RSA is a powerful research technique that is currently being studied at Hospital for Special Surgery (HSS) for joint replacement patients. RSA stands for radiostereometric analysis, which describes a special way of taking two x-rays from different directions at the same time, creating a “stereo” image. RSA x-rays allow surgeons to measure precisely how the body and the implant are interacting. This enables them to accomplish two goals at once – while measuring the patient’s progress, they can also use information from this research to improve implant design and technology for future patients.
Edwin P. Su, MD, explains, “The power of RSA comes from its ability to precisely measure small amounts of movement, thus allowing us to gather a lot of information from a small group of patients”
To precisely measure implant position on RSA images, the surgeon inserts beads into the bone surrounding the implant. These beads are very small - about the size of a poppy seed (see Figure 1) - and made of tantalum, a metal that is used in prosthetic implants and is well tolerated by the body. These beads become stably integrated into the bone and can be used as references with which to detect any change in position of the implant components.
This image shows the relative size of the implanted tantalum beads to a US quarter.
The first set of stereo x-rays are taken post-operatively while the patient is still in the hospital, with additional images taken on subsequent office visits. HSS surgeons rely on colleagues in Sweden to analyze the images and to calculate any movement or shifting of the implant that might have occurred. Previous studies have shown RSA to be accurate to 2/10ths of a millimeter of motion (see Figure 2).
An RSA image of tantalum beads inserted into a joint implant and labeled for detection of movement.
The information gathered from RSA tells the surgeon precisely how much position change has occurred since implantation of the prosthesis. This may also help the surgeon predict how long the implant will last. It can give the patient valuable information about his or her particular prosthesis. RSA will also benefit future patients by providing important data to help researchers in improving joint replacement surgery.
Timothy Wright, PhD, Senior Scientist at HSS, notes that "because of its accuracy and precision, RSA has been shown to be perhaps the most reliable tool for providing an early warning that an implant is failing."
“With this level of accuracy,” adds Mathias Bostrom, MD, “we can assess new implant technology, more accurately predicting which implants will offer good long term performance. It’s a great benefit for the good of patient care.”
The Use of RSA at HSS
RSA is used in clinical studies to reveal critical information about joint implants. Current research projects involving the use of RSA and joint replacement include:
The risks involved with RSA imaging are minimal. RSA has been used for over 30 years in Europe and for 5 years at Hospital for Special Surgery. The only potential risks come from implantation of the marker beads.
The success of RSA has been proven in Europe over many years. Now RSA has become a promising diagnostic tool in this country, and patients undergoing total joint replacements are likely to benefit greatly from this technology for years to come.
“RSA is an emerging technology offering the most accurate assessment of total joint replacement fixation and long term stability,” Dr. Bostrom explains. “It is rapidly becoming the gold standard for assessing radiographic outcome.”
HSS’ contributions in advancing implant technology, refining surgical techniques, and improving patient care are made possible by studying select groups of patients to learn what may benefit others. The information gathered from these clinical tests is valuable to both current and future patients requiring total joint replacements. Patients of the four RSA-approved physicians at HSS - including Dr. Edwin P. Su, Dr. Mathias P. Bostrom, Dr. Thomas P. Sculco, and Dr. Bryan J. Nestor - are eligible to participate in the study if they are appropriate candidates.