Precise Three-Dimensional Morphology of the Male Anterior Anorectum Reconstructed From Large Serial Histologic Sections: A Cadaveric Study

BACKGROUND: Deep anatomic knowledge of the male anterior anorectum is important to avoid urethral injury and rectal perforation in intersphincteric resection or abdominoperineal resection for very low rectal cancer. However, its structure is difficult to understand, because the anorectum, muscles, and urogenital organs are complicatedly and 3-dimensionally arranged. OBJECTIVE: The purpose of this study was to revisit the anatomic information of the male anterior anorectum for intersphincteric resection and abdominoperineal resection with a focus on the spatial muscular morphology. DESIGN: This was a descriptive cadaveric study. SETTINGS: The study was conducted at Ehime and Kyoto universities. PATIENTS: Tissue specimens from 9 male cadavers were included. MAIN OUTCOME MEASURES: Specimens around the anterior anorectum were serially sectioned in the horizontal, sagittal, or frontal plane; large semiserial histologic sections were created at 250-mu m intervals. The series were stained with Elastica van Gieson, and some sections from the series were studied by immunohistochemistry to detect smooth and striated muscles. Two series were digitalized and reconstructed 3-dimensionally. RESULTS: Two regions without a clear anatomic border were elucidated: 1) the anterior region of the external anal sphincter, where the external anal sphincter, bulbospongiosus muscle, and superficial transverse perineal muscle were intertwined; and 2) the rectourethralis muscle, where the smooth muscle of the longitudinal muscle continuously extended to the posteroinferior area of the urethra, which became closest to the anorectum at the prostatic apex level. A tight connection between the striated and smooth muscles was identified at the anterior part of the upper external anal sphincter and anterolateral part of the puborectalis muscle level. LIMITATIONS: This study involved a small sample size of elderly cadavers. CONCLUSIONS: This study clarified the precise spatial relationship between smooth and striated muscles. The detailed anatomic findings will contribute more accurate step-by-step anterior dissection in intersphincteric resection and abdominoperineal resection, especially with the transanal approach, which can magnify the muscle fiber direction and contraction of striated muscle by electrostimulation. MORFOLOGiA TRIDIMENSIONAL PRECISA DEL ANORRECTO ANTERIOR MASCULINO RECONSTRUIDO A TRAVeS DE SECCIONES MAYORES HISTOL & Oacute; GICAS EN SERIE: UN ESTUDIO CADAVeRICOANTECEDENTES: El conocimiento anatomico amplio del anorrecto anterior masculino es importante para evitar lesiones de uretra y perforacion de recto en la reseccion interesfinterica o la reseccion abdominoperineal para cancer de recto bajo. Sin embargo, su estructura es dificil de entender porque el anorrecto, los musculos y los organos urogenitales estan alinados en forma complexa tridimensional. OBJETIVO: Revisar de nuevo el conocimiento anatomico del anorrecto anterior masculino relevante a la reseccion interesfinterica y la reseccion abdominoperineal con un enfoque en la morfologia muscular espacial. DISENO: Estudio descriptivo cadaverico. ENTORNO: Ehime y la Universidad de Kyoto. SUJETOS: Tejido especimenes de nueve cadaveres masculinos. PUNTOS FINALES DE VALORACI & Oacute; N: Las muestras alrededor del anorrecto anterior se seccionaron en serie en planos horizontal, sagital y coronal. Se crearon mayores secciones histologicas en serie a intervalos de 250 mu m. Los especimenes fueron tenidos con Elastica van Gieson, y algunas secciones de la serie se estudiaron mediante inmunohistoquimica para detectar musculos lisos y estriados. Dos series fueron digitalizadas y reconstruidas tridimensionalmente. RESULTADOS: Se demostraron dos regiones sin un borde anatomico definido: (i) la region anterior del esfinter anal externo, donde se entrelazaron el esfinter anal externo, el musculo bulbospongoso y el musculo perineal transverso superficial; y (ii) musculo rectouretral, donde el musculo liso del musculo longitudinal se extiende continuamente a la zona posteroinferior de la uretra, que se acerca mas al anorrecto a nivel del apice prostatico. La conexion estrecha entre los musculos estriados y lisos se identifico en la parte anterior del esfinter anal externo superior y la parte anterolateral del nivel del musculo puborrectal. LIMITACI & Oacute; N: Este estudio incluyo una muestra pequena de cadaveres ancianos. CONCLUSI & Oacute; N: Este estudio aclaro la relacion espacial precisa entre los musculos lisos y estriados. Los hallazgos anatomicos detallados ayudaran para una diseccion anterior paso a paso mas precisa en la reseccion interesfinterica y la reseccion abdominoperineal, especialmente con el abordaje transanal, que puede magnificar la direccion de las fibras musculares y la contraccion del musculo estriado utilizando electroestimulacion.