Mechanical loosening of orthopaedic implants can be predicted by measuring the progressive micromotion of the implant with respect to the bone in the first two years after surgery.The most accurate Roentgen technique for three-dimensional assessment of micromotion is Roentgen Stereophotogrammetric Analysis(RSA).The reported accuracy of RSA ranges between 0.05 and 0.5 mm for translations and between 0.15 and 1.15 deg for rotations.RSA is used to study the effect on prosthetic fixation due to changes in implant design,addition of coatings,or new bone cements.The advantages of RSA are that small patient groups(i.e.25 patients) and short-term(i.e.2 years) clinical studies are in general sufficient to predict the likelihood of long-term(i.e.10 years) mechanical loosening of the prosthesis.This makes RSA an important measurement tool to study new developments in prosthetic design in order to prevent large patient groups from being exposed to potentially inferior designs.In this paper,the basics of the RSA technique will be explained,and some examples of clinical RSA studies will be presented to illustrate the clinical relevance of RSA.