What are the anatomical landmarks for sacroiliac joint injection?

Anatomical Landmarks for Sacroiliac Joint Injection

For fluoroscopy-guided SI joint injections, use the inferior aspect of the joint as your primary target, accessed by rotating the C-arm to a contralateral oblique view of 5-20 degrees to superimpose the posterior and anterior joint lines, creating a lucency that represents your target. 1

Fluoroscopic Technique (Preferred Method)

Patient Positioning and C-Arm Setup

• Position patient prone
• Classic approach: Rotate the C-arm image intensifier to a contralateral oblique view of 5-20 degrees to superimpose the posterior and anterior SI joint lines, yielding a lucency that represents the target 1
• Alternative approach: If you cannot superimpose the joint lines despite exploring a range of oblique angles, the medial joint line may correspond to the posterior aspect of the joint 1

Specific Entry Point

• Target the inferior aspect of the joint (most commonly used approach) 1
• The needle entry point should be approximately 44 mm (1.75 inches) caudal to the posterior superior iliac spine (PSIS) 2
• Alternative access points include the mid-portion or superior-portion of the SI joint, though these are more challenging due to lack of synovium 1

Surface Landmark Relationships

• Horizontal distance from PSIS to back of superior SI joint: 43.9 mm (±5.6 mm) 2
• Horizontal distance from ASIS to front of superior SI joint: 75.4 mm (±8.4 mm) 2
• The joint extends 7.5 mm cephalad and 38.1 mm caudal to the PSIS 2
• The joint extends 35.4 mm cephalad and 10.2 mm caudal to the ASIS 2

Ultrasound-Guided Technique (Alternative)

Two Possible Access Levels

• Upper level: At the level of posterior sacral foramen 1 (S1)
• Lower level: At the level of posterior sacral foramen 2 (S2) - easier to access 3

The lower level (S2) shows higher success rates (90% vs 70% at upper level in cadaveric studies), but if joint alterations prevent visualization at the lower level, the upper level provides an appropriate alternative 3.

Critical Technical Considerations

Image Guidance is Essential

Palpation-guided injections have poor accuracy and should not be used 1. While fluoroscopy shows only modest improvement over anatomic palpation in pain outcomes, image guidance is necessary for diagnostic accuracy and to confirm intra-articular placement 1.

Fluoroscopy vs Ultrasound

Fluoroscopy is preferred because it allows:

• Confirmation of intra-articular needle placement with contrast
• Rule out intravascular needle tip position before injection 1
• Higher accuracy rates (65% vs 50% for ultrasound in confirming intra-articular placement) 1

Volume Considerations

The SI joint has an estimated volume of approximately 1.4 mL (range 0.6-2.7 mL) 1. Injecting beyond this volume means the injectate will reach adjacent unintended structures, potentially affecting therapeutic outcomes.

Common Pitfalls to Avoid

1. Avoid blind palpation-guided injections - they have unacceptably poor accuracy 1
2. Don't target only the superior joint - the inferior aspect is preferred and more accessible 1
3. Confirm contrast flow before injecting therapeutic agents - this ensures intra-articular placement and rules out vascular uptake 1
4. Be aware that cephalad extension of contrast may predict better short-term outcomes (at 2 weeks), though this difference disappears by 8 weeks 1