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.
Highlights
The anatomical body of knowledge is usually stated to have evolved completely
We propose that the analsphincter complex (ASC) is involved in continence and the rectal diaphragm in defecation
The anterior portion of the levator ani muscle (LAM) was well developed, but we observed no perimysial septa between puborectal and pubococcygeal portions that would allow a separation as shown in virtually all textbooks
Summary
The anatomical body of knowledge is usually stated to have evolved completely. considerable differences are noticed in modern anatomical atlases with respect to the architecture of the levator and sphincter ani complex, and the structures that affix the rectum to its surroundings. The structural features of the pelvic floor were bones of contention for well over a century and continue to attract attention [1,2,3,4,5,6,7,8]. Such disparate information, which negatively affects the correct interpretation of imaging data, functional tests, and surgical planning, can probably be accounted for by the deep position and, limited accessibility of the pelvic floor, and the near total reliance on dissection. We used the serial-section approach to settle contentious issues and an interactive 3D-pdf to make the results widely accessible
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