C1-2 (Atlantoaxial) Injections & C2 Nerve Blocks

C1-2 (Atlantoaxial) Joint Injection Technique

The atlantoaxial (AA) joint injection is a technically demanding procedure that requires thorough understanding of craniocervical junction anatomy and careful attention to the vertebral artery.

Step-by-Step Procedure

1. Patient positioning: Prone or lateral decubitus
2. Imaging guidance: Fluoroscopy (AP and lateral views) or CT guidance
3. Target identification: The C1-2 lateral atlantoaxial joint — identified on AP view between the lateral mass of C1 and the superior articular surface of C2
4. Approach: Posterolateral — the needle is advanced to the posterior aspect of the C1-2 joint
5. Needle selection: 25-gauge spinal needle
6. Arthrogram: Inject contrast to confirm intra-articular position
7. Injection: Small volume — 0.5-1.0 mL of local anesthetic and steroid

Safety Considerations

The primary safety concern is the vertebral artery, which courses lateral and then posterior to the C1-2 lateral atlantoaxial joint. After exiting the C2 transverse foramen, the artery loops laterally and posteriorly around the lateral mass of C1 (in the vertebral artery groove on the superior surface of the C1 posterior arch) before turning medially to enter the foramen magnum. This means the artery sits lateral and posterolateral to the LAAJ in its V3 segment, making lateral and posterolateral needle trajectories the highest-risk approaches:

• Avoid the lateral and posterolateral periphery of the joint: The V3 segment of the vertebral artery loops laterally and then posteriorly around the C1 lateral mass before crossing the superior surface of the C1 posterior arch. Lateral or far-posterolateral needle trajectories can encounter the artery.
• Target the lateral half of the joint, not the lateral margin: The IPSIS-recommended target is the junction of the medial two-thirds and lateral one-third of the joint — medial to where the artery loops, but lateral to the thecal sac and cord
• Small volumes: Use minimal injectate to avoid any mass effect
• CT guidance: May provide superior safety for C1-2 injections due to better visualization of the vertebral artery position

IPSIS Technical Details: Lateral Atlantoaxial Joint (LAAJ) Injection

The IPSIS manual provides highly detailed procedural guidance reflecting the anatomic complexity of this injection:

Pre-Procedure Imaging is Essential:

• Advanced imaging (CT or MRI) must be reviewed before the procedure to identify the position of the vertebral artery, thecal sac, and C2 ganglion relative to the target joint
• In fewer than 1% of cases, a vertebral artery loop may course over the posterolateral aspect of the C1-2 joint directly in the needle path — this is a contraindication
• The thecal sac typically covers the medial half of the C1-2 lateral articulation
• The C2 dorsal root ganglion sits directly posterior to the joint

Narrow Safe Corridor:

• The needle must navigate medial to the vertebral artery, lateral to the thecal sac, and lateral or inferior to the C2 DRG
• The safe corridor of access is approximately the lateral half of the joint (junction of medial two-thirds and lateral one-third to the lateral joint margin)
• This creates a very constrained pathway requiring meticulous needle manipulation

Fluoroscopic Target Acquisition:

• On AP imaging, target the junction of the medial two-thirds and lateral one-third of the joint (either C1 inferior articular process or C2 superior articular process)
• The C1-2 joint is noncongruent (C2 articular pillar is convex superiorly), so it is seen crisply over an arc of fluoroscopic rotation
• To find the posterior joint: decline the fluoroscope until the lucent joint space is lost, then rotate cranially until joint lucency and crisp cortical margins are first acquired — this images the posterior aspect the needle must enter
• True AP is verified by centering the dens within the ring of C1 and ensuring C2 pedicles are equidistant from the spinous process

Needle Advancement Strategy:

• Use a 25-27 gauge needle advanced slowly in small increments
• A caudal-to-cranial needle trajectory is recommended to pass underneath the C2 ganglion and avoid the posterior arch of C1
• If neural stimulation is encountered, adjust to a more caudal trajectory
• Multiple needle passes should be avoided to prevent injury to the prominent venous plexus along the posterior C1-2 joint
• Intermittent true lateral views confirm appropriate trajectory and depth

Arthrogram and Injectate:

• Joint volume is frequently less than 1.0 mL (especially in arthritic joints)
• Confirm intra-articular position with only 0.1-0.2 mL of contrast to preserve capacity for diagnostic/therapeutic injectate
• On lateral fluoroscopy, intra-articular contrast flows anteriorly toward the anterior joint recess (may show a meniscoid)
• On AP, contrast localizes between the cortical surfaces of the joint
• If contrast extravasates posteriorly into the epidural space, do not deliver the therapeutic injectate
• Total injectate volume should not exceed 1 mL

Steroid Selection:

• Nonparticulate dexamethasone is strongly recommended due to the needle's proximity to the vertebral artery
• Avoid mixing ropivacaine with dexamethasone as this causes injectate precipitation
• Consider aspirating contrast from the needle hub and replacing with therapeutic injectate to maximize medication delivery within the small joint volume

Practical Challenges:

• Dental fillings and appliances frequently obscure visualization — open-mouthed views or slight head rotation away from the injection side may help
• Overlying C1 posterior arch may cover the target trajectory; caudal-to-cranial angulation helps clear this
• In osteoarthritic joints, the joint space may be nearly obliterated, requiring entry at the most lateral aspect

C2 Nerve Block / Greater Occipital Nerve Block

These are related but distinct procedures:

• C2 nerve block: Targets the C2 dorsal root ganglion or C2 medial branch at the C1-2 level; performed under fluoroscopic guidance. More invasive but more specific.
• Greater occipital nerve (GON) block: Can be performed with landmark/ultrasound guidance at the level of the superior nuchal line. Less invasive than C2 DRG block but targets the more distal nerve.
• C2-3 facet joint injection: The C2-3 facet (third occipital nerve) is another common source of cervicogenic headache — should be considered in the differential.

C1-2 Joint Examination

Physical exam includes testing C1-2 rotation:

• Rotate the patient's head while stabilizing the C2 spinous process
• Pain provoked with this maneuver suggests AA joint pathology
• This isolates rotation at the C1-2 segment from lower cervical motion