With great interest I reviewed the research article by Grimm et al, and have tried to analyze their data from the perspective of clinical applicability.

The use of traction should cease when shoulder dystocia (SD) is recognized. Therefore, the baseline lithotomy model presented is akin to delivery being accomplished without SD being diagnosed, but does not assess the effect of maternal pushing. It would be reassuring to be able to conclude that downward, in-line (axial) traction, presumably applied slowly to a large-for-gestational-age baby, in a presumably average gynecoid pelvis, does not result in sufficient force or stretch considered capable of causing permanent brachial plexus (BP) injury, as predicted by their computer model. However, if the data represented an actual delivery, SD would not be documented (coded) as having occurred.

Had the authors also considered the effect of maternal forces, it is probable that additional force directed more posteriorly, creating a greater angle of downward traction, would have been predicted, resulting in increased BP stretch. If this increased force and angulation reached levels considered capable of overstretching, the clinical analogy would be that of a newborn injured prior to, or without, SD being diagnosed.

Reportedly, SD is 8-10 times more common in patients with a flat pelvis. Had the authors also studied a platypelloid model, it is probable that the shortened antero-posterior diameter and broadened pubic arch would have predicted greater force and BP stretch than in the gynecoid model, enhancing the potential clinical applicability of their research.

For the above reasons I do not believe that the baseline lithotomy model presented has validity.

Beginning in the mid-20th century delivery of the posterior arm and shoulder rotation, in place of traction, reduced the incidence of BP injury by 400%. Grimm et al nicely demonstrate that appropriately executed maneuvers after SD recognition result in less force and less stretch, illustrating why these maneuvers should be used.

Research using sophisticated physical models has shown that “SD itself does not pose additional anterior BP stretch,” when compared to models unaffected by SD, concluding that injury would be “unlikely” unless traction is applied. The computer models presented by Grimm et al, however, did not evaluate nontraction BP stretch or maternal forces. Therefore, I am critical of the commentary (unreferenced) in this “research” article, based on unsubstantiated theory, stating that “maternal forces alone” during SD “are a significant cause of BP stretch” capable of injury.