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Os sacrum (Sacrum) – Anatomy & Conditions

The os sacrum (sacrum) is a triangular bone at the base of the spine. It connects the vertebral column to the pelvis and forms the posterior wall of the pelvic cavity.

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Things worth knowing about "Os sacrum"

The os sacrum (sacrum) is a triangular bone at the base of the spine. It connects the vertebral column to the pelvis and forms the posterior wall of the pelvic cavity.

What is the Os sacrum?

The os sacrum, commonly known as the 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 begins in childhood and is typically complete by around age 30. Together with the coccyx (os coccygis), the sacrum forms the lower end of the spine. On each side, it articulates with the iliac bones of the pelvis via the sacroiliac joints, forming the posterior part of the pelvic ring.

Anatomy and Location

The sacrum has a characteristic wedge-shaped form, curved slightly forward (anteriorly) and tapering toward its lower end. Key anatomical structures include:

Base of the sacrum (Basis ossis sacri): The broad upper surface that articulates with the fifth lumbar vertebra (L5), forming the lumbosacral junction.
Apex of the sacrum (Apex ossis sacri): The narrow lower tip that connects with the coccyx.
Pelvic surface (Facies pelvica): The smooth anterior surface facing the pelvic cavity, featuring four pairs of anterior sacral foramina through which sacral nerve branches exit.
Dorsal surface (Facies dorsalis): The rough posterior surface with the median sacral crest and posterior sacral foramina for the dorsal branches of sacral nerves.
Sacral canal (Canalis sacralis): The internal canal continuing from the vertebral canal, housing the terminal nerve roots of the spinal cord (cauda equina).
Auricular surface (Facies auricularis): The ear-shaped articular surface on each side that forms the sacroiliac joint.

Functions

The sacrum serves several essential functions in the human body:

Weight transmission: It transfers the weight of the upper body and spine to the pelvis and ultimately to the lower limbs.
Stabilization: Together with the sacroiliac joints, it stabilizes the pelvic ring during standing, walking, and movement.
Nerve protection: The sacral canal protects the sacral nerve roots (S1–S5), which supply the pelvic organs, perineum, and lower extremities.
Pelvic structure: The sacrum forms the posterior wall of the lesser pelvis and contributes to the shape of the birth canal.

Common Conditions and Injuries

Sacral Fracture

Fractures of the sacrum typically occur following high-energy trauma such as falls from height or road traffic accidents. In older adults with osteoporosis, low-energy insufficiency fractures are common. Symptoms include pain in the lower back, buttock, and sacral region, and in severe cases, neurological deficits due to sacral canal involvement.

Sacroiliitis

Sacroiliitis is inflammation of one or both sacroiliac joints. It frequently occurs in association with inflammatory rheumatic conditions such as ankylosing spondylitis (Bechterew disease) and causes deep lower back and buttock pain, typically worse in the morning.

Lumbosacral and Sacroiliac Dysfunction

Functional disturbances at the lumbosacral junction or sacroiliac joints are among the most common causes of chronic lower back pain. Treatment typically involves physiotherapy, manual therapy, and targeted strengthening exercises.

Sacral Canal Stenosis

Narrowing of the sacral canal due to degenerative changes or tumors can compress nerve roots, leading to pain, numbness, or weakness in the legs, as well as bladder and bowel dysfunction.

Clinical Relevance

The os sacrum plays an important role across multiple medical specialties:

Obstetrics: The dimensions and shape of the lesser pelvis, partly determined by the sacrum, are critical for the progress of labor and delivery.
Anesthesiology: Caudal anesthesia (epidural anesthesia via the sacral hiatus) is widely used for pelvic surgery and obstetric procedures.
Orthopedics and Trauma Surgery: Fractures, tumors, and degenerative conditions of the sacrum require specialized diagnostics and treatment.
Oncology: The sacrum can be affected by primary bone tumors (e.g., chordoma) or metastatic disease.

Diagnosis

Several imaging techniques are used to evaluate the os sacrum:

X-ray: Initial imaging for suspected fractures or degenerative changes.
Computed Tomography (CT): Provides detailed visualization of bony structures, fractures, and tumors.
Magnetic Resonance Imaging (MRI): Best modality for assessing soft tissues, nerves, intervertebral discs, and inflammatory changes.
Bone Scintigraphy / PET-CT: Used when metastatic disease or systemic inflammatory conditions are suspected.

References

Standring, S. (Ed.) (2020). Gray's Anatomy: The Anatomical Basis of Clinical Practice (42nd ed.). Elsevier.
Moore, K. L., Dalley, A. F. & Agur, A. M. R. (2018). Clinically Oriented Anatomy (8th ed.). Wolters Kluwer.
Denis, F., Davis, S. & Comfort, T. (1988). Sacral fractures: an important problem. Clinical Orthopaedics and Related Research, 227, 67–81.

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