What is Roentgen Stereophotogrammetric Analysis (RSA)?

RSA is a precise stereo x-ray exam comprised of two simultaneous x-rays taken from two different points of view. RSA is primarily used to measure the positional changes (migration) of total joint replacements relative to the bone, with several measurements being taken over a period of two years. To date the most common application of RSA involves monitoring Total Knee Replacements and cemented Total Hip Replacements. However, recently RSA has been used to monitor spinal fusion, total shoulder replacements, total ankle replacements, as well as document the effects of drugs on implant fixation. The use of RSA is being expanded to other areas including spinal assessment, measurement of fracture healing, and measurements of the effect of pharmaceuticals on both bone healing and implant fixation.

Why RSA?

The RSA methodology is the only tool that measures migration with tenfold increase in accuracy compared to other available methods: CT, MRI, conventional x-ray. The migration that is detected by RSA has been shown to highly correlate with implant loosening. RSA is an independent and quantitative measure of implant performance, allowing for non-basis comparison of implants or evaluation of new technology.

RSA x-rays exert a low to very low effective dose of radiation compared to a conventional x-ray. The radiation dosage depends on the type of implant; for a hip implant the radiation received is more than the radiation for a knee implant. Within the first year of having a hip implant, the total radiation dosage received is comparable to the amount of natural radiation any individual receives in their daily living. For a knee implant it can be as low as one percent of the yearly natural radiation received.

How does RSA work?

One millimeter diameter tantalum markers are implanted into the host bone with a simple medical device during surgery. An RSA calibration box is placed between the patient and two x-ray imaging devices. A stereo exam is acquired by simultaneously imaging the patient with the dual x-ray set up. Specialized RSA software is used to calculate the three dimensional position of the implants relative to the bone (Figure 1). The software then uses multiple RSA exposures to calculate how the implant is migrating over a two year period.