Anatomic relationship between the pubic symphysis and ischial spines and its clinical significance in the assessment of fetal head engagement and station during labor

Objective To develop a geometric model from computed tomographic (CT) images in non-pregnant women that would objectively reflect clinical head station in laboring patients, against which to test the accuracy of digital examinations. Method CT scans were performed in 70 non-pregnant women to determine, using a geometric model, which angle in a mid-sagittal transperineal ultrasound (TPU) image best coincided with the mid-point of a line drawn between the ischial spines (zero station). Using a geometric algorithm, TPU angles were then assigned for other clinical stations (-5 to +5). Finally, clinical station was assessed by digital examination and simultaneous TPU assessments in 88 laboring patients to see how closely the clinical examination correlated with the station calculated from the above geometric model. Results The mean angle between the long axis of the symphysis pubis and the mid-point of the line connecting the two ischial spines for the 70 non-pregnant subjects was 99 degrees. The geometric model developed allowed us to build an algorithm to assign a specific set of theoretical angles to each computed station. Relationships between digitally assessed fetal bead station, TPU angle for each station, and the geometrical model created with CT data, are reported. Clinical digital assessment of station correlated poorly with computed station, especially at stations below zero, where it could have greater clinical impact. Conclusion The TPU angle of 99 degrees correlated with zero station, and each station above or below this station could be assigned a specific corresponding angle for reference. Copyright (c) 2009 ISUOG. Published by John Wiley & Sons, Ltd.