Trabecular Microfracture Precedes Cortical Shell Failure in the Rat Caudal Vertebra Under Cyclic Overloading S. R. Kummari Æ A. J. Davis Æ L. A. Vega Æ N. Ahn Æ E. H. Cassinelli Æ C. J. Hernandez Received: 23 December 2008 / Accepted: 3 May 2009 / Published online: 2 June 2009 Ó Springer Science+Business Media, LLC 2009 Abstract Microscopic tissue damage has been observed in otherwise healthy cancellous bone in humans and is believed to contribute to bone fragility and increased fracture risk. Animal models to study microscopic tissue damage and repair in cancellous bone would be useful, but it is currently not clear how loads applied to a whole ani- mal bone are related to the amount and type of resulting microdamage in cancellous bone. In the current study we determine the relationship between applied cyclic com- pressive overloading and the resulting amount of micro- damage in isolated rat tail vertebrae, a bone that has been used previously for in vivo loading experiments. Rat caudal vertebrae (C7–C9, n = 22) were potted in bone cement and subjected to cyclic compressive loading from 0 to 260 N. Loading was terminated in the secondary and tertiary phases of the creep-fatigue curve using custom data-mon- itoring software. In cancellous bone, trabecular micro- fracture was the primary form of microdamage observed with few microcracks. Trabecular microfracture prevalence increased with the amount of cyclic loading and occurred in nine out of 10 specimens loaded into the tertiary phase. Only small amounts of microdamage were observed in the cortical shell of the vertebrae, demonstrating that, under axial cyclic loading, damage occurs primarily in regions of cancellous bone before overt fracture of the bone (macroscopic cracks in the cortical shell). These experi- ments in isolated rat tail vertebrae suggest that it may be possible to use an animal model to study the generation and repair of microscopic tissue damage in cancellous bone. Keywords Trabecular microfracture Á Microdamage Á Creep-fatigue Á Bone potting Á Rat tail loading model Microdamage in the form of microscopic cracks and dif- fuse damage has been observed in vivo in both human cortical bone [1–4] and cancellous bone [5–7]. In cancel- lous bone, microdamage may also take the form of tra- becular microfractures (complete fracture of trabeculae) [8–12]. Microdamage is associated with reductions in the elastic modulus and strength of bone [13] and is believed to influence clinical fracture. Microscopic tissue damage has been implicated as a contributor to clinical fractures, par- ticularly those that are not caused by a single loading event, such as stress fractures and vertebral fractures [14, 15]. Once microdamage is formed in bone, it remains until it is removed by repair processes such as bone remodeling or fracture healing (i.e., callus). As a result, the amount of microdamage in a bone is influenced by both microdamage formation and microdamage repair. The rat ulna loading model [16] is the most popular method used to examine the repair of microdamage in cortical bone [17, 18]. In vivo animal models for studying microdamage in cancellous bone are rare and have so far been limited to studies with implants [19–21]. Only one of these approaches has been used to examine microscopic tissue damage [21]. A disadvantage of techniques using implants is that application of the implant requires dam- aging the cortical shell and surrounding periosteum, S. R. Kummari Á A. J. Davis Á L. A. Vega Á C. J. Hernandez (&) Musculoskeletal Mechanics and Materials Laboratory, Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Glennan 615A, 10900 Euclid Ave., Cleveland, OH 44106, USA e-mail: christopher.hernandez@case.edu N. Ahn Á E. H. Cassinelli Department of Orthopedic Surgery, Case Western Reserve University, Cleveland, OH, USA 123 Calcif Tissue Int (2009) 85:127–133 DOI 10.1007/s00223-009-9257-3