while the head has rotated in the AP diameter at
the outlet. The severity of the injury may depend
on the degree of traction, twisting and extension of
the fetal head. Measurements of traction forces with
the use of force sensing devices have shown that the
applied peak traction forces are about 47 N for routine
deliveries, 69N for difficult deliveries, and 100
N for deliveries with shoulder dystocia, suggesting
that the applied traction is and proportionate to the
severity of the dystocia.19
Even in deliveries not complicated by shoulder
dystocia, the forces during downward traction are
often underestimated, as substantial forces were
found to have been used in many OBPP cases. Direct
compression of the brachial plexus against the
symphysis pubis can also contribute to injury.
OBPP may occur regardless of the maneuvers
used in cases of shoulder dystocia, but the difficulty
to achieve delivery of the shoulders and the need
for additional maneuvers is correlated to the risk
of OBPP. Experiments with pelvic and fetal models
have shown that as the difficulty of the delivery and
the traction forces increase, there is a concentration
of force on the brachial plexus from exogenously
applied lateral flexion and that the force requirements
and the incidence of injury are higher in
wider fetal shoulder girths.20 In contrast, the degree
of brachial plexus stretching is reduced with
the McRoberts maneuver. Slightly more than 10%
while the head has rotated in the AP diameter atthe outlet. The severity of the injury may dependon the degree of traction, twisting and extension ofthe fetal head. Measurements of traction forces withthe use of force sensing devices have shown that theapplied peak traction forces are about 47 N for routinedeliveries, 69N for difficult deliveries, and 100N for deliveries with shoulder dystocia, suggestingthat the applied traction is and proportionate to theseverity of the dystocia.19Even in deliveries not complicated by shoulderdystocia, the forces during downward traction areoften underestimated, as substantial forces werefound to have been used in many OBPP cases. Directcompression of the brachial plexus against thesymphysis pubis can also contribute to injury.OBPP may occur regardless of the maneuversused in cases of shoulder dystocia, but the difficultyto achieve delivery of the shoulders and the needfor additional maneuvers is correlated to the riskof OBPP. Experiments with pelvic and fetal modelshave shown that as the difficulty of the delivery andthe traction forces increase, there is a concentrationof force on the brachial plexus from exogenouslyapplied lateral flexion and that the force requirementsand the incidence of injury are higher inwider fetal shoulder girths.20 In contrast, the degreeof brachial plexus stretching is reduced withthe McRoberts maneuver. Slightly more than 10%
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