The interspinous ligament at L5-S1 connects the spinous process of L5 to the sacral crest, occupying the interspinous space at the lumbosacral junction. It is often hypoplastic or absent at this level compared to more cranial segments.
| Origin | Lower border of the L5 spinous process |
|---|---|
| Insertion | Upper border of the first sacral spinous tubercle (median sacral crest) |
Resists separation of L5 and S1 in flexion
The L5-S1 interspinous ligament is frequently absent or rudimentary in adults, explaining the high mobility at this junction. Interspinous spacer devices (Wallis, Coflex) at L4-L5 cannot be placed at L5-S1 due to the sacral anatomy. The interspinous space at L5-S1 is the target for interlaminar epidural injection at the most common symptomatic level.
Hypoplastic or absent L5-S1 interspinous ligament contributing to hypermobility at the lumbosacral junction, a factor in L5-S1 disc disease and isthmic spondylolisthesis at this level.
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