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Superior articular process of S1 vertebra

The superior articular process of the first sacral vertebra (S1) is a paired bony projection that extends upward from the posterior aspect of the sacrum. It represents the articular connection between the sacrum and the fifth lumbar vertebra (L5), forming the lumbosacral facet joints. These processes are critical for stability of the lumbosacral junction and play a role in weight transmission, movement restriction, and prevention of anterior translation of L5.

Each process terminates in a facet surface, which articulates with the inferior articular processes of L5. The orientation of these facets (more coronally and posteriorly than lumbar vertebrae above) contributes to the unique biomechanics of the lumbosacral junction.

Synonyms

  • Superior facet of S1

  • Articular process of sacrum (cranial)

  • Lumbosacral articular process

Location and Structure

  • Projects upward from the posterior-lateral surface of the S1 vertebra

  • Each process contains a smooth, hyaline cartilage–covered articular facet

  • Facet orientation: posteromedial, nearly in coronal plane, opposing the inferior facets of L5

  • Strong capsule reinforced by the lumbosacral and iliolumbar ligaments

Relations

  • Superiorly: Inferior articular processes of L5 (forming L5–S1 zygapophyseal/facet joints)

  • Inferiorly: Sacral lamina and S1 dorsal surface

  • Anteriorly: Sacral pedicle and neural foramina transmitting L5 spinal nerve roots

  • Posteriorly: Ligamentum flavum and erector spinae muscles

Function

  • Provides a stable articulation with the inferior articular processes of L5

  • Prevents anterior displacement of L5 on S1 (anterolisthesis)

  • Restricts excessive flexion and rotation at the lumbosacral junction

  • Acts as a key load-bearing structure for axial forces transmitted from the lumbar spine to the sacrum and pelvis

Clinical Significance

  • Facet joint arthropathy: Degenerative changes are common at L5–S1 due to high mechanical load

  • Spondylolisthesis: Defects in facet orientation or degeneration predispose to anterior slippage of L5

  • Nerve root compression: Hypertrophy, osteophytes, or joint cysts may impinge on exiting L5 nerve roots

  • Surgical landmark: Critical reference during lumbosacral fixation and spinal decompression procedures

  • Pain syndromes: Lumbosacral facet syndrome may cause chronic low back pain radiating into buttocks

MRI Appearance

T1-weighted images:

  • Cortical bone of superior articular process: low signal intensity (dark)

  • Subchondral bone marrow: intermediate to bright signal depending on fat content

  • Fat planes surrounding capsule: bright

T2-weighted images:

  • Cortical bone: dark (very low signal)

  • Cartilage and joint space: intermediate to bright signal if fluid present

  • Joint effusion or synovitis: bright signal

  • Adjacent muscle and ligaments: intermediate signal

STIR (Short Tau Inversion Recovery):

  • Normal cortical bone: dark

  • Bone marrow edema (trauma, arthropathy, inflammation): bright hyperintensity

  • Fat: suppressed and dark, highlighting abnormal signals

T1 Fat-Sat Post-Contrast:

  • Normal articular processes show minimal enhancement

  • Inflammatory changes (facet synovitis, arthritis): capsular and periarticular enhancement

  • Tumor or infection: heterogeneous enhancement with marrow involvement

3D T2 SPACE / CISS:

  • Facet cortical margins: very low signal (dark)

  • Joint capsule and surrounding CSF/fat: bright, providing contrast

  • Allows high-resolution visualization of facet orientation, cysts, or nerve root impingement

CT Appearance

Non-Contrast CT:

  • Cortical bone: high density (bright white)

  • Articular surface clearly delineated against joint space

  • Osteophytes, sclerosis, or facet hypertrophy easily detected

Post-Contrast CT:

  • Bone itself does not enhance

  • Soft tissue structures (capsule, synovial cysts, or inflammatory tissue) may enhance

  • Useful for detecting erosions, tumors, or infection at the facet joint

MRI image

Superior articular process of S1 vertebra mri coronal image

CT image

Superior articular process of S1 vertebra ct coronal anatomy image-img-00000-00000