Variation of the spatial position computed by Roentgen Stereophotogrammetric Analysis (RSA) under non-standard conditions

Roentgen Stereophotogrammetric Analysis (RSA) has been applied to different kinds of research in order to obtain important information in the biomecha nics field. Operative requirements change according to the type of investig ation and sometimes practical conditions do not always permit one to respec t standard specifications. The aim of this paper is to verify the reliability of the system under non- standard conditions, studying the effects of the focus-to-film distance on the determination of 3D co-ordinates of markers. The application of statistical analysis, consisting of two and three way AN alysis Of VAriance (ANOVA), showed that focus-to-film distance is not a sig nificant source of variation, i.e., the system can correctly compute the ma rker position inside the calibrated space under all conditions of magnifica tion. Since we obtained a fluctuation due to focal distance change of 10(-5 ) mm (LSD for 95% confidence level), we can assert that this source causes a variation strictly within the limits required by clinical investigations. Thus, this study contributes to improving the flexibility of RSA in clinic al applications, making the technical requirements of the radiographic sit- up less constraining. (C) 1999 IPEM. Published by Elsevier Science Ltd. All rights reserved.