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Spondylolisthesis may be divided into two categories: developmental and acquired. The standard accepted classification system for spondylolisthesis is that of Newman, Wiltse, and McNab. This system describes the etiology of the spondylolisthesis and also highlights the anatomic lesion responsible for the slip.

Type I - Dysplastic or Congenital Spondylolisthesis

This type of spondylolisthesis accounts for up to 20% of treated cases. Spondylolysis is not a causal factor of the slippage. Rather, slippage is related to some type of malformation of spinal structures, (e.g., vertebrae, facets, or pars) or elongation of the pars. The pars may be poorly developed and thereby predisposed to cracking or fracture. The presence of dysplastic (abnormal development) sacral facet joints is accompanied by forward translation of one vertebra over another leading to stress on the pars which may then fracture. Spina bifida in some degree is frequently present and scoliosis is noted in up to 50% of cases due to the spinal curvature at the level of the slip.

Type II - Isthmic Spondylolisthesis

This type is due to a defect or fracture in the pars interarticularis, the articulating segment of the vertebrae and includes:

• Subtype IIA - a defect due to a fatigue fracture of the articulating segment and associated with complete bony separation. This is more common in individuals under 50 years of age.

• Subtype IIB - elongation of the articulating segment due to repeated microfractures, but no separation. Bony areas may form around the improperly healed microfractures. The result is an elongated pars which may eventually break and may transform to Type IIA over time.

• Subtype IIC - acute fracture of the articulating segment

Type III - Degenerative Spondylolisthesis

This type of spondylolisthesis is due to degeneration of the disk and is often accompanied by arthritis of the joints. It usually occurs later in life after the age of 50 and often causes spinal stenosis (narrowing of the spinal canal). The location of degeneration is most frequently at the level of L4-L5, though it can also be present at L5-S1 and L3-L4 levels. Some theorize that it related to increased laxity of ligaments due to hormonal changes. As the vertebral parts degenerate, the integrity of the spine is compromised. Degenerative spondylolisthesis causes instability between the segments of the vertebrae and although progression of slippage occurs in approximately 30% of people, it usually does not exceed Grade I. It is diagnosed in females approximately five times more frequently than males.

Type IV - Traumatic Spondylolisthesis

The cause of this type of spondylolisthesis is an acute fracture of some part of the spinal process which causes destabilization of the vertebrae.

Type V - Pathologic Spondylolisthesis

This type of spondylolisthesis results from bone disease (e.g., syphilis, Paget's disease, or Albers-Schoenberg's disease) which causes a destabilization of the spinal structures.

Type VI - Iatrogenic Spondylolisthesis

This is also called postoperative spondylolisthesis and results from surgical intervention. The classification was recently added and results from excessive removal of the posterior elements of the pars or other spinal supporting structures after laminectomy (spinal surgery usually performed to correct disk herniation or nerve compression caused by spinal stenosis). The incidence of iatrogenic spondylolisthesis is thought to be approximately 3-5% of cases of treated spondylolisthesis.