Ligaments of Sacrum and Coccyx

The lumbosacral junction is mechanically imperfect because of its angulation and the consequent sloping platform provided for the fifth lumbar vertebra by the first sacral vertebra (see Plate 3-9). The tendency to subluxation (spondylolisthesis) is resisted by the impingement of the almost sagittally arranged lumbosacral articular processes, and this bony check is strongly augmented by the last intervertebral disk, the anterior and posterior longitudinal ligaments, the ligamenta flava, and the supraspinal and interspinal ligaments. These ligaments are further reinforced by the erector spinae and other muscles and by the iliolumbar ligaments, which are strong bands uniting the transverse processes of the fourth and fifth lumbar vertebrae and the posterior parts of the iliac crests and sacral alae. The iliolumbar ligaments are really the expanded lower margins of the anterior and middle layers of the thoracolumbar fascia that encloses the quadratus lumborum muscles. They blend below with the anterior sacroiliac ligaments.

The sacroiliac joints between the auricular surfaces of the sacrum and ilia are synovial in type. Movements are limited, however, because of the interlocking elevations and depressions on the opposed articular surfaces, the way the sacrum is wedged between the hip bones, and the restraining influence of the anterior, posterior, and interosseous sacroiliac ligaments and the accessory sacrotuberal and sacrospinal ligaments.

The anterior sacroiliac ligament is a thin, wide, fibrous layer reinforcing the anterior part of the articular capsule and stretching from the ala and pelvic surface of the sacrum to the adjoining parts of the iliac bone.

The posterior sacroiliac ligament consists of more superficial, longer bundles and deeper, shorter bundles. The fibers of the long posterior sacroiliac ligament interconnect the posterior superior iliac spine and the lateral parts of the third and fourth sacral segments; its outer fibers interdigitate with those of the sacrotuberal ligament. The short posterior sacroiliac ligament interconnects the medial surface of the iliac bone to the lateral parts of the first and second sacral segments and is often considered to be a part of the interosseous ligament.

The interosseous sacroiliac ligament is formed by short, thick bundles of fibers interconnecting the sacral and iliac tuberosities—the rough areas behind and above the auricular surfaces of both bones. It is the most powerful bond between the bones and, indeed, is one of the strongest ligaments in the body. It lies deep to the posterior sacroiliac ligament and is not shown in the illustration.

The sacrotuberal and sacrospinal ligaments act as accessory ligaments of the sacroiliac joints because they assist in regulating joint movements. The downward thrust at the lumbosacral junction tends to push the upper part of the sacrum down, with coincident upward tilting of its lower part as the sacrum seesaws on a transverse axis through the middle of the sacroiliac joints. The illustration shows how these accessory ligaments anchor the lower sacrum and coccyx to the ischial tuberosity and spine, thus limiting the seesaw movement.

The sacrum and coccyx are connected by a small, fibrocartilaginous intervertebral disk and by thin bands on the anterior, posterior, and lateral sides of the junction—the anterior, posterior, and lateral sacrococcygeal ligaments. The posterior ligament has a superficial part, which partly fills in the sacral hiatus, and a deep part, which represents the posterior spinal longitudinal ligament.

Only gold members can continue reading. Log In or Register to continue