Spondylolisthesis is defined as the anterior displacement of one vertebral body on another. Typically, the posterior portion of the arch, including the inferior facets, remains aligned with the sacrum, while the anterior portion of the vertebral body slips forward. If disruption of the interarticular portion of the vertebral arch occurs without anterior displacement of the vertebral body, it is termed spondylolysis. Spondylolisthesis occurs with a prevalence of 5% in the general population, and is equally distributed between the sexes. Spondylolisthesis is graded based on the degree of anterior displacement of the upper vertebra relative to the lower:
Grade 1: <25% displacement
Grade 11: 25-50% displacement
Grade 111: 50-75% displacement
Grade 1V: >75% displacement

Spondylolisthesis is also divided into five etiologic classifications:

Type I: Dysplastic- congenital abnormality of the upper or lower sacral facets that allows slipping of L5 on S1. No pars interarticularis defect is present.
Type II: Isthmic- bilateral defect in the pars interarticularis allows slipping of L5 on S1. This type is typical of pars interarticularis fractures.
Type III: Degenerative- typically occurs in patients >50 yo with degenerative changes in the facet joints.
Type IV: Traumatic- occurs with posterior vertebral fracture at a site other than the pars interarticularis.
Type V: Pathological- occurs secondary to localized or generalized bone disease.

For radiographic examination of spondylolisthesis, the views of choice include lateral and oblique radiographs of the lumbar spine. The normal oblique view of the LS spine shows a "Scotty Dog" shape (see Figure part 6/6).

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