The zygapophyseal joints, commonly known as facet joints, are a frequent source of chronic axial spine pain. It is estimated that facet joints contribute to approximately 15% to 45% of cases of chronic low back pain. Unlike radicular pain, facet-mediated pain is typically “mechanical” or “pseudo-radicular,” making the clinical history and physical exam crucial for differentiating it from discogenic or sacroiliac pathology.

1. Neuroanatomy: The Medial Branch

Understanding the innervation of the facet joint is the most high-yield anatomical concept for the pain board exam. Each facet joint receives dual innervation from the medial branches of the posterior primary rami.

• The Rule of Two: Each joint is supplied by the medial branch at the same level and the medial branch from one level above.
  • Example: The L4-L5 facet joint is innervated by the L3 medial branch and the L4 medial branch.
• The Path of the Medial Branch: The nerve travels across the junction of the transverse process and the superior articular process (the “eye of the Scottie dog” on oblique fluoroscopy).
• The “Exception” Level: At the L5-S1 level, the joint is innervated by the L4 medial branch and the L5 dorsal ramus, which is found in the groove between the ala of the sacrum and the superior articular process of S1.

2. Clinical Presentation and Physical Exam

Facet pain is generally described as a dull, aching sensation in the axial spine that can radiate into the buttocks or thighs, but rarely below the knee (unlike true sciatica).

• Aggravating Factors: Extension, lateral rotation, and prolonged standing. These maneuvers decrease the volume of the joint and increase the load on the articular surfaces.
• Relieving Factors: Flexion (sitting or leaning forward) often provides relief as it “opens” the joint.
• Physical Exam Maneuver: The Kemp Test (or Facet Loading Test) involves extending and rotating the spine toward the side of the pain to provoke symptoms. While common, its specificity is relatively low, emphasizing the need for diagnostic injections.

3. The Diagnostic Protocol: Medial Branch Blocks (MBB)

Because imaging (X-ray, CT, or MRI) correlates poorly with the actual source of pain—many asymptomatic individuals have “severe” facet arthrosis—diagnostic injections are the gold standard.

The Dual-Block Paradigm

To minimize the “false-positive” rate (which can be as high as 30-40% due to the placebo effect), most guidelines and insurance carriers require two successful diagnostic blocks:

1. Screening Block: Using a short-acting anesthetic (e.g., Lidocaine).
2. Confirmatory Block: Using a long-acting anesthetic (e.g., Bupivacaine).

A “positive” response is typically defined as a $\ge$80% reduction in pain for the duration of the local anesthetic used. If the patient achieves this threshold twice, they are considered a candidate for Radiofrequency Ablation.

4. Therapeutic Intervention: Radiofrequency Ablation (RFA)

RFA, or Rhizotomy, uses high-frequency alternating current to generate thermal energy, creating a heat lesion that coagulates the medial branch nerve.

Procedural Physics

• Continuous (Thermal) RFA: The electrode tip is heated to 80°C for 60 to 90 seconds. This creates a prolate spheroid-shaped lesion.
• Needle Positioning: The needle must be placed parallel to the nerve, not perpendicular. Since the lesion grows radially from the uninsulated tip, parallel placement maximizes the area of the nerve affected by the “heat zone.”
• Grounding Pad: A grounding pad is required to complete the circuit. If the patient has a pacemaker or spinal cord stimulator, specific precautions must be taken to avoid electrical interference.

5. Potential Complications and “Red Flags”

While RFA is generally safe, board questions often focus on procedural errors:

• Motor Testing: Before lesioning, the physician must perform motor testing at 2.0V. If the patient’s leg twitches (multifidus twitch is expected and okay, but a lower extremity twitch is not), the needle is too close to the ventral ramus/spinal nerve and must be repositioned to avoid permanent motor deficit.
• Sensory Testing: Performed at 0.5V to ensure the needle is near the target sensory nerve.
• Post-RFA Neuritis: A common side effect where the patient experiences a “sunburn-like” sensation or increased pain for 1-2 weeks following the procedure. This is usually self-limiting and treated with NSAIDs or gabapentinoids.

6. PM&R Perspective: The Kinetic Chain

In your pain medicine review, it is important to note that facet arthropathy rarely occurs in isolation. It is often the result of Lower Crossed Syndrome, a postural pattern involving:

• Weak gluteals and abdominal muscles.
• Tight iliopsoas and spinal extensors.

This imbalance leads to an anterior pelvic tilt, which increases the lumbar lordosis and chronically “loads” the facet joints. Successful long-term management requires RFA to provide the “pain-free window,” followed by physiatric-led core stabilization and pelvic tilting exercises to prevent recurrence.

---

High-Yield “Board Pearls”

• The Medial Branch does not innervate the skin; therefore, RFA does not cause cutaneous numbness (if done correctly).
• The Multifidus Muscle is the only muscle innervated by the medial branch. Chronic RFA can lead to atrophy of this muscle, which is often seen on follow-up MRIs.
• Cervical Facet Pain: Follows a similar “Rule of Two,” but the Third Occipital Nerve (TON) is the target for the C2-C3 facet joint, which is a common cause of cervicogenic headaches.