Ankylosing Spondylitis - Bamboo Spine

A 38-year-old man with a 15-year history of insidious-onset low back pain, morning stiffness lasting >2 hours, and progressive loss of spinal mobility presents to the orthopedic clinic. HLA-B27 is positive, and inflammatory markers are elevated (CRP 32, ESR 56). Anteroposterior and lateral lumbar spine radiographs reveal the classic "bamboo spine" appearance with complete sacroiliac joint ankylosis. The structures marked **A** in the diagram (symmetric bridging syndesmophytes) are characteristic radiographic findings in this patient's condition. Which of the following best describes the pathophysiological mechanism underlying the formation of these structures marked **A**?