Biomechanics of Brachial Plexus Injuries Due to Shoulder Dystocia.

Brachial plexus injuries during childbirth can be devastating injuries with lifelong consequences. Here we review the biomechanical literature related to this injury and integrate it with recent epidemiological and clinical literature to better understand how intrinsic and extrinsic factors contribute to this injury. Brachial plexus palsy is caused by excessive stretching, tearing, or avulsion of the nerve fibers of the brachial plexus and can lead to temporary or permanent injury to the motor and sensory functions of the upper extremity. Compared to other maternal and fetal factors, the highest risk factor for brachial plexus palsy is shoulder dystocia. The continuum of brachial plexus injuries, from temporary impairments of the C5 and C6 nerve roots to the permanent disruption of the entire brachial plexus, is consistent with a dose-response relationship whereby higher applied birthing forces cause greater degrees of injury. The current biomechanical models of shoulder dystocia and brachial plexus strain have not been validated against experimental data and their results should be treated cautiously. Endogenous forces (e.g., uterine contractions and maternal pushing) and exogenous forces (e.g., clinician-applied traction) generate strain in the brachial plexus, but the rarity of permanent, severe injuries and the reduction of these injuries after clinician training suggest that clinician-applied forces during shoulder dystocia increase the risk of permanent, severe brachial plexus injury. There are currently no reliable biomechanical methods for determining if maternal forces or clinician-applied forces are responsible for less severe types of brachial plexus injury.