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.

Our aim was to investigate the molecular features of progressive severities of cartilage damage, within the phenotype of Anteromedial Gonarthrosis (AMG).

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 H& E staining) 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 an OARSI grade of 0 (normal). The GAG immunoassay showed decreased levels with increasing severity of cartilage damage (p< 0.0001). 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).

In AMG there are distinct areas, demonstrating progressive cartilage loss. 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.

Despite the satisfactory short-term implant survivor-ship, there is an increasing concern that the metal-on-metal hip resurfacing arthroplasty (MoMHRA) release large amount of very small wear particles and metal ions. The periprosthetic soft-tissue masses such as pseudotumours are being increasingly reported. These were found be locally destructive, requiring revision surgery in most patients. It has been suggested that either an immune reaction or cytotoxic effect of chromium(Cr) or cobalt(Co) may play a role in its aetiology. However, the effect of the phagocytosis of implant-associated metal nanoparticles on macrophages has not been elucidated. The aim of this study was to investigate the in vitro viability and proliferative response of murine macrophages to clinically relevant metal nanoparticles and ions.

Materials and Methods: The RAW 264.7 murine macrophage cell line was cultured in MEM at a seeding density of 10E5 cells/cm2. Culture was set up in the presence of either:(1) negative control: medium alone;(2)Cobalt sulphate heptahyrate and chromium chloride hexahydrate (Sigma) at concentrations of 1uM, 10uM, 100uM;(3)Metal nanoparticles sized 30–35nm (American Elements) of cobalt, chromium and titanium at concentrations from 10E7 to 10E14 particles/ml.

At the end of day 1 and 4, two methods were used to quantify cell proliferation and viability. The AlamarBlue assay(Invitrogen) incorporates a fluorimetric growth indicator and the fluorescence signal correlates with metabolic activity of the cells. LIVE/DEAD stain kit(Molecular Probes) contains two fluorescent dyes to stain living cells green and dead cells red. The viability was calculated by the number of live cells divided by total cell numbers. Inter-group comparisons were performed using one-way ANOVA with Tukey post hoc test. Differences at p< 0.05 were considered to be significant.

Results: Compared with control, Alamar blue assay showed inhibition of cell proliferation in all three metal particles (p< 0.05). The Live/Dead staining showed Co nanoparticles were cytotoxic to most of cells Day 1 and Day 4 at 10E11/mL. At 10E13/mL, the Cr group showed cytotoxicity at day 4 (p< 0.05). There was no difference between Ti and control group. The Co2+ and Cr3+ ions led to inhibition to cell proliferation. At 10uM concentration, Co2+ caused a dramatic decrease in cell number. Live/Dead staining showed that Co2+ were toxic to cells (p< 0.05). Cr3+ group showed cytotoxicity at Day 4 (p< 0.05).

Discussion: This study demonstrates that Co and Cr nanoparticles and ions have dose-dependent proliferation and cytotoxic effects on the macrophages in vitro. The cytotoxicity occurred at the high concentration range that is found in the hip aspirates of MoMHRA patients with pseudotumours. This suggests the formation of pseudotumour may be the local sequelae of cytotoxicity due to increased production of metal wear nanoparticles.

Recently, a series of locally destructive soft tissue pseudotumour has been reported in patients following metal-on-metal hip resurfacing arthroplasty (MoMHRA), requiring revision surgery in a high percentage of patients. Based on the histological evidence of lymphocytic infiltration, a delayed hypersensitivity reaction to nickel (Ni), chromium (Cr) or cobalt (Co) has been suggested to play a role in its aetiology. The aim of this study was to investigate the incidence and level of hypersensitivity reaction to metals in patients with pseudotumour.

Materials and Methods: 25 patients were investigated in this Ethics approved study:

Group 1: MoMHRA patients with pseudotumours, detected on the ultrasound and confirmed with MRI (n=6, 5 F:1 M, mean age 53 years);

Lymphocyte transformation tests (LTT) were used to measure lymphocyte proliferation responses to metals. Peripheral blood mononuclear cells were isolated from heparinized blood samples using standard Ficoll–Hypaque® (Pharmacia). The PBMC were cultured at a cell density of 106 cells/mL. Culture was set up in the presence of either:

medium alone;

After 5 days of culture, cells were pulsed with [3H]-thymidine and proliferation was assessed by scintillation counting. The stimulation index (SI) was calculated by the ratio of mean counts per minute of stimulated to unstimulated cultures. A SI value of greater than 2.0 was interpreted as a positive result.

Results: A clinical history of metal allergy was reported in 2/6 in Group 1, 2/13 in Group 2, and none in Group 3. In pseudotumour group, the incidence of reactivity to Ni, Co and Cr was 60%, 17% and 0%, respectively. Within Group 2, the reactivity to Ni, Co and Cr was 69%, 8% and 15%, respectively. One control subject had reactivity to Ni. Inter-group comparisons of mean SI values (Kruskal-Wallis non-parametric analysis of variance) showed no significant differences (p> 0.05).

Discussion: The incidence of enhanced lymphocyte response to metals in patients with MoMHRA was more common than the control group. However, in comparison with non-pseudotumour patients, there was no significant difference in the incidence or the level of lymphocyte reactivity in patients with pseudotumour. We conclude that patients with MoMHRA have an enhanced lymphocyte response to metal ions, reflecting exposure and immune reactivity. However, patients with pseudotumours have a similar proliferative response to those without pseudotumours, which suggests that type IV hypersensitivity may not be the cause of the pseudotumours.

Tribological studies of hip arthroplasty suggest that larger diameter metal-on-metal (MOM) articulations would produce less wear than smaller diameter articulations. Other advantages using these large femoral heads implants include better stability with lower dislocation rates and improved range of motion. The aim of the present study was to compare chromium (Cr), cobalt (Co) and titanium (Ti) ion concentrations up to 1-year after implantation of different large diameter MOM total hip arthroplasty (THA).

Methods: Cr, Co and Ti concentrations were measured using a high resolution mass spectrometer (HR-ICP-MS) by an independent laboratory in 110 patients, randomized to receive a large metal-on-metal articulation unce-mented Ti THA from one of the following companies: Zimmer, Smith & Nephew, Biomet or Depuy. Samples of whole blood were collected pre-operatively, and postoperatively at six months and one year.

Summary of Results: At 6 months, whole blood cobalt levels were: (table removed)

Statistical group comparison revealed significant difference for Cr (p=0.006), Co (p=0.047) and Ti (p=< 0.001). With Biomet implants presenting the best results for Cr and Co and Zimmer the highest Ti level.

Discussion: Different implant factors may influence measured metal ion level in whole blood: articular surface wear and implant passive corrosion. Bearing wear may be related to its diameter, quality of the surface finish, component sphericity, radial clearance, manufacturing process (forged vs cast metal) and metal carbon content. Biomet articulation seems to present the best factors selection. Passive corrosion of exposed metallic surfaces is represented by the elevated Ti levels found in all tested systems (Ti was not part of the bearing surfaces). The plasma sprayed acetabular component surface of the Zimmer’s component seems to be responsible for the significant difference in Ti versus the other implants.

Metal on Metal Hip Resurfacing Arthroplasty (MoMHRA) has gained popularity due to its perceived advantages of bone conservation and relative ease of revision to a conventional THR if it fails. Known MoMHRA-associated complications include femoral neck fracture, avascular necrosis/collapse of the femoral head/neck, aseptic loosening and soft tissue responses such as ALVAL and pseudotumours. This study’s aim was to assess the functional outcome of failed MoMHRA revised to THR and compare it with a matched cohort of primary THRs.

Method: We have revised 53 MoMHRA cases to THR; the reasons for revision were femoral neck fracture (Fracture Group, n=21), soft tissue reaction (Pseudotumour Group, n=16) and other causes (Other Group, n=16: loosening, AVN and infection). Average followup was 2.9 years. These MoMHRA revisions were compared with 103 matched controls from a primary THR cohort; matched for age, gender and length of followup. We compared, using the MannWhitney U test, operative time (OT, measured in minutes), and Oxford Hip Score (OHS) between the revised MoMHRA groups and their individual controls. We also compared, using the Kruskal Wallis test, UCLA Activity Score in the revised MoMHRA groups.

Results: There were no differences between the Fracture Group (mean OT 99.6, SD: 30.4; mean OHS 19.8, SD:9.2) and its controls (mean OT 95.9, SD: 31.8; mean OHS 17.3, SD: 7.5) nor between the Other Group (mean OT 129.4, SD: 36.7; mean OHS 22.2, SD: 9.4) and its controls (mean OT 104.4, SD: 39.2; mean OHS 20.3, SD: 10.1) in terms of OT and OHS. The Pseudotumour Group had significantly longer OT (mean 161.6, SD: 24.5, p< 0.001) and worse outcome (mean OHS 39.1, SD: 9.3, p< 0.001) than its controls (mean OT 113.1, SD: 51.7; mean OHS 20.0, SD: 9.2). In the Fracture Group, there were 3 infections requiring revisions. For the Pseudotumour Group, there were 3 recurrent dislocations, 1 femoral artery stenosis and 3 femoral nerve palsies. In the Other Group, there were 2 periprosthetic fractures. There was significant difference (p< 0.001) in UCLA scores between the MoMHRA groups. The Pseudotumour Group had the lowest mean UCLA score of 3.8 (SD: 1.89). The Fracture Group (mean: 7.0, S.D. 2.0) and the Other Group (mean: 6.7, S.D. 2.1) had similar UCLA scores.

Discussion: The results demonstrate that outcome after revision of MoMHRA is dependent upon the indication for revision. Patients revised for soft tissue reactions had significantly worse outcome. Patients with soft tissue reactions are more likely to experience complications and require further surgical intervention. The pseudotumour associated revisions were associated with a significantly prolonged OT. The overall complication rate for the study groups was quite high, with 11 (21%) revised MoMHRA cases experiencing a complication. The Pseudotumour Group had a higher complication rate (37%).

Patello-femoral instability (PFI) affects 40 individuals per 100,000 population and causes significant morbidity. The causes of patello-femoral instability are multi-factorial, and an isolated anatomical abnormality does not necessarily indicate instability. Patello-femoral subluxation ranges from 0% (stable patella tracking) to 100% (dislocation) and there is an established relationship between the amount of subluxation and anterior knee pain. Traditionally, magnetic resonance (MR) imaging and standard radiographs are used to guide the clinician towards a suitable corrective procedure for PFI. The multi-factorial nature of patello-femoral instability is not addressed with current imaging techniques. This study aims to address which anatomical variables assessed on MR images are most relevant to patello-femoral subluxation. This information will aid surgical decision making, particularly in selecting the most appropriate reconstructive surgery.

A retrospective analysis of MR studies of 60 patients with suspected patello-femoral instability was performed. All patients were graded for degree of subluxation using a dynamic MR scan.

The patient scans were assessed for the presence of a specific range of anatomical variables:

patella alta, (modified Insall-Salvatti)

The Wilk’s Lambda test for multi-variate analysis was used to establish whether any relationship was present between the degree of patello-femoral instability and bony or soft tissue anatomical variables. Non-parametric statistical tests were applied across the groups and within the groups to assess their relative significance.

The following variables showed a significant relationship with patellofemoral subluxation; distance of the VMO from the patella (< 0.001), TTD (< 0.001), patella engagement (0.001), sulcus angles (0.004) and patella alta (0.005).

This study agrees with previous work showing a significant correlation between subluxation and trochlea sulcus angle and TTD.

This is the first study to establish a significant correlation between patella engagement and radiological instability. The lower the percentage engagement of the patella in the trochlea, the greater the degree of patello-femoral instability. Patella engagement showed a more significant relationship with subluxation than patella alta.

We report a new method of predicting patello-femoral instability by measuring the overlap of the patella in the trochlea groove.

Introduction: Metal on metal Hip Resurfacing Arthroplasty (MoMHRA) has gained popularity due to its perceived advantages of bone conservation and relative ease of revision to a conventional THR if it fails. This retrospective study is aimed at assessing the functional outcome of failed MoMHRA revised to THR and comparing it with a matched cohort of primary THRs.

Method: Since 1999 we have revised 53 MoMHRA to THR. The reasons for revision were femoral neck fracture (Group A, n=21), pseudotumour (Group B, n=16) and other causes (Group C, n=16: loosening, avascular necrosis and infection). Average follow-up was 3 years months (1.2–7.3). These revisions were compared with 106 primary THRs which were age, gender and follow-up matched with the revision group in a ratio of 2:1.

Results: The mean Oxford Hip Score (OHS) was 20.1 (12–51) for group A, 39.1 (14– 56) for group B, 22.8 (12–39) for group C and 17.8 (12–45) for primary THR group. In group A, there were three infections requiring further revisions. In group B, there were three recurrent dislocations, three patients with femoral nerve palsy and one femoral artery stenosis. In group C, there were no complications. The differences in clinical and functional outcome between group B and the remaining groups as well as the difference in the outcome between group B and control group were statistically significant (p < 0.05).

Conclusions: THR for failed MoMHRA was associated with significantly more complications, operation time and need for blood transfusion for the pseudotumour group. In addition, the revisions secondary to pseudotumour also had significantly worse functional outcome when compared to other MoMHRA revisions or primary THR.

Introduction: Despite the satisfactory short-term implant survivorship of MoM hip resurfacing arthroplasty, symptomatic abnormal periprosthetic soft-tissue masses relating to the hip joint, ‘pseudotumours’, are being increasingly reported. These were found be locally destructive, requiring revision surgery in 75% of patients. Asymptomatic pseudotumours have not been previously investigated.

Methods: The aims were: (1) to investigate the prevalence of asymptomatic pseudotumours; and (2) to investigate their potential association with the level of metal ions. A total of 160 hips in 123 patients with a mean age 56 years (range 33–73) were evaluated at a mean follow-up of 61 months (range 13–88). Radiographs and OHS were assessed. Patients with a cystic or solid mass detected on the ultrasound/MRI had an aspiration or biopsy performed. Cobalt and chromium levels were analysed using Inductively-Coupled Plasma Spectrometer.

Results: Pseudotumours were found in 6 patients (5F: 1M). In 80% of bilateral cases, it was found in both sides. Histological examination showed extensive necrosis and diffuse lymphocyte infiltration. The presence of pseudotumour was associated with higher serum cobalt (9.2 μg/L vs. 1.9μg/L, p< 0.001) and chromium levels (12.0μg/L vs. 2.1μg/L, p< 0.001); higher hip aspirate cobalt (1182 μg/L vs. 86.2μg/L, p=0.003) and chromium levels (883μg/L vs. 114.8μg/L, p=0.006); and with inferior OHS (23 vs. 14 p=0.08).

Discussion: The prevalence of asymptomatic pseudotumour (5%) was higher than previously reported for the symptomatic pseudotumours (1%). There was a sixfold elevation of serum and a twelve-fold elevation of hip aspirate levels of cobalt and chromium in patients with pseudotumours. This suggests that pseudotumours may be a biological consequence of the large amount of metal debris generated in vivo. The association between pseudotumour and elevated metal ion levels might theoretically be explained by either systemic hypersensitivity responses to metal ions or local cytotoxic effects due to a high level of metal ions.

Introduction: This study explores whether modern magnetic resonance imaging (MRI) with improved cartilage sequencing is able to show a more detailed view of anteromedial osteoarthritis of the knee (AMOA). Preoperative assessment of patients and selection of intervention is very important and preoperative imaging forms an integral part of this. Modern MRI technology may allow us to visualize in great detail the structures and cartilage within the knee, providing a better understanding of the pathoanatomy of AMOA. This will be useful in preoperative assessment and surgical management of patients.

Methods: 50 patients with a radiographic diagnosis of anteromedial osteoarthritis of the knee and had been listed for unicompartmental knee arthroplasty (UKA) had MRI as part of their pre-op workup. At operation all were deemed suitable for UKA using the current Oxford indications. The image sequences were coronal, axial and sagittal with a predetermined cartilage protocol. The state of the ACL, cartilage wear location and pattern, presence of osteophytes and subchondral high signal were assessed.

Results: All the ACLs were visualized and in continuity, however 40% showed intrasubstance high signal.

100% of medial compartments showed full thickness anteromedial loss with preservation of the posteromedial cartilage. When present, the meniscus was extruded in 96% of cases.

Discussion: This study maps the pattern of anteromedial osteoarthritis using modern MRI techniques. This has importance in determining preoperative indications (preservation of ACL and posteromedial cartilage); surgical technique (determine pattern of osteophytes requiring resection) and potentially important for long-term outcome (early lateral compartment changes).

Introduction: Radiolucent lines (RLL) underneath the tibial component are common findings following the Oxford Uni-compartmental Knee Arthroplasty (OUKA)[1]. Many theories have been proposed to explain the cause of RLL, such as poor cementing, osteonecrosis, micromotion, and thermal necrosis, however, the true aetiology and clinical significance remain unclear. We undertook a retrospective study analysing the association between RLL and pre-operative, intra-operative factors, as well as clinical outcome scores.

Method: One hundred and sixty-one knees which had undergone primary Phase 3 medial Oxford OUKA were included in the study. Fluoroscopic radiography films were assessed at five years post-operatively for areas of tibial RLL. The presence of RLL was compared to

patients’ pre-operative demographics for age, weight, height, BMI,

Results: Of the 161 knees in the study, 126 (78%) were found to have tibial RLL. No statistical difference was found between knees with RLL and those without in terms of preoperative demographics, intra-operative factors, or clinical assessment criteria.

Discussion: No clear relationship between RLL, preoperative demographics, and intra-operative factors has been identified in this study. We conclude that tibial RLL following OUKA is a common finding but do not seem to affect medium term clinical outcome.

Introduction: Avascular necrosis of the femoral head after resurfacing hip replacement is an important complication which may lead to fracture or failure. The surgical approach may affect the blood supply to the femoral head. We compared the changes in femoral head oxygenation resulting from the extended posterior approach to those resulting from the anterolateral approach, the trochanteric flip approach and a modified, soft tissue preserving posterior approach.

Methods: We recruited 48 patients who underwent hip resurfacing arthroplasty (HRA) to measure bone oxygen levels. A calibrated gas-sensitive electrode was inserted in the femoral head following division of the fascia lata. Intra-operative X-ray confirmed correct electrode placement. Base-line oxygen concentration levels were recorded immediately after electrode insertion and continuous measurements were then performed throughout surgery. All results were expressed relative to the baseline, which was considered as 100% relative oxygen concentration and changes during surgery through the posterior approach (n=10), the antero-lateral approach (n=12), the trochanteric flip approach (n=15) and the modified posterior approach (n=11) were compared.

Results: The relative oxygen concentration at the end of the procedure was significantly reduced when hip resurfacing was performed through the posterior (22%, SD 31%, p< 0.005) or a modified posterior (35%, SD 31%, p< 0.005) approach, but recovered in the anterolateral (123%, SD 99%, p=0.6) and trochanteric flip group (89%, SD 62%, p=0.5). Sub-group analysis of these two relatively blood preserving approaches showed that intra-operative oxygen concentration was significantly more consistent during surgery through the trochanteric flip approach (p< 0.02).

Discusssion and conclusion: This study has demonstrated that disruption of blood flow to the femoral head during HRA is dependent on the surgical approach. We therefore believe that blood supply preserving approaches (i.e. anterolateral, trochanteric flip) may be associated with a lower risk of avascular necrosis and femoral neck fracture.

In finite element (FE) analysis of long bones it is now common practice to calculate the material properties based on CT data. Although a unique material property is calculated for each element, assigning each element an individual material property results in excessively large models. To avoid this, it is usual to group the elements based on their material properties and to assign each group a single material property (Zannoni 1998). No study has analysed the effect the number of material properties used in a long bone FE model has on the accuracy of the results.

The aim of this study was to evaluate the variation in the calculated mechanical environment as a function of the number of material properties used in an FE model.

An FE mesh of a cadaveric human tibia containing 47,696 ten-node tetrahedron elements and 75,583 nodes was created using CT scans. Material properties were calculated for each element of the mesh based on previous work (Rho 1995, 1996). Eleven FE models were created by varying the number of groups (1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024) the elements were divided into. A single material property was assigned to each group. All models were subject to an axial point load of 300N applied on the medial condyle of the tibial plateau while the distal end was fixed. The variation in maximum and minimum principal strains and deflections, at 17 well distributed surface nodes and at 65 randomly distributed nodes within the bone were plotted against the number of element groups. The total strain energy was also plotted against the number of groups. The errors for strain, deflection, and total strain energy were calculated for each model assuming that the model using 1024 element groups was accurate.

The parameter to converge with the least number of element groups was the total strain energy. At 512 element groups the error was less than 0.001% (0.7% for the two material model). The next to converge were the displacements. Using 512 materials the maximum error in displacement at the surface nodes was 0.001% (4.7% for the 2 material model), while for the internal nodes the maximum error was 0.53% (36.7% for the 2 material model). The least convergence occurred for principal strains. The maximum errors when 512 materials were used were 1.06% (57.7% for the 2 material model) and 3.02% (104.5% for the 2 material model) for the surface and the internal nodes respectively.

This study demonstrates the relationship between the accuracy of calculated mechanical environment and the number of material properties assigned to the model. While this study will allow the analyst to make an informed decision on the number of material properties for modelling the human tibia it also helps examine the validity of previous studies which, usually due to limited resources, used fewer material properties.

Introduction: Although resurfacing hip replacements are widely used there are few little independent outcome data to support this. The aim of this study was to report the 5 year clinical outcome and 7 year survival of an independent series.

Method: 610 Birmingham hip resurfacings were implanted in 532 patients with an average age of 51.8 years (range 16.5–81.6 years) and were followed for between 2 to 8 years; 120 of this series had minimum five year follow-up. Two patients were lost. There were 23 revisions, giving an overall survival of 95% (95% CI 85–99%) at seven years. Fractured neck of femur (n=13) was the most common reason for revision, followed by aseptic loosening (n=4). There were also 3 patients who had failures that were possibly related to metal debris. At a minimum of 5 year follow-up 93% had excellent or good outcome according to the Harris Hip Score. The mean Oxford Hip Score was 16.1 points (SD 7.7) and the mean UCLA activity score was 6.6 points (SD 1.9). There were no patients with definite evidence of radiographic loosening or greater than 10% of neck narrowing.

Discussion: The results demonstrate that with the Birmingham Hip Resurfacing, implanted using the extended posterior approach, the five year survival is similar or better to the reported survival rates for cemented and hybrid THR’s in young patients.

Conclusions: Considering these patients are young and active these results are good and support the use of resurfacing. However, further study is needed to address the early failures; particularly those related to fracture and metal debris.

Introduction: We report on a group of 20 metal-onmetal resurfaced hips (17 patients) presenting with a soft tissue mass associated with various symptoms; these masses we termed pseudotumours.

Methods: All patients underwent plane radiography; CT, MRI and ultrasound investigations were also performed for some patients. Where samples were available histology was performed. Metal ion levels were measured in six patients and one patient had the metal ion levels in the joint fluid measured.

Results: All patients in this series were female. Presentation was variable; the most common symptom was pain or discomfort in the hip region. Other symptoms included spontaneous dislocation, nerve palsy, a noticeable mass or a rash. In all cases a soft tissue mass was present in the region of the hip, this was either solid or cystic. The common histological features were extensive necrosis and lymphocytic infiltration. The blood cobalt and chromium levels varied considerably between the six patients that had these measurements. The median blood chromium level was 3.8 μg/L (range 0.8 to 23 μg/L) and that for cobalt was 11.5 μg/L (range 2.1 to 15 μg/L). The synovial fluid sample taken from a single joint contained much higher metal levels, 701 μg/L for chromium and 329 μg/L for cobalt. Twelve of the 20 cases have so far required revision to a conventional hip replacement.

Discussion: This complication is best imaged with ultrasound, and is not detected by normal xray. We estimate that about 1% of patients develop a pseudotumour in the first five postoperative years. The cause of these pseudotumours is unknown and is probably multifactorial, further work is required to define this; they may be manifestations of a metal sensitivity response. We are concerned that with time the incidence of these pseudo-tumours will increase.

Metal-on-metal (MoM) bearing technology, made of cobalt-chromium (Co-Cr) alloys, is being used in anticipation of extending the durability of hip replacements. Increasingly, concern has been expressed that long term exposure to Co2+ and Cr3+ could cause DNA damage and immune dysfunction; specifically a reduction in the circulating number of CD8+ cytotoxic cells. More recently, we reported that Co2+ and Cr3+ affected the differentiation of osteoclast precursors into bone-resorbing osteoclasts. Despite these observations the effects of metal ions on osteoblast activity have been poorly investigated. The aim of the current study was to elucidate the effects of various metal ions on osteoblast activity in vitro.

Cells of the human osteosarcoma cell line SaOS-2 were cultured in the presence of 0, 1, 10 and 100 μM Co2+ and Cr3+. The morphology, viability, cytokine release (TNFalpha, IL-1beta, IL-6, LIGHT, MIP-1alpha and VEGF) and alkaline phosphatase activity were investigated after 24h and 48h in contact with metal ions. Finally the capacity of SaOS-2 to produce and mineralize a new bone matrix was assessed by the Alizarin red method. All experiments were repeated at least 5 times and the differences between each were determined using non-parametric Mann-Whitney test.

Compared to untreated cultures, although the morphology looked normal after 48h, the viability indicated that Co2+ and Cr3+ ions at high concentrations induced some significant and irreversible damages to the osteoblast cells. Interestingly, any of the cytokines investigated were released in contact with metal ions after 24h or 48h. The alkaline phosphatase activity was significantly increased by low concentrations of Co2+ and decreased by high concentrations of Cr3+ after 24h and 48h. Moreover, the degree of mineralization of a new bone matrix in vitro was significantly reduced when the SaOS-2 cells were exposed to high concentrations of Cr3+, but significantly increased when they were exposed to Co2+.

Our results indicated that irreversible damages are caused to the cells as soon as 24h with high concentrations of metal ions. For osteoblasts cells, Co2+ appeared to be less toxic than Cr3+ at high concentrations.

This study was supported by Furlong Research Charitable Foundation

Statement of purpose: The aim of this study is to evaluate different designs of unicompartmental knee replacement (UKR) by comparing the peak von Mises and contact stresses in polyethylene (PE) bearings over a step-up activity.

Summary of Methods: A validated finite element (FE) model was used in this study. Three UKR designs were modelled: a spherical femoral component with a spherical PE bearing (fully-congruent), a poly-radial femoral component with a concave PE bearing (semi-congruent), and a spherical femoral component with a flat bearing (non-congruent).

Kinematic data from in-vivo fluoroscopy measurements during a step-up activity was used to determine the relative tibial-femoral position as a function of knee flexion angle for each model. Medial and lateral force distribution was adapted from loads measured in-vivo with an instrumented implant during a step-up activity. The affect that varying the bearing thickness has on the stresses in the bearing was investigated. In addition, varus-valgus mal-alignment was investigated by rotating the femoral component through 10 degrees.

Summary of Results: Only the fully congruent bearing experienced peak von Mises and contact stresses below the PE lower fatigue limit (17MPa) for the step-up activity (fully congruent PE peak contact stress, 5MPa). The highest PE contact stresses were observed for the semi-congruent and non-congruent designs, which experienced approximately 3 times the PE lower fatigue limit. Peak PE von Mises stresses for the semi-congruent and non-congruent designs were similar, peaking at approximately 25MPa. Peak PE von Mises stresses were ameliorated with increased bearing thickness. Varus-valgus mal-alignment had little effect on the peak stresses in the three UKR designs.

Statement of Conclusions: Fully congruent articulating surfaces significantly reduce the peak contact stresses and von Mises stresses in the bearing. The FE model demonstrates that fully congruent bearings as thin as 2.5mm can be used without increasing the contact stresses significantly. Fully congruent designs can use thinner bearings and enable greater bone preservation.

Introduction: The results of the mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing with a five year survival of 82%. Therefore, it is recommended that mobile bearings should not be used for lateral UKR. This low survivorship is primarily due to high dislocation rate, all occurring in the first year. A detailed analysis of the causes of bearing dislocation confirmed the elevated lateral tibial joint line to be a contributory factor. 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 to the technique were also introduced including use of a domed tibial component.

Aim: The aim of this study is to compare the outcome of these iterations: the original series [series I], Series II with improved surgical technique and the domed tibial component [Series III].

Method: The primary assessment of outcome was bearing dislocation at one year. One year was chosen as all the dislocations in the first series occurred within a year. In series I, there were 53 knees, in series II 65 knees and in series III 60 knees, all with a minimum of one year follow up.

Results: In series I, there were 6 bearing dislocations (11%) and the average range of movement (ROM) was 110°. In the second series, there were 2 dislocations (3%) and the average ROM was 118°. In the third series, there were no primary dislocations and the average ROM was 125°.

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

Statement of purpose: Finite element (FE) models of bone can be used to evaluate new and modified knee replacements. Validation of FE models is seldom used, and the quantification of modelling parameters has a considerable effect on the results obtained. The aim of this study is to develop a FE model of a cadaveric tibia and validate it against a comprehensive set of experiments.

Summary of Methods: Seventeen tri-axial rosettes were attached to a cleaned, fresh frozen cadaveric human tibia and the tibia was subjected to 13 loading conditions. Deflection and strain data were used for comparison with the FE model. A geometric model was created on the basis of computed tomography (CT) scans. The CT data was used to map 600 orthotropic material properties to the tibia. All experiments were simulated on the FE model. Measured principal strains were compared to their corresponding FE values using regression analysis. The validated tibia model was reduced in size (75mm to the proximal) and then re-modelled to represent only the proximal tibia. This re-modelled tibia was validated against the reduced size FE model. Virtual surgery was performed on the validated proximal model to implant a UKR.

Summary of Results: For the whole tibia model, the regression line for all axial loads combined had a slope of 0.999, an intercept of −6.24 micro-strain, and an R2 value of 0.962. The root mean square error as a percentage was 5%. For the proximal tibia model, correlation coefficients of 0.989 and 0.976 were obtained for the maximum and minimum principal strains respectively.

Statement of Conclusions: An FE model of an implanted proximal tibia has been validated against experimental data. This model is able to accurately predict the deflection and stresses in a replaced knee joint to obtain clinically relevant information. This will provide a virtual model of unicompartmental arthroplasty, where variables such as fixation method and bearing mechanics can be assessed.

Purpose: To compare the site of lesions in medial and lateral unicompartmental osteoarthritis (OA) of the knee.

Methods: Patients with medial (n=35) and lateral (n=15) OA, having unicompartmental knee arthroplasty, were recruited. Intra-operatively, the distance between the anterior, posterior, medial and lateral margins of the full-thickness lesion and reference lines dividing the condyles was measured. The midpoints of lesions were calculated and groups were compared. Lateral radiographs were used to determine the relationship between the lesion site and knee flexion angle (KFA).

Results: Femoral lesion: In lateral OA, the midpoint of lesions was 25.0mm (SD:8.8) posterior to the reference line passing transversely through the apex of the inter-condylar notch. This was significantly different (p< 0.001) from midpoint in medial OA, which was 10.7mm (SD:9.4) posterior to the reference line.

Tibial lesion: In lateral OA, the midpoint of lesions was 2.0mm (SD:6.5) posterior to the reference line passing through the mid-coronal plane of the resected tibia. This was located significantly more posterior (p=0.038) than midpoint in medial OA, which was 2.2mm (SD:5.7) anterior to the reference line.

Knee Flexion Angle: In lateral OA, the midpoint of lesions was on average at 40° flexion and sites of smaller lesions were very variable. The lesion expanded both anteriorly and posteriorly. In medial OA, smaller femoral lesions occurred in full extension and extended further posteriorly with disease progression.

No significant difference was demonstrated in medial and lateral localisation of the lesions.

Conclusion: Medial OA begins near full extension, progresses in a predictable manner and is perhaps initiated by events occurring at heel strike. Lateral OA begins in flexion in a less predictable manner, at KFA above that seen during the gait cycle. The different sites of lesions in medial and lateral OA suggest different aetiology and pathophysiology. Therefore, prevention and treatment strategies should be different.