Mathematic modeling of forces associated with shoulder dystocia: a comparison of endogenous and exogenous sources.

OBJECTIVE: A mathematic model was developed to estimate the compressive pressure on the fetal neck overlying the roots of the brachial plexus by the symphysis pubis during a shoulder dystocia event. The induced pressure was calculated for both exogenous (clinician applied) and endogenous (maternal and uterine) forces during the second stage of labor.

STUDY DESIGN: Intrauterine pressure and clinician-applied force data were taken from the existing literature. A free-body diagram was generated and equilibrium equations were used to calculate the contact pressure between the base of the fetal neck and the symphysis pubis during a shoulder dystocia event.

RESULTS: Clinician-applied traction to the fetal head (exogenous force) led to an estimated contact pressure of 22.9 kPa between the fetal neck and the symphysis pubis. In contrast, uterine and maternal expulsive efforts (endogenous forces) resulted in contact pressures that ranged from 91.1 to 202.5 kPa. The estimated pressures resulting from endogenous forces are 4 to 9 times greater than the value calculated for clinician-applied forces.

CONCLUSION: Neonatal brachial plexus injury is not a priori explained by iatrogenically induced excessive traction. Spontaneous endogenous forces may contribute substantially to this type of neonatal trauma.