Mechanisms of Pelvic Floor Trauma During Vaginal Delivery

Abstract

We review the biomechanics of the second stage of labor and the mechanics of non-superficial injuries that occur during labor. Primiparas are those at greatest risk for injury, but because of the intrusive nature of direct measurements, we have instead used a range of computer models to determine which pelvic floor muscles and nerves are stretched the most and when this occurs during the second stage of labor. The results show that it is the pubovisceral (also known as the pubococcygeal) muscle that is stretched the most, reaching a value exceeding three times its original length near the end of the second stage. Postnatal magnetic resonance imaging confirms the mechanism of injury as being stretch-induced muscle tearing, not muscle compression or denervation, by showing immediate muscle disruption after a difficult labor. Our findings also suggest 75% of women can give birth to any fetal head size without risk of injury, ~5% will need an instrumented delivery, but the remaining 20% will need care to prevent injury. Viscoelastic modeling of distal birth canal dilation behavior under a given force in 28 healthy women shows a remarkable 20-fold variation above and below the mean value in the longer of the two time constants due to as yet unknown hormonal ripening mechanisms. Our current models, are currently better able to predict undesirably long second-stage durations than the risk of muscle injury in any specific patient, indicating a worthwhile topic for future research.