Contribution of collagen fibers to the compressive stiffness of cartilaginous tissues
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ORIGINAL PAPER
Contribution of collagen fibers to the compressive stiffness of cartilaginous tissues Anne M. Römgens · Corrinus C. van Donkelaar · Keita Ito
Received: 25 May 2012 / Accepted: 4 February 2013 / Published online: 27 February 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract Cartilaginous tissues such as the intervertebral disk are predominantly loaded under compression. Yet, they contain abundant collagen fibers, which are generally assumed to contribute to tensile loading only. Fiber tension is thought to originate from swelling of the proteoglycan-rich nucleus. However, in aged or degenerate disk, proteoglycans are depleted, whereas collagen content changes little. The question then rises to which extend the collagen may contribute to the compressive stiffness of the tissue. We hypothesized that this contribution is significant at high strain magnitudes and that the effect depends on fiber orientation. In addition, we aimed to determine the compression of the matrix. Bovine inner and outer annulus fibrosus specimens were subjected to incremental confined compression tests up to 60 % strain in radial and circumferential direction. The compressive aggregate modulus was determined per 10 % strain increment. The biochemical composition of the compressed specimens and uncompressed adjacent tissue was determined to compute solid matrix compression. The stiffness of all specimens increased nonlinearly with strain. The collagen-rich outer annulus was significantly stiffer than the inner annulus above 20 % compressive strain. Orientation influenced the modulus in the collagen-rich outer annulus. Finally, it was shown that the solid matrix was significantly compressed above 30 % strain. Therefore, we concluded that collagen fibers significantly contribute to the compressive stiffness of the intervertebral disk at high strains. This is valuable for understanding the compressive behavior of collagenA. M. Römgens · C. C. van Donkelaar (B) · K. Ito Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands e-mail: [email protected] A. M. Römgens e-mail: [email protected]
reinforced tissues in general, and may be particularly relevant for aging or degenerate disks, which become more fibrous and less hydrated. Keywords Collagen stiffness · Confined compression · Annulus fibrosus · Solid matrix compressibility
1 Introduction Cartilaginous tissues, like articular cartilage, intervertebral disks, and the meniscus, bear and distribute compressive loads. To provide for their function, their extracellular matrix is mainly composed of collagen fibers, proteoglycans, and water (Urban and Roberts 2003; Lorenz and Richter 2006). As people age, cartilaginous tissues start to degenerate, which causes pain and reduces mobility. During degeneration, the biochemical composition changes. Proteoglycan content decreases, while previous studies are not consistent on the changes in collagen content (Urban and Roberts 2003; Antoniou
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