Ring apophysis fractures induced by low-load low-angle repetitive flexion in an ex-vivo cervine model

Ring apophysis fractures of the spine occur in physically-active adolescents causing low back pain and the potential for chronic pain. Many of these fractures occur without memorable trauma, suggesting that the fractures occur during everyday movements and activities. The benign nature of this poorly understood potential mechanism of injury hampers appropriate diagnosis and early treatment. The purpose of this study was to establish an ex-vivo model of ring apophysis fracture and demonstrate that these fractures can be initiated by repetitive non-traumatic loading. Six 5-vertebra cervine lumbar (L1-L5) motion segments were cyclically loaded in low-angle low-load flexion (to 15° flexion, with peak load of 230±50N), a representative movement component of daily activities for both human and deer lumbar spines. Pinned end conditions replicated physiologically realistic loading. Ring apophysis fractures were created under low-load low-angle conditions in healthy vertebrae of similar bone mineral density and a similar degree of skeletal maturity to adolescent humans. All specimens developed ring apophysis fractures, some as early as 1400 cycles. The load-displacement data, and hysteresis loops during the cyclic loading, suggest that the fractures occurred gradually, i.e., without trauma. The ease at which these fractures were created suggests that ring apophysis fractures may be more prevalent than current diagnosis rates. Therefore, clinically, healthcare providers should include the potential for ring apophysis fracture in the differential diagnosis of all physically-active adolescents who present with back pain.