How can a clavicle fracture occur during spontaneous vaginal delivery (SVD) without shoulder dystocia?

Mechanism of Clavicle Fracture During Uncomplicated SVD

Clavicle fractures can occur during spontaneous vaginal delivery without shoulder dystocia primarily due to the mechanical forces exerted on the fetal shoulder girdle as it navigates through the maternal pelvis, particularly when the fetal biclavicular diameter exceeds the pelvic dimensions, independent of impaction at the symphysis pubis. 1

Primary Mechanisms Without Shoulder Dystocia

Direct Compression Forces

• The clavicle is subjected to compressive forces as the fetal shoulders rotate and descend through the birth canal, even during normal delivery mechanics 2
• Laboratory models demonstrate that clavicular fracture can occur when the biclavicular diameter reaches 12.0 cm or greater, with fracture rates of 63% in lithotomy position versus 0% in McRoberts position at this diameter 2
• The relatively uniform pressure exerted by the muscular birth canal during vaginal delivery can generate sufficient force to fracture the clavicle without causing shoulder impaction 3

Birth Weight and Fetal Size

• Fracture risk is directly related to fetal weight, with the average weight of infants sustaining clavicular fractures being 3767 grams 1
• In macrosomic infants (>4500g), the risk of clavicular fracture increases approximately 10-fold compared to normal birth weight infants 3
• The weight of the newborn and delivery technique are the most significant determinants of clavicular fracture, independent of shoulder dystocia occurrence 1

Clinical Evidence Supporting Non-Dystocia Fractures

Epidemiologic Data

• Among 38 clavicular fractures studied over 5.5 years, only 2 were complicated by shoulder dystocia, demonstrating that the majority occur without this complication 1
• Clavicular fractures occurred across all delivery modes: 24 in normal vaginal deliveries with episiotomy, 6 with vacuum extraction, and 8 during cesarean sections 1
• The overall incidence of clavicular fracture is 0.2-3.5% of all births, while shoulder dystocia complicates only 1.4% of vaginal deliveries, indicating these are largely independent events 3, 1

Fracture Patterns in Shoulder Dystocia Cases

• Interestingly, clavicular fracture occurs significantly less often in shoulder dystocia cases without brachial plexus injury (2%) compared to those with brachial plexus injury (17%), suggesting different force vectors are involved 4
• Among 185 coded episodes of shoulder dystocia, only 14 fractured clavicles occurred, while an additional 14 clavicular fractures occurred in deliveries not coded for shoulder dystocia 5

Biomechanical Considerations

Force Distribution

• Fetal neck extension readings are consistently lower than total traction forces recorded during delivery, indicating that lateral flexion forces (not just axial traction) contribute to skeletal injury 2
• These inadvertent flexion forces become most pronounced as delivery difficulty increases, affecting both the brachial plexus and clavicle 2
• Cesarean delivery involves significant forces on fetal limbs through a relatively small uterine incision, which can also cause fractures despite avoiding vaginal canal compression 3

Maternal Positioning Effects

• McRoberts positioning (maternal thigh hyperflexion) consistently reduces shoulder extraction forces and clavicular fracture incidence compared to lithotomy position 2
• At a biclavicular diameter of 12.0 cm, no clavicles fractured at 10 degrees (McRoberts) versus 63% fracture rate at 25 degrees (lithotomy) 2

Clinical Implications

Unpredictability

• Most traditional risk factors for shoulder dystocia have no predictive value for clavicular fracture, and infants at risk are virtually impossible to predict prospectively 5
• No delivery method is without injury risk—fractures occur in spontaneous vaginal delivery, operative vaginal delivery, and cesarean section 1, 5

Management Considerations

• The technique of managing labor and delivery is more important than the presence or absence of shoulder dystocia in determining fracture risk 1
• Attempting to deliver during a uterine contraction can exacerbate problems with head elevation and potentially increase skeletal injury risk 6
• Clavicular fractures typically heal without sequelae, occurring in approximately 10.6% of shoulder dystocia cases when present 7