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.
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.
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