An evaluation of the usefulness of x-ray pelvimetry: comparison of the Thoms and modified Ball methods with manual pelvimetry.

One hundred studies by x-ray pelvimetry of cephalic presentations were retrospectively analyzed. The Thoms method of interpretation was compared to the modified Ball technique, and both were then contrasted with manual assessment of the pelvis, as prognostic indicators for safe vaginal delivery. Uneventful nonoperative vaginal deliveries occurred in 28.6% of patients with either inlet or midpelvic disproportion by the Thoms method, and in 22.5% of women with absolute disproportion in either plane by the modified Ball method. Neither of the two pelvimetric techniques was significantly more accurate than manual assessment of the pelvis in predicting obstetric outcome. The modified Ball technique offered no improvement over the Thoms method. In view of the high rate of false positives in this series, and the potential oncogenic risk of fetal irradiation, argument is presented for a full trial of labor in vertex presentations, except in the most clinically obvious cases of cephalopelvic disproportion (CPD). The data from this study suggest that there is no significant role for x-ray pelvimetry in the diagnosis and management of CPD in cephalic presentations.