People with spinal cord injuries are often told what level of spinal cord injury they have, whether it is a “complete” or “incomplete” injury, and whether it is classified as A, B, C, D, or E according to the American Spinal Cord Injury Association (ASIA). spinal cord injury levels, complete and incomplete What is the significance of spinal cord injury levels, complete and incomplete injuries, and spinal cord injury classifications? In this article I will attempt to explain the currently accepted levels and classifications of spinal cord injury. Spinal Cord and Spinal Cord Segment Levels: The spinal cord is located within the spinal column. The spinal column is made up of a series of vertebrae. The “nerve” segment level of the spinal cord itself is based on the spinal nerve roots that enter and exit the spinal column between the vertebrae. As shown in Figure 1, the spinal cord segments do not necessarily coincide with the vertebrae. The spinal levels are shown on the left, while the spinal cord levels are listed as cervical (red), thoracic (blue), lumbar (green), and sacral (gray). The spinal column has 7 cervical, 12 thoracic, 5 lumbar and 5 sacral segments. The spinal cord is shorter than the spinal canal and usually ends just below the lumbar 1 vertebrae. The cervical 1 spinal nerve root penetrates above the cervical 1 vertebrae. There are no cervical 8 vertebrae, so the cervical 8 spinal nerve root passes between cervical 7 and thoracic 1. The thoracic 1 nerve root passes between thoracic 1 and thoracic 2, and the lumbar 5 nerve root passes between lumbar 1 and sacral 1. The first and second cervical vertebrae support head movement. The cervical 1 vertebrae support the skull and are called the atlantoaxial vertebrae. Posterior to the skull is the occipital bone, and the joint between the occipital bone and the cervical 1 vertebra is called the atlanto-occipital joint. The cervical 2 vertebrae support the movement of the atlantoaxial vertebrae and are called the axial vertebrae. The joint between the cervical 1 and cervical 2 vertebrae is the atlantoaxial joint. The nerves of the cervical spinal cord innervate the diaphragm (cervical 3), deltoid (cervical 4), biceps (cervical 4-5), wrist extensors (cervical 6), triceps (cervical 7), finger flexors (cervical 8), and hand muscles (cervical 8 – thoracic 1). Twelve thoracic vertebrae are attached to the rib cage. The spinal nerve roots form the intercostal nerves, which travel at the base of the ribs, are attached to the intercostal muscles, and are associated with the dermatomes. About 5% of the population has a degenerated 13th rib. The spinal cord ends just below lumbar 1. The spinal cone is the tip of the spinal cord. Below the cone, the spinal nerve roots from lumbar 2 to sacral 5 form the cauda equina. Injuries to the lower lumbar spinal cord often injure the lumbar expansion. Any damage to the lumbosacral spine can result in injury to the lumbosacral bulge.