Roentgen stereophotogrammetry (RSA) is a highly accurate technique for the assessment of three-dimensional migration and micromotion of a
joint replacement
Replacement arthroplasty (from Greek ''arthron'', joint, limb, articulate, + ''plassein'', to form, mould, forge, feign, make an image of), or joint replacement surgery, is a procedure of orthopedic surgery in which an arthritic or dysfunctional ...
prosthesis relative to the bone it is attached to. It was introduced in 1974 by Göran Selvik.
Several studies have found implant migration to be predictive of long-term implant survival and, for most devices, measurement over 2 years might therefore provide a surrogate outcome measure with relatively low numbers of subjects, e.g. from 15 to 25 patients in each group in randomized studies. A smaller number of subjects can be used in these studies as a consequence of the high accuracy of the measurement technique. Because of this, RSA is an important technique in early
clinical trials for screening new joint replacement prostheses.
Methodology
To achieve the high accuracy, the following steps are carried out: Small
radio opaque markers are introduced in the bone and attached to the prosthesis to serve as well-defined artificial landmarks. Two synchronised
x-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
foci are used to obtain a stereo image of the bone and the prosthesis. The positions of the foci are assessed using a calibration object that holds
tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...
markers at accurately known positions. The coordinates of the bone and prosthesis markers are accurately measured and the
three-dimensional position of the markers is reconstructed using software. The change in the position (
translation
Translation is the communication of the meaning of a source-language text by means of an equivalent target-language text. The English language draws a terminological distinction (which does not exist in every language) between ''transla ...
and
rotation) of the prosthesis markers relative to the bone markers is then determined. The reported accuracy of RSA ranges between 0.05 and 0.5 mm for translations and between 0.15˚ and 1.15˚ for rotations (95%
confidence interval). New RSA techniques that avoid the need for attaching markers to the prosthesis have been introduced.
See also
*
Roentgen radiation
*
Stereophotogrammetry
Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant ima ...
*
Dynamic Roentgen stereophotogrammetry
References
External links
Digital Roentgen Stereophotogrammetry: Development, Validation, and Clinical Application
{{DEFAULTSORT:Medical Imaging
Prosthetics
Medical imaging