The sacrum is the triangular bone at the bottom of the spine. It connects the lumbar spine to the pelvis and helps transfer weight from the upper body into the hips and legs.
On each side of the sacrum is a sacroiliac joint, often called an SI joint. These joints connect the sacrum to the pelvic bones. They normally allow only a small amount of motion, but they play an important role in standing, walking, bending, and absorbing stress.
Please review the Lumbosacral spine anatomy section for more information.
Disorders involving the sacrum or sacroiliac joints may cause low back, buttock, groin, pelvic, or leg pain. Because these symptoms can resemble lumbar spine or hip problems, a careful evaluation is often necessary.
Major Diagnosis Types
Below are the major types we see in our clinic. Click on available links to learn about our approach to the problems.
Sacroiliitis
Sacroiliitis is inflammation of one or both sacroiliac joints.
It may occur because of mechanical stress, injury, infection, or an inflammatory condition such as ankylosing spondylitis.
Symptoms may include buttock pain, stiffness, and pain that worsens after prolonged sitting or standing.
Minor Diagnosis Types
These terms are often encountered, but are either less often treated or described within one of the above diagnoses.
Sacral Insufficiency Fracture
A sacral insufficiency fracture occurs when weakened bone fractures under normal daily stress.
These fractures are most commonly associated with osteoporosis and may occur without a major injury.
Risk factors include:
- Advanced age
- Osteoporosis
- Long-term steroid use
- Pelvic radiation
- Previous spinal fusion
- Low body weight
- Metabolic bone disease
Pain is often severe and may be mistaken for lumbar stenosis, hip disease, or muscle strain.
Sacral Tumor
Tumors may arise within the sacrum or spread there from another part of the body.
Examples include:
- Chordoma
- Giant cell tumor
- Metastatic cancer
- Plasmacytoma
- Nerve sheath tumors
Sacral tumors may cause persistent pain, nerve symptoms, bowel or bladder dysfunction, or pelvic instability.
Sacroiliac Joint Arthritis
Arthritis develops when the cartilage and supporting surfaces of the SI joint gradually wear down.
This may occur because of:
- Normal aging
- Previous injury
- Uneven loading
- Prior lumbar fusion
- Inflammatory disease
Arthritis may cause localized pain near the back of the pelvis and may worsen with standing, walking, or climbing stairs.
Sacroiliac Joint Dysfunction
Sacroiliac joint dysfunction occurs when the SI joint becomes painful because of abnormal movement, inflammation, degeneration, or altered stress across the joint.
The joint may move too much, too little, or simply become painful despite relatively normal motion.
SI joint dysfunction is a common cause of pain in the lower back and buttock.
Sacroiliac Joint Instability
Sacroiliac joint instability occurs when the supporting ligaments do not adequately control movement across the joint.
It may develop after:
- Pregnancy
- Pelvic trauma
- Repetitive stress
- Connective tissue disorders
- Previous pelvic surgery
Instability may produce a sensation of shifting, catching, or weakness around the pelvis.
Traumatic Sacral Fracture
A traumatic sacral fracture usually results from a significant fall, motor vehicle collision, or other high-energy injury.
Some fractures are stable, while others may affect the pelvic ring, nearby nerves, or spinal stability.