The sacrum is a distinctive, triangular bone situated at the base of the vertebral column, representing a fusion of five individual vertebrae. This structure is positioned like a wedge, connecting the spine directly to the pelvic girdle. As the central connection point between the upper and lower body, the sacrum plays a fundamental role in human posture and locomotion. It transmits forces and provides stability, supporting overall movement.
Defining the Sacrum’s Structure and Position
The sacrum’s unique form results from the fusion of the five sacral vertebrae (S1 through S5). This process typically begins in the mid-teens and completes between 18 and 30 years of age. The fusion creates a single, inverted triangular bone situated superior to the coccyx (tailbone) and inferior to the fifth lumbar vertebra (L5).
Its broad upper portion, the base, articulates with L5, forming the lumbosacral angle. The bone is wedged firmly between the two hip bones (ilia), forming the two L-shaped sacroiliac (SI) joints on either side.
The sacrum has a noticeable curve, being concave on its anterior (pelvic) surface and convex on its posterior surface. A defining feature is the presence of four pairs of openings, known as the sacral foramina. These foramina allow for the passage of the anterior and posterior branches of the sacral nerves.
These nerves exit the central sacral canal, which is a continuation of the spinal canal, to supply sensation and motor function to the pelvic region and legs. The ridges visible on the sacrum’s surface are remnants of the original vertebral joints and spinous processes that have merged together.
Essential Functions in the Skeletal System
The primary mechanical function of the sacrum is to act as a link in the weight transfer system of the body. It receives the entire weight of the upper torso and head from the spine and distributes this load laterally to the hip bones and down to the lower limbs. This involves the sacrum functioning somewhat like a reverse keystone, where increased weight loading causes it to settle deeper between the ilia, thus increasing stability.
The sacrum anchors the spine while providing a stable base for controlled movement required during walking, running, and standing. Strong ligaments surrounding the sacroiliac joints limit motion to a few degrees, which is sufficient for shock absorption without compromising pelvic stability. This controlled movement is important in women, as the sacrum’s connection to the pelvis allows flexibility during childbirth.
The sacrum also serves a protective function. By forming the posterior wall of the pelvic cavity, it provides a bony shield for internal organs, including portions of the bladder, rectum, and reproductive organs. Furthermore, numerous muscles responsible for posture and movement, such as the gluteus maximus, attach directly to the sacrum, utilizing it as an anchor point.
Common Conditions Affecting the Sacrum
The sacrum is involved in conditions that cause pain and limit mobility due to its role as a weight-bearing junction. The most common pathology is Sacroiliac (SI) Joint Dysfunction, characterized by improper movement of the joints connecting the sacrum to the ilium. This dysfunction can manifest as either hypermobility (too much movement and instability) or hypomobility (fixation and stiffness).
Symptoms of SI joint dysfunction often present as low back or buttock pain, which can radiate down the leg, mimicking sciatica. Causes include pregnancy-related ligament stretching, degenerative changes like arthritis, or trauma from a fall or accident. When the SI joint becomes inflamed, the resulting condition is termed sacroiliitis.
Sacral fractures are less common than other spinal injuries but result from high-impact trauma, such as motor vehicle accidents or significant falls. These fractures can be difficult to diagnose and may lead to neurological issues if the injury involves the sacral canal and the nerves passing through the foramina. Pain in the coccygeal region, or coccydynia, is related to the sacrum, as the coccyx articulates at its inferior apex. This tailbone pain is often linked to localized trauma or postural misalignment.