The Anatomy of the Sacral Vertebrae

The five sacral vertebrae fuse together to form the sacrum

In This Article

Table of Contents

The sacral vertebrae—also called the sacral spine—consists of five sacral vertebrae bones. These bones fuse together to form the sacrum, the shield-shaped bony structure located at the base of the lumbar vertebrae (the five cylindrical bones forming the spine of the lower bank) and connected to the pelvis. The sacral vertebrae are represented by segments S1 through S5 and located between the lumbar vertebrae and the coccyx (tailbone)—the lowest part of the vertebral column.

Anatomy

Your sacral vertebrae develop in the first and second months of embryonic development.

The five vertebrae of the sacral vertebrae are separate early on but start to fuse during adolescence and early adulthood. They form into a single bone by age 30 to complete the fusion process.

The first three vertebrae of the sacral region form the wide lateral wings called the alae. The alae (also called the ala or wing of sacrum) connect with the blades of pelvis—called the ilium. The sacrum also forms the back wall of the pelvis and the joints at the hip bones called the sacroiliac joints.

There are a series of four openings on each side of the sacrum where the sacral nerves and blood vessels run. The sacral canal runs down the center of the sacrum, representing the end of the vertebral canal.

The five segments of the sacral vertebrae affect nerve communication to the lower part of the body. There, numerical levels are often mentioned in imaging studies of the spine. S1 refers to the first sacral bone, S2 to the second sacral bone, and so on. S1 is at the top and S5 is towards the bottom. Each number corresponds with the nerves in that part of the spinal cord.

  • S1 nerves affect the hips and groin.
  • S2 nerves affect the back of the thighs.
  • S3 nerves affect the medial buttock area.
  • S4 and S5 nerves affect the perineal area. The perineum is between the legs. It is the diamond-shaped area containing the anus, and in females, the vagina.

Function

The function of the sacral vertebrae is to secure the pelvic girdle, the basin-like bone structure connecting the truck and the legs, supporting and balancing the trunk, and containing the intestines, bladder, bowel, and internal sex organs. Injuries to this area can affect bowel and bladder control, as well as sexual function, especially in men. Injuries to the sacral vertebrae are complex, often misdiagnosed, and not always appropriately treated.

Common causes of injuries related to the sacral vertebrae include car accidents, sports injuries, trauma, falls, birth defects, osteoporosis, and joint degeneration.

Injuries and damage to S1, S2, S3, S4, or S5 can still leave a person functional, but they primarily affect bowel and bladder function.

Most people with sacral vertebrae injuries tend to live very normal lives. They may need some assistance but can still do well on their own.

Associated Conditions

A person’s sacral vertebrae are generally healthy and rarely fractured, except in the case of serious injuries, such as traumas to the area, including falls and blunt force.

Stress Fractures

People who have osteoporosis or rheumatoid arthritis (RA) are predisposed to stress fractures in the sacrum. Stress fractures, also called hairline fractures, can be described as small cracks or slivers in the bone.

They mostly affect weight-bearing joints. They are less common in the femur, pelvis, or sacrum, but they are common in people with osteoporosis, RA and other rheumatic conditions because of the inflammatory process and reduced bone strength.

Sciatica

Sciatica, a condition that causes back or leg pain, can arise from injury to the area where the lumbar spine (lower back, where the spine curves inward toward the abdomen) and sacral vertebrae connect. This is because this area of the spine is subject to huge amounts of stress and twisting from activity, such as sitting for long periods or playing a sport.

Sacral Nerve Injuries

While there are no spinal cord sections in the sacral vertebrae, the sacral nerves originate in the lumbar spine. Any damage to these nerve roots may cause similar symptoms to those of spinal cord damage.

People with sacral nerve injuries will have symptoms on one or both sides of their bodies. Injuries to the sacral vertebrae may cause loss of function in the hips and/or legs, which affects activities, including walking and driving. Sacral vertebrae injuries may also cause bladder and bowel control dysfunction. However, injuries of the sacral vertebrae aren’t significant enough to affect daily living and self-care.

Symptoms of injuries to the sacral nerve roots may include:

  • Lack of bladder or bowel function
  • Low back pain
  • Leg pain, radiating to the back of the legs
  • Sensory issues in the groin and buttocks

Cauda Equina Syndrome

The cauda equina is a bundle of spinal nerves and spinal nerve rootlets of the second through fifth lumbar vertebrae, the first through fifth sacral nerves, and the coccygeal nerve of the tailbone. Cauda equina syndrome, a rare disorder where something compresses on the spinal nerve roots, usually affects vertebral levels L5 through S1.

The condition is a surgical emergency and without fast treatment, there could be lasting damage that leads to incontinence (loss of bladder control) and permanent leg paralysis.

Symptoms of cauda equina syndrome include:

  • Severe low back
  • Pain, numbness, and weakness in one or both legs that causes balance problems and trouble getting out of a chair
  • Saddle anesthesia: Loss of or altered sensations in the legs, buttocks, inner thighs, back of the legs, or feet that keeps worsening
  • Recent problems with passing urine or waste or incontinence
  • Sexual dysfunction that has suddenly come on

The symptoms of cauda equina syndrome require immediate medical attention. You should seek out emergency medical attention for one or more of the symptoms of this condition.

If you have cauda equina syndrome, you will need surgery to relieve pressure off of the nerves, and the best outcomes are with 48 hours of the start of symptoms.

Treatment

There are treatments for sacral vertebrae injuries.

  • Medication: Non-steroidal anti-inflammatory (NSAID) drugs can help with treating nerve root injuries and reducing pain and inflammation. The quicker these drugs are used, the better a person’s outcome is. This is especially important because of the inflammation affecting the lower part of the spinal cord.
  • Therapy: Physical therapy can encourage strength in affected areas of the sacral vertebrae. Occupational therapy can help a person regain the function they have lost from sacral vertebrae injuries.
  • Surgery: Decompression surgery of the nerves or fusion surgery of the vertebrae can reduce pressure around the spinal nerves and fix any damaged or broken bones.

Sacral injuries can also affect the tailbone. This type of injury—called coccydynia—is likely to cause bruising, discoloration, or fracture of the tailbone.

Home treatment of sacral and tailbone injuries might be all a person needs. However, if these fractures are severe or affect nerves, surgery might be necessary. It is important to remember that bones heal best when a person takes good care of themselves in a variety of helpful ways, including eating right and not smoking.

Was this page helpful?

Article Sources

  1. Okumura M, Ishikawa A, Aoyama T, et al. Cartilage formation in the pelvic skeleton during the embryonic and early-fetal period. PLoS One. 2017 Apr 6;12(4):e0173852. doi:10.1371/journal.pone.0173852

  2. Degmetich S, Bailey JF, Liebenberg E, et al. Neural innervation patterns in the sacral vertebral body. Eur Spine J. 2016 Jun; 25(6): 1932–1938. doi:10.1007/s00586-015-4037-4

  3. Lyders EM, Whitlow CT, Baker MD, et al. Imaging and treatment of sacral insufficiency fractures. American Journal of Neuroradiology. February 2010;31 (2) 201-210. doi:10.3174/ajnr.A1666

  4. Cleveland Clinic. Stress Fractures. Updated June 17, 2015. 

  5. Berg EJ, Ashurst JV. Anatomy, back, cauda equina. Updated December 6, 2018.

Additional Reading