1. 3D LOW DOSE CT BASED ALGORITHM FOR WEAR MEASUREMENTS IN THR – A CLINICAL STUDY V Kannan1, R Richards2, V Sauret2, J P Cobb1 1 Department Of Orthopaedics, Imperial College London, 2 Medical Physics Department, University College London Twenty four patients ( 32 hips ) who had Furlong uncemented hip replacement took part in our study. The mean follow up was 22 months ( max= 12 yrs; min= 0.75 yrs ). All patients had 28 mm diameter ceramic heads. Of the 32 hips, 20 hips had polyethylene inserts and 12 hips had ceramic inserts. The maximum follow up for the polyethylene and ceramic groups were 12 years and 5.5 years respectively. All the patients were scanned using Somatom Sensation 4 scanner using our low dose radiation protocol. Using custom software,3D reconstruction of the components were done and landmark acquisition done on the femoral head, acetabular metal component and the insert (Fig 1,2,3). From these landmarks, a dedicated program was used to calculate the centre of the femoral head in relation to the centre of the acetabular component in all three axes and an indirect measurement of wear obtained INTRODUCTION Wear and loosening are the major causes of long term failure of Total Hip Replacement (THR). In clinical scenario, three dimensional wear analysis of radiographs has its own limitations ( 1,2). Recent ex vivo study of CT scan measurements of wear has shown that it reflects the true 3D wear with accuracy and repeatability of less 1mm(3).We report the results of our in vivo study of three dimensional wear using our custom low dose CT based algorithm developed from our previous experience with knee protocol(4) Measurements obtained by two independent observers showed a strong correlation ( 0.99, p value = 0.001) for the polyethylene group. In the ceramic group the correlation ( 0.69, p value=0.0126) was not as strong as the polyethylene group. (Fig 5) MATERIALS AND METHODS DISCUSSION Three dimensional estimation of wear can be obtained from low dose ct scans with the same accuracy and repeatability (<1 mm) as previously demonstrated in ex vivo studies (3) particularly in polyethylene bearings. Noise reduction with appropriate artefact reduction software may improve the accuracy of this method. RESULTS In the polyethylene group, there was no significant (<1mm) wear in x and y axis. However there was significant evidence of wear in relation to time in the z axis ( max wear = -2.5 mm ) (Fig 4). In the ceramic group,there was no evidence of significant wear in all three axes ACKNOWLEDGEMENT The author had received grant from Furlong Research Foundation, London, UK for the study REFERENCES 1.Clarke JC,Black K, Rennie C, Amstutz HC. Can wear in total hip arthroplasties be assessed from radiographs? Clin Orthop Relat Res 1976;121:126-42 2. Devane PA, Bourne RB, Rorabeck CH, Hardie RM,Horne JG. Measurement of polyethylene wear in metal backed acetabular cups. I. Three-dimensional technique. Clin Orthop Relat Res 1995;319:303- 16 3. Olivecrona L, Jedenmalm A, Aspelin P, Weidenhelm L, Noz ME. Assessing wear of the acetabular cup using Computed Tomography: an ex vivo study 4. Henckel J, Harris SJ, Barrett A, Rodrigeuz F, Davies B, Gomez P, Cobb JP Very low dose CT based planning and outcome measurement in knee arthroplasty.5th Annual Meeting of CAOS-International Proceedings pp 162-4 Helsinki, Finland 2005 Fig 1 Fig 2 Fig 3 Fig 4 Fig 5