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Superior articular process of sacrum

The superior articular process of the sacrum is a paired bony projection arising from the upper part of the sacrum at the level of the first sacral vertebra (S1). It forms the inferior component of the lumbosacral facet joints, articulating with the inferior articular processes of the fifth lumbar vertebra (L5).

It plays an essential role in the stability and alignment of the lumbosacral junction and is an important structure in spinal anatomy, biomechanics, and cross-sectional imaging.

Synonyms

  • Superior articular process of S1

  • Sacral superior facet process

Location

  • Situated on the posterosuperior aspect of the sacrum

  • Arises from the upper part of the first sacral segment (S1)

  • Located posterior to the sacral ala

  • Forms the posterior element of the lumbosacral junction

  • Articulates superiorly with the inferior articular process of L5

Anatomical components

  • Paired bony processes projecting superiorly and posteriorly

  • Articular facet surface:

    • Oriented posteromedially

    • Covered by hyaline cartilage in the facet joint

  • Pedicle and laminar base:

    • Continuous with the posterior arch of S1

  • Facet joint capsule attachment along the margins

Relations

Anteriorly:

  • Sacral ala

  • Body of S1 (indirectly)

Posteriorly:

  • Posterior sacral surface

  • Origin of deep paraspinal muscles

Medially:

  • Sacral canal

  • Spinous process of S1 (rudimentary)

Laterally:

  • Sacral ala

  • Lumbosacral facet joint space

Superiorly:

  • Inferior articular process of L5

Inferiorly:

  • Posterior elements of the sacrum

Structures related to the process

  • L5–S1 facet joints

  • Facet joint capsule

  • Ligamentum flavum (adjacent superiorly)

  • Multifidus and deep paraspinal muscles

  • Dorsal rami of spinal nerves (nearby)

Function

  • Facet articulation: Forms the inferior facet surface of the L5–S1 zygapophyseal joints

  • Stability: Limits excessive rotation and anterior translation at the lumbosacral junction

  • Load transmission: Contributes to transfer of axial load from lumbar spine to sacrum

  • Guidance of motion: Helps direct flexion, extension, and rotation at L5–S1

X-ray appearance

Lumbosacral spine radiographs (AP / lateral / oblique views):

  • Superior articular process: Subtle bony projection at the upper posterior margin of the sacrum

  • Facet joint line: Oblique lucent line at L5–S1 level

  • Symmetry: Paired and symmetric appearance

  • Visualization: Best appreciated on oblique projections

CT appearance (pre-contrast)

Non-contrast CT:

  • Cortical bone: Dense, well-defined margins of the superior articular process

  • Articular surface: Smooth, concave facet contour

  • Facet joint space: Thin, uniform joint line between L5 and S1

  • Trabecular bone: Homogeneous internal structure

  • Anatomical detail: Excellent delineation of orientation and morphology

MRI appearance

T1-weighted images:

  • Cortical bone: Low signal intensity

  • Marrow within the process: Intermediate-to-high signal

  • Facet joint cartilage: Low-to-intermediate signal thin layer

T2-weighted images:

  • Cortical bone: Low signal

  • Marrow: Intermediate signal

  • Facet joint space: High signal from joint fluid if present

STIR:

  • Marrow: Normally suppressed to low signal

  • Surrounding soft tissues: Low-to-intermediate signal

MRI images

Superior articular process of sacrum MRI anatomy Location and Imaging Appearance -img-00000-00000_00002

CT images

Superior articular process of sacrum CT anatomy Location and Imaging Appearance -img-00000-00000

CT images

Superior articular process of sacrum CT anatomy Location and Imaging Appearance -img-00000-00000_00001