In plain language, spondylolisthesis means the abnormal forward placement of a vertebral body in relationship to another vertebral body. It is more than a “slip of a disk”.
To understand the finding, we must first understand the normal orientation of the vertebral bodies to each other, and the nature of the structures of the spine.
The spine has a function of providing structure, flexibility, and protection of the nerves that come out from the brain into the body.
Nerves carry communication from the brain to the various organs, ligament and muscles of the body. In addition, the nerves carry information to tell the brain if the coordination of the various body parts are achieving the intended action, and also protects the body from any harmful insult.
The spine is a collections of functions spine units (FSU), which comprises of two vertebrae, connected together by a disc, two facet joints in the back, and the associated ligaments that hold the facet joints, and the vertebra together. The FSU also protects the nerve tissues that occupy the center of the spine unit, and protects the nerves as the exit out the holes of the FSU (foramen).
In spondylolisthesis, the FSU no longer functions normally, and the vertebral bodies are no longer oriented in alignment. To make that concept easier, if you place two same sized rings on top of each other, that would be the normal orientation. When you look a the rings through the top or bottom opening, you will see a round circle. You can easily see an object passing through the center of the two rings, and the space would be uniform.
In this example, to demonstrate a spondylolisthesis, have one ring placed out of alignment to the other ring, therefore causing the opening hole now narrower in size. The once circular hole is not more oblong. If you further displace the rings from each other, that oblong space will become narrower and smaller. That is the analogy of spondylolisthesis. The vertebra, in the back, consists of a ring that allows the spinal cord or nerves pass through the center. If the vertebral rings shift, that is spondylolisthesis.
Spondylolisthesis can be caused by ligament laxity, or stretching that allows the vertebral ring to shift forward of each other. That usually requires an incompetent disc, and the ligaments associated with the capsules of the facet joints and the various interspinous ligaments.
Spondylolisthesis can be caused by a fracture of the back of the vertebra in the area of the pars intraarticularis, causing extra stress to the disc and ligaments, resulting in the shifting of the vertebral rings on one another.
Spondylolisthesis can the result of a congenital development, a degenerative development, or a traumatic event.
- Hari A, Krishna M, Rajagandhi S, Rajakumar DV. Minimally invasive transforaminal lumbar interbody fusion-indications and clinical experience. Neurol India. 2016 May-Jun;64(3):444-54. PubMed PMID: 27147152