There are five types of spondylolisthesis, categorized based on the etiology: Type I, congenital: caused by agenesis of superior articular facet Type II, isthmic: caused by a defect in the pars interarticularis (spondylolysis) Type III, degenerative: caused by articular degeneration as occurs in conjunction with osteoarthritis Type IV, traumatic: caused by fracture, dislocation, or other injury Type V, pathologic: caused by infection, cancer, or other bony abnormalities Spondylolisthesis usually involves the L3-L4, L4-L5, or most commonly the L5-S1 vertebrae.