[Standardized analysis of acetabulum cup migration in hip endoprosthesis using digital image processing]

Measurement of the migration of the acetabular component following total hip replacement seems to be a useful method of early detection of aseptic loosening. The methods used so far for migration analysis are either too time-consuming or too elaborate in terms of the technical equipment required, and are therefore not suitable for use in day-to-day clinical practice. Analysis software based on the methods described by Nunn and Sutherland was developed with the aid of digitized conventional radiographs of the pelvis. It allows a planar migration measurement including cup inclination following manual marking of only five reference points on a computer screen. A correction for projection enlargement and rotational malpositioning of the patient is performed. All results are stored in a data bank and automatically compared with those recorded at previous follow-ups. Five different acetabular cups were analysed by a conventional and by a digital method for calculation of intra-observer variability. The standard deviation (SD) for horizontal and vertical migration was 0.2-0.4 mm and that for inclination angle 0.8-1.4 degrees with digital evaluation. For manual measurement the SD was 0.4-0.8 mm for migration and 0.8-1.2 degrees for inclination. A pelvis phantom was measured in nine different positions (neutral, 5 degrees and 10 degrees cranial and caudal inclination, 5 degrees and 10 degrees rotation to each side). Four different acetabular cup design were examined, each of which yielded a series of nine measurements. Within these series the SD for biplanar distances varied between 0.1 mm and 1.2 mm and for inclination between 1.1 degree and 2.3 degrees. Intra-observer variability was negligible.(ABSTRACT TRUNCATED AT 250 WORDS)