINE, Cochrane, Embase, and PubMed research databases up to December 2021, as recommended by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Data extraction was focused on survival rates, causes of failure, and functional outcomes. Survival estimates at 10 years were pooled in a meta-analysis, with each series weighted by its variance. Causes of failure were presented as a percentage of the pooled revisions.

Results

A total of 11 studies were identified, encompassing 3,129 cases. Across the 9 studies that had reported a mean follow-up, the mean follow-up was 11.7 years (range, 9.55 to 13.7 years). We found a pooled 10-year survival rate of 95.5% (95% confidence interval, 93.4% to 97.1%). There were 149 revisions among the studies (range, 4 to 38 revisions per study), a rate of 4.8% of the total procedures performed. The 2 main causes of revision were aseptic loosening (20.1% of revisions) and adverse reactions to metal debris (20.1%). There were no revisions for dislocation. Of the studies that reported preoperative functional scores, all reported significant improvement in mean scores postoperatively except for 1 study in which the mean Tegner activity score did not significantly improve.

Conclusions

When performed for appropriate indications, patients undergoing an HRA with use of the BIRMINGHAM HIP Resurfacing System can expect good implant survivorship at 10 years with acceptable functional results and low rates of dislocation and infection. This systematic review, however, confirms concerns regarding adverse reactions to metal debris as a leading cause of revision.

Level of Evidence

Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.

Contemporary, non-navigation, cup orientation techniques improve accuracy and eliminate differences seen between the anterior and posterior approach in THA

BACKGROUND

This study aimed to: (1) Determine the ability to achieve the surgeons' desired cup orientation, without navigation, using contemporary measures with the anterior- (AA) and posterior- approaches (PA); and (2) assess whether surgical approach is associated with cup orientation accuracy, as previously reported, when contemporary measures are used.

METHODS

A prospective, 2-centre, multi-surgeon study of 400 THAs (200 AA; 200 PA) was performed. Intraoperative radiographs were obtained with the AA. A digital inclinometer and 3-point pelvic support were used with the PA. With the PA, intraoperative cup inclination at impaction was recorded. Radiographic inclination/anteversion (RI/RA) was measured from intraoperative radiographs (AA-only) and from postoperative radiographs for all cases. Optimum inclination/anteversion was defined as 40°/20° (±10°). The difference between intra- and postoperative orientations allowed for determination of the difference in pelvic position at impaction.

RESULTS

Optimum RI and RA were achieved in 91.3% (n = 365) and 92% (n = 368) of cases respectively. Optimum cup orientation was detected in 84% of cases (n = 336). There was equivalent ability to achieve cup orientation between approaches (AA: 82.5% vs. PA: 85.5%; p = 0.41). The use of an inclinometer in the PA was associated with a smaller variability of inclination at implantation (10° vs. 14°) and counteracted the greater difference in pelvic position (4.4° vs. 2.1°) seen with the PA.

CONCLUSIONS

Over 80% of cases can have optimum orientation without navigation, using simple, cheap contemporary measures. Such measures eliminate differences between approaches, related to patient position. Narrower implantation angles will further reduce variability in cup orientation achieved.

The anatomical location of cystic pseudotumors and muscle atrophy in metal-on-metal resurfacing hip arthroplasty is related to the surgical approach used for implantation. A subgroup analyses of a randomized controlled trial

BACKGROUND

Pseudotumors and muscle atrophy have been associated with metal-on-metal (MoM) resurfacing hip arthroplasty (RHA). We aimed to investigate the influence of the anterolateral (AntLat) and the posterior (Post) surgical approach on the location, grade and prevalence of pseudotumors and muscle atrophy in MoM RHA.

PATIENTS AND METHODS

Forty-nine patients were randomized to MoM RHA by the AntLat (n = 25) or the Post (n = 24) approach at Aarhus University Hospital. Patients underwent metal artifact reduction sequence (MARS) magnetic resonance imaging (MRI) scans for investigation of location, grade and prevalence of pseudotumors and muscle atrophy. Plain radiographs, metal-ions concentrations and clinical outcome scores were evaluated to compare outcomes of the surgical approaches.

RESULTS

MRI-detected pseudotumors were seen in 7 of 18 patients (39%) in the AntLat group and in 12 of 22 patients (55%) in the Post group (p = 0.33). Pseudotumors were mainly located anterolaterally to the hip joint in the AntLat group and postero-lateral to the hip joint in the Post group. Higher grades of muscle atrophy of the caudal part of the gluteus medius and minimus (p < 0.004) were seen in the AntLat group, and higher grades of muscle atrophy of the small external rotators were seen in the Post group (p < 0.001). The AntLat group had higher anteversion angles of mean 15.3° (range 6.1-7.5) versus mean 11.5° (range 4.9-22.5) in the Post group (p = 0.02). Metal-ion concentrations and clinical outcome scores were similar between groups (p > 0.08).

CONCLUSION

Muscle atrophy and pseudotumor location after MoM RHA follow the surgical approach used for implantation. This knowledge may help differentiate between "normal postoperative appearance" and "MoM disease."

Hip Resurfacing Arthroplasty: Past, Present and Future

The history of hip resurfacing arthroplasty (HRA) has faced numerous challenges and undergone decades of evolution in materials and methods. These innovations have been translated to the successes of current prostheses and represent a surgical and mechanical achievement. Modern HRAs now have long term outcomes with excellent results in specific patient groups as demonstrated in national joint registries. This article reviews the key moments in the history of HRAs with specific emphasis on the lessons learnt, current outcomes and future prospects.

Are serum cobalt and chromium levels predictors for patient-reported outcome measures in the ASR hip resurfacing arthroplasty?

Aims

The aims of this study were to determine if an increasing serum cobalt (Co) and/or chromium (Cr) concentration is correlated with a decreasing Harris Hip Score (HHS) and Hip disability and Osteoarthritis Outcome Score (HOOS) in patients who received the Articular Surface Replacement (ASR) hip resurfacing arthroplasty (HRA), and to evaluate the ten-year revision rate and show if sex, inclination angle, and Co level influenced the revision rate.

Methods

A total of 62 patients with an ASR-HRA were included and monitored yearly postoperatively. At follow-up, serum Co and Cr levels were measured and the HHS and the HOOS were scored. In addition, preoperative patient and implant variables and the need for revision surgery were recorded. We used a linear mixed model to relate the serum Co and Cr levels to different patient-reported outcome measures (PROMs). For the survival analyses we used the Kaplan-Meier and Cox regression model.

Results

We found that an increase of one part per billion (ppb) in serum Co and Cr levels correlated significantly with worsening of the HHS in the following year. This significant correlation was also true for the HOOS-Pain and HOOS-quality of life sub scores. The overall ten-year survival rate in our cohort was 65% (95% confidence interval (CI) 52.5 to 77.6). Cox regression analysis showed a significant hazard ratio (HR) of 1.08 (95% CI 1.01 to 1.15; p = 0.028) for serum Co level. No significance was found with sex or inclination angle.

Conclusion

This study shows that increasing serum Co and Cr levels measured in patients with an ASR-HRA are predictive for deterioration in HHS and HOOS subscales in the following year. Increasing serum Co and Cr should forewarn both surgeon and patient that there is a heightened risk of failure. Continued and regular review of patients with an ASR-HRA implant by measurement of serum Co/Cr levels and PROMs remains essential.

Does functional planning, 3D templating and patient-specific instrumentation improve accuracy in total hip replacement?- a randomized controlled trial

AIMS

Debate continues as to the optimal orientation of the acetabular component in total hip arthroplasty (THA) and how to reliably achieve this. The primary objective of this study was to compare functional CT-based planning and patient-specific instruments with conventional THA using 2D templating.

METHODS

A pragmatic single-center, patient-assessor blinded, randomized control trial of patients undergoing THA was performed. 54 patients (aged 18-70) were recruited to either Corin Optimized Positioning System (OPS) or conventional THA. All patients received a cementless acetabular component. All patients underwent pre- and postoperative CT scans, and four functional X-rays. Patients in the OPS group had a 3D surgical plan and bespoke guides made. Patients in the conventional group had a surgical plan based on 2D templating X-rays. The primary outcome measure was the mean error in acetabular anteversion as determined by postoperative CT scan.

RESULTS

There was no statistically significant difference in the mean error in angle of acetabular anteversion when comparing OPS and conventional THA. In the OPS group, the achieved acetabular anteversion was within 10° of the planned anteversion in 96% of cases, compared with only 76% in the conventional group. The clinical outcomes were comparable between the groups.

CONCLUSION

Large errors in acetabular orientation appear to be reduced when CT-based planning and patient-specific instruments are used compared to the standard technique but no significant differences were seen in the mean error.

Cup orientation following posterior approach THA - the effect of different visual aids and pelvic supports

Introduction

This study aims to compare cup inclination achieved (1) Using two orientation guides, whilst using the same 3-point pelvic positioner and (2) Using two types of pelvic positioners, whilst measuring intra-operative cup inclination with an inclinometer.

Materials and methods

This is a prospective, diagnostic cohort study of a consecutive series of 150 THAs performed through a posterior approach. Two types of 3-point pelvic positioners were used (Stulberg and modified Capello Hip Positioners) and the cup was positioned freehand using one of two orientation guides (mechanical guide or digital inclinometer). Intra-operative inclination was recorded, radiographic cup inclination and anteversion were measured from radiographs. The differences in inclination due to pelvic position (ΔPelvicPosition) and orientation definitions (ΔDefinition) were calculated. Target radiographic inclination and anteversion was 40/20° ± 10°.

Results

There was no difference in radiographic cup inclination/ (p = 0.63) using a mechanical guide or digital inclinometer. However, differences were seen in ΔPelvicPosition between the positioners ((Stulberg: 0° ± 5 vs. Capello: 3° ± 6); p = 0.011). Intra-operative inclination at implantation was different between positioners and this led to equivalent cases within inclination/anteversion targets (Stulberg:84%, Capello:80%; p = 0.48).

Conclusions

With the pelvis securely positioned with 3-point supports, optimum cup orientation can be achieved with both alignment guides and inclinometer. Non-optimal cup inclinations were seen when intra-operative inclinations were above 40° and below 32°, or the ΔPelvicPosition was excessive (> 15°; n = 2). We would thus recommend that the intra-operative cup inclination should be centered strictly between 30° and 35° relative to the floor. Small differences exist between different type of pelvic positioners that surgeons need to be aware off and account for.

Hip Resurfacing: A Single Surgeon U.S. Series With Minimum Ten-Year Follow-up

BACKGROUND

Metal-on-metal hip resurfacing is an alternative to total hip arthroplasty (THA). The aim of this study was to determine implant survivorship, analyze patient-reported outcomes measures and to determine patient satisfaction for patients who underwent metal-on-metal hip resurfacing at a large US academic institution by a single surgeon with a minimum of 10-year follow-up.

METHODS

Patients who underwent hip resurfacing from September 2006 through November 2009 were included. Patient demographics and variables were collected from a prospectively maintained institutional database and patients completed an additional questionnaire with patient-reported outcomes measures.

RESULTS

A total of 350 patients (389 hips) out of 371 (433 hips) with a minimum 10-year follow-up were successfully contacted (94.3% follow-up). Mean age was 53 years, 258 were male (73%). 377 out of 389 hips (96.9%) did not require additional surgery. Gender was significantly related to implant survivorship (males 99.0%, females 90.9%; P < .001). 330 patients (369 hips, 94.8%) were satisfied with their surgery. Males had higher proportion of satisfaction scores (P = .02) and higher modified Harris Hip Score (odds ratio = 2.63 (1.39, 4.98), P = .003). Median modified Harris Hip Score score for non-revised hips was 84.0 [80.0; 86.0] versus those requiring revision, 81.5 [74.0; 83.0], (P = .009).

CONCLUSION

At a minimum 10-year follow-up, hip resurfacing, using an implant with a good track record, demonstrates 99.0% survivorship in male patients with an average age of 52 years. We believe that the continued use of metal-on-metal hip resurfacing arthroplasty in this population is justified by both positive patient reported outcomes and survivorship.

A meta-analysis assessing time for return to sport following hip resurfacing

BACKGROUND

Hip resurfacing arthroplasty (HRA) is associated with excellent functional outcomes and return to pre-disease level of activity. The time for return to sport (RTS) following HRA remains unknown. The aim of this meta-analysis was to establish the time for RTS following HRA.

METHODS

A search was performed on PUBMED, MEDLINE, EMBASE, and the Cochrane Library for trials on HRA and RTS, in the English language, published from the inception of the database to October 2020. In addition, a manual search was performed of relevant sports medicine and orthopaedic journals, and the bibliographies reviewed for eligible trials. All clinical trials reporting on time to RTS following HRA were included. Data relating to patient demographics, methodological quality, operation type, RTS, clinical outcomes, and complications were recorded by two independent reviewers. The PRISMA guidelines for reporting meta-analyses was used to undertake this study.

RESULTS

The initial literature search identified 1559 studies and nine further studies were found. Of these, 11 studies with a total of 659 patients matched the inclusion criteria. Two studies involving a total of 94 patients demonstrated an overall pooled proportion of 91.8% (95% CI 71.8-100) of patients RTS by three months post-operatively. Four studies including a total of 265 patients determined a pooled proportion of 96.8% (95% CI 91.0-99.7) of patients able to RTS by the 6-month post-operative stage. Pooled proportion analysis from all 11 studies comprising 659 patients showed 90.9% (95% CI 82.2-96.9) of patients were able to RTS by final follow up of 3 years.

CONCLUSION

Pooled proportion analysis showed an increasing number of patients were able to RTS after HRA over the first one year after surgery. There remains marked inter and intra-study variations in time for RTS but the pooled analysis shows that over 80% of patients were able to RTS at 6 to 12 months after HRA. The findings of this meta-analysis will enable more informed discussions between patients and healthcare professionals about time for RTS following HRA.

Orthopedic Surgeons’ Accuracy When Orienting an Acetabular Cup. A Comparison with Untrained Individuals

Background and Objectives: Previous studies demonstrated a huge variability among surgeons when it comes to reproducing the position of an acetabular cup in total hip arthroplasty. Our main objective is to determine if orthopedic surgeons can replicate a given orientation on a pelvic model better than untrained individuals. Our secondary objective is to determine if experience has any influence on their ability for this task. Materials and Methods: A group of specialist orthopedic hip surgeons and a group of volunteers with no medical training were asked to reproduce three given (randomly generated) acetabular cup orientations (inclination and anteversion) on a pelvic model. Error was measured by means of a hip navigation system and comparisons between groups were made using the appropriate statistical methods. Results: The study included 107 individuals, 36 orthopedic surgeons and 71 untrained volunteers. The mean error among surgeons was slightly greater as regards both inclination (7.84 ± 5.53 vs. 6.70 ± 4.03) and anteversion (5.85 ± 4.52 vs. 5.48 ± 3.44), although statistical significance was not reached (p = 0.226 and p = 0.639, respectively). Similarly, although surgeons with more than 100 procedures a year obtained better results than those with less surgical experience (8.01 vs. 7.67 degrees of error in inclination and 5.83 vs. 5.87 in anteversion), this difference was not statistically significant, either (p = 0.852 and p = 0.981). Conclusions: No differences were found in the average error made by orthopedic surgeons and untrained individuals. Furthermore, the surgeons’ cup orientation accuracy was not seen to improve significantly with experience.

Survivorship of a Metal-on-Metal Total Hip Implant With Modular Titanium Adapter

BACKGROUND

Use of metal-on-metal (MoM) articulations in total hip arthroplasty (THA) has sharply declined due to high failure rates from metal-related complications. Although certain MoM designs have demonstrated only 46% survival, not all MoM designs have performed the same. The purpose of this study is to evaluate mid-term to long-term survival of a specific MoM implant with a modular titanium taper adapter.

METHODS

A retrospective review was performed on all patients who underwent primary THA at our center with the M²a-Magnum system (Zimmer Biomet, Warsaw, IN). Of 829 patients (956 hips) identified, 754 patients (869 hips) met inclusion criteria of signed research consent, minimum 2-year follow-up, and/or any revision surgery.

RESULTS

Mean follow-up was 11.0 years (range 2-16; ±3.5). Mean cup angle of inclination was 42.8° (range 24°-70°, ±6.3°), with 88.0% reconstructed within the 40° ± 10° safe zone. There were 64 revisions (7.36%): 7 (0.81%) septic and 57 (6.56%) aseptic. Of those, 32 (3.68%) were adverse reactions to metal debris. Kaplan-Meier survival free of revision for all causes was 88.6% at 16 years (95% confidence interval 86.8-90.4). Univariate analysis of risk factors for all-cause, aseptic, and adverse reaction to metal debris revision found no relationship with female gender, age ≥65 years, body mass index >30 kg/m², higher activity level, or inclination angle outlier.

CONCLUSION

The results of this study demonstrate a more favorable mid-term to long-term survivorship with this specific MoM implant compared to other designs. Although our institution no longer performs MoM THA, further investigation into differences in MoM implant designs is warranted.

Revision arthroplasty using a custom-made implant in the course of acetabular loosening of the J&J DePuy ASR replacement system - case report

RATIONALE

Resurfacing arthroplasty using the J&J DePuy ASR system was withdrawn from surgical treatment due to the necessity of frequent revision procedures after its application. There have been many studies concerning treatment of acetabular bone loss using different operating techniques. However, we felt that data of custom - made implant usage in such cases is highly insufficient, and there is lack of evidence on its application in treatment of loosening of the previous implant. The aim of the study was to evaluate the effectiveness of surgical treatment with a custom-made implant in a patient with extensive acetabular bone loss after aseptic loosening of the acetabular component of the J&J DePuy ASR surface prosthesis in the early period of observation.

PATIENT CONCERNS

A 74-year-old patient was taken to the Orthopaedic Trauma Emergency Room due to increasing pain in the right hip for about 3 months. Nine years earlier he underwent resurfacing arthroplasty of the right hip using the J&J DePuy ASR method.

DIAGNOSES

The imaging diagnostics (X-ray, computed tomography, ultrasound) revealed the presence of a pseudotumor and lysis around the acetabular implant, which caused a fracture in the acetabulum.

INTERVENTIONS

Revision arthroplasty of the right hip joint was performed with the removal of the ASR implant. During the procedure extensive bone defects were visualized, preventing the insertion of the revision acetabulum. After extensive plasticization of the defects with the use of allogeneic cancellous chips the "hanging hip" was left with the intention of making another attempt to insert the implant after the reconstitution of the acetabular bone. A computed tomography examination 2.5 years after the ASR removal revealed the lack of an adequate degree of bone remodeling for the planned implant. Arthroplasty using custom - made aMace Acetabular Revision System by Materialize was performed 3 years after the removal of ASR.

OUTCOMES

Optimal implant adherence to the bone base and full osseointegration with the pelvic bone bearing has been achieved. Significant improvement in clinical parameters has been noted, with no complications in the postoperative period.

LESSONS

The use of an individual custom-made implant in extensive acetabular bone loss after aseptic loosening of the acetabular component of the J&J DePuy ASR surface prosthesis in patients is an effective method of surgical treatment.

Computational modelling of hip resurfacing arthroplasty investigating the effect of femoral version on hip biomechanics

Aim

How reduced femoral neck anteversion alters the distribution of pressure and contact area in Hip Resurfacing Arthroplasty (HRA) remains unclear. The purpose of this study was to quantitatively describe the biomechanical implication of different femoral neck version angles on HRA using a finite element analysis.

Materials and methods

A total of sixty models were constructed to assess the effect of different femoral neck version angles on three different functional loads: 0°of hip flexion, 45°of hip flexion, and 90° of hip flexion. Femoral version was varied between 30° of anteversion to 30° of retroversion. All models were tested with the acetabular cup in four different positions: (1) 40°/15° (inclination/version), (2) 40°/25°, (3) 50°/15°, and (4) 50°/25°. Differences in range of motion due to presence of impingement, joint contact pressure, and joint contact area with different femoral versions and acetabular cup positions were calculated.

Results

Impingement was found to be most significant with the femur in 30° of retroversion, regardless of acetabular cup position. Anterior hip impingement occurred earlier during hip flexion as the femur was progressively retroverted. Impingement was reduced in all models by increasing acetabular cup inclination and anteversion, yet this consequentially led to higher contact pressures. At 90° of hip flexion, contact pressures and contact areas were inversely related and showed most notable change with 30° of femoral retroversion. In this model, the contact area migrated towards the anterior implant-bone interface along the femoral neck.

Conclusion

Femoral retroversion in HRA influences impingement and increases joint contact pressure most when the hip is loaded in flexion. Increasing acetabular inclination decreases the area of impingement but doing so causes a reciprocal increase in joint contact pressure. It may be advisable to study femoral neck version pre-operatively to better choose hip resurfacing arthroplasty candidates.

The area method for measuring acetabular cup anteversion: An accurate and autonomous solution

Several radiological methods of measuring anteversion of the acetabular component after total hip arthroplasty have been described, all time-consuming and with varying reproducibility. This study aimed to compare the recently proposed Area method to true cup anteversion as determined by an accelerometer. This study further applied this method programmatically to autonomously determine radiographic cup orientation using two computer programs, then compared these results to hand and accelerometer measurements. 160 anteroposterior pelvis radiographs were taken of a standard Sawbones® pelvis fitted with a total hip arthroplasty system. The acetabular cup was re-oriented between each radiograph, with anteversion ranging from 0° to 90°. An accelerometer was mounted to the cup to measure true cup anteversion. Radiographic anteversion was independently measured via three methods: by hand, linear image processing, and machine learning. Measurements were compared to triaxial accelerometer recordings. Coefficient of determination (R2) was found to be 0.997, 0.991, and 0.989 for hand measurements, the machine learning, and linear image processing, respectively. The machine learning program and hand measurements overestimated anteversion by 0.70° and 0.02° respectively. The program using linear techniques underestimated anteversion by 5.02°. Average runtime was 0.03 and 0.59 s for the machine learning and linear image processing program, respectively. The machine learning program averaged within 1° of cup orientation given a true cup anteversion less than 51°, and within 2° given an anteversion less than 85°. The Area method showed great accuracy and reliability with hand measurements compared to true anteversion. The results of this study support the use of machine learning for accurate, timely, autonomous assessment of cup orientation.

Metal on metal hip resurfacing arthroplasty: Where are we now?

Metal on metal hip resurfacing arthroplasty (HRA) was introduced in an attempt to address potential limitations regarding return to sport, return to high functional activity and premature wear in young patients requiring standard total hip replacement (THR). Around 12% patients undergoing hip replacement surgery are under 55 years of age. By more closely mimicking natural anatomy, and having a metal on metal (MoM) bearing, it was hoped that HRA would address the issues. However, concern has emerged about early failure, high revision rates from local adverse reactions to metallic wear debris and potential systemic consequences of metal ion cardiotoxicity. In this article, we discuss the existing literature in the field, the current clinical evidence surrounding HRA, its indications, clinical outcomes, and risk factors for failure and conclude if it still has a role within orthopaedic hip surgery.

Functional navigation in hip resurfacing

BACKGROUND

Acetabular malpositioning in total hip arthroplasty (THA) can result in edge loading, leading to increased wear, pseudotumours and bearing failure. Historically Lewinnek's safe zone has been the goal for optimal THA cup positioning; however, research indicates that one generic safe zone is not ideal. In hip resurfacing (HR) there is a lack of data regarding optimal position, and studies indicate that edge loading is related to increased inclination.

METHODS

We used a functional positioning protocol to produce patient-specific (PS) dynamic analyses, identifying individual functional ideal acetabular zones. We hypothesized that identifying and implanting into PS functional ideal zones would result in decreased edge loading and improved clinical outcomes. Fifteen adept HRs were implanted using a PS functional positioning protocol. As plans were created for standard THA acetabular position, adjustments were made intra-operatively.

RESULTS

Dynamic post-operative analyses showed 80% of implants were appropriately positioned. The remaining 20% had edge loading, where implants had inclination ≤5° of planned positioning and 10° of planned anteversion. 60% were ≤10° of planned inclination and anteversion. No pre-operative planned positions were within 'Lewinneks's zone', indicating its questionable relevance, especially in HR. A total of 20% of patients developed pain; however, at 1 year post-operatively all patients recorded improved Oxford hip scores.

CONCLUSION

This study confirms that functional positioning in HR consistently results in improved precision and accuracy in achieving the functional acetabular ideal zone, leading to improved clinical outcomes and prevention of edge loading.

Pinnacle Ultamet metal-on-metal total hip arthroplasty survivorship: average 10-year follow-up

Background

It is unclear whether a connection exists between femoral head size, offset, neck length, and cup abduction angles, and rate of revision in metal-on-metal (MoM) total hip arthroplasty (THA) implant systems.

Methods

A retrospective review of MoM THA completed by a single surgeon with a single implant between 2003 and 2008 was conducted. Patient demographics, implant data, radiographs, and revision details were collected at follow-up. Incidence rates for revision and osteolysis were calculated in regard to the femoral head size, stem offset, neck length, and cup abduction angles.

Results

Six hundred and ninety two THAs were identified, with 79% of patients returning for a median follow-up of 10.3 years (interquartile range = 6.0-12.3). The median time to revision was 7.5 years (interquartile range = 5.3-9.9) among 27 total revision surgeries. The overall incidence rate of revision was 5.4 revisions per 1000 person-years, 3.0 revisions per 1000 person-years for adverse local tissue reaction. Hips with a cup abduction angle of ≤40° had revisions at nearly twice the rate of those with an angle of 41°-50° (incidence rate ratio = 1.98, 95% confidence interval: 0.92, 4.29). Hips with a 9 mm neck length had an increased rate of revision (incidence rate ratio = 5.94, 95% confidence interval: 1.33, 26.55) relative to those with a neck length of 0 mm. Rates of osteolysis were similar between implants of different head sizes, neck lengths and cup abduction angles.

Conclusions

MoM implant systems with longer necks and smaller cup abduction angles may lead to increased need for revision. Results from this study suggest a need for closer long-term follow-up of MoM THA systems.

Has the threshold for revision surgery for adverse reactions to metal debris changed in metal-on-metal hip arthroplasty patients? A cohort study of 239 patients using an adapted risk-stratification algorithm

Background and purpose - A risk-stratification algorithm for metal-on-metal hip arthroplasty (MoMHA) patients was devised by US experts to help clinicians make management decisions. However, the proposed algorithm did not cover all potential patient or surgical abnormalities. Therefore we adapted the US risk-stratification algorithm in MoMHA patients revised for adverse reactions to metal debris (ARMD) to determine the variability in the revision threshold, and also whether high-risk patients had inferior outcomes following revision.Patients and methods - We analysed 239 MoMHA revisions for ARMD between 2001 and 2016 from 2 centres with pre-revision blood metal ions and imaging. Patients were stratified (low risk, moderate risk, high risk) using pre-revision factors (implant, radiographic, blood metal ions, cross-sectional imaging) by adapting a published algorithm. The risk categories for each factor were assessed against revision year, revision centre, and post-revision outcomes (re-revision surgery, and any poor outcome).Results - Compared with hips revised before 2012, hips revised from 2012 onwards included more high-risk implants (44% vs. 17% pre-2012), high-risk radiographic features (85% vs. 69% pre-2012), and low-risk metal ions (41% vs. 19% pre-2012). 1 centre more frequently revised patients with high-risk implants (48% vs. 14%) and low-risk blood metal ions (45% vs. 15%) compared with the other. All these comparisons were statistically significant (p < 0.05). With the limited sample size available, implant, radiographic, blood metal ion, and cross-sectional imaging risk groups did not statistically significantly affect the rates of re-revision surgery or frequency of poor outcomes post-revision.Interpretation - When applying the adapted risk-stratification algorithm the threshold for ARMD revision changed over time, presumably due to increasing evidence, patient surveillance, and investigation since 2012. Lower blood metal ion thresholds were used from 2012 for ARMD revisions; however, there was evidence that centres attached different importance to metal ions when managing patients. High-risk patients did not have inferior outcomes following ARMD revision.

Metal ion concentrations after metal-on-metal hip arthroplasty are not correlated with habitual physical activity levels

INTRODUCTION

Metal-on-metal (MoM) hip arthroplasties have shown high clinical failure rates with many patients at risk for a revision and under surveillance for high metal ion concentrations. Implant wear releasing such ions is assumed to be a function of use, i.e. the patient's physical activity. This study aimed to assess whether habitual physical activity levels of MoM patients are correlated with metal ion concentrations and are higher in patients with high (at risk) than in patients with low (safe) metal ion concentrations.

METHODS

A cohort study was conducted of patients with any type of MoM hip prosthesis. Metal ion concentrations were determined using ICP-MS. Habitual physical activity of subjects was measured in daily living using an acceleration-based activity monitor. Outcome consisted of quantitative and qualitative activity parameters.

RESULTS

In total, 62 patients were included. Mean age at surgery was 60.8 ± 9.3 years and follow-up was 6.3 ± 1.4 years. Cobalt concentrations were highly elevated overall (112.4 ± 137.9 nmol/L) and significantly more in bilateral (184.8 ± 106.5 nmol/L) than in unilateral cases (87.8 ± 139.4 nmol/L). No correlations were found between physical activity parameters and metal ion concentrations. Subgroup analysis of patients with low versus high cobalt concentration showed no significant differences in habitual physical activity.

DISCUSSION

No correlation was found between physical activity levels and metal ion concentrations. Implant use by normal habitual activities of daily living seems not to influence metal ion concentrations.

Is the alignment guide technique in total hip arthroplasty sufficient for accurate cup positioning with a modified Watson Jones approach?

PURPOSE:

Cup setting with only an alignment guide has been reported to be inaccurate in the lateral decubitus position in total hip arthroplasty (THA). We assessed the accuracy of cup positioning using only the alignment guide technique via a modified Watson Jones approach in the lateral decubitus position.

METHODS:

Two hundred hips of 189 patients underwent THA from October 2014 to September 2016 via a modified Watson Jones approach. In the final sample, 181 hips of 171 patients (35 males, 136 females) were included in this investigation. The alignment of the cup was evaluated by an anteroposterior radiograph of the pelvis 1 week after surgery. Measurements were divided into safe zone determined by Callanan and Lewinnek.

RESULTS:

There were 168 (92.8%) acetabular cups that were placed within the safe zone for both inclination and anteversion based on the safe zones defined by Lewinnek, and 134 (74%) acetabular cups that were placed within the safe zone defined by Callanan. Multiple logistic analysis showed that the laterality and the addition of the confirmation method were indicators for malpositioning of combined inclination and anteversion.

CONCLUSION:

Our data suggested that even if special tools were not used in the lateral decubitus position, using only the alignment guide enabled cup positioning to be achieved with 92.8% accuracy in the Lewinnek safe zone and 74% accuracy in the Callanan safe zone. Multiple logistic analysis showed that the laterality and the addition of a confirmation alignment guide influenced the accuracy of cup positioning.

No Threshold Exists for Recommending Revision Surgery in Metal-on-Metal Hip Arthroplasty Patients With Adverse Reactions to Metal Debris: A Retrospective Cohort Study of 346 Revisions

BACKGROUND

Surgeons currently have difficulty when managing metal-on-metal hip arthroplasty (MoMHA) patients with adverse reactions to metal debris (ARMD). This stems from a lack of evidence, which is emphasized by the variability in the recommendations proposed by different worldwide regulatory authorities for considering MoMHA revision surgery. We investigated predictors of poor outcomes following MoMHA revision surgery performed for ARMD to help inform the revision threshold and type of reconstruction.

METHODS

We retrospectively studied 346 MoMHA revisions for ARMD performed at 2 European centers. Preoperative (metal ions/imaging) and intraoperative (findings, components removed/implanted) factors were used to predict poor outcomes. Poor outcomes were postoperative complications (including re-revision), 90-day mortality, and poor Oxford Hip Score.

RESULTS

Poor outcomes occurred in 38.5%. Shorter time (under 4 years) to revision surgery was the only preoperative predictor of poor outcomes (odds ratio [OR] = 2.12, confidence interval [CI] = 1.00-4.46). Prerevision metal ions and imaging did not influence outcomes. Single-component revisions (vs all-component revisions) increased the risk of poor outcomes (OR = 2.99, CI = 1.50-5.97). Intraoperative modifiable factors reducing the risk of poor outcomes included the posterior approach (OR = 0.22, CI = 0.10-0.49), revision head sizes ≥36 mm (vs <36 mm: OR = 0.37, CI = 0.18-0.77), ceramic-on-polyethylene revision bearings (OR vs ceramic-on-ceramic = 0.30, CI = 0.14-0.66), and metal-on-polyethylene revision bearings (OR vs ceramic-on-ceramic = 0.37, CI = 0.17-0.83).

CONCLUSION

No threshold exists for recommending revision in MoMHA patients with ARMD. However postrevision outcomes were surgeon modifiable. Optimal outcomes may be achieved if surgeons use the posterior approach, revise all MoMHA components, and use ≥36 mm ceramic-on-polyethylene or metal-on-polyethylene articulations.

Is end-organ surveillance necessary in patients with well-functioning metal-on-metal hip resurfacings?

Aims

Cardiac magnetic resonance (CMR) was used to assess whether cardiac function or tissue composition was affected in patients with well-functioning metal-on-metal hip resurfacing arthroplasties (MoMHRA) when compared with a group of controls, and to assess if metal ion levels correlated with any of the functional or structural parameters studied.

Patients and Methods

In all, 30 participants with no significant cardiac history were enrolled: 20 patients with well-functioning MoMHRA at mean follow-up of 8.3 years post-procedure (ten unilateral, ten bilateral; 17 men, three women) and a case-matched control group of ten non-MoM total hip arthroplasty patients (six men, four women). The mean age of the whole cohort (study group and controls) at the time of surgery was 50.6 years (41.0 to 64.0). Serum levels of cobalt and chromium were measured, and all patients underwent CMR imaging, including cine, T2* measurements, T1 and T2 mapping, late gadolinium enhancement, and strain measurements.

Results

None of the MoMHRA patients showed clinically significant cardiac functional abnormality. The MoMHRA patients had larger indexed right and left end diastolic volumes (left ventricular (LV): 74 ml/m2 vs 67 ml/m2, p = 0.045; right ventricular: 80 ml/m2 vs 71 ml/m2, p = 0.02). There was a small decrease in T2 time in the MoMHRA patients (median 49 ms vs 54 ms; p = 0.0003). Higher metal ion levels were associated with larger LV volumes and with shorter T2 time.

Conclusion

Although cardiac function is not clinically adversely affected in patients with well-functioning MoMHRA, modern imaging is able to demonstrate subtle changes in structure and function of the heart. As these changes correlate with systemic ion measurements, they may be consequences of wear debris deposition. Longer, longitudinal studies are necessary to determine whether cardiac function will become affected. Cite this article: Bone Joint J 2019;101-B:540–546.

Morphometric geometrical analysis to determine the centre of the acetabular component placement in Crowe type IV hips undergoing total hip arthroplasty

AIMS

We analyzed the acetabular morphology of Crowe type IV hips using CT data to identify a landmark for the ideal placement of the centre of the acetabular component, as assessed by morphometric geometrical analysis, and its reliability.

PATIENTS AND METHODS

A total of 52 Crowe IV hips (42 patients; seven male, 35 female; mean age 68.5 years (32 to 82)) and 50 normal hips (50 patients; eight male, 42 female; mean age 60.7 years (34 to 86)) undergoing total hip arthroplasty were retrospectively identified. In this CT-based simulation study, the acetabular component was positioned at the true acetabulum with a radiological inclination of 40° and anteversion of 20°. Acetabular shape and the position of the centre of the acetabular component were analyzed by morphometric geometrical analysis using the generalized Procrustes analysis.

RESULTS

The acetabular shapes of Crowe IV hips were distinctively triangular; the ideal position of the centre of the acetabular component was superior on the posterior bony wall. The first and second relative warps explained 34.2% and 18.4% of the variance, respectively, compared with that of 28.6% and 18.0% in normal hips. We defined the landmark as one-third the distance from top on the posterior bony wall in Crowe IV hips. The average distance from the centre of the acetabular component was 5.6 mm.

CONCLUSION

Crowe IV hips are distinctively triangular; the point one-third from the top on the posterior bony wall was a useful landmark for placing the acetabular component.

Clinical accuracy and precision of hip resurfacing arthroplasty using computed tomography-based navigation

PURPOSE

To avoid malalignment of components during hip resurfacing arthroplasty (HRA), we used a computed tomography (CT)-based navigation system for guidance. This study aimed to evaluate the clinical accuracy and precision of HRA performed using the CT-based navigation systems.

METHODS

HRA was performed on 17 hips guided by the CT-based navigation systems. We measured cup alignment deviation, deviation of the stem position, and alignment from the plan by image matching between pre-operative and post-operative CT images.

RESULTS

Cup anteversion was within 5° of that in the plan in all cases. Cup inclination was within 5° of that in the plan in 82.4% and within 10° in all cases. The angular difference of the stem was within 5° in all cases, and the entry point of the stem was within 4 mm in all cases.

CONCLUSION

The CT-based navigation system for HRA guided accurate component placement according to the plan.

What is appropriate surveillance for metal-on-metal hip arthroplasty patients?

The unexpected high revision rates of large-diameter (femoral head sizes of 36 mm or greater) metal-on-metal hip arthroplasties (MoMHAs) have led to worldwide regulatory authorities recommending regular surveillance, even for asymptomatic individuals. However, these recommendations are not evidence-based and are very costly. The rapidly evolving evidence base requires an update regarding the investigation and management of MoMHA patients. This article is the first of 2 (the second article in this series will consider the threshold for performing revision, and the outcomes following ARMD revision surgery: Matharu et al., Revision surgery of metal-on-metal hip arthroplasties for adverse reactions to metal debris: A clinical update. Acta Orthop 2018; in press), and considers the various investigative modalities used during surveillance, with specific focus on blood metal ion sampling and cross-sectional imaging. No single investigation can universally be used during MoMHA patient surveillance. Recent studies have now provided important information on interpreting blood metal ions (effective in identifying patients at low risk of problems), clarifying the roles of cross-sectional imaging (reserve combined ultrasound and MARS-MRI for complex cases), and providing parameters to safely exclude many asymptomatic patients from regular surveillance. This information will be useful when designing future surveillance protocols for MoMHA patients.

Evidence based recommendations for reducing head-neck taper connection fretting corrosion in hip replacement prostheses

INTRODUCTION

This systematic review seeks to summarise the published studies investigating prosthetic design, manufacture and surgical technique's effect on fretting corrosion at the head-neck taper connection, and provide clinical recommendations to reduce its occurrence.

METHODS

PubMed, MEDLINE and EMBASE electronic databases were searched using the terms taper, trunnion, cone and head-neck junction. Articles investigating prosthetic design, manufacture and surgical technique's effect on fretting corrosion were retrieved, reviewed and graded according to OCEBM levels of evidence and grades of recommendation.

RESULTS

The initial search yielded 1,224 unique articles, and 91 were included in the analysis.

CONCLUSIONS

There is fair evidence to recommend against the use of high offset femoral heads, larger diameter femoral heads, and to pay particular consideration to fretting corrosion's progression with time and risk with heavier or more active patients. Particular to metal-on-metal hip prostheses, there is fair evidence to recommend positioning the acetabular component to minimise edge loading. Particular to metal-on-polyethylene hip prostheses, there is fair evidence to recommend the use of ceramic femoral heads, against use of cast cobalt alloy femoral heads, and against use of low flexural rigidity femoral stems. Evidence related to taper connection design is largely conflicting or inconclusive. Head-neck taper connection fretting corrosion is a multifactorial problem. Strict adherence to the guidelines presented herein does not eliminate the risk. Prosthesis selection is critical, and well-controlled studies to identify each design parameter's relative contribution to head-neck taper connection fretting corrosion are required.

Blood Metal Ion Thresholds to Identify Patients with Metal-on-Metal Hip Implants at Risk of Adverse Reactions to Metal Debris: An External Multicenter Validation Study of Birmingham Hip Resurfacing and Corail-Pinnacle Implants

BACKGROUND

The authors of recent studies have reported newly devised implant-specific blood metal ion thresholds to predict adverse reactions to metal debris (ARMD) in patients who have undergone unilateral or bilateral metal-on-metal (MoM) hip arthroplasty. These thresholds were most effective for identifying patients at low risk of ARMD. We investigated whether these newly devised blood metal ion thresholds could effectively identify patients at risk of ARMD after MoM hip arthroplasty in an external cohort of patients.

METHODS

We performed a validation study involving 803 MoM hip arthroplasties (323 unilateral Birmingham Hip Resurfacing [BHR], 93 bilateral BHR, and 294 unilateral Corail-Pinnacle implants) performed in 710 patients at 3 European centers. All patients underwent whole-blood metal ion sampling, and were divided into 2 groups: those with ARMD (leading to revision or identified on imaging; n = 75) and those without ARMD (n = 635). Previously devised implant-specific blood metal ion thresholds (2.15 μg/L of cobalt for unilateral BHR; 5.5 μg/L for the maximum of either cobalt or chromium for bilateral BHR; and 3.57 μg/L of cobalt for unilateral Corail-Pinnacle implants) were applied to the validation cohort, and receiver operating characteristic curve analysis was used to establish the discriminatory characteristics of each threshold.

RESULTS

The area under the curve, sensitivity, specificity, and positive and negative predictive values for the ability of each implant-specific threshold to distinguish between patients with and without ARMD were, respectively, 89.4% (95% confidence interval [CI] = 82.8% to 96.0%), 78.9%, 86.7%, 44.1%, and 96.9% for unilateral BHR; 89.2% (CI = 81.3% to 97.1%), 70.6%, 86.8%, 54.5%, and 93.0% for bilateral BHR; and 76.9% (CI = 63.9% to 90.0%), 65.0%, 85.4%, 24.5%, and 97.1% for unilateral Corail-Pinnacle implants. Using the implant-specific thresholds, we missed 20 patients with ARMD (2.8% of the patients in this series). We missed more patients with ARMD when we used the fixed thresholds proposed by regulatory authorities: 35 (4.9%) when we used the U.K. threshold of 7 μg/L for both cobalt and chromium (p = 0.0003), 21 (3.0%) when we used the U.S. threshold of 3 μg/L for both cobalt and chromium (p = 1.0), and 46 (6.5%) when we used the U.S. threshold of 10 μg/L for both cobalt and chromium (p < 0.0001).

CONCLUSIONS

This external multicenter validation study confirmed that patients with blood metal ion levels below new implant-specific thresholds have a low risk of ARMD after MoM hip arthroplasty. Using these implant-specific thresholds, we missed fewer patients with ARMD compared with when the thresholds currently proposed by regulatory authorities were used. We therefore recommend using implant-specific blood metal ion thresholds when managing patients who have undergone MoM hip arthroplasty.

LEVEL OF EVIDENCE

Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Use of iPhone technology in improving acetabular component position in total hip arthroplasty

Improper acetabular cup positioning is associated with high risk of complications after total hip arthroplasty. The aim of our study is to objectively compare 3 methods, namely (1) free hand, (2) alignment jig (Sputnik), and (3) iPhone application to identify an easy, reproducible, and accurate method in improving acetabular cup placement. We designed a simple setup and carried out a simple experiment (see Method section). Using statistical analysis, the difference in inclination angles using iPhone application compared with the freehand method was found to be statistically significant (F[2,51] = 4.17, P = .02) in the “untrained group”. There is no statistical significance detected for the other groups. This suggests a potential role for iPhone applications in junior surgeons in overcoming the steep learning curve.

Which factors influence the rate of failure following metal-on-metal hip arthroplasty revision surgery performed for adverse reactions to metal debris? an analysis from the National Joint Registry for England and Wales

Aims

To determine the outcomes following revision surgery of metal-on-metal hip arthroplasties (MoMHA) performed for adverse reactions to metal debris (ARMD), and to identify factors predictive of re-revision.

Patients and Methods

We performed a retrospective observational study using National Joint Registry (NJR) data on 2535 MoMHAs undergoing revision surgery for ARMD between 2008 and 2014. The outcomes studied following revision were intra-operative complications, mortality and re-revision surgery. Predictors of re-revision were identified using competing-risk regression modelling.

Results

Intra-operative complications occurred in 40 revisions (1.6%). The cumulative five-year patient survival rate was 95.9% (95% confidence intervals (CI) 92.3 to 97.8). Re-revision surgery was performed in 192 hips (7.6%). The cumulative five-year implant survival rate was 89.5% (95% CI 87.3 to 91.3). Predictors of re-revision were high body mass index at revision (subhazard ratio (SHR) 1.06 per kg/m² increase, 95% CI 1.02 to 1.09), modular component only revisions (head and liner with or without taper adapter; SHR 2.01, 95% CI 1.19 to 3.38), ceramic-on-ceramic revision bearings (SHR 1.86, 95% CI 1.23 to 2.80), and acetabular bone grafting (SHR 2.10, 95% CI 1.43 to 3.07). These four factors remained predictive of re-revision when the missing data were imputed.

Conclusion

The short-term risk of re-revision following MoMHA revision surgery performed for ARMD was comparable with that reported in the NJR following all-cause non-MoMHA revision surgery. However, the factors predictive of re-revision included those which could be modified by the surgeon, suggesting that rates of failure following ARMD revision may be reduced further.

Cite this article: Bone Joint J 2017;99-B:1020–7.

The influence of spine-hip relations on total hip replacement: A systematic review

Sagittal pelvic kinematics along with spino-pelvic angular parameters have recently been studied by numerous investigators for their effect on total hip replacement (THR) clinical outcomes, but many issue of spine-hip relations (SHR) are currently unexplored. Therefore, our review aims at clarifying the following questions: is there any evidence of a relationship between articular impingement/dislocation risk in primary THR and (1) certain sagittal pelvic kinematics patterns, (2) pelvic incidence, and (3) types of SHRs? A systematic review of the existing literature utilising PubMed and Google search engines was performed in January 2017. Only clinical or computational studies published in peer-reviewed journals over the last five years in either English or French were reviewed. We identified 769 reports, of which 12 met our eligibility criteria. A review of literature shows that sagittal pelvic kinematics, but not the pelvic incidence, influences the risk of prosthetic impingement/dislocation. We found no study having assessed the relationship between this risk and the types of SHRs. Sagittal pelvic kinematics is highly variable among individuals and certain kinematic patterns substantially influences the risk of prosthetic impingement/dislocation. Recommendations for cup positioning are therefore switching from a systematic to a patient-specific approach, with the standing cup orientation Lewinneck safe zone progressively giving way to a new parameter of interest: the functional orientation of the cup. Based on a recently published classification for SHRs, We propose a new concept of "kinematically aligned THR" for the purposes of THR planning. Further studies are needed to investigate the relevance of such a classification towards the assumptions and hypothesis we have made. Level of evidence,- Level IV, systematic review of level III and IV studies.

Patient and Radiographic Factors Help to Predict Metal-on-Metal Hip Resurfacings with Evidence of a Pseudotumor

BACKGROUND

The role of radiographs in the follow-up of patients with metal-on-metal hip resurfacing (MoMHR) implants is unclear. We investigated whether a combination of patient and radiographic factors predicted MoMHRs with evidence of a pseudotumor.

METHODS

We performed a retrospective single-center case-control study of 384 MoMHRs. The pseudotumor group of 130 hips all had evidence of a symptomatic pseudotumor on cross-sectional imaging, with the diagnosis confirmed at revision. The nonpseudotumor group of 254 hips (a subgroup of these hips were previously reported on) all had normal findings on cross-sectional imaging. Radiographs taken immediately prior to revision were assessed in the pseudotumor group and were compared with radiographs taken at the time of normal cross-sectional imaging in the nonpseudotumor group. Two blinded independent observers analyzed the radiographs for signs of failure, with excellent interobserver agreement. Logistic regression modeling identified the patient and radiographic predictors of revision for pseudotumor.

RESULTS

Hips with a pseudotumor more commonly had abnormal findings on radiographs compared to hips without a pseudotumor (80.0% compared with 63.4%; p = 0.001). Patient and radiographic factors predictive of revision for pseudotumor in the multivariable model were female sex (odds ratio [OR], 3.14; 95% confidence interval [CI], 1.85 to 5.35; p < 0.001), high inclination (OR, 1.04 per degree; 95% CI, 1.01 to 1.07 per degree; p = 0.006), acetabular osteolysis (OR, 5.06; 95% CI, 2.14 to 12.0; p < 0.001), femoral osteolysis (OR, 17.8; 95% CI, 5.09 to 62.2; p < 0.001), and acetabular loosening (OR, 3.35; 95% CI, 1.34 to 8.35; p = 0.009). Factors predictive of not having a pseudotumor were anteversion of ≥5° (5° to <10°: OR, 0.31; 95% CI, 0.12 to 0.77; p = 0.012; and ≥10°: OR, 0.32; 95% CI, 0.15 to 0.70; p = 0.004) and heterotopic ossification (OR, 0.19; 95% CI, 0.05 to 0.72; p = 0.015). The final multivariable model was well calibrated (p = 0.589), with good discriminatory ability (area under the curve = 0.801; sensitivity = 74.4%; specificity = 71.7%).

CONCLUSIONS

A combination of patient and radiographic factors provided useful information for distinguishing between MoMHRs with and without evidence of a pseudotumor. Surgeons may wish to consider these specific patient and radiographic factors before proceeding with cross-sectional imaging. Radiographs are important when assessing patients with MoMHR implants and should be included in all follow-up protocols.

LEVEL OF EVIDENCE

Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.

Can blood metal ion levels be used to identify patients with bilateral Birmingham Hip Resurfacings who are at risk of adverse reactions to metal debris?

AIMS

We investigated whether blood metal ion levels could effectively identify patients with bilateral Birmingham Hip Resurfacing (BHR) implants who have adverse reactions to metal debris (ARMD).

PATIENTS AND METHODS

Metal ion levels in whole blood were measured in 185 patients with bilateral BHRs. Patients were divided into those with ARMD who either had undergone a revision for ARMD or had ARMD on imaging (n = 30), and those without ARMD (n = 155). Receiver operating characteristic analysis was used to determine the optimal thresholds of blood metal ion levels for identifying patients with ARMD.

RESULTS

The maximum level of cobalt or chromium ions in the blood was the parameter which produced the highest area under the curve (91.0%). The optimal threshold for distinguishing between patients with and without ARMD was 5.5 µg/l (83.3% sensitivity, 88.4% specificity, 58.1% positive and 96.5% negative predictive values). Similar results were obtained in a subgroup of 111 patients who all underwent cross-sectional imaging. Between 3.2% and 4.3% of patients with ARMD were missed if United Kingdom (7 µg/l) and United States (10 µg/l) authority thresholds were used respectively, compared with 2.7% if our implant specific threshold was used, though these differences did not reach statistical significance (p ≥ 0.248).

CONCLUSION

Patients with bilateral BHRs who have blood metal ion levels below our implant specific threshold were at low-risk of having ARMD. Cite this article: Bone Joint J 2016;98-B:1455-62.

A 5-year survival analysis of 160 Biomet Magnum M2 metal-on-metal total hip prostheses

BACKGROUND

Large-head metal-on-metal (MoM) total hip arthroplasties (THA) are associated with high failure rates and possible pseudotumour formation. This study reports the first results of 160 Biomet Magnum M2 large-head MoM total hip articulations.

PATIENTS AND METHODS

From 2006 to 2010 the Reinier de Graaf Hospital implanted 160 large-head Magnum M2 MoM THAs (Biomet Inc. Warsaw, Indiana, USA) in 150 patients. These patients were recalled after a warning from the Dutch Orthopaedic Association. Patients were offered a clinical and radiographic assessment of the hip prosthesis, serum control on cobalt and chromium ions, and an ultrasound of the hip. If indicated, additional MARS-MRI or CT scan was performed. Descriptive statistical analysis, correlations, t-tests, non-parametric tests and implant survival were calculated.

RESULTS

The mean follow-up was 6.1 years (4.8-8.4). A cumulative survival rate of 93.1% (95% CI: 88.3-98%) was found after 5 years. Reasons for revision were loosening, pain, infection and pseudotumour formation. The prevalence of pseudotumour formation around the prostheses was 8.75%.

CONCLUSIONS

This study reports the first results of 160 MoM THAs implanted in our clinic from 2006-2010. In total, 13 (8.1%) of the THAs were eligible for revision after the recall. In most patients the reason for revision was pseudotumour formation. A total of 14 (8.75%) pseudotumours were diagnosed at the first recall. These results show that a comprehensive follow-up strategy is essential for MoM THAs to promptly identify and manage early complications.

Predicting high blood metal ion concentrations following hip resurfacing

PURPOSE

To determine whether gender, femoral head size, acetabular inclination, and time since surgery predicted high blood metal ion concentrations following Birmingham Hip Resurfacing (BHR).

METHODS

BHR patients with unilateral bearings at one specialist centre with blood cobalt and chromium concentrations measured up to May 2013 were included. This comprised a mixed (at-risk) group including symptomatic patients and asymptomatic individuals with specific clinical and/or radiological findings. Blood sampling was at a mean of 7.5 years (range 1-15.4 years) postoperatively.

RESULTS

Of 319 patients (mean age 49.3 years; 53% male), blood metal ions greater than 7 µg/l were observed in 9% (n = 28). Blood metal ions were significantly higher in females (p&lt;0.001), femoral head sizes ≤48 mm (p&lt;0.01), and cup inclinations &gt;55° (p&lt;0.001). Linear regression demonstrated femoral head size was responsible for the highest proportion of variance in blood metal ions (cobalt p&lt;0.001, R2 = 8%; chromium p&lt;0.001, R2 = 11%). Analysis of femoral head size and inclination together demonstrated 36% of BHRs with head sizes of 38-44 mm and inclination &gt;55° had blood metal ions &gt;7 µg/l. BHR 10-year survival for this at-risk group was 91% (95% confidence intervals 86.0%-95.0%) with 30 hips revised.

CONCLUSIONS

If blood metal ions are used to screen hip resurfacing patients for adverse reactions to metal debris it is recommended those with small femoral head sizes (38-44 mm) and high acetabular inclinations (&gt;55°) are targeted. These findings require validation in other cohorts as they may not be applicable to all hip resurfacing devices given the differences in radial clearance, coverage arc, and metallurgy.

Surgeons' Accuracy in Achieving Their Desired Acetabular Component Orientation

BACKGROUND

Wide variability in cup orientation has been reported. The aims of this study were to determine how accurate surgeons are at orientating the acetabular component and whether factors such as visual cues and the side of operating table improved accuracy.

METHODS

A pelvic model was positioned in neutral alignment on an operating table and was prepared as in a posterior approach. Twenty-one surgeons (9 trainers and 12 trainees) were tasked with positioning an acetabular component in a series of target orientations. The orientation of the component was measured using stereophotogrammetry, and the difference between the achieved orientation and the target orientation was calculated. Tasks included stating the surgeon's preferred orientation and thereafter placing the cup in that orientation, reproducing visual cues (transverse acetabular ligament and alignment guide), altering orientation by 10°, and estimating orientation while on the assistant's side.

RESULTS

The preferred inclination was 42° and the preferred anteversion was 21°. On average, surgeons decreased the inclination by 4° and increased the anteversion by 11° when tasked with replicating their desired orientation. The variability (defined as 2 standard deviations) in achieving a target orientation was 14°. The use of visual cues, such as the transverse acetabular ligament or the alignment guide, significantly improved accuracy to 1° for anteversion (p < 0.001) and -3° for inclination (p = 0.003). In addition, the use of an alignment guide reduced the variability by one-third. Trainees and trainers had similar accuracy and variability. There was greater variability in assessing cup inclination when standing on the assistant's side compared with the surgeon's side of the table, which has implications for training.

CONCLUSIONS

Surgeons overestimate operative inclination and underestimate anteversion, which is of benefit, as this, on average, helps to achieve the desired radiographic cup orientation. Although the use of visual cues helps, conventional techniques result in a large variability in acetabular component orientation. New and better guides and methods for training need to be developed.

Prevalence of and Risk Factors for Hip Resurfacing Revision: A Cohort Study Into the Second Decade After the Operation

BACKGROUND

Most metal-on-metal hip resurfacing (MoMHR) designs have experienced high short-term failure rates because of pseudotumors. The impact of this complication into the second decade after the procedure is unknown. We investigated (1) the prevalence of, and risk factors for, all-cause and pseudotumor-related revision at up to 15 years following MoMHR and (2) whether risk factors were sex-specific.

METHODS

This single-center prospective cohort study included 1,429 MoMHRs (1216 patients; 40% female) implanted between 1999 and 2009. Patients were contacted in 2010 and 2012 as per national recommendations. Patients with symptoms related to the hip and/or suboptimal Oxford Hip Scores (≤41 of 48 points) underwent cross-sectional imaging and blood metal-ion sampling. Revision diagnoses were established using operative and histopathological findings. Multivariate Cox proportional hazard models were used to assess the association of predictor variables with the time to all-cause and pseudotumor-related revisions.

RESULTS

One hundred and eighty MoMHRs (12.6%) were revised for all causes, and 111 (7.8% of the series and 61.7 % of all revisions) were revised because of pseudotumor. Survival analysis showed the 15-year cumulative revision rate for all causes to be 19.5% (95% confidence interval [CI] = 16.2% to 23.2%) and the 15-year rate of revision due to pseudotumor to be 14.0% (95% CI = 11.0% to 17.7%). Small femoral head size (hazard ratio [HR] per 2 mm = 0.92, 95% CI = 0.88 to 0.97; p = 0.003) and certain implant designs (HR = 1.55 to 3.01; p ≤ 0.029) significantly increased the all-cause revision risk. Female sex (HR = 2.03, 95% CI = 1.19 to 3.44; p = 0.009) and young age (HR per year = 0.98, 95% CI = 0.96 to 1.00; p = 0.020) significantly increased the pseudotumor-related revision risk but not the all-cause revision risk. Risk factors for all-cause and pseudotumor-related revision were sex-specific. In females, small femoral head size (p = 0.014) increased the all-cause revision risk, and young age was the only predictor of pseudotumor-related revision (p = 0.019). In males, implant design was the only predictor of all-cause revision (p ≤ 0.015) and pseudotumor-related revision (p = 0.001).

CONCLUSIONS

The prevalence and rates of revision for all causes and pseudotumor were high at up to 15 years following MoMHR. Predictors of revision differed between all-cause and pseudotumor-related revisions and were sex-specific. These factors must be appropriately weighted when risk-stratifying patients with MoMHRs for surveillance.

LEVEL OF EVIDENCE

Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.

Difference in the acetabular cup orientation in standing and supine radiographs

Acetabular orientation changes with that of the pelvis during lying and standing. This study was designed to measure these changes. We assessed 17 BHR replacements using EBRA software. The mean acetabular anteversion was more (p = 0.02) on erect than supine radiographs. Linear regression analysis showed that anteversion and inclination increased in some while decreased in others, and Bland and Altman analysis showed wide limits of agreement. The changes in acetabular orientation are thus subject to significant variations between the patients. We suggest studying the factors affecting acetabular orientation in standing to help reduce joint reaction forces and improve outcomes.

Gait alterations can reduce the risk of edge loading

Following metal-on-metal hip arthroplasty, edge loading (i.e., loading near the edge of a prosthesis cup) can increase wear and lead to early revision. The position and coverage angle of the prosthesis cup influence the risk of edge loading. This study investigates the effect of altered gait patterns, more specific hip, and pelvis kinematics, on the orientation of hip contact force and the consequent risk of antero-superior edge loading using muscle driven simulations of gait. With a cup orientation of 25° anteversion and 50° inclination and a coverage angle of 168°, many gait patterns presented risk of edge loading. Specifically at terminal double support, 189 out of 405 gait patterns indicated a risk of edge loading. At this time instant, the high hip contact forces and the proximity of the hip contact force to the edge of the cup indicated the likelihood of the occurrence of edge loading. Although the cup position contributed most to edge loading, altering kinematics considerably influenced the risk of edge loading. Increased hip abduction, resulting in decreasing hip contact force magnitude, and decreased hip extension, resulting in decreased risk on edge loading, are gait strategies that could prevent edge loading. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1069-1076, 2016.

Standing radiological analysis with a low-dose biplanar imaging system (EOS system) of the position of the components in total hip arthroplasty using an anterior approach

Aims

The primary aim of this study was to analyse the position of the acetabular and femoral components in total hip arthroplasty undertaken using an anterior surgical approach.

Patients and Methods

In a prospective, single centre study, we used the EOS imaging system to analyse the position of components following THA performed via the anterior approach in 102 patients (103 hips) with a mean age of 64.7 years (sd 12.6). Images were taken with patients in the standing position, allowing measurement of both anatomical and functional anteversion of the acetabular component.

Results

The mean inclination of the acetabular component was 39° (standard deviation (sd) 6), the mean anatomical anteversion was 30° (sd 10), and the mean functional anteversion was 31° (sd 8) five days after surgery. The mean anteversion of the femoral component was 20° (sd 11). Anatomical and functional anteversion of the acetabular component differed by >  10° in 23 (22%) cases. Pelvic tilt was the only pre-operative predictive factor of this difference.

Conclusion

Our study showed that anteversion of the acetabular component following THA using the anterior approach was greater than the recommended target value, and that substantial differences were observed in some patients when measured using two different measurement planes. If these results are confirmed by further studies, and considering that the anterior approach is intended to limit the incidence of dislocation, a new correlation study for each reference plane (anatomical and functional) will be necessary to define a ‘safe zone’ for use with the anterior approach. Take home message: EOS imaging system is helpful in the pre-operative and post-operative radiological analysis of total hip arthroplasty. Cite this article: Bone Joint J 2016;98-B:326–333.

Hip Resurfacing Implant Orientation Analysis: A Comparison of a Computer-Added Design Technique and Computed Tomography

BACKGROUND

Accurate acetabular component orientation in hip resurfacing is mandatory. The aim of this study is to analyze if interpretation of pelvic radiographs with computer-added design (CAD) software is comparable to computed tomography (CT) in measurement of acetabular anteversion and inclination of a Birmingham Hip Resurfacing (BHR) hip.

METHODS

A consecutive series of 49 patients (50 hips) who underwent hip resurfacing arthroplasty between 2005 and 2007 with the BHR system were retrospectively included. The surgical procedure was performed by 1 orthopedic surgeon in the beginning of his learning curve. Computer-added design software was used to measure acetabular component orientation on an anteroposterior pelvic radiograph. These measurements were compared with CT measurements. We calculated the correlation between the CAD software and CT analysis. The degree of underestimation or overestimation was determined, and a Bland-Altman plot was created to visualize the agreement between CAD software and CT results.

RESULTS

We analyzed 50 BHR hips with mean inclination of 54.6° and 55.6° and mean anteversion of 24.8° and 13.3° measured by CT and CAD, respectively. Pearson correlation coefficient for inclination was 0.69 (P < .001) and for anteversion 0.81 (P < .001). Computer-added design showed a mean underestimated anteversion of 11.6° (P < .001). There was no significant underestimation or overestimation of inclination with CAD analysis compared to CT measurements.

CONCLUSION

The CAD software is useful to assess acetabular inclination in hip resurfacing but underestimates anteversion.

How Should We Follow-Up Asymptomatic Metal-on-Metal Hip Resurfacing Patients? A Prospective Longitudinal Cohort Study

Current surveillance for metal-on-metal hip resurfacing (MoMHR) patients is not evidence based. This study established changes that occurred in 152 asymptomatic MoMHRs using repeat ultrasound and patient-reported outcomes. Factors associated with (1) ultrasound progression and (2) developing new pseudotumors were analyzed. Patients underwent repeat assessments 4.3 years later. Ultrasound progression was observed in 19% (n = 29), with 10% (n = 15) developing new pseudotumors. Key predictors of ultrasound progression included high blood cobalt (P = .00013) and chromium (P = .00065), and high initial ultrasound grade (P = .003) and volume (P = .036). No asymptomatic MoMHRs with initially normal metal ions (<2 μg/L) and normal ultrasounds (33% of cohort) developed new pseudotumors. This patient subgroup does not require repeat follow-up within 5 years.

Effects of coordinate-system construction methods on postoperative computed tomography evaluation of implant orientation after total hip arthroplasty

OBJECTIVE

In total hip arthroplasty, it is important to assess postoperative implant orientation. The computed tomography-based (CT-based) three-dimensional (3D) templating method using 3D preoperative planning software is generally recommended. In this method, postoperative implant orientation within a bony coordinate system can be measured by overlaying a 3D computerized model of the implant on a real postoperative CT image of the implant. The bony coordinate system consists of several reference points (RPs) marked on a CT image of the bone surface. Therefore, preoperative and postoperative coordinate systems do not always match. We investigated how the difference between coordinate systems constructed from RPs chosen by manual methods (M1 and M2) and those constructed by the computer matching method influences the results of measurement validation.

METHODS

In M1, postoperative RPs were chosen without a specific tool in a single planning module. In M2, postoperative RPs were chosen with as little deviation as possible from preoperative RPs, verifying preoperative RPs on another monitor.

RESULTS

M1 and M2 produced mean errors in acetabular cup inclination of 0.7° ± 0.5° and 0.5° ± 0.3°, respectively, and mean errors in cup anteversion of 1.3° ± 1.2° and 0.5° ± 0.4°, respectively, which were statistically significant differences. M1 and M2 produced mean errors in femoral stem anteversion of 2.4° ± 2.0° and 2.7° ± 2.1°, respectively, not a significant difference, but these errors were larger than errors in cup orientation.

DISCUSSION

We recommend referring to preoperative RPs when choosing postoperative RPs. Surgeons must be aware that for evaluation of postoperative stem anteversion, manual methods may produce considerable error.

Follow-Up of Metal-on-Metal Hip Arthroplasty Patients Is Currently Not Evidence Based or Cost Effective

Over one-million patients worldwide have received metal-on-metal (MoM) hip arthroplasties with a significant proportion requiring revision surgery in the short-term for adverse reaction to metal debris (ARMD). Worldwide authorities have subsequently issued follow-up guidance for MoM hip patients. This article compares follow-up guidelines for MoM hips published by five worldwide authorities, analyses these protocols in relation to published evidence, and assesses the financial implications of these guidelines. A number of major differences exist between authorities regarding patient follow-up, with vast cost differences between protocols (£84 to £988/patient/year for stemmed MoM hips and £0 to £988/patient/year for hip resurfacing). Current worldwide guidance is neither evidence-based nor financially sustainable with most protocols lacking the sensitivity to detect asymptomatic ARMD lesions.

2D measurements of cup orientation are less reliable than 3D measurements

BACKGROUND AND PURPOSE

2D analysis of metal-on-metal (MoM) hip arthroplasty (HA) has been conducted in several large series on conventional radiographs with the use of Ein Bild Roentegen Analyse (EBRA) software, but there have been no comparisons with 3D analysis in the literature. The main aim of this study was to quantify the agreement in measurements of cup version of large-diameter MoM hips obtained by EBRA and by 3D computed tomography (3D-CT). The secondary aim was to quantify the agreement for cup inclination. Lastly, we wanted to determine the inter- and intra-observer reliability of both methods.

PATIENTS AND METHODS

87 MoM hips in 81 patients were analyzed for cup inclination and version in 2D on conventional radiographs using EBRA software. The results were compared with 3D measurements using CT.

RESULTS

Cup version was underestimated by EBRA when compared to 3D-CT, by 6° on average with the pelvis supine and by 8° on average with the pelvis orientated to the anterior pelvic plane (APP). For inclination, the mean difference was no more than 1°. 53% of hips were within a 10° safe zone of 45° inclination and 20° version when measured by 3D-CT with the pelvis supine (and 54% with the pelvis in the APP). The proportion was only 24% when measured by EBRA. Inter- and intra-observer reliability of cup version is poorer using 2D analysis than when using 3D-CT.

INTERPRETATION

Errors in version in 2D were due to the difficulty in delineating the cup rim, which was obscured by a large-diameter metal head of the same radio-opacity. This can be overcome with 3D analysis. The present study demonstrates that measurements using EBRA have poor agreement and are less reliable than those with 3D-CT when measuring cup version and inclination in MoM hips.

Birmingham Hip Resurfacing: A Single Surgeon Series Reported at a Minimum of 10 Years Follow-Up

We report outcomes on 120 Birmingham Hip Resurfacings (BHRs) (mean age 50 years) at a minimum of ten-years follow-up. Cases were performed by one surgeon and included his learning curve. Six hips were revised, with no revisions for infection, dislocation, or adverse reaction to metal debris. Ten-year survival was 94.2% (95% confidence interval (CI) 88.8%-98.7%) for all revisions and 96.1% (95% CI 91.5%-99.8%) for revisions for aseptic loosening. Gender (P = 0.463) and head size (P = 0.114) did not affect revision risk. Mean post-operative Harris hip score was 84.0. Contrary to previous independent reports, good outcomes into the second decade were achieved with the BHR in both men and women. Longer term follow-up will confirm whether these promising outcomes in women continue.

High failure rate of the R3 metal-on-metal total hip arthroplasty

We report our failures with the use of the R3 metal-on-metal bearing. Forty six patients had an R3 acetabular system metal-on-metal THR in our centre between March 2007 and March 2009. All operations were performed using femoral components and appropriately matched femoral heads manufactured by Smith and Nephew. Twelve patients underwent revision surgery for adverse reaction to metal debris (ARMD). The median acetabular inclination was 40 degrees (range 21.1-49.1) and the median acetabular anteversion was 7.5 degrees (range 3.3-10.4). The median serum Cobalt was 9.9 µg/L (range 3.1-45) and the median serum Chromium was 5.8 µg/L (range 1.8-19.3). The time to revision was 39.2 months (range 13-53). Our current failure rate is 24%.

Revision of metal-on-metal hip replacements and resurfacings for adverse reaction to metal debris: a systematic review of outcomes

PURPOSE

This systematic review assessed evidence on outcomes following revision of MoM hip resurfacings (HRs) and total hip replacements (THRs) for adverse reaction to metal debris (ARMD).

METHODS

Four electronic databases were searched between January 2009 and July 2013 to identify studies reporting clinical outcomes following revision of MoM HRs and THRs for ARMD. Only studies reporting cohorts with more than 10 metal-on-metal (MoM) hips revised for ARMD were included. Outcomes of interest following ARMD revision were: (1) complication rates; (2) re-revision rates; (3) surgical intervention other than re-revision; (4) functional outcome.

RESULTS

Of 148 unique studies identified, six studies were eligible for inclusion containing 216 MoM hips (197 HRs and 19 THRs) revised for ARMD. Mean follow-up time from ARMD revision ranged between 21-61 months. Complication rates were 4%-50% for HR and 68% for THR. Re-revision rates were 3%-38% for HR and 21% for THR. Dislocation (n = 14), ARMD recurrence (n = 11), and acetabular loosening (n = 9) were the three commonest complications and indications for re-revision. All six studies reported between one and three cases of ARMD recurrence during follow-up. One study specifically reported on performing procedures other than re-revision with 26% requiring closed reductions for dislocated THRs. Functional outcomes following ARMD revision were good or satisfactory in all but two studies.

CONCLUSIONS

Limited evidence exists regarding outcomes following revision of MoM hips for ARMD, especially for THRs. This should be addressed in future studies and may be important when counselling asymptomatic individuals in whom revision is considered for raised blood metal ions.