Apportionment estimates the degree to which each of various occupational or nonoccupational factors may have caused or contributed to a particular impairment. Apportioning spinal impairment among different injuries often is challenging. The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Third Edition, relied on the Range of Motion (ROM) Model, which is included in the Fourth Edition for use only in 2 circumstances: first, if the spine has a disease and no injury has occurred (eg, ankylosing spondylitis) and, second, as a tie breaker. The AMA Guides, Fourth Edition, introduced a new approach, the Injury Model or Diagnosis-related estimates (DRE) Model. The Injury Model is different from other approaches in the AMA Guides because when the individual reaches Maximal Medical Improvement (MMI), the degree of persisting anatomic or physiologic loss is rated. In contrast, the Injury Model rates the severity of the original injury, not the degree of recovery. Recurrence is common after significant back injury, so even if the individual seems to be doing well when rated, years later the functional result may be different; these concerns explain the instructions to rate soft-tissue back injury using objective findings and true radiculopathy using the Injury Model.