Residency · Residency · Chronic Pain Management
Epidural Steroid Injections: Transforaminal Approach
Introduction
The transforaminal epidural steroid injection (TFESI) delivers corticosteroid and local anesthetic into the epidural space via the neural foramen, depositing medication directly in the ventral epidural space adjacent to the affected nerve root. Compared to the interlaminar approach, the transforaminal technique offers more selective nerve root coverage, greater ventral epidural spread, and theoretically higher local concentration of steroid at the site of disc-nerve root pathology. These advantages come with a serious tradeoff: the transforaminal approach carries the risk of catastrophic neurologic complications, including spinal cord infarction from inadvertent intra-arterial injection, particularly when particulate steroids are used.
Anatomy of the Neural Foramen
Foramen Boundaries
The neural foramen is bounded superiorly by the pedicle of the vertebra above, inferiorly by the pedicle of the vertebra below, anteriorly by the vertebral body and intervertebral disc, and posteriorly by the superior articular process (SAP) of the vertebra below and the ligamentum flavum.
Contents of the Foramen
The foramen contains the exiting nerve root, with the dorsal root ganglion typically residing in the superior and anterior portion. Radicular arteries traverse the foramen, including the segmental medullary arteries. The artery of Adamkiewicz (arteria radicularis magna) is the dominant segmental artery supplying the anterior spinal artery in the lower thoracic and upper lumbar region. It most commonly arises from T9-T12 and is left-sided in approximately 80% of individuals. The foramen also contains radicular veins, lymphatics, and fat.
Safe Triangle
The so-called "safe triangle" is defined by the inferior margin of the pedicle above (superior border), the lateral edge of the neural foramen (lateral border), and the exiting nerve root (medial/inferior border). The needle target for the standard subpedicular approach is the superolateral aspect of the foramen, inferior to the pedicle, within this triangle. Despite its name, the safe triangle is not free of vascular structures -- radicular arteries traverse this zone in a significant percentage of foramina.
<image>Detailed anatomic illustration of the lumbar neural foramen in oblique view showing the safe triangle boundaries (inferior pedicle margin superiorly, nerve root inferiorly and medially, lateral foramen edge laterally), with the exiting nerve root, dorsal root ganglion, radicular artery, and epidural veins labeled, a needle positioned in the subpedicular approach targeting the superolateral foramen, and a small inset showing the artery of Adamkiewicz course along the left lower thoracic foramen</image>
Technique
Subpedicular Approach (Standard)
The patient is positioned prone on the fluoroscopy table. An AP view is obtained to identify the target level, and cranial-caudal tilt is adjusted to square the endplates. The C-arm is then rotated obliquely (15-30 degrees ipsilateral for lumbar, less for thoracic) to bring the superior articular process to the mid-pedicular line, creating a clear view of the neural foramen. The target point is the 6 o'clock position of the pedicle (its inferior margin) on the oblique view.
A 22- or 25-gauge spinal needle is advanced using a coaxial (gun-barrel or tunnel vision) technique toward the inferior aspect of the pedicle. On AP view, the needle tip should be at the lateral pedicular line and must not be advanced medial to the mid-pedicular line to avoid the thecal sac. On lateral view, the tip should be in the posterior and superior aspect of the foramen, at the junction of the posterior one-third and anterior two-thirds. After confirming position, 0.5-1 mL of contrast is injected under live fluoroscopy or digital subtraction angiography (DSA) to confirm epidural spread.
Infraneural (Kambin's Triangle) Approach
This alternative targets the inferoposterior aspect of the foramen, below the exiting nerve root. The needle passes through Kambin's triangle, bounded by the exiting nerve root superiorly, the SAP posteriorly, and the endplate of the vertebral body below inferiorly. This approach may reduce the risk of nerve root contact and vascular injection in some anatomic configurations and is gaining interest for safety considerations, though it is less widely practiced than the subpedicular approach.
Cervical Transforaminal Technique
Cervical transforaminal injections carry higher risk than lumbar due to the proximity of the vertebral artery (anterior to the foramen) and radicular arteries supplying the anterior spinal artery. The needle target is the posterior-superior aspect of the foramen on oblique view. A 25-gauge needle is used with a slight posterior approach to avoid the vertebral artery. DSA is strongly recommended at cervical levels to detect intravascular injection. Some experts recommend an anterior oblique approach to avoid the posterior vertebral artery loop.
Contrast Flow Patterns
Desired Epidural Pattern
On AP view, correctly placed contrast outlines the nerve root sleeve and extends medially into the ventral epidural space. On lateral view, contrast spreads along the anterior epidural space dorsal to the vertebral body. The characteristic pattern shows a peri-radicular contrast outline confirming nerve root sheath and epidural deposition.
Intravascular Patterns
Real-time vascular uptake is seen as contrast rapidly clearing from the injection site, with opacification of segmental veins or arteries. This occurs in approximately 8-23% of lumbar transforaminal injections and may be subtle. Digital subtraction angiography increases detection sensitivity compared to real-time fluoroscopy alone. Detecting intravascular uptake is critical because intra-arterial injection of particulate steroid can cause spinal cord infarction.
Intrathecal Pattern
Dense, homogeneous contrast in the subarachnoid space with rapid cephalad spread indicates dural puncture. The procedure should be aborted and the patient monitored.
<image>Fluoroscopic image comparison panel showing three contrast patterns during lumbar transforaminal epidural injection: (1) correct epidural spread with nerve root sleeve outline and ventral epidural flow on AP view with corresponding lateral view showing anterior epidural contrast, (2) intravascular uptake pattern on AP view showing rapid vascular opacification with washout, and (3) digital subtraction angiography frame demonstrating a subtle vascular blush that would be missed on standard fluoroscopy, each panel annotated with key identifying features</image>
Particulate vs. Non-Particulate Steroids
Particulate Steroids
Methylprednisolone acetate (Depo-Medrol), triamcinolone acetonide (Kenalog), and betamethasone acetate/sodium phosphate (Celestone Soluspan) contain insoluble crystalline particles ranging from 0.5 to over 100 micrometers in size, with methylprednisolone having the largest particles. If injected intra-arterially, these particles can aggregate and form emboli. Embolic occlusion of a radicular artery or the anterior spinal artery can cause spinal cord infarction, paraplegia, or death. The potential advantage of particulate steroids is longer-lasting local anti-inflammatory effects due to sustained depot release.
Non-Particulate Steroids
Dexamethasone sodium phosphate is a true solution without crystalline particles. Its particle-free formulation eliminates the risk of embolic spinal cord infarction if inadvertent intra-arterial injection occurs. The FDA Safe Use Initiative and multiple specialty society guidelines recommend non-particulate steroids for all transforaminal injections, particularly at cervical levels. Some evidence suggests non-particulate steroids may provide shorter duration of relief compared to particulate formulations, though this remains debated. The standard dose for transforaminal injection is dexamethasone 4-8 mg.
Current Consensus
| Steroid | Type | Particle Size | Cervical TFESI | Lumbar TFESI | Key Consideration |
|---|---|---|---|---|---|
| Dexamethasone sodium phosphate | Non-particulate (solution) | None | Mandatory | Strongly recommended | No embolic risk; may have shorter duration |
| Betamethasone (Celestone Soluspan) | Mixed (particulate + solution) | 1–10 µm | Contraindicated | Use with caution | Contains both acetate and phosphate fractions |
| Triamcinolone acetonide (Kenalog) | Particulate | 1–100 µm | Contraindicated | Use with caution after neg. vascular test | Depot effect may prolong relief |
| Methylprednisolone acetate (Depo-Medrol) | Particulate | 20–100+ µm | Contraindicated | Avoid (largest particles) | Highest embolic risk |
For cervical transforaminal injections, non-particulate steroid (dexamethasone) is mandatory -- there is no clinical scenario where particulate steroid is justified at the cervical level. For lumbar transforaminal injections, non-particulate steroid is strongly recommended. If particulate steroid is used at the lumbar level, it should only be after negative aspiration, negative contrast injection on live fluoroscopy, and ideally negative DSA. For the first injection in a series, non-particulate steroid should always be used until the vascular anatomy at the target level has been characterized.
Catastrophic Complications
Spinal Cord Infarction
Spinal cord infarction results from intra-arterial injection of particulate steroid into a radicular artery supplying the anterior spinal artery. The artery of Adamkiewicz is the most critical vessel; its occlusion leads to anterior spinal artery syndrome with motor paralysis, loss of pain and temperature sensation, and bowel and bladder dysfunction below the level of the lesion. This complication has been reported at cervical, thoracic, and lumbar levels. Prevention requires using non-particulate steroids, confirming epidural spread with contrast under live fluoroscopy or DSA, aspirating before injection, and injecting slowly with serial aspiration.
Other Serious Complications
Direct needle trauma can cause spinal cord or nerve root injury, which is minimized by proper fluoroscopic technique and maintaining sedation levels light enough that the patient can report paresthesia. Epidural hematoma is a risk in patients on anticoagulants or with bleeding disorders. Infection (epidural abscess, meningitis) is rare with aseptic technique. Dural puncture can cause post-dural puncture headache. Allergic reactions to contrast, steroid, or local anesthetic are possible but uncommon.
<image>Safety protocol diagram illustrating the step-by-step safeguards for transforaminal epidural steroid injection: (1) proper needle placement confirmed on AP and lateral fluoroscopy, (2) aspiration test for blood or CSF, (3) contrast injection under live fluoroscopy with epidural spread confirmation, (4) digital subtraction angiography when available, (5) non-particulate steroid selection decision tree, and (6) slow injection with intermittent aspiration, with red warning boxes highlighting the critical decision points where intravascular uptake or intrathecal spread requires needle repositioning or procedure termination</image>
Clinical Pearls
The transforaminal approach delivers medication to the ventral epidural space more reliably than the interlaminar approach, making it the preferred technique for foraminal and lateral disc herniations compressing a specific nerve root. Non-particulate steroid (dexamethasone) must always be used for cervical transforaminal injections -- there is no justification for particulate steroid at this level. Intravascular uptake occurs in up to 23% of lumbar transforaminal injections, so contrast must always be injected under live fluoroscopy, with DSA used when available to detect subtle vascular patterns. The needle tip should never advance medial to the mid-pedicular line on AP view to avoid entering or contacting the thecal sac and cord. The patient should be maintained at a sedation level that allows reporting of paresthesia or pain during needle advancement, because general anesthesia eliminates this critical safety feedback.
References
- Rathmell JP, Benzon HT, Dreyfuss P, et al. Safeguards to prevent neurologic complications after epidural steroid injections: consensus opinions from a multidisciplinary working group and national organizations. Anesthesiology. 2015;122(5):974-984.
- Scanlon GC, Moeller-Bertram T, Romanowsky SM, Wallace MS. Cervical transforaminal epidural steroid injections: more dangerous than we think? Spine. 2007;32(11):1249-1256.
- Manchikanti L, Malla Y, Wargo BW, et al. A prospective evaluation of complications of 10,000 fluoroscopically directed epidural injections. Pain Physician. 2012;15(2):131-140.
- Benzon HT, Chew TL, McCarthy RJ, Benzon HA, Walega DR. Comparison of the particle sizes of different steroids and the effect of dilution: a review of the relative neurotoxicities of the steroids. Anesthesiology. 2007;106(2):331-338.
