Objective
The objective of the study was to determine how standard shoulder dystocia maneuvers affect delivery force and brachial plexus stretch.
Study Design
A 3-dimensional computer model of shoulder dystocia was developed, including both a fetus and a maternal pelvis. Application of suprapubic pressure, rotation of the infant’s shoulders, and delivery of the posterior arm following shoulder dystocia were each modeled, and delivery force and brachial plexus stretch were predicted.
Results
Compared with lithotomy alone, all maneuvers reduced both the required delivery force and brachial plexus stretch. The greatest effect was seen with delivery of the posterior arm, which showed a 71% decrease in anterior nerve stretch (3.9% vs 13.5%) and an 80% decrease in delivery force.
Conclusion
The standard maneuvers met the objective of reducing the necessary delivery force compared with the lithotomy position alone. Brachial plexus stretch is also reduced when these maneuvers are used rather than continuing the delivery in lithotomy position.
Shoulder dystocia has long been recognized as an obstetrical emergency that puts both the mother and fetus at increased risk of injury. For neonates, shoulder dystocia increases the risk of brachial plexus injury; clavicle or humerus fracture; and, if not resolved in a timely fashion, central nervous system injury or death.
In the past 10-15 years, epidemiological data, case studies, and computer modeling have all provided support to the theory that brachial plexus stretch and injury can result from maternal forces when the infant’s shoulder has an impact with the bony pelvis of the mother. However, it is also well established that clinician-applied exogenous forces can also cause significant stretch in the brachial plexus and may result in injury.
Over the past 50 years, various maneuvers have been introduced to assist with the delivery of an infant following a shoulder dystocia, including the McRoberts maneuver, Wood’s Screw, Rubin maneuver, application of suprapubic pressure, and delivery of the posterior arm.
The results of a number of studies have indicated that these maneuvers reduce the force that is required to deliver an infant following a shoulder dystocia. However, the effect of these maneuvers on brachial plexus stretch, an important predictor of injury risk, has not been thoroughly examined.
Because brachial plexus stretch cannot be investigated in a clinical delivery, computer models provide a method through which the effect of various delivery scenarios can be investigated. This study took advantage of a computer model of shoulder dystocia to examine the effect of fetal maneuvers that are typically used in the reduction of a shoulder impingement.
Materials and Methods
Using a 3-dimensional computer model previously described, the effect of various clinician-applied maneuvers on brachial plexus stretch was investigated. The model, developed using MADYMO software (TNO-MADYMO, Rijswijk, The Netherlands), is based on the anatomy of a 90th percentile infant and the pelvis of a 50th percentile mother ( Figure 1 ).
The development of the fetal model began with MADYMO’s model of a 9 month old crash test dummy, which was then modified to match the desired size for a neonate and included more biofidelic properties for the neck and the shoulder region. The brachial plexus was modeled as a single, 7.5 cm long, nonlinear spring that was connected between the C5 and C6 intervertebral joint and the midpoint of the humerus. Each shoulder was modeled as a fully articulated clavicle, humerus, and scapula to allow biofidelic shoulder movement during the delivery process.
Mechanical properties for the fetal neck and brachial plexus elements in the model were taken from the biomechanical literature. Pintar et al measured both bending and axial stiffness in a caprine (goat) model of the pediatric neck, providing scaled values that have been used frequently for predicting infant responses in child safety seats. Theirs is currently the only study that provides data on both modes of loading down for infants.
More recently, Luck et al conducted axial testing on postmortem human subjects that included neonatal and infant specimens. They concluded that the animal models previously used, which included the caprine model, provided consistent values for stiffness of the cervical spine when compared with actual human data. The study selected for the peripheral nerve properties was chosen because of its full characterization of the mechanical response in a fresh nerve.
Similar studies are not available for human nerves because of the need to test the specimens immediately after the subject dies to avoid the rapid degradation that occurs in nerve tissue. The stiffness of the neonatal shoulder was estimated to be 2.5% of an adult shoulder. This scaling is based on the mass and geometric difference between the 90th percentile infant and the 50th percentile male. Scaling based on these 2 anthropomorphic variations is a practice commonly used within the field of injury biomechanics.
The maternal pelvis was modeled as a rigid structure. The geometry was developed based on computer-aided design data derived from a progressive computed tomography scan of a median female pelvis. The anterior-posterior dimension of the outlet was 12.5 cm. Data on soft tissue resistance of the birth canal are not available; therefore, an initial estimate of this effect was made by introducing coefficients of friction of 0.6 for the sacrum and 0.2 for the pubic symphysis. Lithotomy was defined as an angle of 45° between the symphysis pubis and the horizontal axis ( Figure 2 , A).
Once developed, the fetal model was positioned within the maternal pelvis so that impact between the anterior shoulder and symphysis pubis would occur, simulating a shoulder dystocia. The bisacromial axis of the infant’s shoulders was oriented in an anterior-posterior position. Clinician-applied traction was modeled as a downward, axial force acting at a 45° angle from the horizontal.
The axis of the infant’s spine was maintained to prevent bending of the neck. The traction force was applied as 10 seconds of loading followed by 10 seconds of unloading, in a triangular pulse. The effect of gravity was included in the neonatal model.
Four different delivery scenarios were modeled ( Figure 2 ):
- 1
Lithotomy position with axially applied traction (no maneuvers), baseline.
- 2
Application of suprapubic pressure (SPP).
- 3
Rotation to an oblique position.
- 4
Delivery of the posterior arm.
All of the maneuvers were modeled based on the lithotomy position of the maternal pelvis; thus, they were not performed in conjunction with McRobert’s position in this study. Suprapubic pressure was modeled as a downward-directed force through a soft tissue element located directly superior to the pubic bone of the maternal pelvis. The force was applied for 5 seconds followed by 5 seconds of unloading, using a triangular pulse.
Oblique positioning of the shoulders was simulated by rotating the neonatal model by 15° about its long axis. This aligned the fetus’ shoulders with a wider opening within the pelvic inlet. Delivery of the posterior arm was modeled by fully abducting the posterior arm of the infant before the simulation was started. In each case, a clinician-applied force was applied to the fetal head in conjunction with the maneuver and was increased in steps until the infant delivered. Successful delivery was defined as forward motion of the anterior clavicle with respect to the pubic symphysis.
The model was used to predict the clinician-applied force required to achieve delivery and the resulting strain to the brachial plexus. Strain is defined as the percent stretch within the nerve, and is calculated from the equation:
ε = ( l i − l f ) / l i
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