Migration Analysis of a Hybrid Hip Replacement Evaluating Stem and Ancillary Acetabular Screw Fixation-A Randomized Controlled CT-RSA Trial

Hip and knee osteoarthritis OA is one of the leading causes of disability worldwide. The lifetime risk of developing hip OA is estimated to be 25%, in people who reach the age of 85. Total hip replacement THR is considered to be a safe and reliable surgical treatment for end stage hip OA and was described as "Surgery of the century". One of the challenges in hip replacement surgery is to ensure the longevity and durability of the implants used. New implants are designed to improve the primary stability and long-term survival of the implant. Early failure of implants and the need for revision surgery, is a major concern. Revision hip replacement surgery is a surgical procedure with high rates of postoperative complications and overall re-revision rate of 15,8% within 1 year. In the Norwegian arthroplasty register 10 year failure rate of hip revision prostheses is 26%. Thus, the longevity and durability of implants is a continuous challenge in which the ultimate goal is to develop well-functioning implants which will last a lifetime.

The purpose of this study is to compare implant migration and early fixation of a hybrid hip replacement combining the uncemented ACE acetabular component and the cemented Avanteon femoral stem, by using CT based Radiostereometric Analysis, a new innovative method for evaluating implant migration. Our study may predict the long term survival of the implants used. This will be the first ever CT-RSA study in Norway and one of the first world-wide, evaluating hip implants.

Radiostereometric analysis RSA is a method used to measure implant migration and micromotion after joint replacement surgery, with high precision and accuracy. It is considered the gold standard for to assess migration in orthopedic implants. Early implant migration predicts later loosening, and failure of the implant. Subsidence of the femoral stem of more than 1.2 mm at 24m months is predictive of revision. Proximal migration of the acetabular component of up to 0.2mm is considered acceptable, 0.2-1mm is considered at risk, and above 1mm is unacceptable. RSA is thereby used as an early measure of longevity and durability of new implants. However, RSA is an expensive and time consuming procedure. It requires special equipment such as RSA cages, software, and special x-ray setup. It also requires specially trained personnel to do the examination. Loss of data due to occlusion of tantalum beads from metallic implants, is unavoidable in RSA. Over the last years CT-RSA has evolved to overcome some of the challenges of RSA. CT-RSA has been validated to have similar or even higher precision and accuracy than RSA in both hip and knee implant analysis.By using CT-RSA we can evaluate implant migration without the need for implant or bone markers, by using bone surface anatomy. One of the authors (FDØ) has recently shown that a vast decrease of effective radiation doses of CT-RSA in total knee arthroplasty yields equal precision in a cadaver study (publication in press). We assume this applies to CT-RSA of the hip as well. We therefore would like to perform a study on ultra low dose CT-RSA of hip implants, to evaluate the precision. This could have a major impact for future CT-RSA studies of the hip, since we potentially could reduce the effective radiation dose.

Previous cadaveric studies have shown that screw fixation increases initial stability of acetabular components. In long term, press-fit screwless fixation achieve sufficient stability.The use of augmented screw fixation may increase the risk of particle debris and osteolysis around the acetabular component. The use of ancillary screw fixation may thereby increase the risk of loosening, and need for major revision surgery. There is also a risk for iatrogenic damage to nearby vessels and pelvic organs, due to screw misplacement.

The Avanteon JRI is a highly polished double tapered femoral stem, designed by force-closed principles. The stem is intended to subside gradually in the cement mantle over time. Highly polished femoral stems have shown excellent results, and lower revision rates compared to shape-closed design stems. New implants are developed to increase early fixation and long term survival, to avoid the need for revision surgery.