Spondylolisthesis generally is defined as an anterior or posterior slipping or displacement of one vertebra on another. A unilateral or bilateral defect of the pars interarticularis without displacement of the vertebra is known as spondylolysis or, less frequently, spondyloschisis. The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) notes that the two common types of spondylolisthesis are isthmic (related to the presence of spondylolysis and occurring in the immature spine) and degenerative spondylolisthesis (usually found in older individuals with degenerative changes, usually at the fourth or fifth lumbar vertebra). The AMA Guides classifies spondylolisthesis based on the etiology of the slip: congenital, isthmic, degenerative, traumatic, pathologic, and iatrogenic/postsurgical. In addition, grades ranging from I to V are assessed based on the severity of the displacement of the vertebra above on the vertebra below. Some of the confusion concerning terminology used in the AMA Guides regarding alteration of motion segment integrity arises because multiple different but similar terms exist, eg, those from the American Academy of Orthopaedic Surgeons vs the AMA Guides. The AMA Guides specifies that an x-ray finding of spondylolisthesis alone does not equal alteration of motion segment integrity, and a Diagnosis-related estimate (DRE) is the appropriate method for assessing spondylolisthesis (except in rare occurrences of multilevel spinal involvement, when the range-of-motion method is recommended).