Table 4.

Classification of Spondylolisthesis by Etiology.^a

Type	Etiology	Pathogenesis
I	Dysplastic	Congenital defect in the neural arch
II	Isthmic	Pars interarticularis defect
IIa		Stress fracture of the pars (spondylolysis)
IIb		Repeated microtrauma and nonlinear forces cause elongation of the pars
IIc		Acute traumatic fracture of the pars
III	Degenerative	Degeneration of the facet joint complex (capsule, ligaments)
IV	Traumatic	Acute traumatic fracture of posterior column, but not the pars
V	Pathologic	Infection, neoplasm, endocrine disorder, or other pathology causes vertebral instability
VI	Postsurgical	Postsurgical lumbar instability

^a Pars interarticularis defects in athletes correspond to type IIa spondylolisthesis.