Sacral Segments (S1-S5)
Vertebrae Sacrales
location_on Posterior pelvis, fused to form the sacrum
The five sacral vertebrae (S1-S5) fuse to form the sacrum, but individually they retain identifiable features including transverse lines (fusion sites), anterior and posterior sacral foramina, and a progressively diminishing vertebral canal. The S1 segment is the largest and bears the sacral promontory; S2-S4 give rise to the sacral parasympathetic outflow (pelvic splanchnic nerves) that controls bladder, bowel, and sexual function.
Key Anatomical Features
- S1 is the largest segment and bears the sacral promontory on its anterior superior surface
- S2 marks the level where the dural sac (thecal sac) typically ends
- S2-S4 contribute the parasympathetic pelvic splanchnic nerves
- Transverse ridges on the anterior surface mark the fusion sites between segments
- Sacral hiatus at S5 is the opening used for caudal epidural injections
- Four pairs of sacral foramina transmit the sacral nerve roots
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Piriformis | Anterior surfaces of S2-S4 segments | Laterally rotates the hip |
| Coccygeus | Lateral S4-S5 segments | Supports the pelvic floor |
| Levator ani | Pelvic surface near S3-S5 | Supports pelvic organs and controls the anorectal angle |
| Multifidus (sacral fibers) | Dorsal sacral surface | Extends and stabilizes the lumbosacral junction |
| Erector spinae (sacral origin) | Posterior sacral surface | Extends the vertebral column |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| S1-L5 lumbosacral joint | Cartilaginous symphysis with intervertebral disc | L5 vertebra |
| Sacroiliac joints | Synovial anteriorly, syndesmosis posteriorly | Ilium bilaterally |
| Sacrococcygeal joint | Secondary cartilaginous (often fuses) | Coccyx |
Common Pathologies
Sacral insufficiency fracture
Stress fracture of the sacral ala in osteoporotic patients, often bilateral. Produces characteristic H-shaped uptake on bone scan (Honda sign). Frequently misdiagnosed as lumbar disc disease.
Sacral nerve root injury
Trauma or tumors affecting S2-S4 cause cauda equina syndrome with bladder and bowel dysfunction, saddle anesthesia, and sexual dysfunction.
Sacralization of L5
Congenital fusion of the L5 transverse process to the sacrum (present in 4-6% of people), which can be a source of low back pain from the pseudoarticulation (Bertolotti syndrome).
Sacral agenesis
Congenital absence of part or all of the sacrum, associated with maternal diabetes. Ranges from minor coccygeal absence to complete sacral agenesis with neurological deficits.
Clinical Relevance
The S2 level corresponds to the termination of the dural sac, which is important for lumbar puncture and epidural technique. The S2-S4 nerve roots carry the parasympathetic outflow to the bladder, rectum, and genitalia; injury to these roots causes neurogenic bladder and bowel. Caudal epidural anesthesia is performed through the sacral hiatus (at the S4-S5 level) and is commonly used in pediatric surgery. The dorsal sacral foramina are targets for sacral neuromodulation (InterStim) for overactive bladder.
Development and Ossification
Each sacral vertebra ossifies from three primary centers (body and two costal elements) appearing between weeks 8 and 12 of fetal development. Fusion of adjacent segments begins at puberty from the lower segments upward and completes by about age 30. The S1-S2 junction is typically the last to fuse.
Did You Know?
- The S2-S4 sacral segments control bladder, bowel, and sexual function through the pelvic splanchnic nerves
- The sacral hiatus at the bottom of the sacrum has been used for caudal anesthesia since the early 1900s
- Sacralization of L5 is present in about 4-6% of the population and is often an incidental radiographic finding
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