Spondylolisthesis (also known as anterolisthesis) is defi ned as an anterior displacement of a vertebra relative to the vertebra below, whereas the reverse, i.e. when the superior vertebra slips posterior to that below, is called retrolisthesis (Butt and Saifuddin 2005).
|1. Dysplastic||Congenital dysplasia of the articular processes|
|2. Isthmic||Defect in the pars articularis|
|3. Degenerative||Degenerative changes in the facet joints|
|4. Traumatic||Fracture of the neural arch other than the pars articularis|
|5. Pathological||Weakening of the neural arch due to disorders of the bone|
|6. Iatrogenic||Excessive removal of bone following spinal decompression|
Grading of Lumbar Spondylolisthesis
The forward slip of the upper vertebra is measured using the method of Meyerding, or the method described by Taillard. Using the method of Meyerding, the anteroposterior (AP) diameter of the superior surface of the lower vertebra is divided into quarters and a grade of I– IV is assigned to slips of one, two, three or four quarters of the superior vertebra, respectively.
Isthmic Spondylolisthesis: Spondylolysis Isthmic spondylolisthesis occurs when a bilateral defect in the pars interarticularis is present. The pars interarticularis (also known as the isthmus) is the part of the neural arch that joins the superior and inferior articular processes. A defect at this point functionally separates the vertebral body, pedicle, and superior articular process from the inferior articular process and remainder of the vertebra. Thus, the defect cleaves the vertebra into two parts. The portion of the vertebra posterior to the defect remains fi xed, and the anterior portion has the potential to slip forward relative to the posterior structures and the spine below. Isthmic spondylolisthesis can be further categorized into three subtypes (Wiltse et al. 1976).
In type 2a, a lytic pars defect is present, due to a congenital weakness in the bone and/or repeated mechanical strain.
In type 2b, the elongated pars is a true stress fracture of the pars.
This is the commonest cause of lumbar spondylolisthesis above the age of 50. Degenerative changes of the facet joints and associated disc degeneration are responsible for the anterior slip, which is usually mild.