Clinical studies have shown that early motion (migration) of un-cemented implants is related to later loss of the implant.

Prosthesis magration can be examined by the use of marker-based radio stereometric analysis (RSA).

In order to carry out this examination the joint prostheses are marked with tantalum balls which can be recognized by the computer software RSA-CMS (Medis Specials, Holland). Furthermore, tantalum balls are placed in the bone near the prosthesis.

By means of a stereometric picture, taken immediately after the oparation, the prosthesis' initial position in relation to fixation points in the bone is determined by means of a test grid.

Control stereometric pictures are taken according to determined time intervals. By comparing the pictures the prosthesis' displacement from the bone fixation points can be determined in three dimensions as a function of time. If the prosthesis migrates from the surrounding bone a mechanical solution may be contemplated.

Today researchers and clinicians agree that the effect of new surgical methods at the insertion and design of a prosthesis should be evaluated through migration studies before they can be recommended for general use. 

By means of RSA it is possible to gain a very precise evaluation of the prosthesis migration, i.e. down to 0.11 mm.

The newst development within the RSA method is Model Based RSA (Medis Specials, Holland), which makes it possible to match a model of the prosthesis (CAD or reverse engineered) on the radio stereometric picure of the patient's implant.

The prosthesis' migration is determined in relation to tantalum bone markers which are placed in the bone during the operation. The system has shown to have an equivalent or better accuracy than the Marker Based RSA.

In Aarhus we have done a methological examination of cemented knee prostheses. We have found a general matching defect between the model and the implant on the 0.01 mm radio stereometric picture.

The analysis with Model Based RSA takes approx. one hour per radio stereometric picture, which is faster than or equivalent to Marker Based RSA. The analyses are done by experienced laboratory technicians.

RSA is a good predictor on the long-term prognosis concerning aseptic solution and studies concerning RSA. Mechanical solution shows that a short follow-up period of two to five years is sufficient when determining the long-term course for both knee and hip prostheses.

Contact Maiken Stilling for further information.