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Posterior sacral foramina

The posterior sacral foramina are a series of paired openings located on the dorsal surface of the sacrum, aligned laterally to the median sacral crest. They represent the exit points for the posterior (dorsal) rami of the sacral spinal nerves (S1–S4). Each sacral segment contributes one foramen on each side, usually four pairs in total.

The foramina transmit the dorsal rami, which provide motor innervation to deep back muscles and sensory innervation to skin of the posterior sacrum, buttock, and adjacent regions. They also transmit small vascular branches.

Anatomically, the posterior sacral foramina correspond to the anterior sacral foramina on the ventral surface, which transmit the ventral rami. Their location is important in nerve blocks, sacral fractures, sacralization anomalies, and surgical approaches to the sacrum.

Synonyms

  • Dorsal sacral foramina

  • Posterior foramina of sacrum

Function

  • Allow passage of the posterior rami of sacral spinal nerves (S1–S4)

  • Provide motor innervation to multifidus and erector spinae muscles of sacral region

  • Provide sensory innervation to overlying skin and gluteal region

  • Serve as landmarks in spinal anesthesia, sacral nerve stimulation, and sacral surgery

MRI Appearance

T1-weighted images:

  • Posterior sacral foramina appear as small round low-signal openings in the dorsal sacrum

  • Nerve roots within foramina show intermediate signal intensity against surrounding hypointense cortical bone

T2-weighted images:

  • Nerves within foramina appear intermediate to slightly hyperintense

  • CSF signal is not typically present beyond the sacral canal, aiding contrast between nerve and bone

STIR:

  • Fat suppression highlights foramina by suppressing perineural fat signal

  • Helps detect perineural cysts, edema, or nerve inflammation

T1 Fat-Saturated (Pre-contrast):

  • Nerves show intermediate signal against suppressed fat background

  • Foramina margins remain hypointense due to cortical bone

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal nerves show minimal enhancement

  • Abnormal enhancement suggests neuritis, tumor, or vascular lesion

MRI Non-Contrast 3D Imaging:

  • Provides 3D reconstruction of foraminal alignment

  • Useful for nerve mapping, congenital anomalies, and surgical planning

CT Appearance

CT Pre-Contrast:

  • Foramina appear as well-corticated osseous openings on the dorsal sacrum

  • Demonstrates number, size, and alignment clearly

  • Useful in detecting fractures, congenital anomalies, or foraminal narrowing

CT Post-Contrast:

  • Nerves themselves are not directly enhanced, but surrounding vascular structures may opacify

  • Highlights soft tissue abnormalities extending into foramina

  • 3D CT reconstructions are helpful for preoperative navigation and trauma assessment

MRI image

Posterior sacral foramina MRI axial anatomy image -img-00000-00000

CT image

Posterior sacral foramina CT axial image

CT VRT 3D image

Posterior sacral foramina 3d ct image