Ankylosing Spondylitis: Pathogenesis and Radiographic Hallmarks

Early Radiographic Findings: Sacroiliitis

Bilateral, symmetrical inflammation of the sacroiliac joints is universally the initial absolute requirement for a radiographic diagnosis of AS.

• Pseudowidening: The earliest detectable plain film change. Inflammatory hyperemia causes subchondral bone resorption, making the joint space appear falsely widened and ill-defined.
• Subchondral Erosions: Irregular 'postage stamp' erosions aggressively develop, predominantly eroding the thinner iliac side of the complex SI joint before affecting the thicker sacral side.
• Reactive Sclerosis: As the body attempts to heal the lesions, thick, opaque bands of dense reactive osteosclerosis aggressively form adjacent to the erosions.
• Ankylosis: In advanced severe stages, the erosions are entirely bridged by new bone, completely obliterating the joint space and physically fusing the sacrum to the ilium.

Spinal Manifestations

The inflammatory cascade systematically ascends the spine, typically progressing predictably from the lumbar region directly upward to the cervical spine.

• Romanus Lesions and Shiny Corners: Early focal inflammatory erosions specifically located at the superior and inferior anterior margins of the vertebral bodies (Romanus lesions). Following healing, these eroded corners become distinctly sclerotic, appearing intensely white on radiographs (the 'shiny corner' sign).
• Squaring of Vertebrae: Chronic anterior longitudinal ligament inflammation stimulates bone resorption along the normally concave anterior vertebral borders, rendering the affected vertebral bodies structurally box-like and exceptionally 'square' on lateral views.
• Syndesmophytes: The hallmark of AS. Delicate, thin, vertically oriented continuous sheets of ossification that bridge adjacent vertebral bodies by directly tracking along the outer annular fibers of the intervertebral discs.
• The Bamboo Spine: An extremely late, classic, and often devastating radiographic appearance. The continuous, uninterrupted formation of bridging syndesmophytes completely fuses the entire spinal column into a single solid, rigid bony tube resembling a mature bamboo stalk.