Evolution of the human pelvis and obstructed labor: new explanations of an old obstetrical dilemma

Without cesarean delivery, obstructed labor can result in maternal and fetal injuries or even death given a disproportion in size between the fetus and the maternal birth canal. The precise frequency of obstructed labor is difficult to estimate because of the widespread use of cesarean delivery for indications other than proven cephalopelvic disproportion, but it has been estimated that at least 1 million mothers per year are affected by this disorder worldwide. Why is the fit between the fetus and the maternal pelvis so tight? Why did evolution not lead to a greater safety margin, as in other primates? Here we review current research and suggest new hypotheses on the evolution of human childbirth and pelvic morphology. In 1960, Washburn suggested that this obstetrical dilemma arose because the human pelvis is an evolutionary compromise between two functions, bipedal gait and childbirth. However, recent biomechanical and kinematic studies indicate that pelvic width does not considerably affect the efficiency of bipedal gait and thus is unlikely to have constrained the evolution of a wider birth canal. Instead, bipedalism may have primarily constrained the flexibility of the pubic symphysis during pregnancy, which opens much wider in most mammals with large fetuses than in humans. We argue that the birth canal is mainly constrained by the trade-off between 2 pregnancy-related functions: while a narrow pelvis is disadvantageous for childbirth, it offers better support for the weight exerted by the viscera and the large human fetus during the long gestation period. We discuss the implications of this hypothesis for understanding pelvic floor dysfunction. Furthermore, we propose that selection for a narrow pelvis has also acted in males because of the role of pelvic floor musculature in erectile function. Finally, we review the cliff-edge model of obstetric selection to explain why evolution cannot completely eliminate cephalopelvic disproportion. This model also predicts that the regular application of life-saving cesarean delivery has evolutionarily increased rates of cephalopelvic disproportion already. We address how evolutionary models contribute to understanding and decision making in obstetrics and gynecology as well as in devising health care policies.

Fluoroscopic iliosacral screw placement made safe

AIM

Unstable posterior pelvic ring injuries should be stabilised successfully by percutaneous iliosacral screwing. The intervention takes place under intraoperative fluoroscopic guidance. The inlet and outlet views are crucial and are performed by tilting the image intensifier. Safely interpreting fluoroscopic views can be challenging in certain clinical scenarios. We demonstrated on a series of patients howpreoperative CT scans can be used to anticipate the appropriate intraoperative inlet and outlet fluoroscopic views and positioning of the patient on the operating table, thereby avoiding possible operating table obstacles.

MATERIALS AND METHODS

We analysed at random 30 pelvic CT scans from patients of different ages and both sexes, utilising the sagittal reconstructions. Inlet and outlet angle measurements were calculated on the scans to determine the appropriate intraoperative inlet and outlet views.

RESULTS

The analysed CT scans showed an average inlet view of 22.3° (range 10.4°-39.8°) and an average outlet view of 42.3° (range 31.5°-53.1°). Sex and age had no influence on results. The calculated required free space under the operating table for unobstructed tilting of the C-arm was a minimum of 145cm.

CONCLUSION

The significant anatomic variations of the posterior pelvic ring have been well documented in the literature. The angles required to obtain appropriate intraoperative inlet and outlet views are not perpendicular and differ greatly from traditional settings, which directed the beam 45° caudally and 45° cranially. The fluoroscopic beam would need to be angled differently in each patient to obtain ideal cardinal views that ultimately assist in safe iliosacral screw placement. To avoid collision of the C-arm with the operating table, it is essential to provide secure free space under the operating table of at least 145cm.