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 pre ssure on the fetal neck overlying the roots of the brachial plexus by the s ymphysis pubis during a shoulder dystocia event. The induced pressure was c alculated for both exogenous (clinician applied) and endogenous (maternal a nd uterine) forces during the second stage of labor. STUDY DESIGN: Intrauterine pressure and clinician-applied force data were t aken from the existing literature. A free-body diagram was generated and eq uilibrium 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 th e symphysis pubis. In contrast, uterine and maternal expulsive efforts (end ogenous 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 ia trogenically induced excessive traction. Spontaneous endogenous forces may contribute substantially to this type of neonatal trauma.