Location and Relationships
The external anal sphincter is located in the anal region of the perineum.
- Rectum (rectum) — the sphincter surrounds its terminal part.
- Perineal body (centrum tendineum perinei) — the structure into which the sphincter fibers blend.
- Levator ani muscle (m. levator ani) — forms the deepest layer of the pelvic floor muscles.
Sphincteric Apparatus of the Rectum
The sphincteric apparatus of the rectum includes:
- External anal sphincter (m. sphincter ani externus) — composed of skeletal muscle, under voluntary control, and surrounds the termination of the bowel.
- Internal anal sphincter (m. sphincter ani internus) — formed by smooth muscle, involuntary. Circular muscle bundles are located 3–4 cm from the anal opening.
- Third sphincter / Houston's/Hepner's muscle (m. sphincter ani tertius) — a thickening of longitudinal muscle fibers located 10 cm from the anal opening, corresponding to the perineal flexure of the rectum.
Topography and Surrounding Structures
The external anal sphincter serves as an important topographic landmark of the perineum.
- Ischioanal fossae (fossa ischiorectalis) — located on both sides of the perineal part of the rectum.
- Pelvic floor muscles — at the level of the pelvic diaphragm, fibers of the m. levator ani blend into the muscular layer of the rectum. The levator ani muscle includes the pubococcygeus, iliococcygeus, and puborectalis/ischiococcygeus components.
- Anococcygeal ligament (lig. anococcygeum) — listed among the perineal landmarks.
Clinical Significance and Obstetric Trauma
The condition of the external sphincter is critical in obstetrics and operative surgery:
- Perineal tears: In third-degree perineal tears, the external anal sphincter (m. sphincter ani externus) is specifically listed among the disrupted structures.
- Surgical repair: When repairing a third-degree tear, the rectum is repaired first: sutures are placed extramucosally through the edges of the intestinal wound without piercing the mucosa. Next, catgut sutures are placed across the edges of the m. sphincter ani externus. This step effectively converts a third-degree tear into a second-degree tear. Subsequently, the vagina, deep transverse perineal muscle, medial margins of the m. levator ani, and superficial transverse muscle along with subcutaneous tissue are closed layer by layer.
- Central perineal rupture: In this rare injury, the fetus is delivered through a pathological opening in the center of the perineum. The anal sphincter and posterior labial commissure remain intact, while severe trauma occurs to the perineal muscles.
- Rectal surgeries: During the perineal phase of rectal surgery, the anal opening is closed with a purse-string suture, and two elliptical incisions are made around the anus.