Diagnostic Challenges in a Novice Ultra-Cyclist: Transitioning From an Overuse Injury Hypothesis to HLA-B27–Negative Axial Spondyloarthritis: A Case Report

BACKGROUND: Ultra-endurance cyclists frequently present with overuse injuries, yet bone marrow edema at the medial ilium is highly uncommon and may indicate underlying pathology. CASE PRESENTATION: A 31-year-old cyclist developed atraumatic groin pain migrating to the gluteal region. Despite targeted physiotherapy, symptoms persisted. MRI showed medial iliac bone marrow edema with a suspected stress-fracture line. Given the low-impact nature of cycling, further evaluation was pursued. The subsequent CT-scan was suggestive of unilateral sacroiliitis. OUTCOME AND FOLLOW-UP: Despite reduced cycling, targeted stabilization exercises, and NSAIDs, the condition progressed to bilateral sacroiliitis on repeat MRI. He was diagnosed with HLA-B27–negative axial spondyloarthritis and responded well to TNF-α inhibitor therapy. DISCUSSION: This case illustrates that, even in the presence of substantial risk factors for overuse injuries associated with ultra-endurance cycling, persistent symptoms that cannot be adequately explained by overload should prompt consideration of alternative etiologies, including inflammatory pathologies.