Elsevier

Journal of Biomechanics

Volume 41, Issue 15, 14 November 2008, Pages 3145-3151
Journal of Biomechanics

Two-dimensional strain fields on the cross-section of the bovine humeral head under contact loading

https://doi.org/10.1016/j.jbiomech.2008.08.031Get rights and content

Abstract

The objective of this study was to provide a detailed experimental assessment of the two-dimensional cartilage strain distribution on the cross-section of immature and mature bovine humeral heads subjected to contact loading at a relatively rapid physiological loading rate. Six immature and six mature humeral head specimens were loaded against glass and strains were measured at the end of a 5 s loading ramp on the textured articular cross-section using digital image correlation analysis. The primary findings indicate that elevated tensile and compressive strains occur near the articular surface, around the center of the contact region. Few qualitative or quantitative differences were observed between mature and immature joints. Under an average contact stress of ∼1.7 MPa, the peak compressive strains averaged −0.131±0.048, which was significantly less than the relative change in cartilage thickness, −0.104±0.032 (p<0.05). The peak tensile strains were significantly smaller in magnitude, at 0.0325±0.013. These experimental findings differ from a previous finite element analysis of articular contact, which predicted peak strains at the cartilage–bone interface even when accounting for the porous-hydrated nature of the tissue, its depth-dependent inhomogeneity, and the disparity between its tensile and compressive properties. These experimental results yield new insights into the local mechanical environment of the tissue and cells, and suggest that further refinements are needed in the modeling of contacting articular layers.

Introduction

Despite a wealth of studies on the biomechanics of articular cartilage, it is interesting that detailed measurements of the strain distribution in articular layers under physiological loading conditions have yet to be reported. Here, physiological loading conditions refer to relatively rapid loading rates (but not impact loading) and contact stress magnitudes achieved during normal activities of daily living. Load magnitudes (Cooney and Chao, 1977; Paul, 1976; Poppen and Walker, 1978; Rydell, 1973) and articular contact tractions (Ahmed and Burke, 1983; Ahmed et al., 1983; Brown and Shaw, 1983, Brown and Shaw, 1984; Clark et al., 2002; Huberti and Hayes, 1984, Huberti and Hayes, 1988; Matthews et al., 1977) have been reported for a number of activities of daily living. However, there are only a few reports of measurements of relative cartilage deformation under physiological conditions, using radiographic techniques (Armstrong et al., 1979), ultrasound (Macirowski et al., 1994) and magnetic resonance imaging, (Eckstein et al., 1999, Eckstein et al., 2000), typically reporting relative deformations ranging from 6% to 20%. In contrast, the relative change in thickness of cartilage under prolonged static loading is on the order of 57% (Herberhold et al., 1998, Herberhold et al., 1999). However, this type of loading is not physiological and large changes in thickness under static conditions are not representative of in vivo cartilage deformation.

Cartilage deformation, as measured by change in thickness, has been characterized under various loading conditions using radiographic methods (Armstrong et al., 1979; Wayne et al., 1998), ultrasound (Macirowski et al., 1994; Suh et al., 2001), cryofixation followed by scanning electron microscopy (Kaab et al., 1998), chemical fixation followed by light microscopy (Clark et al., 2003), and magnetic resonance imaging (Eckstein et al., 2000; Herberhold et al., 1998, Herberhold et al., 1999).

Local strain measurements in articular cartilage explants have been reported under equilibrium static loading conditions using optical methods, by tracking distances between cells or cell nuclei (Guilak et al., 1995; Schinagl et al., 1996, Schinagl et al., 1997) or with digital image correlation (DIC) (Bae et al., 2003; Canal et al., 2003; Erne et al., 2005; Wang et al., 2002, Wang et al., 2003). These studies have yielded valuable insights into the equilibrium properties of the tissue, but the equilibrium response is not representative of physiologic loading conditions, and the explant geometry used in these studies was not representative of the articular geometry. More recently, strain fields in cartilage explants have been reported using magnetic resonance tagging (Neu et al., 2005) under steady-state dynamic loading. This approach holds great promise for delivering three-dimensional (3D) dynamic strain measurements in situ but has only been applied to small explants to date.

The objective of this study is to complement these valuable literature findings by providing a detailed experimental assessment of the two-dimensional (2D) cartilage strain distribution on the cross-section of immature and mature bovine humeral heads subjected to contact loading at a physiological loading rate. Mature and immature joints are compared to explore whether significant differences may arise due to the well-recognized evolution in material properties (Kempson, 1991; Williamson et al., 2003) and composition (Front et al., 1989; Garg and Swann, 1981; Wachtel et al., 1995) with age. The strain measurement method employed in this study, which uses DIC, builds upon our earlier application in the study of cartilage explants under static loading (Wang et al., 2002). The characterization of localized strains from experiments can aid in the understanding of the biomechanics of articular cartilage under physiological loading conditions, the structure–function relationships of this tissue, and the interpretation of degenerative and acute failure patterns observed in osteoarthritis and traumatic injuries. It also sets the stage for refining constitutive models of articular cartilage.

Section snippets

Specimen preparation

Humeral heads were dissected from 6-week-old calf joints (N=6) and mature 3-year-old shoulder joints (N=4). All samples were observed to be normal by visual inspection. The humeral heads were sectioned by band saw to create samples with smooth cross-sectional areas of the articular layer and underlying bone. The resulting sample groups were labeled ‘immature’ (n=6) and ‘mature’ (n=6, with two joints providing two samples each). The cartilage layers were kept moist with phosphate buffered saline

Results

The temporal evolution in the axial normal strain Eyy under the prescribed ramp-and-hold loading profile is depicted for a representative immature specimen in Fig. 4a–c, showing that the strain magnitude increased with time after the applied load had reached its peak value at 5 s. Repeatability measurements, illustrated in this representative specimen (Fig. 4a–c, versus d–f), demonstrated that reloading the tissue sample (after a 5 min recovery) and evaluating the strain distribution anew

Discussion

In this study, strain distributions throughout full thickness cross-sections of the articular layer of the bovine humeral head were characterized under physiological loading rates (average contact stresses of ∼1.5–2.0 MPa applied over 5 s) which, for humans, would represent moderate activities of daily living (Ahmed and Burke, 1983; Brown and Shaw, 1983). Both mature and immature joints were analyzed to investigate how age related differences in material properties and composition affect strain

Conflict of interest statement

The authors of this manuscript do not have any conflicts of interest with regard to this study and the materials contained herein.

Acknowledgments

This study was funded by the National Institute of Arthritis, Musculoskeletal and Skin Diseases of the US National Institutes of Health (AR46532). This material is also based upon work supported under a National Science Foundation Graduate Research Fellowship. We would like to thank Ms. Christine Schwaller and Ms. Elizabeth Chorney for their assistance in this study.

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