Objectives

Pseudotumours (abnormal peri-prosthetic soft-tissue reactions) following metal-on-metal hip resurfacing arthroplasty (MoMHRA) have been associated with elevated metal ion levels, suggesting that excessive wear may occur due to edge-loading of these MoM implants. This study aimed to quantify in vivo edge-loading in MoMHRA patients with and without pseudotumours during functional activities.

The aim of this retrospective cohort study was to identify any difference in femoral offset as measured on pre-operative anteroposterior (AP) radiographs of the pelvis, AP radiographs of the hip and corresponding CT scans in a consecutive series of 100 patients with primary end-stage osteoarthritis of the hip (43 men and 57 women with a mean age of 61 years (45 to 74) and a mean body mass index of 28 kg/m² (20 to 45)).

Patients were positioned according to a standardised protocol to achieve reproducible projection and all images were calibrated. Inter- and intra-observer reliability was evaluated and agreement between methods was assessed using Bland-Altman plots.

In the entire cohort, the mean femoral offset was 39.0 mm (95% confidence interval (CI) 37.4 to 40.6) on radiographs of the pelvis, 44.0 mm (95% CI 42.4 to 45.6) on radiographs of the hip and 44.7 mm (95% CI 43.5 to 45.9) on CT scans. AP radiographs of the pelvis underestimated femoral offset by 13% when compared with CT (p < 0.001). No difference in mean femoral offset was seen between AP radiographs of the hip and CT (p = 0.191).

Our results suggest that femoral offset is significantly underestimated on AP radiographs of the pelvis but can be reliably and accurately assessed on AP radiographs of the hip in patients with primary end-stage hip osteoarthritis.

We, therefore, recommend that additional AP radiographs of the hip are obtained routinely for the pre-operative assessment of femoral offset when templating before total hip replacement.

INTRODUCTION

Mobile bearing unicompartmental knee replacement (UKR) is an accepted treatment for patients with isolated medial unicompartmental knee osteoarthritis (OA) with a full thickness cartilage loss. The aim of this study was to determine if this recommendation was correct and if the procedure could be used for partial-thickness cartilage loss.

The aim of this study was to investigate the effect of training on the arthroscopic performance of a group of medical students and to determine whether all students could be trained to competence. Thirty-three medical students with no previous experience of arthroscopy were randomised to a ‘Trained’ or an ‘Untrained’ cohort. They were required to carry out 30 episodes of two simulated arthroscopic tasks (one shoulder and one knee). The primary outcome variable was task success at each episode. Individuals achieved competence when their learning curve stabilised. The secondary outcome was technical dexterity, assessed objectively using a validated motion analysis system. Six subjects in the ‘Untrained’ cohort failed to achieve competence in the shoulder task, compared with one in the ‘Trained’ cohort. During the knee task, two subjects in each cohort failed to achieve competence. Based on the objective motion analysis parameters, the ‘Trained’ cohort performed better on the shoulder task (p < 0.05) but there was no significant difference for the knee task (p > 0.05).

Although specific training improved the arthroscopic performance of novices, there were individuals who could not achieve competence despite focused training.These findings may have an impact on the selection process for trainees and influence individual career choices.

Since the Oxford knee was first used unicompartmentally in 1982, a small number of bearings have fractured. Of 14 retrieved bearings, we examined ten samples with known durations in situ (four Phase 1, four Phase 2 and two Phase 3). Evidence of impingement and associated abnormally high wear (> 0.05 mm per year) as well as oxidation was observed in all bearings. In four samples the fracture was associated with the posterior radio-opaque wire. Fracture surfaces indicated fatigue failure, and scanning electron microscopy suggested that the crack initiated in the thinnest region. The estimated incidence of fracture was 3.20% for Phase 1, 0.74% for Phase 2, 0.35% for Phase 3, and 0% for Phase 3 without the posterior marker wire. The important aetiological factors for bearing fracture are impingement leading to high wear, oxidation, and the posterior marker wire. With improved surgical technique, impingement and high wear should be prevented and modern polyethylene may reduce the oxidation risk. A posterior marker wire is no longer used in the polyethylene meniscus. Therefore, the rate of fracture, which is now very low, should be reduced to a negligible level.

Patella subluxation assessed on dynamic MRI has previously been shown to be associated with anterior knee pain. In this MRI study of 60 patients we investigated the relationship between subluxation and multiple bony, cartilaginous and soft-tissue factors that might predispose to subluxation using discriminant function analysis.

Patella engagement (% of patella cartilage overlapping with trochlea cartilage) had the strongest relationship with subluxation. Patellae with > 30% engagement tended not to sublux; those with < 30% tended to sublux. Other factors that were associated with subluxation included the tibial tubercle-trochlea notch distance, vastus medialis obliquus distance from patella, patella alta, and the bony and cartilaginous sulcus angles in the superior part of the trochlea. No relationship was found between subluxation and sulcus angles for cartilage and bone in the middle and lower part of the trochlea, cartilage thicknesses and Wiberg classification of the patella.

This study indicates that patella engagement is a key factor associated with patellar subluxation. This suggests that in patients with anterior knee pain with subluxation, resistant to conservative management, surgery directed towards improving patella engagement should be considered. A clinical trial is necessary to test this hypothesis.

The contraindications for unicompartmental knee replacement (UKR) remain controversial. The views of many surgeons are based on Kozinn and Scott’s 1989 publication which stated that patients who weighed more than 82 kg, were younger than 60 years, undertook heavy labour, had exposed bone in the patellofemoral joint or chondrocalcinosis, were not ideal candidates for UKR. Our aim was to determine whether these potential contraindications should apply to patients with a mobile-bearing UKR. In order to do this the outcome of patients with these potential contraindications was compared with that of patients without the contraindications in a prospective series of 1000 UKRs. The outcome was assessed using the Oxford knee score, the American Knee Society score, the Tegner activity score, revision rate and survival.

The clinical outcome of patients with each of the potential contraindications was similar to or better than those without each contraindication. Overall, 678 UKRs (68%) were performed in patients who had at least one potential contraindication and only 322 (32%) in patients deemed to be ideal. The survival at ten years was 97.0% (95% confidence interval 93.4 to 100.0) for those with potential contraindications and 93.6% (95% confidence interval 87.2 to 100.0) in the ideal patients.

We conclude that the thresholds proposed by Kozinn and Scott using weight, age, activity, the state of the patellofemoral joint and chondrocalcinosis should not be considered to be contraindications for the use of the Oxford UKR.

Lately, concerns have arisen following the use of large metal-on-metal bearings in hip replacements owing to reports of catastrophic soft-tissue reactions resulting in implant failure and associated complications. This review examines the literature and contemporary presentations on current clinical dilemmas in metal-on-metal hip replacement.

The Oxford unicompartmental knee replacement (UKR) was designed to minimise wear utilising a fully-congruent, mobile, polyethylene bearing. Wear of polyethylene is a significant cause of revision surgery in UKR in the first decade, and the incidence increases in the second decade. Our study used model-based radiostereometric analysis to measure the combined wear of the upper and lower bearing surfaces in 13 medial-compartment Oxford UKRs at a mean of 20.9 years (17.2 to 25.9) post-operatively.

The mean linear penetration of the polyethylene bearing was 1.04 mm (0.307 to 2.15), with a mean annual wear rate of 0.045 mm/year (0.016 to 0.099). The annual wear rate of the phase-2 bearings (mean 0.022 mm/year) was significantly less (p = 0.01) than that of phase-1 bearings (mean 0.07 mm/year).

The linear wear rate of the Oxford UKR remains very low into the third decade. We believe that phase-2 bearings had lower wear rates than phase-1 implants because of the improved bearing design and surgical technique which decreased the incidence of impingement. We conclude that the design of the Oxford UKR gives low rates of wear in the long term.

This prospective study describes the outcome of the first 1000 phase 3 Oxford medial unicompartmental knee replacements (UKRs) implanted using a minimally invasive surgical approach for the recommended indications by two surgeons and followed up independently. The mean follow-up was 5.6 years (1 to 11) with 547 knees having a minimum follow-up of five years. At five years their mean Oxford knee score was 41.3 (sd 7.2), the mean American Knee Society Objective Score 86.4 (sd 13.4), mean American Knee Society Functional Score 86.1 (sd 16.6), mean Tegner activity score 2.8 (sd 1.1). For the entire cohort, the mean maximum flexion was 130° at the time of final review.

The incidence of implant-related re-operations was 2.9%; of these 29 re-operations two were revisions requiring revision knee replacement components with stems and wedges, 17 were conversions to a primary total knee replacement, six were open reductions for dislocation of the bearing, three were secondary lateral UKRs and one was revision of a tibial component. The most common reason for further surgical intervention was progression of arthritis in the lateral compartment (0.9%), followed by dislocation of the bearing (0.6%) and revision for unexplained pain (0.6%). If all implant-related re-operations are considered failures, the ten-year survival rate was 96% (95% confidence interval, 92.5 to 99.5). If only revisions requiring revision components are considered failures the ten-year survival rate is 99.8% (confidence interval 99 to 100).

This is the largest published series of UKRs implanted through a minimally invasive surgical approach and with ten-year survival data. The survival rates are similar to those obtained with a standard open approach whereas the function is better. This demonstrates the effectiveness and safety of a minimally invasive surgical approach for implanting the Oxford UKR.

Pseudotumour is a rare but important complication of metal-on-metal hip resurfacing that occurs much more commonly in women than in men. We examined the relationship between head-neck ratio (HNR) and pseudotumour formation in 18 resurfaced hips (18 patients) revised for pseudotumour and 42 asymptomatic control resurfaced hips (42 patients).

Patients in whom pseudotumour formation had occurred had higher pre-operative HNR than the control patients (mean 1.37 (sd 0.10) vs mean 1.30 (sd 0.08) p = 0.001). At operation the patients with pseudotumours had a greater reduction in the size of their femoral heads (p = 0.035) and subsequently had greater neck narrowing (mean 10.1% (sd 7.2) vs mean 3.8% (sd 3.2) p < 0.001). No female patient with a pre-operative HNR ≤ 1.3 developed a pseudotumour.

We suggest that reducing the size of the femoral head, made possible by a high pre-operative HNR, increases the risk of impingement and edge loading, and may contribute to high wear and pseudotumour formation. As the incidence of pseudotumour is low in men, it appears safe to perform resurfacing in men. However, this study suggests that it is also reasonable to resurface in women with a pre-operative HNR ≤ 1.3.

We have evaluated the difference in the migration patterns over two years of two cementless stems in a randomised, controlled trial using radiostereophotogrammetric analysis (RSA). The implants studied were the Furlong HAC stem, which has good long-term results and the Furlong Active stem, which is a modified version of the former designed to minimise stress concentrations between the implant and bone, and thus to improve fixation.

A total of 23 Furlong HAC and 20 Furlong Active stems were implanted in 43 patients. RSA examinations were carried out immediately post-operatively and at six, 12 and 24 months post-operatively.

The subsidence during the first year in the Furlong HAC stem, was approximately one-third that of the Furlong Active stem, the measured mean subsidence of the femoral head at six months being 0.27 mm (95% confidence interval (CI) 0.03 to 0.51) and 0.99 mm (95% CI 0.38 to 1.60), respectively (p = 0.03). One Active stem continued to subside during the second year. All hips, regardless of the type of stem were clinically successful as judged by the Oxford hip score and a derived pain score without any distinction between the two types of stem.

The initial stability of the Furlong Active stem was not as good as the established stem which might compromise osseo-integration to the detriment of long-term success. The changes in the geometry of the stem, to minimise stress have affected the attainment of initial stability.

Pseudotumours are a rare complication of hip resurfacing. They are thought to be a response to metal debris which may be caused by edge loading due to poor orientation of the acetabular component. Our aim was to determine the optimal acetabular orientation to minimise the risk of pseudotumour formation.

We matched 31 hip resurfacings revised for pseudotumour formation with 58 controls who had a satisfactory outcome from this procedure. The radiographic inclination and anteversion angles of the acetabular component were measured on anteroposterior radiographs of the pelvis using Einzel-Bild-Roentgen-Analyse software. The mean inclination angle (47°, 10° to 81°) and anteversion angle (14°, 4° to 34°) of the pseudotumour cases were the same (p = 0.8, p = 0.2) as the controls, 46° (29° to 60°) and 16° (4° to 30°) respectively, but the variation was greater. Assuming an accuracy of implantation of ± 10° about a target position, the optimal radiographic position was found to be approximately 45° of inclination and 20° of anteversion. The incidence of pseudotumours inside the zone was four times lower (p = 0.007) than outside the zone.

In order to minimise the risk of pseudotumour formation we recommend that surgeons implant the acetabular component at an inclination of 45° (± 10) and anteversion of 20° (± 10) on post-operative radiographs. Because of differences between the radiographic and the operative angles, this may be best achieved by aiming for an inclination of 40° and an anteversion of 25°.

Purpose: This study was designed to establish the poly-ethylene wear rates in the Oxford medial unicompert-mental knee replacement.

Introduction: The Oxford meniscal bearing knee was introduced as a design to reduce polyethylene wear. There has been one previous retrieval study of the Oxford UKA, which reported very low wear rates in some specimens, but abnormal patterns of wear in others, including impingement. There has been no further investigation of these abnormal wear patterns.

Methods: Forty-seven bearings were retrieved from patients who had received a medial Oxford UKA for anteromedial osteoarthritis of the knee, none of which had previously been studied. Mean time to revision was 8.4 years (SD 4.1) and 20 had been implanted for over 10 years. The macroscopic pattern of polyethylene wear and the linear penetration (dial gauge measurement) was recorded for each bearing.

Results: The mean linear penetration rate (LPR) was 0.07mm/year. The patterns of wear fell into 4 categories, each with a different LPR; 1) No abnormal macroscopic appearance, n=16 (LPR = 0.01mm/year), 2) Abnormal macroscopic wear with extra-articular impingement, n=16 (LPR = 0.05mm/year), 3) Abnormal macroscopic wear with intra-articular impingement, n=6 (LPR = 0.10mm/year), 4) Abnormal macroscopic wear with impingement and signs of incongruous articulation, n=9 (LPR = 0.14mm/year). The differences in LPR were statistically significant (p< 0.05).

Conclusion: The results show that very low polyethylene wear rates are possible if the device functions normally. However if the bearing displays abnormal function (extra-articular, intra-articular impingement or incongruous articulation) wear rates increase significantly.

Introduction: Patellofemoral joint subluxation is associated with pain and dysfunction. The causes of patel-lofemoral subluxation are poorly understood and multi-factorial, arising from abnormalities of both bone and soft tissues. This study aims to identify which anatomical variables assessed on Magnetic Resonance (MR) images are most relevant to patellofemoral subluxation.

Method: A retrospective analysis of MR studies of 60 patients with suspected patellofemoral subluxation was performed. All patients were graded for the severity/ magnitude of radiological subluxation using a dynamic MR scan (Grade 0 [nil] to Grade 3 [subluxed]. The patient scans were assessed using a range of anatomical variables, these included:

Patella alta,

Results: The distance of the VMO from the patella (p < 0.001), TTD (p < 0.001) and Patella Engagement (p < 0.001) showed highly significant relationships with patellofemoral subluxation.

Conclusions: The following three anatomical variables are associated with patellofemoral subluxation: the distance of the VMO muscle from the patella, TTD and Patella Engagement.

This is the first study to establish that patella engagement is related to PFJ subluxation showing that the lower the percentage engagement of the patella in the trochlea, the greater the severity/magnitude of patellofemoral subluxation. The finding provides greater insight into the aetiology and understanding of the mechanism of symptomatic PFJ subluxation.

The cause of fracture of the femoral neck after hip resurfacing is poorly understood. In order to evaluate the role of avascular necrosis we compared 19 femoral heads retrieved at revision for fracture of the femoral neck and 13 retrieved for other reasons.

We developed a new technique of assessing avascular necrosis in the femoral head by determining the percentage of empty osteocyte lacunae present. Femoral heads retrieved as controls at total hip replacement for osteoarthritis and avascular necrosis had 9% (sd 4; n = 13) and 85% (sd 5; n = 10, p < 0.001) empty lacunae, respectively.

In the fracture group the percentage of empty lacunae was 71% (sd 22); in the other group it was 21% (sd 13). The differences between the groups were highly significant (p < 0.001).

We conclude that fracture after resurfacing of the hip is associated with a significantly greater percentage of empty osteocyte lacunae within the trabecular bone. This indicates established avascular necrosis and suggests that damage to the blood supply at the time of surgery is a potent risk factor for fracture of the femoral neck after hip resurfacing.

Anteromedial Osteoarthritis of the Knee (AMOA) is a distinct phenotype of OA. Within this pattern of disease, the anterior third of the medial tibial plateau exhibits full thickness cartilage loss. The middle third has damaged partial thickness cartilage, and the posterior third has retained cartilage, which is seen on macroscopic visual assessment to be normal. This study investigates the molecular features of progressive severities of cartilage damage within this phenotype.

Ten medial tibial plateau specimens were collected from patients undergoing unicompartmental knee replacements. The cartilage within the area of macroscopic damage was divided into equal thirds: T1(most damaged), to T3 (least damaged). The area of macroscopically undamaged cartilage was taken as a 4th sample, N. The specimens were prepared for histological (Safranin-O) and immunohistochemical analysis (Type I and II Collagen, proliferation and apoptosis). Immunoassays were undertaken for Collagens I and II and GAG content. Real time PCR compared gene expression between areas T and N.

There was a decrease in OARSI grade across the four areas, with progressively less fibrillation between areas T1, T2 and T3. Area N had a grade of 0 (normal). The GAG immunoassay showed decreased levels with increasing severity of cartilage damage (p< 0.0001). Proliferation and apoptosis, as expected, were increased in the more damaged areas. There was no significant difference in the Collagen II content or gene expression between areas. The Collagen I immunohistochemistry showed increased staining within chondrocyte pericellular areas in the undamaged region (N) and immunoassays showed that the Collagen I content of this macroscopically and histologically normal cartilage, was significantly higher than the damaged areas (p< 0.0001). Furthermore, real time PCR showed a significant increase in Collagen I expression in the macroscopically normal areas compared to the damaged areas (p=0.04).

We conclude that in this phenotype the Collagen I increase, in areas of macroscopically and histologically normal cartilage, may represent very early changes of the cartilage matrix within the osteoarthritic disease process. This may be able to be used as an assay of early disease and as a therapeutic target for disease modification or treatment.

The presence of pseudotumours, which are soft-tissue masses relating to the hip, after metal-on-metal hip resurfacing arthroplasty has been associated with elevated levels of metal ions in serum, suggesting that pseudotumours occur when there is increased wear. We aimed to quantify the wear in vivo of implants revised for pseudotumours (eight) and of a control group of implants (22) revised for other reasons of failure.

We found that the implant group with pseudotumours had a significantly higher rate of median linear wear of the femoral component at 8.1 μm/year (2.75 to 25.4) than the 1.79 μm/year (0.82 to 4.15; p = 0.002) of the non-pseudotumour group. For the acetabular component a significantly higher rate of median linear wear of 7.36 μm/year (1.61 to 24.9) was observed in the pseudotumour group compared with 1.28 μm/year (0.81 to 3.33, p = 0.001) in the other group. Wear of the acetabular component in the pseudotumour group always involved the edge of the implant, indicating that edge-loading had occurred.

Our findings are the first direct evidence that pseudotumour is associated with increased wear at the metal-on-metal articulation. Furthermore, edge-loading with the loss of fluid-film lubrication may be an important mechanism of generation of wear in patients with a pseudotumour.

We report the consensus of surgical opinions of an international faculty of expert metal-on-metal hip resurfacing surgeons, with a combined experience of over 18 000 cases, covering required experience, indications, surgical technique, rehabilitation and the management of problematic cases.

The Oxford Unicompartmental Knee replacement (UKR) was introduced as a design to reduce polyethylene wear. There has been one previous retrieval study involving this implant, which reported very low rates of wear in some specimens but abnormal patterns of wear in others. There has been no further investigation of these abnormal patterns. The bearings were retrieved from 47 patients who had received a medial Oxford UKR for anteromedial osteoarthritis of the knee. None had been studied previously. The mean time to revision was 8.4 years (sd 4.1), with 20 having been implanted for over ten years. The macroscopic pattern of polyethylene wear and the linear penetration were recorded for each bearing. The mean rate of linear penetration was 0.07 mm/year. The patterns of wear fell into three categories, each with a different rate of linear penetration; 1) no abnormal macroscopic wear and a normal articular surface, n = 16 (linear penetration rate = 0.01 mm/year); 2) abnormal macroscopic wear and normal articular surfaces with extra-articular impingement, n = 16 (linear penetration rate = 0.05 mm/year); 3) abnormal macroscopic wear and abnormal articular surfaces with intra-articular impingement +/− signs of non-congruous articulation, n = 15 (linear penetration rate = 0.12 mm/year). The differences in linear penetration rate were statistically significant (p < 0.001).

These results show that very low rates of polyethylene wear are possible if the device functions normally. However, if the bearing displays suboptimal function (extra-articular, intra-articular impingement or incongruous articulation) the rates of wear increase significantly.