Introduction: Metal on metal hip resurfacing arthroplasty(MoMHRA) is an alternative option to THR in the treatment of young adults with OA. A recognised MoMHRA complication is the development of an inflammatory pseudotumour(IP). Diagnosis is made with the aid of US and/or MRI. To-date, no radiographic indication of the presence of IP has been identified. Neck thinning is a recognised phenomenon in MoMHRA hips not associated with any adverse clinical events. Its pathogenesis is considered multi-factorial. Our aim was to establish whether excessive neck narrowing is associated with the presence of a pseudotumour.

Methods: Twenty-seven hips (26 patients) with IP confirmed clinically, radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with an asymptomatic MoMHRA cohort (Control n=60). For all patients, prosthesis-neck-ratio(PNR) was measured on plain AP pelvic radiographs post-operatively and at follow-up as previously described and validated.

Results: All IP patients (4M:23F) and all (12M:48F) but two controls had a posterior approach at the time of MoMHRA. Post-operatively, there was no difference in the PNR between the two groups (p=0.19). At an average follow up of 3.5 years (range:0.7–8.3), IP patients(mean 1.26, 1.10–1.79) had a significantly higher (p< 0.0001) PNR in comparison to their controls(mean 1.14, 1.03–1.35). Greater neck narrowing occurred in both genders. IP necks had narrowed by an average of 8% (range:3–23). The degree of neck narrowing was correlated with length of survival of implant (p=0.001).

Discussion: This study shows a strong association between IP and neck narrowing. Processes such as impingement and increased wear are considered to be involved in the pathogenesis of both IP and neck narrowing. Furthermore, the presence of an IP, could lead to altered vascularity via a mass effect and further contribute to neck narrowing. Neck narrowing in symptomatic MOMHRA patients should alert surgeons of the possible presence of IP.

Metal on metal hip resurfacing (MMHR) is a popular procedure for the treatment of osteoarthritis in young patients. Several centres have observed masses, arising from around these devices, we call these inflammatory pseudotumours. They are locally invasive and may cause massive soft tissue destruction. The aim of this study was to determine the incidence and risk factors for pseudotumours that are serious enough to require revision surgery.

In out unit, 1,419 MMHRs were performed between June 1999 and November 2008. All revisions were identified, including all cases revised for pseudotumour. Pseudotumour diagnosis was made by histological examination of samples from revision. A Kaplan-Meier survival analysis was performed, Cox regression analysis was used to estimate the independent effects of different factors.

The revision rate for pseudotumour increased with time and was 4% (95% CI: 2.2% to 5.8%) at eight years. Female gender was a strong risk factor: at eight years the revision rate for pseudotumours in men was 0.5% (95% CI 0% to 1.1%), in women over 40 it was 6% (95% CI 2.3% to 10.1%) and in women under 40 it was 25% (95% CI 7.3% to 42.9%) (p< 0.001). Other factors associated with an increase in revision rate were, small components (p=0.003) and dysplasia (p=0.019), whereas implant type was not (p=0.156).

We recommend that resurfacings are undertaken with caution in women, especially those younger than 40 years of age, but they remain a good option in men. Further work is required to understand the patho-aetiology of pseudotumours so that this severe complication can be avoided.

Introduction: Edge-loading, a phenomenon whereby the femoral component comes into contact with the edge of the acetabular component, has been suggested to increase wear in metal-on-metal hip resurfacing arthroplasty (MoMHRA). Pseudotumours (soft-tissue mass relating to the hip joint) have been associated with elevated serum and hip aspirate metal ion levels. This study aimed to investigate in vivo edge-loading in MoMHRA patients with pseudotumours by quantifying dynamic loci of the hip joint segment force relative to the acetabular component during functional activities.

Materials and Methods: A total of 21 MoMHRA patients (30 hips) in two groups were investigated in this Ethics approved study:

6 patients with pseudo-tumours detected using ultrasound/MRI;

Three-dimensional lower limb motion analysis (12 camera Vicon System) was performed to estimate hip joint segment force during walking, chair-rising and stair-climbing. CT scans were used to determine each patient’s specific hip joint centre and acetabular component orientation. Edge-loading was defined to occur when a hip joint segment force vector/ cup intersection was located within 10% of the cup radius from the edge of the cup. Serum cobalt and chromium levels were analysed using Inductively-Coupled Plasma Spectrometer.

Results: Edge-loading in the pseudotumour group occurred with significantly (p=0.02) longer (4-fold increase) duration as well as greater magnitude (7-fold increase) of force, compared to the non-pseudotumour group. The duration and force of the edge-loading were activity-dependent, with proportionally greater difference observed during stair climbing. The acetabular cup orientation values in the pseudotumour group were found within the safe zone of Lewinnek in one third of the hips with the remaining two thirds outside the safe zone. The presence of pseudotumour was associated with:

significantly higher median serum cobalt levels: 14.3ug/l (range 10.6–64.1) vs. 1.9ug/l (range 1.2–5.0), p< 0.001;

Discussion: Edge-loading in MoMHRA patients with pseudotumours occurred in vivo with significantly longer duration and greater magnitude of force impulse compared to the patients with a well functioning MoMHRA during activities of daily living. This suggests that edge-loading may be an important mechanism that leads to localised high wear, with subsequent elevation of metal ion levels in MoMHRA patients with pseudotumours. Although the acetabular component malposition, such as increase in both inclination and anteversion angles, appears to be an important factor in edge-loading, the aetiology of edge-loading is likely to be multi-factorial.

Introduction: A unique failure mode of hip resurfacing is femoral neck fracture. These tend to occur early after surgery during normal activities. One theory regarding fracture occurrence includes the introduction of stress magnifiers in the form of notches on the superior neck. The presence of a notch can arise from reaming or from removal of osteophytes during surgery. The aim of the present study was to investigate the effect of notching the femoral neck, following resurfacing by using a finite element (FE) model.

Methods: A physiological load case was simulated in the FE model of a femur, implanted with a cemented hip resurfacing system. Twelve implant alignments were modelled: an ideal implant alignment with no notch, and a 1 mm, 3 mm, 5 mm and 7 mm superior notch; 5° anteversion, 5° and 10° degrees retroversion; 5° and 10° degrees in varus and valgus. These models were compared to that of an intact femur for baseline analysis.

The intact femur geometry was derived from a CT dataset of a cadaveric femur and CT numbers were converted into a realistic distribution of material properties. The FE intact mesh was based on an experimentally validated mesh of a human femur. The femur was segmented into 22 neck sections.

The loading condition was modelled to represent an instant at 10% of gait where all muscle forces were included. The femoral neck regions were compared between the models to evaluate the effect of notch sizes on stress distribution. Maximum tensile stresses were compared to the ultimate tensile stress (UTS) of cortical and cancellous bone.

Results: As the notch size increased the peak and average 1st (tensile) and 3rd (compressive) principal stress increased along the superior portion of the femoral neck. For the 5 mm superior notch, the maximum 1st principal stress increased by 283% and 154% when compared to that of the ideally aligned implant and the intact femur respectively. The largest increase of tensile stress was observed when the implant was mal-aligned in 10° of varus; this resulted in a 768% increase in stress compared to the ideally implanted model.

Discussion: The introduction of a superior notch causes a stress concentration on the femoral neck. Although the stress concentration is pronounced, a notch on the superior aspect of the femoral neck may not lead to fracture following resurfacing; the UTS of cortical bone is 100MPa, and the UTS of cancellous bone is between 2MPa and 20MPa. Peak stresses in the model are well below the UTS of cortical bone, and for damage to accumulate in cancellous bone, energy absorption in the ‘honey-comb’ structure of trabecular bone must be considered. Varus mal-alignment resulted in the largest increase in tensile stress on the superior aspect of the neck, and has been associated with femoral neck fracture; this type of mal-alignment may be critical when considering femoral neck fractures.

Introduction: The results of the mobile bearing Oxford partial knee replacement (PKR) in the lateral compartment have been disappointing with a five year survival of 82%. Bearing dislocation is a particular concern, and to address this issue a new domed implant was introduced with a modified surgical technique. The aim of this study was to compare the risk of dislocation between a domed and flat lateral PKR.

Methods: Separate kinematic models were generated for the domed and flat bearings. The femoral component, tibial tray and bearing were aligned in a neutral position; the flat bearing was positioned centrally on the tibial tray and 2 mm from the side wall; the domed bearing was placed concentrically on the domed tibial tray. Dislocation in the Posterior (A-P), Lateral (M-L) and Medial against the tray wall (L-M-wall) were investigated. For each dislocation the tibial tray was restrained in all degrees of freedom (DOF) and the femoral component was restrained in five DOF; A-P and M-L displacements; A-P, M-L and Superior-Inferior (S-I) rotations. The bearing was restrained from rotating about the S-I axis for each dislocation. For the L-M-wall dislocation the underside of the bearing was held in contact with the tibial tray wall such that the lowest S-I displacement of the femoral component was achieved. The least amount of distraction required for bearing dislocation to occur was calculated for the seven bearing sizes available. The effect of medial-lateral positioning of the femur on dislocation was investigated.

Results: The minimum femur distraction to cause A-P flat and domed bearing dislocation ranged from 4.68mm to 3.91mm and 6.29mm to 5.59mm respectively as the bearing thickness increased from 3.5mm to 9.5mm. The minimum femur distraction to cause L-M-wall flat and domed bearing dislocation ranged from 3.42mm to 4.16mm and 4.55mm to 5.44mm respectively as the bearing thickness increased from 3.5 mm to 9.5 mm. The femur distraction required for L-M-wall bearing dislocation increased from 4.55mm to 6.3mm with a 2 mm medial movement of the femoral component. A 4 mm lateral movement of the femoral component decreased the distraction from 4.55mm to 2.35mm.

Discussion: A domed bearing can lead to an increased femoral distraction of between 25% and 37%, significantly reducing the likelihood of dislocation. This may be significant during everyday activities and demonstrates that the new domed design should reduce the incidence of bearing dislocation by increasing the amount of entrapment; our current series of 200 patients has no dislocations. Increasing the thickness of the bearing has a small effect on the distraction required to allow bearing dislocation. The medial-lateral placement of the femoral component has a pronounced effect on the femoral distraction required for bearing dislocation over the tray wall; medial placement of the femoral component is advisable.

Introduction: Anteromedial gonarthrosis is a common well described pattern of knee osteoarthritis with cartilage wear beginning in the anteromedial quadrant of the medial tibial plateau in the presence of an intact and functioning ACL. It is well known that mechanical factors such as limb alignment and meniscal integrity affect the progression of arthritis and there is some evidence that the morphology of the tibial plateau may be a risk factor in the development of this disease. The extension facet angle is the angle of the downslope of the anterior portion of the medial tibial plateau joint surface in relation to the middle portion on a sagittal view. If this is an important factor in the development of AMG there may be potential for disease modifying intervention.

This study investigates if there is a significant difference in this angle as measured on MRI between a study cohort with early AMG (partial thickness cartilage damage and intact ACL) and a comparator control cohort of patients (no cartilage damage and ACL rupture).

Methods: 3 Tesla MRI scans of 99 patients; 54 with partial thickness cartilage damage and 44 comparitors with no cartilage damage (acute ACL rupture) were assessed. The extension facet angle was measured (Osirix v3.6) using a validated technique on two consecutive MRI T2 sagittal slices orientated at the mid-coronal point of the medial femoral condyle. (InterClass Correlation 0.95, IntraClass Correlation 0.97, within subject variation of 1.1° and coefficiant of variation 10.7%). The mean of the two extension angle values was used. The results were tabulated and analysed (R v2.9.1).

Results: Of the 99 knees, 38 were female and 61 male; 44 left knees and 55 right. The mean extension facet angle for the partial thickness group was 12.7° (SD 3.35) and for the comparator group 8.7° (SD 3.09). There was a significant difference between these 2 groups (Mann Whitney U, p< 0.001). Although there were significantly more men than women in the comparator group, stratification analysis showed that there was no effect of gender on the mean extension facet angle.

Discussion: There is a significance difference in the extension facet angle between patients with AMG with only partial thickness cartilage loss and a comparator group. This has not been shown in a study group of this size before. Since none of the subjects had full thickness cartilage loss it is unlikely that this difference is due to bone attrition changing the angle as part of the disease process but this is an important area for further study. We believe that a higher medial tibial extension facet angle alters the mechanics within the medial compartment, placing these patients at higher risk of developing AMG. This may present an opportunity for risk factor modification, for example osteotomy.

Introduction: Pseudotumours (soft-tissue masses relating to the hip joint) following metal-on-metal hip resurfacing arthroplasty (MoMHRA) have been associated with elevated serum and hip aspirate metal ion levels, suggesting that pseudotumours occur when there is increased wear. This study aimed to quantify in vivo wear of implants revised for pseudotumours and a control group of implants revised for other reasons of failure.

Methods: A total of 30 contemporary MoMHRA implants in two groups were investigated in this Institutional Review Board approved study:

8 MoMHRA implants revised due to pseudotumour;

The linear wear of retrieved implants was measured using a Taylor-Hobson Roundness machine. The average linear wear rate was defined as the maximum linear wear depth divided by the duration of the implant in vivo.

Results: In comparison with the non-pseudotumour implant group, the pseudotumour implant group was associated with:

significantly higher median linear wear rate of the femoral component: 8.1um/year (range 2.75–25.4um/year) vs. 1.79um/year (range 0.82–4.15um/year), p=0.002; and

Similarly, differences were also measured in absolute wear values. The median absolute linear wear was significantly higher in the pseudotumour implant group:

21.05um (range 2.74–164.80um) vs. 4.44um (range 1.50–8.80um) for the femoral component, p=0.005; and

Wear on the acetabular cup components in the pseudotumour group always involved the edge, indicating edge-loading of the bearing. In contrast, edge-loading was observed in only one acetabular component in the non-pseudotumour group of implants. The deepest wear was observed well within the bearing surface for the rest of the non-pseudotumour group. The difference in the incidence of edge-loading between the two groups was statistically significant (Fisher’s exact test, p=0.03).

Discussion: Significantly greater linear wear rates of the MoMHRA implants revised due to pseudotumour support the in vivo elevated metal ion concentrations in patients with pseudotumours. This study provides the first direct evidence to confirm that pseudotumour is associated with increased wear at the MoM articulation. Furthermore, edge-loading with the loss of fluid film lubrication may be the dominant wear generation mechanism in patients with pseudotumour.

Introduction: Approximately 20% of unicompartmental knee replacement (UKR) revisions are related to polyethylene wear. The Phase 1 Oxford UKR was introduced as a design against wear, with a fully congruent mobile bearing. The Phase 2 implant was introduced with new instrumentation (femoral mill) and changes to the bearing shape (lower anterior wall) to reduce the incidence of anterior impingement. We have previously shown that the Oxford UKR has a wear rate of 0.02 mm/year at ten years, in well functioning devices, but that higher wear rates can be seen with impingement or if the congruous articulation is lost. The aim of this study was to determine the 20 year in-vivo wear of the Oxford Phase 1 and Phase 2 UKR, using Roentgen Stereophotogrammetric Analysis (RSA).

Method: We measured the in-vivo wear of 6 Phase 1 (5 patients, mean age 65.24 years) and 7 Phase 2 (4 patients, mean age 63.43) Oxford UKR bearings. Average time since surgery was 22.37 years and 19.46 years for the Phase 1 and Phase 2 implants respectively. Selection criteria included patients who were mobile, with an exercise tolerance greater than 100m as per the American Knee Society Score (AKSS) functional questionnaire. RSA x-rays were taken with the knee in the normal anatomical position on standing and with the knee flexed to 30o. The Oxford knee score (OKS) and AKSS were gained at the RSA examination. Phase 1 and 2 components were reverse engineered by laser scanning, and converted to CAD models. The CAD models of the tibia and femur were pose-estimated in the RSA software (Medis Specials, Leiden, Netherlands). A sphere was fit to the femoral component and the minimum bearing thickness was determined by measuring the shortest perpendicular distance between the sphere and the plane contained on the tibial tray articular surface. The linear wear for each bearing was calculated by subtracting the measured thickness from the corrected nominal bearing thickness. Non-parametric statistics were used to compare the two Phases.

Results: There was no significant difference in age, OKS and AKSS between the two groups. The median wear rate was 0.078 mm/year for Phase 1 and 0.023 mm/year for Phase 2. This difference was statistically significant (p = 0.027).

Discussion: The difference in wear rate is explained by impingement in Phase 1, which was reduced by design changes with the introduction of Phase 2; the Phase 2 is designed to avoid impingement between the femur and the bearing. This study demonstrates that very low wear rates can be maintained with the Phase 2 implant to the end of the second decade after implantation. This is of particular importance when the device is used in younger patients and demonstrates that the Oxford UKR can be a definitive implant for the treatment of isolated compartmental osteoarthritis.

Introduction: Partial Knee Replacement (PKR) is an appealing alternative to Total Knee Replacement (TKR) when the patient has isolated compartment osteoarthritis (OA). In nearly all cases there is a radiolucency observed between the tibial tray wall and the boney interface. The reasons why radiolucencies appear are unknown, but the bone will adapt to its altered mechanical environment by bone remodelling in accordance with ‘Wollf’s Law’. The aim of this study was to investigate the mechanical environment of the tibia bone adjacent to the tray wall, following cemented and cementless PKR, in order to determine whether this region of bone resorbs.

Methods: A validated finite element (FE) model of a cadaver tibia implanted with an Oxford PKR was used in this study. Kinematic data from fluoroscopy measurements during a step-up activity were used to determine the relative tibio-femoral positioning for the Oxford PKR model. Load data were adapted from the in-vivo measured loads using an instrumented implant during a step-up activity. The standard operating protocol was simulated for the Oxford PKR FE models, with the tibial tray implanted in a neutral position. The tibia was sectioned around the tray. Zone 7 was defined as parallel to the vertical tray wall, corresponding to the region on screened x-rays where radiolucencies are observed. It was assumed that the bone in the implanted tibia will attempt to normalise its stress-strain patterns locally to its equilibrium state, the intact tibia, for the same loading conditions. Forty patients (20 cemented, 20 cementless) who had undergone PKR were randomly selected from a database, and their screened x-rays assessed for radiolucency in region 7.

Results: The SED in region 7 was 80% lower in the cemented and cementless tibia, compared to the intact tibia (Figure 2). The maximum tensile stress was 63% lower in the cemented and cementless tibia, compared to the intact tibia. The corresponding maximum compressive stress was 52% lower. Radiolucency was observed in all forty radiographs in region 7.

Discussion: After implantation with a cemented or cementless PKR the bone strains and SED in region 7 are reduced. This reduction may provide the signal for adaptive bone remodelling and bone will be resorbed from this region, decreasing the volume and increasing the SED. Bone resorption will continue until the equilibrium state is reached. If a ‘lazy’ zone between 35% and 50% of the remodelling signal is considered, bone resorption will still occur due to the large decrease in SED for this region. For region 7 to return its SED to the equilibrium state, its volume will need to be reduced by 80%. This is likely to be the reason why a radiolucency is observed clinically in this region in almost every case, whether a cemented or cementless implant is used.

Introduction: The use of second generation highly cross-linked polyethylene (HXLPE) is now commonplace for total hip arthroplasty, however there is no long-term data to support its use. Hip simulator studies suggest that the wear rate of HXLPE is ten times less than conventional polyethylene (UHMWPE). The outcomes of hip simulator studies are not always reproducible in vivo. Long term clinical data is required, as there is emerging clinical data, which suggests that some types of second generation HXLPE may have increased wear after 5 years.

Method: A prospective double blind randomised control trial was conducted using Radiostereometric analysis (RSA). Fifty-four subjects were randomised to receive hip replacements with either UHMWPE liners or HXLPE liners. All subjects received a cemented CPT stem and uncemented Trilogy acetabular component (Zimmer, Warsaw, IN, USA). The 3D penetration of the head into the socket was determined to a minimum of 7 years.

Results: The total liner penetration was significantly different at 7 years (p=0.01) with values of 0.33mm (SD 0.17mm) for the HXLPE group and 0.51mm (SD 0.14mm) for the UHMWPE group. The steady state wear rate from 1 year onwards was significantly lower for HXLPE (0.003 mm/yr, SD 0.04 mm/yr) than for UHMWPE (0.03 mm/yr, SD 0.03 mm/yr) (p=0.01). The direction of wear was in the antero-medial direction in both groups.

Conclusion: We have previously demonstrated that the penetration in the first year is creep-dominated, from one year onwards the majority of penetration is due to wear. The wear rate of this second generation HXLPE approaches that of metal on metal bearings. Second-generation HXLPE may have the potential to reduce the risk of revision surgery, due to wear debris induced osteolysis.

Introduction: Metal on metal hip resurfacing arthroplasty-induced pseudotumours are a serious complication, which occur in 4% of patients who undergo this procedure. The aim of this study was to measure the 3D in vivo wear on the surface of resurfacing components revised for pseudotumour, compared to a control group.

Method: Thirty-nine hip resurfacing implants were examined; these were sourced from our institutions prosthesis retrieval bank. They were divided into two groups; 22 patients with a clinical and histopathological diagnosis of pseudotumour and 17 controls. Patient demographics and time to revision were known. Three dimensional contactless metrology (Redlux™ Ltd) was used to scan the surface of the femoral and acetabular components, to a resolution of 20 nanometers. The location, depth and area of the wear scar was determined for each component. Volumetric wear was determined, along with the presence of absence of edge-loading. A separate blinded analysis to determine the presence of absence of impingement was performed by one of the authors. ANOVA was used to test for differences in wear and Fishers Exact test was used to compare the incidence of edge-loading between the groups.

Results: The volumetric wear rate for femoral component of the pseudotumour group was 4.7mm3/yr (SD3.5) and 1.7 mm3/yr (SD1.5) for the control group (p=0.03). In the pseudotumour group, the volumetric wear rate of the acetabular component was 3.5 mm3/yr (SD3.6) compared to 0.02 mm3/yr (SD0.07) for the control group (p=0.01). Edge-loading was detected in 74% of acetabular components in the pseudotumour group and 22% of those in the control group (p=0.01). Anterior or posterior edge-loading, consistent with impingement was present on the femoral components of 73% of patients in the pseudotumour group and 22% in the control group (p=0.01).

Discussion: This work demonstrates that implants revised for pseudotumour have significantly higher volumetric wear rates than controls. They also have a significantly higher incidence of edge-loading and impingement than controls. Edge-loading significantly increases wear. We suggest that pseudotumours are caused by high concentrations of metal wear debris, which have been shown to have a toxic effect on osteocytes and macrophages. This is the one of the first studies to demonstrate a clear link between pseudotumours and increased bearing surface wear. It is also the first to demonstrate that edge-loading, due to impingement, occurs in a significant number of patients who develop this condition. Improved implantation techniques and resurfacing designs may help avoid this serious complication of hip resurfacing.

Introduction: The development of Inflammatory Pseudotumour (IP) is a recognised complication following Metal on Metal Hip Resurfacing Arthroplasty (MoMHRA), thought to occur secondary to wear and elevated ion levels. Studies have shown that acetabular component orientation influences the wear of metal-on-metal hip replacement bearings. The aims of this study were to investigate the significance of cup orientation in the development of IP, and to identify a ‘safe-zone’ for cup placement with lower-risk for IP development.

Methods: Twenty six patients (n=27 hips) with IP confirmed radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with a cohort of asymptomatic MoMHRA patients (Control n=58). Radiographic acetabular anteversion and inclination were measured using EBRA. We calculated the distance in degree space of each acetabular component from the optimum position of 40° inclination and 20° anteversion, recommended by the designers, and thus compared acetabular component position between the two groups. Three different zones were tested as possibly optimum for acetabular placement. These were Lewinneck’s Zone (LZ) (inclination/anteversion; 30–50°/5–25°), and two zones defined by ±5° (Zone 1) or ± 10° (Zone 2) about the suggested target of 40°/20°. An optimal placement zone was determined based on a significant difference in IP incidence between components in the zone versus those outside.

Results: There was a wide range in cup orientations; mean inclination and anteversion were similar in the two groups: IP 47.5° (10.1°–80.6°)/14.1° (4.1°–33.6°) Vs Control 46.1° (28.8°–59.8°)/15.6° (4.3°–32.9°). Acetabular components in the IP group were significantly further away from the optimum position of 40°/20° in comparison to the controls (p=0.023). There was no difference in IP incidence between cups positioned within (IP:13/27, Control:35/58) or out of LZ (p=0.09) and within (IP: 2/27, Control: 10/58) or out of Zone 1 (p=0.156). Cups placed in Zone 2 (IP:6/27, Control:27/58) had significantly lower IP incidence versus those outside this zone (p=0.01). The odd’s ratio of developing IP when the cup is positioned out-of Zone 2 was 3.7.

Discussion: This study highlights the importance of ace-tabular component orientation in IP development. On the whole, patients with pseudotumour had acetabular components that were further away from the optimum position in comparison to the controls. However, a small number of IP patients had well-placed components implying that additional factors, possibly patient and/or gender specific, are involved in the development of pseudotumour. Furthermore, we defined an optimum, ‘safe-zone’ of ±10° around the cup position of 40°/20°. Patients with acetabular components outside this safe zone have an increased risk of IP development.

Introduction: Low-molecular-weight heparin is commonly used for thromboprophylactic therapy post orthopaedic surgery. Studies in the past have suggested that it may have a negative effect on osteoblasts and some have implicated its use with the risk of developing osteoporosis. Recently, Rivaroxaban, an oral Factor Xa inhibitor is gaining impetus for antithrombotic therapy over the last year and has been recommended for licensing by the FDA for this purpose. The effect of Rivaroxa-ban on bone and osteoblasts, if any, remains to be seen.

Methods: In a standardized in vitro model, human osteoblasts were cultured and exposed to a range of Enoxaparin and Rivaroxaban concentrations including their therapeutic dose. We evaluated the effects of these drugs on osteoblastic proliferation and activity using CellTiter 96 AQueous non-radioactive cell proliferation (MTS) and alkaline phosphatase assays respectively. Gene expression of Runt-related transcription factor 2 (Runx2), osteocalcin and bone morphogenetic protein 2 (BMP-2) were evaluated using Real time-polymerase chain reaction (RT-PCR) studies. Statistical analyses (t-test) were conducted using Microsoft Excel 2007.

Results: Rivaroxaban and Enoxaparin significantly reduced alkaline phosphatase activity (p< 0.05) however, no negative effects on osteoblastic proliferation was seen at all concentrations of both drugs. Rivaroxaban decreased Osteocalcin and Runx2 mRNA expression levels at 24 hours at therapeutic concentrations (p< 0.05). This effect was similarly found at therapeutic levels of Enoxaparin. Both Rivaroxaban and Enoxaparin significantly reduced BMP-2 mRNA expression both at 24 hours and 7 days at therapeutic concentrations. (p< 0.05).

Conclusion: Our study suggests that Rivaroxaban has similar negative effects on osteoblasts compared to Enoxaparin in the early stages. The increased duration of recommended Rivaroxaban therapy (2 and 5 weeks) post arthroplasty compared to Enoxaparin therapy (around 1 week) may have a more pronounced effect on bone homeostasis.

Introduction: Metal-on-metal hip resurfacing arthroplasty (MoMHRA) has gained popularity as an alternative to THR for younger patients with osteoarthritis. A growing concern has been the association of MoMHRA with the development of inflammatory pseudotumours (IP), especially in women. These have been linked to metal-on-metal wear, which can be related to metal ion concentrations. Elevated metal wear debris levels may result from impingement, rim contact and edge loading. Head-neck ratio (HNR) is a predetermining factor for range of movement and impingement. Neck thinning is a recognised phenomenon post-MoMHRA and we have found an association of IP with increased neck thinning based on a case control study. Our aims were to identify HNR changes a hip undergoes when resurfaced and at follow up; and whether greater neck thinning at follow-up could be associated with the presence of elevated metal ions.

Methods: A cohort of 91 patients (57M:34F) with unilateral MoMHRAs were included in this study. Blood tests were obtained at a mean follow up of 3.9 years (range 1.7–7 years) and serum (Co:Cr) ion levels were measured (ICPMS). High metal ion concentrations were defined as Co> 4.1ppb and Cr> 5.2ppb. For all patients, head-neck ratio (HNR) was measured on plain anterio-posterior pelvic radiographs pre-operatively, immediately post-operatively and at follow-up.

Results: Female patients had significantly bigger HNR pre-op (mean=1.35, range:1.22–1.64) compared to males(mean=1.22, range:1.05–1.38) (p< 0.01). Immediately post-op, female HNRs (mean: 1.26, range: 1.14–1.34) were not different to male patients(mean=1.24, range=1.11–1.38) (p=0.11). At follow-up HNR was once again significantly bigger (p< 0.01) in females (mean=1.35, range: 1.21–1.49), compare to males (mean=1.27, range:1.11–1.38). HNR alterations with operation (p=0.00) and at follow-up (p< 0.01) were significantly bigger in female patients. Furthermore, there was a significant correlation between high ion levels and HNR change at follow-up for both Co (p=0.02) and Cr (p< 0.01).

Conclusion: This study identified gender-specific changes in HNR that resurfaced hips undergo, not previously documented. Female hips have greater HNR pre-operatively, compared to male hips, and appear to be biomechanically disadvantaged when resurfaced. A decrease in HNR with resurfacing could result in impingement and lead to processes, known to be more prevalent in females, such as neck thinning, increased wear and IP development. In addition, we highlight a correlation between high ion levels and greater neck thinning at follow-up. Increased neck thinning in symptomatic MoMHRA hips could be secondary to increased wear and should be investigated further radiologically for the presence of IP.

Introduction: Metal on metal hip resurfacing arthroplasty (MoMHRA) has become an alternative option to THR in the treatment of young adults with OA. A recognised MoMHRA complication is the development of an inflammatory pseudotumour (IP). IPs can be cystic (predominantly posterio-laterally located), solid (mostly anteriorly located) or mixed in nature. Diagnosis is made with the aid of US and/or MRI. To-date, no radiographic aid in the diagnosis of IP has been identified. Neck thinning is a recognised phenomenon following MoMHRA, occurring in up to 90% of resurfaced hips, which has not been associated with any adverse clinical events. Its pathogenesis is considered multi-factorial secondary to stress shielding, impingement, pressure effect on cancellous femoral neck, bone necrosis secondary to femoral preparation and altered vascularity/AVN. Our aim was to establish whether neck thinning is associated with the presence of a pseudotumour.

Methods: Thirty-one hips (30 patients) with IP confirmed clinically, radiologically, intra-operatively and histologically were matched for sex, age, pre-operative diagnosis, component size and follow-up with an asymptomatic MoMHRA cohort without pseudotumour (Control n=60). Radiological and operative findings at the time of revision of all IP patients were reviewed regarding location of pseudotumour; 4 different locations were defined: anteriorly-extending, posteriorly-extending, anteriorly & posteriorly-extending and within joint only. For all patients, prosthesis-neck ratio (PNR) at follow-up was measured on plain AP pelvic radiographs as previously described and validated.

Results: All IP patients (6M:24F) and all (12M:48F) but two controls had a posterior approach at the time of MoMHRA. Mean femoral component size was 46 mm for both groups. At an average follow up of 3.5 years (0.7–8.3), IP patients (mean 1.26, 1.10–1.79) had a significantly higher (p< 0.0001) PNR in comparison to their controls (mean 1.14, 1.03–1.35). Greater neck thinning had occurred in both IP-males (p< 0.001) and IP-females (p=0.002) in comparison to their controls. Location of IP and hence nature did not appear to have an effect on the degree of neck thinning.

Discussion: This study shows that IP patients had significantly narrower femoral necks at follow-up. Processes, such as impingement and increased wear that are thought to contribute to the process of neck narrowing are also thought to be factors in IP development. Furthermore, the presence of an IP, could lead to altered vascularity via a mass effect and further contribute to neck narrowing. Interestingly, nature of IP did not have a significantly affect PNR. Although one cannot be certain whether neck narrowing is a consequence or a contributing factor for IP development, their association is significant. Surgeons should consider the possibility of pseudotumour in symptomatic MoMHRA patients with neck narrowing.

Introduction: Metal-on-metal hip resurfacing arthroplasty (MoMHRA) has gained popularity as an alternative to THR for younger patients with osteoarthritis. A growing concern has been the association of MoMHRA with the development of inflammatory pseudotumours (IP), especially in women. These have been linked to metal-on-metal wear, which can be related to metal ion concentrations. Although cup orientation has been shown to influence wear, the optimum cup position has not been clearly defined. We have identified an optimal cup orientation to minimise IP risk, based on a case controlled study, for inclination/anteversion within ±10° of 40°/20°. Our aim was to see if this optimal position results in lower metal ions, and to identify the boundary of an optimal placement zone for low wear.

Methods: A cohort of 104 patients (60M: 44F) with unilateral MoMHRA was included in this study. Blood tests were obtained at a mean follow up of 3.9 years (range 1.7–7 years) and serum Co and Cr ion levels were measured (ICPMS). High metal ion concentrations were defined as Co> 4.1ppb and Cr> 5.2ppb. Radiographic cup inclination and anteversion were measured using EBRA. The differences in ion levels between different cup orientation zones were investigated. Three orientation zones were defined centered on the target orientation of 40°/20°: Z1 within ±5°, Z2 outside ±5°/within ±10° and Z3: within ±10°.

Results: There was a wide range of cup placements; mean inclination/anteversion were 46.3°(21.5°–64.6°)/15°(2.7°–35.6°). Cr levels, but not Co, were higher in female patients (p=0.002) and those with small femoral components (< 50mm, p =0.03).

For the whole cohort, there was no significant difference in ion levels (Cr: p=0.092. Co=0.075) between cups positioned within Z3 (n=58) versus those outside (n=46 mean). Male patients with cups within Z3 (n=27) had lower ion levels in comparison to those outside Z3, which were significantly lower for Co (p=0.049) but not Cr (p=0.084). Female patients had similar levels within and out of Z3 for both ions (Cr: p=0.83, Co: p=0.84). However, patients with cups within Z1 (n=13) had significantly lower Co (p=0.005) and Cr (p=0.001) than those outside Z1 (n=95). Interestingly, Co levels were significantly lower in Z1 (n=13) in comparison to Z2 (n=33) (p=0.048) but Cr levels were not different (p=0.06).

Discussion: MoMHRA cups placed with ±5° of the ideal position of 40°/20°gave rise to significantly lower metal ions indicating lower wear within this narrow zone, in both sexes. This safe zone, could be extended to ±10° for male patients only. Gender specific factors, such as pelvic anatomy and joint flexibility, could be responsible for the narrower ‘safe’ zone seen in females. The narrower safe zone coupled with smaller components implanted are factors contributing to higher ion levels and hence the increased incidence of IP seen in females.

The results of mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing (five-year survival: 82%). Therefore, it is recommended that mobile bearings should not be used for lateral UKR. This low survivorship is primarily due to a high dislocation rate. A detailed analysis confirmed the elevated lateral tibial joint line to be a contributory factor to bearing dislocation. A new surgical technique was therefore introduced in which care was taken neither to remove too much bone from the distal femur nor to over tighten the knee and therefore ensure that the tibial joint line was not elevated. Other modifications included use of a domed tibial component.

The aim of this study is to compare the outcome of these iterations: the original series (series I), those with improved surgical technique (series II) and the domed tibial component (series III). The primary outcome measure was bearing dislocation at one year. One year was chosen as all the dislocations in the first series occurred within a year. In the original series (n=53), implanted using a standard open approach, there were six dislocations in the first year, the average flexion 110°, and 95% had no/mild pain on activity. In the second series (n=65), there were 3 dislocations, the average flexion was 117°, and 80% had no/mild pain on activity.

In the third series with the modified technique and a convex domed tibial plateau, there was one dislocation, average flexion was 125° and 94% had no/mild pain on activity. At four years the cumulative primary dislocation rates were 10%, 5% and 0% respectively, and were significantly different (p=0.04).

The improved surgical technique and implant design has reduced dislocation rate to an acceptable level so a mobile bearing can now be recommended for lateral UKR.

Roentgen Stereophotogrammetric Analysis (RSA) can predict long-term outcome of prostheses by measuring migration over time. The Exeter femoral stem is a double-tapered highly polished implant and has been shown to subside within the cement mantle in 2 year RSA studies. It has a proven track record in terms of long-term survivorship and low revision rates. Several studies have demonstrated excellent clinical outcomes following its implantation but this is the first study to assess stem migration at 10 years, using RSA.

This is a single-centre study involving 20 patients (mean age: 63 years, SD=7) undergoing primary total hip replacement for degenerative osteoarthritis using the lateral (Hardinge) approach. RSA radiographs were taken with the patient bearing full weight post-operatively, at 3, 6, 12 months and at 2, 5 and 10 years follow-up. The three-dimensional migration of the Exeter femoral stem was determined.

The mean Oxford Hip Score at 10 years was 43.4 (SD=4.6) and there were no revisions. The stems subsided and rotated internally during a 10-year period. The mean migrations of the head and tip of the femoral stem in all three anatomic directions (antero-posterior, medio-lateral & supero-distal) were 0.69 mm posterior, 0.04 mm lateral and 1.67 mm distal for the head and 0.20 mm anterior, 0.02 mm lateral and 1.23 mm distal for the tip. The total migration at 10 years was 1.81 mm for the head and 1.25 mm for the tip.

The Exeter femoral stem exhibits migration which is a complex combination of translation and rotation in three dimensions. Comparing our 10 year with our previous 2 year migration results, the Exeter stems show continued, but slow distal migration and internal rotation. The subsidence continues to compress the cement and bone-cement interface which maintains secure fixation in the long term.

About ten years ago we introduced sophisticated instrumentation and an increased range of component sizes for the Oxford unicompartmental knee replacement (UKR) to facilitate a minimally invasive surgical (MIS) approach. The device is now routinely implanted through an incision from the medial pole of the patella to the tibial tuberosity. This has resulted in a more rapid recovery and an improved functional result. As the access to the knee is limited there is a concern that the long term results may be compromised. The aim of this study was to determine the 10 year survival.

A prospective follow up of all Phase 3 minimally invasive Oxford UKR implanted by two senior authors (DWM & CAFD) has been undertaken. So far 1015 UKRs have been implanted for anteromedial osteoarthritis. All patients received a cemented implant through a MIS approach and were followed up prospectively by an independent observer. The data was collected prospectively regarding pre-operative status, complications and clinical as well as functional outcome at predetermined intervals.

The average age of patients was 66.4 years (range: 33 – 88) with mean Oxford Knee Score 41 (SD: 7.9) at the time of last follow up, Knee Society Score (objective) of 84 (SD: 13) and Knee Society Score (functional) of 83 (SD: 21). At ten years the survival of this cohort is 96%. There were 22 revisions including 7 for progression of arthritis, 5 for infection, 5 for bearing dislocation, 4 for unexplained pain and one for rupture of ACL secondary to trauma.

We conclude that the Oxford Knee can be implanted reliably through a minimally invasive approach, giving excellent long term results.

Introduction: Symptomatic abnormal soft-tissue masses relating to the hip joint, such as those described as pseudotumours, are being increasingly reported following metal-on-metal hip resurfacing arthroplasty (MoMHRA). These were found to be locally destructive, requiring revision surgery in a high proportion (75%) of patients. Lymphocyte infiltrations seen in pseudotumours were similar to aseptic lymphocyte vascular associated lesion (ALVAL), which is thought to represent a T-lymphocyte-mediated delayed type hypersensitivity. Therefore, a delayed hypersensitivity reaction to nickel (Ni), chromium (Cr) or cobalt (Co) has been suggested to play a role in pseudotumour aetiology. In patients with bilateral MoMHRA who presented with symptoms on one side, subsequent scans have demonstrated pseudotumours both on the symptomatic and asymptomatic side. Thus, there are concerns that there may be an appreciable number of asymptomatic pseudotumours that surgeons are unaware of and these may eventually become symptomatic.

Aim: The aims of this study were:

to determine the prevalence of asymptomatic pseudotumours after MoMHRA; and

Methods: A total of 201 MoMHRA implanted hips in 158 patients (97 male, 61 female) with a mean age of 56 years (range 33–73 years) were evaluated. The mean follow-up was 61 months (range 13–88 months). Resurfacing devices implanted included 128 Birmingham Hip Resurfacing, 66 Conserve Plus and seven ReCap. The control groups included additional 20 patients, 10 male and 10 female (a mean age 68 years, range 57–80 years) with metal-on-polyethylene total hip arthroplasty and a further 22 age-matched patients (a mean age 55 years) without any metal implants. Ultrasound was used as the initial imaging modality and MRI was used to assess the extent of the identified masses. Patients with a soft-tissue mass had ultrasound-guided aspiration or core biopsy performed. Venous blood samples were collected in all patients for serum cobalt and chromium ion levels analysis using Inductively-Coupled Plasma Mass Spectrometer and lymphocyte transformation tests (LTT). The Oxford Hip Score (OHS) was used to measure the functional outcomes of patients. Acetabular component abduction angle was measured from standardised anteroposterior pelvis radiographs.

Results: Prevalence – Pseudotumours were found in 7 patients (6 female and 1 male). The overall prevalence of asymptomatic pseudotumours was 4%, with a relatively very high (30%) prevalence in females with bilateral implants. Histological examinations showed extensive necrosis of connective tissue, in which there were scattered aggregates of metal particles and a diffuse lymphocyte infiltrate.

Metal Ion Levels – The presence of pseudotumour was associated with significantly higher median serum cobalt levels (9.2mg/L vs. 1.9mg/L, p< 0.001), chromium levels (12.0mg/L vs. 2.1mg/L, p< 0.001), hip aspirate cobalt levels (1182 mg/L vs. 86.2mg/L, p=0.003), and aspirate chromium levels (883mg/L vs. 114.8mg/ L, p=0.006), as well as with inferior functional scores (OHS 41 vs. 47 p< 0.001). There was no significant difference in acetabular cup inclination angle (p=0.51). Lymphocyte Reactivity: A higher incidence and level of enhanced lymphocyte reactivity to Ni (p=0.001), but not to Co or Cr (the principal elements in the CoCr alloy used in metal-on-metal hip resurfacing implants), was found in patients with MoMHRA compared to the patients without MoM implants. However, lymphocyte reactivity to Co, Cr and Ni did not significantly differ in patients with pseudotumours compared to those patients without pseudotumours.

Conclusion: The prevalence of asymptomatic pseudotumours in females was high, especially in females with bilateral MoMHRA implants (30%). The patients with ‘asymptomatic’ pseudotumours were in fact mildly symptomatic. Lymphocyte reactivity to Co, Cr and Ni did not differ in patients with pseudotumour compared to those patients without pseudotumours, suggesting that systemic hypersensitivity type IV reactions, mediated by lymphocyte reactivity to these metals, is not the dominant mechanism in pathogenesis of the soft tissue pseudotumours. Furthermore, pseudotumours were not detected in those patients who had normal levels of cobalt and chromium ions. This suggests that pseudotumours do not occur if MoM articulations are well functioning. Therefore, pseudotumours are likely to be a biological consequence of the large amount of metal debris generated in vivo due to excessive wear.