The Netherlands Orthopedic Society: S'Hertogenbosch, January 11–12, 1996 (original) (raw)

1996, Acta Orthopaedica Scandinavica

the Netherland? Micromoveinents of prostheses can be measured accurately by means of Roentgen Stereophotogrammetric Analysis (RSA). To this purpose, the prosthesis and the surrounding bone are marked with tiny metal pellets. The marking of the prosthesis, in particular, is regarded as disadvantageous. For this reason, for the Mallory Head hip prosthesis an adapted RSA method was developed, which requires no marking of the prosthesis but which uses prominent points of the prosthesis itself. A study was carried out in order to establish if the adapted RSA method is accurate enough for the measurement of micromovements of the Mallory Head hip prosthesis. Maferial and method: With the adapted RSA method, the spatial position of the prosthesis is determined using measuring points on the head, the stein and the tip of the prosthesis. To test the method, RSA photographs were made of a phantom placed in nine different positions. The phantom consisted of the femoral component of the Mallory Head hip prosthesis glued in a perspex cylinder. A number of tantalum markers were placed on the outline of the cylinder. The measurements of the positions of the markers and of the prominent measuring points of the prosthesis in the photograph were carried out using an accurate measuring table (Zeiss Planicomp 100). The calculated position of the cylinder in space was the golden standard with which the position of the prosthesis was compared. Results: As the prosthesis was bonded inside the cylinder with glue, no relative movement between the prosthesis and the cylinder proved possible. However, relative movements were nevertheless measured as the result of measuring errors. The magnitude of these relative movements was a measure of the magnitude of the measuring error. The relative movements of the prosthesis in relation to the cylinder were determined in eight cases, and expressed in spiral axis parameters. Two of the spiral axis parameters were the translation along and rotation round the spiral axis. The mean relative translation amounted to 0.005 inin, the mean relative ro-Copyright 0 Scandinavian University Press 1996. ISSN 0001 tation to 0.17 degrees. The 9 5 8 confidence intervals of these two paraineters ranged from-0.04 to 0.05 mm and from 0.00 to 0.36 degrees, respectively. Conclusinrz: The newly developed adapted RSA method proved to be accurate enough to determine the micromovements of the Mallory Head prosthesis. The exactitude of measurements in patients still remains to be investigated. In practice, the image quality will probably he less than that obtained with the measurements in this experimental setup. It is expected, however, that measurements with the adapted RSA technique in vivo will have an exactitude comparable with that of RSA measurements with pellet-marked prostheses.-6470. Printed in Swedenall rights reserved.