journal homepage: www.elsevier.com/locate/jmbbm Available online at www.sciencedirect.com Impact induced failure of cartilage-on-bone following creep loading: A microstructural and fracture mechanics study Ashvin Thambyah n , Geran Zhang, Woong Kim, Neil D. Broom Experimental Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, Private Bag 92019, Auckland, New Zealand article info Article history: Received 3 April 2012 Received in revised form 9 June 2012 Accepted 12 June 2012 Keywords: Crack propagation Vulnerable joint DIC SEM Fracture mechanics abstract Cartilage-on-bone samples obtained from healthy bovine patellae, with or without prior static compression (i.e. creep) at 2 MPa for 3 h, were delivered a single impact via an instrumented pendulum indenter at a velocity of 1.13 m/s and an energy of 2.2 J. Mechanical data was obtained and microstructural assessment of the region of failure was carried out using differential interference contrast (DIC) optical imaging. In addition, a fibrillar-level structural analysis using scanning electron microscopy (SEM) was conducted on a control batch of non- impacted samples that were subjected to either creep or non-creep loading protocols. Arising from the impact event the deepest levels of crack penetration into the articular cartilage occurred in those samples subjected to prior creep loading. Further the crack depth was inversely proportional to the rebound velocity of the indenter. By contrast, those impacted samples not subjected to prior creep loading had only short obliquely patterned microcracks confined to the upper one-third of the full cartilage depth. Ultrastructurally the creep-loaded cartilage matrix exhibited a substantial radial collapse or compaction of the fibrillar network in its primary radial zone. The increase in crack length in the prior creep-loaded cartilage is consistent with a reduction in its dissipative properties as indicated by a reduction in rebound velocity. An interpretation is offered in terms of classical fracture mechanics theory. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Activities of daily living can include extended periods of sitting, squatting and standing, and all involve some form of sustained loading of the joint tissues. In vivo studies show that the hip and knee joint experience significant contact forces of between 1 and 2 times bodyweight even in seemingly relaxed positions such as sitting and standing (Bergmann et al., 2001; Varadarajan et al., 2008). Further, equivalent contact stresses in these same joints have been estimated to be as much as 3–8 MPa (Thambyah et al., 2005; Sparks et al., 2005; Yoshida et al., 2006). In view of such physiological levels of stress we might ask whether such large and sustained contact stresses might make the joint tissues more susceptible to subsequent injury following a more dynamic or transient mode of loading. Expressing the problem in functional terms how might passive activities of daily living such as sitting, squatting or standing alter the internal state of the joint tissues so as to increase their susceptibility to injury from sudden loading? By combining an analysis of the structural effects of loading arising from the activities of daily living with joint tissue vulnerability to injury may contribute to our understanding of the mechanical factors associated with the development of osteoarthritis (OA). For example it has been shown that popula- tion groups that habitually squat for long periods are at a greater risk of developing knee OA than those who do not (Zhang et al., 2004; Coggon et al., 2000; Cooper et al., 1994; Anderson and Felson, 1988; Felson, 1990); and while many studies investigating 1751-6161/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jmbbm.2012.06.007 n Corresponding author. E-mail address: ashvin.thambyah@auckland.ac.nz (A. Thambyah). journal of the mechanical behavior of biomedical materials ] ( ]]]] ) ]]] – ]]] Please cite this article as: Thambyah, A., et al., Impact induced failure of cartilage-on-bone following creep loading: A microstructural and fracture mechanics study. Journal of the Mechanical Behavior of Biomedical Materials (2012), http://dx.d oi.org/10.1016/j.jmbbm.2012.06.007