Sacrum
Os Sacrum
location_on Posterior pelvis, between the two hip bones, at the base of the vertebral column
The sacrum is a large triangular bone formed by the fusion of five sacral vertebrae (S1-S5). It forms the posterior wall of the pelvis and transmits the weight of the upper body to the hip bones through the sacroiliac joints. The sacral canal contains the cauda equina nerve roots, and the sacral foramina transmit the sacral spinal nerves. Sexual dimorphism is marked: the female sacrum is wider and less curved.
Key Anatomical Features
- Sacral promontory is the anterior projection of S1, a key obstetric landmark
- Four pairs of anterior and posterior sacral foramina transmit sacral nerves
- Auricular surface on each side articulates with the ilium at the sacroiliac joint
- Sacral canal is the continuation of the vertebral canal within the sacrum
- Sacral hiatus is the opening at the inferior end of the sacral canal used for caudal epidural injections
- Median sacral crest represents the fused spinous processes
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Piriformis | Anterior surface of S2-S4 | Laterally rotates and abducts the hip |
| Multifidus | Posterior sacral surface | Stabilizes the lumbosacral junction |
| Erector spinae | Posterior sacral surface and sacral crests | Extends the spine |
| Gluteus maximus | Posterolateral sacrum and coccyx | Extends and laterally rotates the hip |
| Iliacus | Ala (wing) of sacrum | Flexes the hip |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Lumbosacral joint | Cartilaginous symphysis | L5 vertebra |
| Sacroiliac joint | Synovial (anterior) and syndesmosis (posterior) | Ilium of hip bone |
| Sacrococcygeal joint | Cartilaginous symphysis | Coccyx |
Common Pathologies
Sacral fracture
Usually from high-energy trauma or insufficiency (osteoporotic) fractures. Sacral insufficiency fractures cause low back and buttock pain and are often missed on plain radiographs.
Sacroiliac joint dysfunction
Pain arising from the sacroiliac joint is a common cause of lower back pain, often difficult to distinguish from lumbar disc disease. Confirmed by diagnostic SI joint injection.
Sacral tumors
Chordomas are the most common primary malignant tumor of the sacrum, arising from notochord remnants. Giant cell tumors and nerve sheath tumors also occur.
Tarlov (perineural) cysts
Fluid-filled cysts on sacral nerve roots, found incidentally in up to 5% of MRI scans. Usually asymptomatic but can occasionally cause radiculopathy.
Clinical Relevance
The sacral promontory is a critical obstetric landmark used to measure the anteroposterior diameter of the pelvic inlet (conjugate diameter). Caudal epidural anesthesia is administered through the sacral hiatus, located by palpating the sacral cornua. Sacral insufficiency fractures are an underdiagnosed cause of low back pain in elderly osteoporotic patients and show a characteristic H-shaped pattern on bone scan.
Development and Ossification
Each sacral vertebra ossifies from three primary centers (body and two costal elements). Additional centers appear for the lateral surfaces (costal elements) and auricular surfaces. Fusion of the five sacral segments begins around puberty (S4-S5 first) and proceeds superiorly, completing by about age 30. The S1-S2 junction is the last to fuse.
Did You Know?
- Sacrum means sacred bone in Latin, possibly because it was believed to be the last bone to decay or because it was used in ancient sacrificial rites
- The sacrum has significant sexual dimorphism: the female sacrum is wider, shorter, and less curved to accommodate childbirth
- The sacral promontory can be palpated during a vaginal examination and is used to assess pelvic dimensions for delivery
Scan this bone yourself
Download Osteo+ and identify the sacrum instantly with your camera. Get all the details above and more from a single photo.