A nanoindentation technique for functional evaluation of cartilage repair tissue
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Alfred Kuo, Juan J. Rodrigo, and A. Hari Reddi Department of Orthopaedic Surgery, University of California at Davis, Sacramento, California 95817
Michael Ries Department of Orthopaedic Surgery, University of California at San Francisco, San Francisco, California 94143
Lisa Pruitt Departments of Bioengineering and Mechanical Engineering, University of California at Berkeley, Berkeley, California 94720 (Received 16 June 2003; accepted 8 October 2003)
Cartilage repair is an ongoing challenge in orthopaedic research. Both biological and mechanical properties are needed for full functional characterization of a tissue replacement. However, assessment of cartilage repair tissue has relied primarily on histological markers because mechanical properties are difficult to measure in small tissue regions. Nanoindentation is a technique that is specialized for measuring local mechanical properties in small specimens. In this study, nanoindentation was used to measure the mechanical properties of cartilage repair tissue following microfracture treatment of chondral defects in three rabbit knees. The mechanical properties were then correlated with histology findings from the same tissue samples. Nanoindentation provided complementary information to histology. In particular, the nanoindentation results showed that the mechanical properties of the repair tissue were inferior to the control tissue properties, even though some tissue regions were histologically similar to the control tissue.
I. INTRODUCTION
Cartilage repair is an ongoing challenge in orthopaedic research. Many different techniques are under investigation for stimulating cartilage repair in localized chondral defects. These range from approaches that utilize the body’s natural pathways to stimulate cartilage repair, such as debridement and microfracture, to tissue engineering of new cartilage constructs from cells and matrix molecules.1 To compare different repair techniques, suitable methods are needed to evaluate the functionality of the repair tissue that grows in chondral defects after treatment. The functionality of the tissue can be characterized by both mechanical and biochemical markers. The mechanical properties of cartilage repair tissue are often difficult to assess through traditional mechanical testing techniques due to the small size and irregular shape of repair cartilage specimens. Mechanical testing may also damage the repair tissue.2 The challenge of mechanical testing has led many researchers to rely on tissue architecture, as assessed through histology, as the J. Mater. Res., Vol. 19, No. 1, Jan 2004
primary marker for tracking cartilage repair.3–9 However, knowledge of the mechanical properties of repair cartilage relative to healthy or nondiseased cartilage is equally essential for determining the overall success of a cartilage repair technique.8,9 Hence, a technique is needed that can be used to measure the mechanical properties of cartilage repair tissue in a wide range of animal models. Nanoindentation is a mechanical testing technique
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