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Accessory process of vertebrae

The accessory process is a small bony projection found on the posterior aspect of the base of the transverse process of lumbar vertebrae. It represents a vestigial structure, thought to be a remnant of lumbar rib elements. Though small, it serves as an important landmark for muscular and ligamentous attachments in the lumbar spine and is relevant in spine surgery and imaging interpretation.

Synonyms

  • Accessory tubercle of lumbar vertebra

  • Posterior transverse tubercle

  • Lumbar accessory process

Location and Structure

  • Found only in the lumbar vertebrae

  • Located at the posterior base of each transverse process

  • Appears as a small rounded or elongated projection, most prominent in mid-lumbar vertebrae (L1–L4)

  • Lies close to the mammillary process, which is positioned more posteriorly on the superior articular process

Relations

  • Anteriorly: Body and pedicle of lumbar vertebra

  • Posteriorly: Mammillary process and lamina

  • Superiorly: Superior articular process and facet joint

  • Inferiorly: Transverse process extending laterally

Function

  • Serves as the site of attachment for the longissimus muscle (part of erector spinae)

  • Provides additional surface area for small stabilizing muscles and ligaments of the lumbar spine

  • Contributes indirectly to spinal postural stability and motion control

Clinical Significance

  • Acts as a radiological landmark in lumbar spine imaging

  • Important in differentiating normal anatomy from fractures, accessory ossicles, or pathology

  • May be a source of confusion in spinal interventions if mistaken for other bony processes

  • In degenerative conditions, accessory processes may appear hypertrophic or obscured

  • Serves as an orientation point during facet joint injections and lumbar surgical approaches

MRI Appearance

T1-weighted images:

  • Accessory process appears as low signal intensity cortical bone with adjacent bright fat planes

  • Surrounded by intermediate signal marrow within transverse process base

T2-weighted images:

  • Cortical bone remains dark (low signal)

  • Surrounding fat and CSF appear bright, enhancing its visibility

  • Adjacent muscle attachments show intermediate signal

STIR (Short Tau Inversion Recovery):

  • Cortical bone: dark signal

  • Any surrounding edema, enthesopathy, or muscle inflammation: bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal accessory process does not enhance

  • Pathology (inflammatory changes, tumor infiltration) may show enhancement in adjacent soft tissues or marrow

3D T2 SPACE / CISS:

  • Accessory process visualized as a dark bony projection at base of transverse process

  • Surrounded by bright CSF and fat, allowing high-contrast identification of spinal landmarks

CT Appearance

Non-Contrast CT:

  • Accessory process appears as a small bony outgrowth at the posterior base of the transverse process

  • Clearly visualized due to high resolution of cortical bone

  • Useful for detecting fractures, bony hypertrophy, or congenital variations

Post-Contrast CT:

  • Bony structure itself does not enhance

  • Adjacent soft tissue pathology (inflammatory, infectious, or neoplastic) may enhance and obscure its borders

  • Excellent for surgical planning and anatomical localization

MRI images

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CT image

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