Iliococcygeus

The iliococcygeus is the most posterior and lateral component of the levator ani muscle group. Though thinner and more aponeurotic than the puborectalis and pubococcygeus, it plays an important role in lifting the pelvic floor and supporting the pelvic organs. Its fibers originate from the tendinous arch of the obturator fascia and course medially to insert onto the anococcygeal ligament and coccyx. The iliococcygeus contributes to the integrity of the pelvic diaphragm and assists with continence and pelvic stability.

Structure

The iliococcygeus is a paired, fan-shaped muscle. Unlike the puborectalis and pubococcygeus, it is often less muscular and more tendinous in texture. It originates from a linear thickening of the obturator internus fascia known as the tendinous arch (arcus tendineus levator ani) and from the ischial spine. Its fibers pass medially and posteriorly to insert into the anococcygeal ligament and the coccyx.

Attachments

Location

The iliococcygeus is the most lateral part of the levator ani. It lies posterior and lateral to the pubococcygeus and puborectalis muscles. Superiorly, it is bordered by the pelvic sidewall and obturator internus muscle. Inferiorly, it contributes to the muscular layer of the pelvic floor, meeting its contralateral counterpart at the midline raphe posterior to the anal canal.

Relations

Function

• Pelvic Floor Elevation: Contracts to elevate the pelvic floor, supporting pelvic organs during increased intra-abdominal pressure
• Pelvic Organ Support: Helps maintain the position of the bladder, uterus, rectum, and small bowel
• Reinforcement of the Anococcygeal Ligament: Helps stabilize the posterior midline pelvic floor
• Continence Support: Assists in urinary and fecal continence by contributing to the sphincter mechanism of the pelvic floor

Innervation

• Nerve to levator ani (S3–S4): Primary motor supply

Vascular Supply

• Inferior gluteal artery
• Internal pudendal artery
• Venous drainage parallels arteries to the internal iliac vein

Histology

The iliococcygeus consists primarily of skeletal muscle fibers with a high proportion of Type I fibers, suitable for endurance and postural tone. However, it is generally thinner and flatter than the other levator ani components and contains more aponeurotic tissue. This structural composition reflects its role as a stabilizer rather than a primary mover.

Development

The iliococcygeus develops from the paraxial mesoderm of the sacral somites. It shares a common embryologic origin with other levator ani muscles. By the 9th week of gestation, the muscle begins to separate from adjacent fascial layers and assumes its adult-like attachments along the tendinous arch and coccyx. Its development is influenced by fetal positioning and the descent of pelvic viscera.

Role in Imaging

• Visible on MRI as a thin band of muscle extending from the pelvic sidewall to the anococcygeal ligament
• Can be assessed in 3D transperineal ultrasound for defects or thinning in patients with pelvic floor disorders
• Loss of iliococcygeus continuity may be seen in levator avulsion following childbirth

Clinical Significance

• Pelvic Organ Prolapse: While less commonly torn than pubococcygeus, defects or thinning of iliococcygeus contribute to lateral support loss and uterine or vaginal prolapse
• Pelvic Floor Weakness: Global weakening of levator ani, including iliococcygeus, can result in decreased pelvic floor elevation and poor sphincter control
• Levator Avulsion Injury: Avulsion of levator ani muscles, often including iliococcygeus, can be seen postpartum on imaging and correlates with chronic dysfunction
• Pelvic Pain Syndromes: In hypertonic states, iliococcygeus may contribute to myofascial pelvic pain and may respond to pelvic floor physiotherapy
• Surgical Consideration: The iliococcygeus is sometimes used as a landmark or suture anchor point in procedures for pelvic reconstruction