RSA Introduction

Clinical use of RSA may be divided into the following steps:

1. Small spherical markers (0.5-1.0 mm in diameter) made of Tantalum are inserted into the patient and/or attached to the prosthesis. The markers are grouped into segments, which may be used to define an orthopaedic entity, e.g. the femur. Insertion may be done during surgery, but also non-surgically with an insertion device.
2. The patient is X-rayed from two angles simultaneously together with a reference cage, producing two radiographs. See the illustration below.
3. The position of the marker projections are measured on the films.
4. The projected positions of the fiducial markers and control points attached to the cage are used to establish the exposure geometry, i.e. the relationship between the X-ray foci, the reference cage, and the X-ray films.
5. The three-dimensional coordinates of the patient markers are reconstructed from their projections.
6. Given the result of a previous RSA examination, the motion between different segments may be calculated.