The sacrum is a triangular bone at the base of the spine. It is a foundational component of the axial skeleton, acting as the connecting point between the upper body and the lower limbs. This bone is securely wedged between the two large hip bones, forming the posterior wall of the pelvic girdle. Understanding the sacrum’s anatomy is key to comprehending human posture, movement, and common sources of lower back discomfort.
The Sacrum’s Exact Location and Structure
The sacrum is positioned directly below the fifth lumbar vertebra (L5) and above the coccyx, or tailbone. It is found at the back of the pelvis, flanked on either side by the ilium, the large, blade-shaped bones of the hip. This strategic location makes the sacrum an integral part of the bony ring that comprises the pelvis.
The unique structure of the sacrum results from a developmental process where five separate vertebral segments (S1 through S5) fuse together. This fusion typically begins in the mid-teens and is completed by the early to mid-twenties, creating one strong, immobile bone. The resulting shape is that of an inverted triangle, wider and thicker at the top and tapering toward the bottom.
The broad, upper end of the bone is called the base, which forms a joint with the L5 vertebra. The narrow, pointed lower tip is the apex, which connects to the coccyx. The surface is marked by four pairs of small openings, or sacral foramina, which allow for the passage of sacral nerves and blood vessels into the pelvis and lower body.
Primary Roles in Body Movement and Support
The primary function of the sacrum is to act as the central anchor point for the upper body’s weight. It receives compressive forces generated by the head, torso, and arms through the lumbar spine. This load is then distributed laterally through the pelvis and into the femurs, allowing for standing and walking.
The sacrum’s curved shape contributes to the spine’s natural S-curve. This curvature helps absorb shock and maintain balance during dynamic activities like running or jumping. The bone also serves as an attachment site for various muscles and ligaments that influence movement and stability in the lower back and hips.
Key Anatomical Connections
The sacrum forms joints with four other bones: the fifth lumbar vertebra, the coccyx, and the two iliac bones of the pelvis. The most significant connections are the two L-shaped sacroiliac (SI) joints, where the sacrum articulates with the ilium. These joints are reinforced by some of the strongest ligaments in the human body, including the interosseous sacroiliac ligament, which provides substantial stability.
While the SI joints are designed for stability and load transfer, they allow for a small amount of movement, typically limited to two to four millimeters. This slight motion is necessary to absorb rotational and torsional forces during gait and is also important during childbirth. The base of the sacrum connects to the lumbar spine via the lumbosacral joint, which is a major junction for transferring weight.
Understanding Sacral Pain and Dysfunction
Pain originating in the sacral region is a common source of lower back discomfort, often stemming from the sacroiliac (SI) joints. SI joint dysfunction occurs when there is an alteration in the joint’s normal motion, manifesting as either too much movement (hypermobility) or too little movement (hypomobility). This dysfunction typically results in pain felt in the lower back, buttock, or hip, often on only one side.
The pain associated with SI joint dysfunction can sometimes radiate down the leg, mimicking sciatica symptoms. True sciatica is pain along the path of the sciatic nerve, which branches from the sacral plexus—a network of nerves near the sacrum. Because the L5 and S1 spinal nerves are located in close proximity to the SI joint, irritation can affect these nerves, leading to pain, tingling, or weakness in the lower extremity.
Common causes of sacral pain and SI joint dysfunction include trauma (such as a fall onto the buttocks) or repetitive stress from activities that place uneven strain on the pelvis. Pregnancy is a frequent factor, as hormones loosen the ligaments, leading to hypermobility and instability. Poor posture or uneven leg length can also contribute to misalignment and stress on the sacral connections, leading to chronic discomfort.