Transitional Female Sacrum: Dimensions, Alterations in Dorsal Pelvic Structure, and Potential Obstetric Implications

Objectives: The space available within the female pelvis is determined by the morphology of all bony components constituting the pelvic skeleton. Although several studies have investigated the impact of hip-bone structural variations in on parturition, the potential effects of commonly occurring lumbosacral transitional vertebrae (LSTV) variations have been underreported. This descriptive morphometric study reports dimensions of the female sacrum associated LSTV variations and suggests their probable mechanistic effects on normal labor. Methods: One hundred and twelve female osteological sacral specimens with LSTV were examined for the type of transitional anomaly. Position, height, and surface areas of the auricular surfaces, interauricular distances, sacral heights, S1 body width and upper surface areas, and articulating areas of S1 facets were measured and compared with data from non-transitional samples. Results: Female LSTV predominantly presented as accessory L5-S1 articulations (unilateral and bilateral) and degrees of lumbarization (separation of the first sacral segment). Since some of these alterations were found to be significantly associated with changes in sacral size, these features, in conjunction with cranial shift of the auricular surfaces, may be associated with overall in dorsal pelvic dimensions. Conclusions: Structural alterations identified in female LSTV sacrum may change dorsal pelvic dimensions and thus, the availability of dorsal pelvic space, potentially altering the biomechanics of normal labor