Pudendal nerve stretch during vaginal birth: a 3D computer simulation

Am J Obstet Gynecol. 2005 May;192(5):1669-76. doi: 10.1016/j.ajog.2005.01.032.

Abstract

Objective: The purpose of this study was to determine the increase in pudendal nerve branch lengths using a 3D computer model of vaginal delivery.

Study design: The main inferior rectal and perineal branches of the pudendal nerve were dissected in 12 hemi-pelves from 6 adult female cadavers. Their 3D courses were digitized in the 4 specimens with the most characteristic nerve branching pattern, and the data were imported into a published 3D computer model of the pelvic floor. Each nerve branch was then represented by a stretchable cord with a fixation point at the ischial spine. The length change in each branch was then quantified as the fetal head descended through the pelvic floor. The maximum nerve strains ([final length minus original length/original length] x 100) were calculated for 5 degrees of perineal descent: reference descent from the literature, 1.25 cm and 2.5 cm caudal and cephalad. The effect of alternative fixation points on resultant nerve strain was also studied.

Results: The inferior rectal branch exhibited the maximum strain, 35%, and this strain varied by 15% from the scenario with the least perineal descent to that with the most perineal descent. The strain in the perineal nerve branch innervating the anal sphincter reached 33%, while the branches innervating the posterior labia and urethral sphincter reached values of 15% and 13%, respectively. The more proximal the nerve fixation point, the greater the nerve strain.

Conclusion: During the second stage: (1) nerves innervating the anal sphincter are stretched beyond the 15% strain threshold known to cause permanent damage in appendicular peripheral nerve, and (2) the degree of perineal descent is shown to influence pudendal nerve strain.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Aged
  • Aged, 80 and over
  • Anal Canal / innervation*
  • Cadaver
  • Computer Simulation
  • Female
  • Genitalia / innervation*
  • Humans
  • Imaging, Three-Dimensional
  • Middle Aged
  • Models, Neurological*
  • Nervous System / physiopathology*
  • Parturition*
  • Rectum / innervation*
  • Stress, Mechanical