Aims

The extended wait that most patients are now experiencing for hip and knee arthroplasty has raised questions about whether reliance on waiting time as the primary driver for prioritization is ethical, and if other additional factors should be included in determining surgical priority. Our Prioritization of THose aWaiting hip and knee ArthroplastY (PATHWAY) project will explore which perioperative factors are important to consider when prioritizing those on the waiting list for hip and knee arthroplasty, and how these factors should be weighted. The final product will include a weighted benefit score that can be used to aid in surgical prioritization for those awaiting elective primary hip and knee arthroplasty.

One potential approach to addressing the current hip and knee arthroplasty backlog is via adoption of surgical prioritisation methods, such as use of pre-operative health related quality of life (HRQOL) assessment. We set out to determine whether dichotomization using a previously identified bimodal EuroQol Five-Dimension (EQ-5D) distribution could be used to triage waiting lists.

516 patients had data collected regarding demographics, perioperative variables and patient reported outcome measures (pre-operative & 1-year post-operative EQ-5D-3L and Oxford Hip and Knee Scores (OHS/OKS). Patients were split into two equal groups based on pre-operative EQ-5D Time Trade-Off (TTO) scores and compared (Group1 [worse HRQOL] = −0.239 to 0.487; Group2 [better HRQOL] = 0.516 to 1 (best)). The EQ5D TTO is a widely used and validated HRQOL measure that generates single values for different combinations of health-states based upon how individuals compare x years of healthy living to x years of illness.

We identified that those in Group1 had significantly greater improvement in post-operative EQ-5D TTO scores compared to Group2 (Median 0.67vs.0.19; p<0.0001 respectively), as well as greater improvement in OHS/OKS (Mean 22.4vs16.4; p<0.0001 respectively). Those in Group2 were significantly less likely to achieve EQ-5D MCID attainment (OR 0.13, 95%CI 0.07–0.23; p<0.0001) with a trend towards lower OHS/OKS MCID attainment (OR 0.66, 95%CI 0.37–1.19; p=0.168). There was no statistically significant difference in adverse events.

These finding suggest that a pre-operative EQ-5D cut-off of ≤0.487 for hip and knee arthroplasty prioritisation may help to maximise clinical utility and cost-effectiveness in a limited resource setting post COVID-19.

The rising prevalence of osteoarthritis, associated with an ageing population, is expected to deliver increasing demand across Scotland for arthroplasty services in the future. Understanding the scale of potential change to operative workflow is essential to ensure adequate provision of services and prevent prolonged waiting times that can cause patient harm. This future service demand for primary and revision hip arthroplasty across Scotland, and the rest of the U.K., is hitherto unknown.

We set out to provide projections of future primary & revision hip arthroplasty out to 2038 utilising historical trend data (2008–2018) from the Scottish Arthroplasty Project. All analyses were performed using the Holt's exponential smoothing projection method with the forecast package in R statistics. Results were adjusted for projected future population estimates provided by National Records of Scotland. Independent age & sex group predictions were also performed. All results are presented per 100,000 population at-risk per year (/100k/year).

The predicted rise of primary hip arthroplasty for all ages is from 120/100k/year in 2018 to 152/100k/year in 2038, a 27% increase. Based on a static 3 day length of stay average this would see 4280 additional patient bed days required for primary hip arthroplasty patients per annum. The number of revision hip arthroplasty procedures for all ages is projected to fall from 14/100k/year to 4/100k/year based on historical trend data. This does not however take into account the suspect increase in primary arthroplasty numbers that is likely to influence future revision rates.

Anticipated future demand for primary hip arthroplasty will require significant additional resource and funding to prevent deterioration in quality of care and an increase in patient wait times. Demand for revision arthroplasty is set to decrease, likely on account of improved implant materials, technique, and understanding of best practice to minimise complication risk. This doesn't however take into account the impact of the complex interaction between an increasing primary arthroplasty rate and revision risk. Understanding presented projections of changes to arthroplasty demand is key to future service delivery.

The aim of this study was to identify the effect of COVID-19 lockdown on the rates, types, mechanisms and mortality of musculoskeletal trauma across Scotland.

Data for all musculoskeletal trauma requiring operative treatment was collected prospectively from five orthopaedic units across Scotland during the initial lockdown period (23/03/2020-28/05/2020). This was compared with data for the same timeframe in 2018 and 2019. Data collected included all cases requiring surgery, injury type, mechanism of injury, and inpatient mortality.

1315 patients received operative treatment in 2020 compared to 1791 in 2019 and 1719 in 2018. The numbers of all injury types decreased, but the relative frequency of hip fractures increased(36.3% 2020 vs 30.2% 2019, p<0.0001 & 30.7% 2018, p<0.0001). Significant increases were seen in proportion of DIY-related injuries(3.1% 2020 vs 1.7% 2019, p=0.01 & 1.6% 2018, p<0.01) and injuries caused by falls(65.6% 2020 vs 62.6% 2019, p=0.08 & 61.9% 2018, p=0.05). Significant decreases were seen in proportion of RTCs(2.6% 2020 vs 5.4% 2019, p<0.0001 & 4.2% 2018, p=0.02) and occupational injuries(1.8% 2020 vs 3.0% 2019, p=0.03 & 2.3% 2018, p=0.01). A significant increase in proportion of self-harm injuries was seen(1.7% 2020 vs 1.1% 2019, p=0.19 & 0.5% 2018, p<0.0001). Mortality of trauma patients was significantly higher in 2020 (4.9%), than in 2019 (3.2%, p=0.02) and 2018 (2.6%, p<0.0001).

In conclusion, lockdown has resulted in a marked reduction of musculoskeletal trauma requiring surgery in Scotland. There have been major changes in types and mechanisms of injury, and mortality of trauma patients has risen significantly.

The rising prevalence of osteoarthritis, associated with an ageing population, is expected to deliver increasing demand across Scotland for primary hip and knee arthroplasty in the future.

Understanding the scale of potential change to operative workflow is essential to ensure adequate provision of services, and prevent prolonged waiting times that can cause patient harm.

We therefore set out to provide projections of future primary hip and knee arthroplasty out to 2038 utilising historical trend data (2008–2018) from the Scottish Arthroplasty Project. All analyses were performed using the Holt's exponential smoothing projection method with the forecast package in R statistics. Results were adjusted for projected future population estimates provided by National Records of Scotland. Independent age & sex group predictions were also performed. All results are presented per 100,000 population at-risk per year (/100k/year).

The predicted rise of primary hip arthroplasty for all ages is from 120/100k/year in 2018 to 152/100k/year in 2038, a 27% increase. The predicted rise of primary knee arthroplasty for all ages is from 164/100k/year in 2018 to 220/100k/year in 2038, a 34% increase. Based on a static 3 day length of stay average this would see 4280 additional patient bed days for hips, and 7392 for knees, required nationally per year by 2038. The associated supplementary cost to the NHS is anticipated to be around £21 million per annum.

Knowledge of increasing resource utilisation and cost associated with predicted future demand for primary hip and knee arthroplasty provides key information for service organisation and delivery.

Over the last decade stemless shoulder arthroplasty has become increasingly popular. However, stability of metaphyseal loading humeral components remains a concern. This study aimed to assess the stability of the Affinis stemless humeral component using Radiostereometric analysis (RSA).

Patients underwent total shoulder arthroplasty via a standardised technique with a press-fit stemless humeral component and a cemented pegged glenoid. Tantalum beads were inserted into the humerus at the time of operation. RSA of the relaxed shoulder was completed at weeks 1, 6, 13, 26, 52 and 104 post-operatively. Stressed RSA with 12 newtons of abduction force was completed from week 13 onwards. ABRSA 5.0 software (Downing Imaging Limited, Aberdeen) was used to calculate humeral component migration and induced movement.

15 patients were recruited. Precision was: 0.041, 0.034, 0.086 and 0.101 mm for Superior, Medial, Posterior and Total Point Motion (TPM) respectively. The mean TPM over 2 years was 0.24 (0.30) mm, (Mean (Standard deviation)). The mean rate of migration per 3 month time period decreased from 0.45 (0.31) to 0.02 (0.01) mm over 2 years. Mean inducible movement TPM peaked at 26 weeks at 0.1 (0.08) mm, which reduced to 0.07 (0.06) mm by 104 weeks when only 3 patients had measurable inducible motion. There was no clear trend in direction of induced movement. There were no adverse events or revisions required.

We conclude migration of the humeral component was low with little inducible movement in the majority of patients implying initial and 2 year stability of the stemless humeral component.

Aims

The UK government declared a national lockdown on 23 March 2020 to reduce transmission of COVID-19. This study aims to identify the effect of lockdown on the rates, types, mechanisms, and mortality of musculoskeletal trauma across Scotland.

Dupuytrens disease is a fibrosing condition of the palmar aponeurosis and its extensions within the digits. Normal fascial fibres running longitudinally in the subcutaneous tissues of the palm become thickened and form the characteristic nodules and cords pathognomonic of Dupuytrens disease. A wide variety of surgical interventions exist, of these the partial fasciectomy remains the most conventional and widely used technique. Minimally invasive surgical treatments such as needle fasciotomy are, however, becoming increasingly popular. Dupuytrens disease remains a challenging condition to treat as recurrence is universally found with all surgical interventions. Although recurrence may be related to the severity of the disease, there are currently no research tools other than clinical examination to examine changes in the diseased tissue postoperatively and predict likelihood of long-term success.

Magnetic Resonance Imaging (MRI) may be of value for the study of Dupuytren disease, at present its use has been greatly underexplored. We wished to carry out a pilot study in order to examine the possibility of using 3.0 Tesla MRI to study Dupuytren tissue and then furthermore to examine the potential changes post-operatively following percutaneous fasciotomy.

Five patients set to undergo percutaneous needle fasciotomy were recruited and consented for the study. All patients underwent MRI scanning of the affected hand pre-operatively and at two weeks post-operatively. Scanning was carried out in the 3.0 Tesla research MRI scanner at Aberdeen Royal Infirmary. Patients were placed prone in the MRI scanner with the hand outstretched above the head in the so-called “Superman” position. A specially designed wrist and hand coil was used. Under the expertise of radiographers and physicists, image capture encompassed four novel scanning sequences in order to make a volumetric three-dimensional image sample of the affected hand. MIPAV software (Bethesda, Maryland) was used for image analysis.

Scanning revealed well defined anatomy. The Dupuytren cord arose from the palmar aponeurosis tissue which is deep to the palmar skin and subcutaneous tissue. It was distinctly different to deep structures such as the flexor tendons and intrinsic hand muscles which appeared with a uniform low and high signal respectively. The Dupuytren tissue had a heterogeneous signal on both T1 and T2 images. On T1 the tissue signal appeared high to intermediate, similar to that of bone and muscle, but low areas of signal were observed diffusely in an irregular fashion throughout. On T2 the tissue had a low signal throughout with some focal areas of high signal. Dupuytren tissue was mapped using MIPAV software for pre- and post-operative comparisons. Signal intensity, surface area and volume of the cords and fasciotomy sites were explored.

Our initial results suggest MRI can be used to study Dupuytren tissue. Such a research tool may be of use to study the natural history of Dupuytren disease and furthermore, the response to medical and surgical interventions.

The Adora RSA (NRT, Denmark) is a new stereo X-ray system custom built for Radeostereometry. Images are acquired using CXDI50C digital detectors (Canon, Netherlands). Analysis software was written locally to detect both Tantalum markers and the spherical head of the hip implant, and for RSA reconstruction and kinematic analysis.

To assess geometric reproducibility, a planar grid phantom was constructed with 1400 2mm markers in a grid pattern over a 350 by 430 mm glass plate. Additionally 25 tantalum markers of each diameter 1.0, 0.8 and 0.5 mm were added within a 120mm square of the grid. The phantom was imaged repeatedly with translation and rotation over the detector. For small phantom movements of up to 10mm over the detector, very small measurement errors were observed of median 2 microns, maximum 6 microns. For larger movements, the errors increased to median 5 microns and maximum 50 microns. Errors also increased with decreasing exposure.

For RSA validation, an acetabular PE cup was cemented to a Sawbone pelvis. Tantalum markers were inserted into the pelvis (10), cement (4), and cup (10). A 28mm metal head was fixed to the cup. The phantom was imaged repeatedly without movement, then moved in translation (up to 100 mm) and rotation (all axes, up to 45 degrees), and with full X-ray repositioning. Precision errors were calculated on the assumption of no relative movement between components.

Results are given for repositioning movement categorised as none, small (less than 25mm or 15 degrees), medium (less than 50mm or 30 degrees), and large. For the head, the mean total point motion error was 4, 10, 14 and 24 micrometers. Mean error of segment fitting was less than 60 microns with no markers rejected from the composite segment of 24 markers. Cup migration total translation error was 10, 16, 24, and 35 micrometers with rotation errors less than 0.05 degrees.

Observed RSA errors were small, increasing with phantom movement. This is consistent with the geometric uniformity tests. X-ray exposure and tissue thickness were also identified as factors in precision. We conclude this system has excellent precision for Radiostereometry.

The reinfusion of perioperative cell salvage is one method employed to reduce exposure to donor blood. Data on the safety of this process, however, are scant. Notably, the effect of intraoperative, washed cell salvage reinfusion on prothrombotic markers has not been demonstrated. The risk of postoperative venous thromboembolism following major orthopaedic operations is not insignificant. The study objective was to assess the effect of cell salvage reinfusion on coagulation and platelet activation.

Twenty-one patients undergoing elective primary hip operations were recruited. Nine patients received washed cell salvage intraoperatively, and were compared with 12 patients undergoing similar surgery that did not. Two patients in the cell salvage group also received postoperative, unwashed cell salvage. Blood samples were collected pre-operatively, immediately post-operatively, and one day post-operatively for assays of platelet activation markers, P-selectin expression and fibrinogen binding by flow cytometry in diluted whole blood; coagulation activation marker, thrombin-antithrombin complex (TAT); D-dimer by ELISA, thrombin generation by chromogenic assay, and full blood count. Samples of cell salvage material were also analysed for prothrombotic markers.

There were no significant differences between the groups preoperatively. Postoperatively haemoglobin levels did not differ significantly between the cell salvage group and controls. Postoperative TAT and D-dimer were significantly higher in the cell salvage group compared with controls (p<0.05). One day postoperatively, there were significantly higher platelet P-selectin expression (p=0.006) and platelet fibrinogen binding (p=0.004) in the cell salvage group compared with controls. The white cell count (WCC) was also significantly higher (p=0.04). In the intraoperative washed cell salvage material, and in postoperative cell salvage, the platelet count was low, but significant proportions of platelets were activated, and levels of D-dimer were elevated compared with venous blood. The postoperative salvage material also contained high levels of TAT.

The results from this pilot study show the induction of a prothrombotic state following reinfusion of intraoperative, washed cell salvage in recipients undergoing primary elective hip operations. An inflammatory response to reinfusion is also indicated by the raised WCC. Further investigation into the safety of cell salvage is indicated.

Prosthesis migration and acetabular cup wear are useful short term measurement which may predict later implant outcome. However, the significance of the magnitude and pattern of the migration is very much dependent on the specific design studied. This study aimed to characterise patterns of migration by following four cemented femoral stem designs using Radiostereometry (RSA) within a prospective randomised longitudinal trial.

164 patients undergoing cemented femoral hip replacement for osteoarthritis were randomised to receive either an Exeter (Howmedica Stryker), Ultima Tapered Polished Stem (TPS) (Depuy), Ultima Straight Stem (USS) (Johnson and Johnson) or Elite Plus (Depuy) stem. Each subject received the OGEE PE cemented acetabular component (Depuy). RSA examinations were performed at 1 week and 6, 12, 18, 24 and 60 months post surgery. They were analysed using the UMRSA system (RSA Biomedical AB, Umea, Sweden), and our local geometric stem measurement software. 149 patients had RSA measurements available to 2 years, and 96 patients to 5 years. Differences were analysed using mixed linear modelling (SPSS).

Median linear proximal cup wear rate reduced to a minimum of 0.02-0.06mm/year in year two. Between 2 and 5 years the wear rate increased, being significantly higher for the Elite.

Cup migration was small but continuous. At 2 years it was median 0.3mm proximally, increasing to 0.5 mm at 5 years. Median rotations were less than 0.3 degrees.

Proximal migration was positive and increasing at all time points for all stems. For the tapered polished designs, while the overall magnitude was significantly higher, the rate of migration significantly decreased, whereas for the other stem designs it did not.

The TPS stem showed a tendency for posterior tilt which was significant compared to the other stems at 5 years.

All stems tended to retroversion, with the USS significantly less than the others and the Elite showing and relative increase at 5 years.

In summary migration patterns are characterised by the stem design, including where there were only small changes between designs. We are now testing measured migrations as predictors of outcome, and will continue to follow this group of patients to 10 years.

Due to its popularity of intramedullary nails (IMN) high success rate, newer design (titanium) IMN system have been introduced to replace stainless steel system. However the stability provided by the titanium IMN

may not be adequate, there by influencing the union rate.

We aimed to compare the results of both IMN systems via prospective clinical study and biomechanical testing using RSA.

Introduction: Total hip arthroplasty is one of the most frequently performed surgical procedures, with implants usually giving over 90% survival at 10 years. The failure rate is primarily due to aseptic loosening often associated with progressive bone stock loss. Impaction of cancellous morselized allografts with cement can be used for revision total hip arthroplasty in such cases. There is increasing interest in the use of synthetic bone graft substitutes as extenders to allograft due to the shortage and variable quality of allograft. A chemically-pure synthetic calcium phosphate (CaP) allograft extender is compared with allograft alone for acetabular and/or femoral revisions using the Impaction Grafting technique.

Methods: 96 hips in 94 patients underwent revision hip arthroplasty using impaction grafting at 4 sites. Hip revision using impaction grafting was carried out using the Exeter X-Change Instrumentation system, using an Exeter Stem and/or a cemented polyethylene cup. Patients were randomized to receive allograft or CaP plus allograft in a 50:50 volume. Clinical and radiographic assessment was conducted pre-operatively and immediately post-operatively and at 6, 12 and 24 months. Clinical assessments included Harris Hip, Oxford and Charnley modified Merle d’Aubigne scores. Clinical complications were also recorded. Radiographs were assessed for the graft quality, radiolucent lines, lyses or migration, and incorporation of graft. The X-rays were also independently reviewed by an experienced author.

Results: The mean age was 70.2 years (range 41–89 years) with 58 males and 36 females. A total of 40 femoral revisions and 88 acetabular revisions were conducted. There were 46 patients in the ApaPore group, 49 patients in the allograft group and 1 patient who received CaP/allograft and allograft for an acetabular and femoral revision respectively.

No deep infections or adverse events due to the CaP were reported, with no significant difference in complication rates including revision and re-operations. No significant difference in acetabular migration, femoral subsidence, radiolucencies and lyses between the groups was observed. The independent review found no difference between the groups in terms of migration. The bone density was apparently greater for the ApaPore group at 12 months (p=0.001) and 24 months (p=0.012) although the significance of this is unclear. No significant difference in the clinical measures was observed between the groups.

Conclusion: CaP is comparable with allograft in terms of performance and safety when used as an allograft extender for total hip revision arthroplasty using impaction grafting.

Introduction: Impaction allografting is an established method for restoring deficient bone during revision arthroplasty of the hip. Graft augmentation with synthetic materials has been proposed and evaluated experimentally. Our aim was to assess clinically whether migration and wear of implants with a synthetic graft mix would be equivalent to pure allograft.

Materials and Methods: Patient inclusion criteria were: acetabular and or femoral defects from aseptically loosened primary THR undergoing cemented revision with impaction grafting; age 55–80 years; initial diagnosis of osteo arthritis; good health with reasonable daily activity level. Patients were randomized to receive either pure allograft or a 50% mixture with a porous hydroxyapatite material (Apapore60, Apatech, UK). Revision was with the Exeter stem, antibiotic Simplex cement (Stryker Howmedica Ltd), and Ogee cup (Depuy Int. Ltd., Leeds, UK). Tantalum markers were inserted into the pelvis, cup, femur, cement and graft in order to measure migration with radiostereometry (RSA). Patients received a sequence of RSA examinations up to 2 years. A total of 26 patients (18 for the femur) have been analysed.

Results: At 2 years no significant differences were seen in cup wear, or migration of the cup, femoral and pelvic graft markers. Stem migration occurred mainly at the stem cement interface. The rate of distal migration for the femoral head was greater for pure allograft in the first year (p < 0.05), however this rate reduced significantly in year two. Higher medial migration in year one was observed for the proximal medial cement mantle for allograft (p < 0.05).

Conclusions: Improved stability and normal cup wear were observed in a randomized clinical study for a synthetically augmented allograft. This agrees with experimental findings¹. Longer follow up with increased patient numbers is recommended to confirm these findings.

Introduction: Bone loss following hip replacement is common. Its role in prosthesis stability and survival is a concern. DXA allows the assessment of adaptive bone remodelling and resorption. We prospectively examined longitudinal bone density changes affecting the Elite Plus [DePuy International Ltd], Leeds, UK), the Ultima Straight Stem (USS) [DePuy], the Exeter [Stryker Howmedica International Ltd, London, UK] and the Ultima Tapered Polished Stem (TPS) [DePuy] to determine whether bone loss was design dependent.

Methods: 164 patients were randomised to one of 4 stem designs. Patients received DXA peri-prosthetic examinations using the LUNAR EXPERT-XL at 7 days, then at

6, 12, 18 and 24 months. Custom analysis software was used to improve precision. Zones were excluded if identified as affected by heterotopic ossification post surgery. For each group the mean and standard error was determined for each follow up. 137 had follow up data to 24 months.

Results: Considering the whole peri-prosthetic region, all stems lost 5–9% BMD in the first 6 Months. The USS lost the most bone and the tapered stems, the Exeter and the TPS the least. Over the next 18 months changes were no more than 2%. The greatest losses were in the proximal zones, ranging between 8 and 20%. The USS lost significantly more (p < 0.05) than the others at 24 months. In zone 5 the TPS exhibited significantly higher recovery than the other designs at 24 months.

Discussion: Whilst the bone remodelling of the two tapered designs was largely equivalent, they exhibited a significant difference in the distal medial region. This study shows evidence that non-subsiding stems lose more bone than tapered subsiding designs.

Introduction: Heterotopic ossification (HO) is common post total hip replacement. Dual energy x-ray absorptiometery (DXA) is an established technique used to assess peri-prosthetic, bone mineral density (BMD) changes in the femur following surgery. The effect of HO on these measurements has not previously been reported. In this study we investigated the incidence and distribution of HO and the extent to which it affected peri-prosthetic DXA results.

Methods: As part of a two-year, prospective, primary cemented THR trial, 137 patients were assessed at six month intervals with DXA as well as anterior-posterior and lateral radiographs. HO was identified from radiographs and then further localised to the seven femoral Gruen zones using DXA subtraction imaging.

Results: HO affected at least one Gruen zone in 46% of study patients. The mean BMD in zone 1 dropped to 89% of the postoperative value in unaffected patients by six months whereas it actually increased to 102% in the HO patients (p< 0.001). This 12% difference persisted throughout the duration of the trial. Zone two was affected in 17 patients and a significant gain of 5% was noted in these patients at 12 months (p< 0.05). From observation of AP x-rays alone, 12% of the HO cases would have been missed.

HO was found to be significantly associated with male gender and increasing age at time of operation. Surgical approach to the hip also had an effect, osteotomies producing a greater incidence of HO formation.

Discussion: HO has a dramatic effect on BMD values determined by DXA. This has not previously been reported and may have affected other peri-prosthetic studies. We recommend careful screening for HO when reporting femoral peri-prosthetic BMD changes.

Introduction: Dual Energy x-ray absorptiometery (DXA) is a useful tool for the assessment of peri-prosthetic bone mineral changes following total joint replacement. In order to assess these changes the precision of the DXA technique must be optimised. While patient positioning is an important factor, the role of the analysis software should also be considered. We developed and applied a new image analysis method to data from the EXPERT-XL fan beam densitometer (LUNAR GE, USA) aiming to improve reproducibility of bone region and tissue type determination by the analysis software.

Methods: 60 patients with cemented THR received repeat same day DXA examinations. These were initially analysed strictly according to the manufacturer’s femoral peri-prosthetic protocol. A modification of this protocol was attempted allowing further small corrections to the tissue typing by the operator. The scans were then reanalysed using locally developed image analysis to accurately determine the bone, prosthesis and Gruen zone boundaries. The coefficient of variation (CV) was calculated from the differences of the repeat examinations for each of the seven Gruen zones and for the whole peri-prosthetic region.

Results: The average zone CV was 5%. The poorest was zone 1 (10%) and best zone 4 (2%). With the operator corrections there was an overall 4% improvement. With our method there was an overall 40% reduction in variation (average CV 3%, maximum 4%, minimum 2%). The whole region CV was 3.1% for the standard method 2.7% modified and 1.3% for our method.

Discussion: Our method significantly improved the reproducibility of EXPERT analysis. This study demonstrates the high dependency of DXA precision on robust regional analysis.

Synthetic graft expanders have recently been developed for use in impaction grafting revision hip arthroplasty, but their true role has yet to be determined.

We performed a series of experiments to investigate the properties of one such porous hydroxyapatite material (IG-Pore, ApaTech Ltd). IG-Pore was mixed with fresh-frozen human allograft chips and impacted into composite femoral models with a similar biomechanical profile to human bone (Sawbones Europe). Exeter hip prostheses (Stryker Howmedica Ltd) were implanted with cement and each model was axially loaded for 18000 cycles at physiological levels using an Instron servohydraulic materials testing machine. Four test groups with 0%, 50%, 70% and 90% IG-Pore were used, and there were eight femora in each group.

Pre- and post-loading radiostereometric analysis was performed to characterise migration of the prosthesis. Total subsidence was measured and was separated into that occurring at the prosthesis-cement and cement-femur interfaces. Cyclical compression and expansion of the graft-containing models was measured using the Instron.

Median values (interquartile range) for total subsidence were 0.43 mm (0.28 to 0.55) for the pure allograft group, 0.31 mm (0.20 to 0.55) for the 50% IG-Pore group, 0.23 mm (0.07 to 0.34) for the 70% allograft group and 0.13 mm (0.06 to 0.18) for the 90% IG-Pore group. These differences were statistically significant (p=0.034, Kruskal-Wallis). Subsidence at the prosthesis-cement interface was also lower for IG-Pore containing models (p=0.019, Kruskal-Wallis), although there was no significant difference at the cement-femur interface. Specimens with a higher proportion of IG-Pore showed smaller cyclical movements on loading (p=0.005, ANOVA).

Higher proportions of IG-Pore do appear to reduce subsidence in a mechanical model of impaction grafting. A randomised clinical trial using RSA to compare a 50% IG-Pore/allograft mix with pure allograft is in progress to investigate the use of this material as a bone graft expander in the clinical setting.

Introduction: Femoral neck failure due to avascular necrosis (AVN) is one of the most significant complications following resurfacing hip arthroplasty. It is likely that the surgical approach is one of the factors influenc-ing the development of AVN. Positron emission tomography (PET) is the only form of imaging that allows visualisation of bone metabolic activity deep to a metal surface.

Objectives: To establish the reliability and accuracy of PET using fluorine-18 to evaluate viability of the femoral head and neck after resurfacing hip arthroplasty. To assess the viability of ten proximal femora after Birmingham resurfacing hip arthroplasty via a modified lateral approach.

Design: A convenience case series of ten patients taken from the first fifteen from one orthopaedic surgeon’s experience of Birmingham resurfacing hip arthroplasty.

Setting: The PET unit of a major urban teaching hospital with a large academic orthopaedic department.

Participants: Patients that had undergone unilateral Birmingham resurfacing hip arthroplasty via a modified lateral approach were asked to volunteer for the study. The main criterion for inclusion was ease of attendance for imaging.

Intervention: Participants were given a single intravenous dose of 250MBq fluorine-18. After a period of 40 minutes uptake time, PET images of adjacent, sequential 10cm transverse sections including both acetabulae and proximal femora were obtained.

Main Outcome Measures: Images were reconstructed to allow relative quantification of uptake between operated and non-operated femoral heads and necks.

Results: PET imaging was successful in all subjects and demonstrated activity within the resurfaced femoral heads and femoral necks. No evidence of AVN was found.

Conclusions: Static positron emission tomography using fluorine-18 is an accurate and reliable method of assessing femoral head and neck viability after resurfacing hip arthroplasty. No evidence of avascular necrosis was found in this initial series of patients that had undergone Birmingham resurfacing hip arthroplasty via a modified lateral approach.

Impaction grafting procedures have found a widespread role in revision hip arthroplasty. Synthetic graft expanders have recently been introduced, but the optimal ratio of expander to allograft is unknown.

We performed a series of in vitro experiments to investigate the optimal ratio for one commercially available porous hydroxyapatite material (IG-Pore, ApaT-ech Ltd). IG-Pore was mixed with fresh frozen human allograft chips from osteoarthritic femoral heads and with blood. Graft was impacted into fibre-glass femoral models (Sawbones Europe) with a similar biomechanical profile to human bone, and Exeter hip prostheses (Stryker Howmedica Ltd) were cemented in place. Each model was loaded using an Instron servohydraulic materials testing machine for 18000 cycles. The magnitude and frequency of the loading cycle was based on physiologically measured values. Four test groups with 0%, 50%, 70% and 90% IG-Pore were used, with eight femora in each group.

Tantalum marker beads were attached to the prosthesis, the femoral model and the cement mantle, and radio-stereometric analysis (RSA) was performed pre- and post- loading to determine migration and rotation of the prosthesis in each axis. Pre-loading films were repeated in sixteen cases for precision measurements, and twelve specimens had delayed post-loading films performed to measure any re-expansion of the unloaded graft.

The primary end-point was RSA-measured subsidence of the prosthesis, defined as vertical movement of the tip. Median subsidence was 0.43mm, 0.31mm, 0.24mm and 0.13mm in the 0%, 50%, 70% and 90% IG-Pore groups respectively (P=0.034, Kruskal-Wallis test). The precision, given as the median absolute difference, was 0.0065mm.

All specimens showed a cyclical compression and expansion throughout the loading cycle. Specimens with a higher proportion of IG-Pore tended to be more resistant to this and the mean values for cyclical movement were 1.76 0.27mm, 1.65 0.21mm, 1.57 0.18 mm and 1.45 0.14mm for the 0%, 50%, 70% and 90% IG-Pore groups.

Higher proportions of IG-Pore appear to reduce subsidence in impaction grafting. Other issues such as the handling qualities of the graft and the biological effect of synthetic materials also need to be considered, however. A randomised clinical trial using RSA to evaluate a 50% IG-Pore/allograft mix in comparison to pure allograft is ongoing in our institution, and we hope that this will answer some of these questions definitively.