First, a quick explanation of spondylolisthesis might be helpful. Normally, the bones of the spine (the vertebrae) stand neatly stacked on top of one another. Ligaments and joints support the spine. Spondylolisthesis alters the alignment of the spine. In this condition, one of the spine bones slips forward over the one below it. As the bone slips forward, the nearby tissues and nerves may become irritated and painful.
The reason this type of slippage develops isn’t always clear. Spondylolisthesis may very rarely be congenital, which means it is present at birth. It can also occur in childhood as a result of injury. In older adults, degeneration of the disc and facet (spinal) joints can lead to spondylolisthesis
Any of the vertebrae can slip forward but in young people (under 20 years old), spondylolisthesis usually involves slippage of the fifth lumbar vertebra over the top of the sacrum. There are several reasons for this.
First, the connection of L5 and the sacrum forms an angle that is tilted slightly forward, mainly because the top of the sacrum slopes forward. This angle is referred to as the lumbosacral kyphosis or LSK. Second, the slight inward curve of the lumbar spine creates an additional forward tilt where L5 meets the sacrum. Finally, gravity attempts to pull L5 in a forward direction, which can be seen on X-ray as the slip angle.
All three of these bony alignments can be measured using X-rays. It can be confusing when the surgeon tries to explain all three angles and changes from previous X-rays to the present. The bottom-line is that the more severe these angles are, the more severe the spinal deformity. Keeping an eye on the measurements for each one of these angles helps the surgeon see if and when the problem is getting worse instead of better. At that point, surgical treatment becomes a consideration.