Radiostereometric Analysis (RSA) was developed by a Swedish researcher, in the mid 1970’s as a method for determining an object’s exact location within a person’s body. RSA involves two x-ray sources aimed at opposing angles which converge on the area of interest (Figure 1). Much like our vision, humans see an individual image from each eye and the brain combines these two images into one, giving our sight depth perception. Where RSA differs from human vision is in the analysis of those two images. Computers and pixels are used to very precisely map out points and objects on the images and then calculate a three-dimensional position of the object based on the intersection of the x-ray lines (Figure 2).
RSA is accurate to within 0.02 mm experimentally and to within 0.1 – 0.3 mm clinically. Therefore, RSA is extremely useful for measuring very small amounts of implant migration in various applications ranging from joint replacement surgery (hip, knee, shoulder, ankle and other joint replacements) to fracture repair surgery to spine surgery.
During an orthopaedic surgery (e.g. joint replacement, fracture repair, spine surgery), the surgeon will insert a small number (usually less than 10) of tantalum marker beads. These beads are smaller than 1 mm in diameter (Figure 3) and are placed into the patient’s bone (Figure 4) surrounding an implanted object such as a hip stem or cup, a femoral nail or plate, or a spinal fusion screw. These markers serve as the reference points within the bone (non-moving) to which the implanted object is compared. By measuring the amount of motion between the bone and the implant over time, researchers and surgeons can determine if the implant is properly fixed.
During RSA imaging, the patient is placed above (Figure 5) or in front of an RSA calibration cage (Figure 6). Two separate images are obtained from an RSA exam. These images are analyzed to determine the relative motion between the reference beads in the bone and the moving implant (Figure 7).
The difference between an initial RSA exam – about 1 to 6 weeks after the surgery – and a follow-up exam (3+ months post-operation) will determine the distance the implant has travelled into or out of the bone as well as any rotation or tilt of the implant with respect to the bone. The total amount or the annual rate of implant motion indicates if the implant is well fixed. If the implant has migrated substantially between RSA exams, this generally indicates that it is loose to some degree and may become fully loose in the future, causing the patient pain and often resulting in revision surgery.
One of the most valuable aspects of RSA is its ability to detect early implant loosening within the first two years following surgery, thereby “predicting” the number of future revision surgeries. This ability is extremely important for determining the projected survivorship (longevity) of newly introduced orthopaedic implants which have not yet been tested in patients. Because of the high accuracy of RSA, clinical RSA studies require only a small number of patients in order to answer the study hypothesis; typically between 20 and 30 patients per study group. By using RSA as a diagnostic tool in preliminary clinical trials, surgeons can drastically reduce the number of patients that are exposed to an untested, potentially poor-performing implant, while still obtaining the answers they need.
Advancement on the traditional marker-based RSA methodology was developed in The Netherlands in 2001; known as model-based RSA (Figure 2). This new methodology uses computer-generated models of the implant to determine motion of the implant with respect to the markers inserted in the bone. The traditional marker-based method required tantalum beads be rigidly attached to the implant itself, whereas the model-based approach no longer requires any modifications to the implant which are costly and often a headache for implant manufacturers. Model-based RSA simply relies on the projected silhouette of the implant in the x-ray images for precise location. The advent of this new technology has made RSA applicable to virtually all implanted orthopaedic devices.
Learn more about the Science behind RSA Technology… (link to science)