Houston Methodist. Leading Medicine

General Spine News

Getting an Angle on Spines That Slip

Some people develop a defect in one of the lower back bones. The defect is a crack that forms in the bony ring on the back of the spinal column. The area affected is called the pars interarticularis, so doctors sometimes refer to this condition as a "pars defect."
When a pars defect is present in only one side of the spine bone, the condition is called spondylolysis. When the break occurs on both sides of the bony ring, the vertebra above can start to slip forward on the one below it. This is called spondylolisthesis.

Doctors are trying to find a way to predict when patients with a crack in one side of the bony ring are at risk for spondylolisthesis. By using X-rays, researchers have been able to photograph and measure different angles in the bones of the back. One of these angles is called the pelvic incidence. The pelvic incidence is a measure of the sacrum in relation to the hips. The sacrum is the triangular-shaped bone at the base of the spine, between the two pelvis bones. This angle is important in deciding how the spine, the sacrum, and the hips line up with each other when viewed from the side.

The researchers found that the worst cases of spondylolisthesis had the most pelvic incidence. In the normal adult, the hip is positioned directly below the last lumbar vertebra. In spondylolisthesis, the hip joint is forward of its normal position.

The presence of pelvic incidence may be a predictor of who will progress from a crack to a slip of the vertebra. Doctors still can't say for sure, because it isn't clear whether pelvic incidence is a cause or an effect. More studies are needed to answer this question.


Darrell S. Hanson, MD, et al. Correlation of Pelvic Incidence With Low- and High-Grade Isthmic Spondylolisthesis. In Spine. September 15, 2002. Vol. 27. No. 18. Pp. 2026-2029.

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