What are the step‑by‑step instructions for performing a transforaminal epidural steroid injection (TFESI) at the S1 level under fluoroscopic guidance in an adult patient with S1 radicular pain and no contraindications?

S1 Transforaminal Epidural Steroid Injection: Step-by-Step Fluoroscopic Technique

The S1 TFESI should be performed using either the oblique "Scotty dog" approach or a hybrid ultrasound-fluoroscopy technique, both of which provide faster needle placement and adequate epidural spread compared to traditional anteroposterior approaches. 1, 2

Pre-Procedure Requirements

Patient Selection Verification

• Confirm MRI evidence of S1 nerve root compression correlating with clinical radicular symptoms (pain radiating below the knee) 3
• Document failure of at least 4-6 weeks of conservative therapy including physical therapy 3
• Verify fluoroscopic guidance availability, which is mandatory for all transforaminal injections to ensure proper needle placement and minimize complications 3, 4

Informed Consent Discussion

• Discuss specific risks including dural puncture, insertion-site infections, cauda equina syndrome, sensorimotor deficits, discitis, epidural granuloma, retinal complications, and rare catastrophic complications including paralysis and death 3, 4
• Explain that the procedure is part of multimodal pain management, not a standalone treatment 3

Equipment Setup

• Sterile preparation tray with local anesthetic 3
• 22-25 gauge spinal needle (3.5-5 inch) 1
• C-arm fluoroscope positioned for optimal visualization 3
• Non-ionic contrast medium 1
• Steroid medication mixed with local anesthetic 3
• Optional: Ultrasound machine for hybrid technique 1

Step-by-Step Procedure

Step 1: Initial C-Arm Positioning

Oblique "Scotty Dog" Approach (Preferred for Speed):

• Rotate the C-arm ipsilaterally to visualize the S1 "Scotty dog" view 2
• This approach reduces needle passage time (24.4 seconds vs 47.8 seconds) and total procedure time (93.3 seconds vs 160 seconds) compared to AP approach 2
• Note that obese patients (higher BMI) may require longer procedure times 2

Alternative Hybrid Ultrasound-Fluoroscopy Technique:

• Use ultrasound initially to identify sacral foramen anatomy and mark the needle entry point 1
• Switch to fluoroscopy for confirmatory imaging and final needle positioning 1
• This technique reduces radiation exposure while maintaining accuracy 1

Step 2: Skin Entry Point Marking

• Mark the skin entry point using fluoroscopic guidance at the target trajectory 3
• For oblique approach: target the superior-anterior aspect of the S1 foramen 3
• The typical cephalad angle is 16.25° ± 6.75° (range 5-27°) and oblique angle is 2.48° ± 2.62° (range 0-7°) 1

Step 3: Local Anesthesia

• Infiltrate the skin and subcutaneous tissues with local anesthetic along the anticipated needle trajectory 3
• Maintain strict sterile technique throughout 3

Step 4: Needle Advancement

• Insert the needle at the predetermined angle toward the superior-anterior aspect of the S1 neural foramen 3
• Advance the needle incrementally with intermittent fluoroscopic confirmation 3
• The mean number of needle insertion attempts should be approximately 1.24 ± 1.25 1

Step 5: Needle Position Confirmation

• Obtain AP fluoroscopic view: confirm needle tip is at the superior-anterior aspect of the foramen 3
• Obtain lateral fluoroscopic view: verify needle tip position relative to the sacral anatomy 3
• Document the final needle position 3

Step 6: Contrast Injection and Vascular Check

• Aspirate to check for blood or cerebrospinal fluid 3
• Inject 0.5-1.0 mL of non-ionic contrast medium under live fluoroscopy 1
• Confirm negative intravascular flow (no vascular uptake pattern) 3, 1
• Observe contrast spread along the S1 nerve root and into the epidural space 3
• All successful injections should exhibit epidural spread of contrast media 1

Step 7: Therapeutic Injection

• After confirming appropriate contrast spread and negative vascular uptake, inject the steroid-local anesthetic mixture 3
• Document the contrast spread pattern (should include lateral recess and anterior epidural space) 3

Step 8: Post-Procedure Documentation

• Record final needle position with saved fluoroscopic images 3
• Document contrast spread pattern 3
• Note total radiation exposure and procedure time 2

Critical Safety Considerations

Vascular Injection Risk

• The incidence of intravascular injection during S1 TFESI ranges from 11.4% to 22.8% 2
• Always perform real-time fluoroscopy during contrast injection to detect vascular uptake 1

Radiation Exposure

• The oblique "Scotty dog" approach may result in higher radiation exposure (80.8 ± 26.8 Cgy/cm²) compared to other approaches 5
• The hybrid ultrasound-fluoroscopy technique mitigates radiation exposure 1

Anatomical Variations

• The S1 foramen anatomy can vary significantly between patients 1
• Ultrasound pre-visualization can help identify anatomical variations before needle insertion 1

Expected Outcomes

• Patients should be counseled that pain relief assessment periods typically range from 2 weeks to 3 months 3
• Repeat injection is only appropriate if the initial injection resulted in at least 50% pain relief lasting at least 2 months 3