Changes in Bone Fatigue Resistance due to Collagen Degradation* Chrystia Wynnyckyj, 1,2 Thomas L. Willett, 1 Sidney Omelon, 3 Jian Wang, 1 Zhirui Wang, 2 Marc D. Grynpas 1,2 1 Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Room 840, Toronto, Ontario, Canada M5G 1X5, 2 Materials Science and Engineering, University of Toronto, Toronto, Ontario, Canada, 3 Chemical and Biological Engineering, University of Ottawa, Ottawa, Ontario, Canada Received 27 May 2010; accepted 28 June 2010 Published online 27 August 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/jor.21228 ABSTRACT: Clinical tools for evaluating fracture risk, such as dual energy X-ray absorptiometry (DXA) and quantitative ultrasound (QUS), focus on bone mineral and cannot detect changes in the collagen matrix that affect bone mechanical properties. However, the mechanical response tissue analyzer (MRTA) directly measures a whole bone mechanical property. The aims of our study were to investi- gate the changes in fatigue resistance after collagen degradation and to determine if clinical tools can detect changes in bone mechanical properties due to fatigue. Male and female emu tibiae were endocortically treated with 1 M KOH for 1–14 days and then either fatigued to failure or fatigued to induce stiffness loss without fracture. Partial fatigue testing caused a decrease in modulus measured by mechanical testing even when not treated with KOH, which was detected by MRTA. At high stresses, only KOH-treated samples had a lower fatigue resistance compared to untreated bones for both sexes. No differences were observed in fatigue behavior at low stresses for all groups. KOH treatment is hypothesized to have changed the collagen structure in situ and adversely affected the bone. Cyclic creep may be an important mechanism in the fast deterioration rate of KOH-treated bones, as creep is the major cause of fatigue failure for bones loaded at high stresses. Therefore, collagen degradation caused by KOH treatment may be responsible for the observed altered fatigue behavior at high stresses, since collagen is responsible for the creep behavior in bone. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J. Orthop. Res. 29: 197–203, 2011 Keywords: bone fatigue; collagen degradation; mechanical response tissue analyzer (MRTA); potassium hydroxide (KOH) Bone adapts its mass, architecture, and mechanical properties in response to mechanical loading. While bone is protective against impact, bone is also suscep- tible to fatigue, a process by which repetitive loading damages the bone matrix. This repetitive loading of bone leads to microcrack formation and accumulation. 1 The fatigue behavior of bone resembles that of com- posite materials, exhibiting a gradual loss of stiffness and strength throughout cyclic loading due to fatigue damage accumulation. 1 Bone accumulates damage over time and eventually fails below its theoretical static strength. When there is time-evolving damage, bone may also experience creep rupture. 1 Creep is the gradual increase in material strain developed over time when a material is exposed to a constant stress. Cyclic creep involves the application of a cyclic force, resulting in permanent (plastic) damage. Creep is a known charac- teristic of materials such as polymers. 2 In bone, collagen is abundant and displays polymeric properties. 3 It has been suggested that the collagen component of bone is responsible for its observed creep behavior. 4,5 Since col- lagen plays a major role in bone biomechanics, 6–8 it is important to identify the role(s) that collagen may have in fatigue fractures. A great deal of interest exists in developing tech- nologies capable of measuring in vivo bone mechanical properties, such as stiffness, that predict the risk of sus- taining a fragility fracture. Current technologies such * The authors have no conflicts of interest. Correspondence to: Marc D. Grynpas (T: 416-586-4800 ext 4464; F: 416-586-8844; E-mail: grynpas@mshri.on.ca) © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. as dual energy X-ray absorptiometry (DXA) and quan- titative ultrasound (QUS) measure only bone mineral density (BMD), a surrogate measure of fracture risk. Bone fragility depends not only on its mineral con- tent but also on its matrix properties, architecture, and geometry. 9 Therefore, evaluating mineral content alone is insufficient to predict changes in bone quality. 10 An alternative technology for predicting changes in bone quality is the mechanical response tissue analyzer (MRTA). The MRTA is a radiation-free, noninvasive instru- ment that directly measures an important mechanical property, the cross-sectional bending stiffness (EI) of long bones. EI is the product of the elastic modulus, E, and the areal cross-sectional moment of inertia, I. EI of a long bone is predictive of its maximum strength 11 ; thus, EI measurements can be used to assess bone quality. Furthermore, the determination of EI could effectively evaluate fracture risk. 11–14 To study the contribution of collagen to bone mechan- ical properties and the ability of the MRTA to detect changes in the collagen matrix, we developed a model using the emu tibiae. The emu tibial morphology is lamellar bone with osteons (Fig. 1). In this model, endo- cortical bone collagen was chemically modified with 1 M potassium hydroxide (KOH) while maintaining the min- eral content unaltered. 15,16 While this treatment is not physiological, it helps to understand the mechanisms by which collagen degradation affects bone mechanical properties. The emu tibia was chosen due to its size and approximate cylindrical shape, making it ideal for devices designed to accommodate human long bones, such as the MRTA. JOURNAL OF ORTHOPAEDIC RESEARCH FEBRUARY 2011 197