Wear and migration in total hip arthroplasty measured with Radiostereometric Analysis. Methodological aspects and clinical studies

Abstract

Total hip arthroplasty (THA) aims to relieve pain, increase hip function and improve quality of life for patients with end- stage osteoarthritis of the hip who do not benefit from non-surgical treatment. Despite constant improvements in prosthetic component design and surgical techniques, prosthetic loosening, wear and dislocation remain the most common complications of a hip replacement procedure. Roentgen Stereophotogrammetric Analysis (RSA) is a highly accurate technique that enables three-dimensional measurements of skeletal or implant micromovement with respect to the host bone in vivo. The method has been used in clinical research for the evaluation of penetration of the femoral head into the acetabular component of a total hip replacement as a result of polyethylene creep and wear. Wear has been identified as a common cause of aseptic implant loosening and late revision. The RSA method has the ability accurately to measure bone and implant movements, rendering it a very useful tool in the evaluation of new implants and surgical techniques. Several studies have shown that RSA measurements of migration can be used to predict the risk of future revision based on a small group of patients and in a relatively short period of time. This thesis aims to study different aspects of RSA methodology with a potential influence on precision and accuracy. The opportunity to obtain clinically relevant information from studies of absolute motions of the pelvic segment over time and between supine and standing positions was explored. The RSA method was also applied in a randomised, clinical study of liner wear. Further, the method was used for a comparative analysis of two acetabular implants with differing properties relating to shell coating (RingLoc and Regenerex) for the development of radiolucent lines at the implant-bone interface and cup migration up to five years. In Study I, femoral head penetration into cups with liners made of either vitamin E-infused highly cross-linked polyethylene (E1) or compression-annealed highly cross-linked polyethylene, ArComXL, was evaluated in 61 patients (70 hips) in a randomised, clinical study. At two years, the femoral head penetration did not differ between the groups. Study II aimed to evaluate differences in the femoral head penetration rate between E1 and ArComXL with the emphasis on changes occurring between two and five years after THA. E1 demonstrated low proximal and total head penetration after the minimum five-year follow-up. The penetration rate was significantly lower for the E1 compared with the ArComXL liners. Both polyethylene types demonstrated increased liner penetration between two and five years. In Study III, wear and migration in four uncemented cups with various geometries that are commonly used for THA in Sweden were evaluated. Further, the influence of cup design on the precision of model-based RSA (MBRSA), using marker-based RSA as a reference, was also evaluated. For three of the studied designs, the precision of MBRSA was as good as the reference method. Poorer precision was observed for the fourth cup design (RingLoc), which indicated that the resolution of MBRSA may vary depending on the geometry of the implant or the surface coating. In Study IV, the change in pelvic tilt angle (PTA) in a supine position and between supine and standing positions was evaluated in 106 patients up to seven years after THA using RSA. The pelvic tilt changed over time both when supine and when rising from supine to standing. At six months, the mean anterior tilt from supine to standing was 3.6° ± 3.8°, which increased to 6.4° ± 3.9° at seven years. In individual patients, this change reached about 11.0 degrees when supine and 18.0 degrees when standing. In Study V, the migration of cups coated with a porous titanium layer (Regenerex) was recorded using a porous plasma-sprayed cup (RingLoc) as a reference in 92 patients (hips). We also evaluated any possible associations between the occurrence of progressive radiolucent lines and the migration of the acetabular component up to five years measured with RSA. Cups with a porous titanium coating demonstrated smaller absolute values of medial or lateral migration. The plasma-sprayed cups migrated more anteriorly than the porous titanium cups. Increasing relative length of radiolucent lines between the postoperative examination and five years follow-up of at least 20 percent of the interface was associated with increasing anterior or posterior cup migration.