Selective dorsal rhizotomy (SDR) is a procedure for the treatment of spasticity in some children with cerebral palsy. The incidence of cerebral palsy ranges from 2 - 2.5 per thousand school-age children. Of these children about 75% will have a spastic variety. Some of these children may benefit by reducing their spasticity through selective dorsal rhizotomy.

Anatomy

• The normal lumbar spine is composed of five building blocks called vertebrae that sit on the sacrum, which is the back part of the pelvic bone. (Figure 1)
  
• Each vertebra is constructed of a body, lamina, and pedicles, which surround an opening, the spinal canal.(Figure 2)
  
• A spinous process extends posteriorly (towards the back) from the lamina in the midline
  
• Separating any two vertebral bodies is a soft elastic material called a disk. (Figure 3). There are five lumbar disks
  
• On each side of the back of the spinal canal and linking one vertebra to the next are a series of small joints called facets
  
• Through the spinal canal pass the nerve roots that emerge from the end of the spinal cord (conus medullaris) and go to form the nerves to the legs. (Figure 4)
  
• Each root exits the spinal canal through a 'hole' in the side of the canal formed by two adjacent vertebrae called a foramen
  
• The roots float in fluid and are contained within a fibrous sac called the dura
  
• Each nerve root is composed of a dorsal (back) and ventral (front) portion. The dorsal roots conduct sensation (pain, temperature and position as well as sensation from muscles and tendons) and enter the spinal cord along its dorsal side. (Figure 5) The ventral roots conduct impulses from the spinal cord to muscles
  
• A sensory nerve from muscle or tendon enters the spinal cord and connects directly to a motor nerve going to the same muscle (called a monosynaptic reflex arc). This reflex is modified by impulses from the brain and other levels of the spinal cord that result in maintaining normal muscle tone (Figure 5)
  
• Both the dorsal and ventral roots are made up of smaller fibers called rootlets
  
• With normal tone, muscles that move a limb in one direction (agonist muscles) are balanced by muscles that move the limb in the opposite direction (antagonist muscles)

Figure 1 - There are five lumbar vertebrae. The disks are numbered according to the number of the vertebra above it.	Figure 2 - Construction of a typical lumbar vertebra.
Figure 3 - Two adjacent vertebrae showing a disk and intervertebral foramen with exiting nerve root.	Figure 4 - Cross-section through a vertebra showing the spinal canal and nerve roots of the cauda equina.
Figure 5 - Diagrammatic representation of a monosynaptic reflex arc composed of a sensory neuron from muscle and tendon going into the spinal cord and connecting to a motor neuron exiting the spinal cord going to muscle. Stimulating the tendon causes the muscle to contract. The long tracts represent neurons that pass up and down the spinal cord and modify the action of the reflex arc. Injury to these tracts usually causes the reflex arc to become hyperactive (spastic). © T. Graves