The sacrum and coccyx are unlike other bones in your spinal column. The sacrum, sometimes called the sacral vertebra or sacral spine (S1), is a large, flat triangular shaped bone nested between the hip bones and positioned below the last lumbar vertebra (L5). The coccyx, commonly known as the tailbone, is below the sacrum.
Individually, the sacrum and coccyx are composed of smaller bones that fuse (grow into a solid bone mass) together by age 30. The sacrum is made up of 5 fused vertebrae (S1-S5) and 3 to 5 small bones fuse creating the coccyx. Both structures are weight-bearing and integral to functions such as walking, standing and sitting.
Sacrum and Lumbosacral Spine
The sacrum is located in between the right and left iliac bones (hips) and forms the back of the pelvis. The sacrum, along with the coccyx and 2 sacroiliac joints make up the pelvic girdle. The top of the sacrum (S1) joins the last lumbar vertebra (L5) and together create the lumbosacral spine.
Where S1 joins L5 helps form the lumbosacral curves: lumbar lordosis and lumbar kyphosis. Lordotic and kyphotic curvature works together to support the upper body, bear and distribute weight/forces, and helps maintain spinal balance and functional flexibility.
Lordosis is the inward curve. Too much lordosis can cause swayback that is sometimes associated with spondylolisthesis.
Loss of lordosis can cause spinal imbalance and may lead to flatback syndrome.
The term kyphosis refers to a normal outward curve.
The sacrum’s location—at the intersection of the spine and pelvis—means that it plays an especially important role to both your low back and hips. The sacrum’s joints are weight-bearing and help to stabilize this region of the spinal column. Like other spinal levels, ligaments, tendons and muscle help support and stabilize joint movement.
Lumbosacral joint: This joint occurs at L5 and S1—it essentially connects the lumbar spine to the sacrum.
There is a great amount of pressure at this meeting point, as the curve of your spine shifts at L5-S1 from lordotic (lumbar lordosis, forward curve) to kyphotic (sacral kyphosis, backward curve). The L5-S1 level is weight-bearing and absorbs and distributes the upper body’s weight at rest and movement. This is one reason why disc herniation and spondylolisthesis are more common at L5-S1.
Sacroiliac (SI) joints: The SI joints connect the sacrum to the left and right sides of the pelvis. Unlike other joints in the body (eg, knees), the span of movement of either SI joint is minimal. These joints are essential to walking, standing, and hip stability. Sacroiliitis and SI joint dysfunction are two spinal disorders related to the sacroiliac joints.
Other spinal disorders related to the sacral spine include sciatica, Tarlov cysts, and spinal chordoma—a common type of spinal bone cancer.
Coccyx’s Role and Function
The coccyx, or tailbone, is located just below the sacrum. Though it’s much smaller than the sacrum, it too has an important weight-bearing role. The coccyx helps support your weight while you sit. If you lean back while sitting, such as reclining in a chair, the pressure on your coccyx increases.
An injury in this coccygeal region can cause tailbone pain, which is known as coccydynia. It is often characterized by inflammation of the coccyx’s connective tissue resulting in tailbone pain that worsens when sitting. Tailbone fracture that may occur from a traumatic event, such as a fall, can also cause this pain.
Sacral and Coccygeal Nerves
The spinal cord ends at L1-L2 creating the cauda equina; a bundle of spinal nerves resembling the tail of a horse. In the sacrum, are the sacral nerves; called the sacral plexus—the term “plexus” simply means a network of nerve structures. The sacral plus with the lumbar plexus create the lumbosacral plexus. The sciatic nerve is the largest nerve in the sacral plexus. Compression of the sciatica nerve causes a group of symptoms called sciatica that are notoriously known for lower back and leg pain. The coccygeal nerve is the one serving the tailbone (ie, coccyx).
There are 5 sacral nerves (part of the spinal cord) numbered S1 through S5. The first sacral spinal nerve serves your groin area and hips; S2 the back of your thighs; S3 the middle of the buttock area; and S4 and S5 the anus and vagina.
An injury or trauma to the sacral spine may cause stress fractures or more serious bone fracture. These fractures are painful and may cause sacral nerve compression. Symptoms may include lower back pain, leg pain, bowel or bladder dysfunction and/or unusual feelings in the buttock(s) or groin. Osteoporosis and/or spinal inflammatory arthritis may increase your risk for a sacral fracture.
