Variation of pelvic diameters due to different scanning positions--the experimental study

The distortion of human pelvis X ray scans, due to different scanning positions, can cause huge mistakes in estimation of pelvic diameters. The aim of the study was to quantify distortion of pelvic diameters in relation to scanning inclination angles. Twenty anatomically defined spots on the pelvis of a young male cadaver, freed of soft tissues, were marked with 3 mm metal balls. The digitalized X-ray scans were made with seven different but similar inclination angles, and marked spots were recognized by computer software. Obstetrical-gynecological (Ob-Gyn), horizontal and vertical diameters were measured between marked spots, and percentages of distortion were calculated for each new scanning position. Twenty seven distances on human pelvis from seven X-ray scans varied from -35.9% to 28.3%, on average 0.47%. This study has pointed to a high variation of vertical pelvic dimensions (4.94 +/- 5.73%), consequently making them unreliable in the estimation of general pelvic shape, and low variation of horizontal dimensions (0.92 +/- 0.61%). Generally, the percentage of variation of pelvic dimensions highly increases with inclination angle, in frontal and sagittal plane. Alteration of scanning distance by 4 cm has a weak influence on pelvic diameters. The most reliable Ob-Gyn pelvic diameter was conjugata diagonalis, then diameters obliqua prima and secunda, with an average length deviations of 3.4, 4.0, and 6.0% respectively. The conjugata anatomica was the most unreliable with an average variation of 11.5%.