How are fluoroscopic views used for S1 through S4 foramina?

Fluoroscopic Views for S1-S4 Foramina Visualization

Fluoroscopic guidance for S1 through S4 foramina visualization requires specific angles and anatomical landmarks to ensure safe and accurate needle placement during interventional procedures.

Optimal Fluoroscopic Views for Sacral Foramina

S1 Foramen

• Anteroposterior (AP) View: Traditional approach but has limitations
• Oblique View (25-30°): Preferred approach that reveals the "S1 Scotty dog" anatomical landmark 1
  • The oblique view significantly improves visualization of the S1 foramen
  • Often requires caudal tilt to properly visualize the foramen 1
  • Increases the visible transverse dimension of the foramen from 6.23mm (PA view) to 7.90mm (oblique view) 2

Medial vs. Lateral Approach for S1

• Medial approach is safer with significantly lower intravascular injection rates (4.9% vs 38.6%) 3
• Lateral approach has higher risk of vascular complications
• Important anatomical consideration: A bony flange covers approximately one-third of the S1 foramen on its medial aspect 2

S2-S4 Foramina

• AP view with caudal tilt: Helps visualize the lower sacral foramina
• Oblique views (20-30°): May improve visualization of these smaller foramina
• Sequential visualization moving caudally from S1

Technical Considerations

Radiation Safety

• Use intermittent-pulse fluoroscopy to minimize radiation exposure 4
• Employ the ALARA principle (As Low As Reasonably Achievable) 4
• Minimize beam-on time and use optimal collimation 4
• Vary the site of radiation entrance port during longer procedures 4
• Monitor radiation dose, especially when exceeding 4 Gy to the interventional reference point 4

Procedural Techniques

• Needle Placement:

  • For S1: Target the lateral aspect of the foramen to avoid the bony flange 2
  • Safe depth for S1 neural foramen is approximately 27.0 ± 2.1 mm 5
  • S1 neural foramen is located caudal to the L5 pedicle at approximately 1.7 times the distance between L4 and L5 pedicles 5

• Contrast Administration:

  • Use digital subtraction when available to identify vascular uptake
  • Lower contrast volumes (1.46 ± 0.48 mL) are required when using the medial approach 3

Clinical Applications

Transforaminal Epidural Steroid Injections

• Looking for the "S1 Scotty dog" facilitates predictable visualization of the foramen 1
• Medial needle placement provides better epidural flow of contrast 1
• Simultaneous visualization allows for needle placement to both L5 and S1 foramina in one view 1

Sacroiliac Joint Procedures

• 2D-fluoroscopic guidance is safe and efficient for iliosacral screw osteosynthesis 6
• Requires thorough preoperative evaluation of sacral morphology 6
• AP, lateral, inlet and outlet views must allow recognition of all anatomical landmarks 6

Potential Complications and Pitfalls

• Anatomical Variations: The bony flange at S1 dorsal foramen may be invisible on fluoroscopy due to minimal cortical bone 2
• Vascular Risks: Higher rates of intravascular injection during S1 procedures compared to lumbar levels 3
• Technical Challenges:
  • Ligamentous tissue extending from the bony flange covers the remainder of the foramen 2
  • Double screw osteosynthesis in S1 should be approached with caution 6

By understanding these specific fluoroscopic approaches and anatomical considerations, clinicians can optimize visualization of the S1-S4 foramina while minimizing procedural risks and radiation exposure.