Stereo Orthopaedic Radiography (SOR) (also known as Roentgen Stereophotogrammetric Analysis or RSA) is a methodology which uses two x-ray images taken at the same time for the purpose of making accurate 3D measurements. Much like having two eyes vs. one, SOR allows for accurate depth perception.
There are several ways to perform SOR. The traditional and most accurate method involves implanting tiny markers (tantalum balls of 1.0 mm diameter) into a patient’s bone during surgery and attaching these markers to an implant. This allows for accurate measurement of the motion of the implant relative to the bone via the markers. Early excessive motion has been shown to predict implant loosening later on. Wear of the implant’s insert can be measured with this methodology as well.
Implants can also be measured directly eliminating the need to attach markers which can be a costly endeavor due to regulatory requirements. Direct measurement is only possible if accurate CAD models or reverse engineered models are available. This results in a small but acceptable loss in accuracy for most applications.
Both previous methods require a surgical procedure. For pre-operative measurements no implants of markers are available to perform the measurements. This is the most challenging form of SOR, because there is less contrast between bones and soft tissue and there is no prior knowledge of the exact shape of the bone. Shape models, however, can be reconstructed from 3D imaging data such as CT and MRI, but this is a prohibitively labor-intensive method. Halifax Biomedical Inc. is developing a novel technique that will estimate the shape of the bone, based on knowledge of the anatomy of that bone, and the bone’s position and orientation in the same analysis eliminating the need for 3D imaging.