The Migration Of CAD-CAM Hips Stems In Patients With Juvenile Rheumatoid Arthritis Over A Follow-Up Period Of Ten Years

Background and Aims: The clinical results obtained after total hip arthroplasty in patients with juvenile rheumatoid arthritis were not encouraging when cemented implants were used. Therefore custom designed femoral components (uncemented and hydroxyapatite-coated) using computer-assisted-design and computer-assisted-manufacture (CAD-CAM) were used for these patients who required total hip replacement (THR). The minimal follow-up for these patients is now 10 years. The clinical results obtained with these implants were excellent. The purpose of this study is threefold; firstly to correlate the degree of migration of the femoral component with the clinical results, secondly to determine if CAD-CAM femoral components can achieve implant stability over 10 years and thirdly to determine if there in a correlation between the extent of migration and the degree of pain experienced by the patient. Methods: the hips were assessed clinically according to the Hospital for Special Surgery (HSS) hip rating system preoperatively and then at 1 year, 3 years, 5 years and 10 years postoperatively. The pain component of the HSS system was recorded separately in addition to the total HSS score. Migration of hip stems was measured by a technique developed in the Biomedical Engineering Department (BME) of the Institute of Orthopaedics and Musculoskeletal science, University College London (UCL). It involved digitisation of 16 points on an anteroposterior (AP) view radiograph in a manner prompted by the computer. The head diameter of the femoral component was inserted into the computer before digitisation. A specially developed computer program then used this information together with that obtained by digitisation to measure the distance between the top of the greater trochanter and the lateral aspect of the hip stem. This information was then used to measure the migration. Results: there was no significant correlation between the degree of migration and the clinical results at the end of the 10 year follow-up (r = 0.032, P = 0.881). However the migration of the CAD-CAM components over the 10 year period was significant (p< 0.05). This means CAD-CAM components were unable to achieve implant stability over the 10 year period. There was also no correlation between the degree of migration and the extent of pain experienced by patients at 10 years follow-up (r = -0.213, P = 0.308). Summary and conclusions: we conclude that the clinical results and degree of pain experienced by JIA patients with a THR are not related to the amount of CAD-CAM hip stem migration. Our study also established that CAD-CAM components undergo significant progressive migration over 10 years. However this rate of migration was not high enough to lead to radiological failure in most cases. Certain limitations were present in the method used to measure migration, thus the results of this study should be interpreted with caution.