3D Topography of the Young Adult Anal Sphincter Complex Reconstructed from Undeformed Serial Anatomical Sections

Abstract

BackgroundPelvic-floor anatomy is usually studied by artifact-prone dissection or imaging, which requires prior anatomical knowledge. We used the serial-section approach to settle contentious issues and an interactive 3D-pdf to make the results widely accessible.Method3D reconstructions of undeformed thin serial anatomical sections of 4 females and 2 males (21–35y) of the Chinese Visible Human database.FindingsBased on tendinous septa and muscle-fiber orientation as segmentation guides, the anal-sphincter complex (ASC) comprised the subcutaneous external anal sphincter (EAS) and the U-shaped puborectal muscle, a part of the levator ani muscle (LAM). The anococcygeal ligament fixed the EAS to the coccygeal bone. The puborectal-muscle loops, which define the levator hiatus, passed around the anorectal junction and inserted anteriorly on the perineal body and pubic bone. The LAM had a common anterior attachment to the pubic bone, but separated posteriorly into puborectal and “pubovisceral” muscles. This pubovisceral muscle was bilayered: its internal layer attached to the conjoint longitudinal muscle of the rectum and the rectococcygeal fascia, while its outer, patchy layer reinforced the inner layer. ASC contraction makes the ano-rectal bend more acute and lifts the pelvic floor. Extensions of the rectal longitudinal smooth muscle to the coccygeal bone (rectococcygeal muscle), perineal body (rectoperineal muscle), and endopelvic fascia (conjoint longitudinal and pubovisceral muscles) formed a “diaphragm” at the inferior boundary of the mesorectum that suspended the anorectal junction. Its contraction should straighten the anorectal bend.ConclusionThe serial-section approach settled contentious topographic issues of the pelvic floor. We propose that the ASC is involved in continence and the rectal diaphragm in defecation.

Architecture of structures in the urogenital triangle of young adult males; comparison with females

“…; Wu et al. ). Near their attachment to the perineal body, the superficial and deep portions of the puborectal muscle were hard to separate.…”

“…The architecture of the muscles of the pelvic floor in males resembled that in females, including the presence of a rectal diaphragm consisting of the rectoperineal, rectococcygeal, and bilaterally the internal layers of the pubovisceral muscle (Wu et al. ). The differences between our description and that present in Anatomical Terminology (FCAT, ) are summarized in Table .…”

“…Another unexpected feature was the approximately twofold smaller volume of the LAM in males than females (Wu et al. ), even though overall body muscle mass is, after correction for size, ~ 25% greater in males than in females (Janssen et al. ).…”

“…The perineal body was still relatively small in fetuses and remained about twofold smaller in males than in age‐matched females (Wu et al. ; cf. Oh & Kark, ).…”

“…) that is interlaced with well‐developed fibrous septa (De Blok, ), which can be tightened by the superficial transverse perineal muscle (Wu et al. ).…”