Sacrum – Anatomy, Function and Disorders
The sacrum is a large, wedge-shaped bone at the base of the spine that connects the vertebral column to the pelvis and plays a key role in stability and load transfer.
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The sacrum is a large, wedge-shaped bone at the base of the spine that connects the vertebral column to the pelvis and plays a key role in stability and load transfer.
What is the Sacrum?
The sacrum, also known by its Latin name Os sacrum, is a large, triangular bone located at the base of the vertebral column. It is formed by the fusion of five sacral vertebrae (S1–S5), a process that is typically completed between the ages of 18 and 30. The sacrum forms the posterior part of the pelvis and connects the spine to the two hip bones via the sacroiliac joints (SIJ).
Anatomy and Location
The sacrum is situated below the fifth lumbar vertebra (L5) and above the coccyx (tailbone). Its key anatomical parts include:
- Sacral base: the broad, upper portion that articulates with L5
- Sacral apex: the narrow, lower tip that connects to the coccyx
- Pelvic surface: the smooth, concave anterior surface facing the pelvic cavity
- Dorsal surface: the rough, convex posterior surface with palpable bony ridges
- Sacral foramina: four pairs of openings on each side through which sacral nerves pass
The sacrum is firmly joined to the pelvis via the sacroiliac joints. These joints allow only minimal movement but are essential for transferring forces between the trunk and the lower limbs.
Function
The sacrum fulfills several important functions in the human body:
- Weight-bearing and load transfer: It transmits the weight of the upper body to the pelvis and legs.
- Protection of sacral nerves: The sacral canal houses nerve roots of the cauda equina, which supply the pelvis, legs, bladder, and bowel.
- Support during sitting: Together with the coccyx, the sacrum forms the base of the sitting area.
- Birth canal: The concave inner surface of the sacrum forms part of the birth canal, making it particularly relevant in obstetrics.
Common Conditions and Disorders
Sacral Fracture
A sacral fracture most commonly results from high-impact trauma such as falls or motor vehicle accidents. In elderly individuals, osteoporosis-related insufficiency fractures are common. Symptoms may include severe pain, numbness, and in serious cases, nerve damage leading to weakness or loss of bladder and bowel control.
Sacroiliac Joint Dysfunction (SIJ Syndrome)
SIJ syndrome refers to pain originating from the sacroiliac joint, often radiating into the lower back, buttocks, or legs. It is a frequent cause of low back pain and can be triggered by overuse, pregnancy, or inflammatory conditions.
Sacroiliitis
Sacroiliitis is an inflammation of the sacroiliac joints and is often associated with rheumatic diseases such as ankylosing spondylitis (Bechterew disease). It causes chronic back pain and stiffness, particularly in the morning.
Coccydynia and Sacral Pain
Pain in the region of the sacrum and coccyx, known as coccydynia, can be caused by falls onto the buttocks, prolonged sitting on hard surfaces, or childbirth.
Tumors and Metastases
The sacrum can be affected by primary bone tumors such as chordoma or by metastases from other cancers. These can weaken the bone and cause nerve-related pain.
Diagnosis
Several diagnostic methods are used to evaluate the sacrum:
- X-ray: the first-line imaging tool for suspected fractures or structural changes
- Magnetic resonance imaging (MRI): ideal for assessing soft tissues, nerve roots, and early signs of inflammation
- Computed tomography (CT): provides detailed bone imaging, especially useful for complex fractures
- Bone scintigraphy / SPECT-CT: used to detect stress reactions, inflammation, or metastases
Treatment
Treatment of sacral conditions depends on the underlying cause:
- Conservative management: pain relief medications, physiotherapy, rest, and physical therapy for SIJ syndrome or uncomplicated fractures
- Interventional procedures: targeted injections (e.g., SIJ injections) for pain relief
- Surgical treatment: required for unstable fractures, severe nerve injuries, or tumors
- Medication for sacroiliitis: nonsteroidal anti-inflammatory drugs (NSAIDs); biologics for ankylosing spondylitis
References
- Standring S (ed.). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 42nd edition. Elsevier, 2020.
- Simopoulos TT et al. A Systematic Evaluation of Prevalence and Diagnostic Accuracy of Sacroiliac Joint Interventions. Pain Physician, 2012; 15(3): E305–E344.
- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy. 8th edition. Wolters Kluwer, 2018.
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