Objective evaluation of the shoulder dystocia phenomenon: effect of maternal pelvic orientation on force reduction.

This report describes the use of maternal pelvic and fetal models, a tactile sensing glove, and a microcomputer data acquisition system to measure fetal shoulder extraction forces. Sixty-nine experiments were carried out in the laboratory setting to simulate vaginal delivery of the aftercoming fetal shoulders. The tests were conducted using a variety of fetal biclavicular diameters (10-13 cm) and maternal pelvic angle positions (McRoberts, 10 degrees; lithotomy, 25 degrees). When comparing lithotomy versus McRoberts positioning, there was a consistent reduction in force needed to extract the fetal shoulders with the latter maneuver. No simulated clavicles were fractured during shoulder delivery until a biclavicular diameter of 12.0 cm was reached. At this point, five of eight clavicles (63%) were fractured at 25 degrees and zero of seven (0%) were fractured at 10 degrees (P less than .025). For all 69 experiments, fetal neck extension readings were consistently lower than the total traction forces recorded by the tactile sensing glove. This suggests that, in addition to the axially oriented fetal neck forces, a component of flexion (lateral force) was also present. As the difficulty of shoulder delivery increased, the impact of these inadvertent flexion forces became most pronounced at the level of the brachial plexus. This is the first study to measure shoulder extraction forces reproducibly using a laboratory model for shoulder dystocia and to describe the pathophysiology of specific neonatal injuries from a force perspective. The results document objectively that McRoberts positioning reduces shoulder extraction forces, brachial plexus stretching, and the incidence of clavicular fracture.