JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS 5 (2012) 206–215 Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jmbbm Research paper Macro-, micro- and ultrastructural investigation of how degeneration influences the response of cartilage to loading Ashvin Thambyah ∗ , Joy-Yi Zhao, Scott L. Bevill, Neil D. Broom Tissue Mechanics Laboratory, Department of Chemical and Materials Engineering, University of Auckland, New Zealand ARTICLE INFO Article history: Received 16 June 2011 Received in revised form 26 August 2011 Accepted 30 August 2011 Published online 13 September 2011 Keywords: Cartilage matrix biomechanics Channel indentation Tangential layer Fibrillar interconnectivity Degeneration Destructuring ABSTRACT Articular cartilage functions as a load-bearing tissue by virtue of a functional coupling between its hydrated proteoglycan component and its zonally differentiated fibrillar network. How degeneration influences this relationship at the macro-, micro-, and ultrastructural levels is investigated in this study. Healthy bovine patellae (N = 9) and patellae exhibiting varying degrees of degeneration (N = 16) formed the basis of the study. Cartilage-on-bone blocks obtained from each patella were subjected to creep loading under a nominal stress of 4.5 MPa via a rectangular planar indenter which incorporated a narrow channel relief space to create a defined region where the cartilage would not be directly loaded. Following the attainment of creep equilibrium each sample was chemically fixed while under load so as to preserve the deformed state of the cartilage matrix. The structural response of the matrix was then analysed using differential interference contrast (DIC) optical microscopy and scanning electron microscopy (SEM). The morphology of the cartilage matrix extruded into the channel relief region was dramatically influenced by the severity of degeneration. The microscopic and ultrastructural characteristics of the extruded matrix showed that the load response of bulk cartilage is determined primarily by the microstructural integrity of the strain-limiting tangential layer and the nano-level interconnectivity of the fibrillar network. In conclusion, this study showed that three mechanically significant structural features of cartilage are important: (1) the strain limiting surface layer; (2) the micro-level boundaries in its zonally differentiated structure, and (3) the extent of fibrillar interconnectivity. Degeneration degrades or destroys the articular surface and ‘destructures’ the fibrillar network such that the latter functions less effectively as a proteoglycan entrapment system. c ⃝ 2011 Elsevier Ltd. All rights reserved. 1. Introduction The structural response of cartilage to loading has been the subject of various scalar levels of analysis. Ranging from the macro through to the fibrillar level, studies have examined ∗ Correspondence to: Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand. Tel.: +64 9 9235379; fax: +64 9 3737463. E-mail address: ashvin.thambyah@auckland.ac.nz (A. Thambyah). overall joint function right down to the cell-level behaviour (e.g. Radin, 1972, Setton et al., 1999, Herzog et al., 2004, Kim et al., 2008, Le Roux et al., 2000 and Glaser and Putz, 2002). The assumption is that important biomechanical principles must exist which functionally integrate this large, multiscalar 1751-6161/$ - see front matter c ⃝ 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jmbbm.2011.08.024