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Chapter Summary
• Context: Axial back pain is extraordinarily common, but only a fraction of patients have an underlying inflammatory etiology. Identifying this subset is vital because axial spondyloarthritis (axSpA) is a chronic inflammatory disorder capable of causing persistent pain, progressive stiffness, functional limitation, and structural ankylosis. Its pathophysiology involves complex interactions between genetic predisposition (notably HLA-B27), environmental triggers, biomechanical stress, and immune activation centered at the entheses, where inflammation and pathological new bone formation occur in tandem. Diagnosis can be difficult because no single laboratory test or imaging modality independently confirms or excludes the disease. Clinical history, peripheral and extra-articular manifestations, HLA-B27, acute phase reactants, and sacroiliac imaging must be interpreted in context. MRI has enhanced the detection of active inflammation prior to radiographic damage, but carries risks of overinterpretation. This chapter emphasizes integrated diagnosis and multidisciplinary care between rheumatology, radiology, physical therapy, and spine surgery.
• Chapter Objective: Recognize the inflammatory pattern of axial spinal involvement with emphasis on axial spondyloarthritis; understand enthesis-centered pathophysiology; accurately interpret clinical features, laboratory markers, and radiographic/MRI findings of the sacroiliac joints; differentiate mimicking conditions; and understand conservative management principles and indications for surgical evaluation.
• Enthesis, inflammation, and new bone formationThe enthesis is the primary site of initial inflammatory cascade. In genetically predisposed individuals, biomechanical stress and environmental factors trigger immune activation and cytokine release. Persistent inflammation drives both subchondral bone erosions and subsequent new bone formation (syndesmophytes), explaining the co-occurrence of structural damage and progressive ankylosis in axSpA.
• The clinical pattern mattersInflammatory back pain typically features insidious onset before age 45, chronic duration, morning stiffness lasting >30 minutes, improvement with exercise, and lack of relief with rest. Table 59.1 compiles clinical criteria sets (ASAS, Calin, Berlin). Peripheral arthritis, enthesitis, and dactylitis, along with extra-articular features (anterior uveitis, psoriasis, inflammatory bowel disease), complete the spondyloarthritis spectrum.
• Physical examination and its limitationsMobility measurements (Schober test, chest expansion, occiput-to-wall distance) document functional restriction, but provocative sacroiliac joint tests lack sufficient sensitivity and specificity to confirm or exclude sacroiliitis. In advanced disease, postural abnormalities reflect rigid spinal ankylosis.
• Integrated diagnosisHLA-B27 increases post-test probability in appropriate clinical scenarios, but its absence does not exclude axSpA. Inflammatory markers (CRP, ESR) can be normal in up to 50% of active cases. Plain radiography of sacroiliac joints remains key for identifying structural damage (New York criteria, Figure 59.1 and Table 59.2). When clinical suspicion is high and radiographs are negative or equivocal, MRI can demonstrate active subchondral bone marrow edema (Figure 59.2, Table 59.3).
• Avoiding imaging overdiagnosisBone marrow edema alone does not automatically equate to axial spondyloarthritis. Mechanical stress, sports, osteitis condensans ilii, degenerative changes, and infection can produce similar findings. Sacroiliac MRI must be interpreted strictly in light of clinical context, lesion anatomical distribution (periarticular vs non-joint areas), and presence of accompanying structural lesions.
• TreatmentStructured exercise, physical therapy, and lifestyle modifications form the foundation across all stages. Pharmacological therapy, managed by rheumatologists, progresses from NSAIDs to biologic DMARDs (TNF inhibitors, IL-17 inhibitors) and targeted synthetic agents (JAK inhibitors) based on disease activity. Surgery is reserved for severe structural consequences: unstable fractures, fixed kyphotic deformities, or secondary neurological deficits.
• Clinical Application: In clinical practice, the first challenge is differentiating mechanical back pain from an inflammatory pattern. A young adult presenting with chronic low back pain, prolonged morning stiffness, and relief with activity warrants targeted investigation even if lumbar radiographs appear unremarkable. HLA-B27 and acute phase reactants should be interpreted as probabilistic tools rather than binary tests. Sacroiliac MRI interpretation requires caution: nonspecific bone marrow edema in runners or postpartum women should not be misdiagnosed as lifelong spondyloarthritis without clinical concordance. Early and accurate diagnosis enables the timely initiation of targeted physical therapy and anti-inflammatory or biologic therapy before irreversible syndesmophyte bridging occurs. When severe fixed deformity or spinal fractures occur, the spine surgeon joins an established multidisciplinary management plan, ensuring systemic inflammation is controlled perioperatively.
