Sacroiliac joint

Anterior sacroiliac ligament

The anterior sacroiliac ligament is composed of many thin strands that form a strong, broad and flat band. It lies on the pelvic surface of the joint, forming the anteroinferior component of the joint capsule. The anterior sacroiliac ligament extends from the ala of the ilium, just anterior to the auricular surface, to the pelvic surface of the sacrum.

The part of the pelvic surface that lies between the auricular surface and the upper rim of the greater sciatic notch often shows a rough preauricular sulcus that provides an attachment site (origin) for the lower fibers of the anterior sacroiliac ligament that then insert onto the third sacral segment. This part of the ligament is particularly well developed, whereas the rest of the ligament is rather thin.

Posterior sacroiliac ligament

The posterior sacroiliac ligament is a compound ligament composed of three distinct bands. It lies posterior and superior to the joint, filling the space between the corresponding tuberosities of the ilium and the sacrum. The posterior sacroiliac ligament is much thicker and stronger than its anterior counterpart. The bands that compose this ligament are as follows:

• Interosseous sacroiliac ligament: a short but very strong band that spans the gap between the sacrum and ilium on the posterior side of the joint by attaching on their tuberosities, just behind the auricular surfaces. Occasionally, one or two accessory joint cavities can be identified within the ligament.
• Short posterior sacroiliac ligament: a short ligament found superficial to the interosseous sacroiliac ligament. It fills the upper part of the gap between the ilium and sacrum. The short posterior sacroiliac ligament originates on the first and second transverse tubercles of the sacrum and extends horizontally to attach on the tuberosity of the ilium.
• Long posterior sacroiliac ligament: a long band that is the most superficial of the posterior sacroiliac ligaments. It extends from the posterior superior iliac spine to the third and fourth transverse tubercles of the sacrum.

Accessory ligaments

The accessory ligaments aid the main ligaments and contribute to the stability of the sacroiliac joint. The main function of these ligaments is to prevent forward tilting of the sacral promontory.

• Sacrotuberous ligament: a flat, triangular ligament that has several superior attachments; one band of the ligament arises on the gap between the posterior superior and posterior inferior iliac spines, where it is partially blended with the posterior sacroiliac ligaments. A second band arises on the lateral side of the sacrum below the auricular surface, and the third on the lateral side of the upper part of the coccyx. The three bands converge and extend inferiorly, forming a triangular shape before inserting on the lower margin of the ischial ramus. Right before the insertion occurs, the bands diverge again, thereby prolonging the inferior attachment point. This prolonged attachment is called the falciform process and is situated just below the pudendal canal, where it blends with the fascial sheath of the internal pudendal vessels and pudendal nerve. The posterior surface of the ligament is blended with the gluteus maximus, whereas the inferior fibers of the ligament continue into the tendon of biceps femoris.
• Sacrospinous ligament: a thin, triangular band found deep to the sacrotuberous ligament. It originates from the border of the lower sacral and upper coccygeal segments, anterior to the sacrotuberous ligament. The ligament is broad in its origin, but narrows as it extends laterally to insert onto the ischial spine. The sacrospinous ligament is thought to be a fibrous part of the coccygeus muscle, as it is largely blended with it.

Besides their main function, the sacrotuberous and sacrospinous ligaments also transform the greater and lesser sciatic notches into the greater and lesser sciatic foramina, respectively.

• Greater sciatic foramen: an opening bounded by the posterior border of the iliac bone anterosuperiorly, the sacrotuberous ligament posteromedially, and the sacrospinous ligament and ischial spine inferiorly. The contents of this foramen are the piriformis muscle, gluteal vessels and nerves, internal pudendal vessels, pudendal nerve, sciatic and posterior femoral cutaneous nerves and the nerves to obturator internus and quadratus femoris.
• Lesser sciatic foramen: an opening bounded by the ischial body anteriorly, spine of the ischium and sacrospinous ligament superiorly, and the sacrotuberous ligament posteriorly. Its contents include the tendon of obturator internus, the nerve to obturator internus, and the internal pudendal vessels and pudendal nerve.

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