RSA Science

Since being invented in 1973 to measure skeletal growth and kinematics of the human skeleton, RSA has taken on an entirely new role in joint replacement evaluation and testing. The most significant ability of RSA is its “predictive” qualities. Owing to its high accuracy, the system can detect within the first 2 years following surgery if an implant is improperly fixed to the bone based on a very small threshold of migration between RSA exams. Continuous migration of an implant, in most cases, indicates that the implant may become completely detached from the bone in the long-term, resulting in painful revision surgery for the patient. Many publications from researcher groups in Europe have proved this predictive ability.

Total Knee Replacement

“In the revision group (15 of 155 cases), 14 components had migrated continuously and at one year significantly more than those in the first group [measured with RSA]. These findings suggest that mechanical loosening begins early in the postoperative period.” 

“We performed two parallel systematic reviews and meta-analyses to determine the association between early migration of tibial components and late aseptic revision. The results showed that for every mm increase in migration there was an 8% increase in revision rate. Consequently, migration up to 0.5 mm was considered acceptable during the first postoperative year, while migration of 1.6 mm or more was unacceptable. The proposed migration thresholds can be implemented in a phased, evidence-based introduction of new types of knee prostheses, since they allow early detection of high-risk TKAs while exposing only a small number of patients.” Quoted from: Pijls, B.G., et al., Early migration of tibial components is associated with late revision: a systematic review and meta-analysis of 21,000 knee arthroplasties. Acta Orthop, 2012. 83(6): p. 614-24.

Total Hip Replacement

“We examined 118 patients at a mean of nine years after hip replacement, including the 19 patients from the original roentgen stereophotogrammetric study. All the patients previously shown in the roentgen stereophotogrammetric study to have high posterior head migration went on to failure. Our study… confirms that early evaluation of new components is important and that roentgen stereophotogrammetric analysis is a good tool for this.” Quoted from: Hauptfleisch, J., et al., The premature failure of the Charnley Elite-Plus stem: a confirmation of RSA predictions. J Bone Joint Surg Br, 2006. 88(2): p. 179-83.

“We performed 2 parallel systematic reviews and meta-analyses to determine the association between early migration of acetabular cups and late aseptic revision. For every mm increase in 2-year proximal migration, there was a 10% increase in revision rate. Consequently, proximal migration of up to 0.2 mm was considered acceptable and proximal migration of 1.0 mm or more was considered unacceptable. The proposed migration thresholds can be implemented in a phased evidence-based introduction, since they allow early detection of high-risk cups while exposing a small number of patients.” Quoted from: Pijls, B.G., et al., Early proximal migration of cups is associated with late revision in THA: a systematic review and meta-analysis of 26 RSA studies and 49 survivalstudies. Acta Orthop, 2012. 83(6): p. 583-91.

Argument for “phased” introduction of new implants

“Although the overall survival of knee and hip prostheses at ten years averages 90%, recent problems with several hip and knee prostheses have illustrated that the orthopaedic community, industry, and regulators can still further improve patient safety. Given the early predictive properties of roentgen stereophotogrammetric analysis (RSA) and the meticulous follow-up of national joint registries, these two methods are ideal tools for such a phased clinical introduction. The five-year rate of revision of RSA-tested total knee replacements was compared with that of non-RSA-tested total knee replacements. There was a 22% to 35% reduction in the number of revisions of RSA-tested total knee replacements as compared with non-RSA-tested total knee replacements in the national joint registries. Assuming that the total cost of total knee arthroplasty is $37,000 in the United States, a 22% to 35% reduction in the number of revisions (currently close to 55,000 annually) could lead to an estimated annual savings of over $400 million to the health-care system. The phased clinical introduction of new prostheses with two-year RSA results as a qualitative tool could lead to better patient care and could reduce the costs associated with revision total knee arthroplasty.” Quoted from: Nelissen, R.G., et al., RSA and registries: the quest for phased introduction of new implants. J Bone Joint Surg Am, 2011. 93 Suppl 3: p. 62-5.