title: Internal and external anal sphincter anatomy as it relates to midline obstetric lacerations.
abstract: OBJECTIVE
  To examine the anatomy of the internal and external anal sphincters in the area of midline obstetric lacerations, to gain insight into sphincter damage and repair.
  
  
  METHODS
  The length, craniocaudal extent, and overlap of the internal and external anal sphincters in the perineal body were measured in 17 cadavers. Further anatomic observations were made in four sets of whole pelvis cross-sections taken in the sagittal, coronal, and transverse planes. During the repair of 20 acute fourth-degree lacerations, observations were made to determine the internal sphincter visibility following birth.
  
  
  RESULTS
  The external and internal and sphincters overlap by 17.0 mm (standard deviation [SD] 6.9), with the internal sphincter lying between the external sphincter and the anal canal. The internal sphincter extends an additional 12.2 mm (SD 5.9) cranial to the proximal extent of the external sphincter, whereas the caudal margin of the internal sphincter lies 3.7 mm (SD 7.2) cranial to the distal margin of the external sphincter. In pregnant women who sustained a fourth-degree laceration, we found that the internal sphincter can be identified as a rubbery white layer adjacent to the anal submucosa lying between the external sphincter and the anal canal.
  
  
  CONCLUSIONS
  The internal anal sphincter lies between the anal mucosa and the external anal sphincter and extends more than a centimeter above the cranial margin of the external sphincter, a region where it is disrupted when a fourth-degree obstetric laceration has occurred.
authors.affiliation: Department of Obstetrics and Gynecology, University of Michigan Medical Center, Ann Arbor, USA. delancey@umich.edu
authors.forename: J O
authors.initials: JO
authors.lastname: Delancey
authors.forename: M R
authors.initials: MR
authors.lastname: Toglia
authors.forename: D
authors.initials: D
authors.lastname: Perucchini
mesh_terms: D000368:Aged; D001003:Anal Canal / Q000033:anatomy & histology* / Q000293:injuries*; D002102:Cadaver; D004841:Episiotomy / Q000379:methods*; D005260:Female; D006801:Humans; D015601:Injury Severity Score; D007744:Obstetric Labor Complications / Q000145:classification / Q000209:etiology* / Q000473:pathology* / Q000517:prevention & control; D010502:Perineum / Q000033:anatomy & histology* / Q000293:injuries*; D011247:Pregnancy; D014950:Wounds, Penetrating / Q000145:classification / Q000209:etiology* / Q000473:pathology* / Q000517:prevention & control