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Lumbar Accessory Process

processus accessorius vertebrae lumbalis

The accessory process of the lumbar vertebra is a small bony projection on the posterior surface of the transverse process, just lateral to the mamillary process and superior articular process. It provides the groove through which the medial branch of the posterior ramus runs in conjunction with the mamillary process, forming the mamillo-accessory groove.

Anatomical Data

Structure & Landmarks

Articulations	No joint; muscular attachment on the transverse process
Landmarks	Posterior surface of transverse process Medial branch nerve groove (with mamillary process) Variably developed, most prominent at L4-L5
Ossification	Formed from the neural arch ossification centre, fusing to the transverse process by age 6. The accessory process is best developed at L3-L5 and may be absent at higher levels.

Clinical Relevance

Clinical Notes

The mamillo-accessory ligament bridges the accessory process and the mamillary process, forming a fibro-osseous tunnel through which the medial branch nerve runs at some lumbar levels. Ossification of this ligament (present in up to 10% at L5) creates a bony tunnel that can entrap the medial branch nerve, causing facet-mediated pain unresponsive to conventional treatment. Identification of this ossification on CT guides radiofrequency ablation needle placement.

Pathology

Common Injuries & Conditions

Lumbar Transverse Process Fracture

Trauma to the transverse process region may involve the accessory process along with the rest of the transverse process.

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