Facet Arthropathy

Definition Facet arthropathy (zygapophyseal joint osteoarthritis) is degenerative disease of the posterior synovial joints of the spine, characterized by cartilage loss, subchondral sclerosis, osteophyte formation, and capsular hypertrophy. It is a major contributor to axial mechanical low back pain and spinal stenosis.

Epidemiology

• Prevalence: Present in 40-85% of adults over 60 on imaging (many asymptomatic).
• Contribution to LBP: Facets responsible for 15-40% of chronic low back pain cases.
• Level Distribution: L4/5 (most common) greater than L5/S1 greater than L3/4.
• Progression: Accelerated by obesity, heavy labor, prior spine surgery (adjacent segment disease).

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Facet Joint Anatomy

• Type: Synovial diarthrodial joint with hyaline cartilage.
• Orientation: Sagittal in lumbar spine (resists rotation), coronal in thoracic (allows rotation).
• Innervation: Dual innervation by medial branches of dorsal rami from levels above and below (e.g., L4/5 facet innervated by L3 and L4 medial branches).
• Capsule: Richly innervated with mechanoreceptors and nociceptors.

Biomechanical Function

• Load Sharing: Facets bear 0-16% of axial load in neutral position, increasing to 40% in extension.
• Motion Control: Primary resistors of rotation and excessive translation.
• Stability: Prevent anterior translation of superior vertebra (resist shear).

Degenerative Cascade

Facet arthropathy develops through a well-defined degenerative cascade:

1. Initiation: Primary cartilage wear from aging, repetitive loading, or trauma triggers chondrocyte dysfunction.
2. Synovial Inflammation: Cartilage breakdown products stimulate synovitis, causing pain and joint effusion.
3. Subchondral Changes: Bone sclerosis and subchondral cyst formation develop as cartilage protection is lost.
4. Osteophyte Formation: Marginal osteophytes form as a compensatory response to increased stress.
5. Capsular Hypertrophy: Joint capsule thickens, contributing to lateral recess stenosis.
6. Instability: Advanced degeneration may lead to subluxation and dynamic instability.

Disc-Facet Relationship

The disc and facet joints form a "three-joint complex" at each spinal level:

• Disc degeneration leads to loss of height, increasing facet joint load
• Increased facet load accelerates facet degeneration
• Facet degeneration causes posterior element instability
• Instability further accelerates disc degeneration (vicious cycle)

Stenosis Mechanism

Facet hypertrophy contributes to spinal stenosis through:

• Central Stenosis: Bilateral facet hypertrophy with ligamentum flavum thickening creates "trefoil" canal
• Lateral Recess Stenosis: Superior articular process hypertrophy compresses traversing nerve root
• Foraminal Stenosis: Subluxation and osteophytes narrow the neural foramen

Both the interventional pathway and the viva follow-ups invoke "conventional versus cooled RFA" without defining the difference. The distinction is one of lesion geometry, which in turn dictates how the electrode must be positioned relative to the medial branch.

Conventional (thermal) RFA heats the active tip to roughly 80-90°C for 60-90 seconds. The thermal lesion forms radially around the shaft of the active tip and extends only minimally beyond its end. Because the hottest zone hugs the side of the cannula, the electrode must be placed parallel and tangential to the nerve as it crosses the SAP-TP junction, to capture the greatest length of medial branch. Lesion size grows with a larger-gauge cannula, a longer active tip, higher temperature, and longer duration; several overlapping lesions are often made to allow for anatomical variability in the exact course of the nerve.

Cooled (water-cooled) RFA circulates chilled water through the electrode to prevent charring and the impedance rise that limits conventional lesions. This deposits more energy and creates a larger, more spherical lesion that projects distally beyond the tip (typically run at a set tip temperature near 60°C). The larger lesion volume permits a more perpendicular approach and, in theory, more reliable capture of a variably located nerve, at the cost of longer procedure time and higher consumable cost. Comparative trials are limited and have not established clear clinical superiority of one modality over the other.

Lesioning discipline — regardless of modality, sensory stimulation (50 Hz, seeking concordant paraesthesia at low voltage) and motor stimulation (2 Hz, confirming the absence of lower-limb muscle contraction that would signal proximity to the ventral ramus) are performed before ablation to confirm safe, accurate targeting.

Pulsed RFA (bursts keeping tissue below roughly 42°C, non-neurodestructive) is generally considered insufficient for durable medial branch denervation and is not the standard for facet pain; continuous thermal or cooled lesioning is preferred.

The interventional pathway develops only medial branch blocks, yet the FACTS trial, the multispecialty consensus, and the guideline table all contrast intra-articular (IA) facet injection with medial branch block (MBB). Distinguishing the two — and knowing why one is chosen before ablation — is a common viva point.

• Intra-articular facet injection places local anaesthetic (with or without corticosteroid) directly into the facet joint capsule under fluoroscopic or CT guidance. Joint capacity is small (approximately 1-2 mL); over-distension can rupture the capsule. It anaesthetises the joint itself and carries a therapeutic aspiration (the steroid), though evidence for durable benefit is weak.
• Medial branch block anaesthetises the medial branch nerves that supply the joint, not the joint cavity, using a small volume (0.5-1 mL) at the SAP-TP junction of the two segmental nerves involved (dual innervation).

Why MBB is preferred as the prognostic test before RFA: the medial branch is the exact structure that radiofrequency ablation denervates, so a positive MBB maps directly onto the RFA target, whereas a positive IA injection does not. The 2020 multispecialty consensus concluded that MBB is more predictive of RFA success than IA injection. In the FACTS RCT, positive-block rates were similar for IA (54%) and MBB (55%), and post-RFA responder rates were also similar (51% for IA vs 56% for MBB) — but neither block was therapeutic, reinforcing that blocks are used to select candidates rather than to treat.

Conservative Management:

• 40-60% achieve satisfactory improvement with PT, NSAIDs, and activity modification.
• Better outcomes in younger patients (under 50), shorter symptom duration (under 6 months).

Radiofrequency Ablation:

• 60-70% report 50% or greater pain reduction at 6-12 months.
• Average duration of relief: 9-12 months.
• Repeat RFA often effective but diminishing returns after third ablation.

Surgical Fusion:

• 70-80% good to excellent outcomes when performed for structural indications (stenosis, instability).
• Poorer outcomes for isolated facet pain without structural pathology (30-50% satisfaction).

Global Epidemiology

• Facet osteoarthritis is near-universal with age: CT prevalence rises from roughly 24% under 40 to 89% at 60-69 years (Framingham cohort), with the highest burden at L4-L5.
• Imaging facet OA correlates poorly with symptoms; facet-mediated pain is estimated to account for 10-40% of chronic low back pain, the wide range reflecting differing diagnostic-block criteria.
• Lumbar fusion and facet-intervention rates vary several-fold between countries and even between regions of the same country, reflecting practice variation rather than disease burden.

The RFA efficacy debate Observational series and earlier reviews report 50% or greater relief in 60-70% of well-selected patients, but the two highest-quality pragmatic RCTs (FACTS 2018 and MINT 2017) found no clinically important benefit over sham/exercise. Critics argue MINT used a liberal 50% block threshold and possibly suboptimal lesioning; proponents of the RCTs argue prior series were confounded by selection bias and placebo response. The honest exam answer: RFA may help carefully selected patients but the strongest evidence is sobering, and routine use is not supported.

Diagnostic block threshold (50% vs 80%) No consensus exists. An 80% (dual-block) standard maximises specificity but yields more false negatives and is costly; a single 50% block (used in MINT) increases sensitivity but raises false positives to 30-40%. Guidelines deliberately leave this to local protocol.

Is "isolated facet pain" a real entity? Facet OA almost always coexists with disc degeneration (three-joint complex), and imaging facet OA does not correlate with pain. Whether a purely facet-mediated pain phenotype exists - and whether it ever justifies fusion - remains contested. Fusion for axial pain alone is widely discouraged.

Grading reliability Inter-rater agreement for the Fujiwara (MRI) and Pathria (CT) grading systems is only fair to moderate, limiting their use for treatment decisions.