Ring apophysis fractures induced by low-load low-angle repetitive flexion in an ex-vivo cervine model Nicole C. Corbiere a , Stacey L. Zeigler b , Kathleen A. Issen a , Arthur J. Michalek a , Laurel Kuxhaus a,n a Department of Mechanical & Aeronautical Engineering, Clarkson University, Potsdam, NY, USA b Department of Physical Therapy, Clarkson University, Potsdam, NY, USA article info Article history: Accepted 13 March 2016 Keywords: Cervine Nontraumatic vertebral fracture initiation Deer vertebrae Low back pain Bone mineral density abstract 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 under- stood 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 230 750 N), 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. & 2016 Elsevier Ltd. All rights reserved. 1. Introduction Low back pain affects approximately 30–40% of Americans in a given year and is the leading physical cause of physician visits. This condition costs an estimated $34 billion annually in direct US health care costs alone, with indirect costs due to lost worker productivity expected to be much larger (Gaskin and Richard, 2012). Among the leading causes of low back pain is degeneration of the intervertebral discs (IVDs). Fractures of the vertebral carti- laginous endplates make the adjacent discs particularly suscep- tible to degeneration (Haschtmann et al., 2008), as the endplates are the primary pathway for disc nutrition (Urban et al., 1977). Endplate fractures result from acute or chronic overload of the vertebral column and may be broadly classified as burst fractures occurring at the center of the endplate or ring apophysis fractures, which initiate along the periphery. Ring apophysis fractures (RAFs) of the lumbar vertebrae are avulsion type fractures to the posterior aspect of the lumbar vertebral body, which initiate at the ring apophysis and propagate either anteriorly or through the vertebral endplate (Takata et al., 1988). These fractures result in low back pain, limited mobility, often present along with intervertebral disc herniation (Asazuma et al., 2003; Singhal et al., 2013; Wu and Ma, 2011), and are rarely visible on plain X-rays (Akhaddar et al., 2011). While RAFs typically result from trauma in adults, adolescents who present with these injuries can often not recall a single high energy event preceding the onset of symptoms (Alvarenga et al., 2014; Asazuma et al., 2003; Martinez-Lage et al., 1998), suggesting the possibility that the fracture resulted from accumulation of microfractures from repeated loading at moderate amplitudes. In adolescents, a dominant risk factor for RAFs resulting from sub-traumatic load is incomplete ossification of the ring apophysis (ossification com- pletes at approximately age eighteen) (Takata et al., 1988); addi- tionally, the ring apophysis carries high tensile stresses under physiological flexion (Sairyo et al., 2006). Given the difficulty of diagnosis, and the fact that many clin- icians are unfamiliar with non-traumatic RAF as an entity (Wu and Ma, 2011), these fractures are uncommon but likely under- diagnosed. In some instances, clinicians may dismiss the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com Journal of Biomechanics http://dx.doi.org/10.1016/j.jbiomech.2016.03.022 0021-9290/& 2016 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ1 315 268 6602; fax: þ1 315 268 6695. Journal of Biomechanics 49 (2016) 1477–1484