Imaging guidance and diagnostics
Safe execution of spinal injections is impossible without radiological navigation. There are two main methods of guidance:
- Computed tomography (CT): provides superior soft-tissue contrast resolution.
- Fluoroscopy: performed faster and allows real-time tracking of needle advancement and contrast distribution.
Before administering the therapeutic agent, a test injection of a contrast medium is mandatory. It is critically important for the physician to distinguish between two diagnostic patterns:
- Epidurography (Epidurography): contrast spreads within the epidural fat. In the anteroposterior view, it outlines the exiting nerves beyond the intervertebral foramina; in the lateral view, it is located between the thecal sac (thecal sac) and the posterior longitudinal ligament.
- Myelography (Myelography): occurs with accidental intrathecal injection. The contrast mixes with the cerebrospinal fluid, outlining the subarachnoid space and nerve root sleeves within the spinal canal.
Approaches and technique
The choice of approach depends on the clinical presentation and the localization of the pathology (e.g., disc herniation).
Interlaminar approach Performed with the patient in the prone or lateral decubitus position (fetal position). A 22-G spinal needle or an 18–20-G Tuohy needle is used. The needle passes through the interspinous ligament and the ligamentum flavum. The ligamentum flavum consists predominantly of collagen fibers and offers strong resistance. Entry into the dorsal epidural space is verified by the loss-of-resistance technique and contrast injection.
Transforaminal approach Used for foraminal and extraforaminal herniations. The X-ray tube is rotated into an oblique projection to obtain the "scotty dog" (scotty dog) silhouette. The needle is directed into the dorsocranial quadrant of the intervertebral foramen. The anatomic landmark is the «safe triangle» (Safe triangle):
- Cranial boundary: pedicle of the vertebra and transverse process.
- Lateral boundary: line between the lateral margins of the superior and inferior pedicles.
- Inferomedial boundary: spinal nerve root.
Depth control is performed in the lateral projection to avoid paresthesia.
In complex cases, such as marked post-surgical scarring (failed back surgery syndrome), the caudal approach via the sacral hiatus is used.
Complications and selection of steroid preparations
Although the procedure is considered safe (50–70% efficacy), it carries certain risks. A frequent complication (2.5%) is inadvertent puncture of the dura mater, causing transient headache.
The most formidable and catastrophic complication is paraplegia (or tetraplegia/death in cervical injections). The pathogenesis is associated with the use of suspension (microparticulate) steroids (triamcinolone, betamethasone, methylprednisolone) during transforaminal injections. Accidental intra-arterial injection of the suspension causes embolization of a radicular artery (e.g., the artery of Adamkiewicz — arteria radicularis magna), leading to irreversible ischemic myelitis.
To minimize this risk at the cervical and upper lumbar levels, the drug of choice is dexamethasone, a non-suspension steroid that does not form microparticles. Modern guidelines require the use of high-quality fluoroscopy (ideally digital subtraction angiography) to immediately detect a vascular contrast distribution pattern and correct the needle position.
Pulsed radiofrequency ablation (PRF)
In cases of chronic radiculopathy, when the efficacy of steroids alone diminishes, pulsed radiofrequency ablation of the dorsal root ganglion is used.
Unlike thermal continuous RFA (which destroys the nerve), PRF operates at a temperature not exceeding 42°C and does not cause histological destruction. The mechanism of action is based on neuromodulation—exposure to an intense electric field (up to 185,000 V/m), which induces long-term depression of synaptic transmission. This effectively relieves neuropathic pain without side effects.
Briefly: answers to topical questions
Through which ligaments does the needle pass during the interlaminar approach?
When performing an epidural injection via the interlaminar approach, the needle sequentially passes through two main ligaments.
- Interspinous ligament (interspinous ligament) — crossed at the initial stage of instrument advancement.
- Ligamentum flavum (ligamentum flavum) — consists almost entirely of collagen fibers and offers the greatest resistance to the needle before entering the dorsal epidural space.
In which cases is the caudal approach via the sacral hiatus used?
The caudal approach via the sacral hiatus (sacral hiatus) is used as an alternative method under fluoroscopic guidance. It is indicated in the following cases:
- In complex clinical situations.
- With marked postoperative epidural scarring.
- In failed back surgery syndrome (Failed back surgery syndrome — FBSS) for catheter advancement and more selective infiltration of glucocorticosteroids.
What complications can occur during spinal injections?
Various side effects can occur during spinal injections, which are divided into several groups.
- Frequent complications include inadvertent dural puncture (dura mater), causing transient headache, and temporary exacerbation of radicular pain.
- Rare complications — arachnoiditis, aseptic or infectious meningitis, epidural abscess, and systemic steroid side effects.
- Catastrophic complications — paraplegia during lumbar injections (due to ischemia of the terminal spinal cord), tetraplegia, or death during cervical interventions.
To remember the boundaries of the "safe triangle" for transforaminal injection, imagine a tent: the roof is bones (pedicle and process), the wall is the line between the pedicles, and the floor is the spinal nerve itself.
Class and exam questions
Why should suspension steroids not be used for transforaminal injections in the cervical and upper lumbar regions?
Suspension steroids (e.g., betamethasone) contain microparticles. If accidentally introduced into a feeder artery, they cause its embolization and spinal cord ischemia, which can lead to paraplegia.
Why is a test dose of contrast administered before the drug injection?
It is necessary to rule out accidental intravascular or intrathecal (subarachnoid) placement of the needle tip, as well as to visualize the spinal nerve.
What does the physician feel when the needle passes through the ligamentum flavum?
The physician feels strong resistance because the ligament consists of dense collagen fibers. Upon penetration into the epidural space, the resistance abruptly disappears (loss-of-resistance technique).
What else to study on this topic
- Anatomic variants of the artery of Adamkiewicz and spinal cord blood supply
- Intra-articular facet injections and medial branch block
- Failed back surgery syndrome (FBSS) and features of the caudal approach
- Comparison of pulsed (PRF) and thermal (CRF) radiofrequency ablation in pain management
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