Study of the nano-mechanical properties of human knee cartilage in different wear conditions M. Wang a,n , Z. Peng a , J. Price b , N. Ketheesan c a School of Mechanical and Manufacturing Engineering, The University of New South Wales, Australia b Townsville Orthopaedic Clinic, Townsville, Australia c Microbiology and Immunology, School of Veterinary and Biomedical Sciences, James Cook University, Australia article info Article history: Received 31 August 2012 Received in revised form 6 December 2012 Accepted 8 December 2012 Available online 19 December 2012 Keywords: Effective indentation modulus Wear of human cartilage Atomic force microscopy OA diagnosis abstract Osteoarthritis (OA) is a wear related disease that affects millions of people worldwide. Mechanical properties of cartilage surface play an important role in osteoarthritis initiation and progression. The objective of this study was to investigate the effective indentation modulus and its change in relation to the wear conditions of human knee cartilage. Following informed consent, samples were collected during orthopaedic surgery. Healthy and osteoarthritic cartilage samples at different OA stages were studied using atomic force microscopy. Unique to existing studies, the effective indentation moduli of the uppermost top surfaces of the healthy and osteoarthritic cartilage samples were measured. This study has shown that the modulus of the top surface changed significantly with wear severity progression. The findings have indicated that the effective indentation modulus measured at the nano- scale could be used to reveal changes in OA and to assist in understanding the wear process. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Wear of cartilage is the primary cause of osteoarthritis (OA). A normal cartilage in human knee joint has an excellent load carrying and lubrication capability. However, the repair ability of the cartilage is limited. When the regeneration of the cartilage cannot pace with its wear rate, cartilage degrades and OA occurs. In cartilage degeneration, collagen network loosening, abnormal proteoglycan expansions and increases in permeability take place. This is accompanied by alternations of the properties of the extracellular matrix in the cartilage [1]. These changes cause the cartilage to malfunction. It is believed that the changes originate at a nanometre level [2] before any symptomatic and functional impairments are detected at a micro- and macro- scale [3–6]. It is therefore important to examine the properties of the cartilage at a nanometre scale. Atomic force microscopy (AFM) is a suitable tool for studying the nano-mechanical properties and surface topographies of a biological material, such as cartilage. However, very limited work has been done on human cartilage. Stolz et al. are one of few researchers and pioneers in using AFM to study the mechanical properties of human cartilage surface [2]. In their work, an applied load of  3 mN was used resulting in an indentation depth of  2 mm. A decreasing trend of the nano- stiffness was observed from healthy cartilages to OA grade 3 cartilages. Most recently, Wen et al. [7] studied the elastic modulus of collagen fibrils of healthy and diseased (OA grades 1–3) cartilages. They reported that the individual collagen fibril of an OA cartilage was much stiffer than that of the healthy ones with an intact collagen structure. Furthermore, although it is well known that the top surface of human cartilage plays a key role in determining the mechanics of cartilage articulating process [1], its mechanical properties and surface structures at a nano-metre scale have not been well studied. To date, the nano-mechanical properties of the uppermost surface (in several hundred nanometre range) of the cartilage and their alterations with progression of OA are unknown. This study aimed to investigate the stiffness of the top surface of human cartilage surfaces at healthy, mild and severe OA conditions. The information assists further understandings of the changes in the nano-mechanical property of the cartilage in the OA progress and at the different wear conditions. 2. Methods and materials 2.1. Sample preparation Human femoral condyle knee joint samples were obtained from five patients in their total knee replacement operations under a Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/wear Wear 0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.12.015 n Corresponding author. Tel.: þ61 2 9385 5054; fax: þ61 2 96631222. E-mail addresses: meiling.wang@unsw.edu.au, wangmling@126.com (M. Wang). Wear 301 (2013) 188–191