Transforaminal injection is often used interchangeably with the term selective nerve root injection. The spinal nerves exit the bony spinal canal through the intervertebral foramen. Just lateral to the foramen, a small volume of injectate can be placed directly adjacent to a single nerve root. Blocking a single nerve root with local anesthetic can be used diagnostically to clarify which nerve root is contributing to clinical symptoms in patients with pathology at multiple levels and a confusing pattern of symptoms. In this way, selective nerve root injection can be used to assist the surgeon’s decision making when pondering the proper operative approach. Interpretation of the results must be carried out cautiously, as the potential space surrounding the nerve roots in the paravertebral region is contiguous with the epidural space. Indeed, as the volume of injectate is increased, the material spreads laterally along the nerve root and proximally through the intervertebral foramen to the epidural space directly surrounding the nerve root. Although some physicians conduct selective nerve root injection just outside the intervertebral foramen and transforaminal injection by advancing the needle tip a few millimeters further to enter the foramen, this distinction likely carries little practical meaning. Even a small volume of material injected at either location often enters the epidural space by contiguous spread.
The most common application of transforaminal injection is to inject steroids. The rationale for injecting steroids is that they suppress inflammation of the nerve, which, in many instances is believed to be the basis for radicular pain. The rationale for using a transforaminal route of injection rather than an interlaminar route is that the injectate is delivered directly onto the target nerve, which ensures that the medication reaches the site of the suspected pathology in maximum concentration.