The lumbar PLL widens at disc levels (providing some resistance to central disc herniation) and narrows at vertebral body levels, producing the hourglass shape on imaging. Its narrowing at the vertebral body leaves the posterolateral disc uncovered — explaining why posterolateral disc herniation is far more common than central herniation at the lumbar levels. The PLL must be removed in anterior lumbar interbody fusion (ALIF) for adequate disc space clearance.
| Origin | Posterior vertebral bodies (continuous from C2 to the sacrum) — widened at disc levels to cover the disc anulus |
|---|---|
| Insertion | Continuous band narrowing at the vertebral body levels (wider at discs, narrow at vertebral bodies — the classic hourglass shape) |
| Actions | Resists posterior disc herniation centrally; limits lumbar flexion |
|---|
The PLL is thinner and narrower in the lumbar spine compared to the cervical and thoracic spine, providing less resistance to posterolateral disc prolapse. In ALIF surgery, the PLL is carefully removed with a curette after disc excision to allow the cage to impact into the disc space. Injury to the epidural veins behind the PLL requires prompt haemostasis.
Narrow lumbar PLL leaving posterolateral disc uncovered, explaining the higher incidence of posterolateral disc herniation compared to central herniation.
