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Research Papers

# Cartilage Collagen Matrix Reorientation and Displacement in Response to Surface Loading

[+] Author and Article Information
C. J. Moger1

School of Physics, University of Exeter, Stocker Road, Exeter, Devon EX4 4QL, UKc.j.moger@ex.ac.uk

K. P. Arkill, R. E. Ellis, E. M. Green, C. P. Winlove

School of Physics, University of Exeter, Stocker Road, Exeter, Devon EX4 4QL, UK

R. Barrett, P. Bleuet

European Synchrotron Radiation Facility (ESRF), BP 220, F-38043 Grenoble Cedex, France

1

Corresponding author.

J Biomech Eng 131(3), 031008 (Jan 07, 2009) (9 pages) doi:10.1115/1.3049478 History: Received August 09, 2007; Revised June 01, 2008; Published January 07, 2009

## Abstract

An investigation of collagen fiber reorientation, as well as fluid and matrix movement of equine articular cartilage and subchondral bone under compressive mechanical loads, was undertaken using small angle X-ray scattering measurements and optical microscopy. Small angle X-ray scattering measurements were made on healthy and diseased samples of equine articular cartilage and subchondral bone mounted in a mechanical testing apparatus on station ID18F of ESRF, Grenoble, together with fiber orientation analysis using polarized light and displacement measurements of the cartilage matrix and fluid using tracers. At surface pressures of up to approximately 1.5 MPa, there was reversible compression of the tangential surface fibers and immediately subjacent zone. As load increased, deformation in these zones reached a maximum and then reorientation propagated to the radial deep zone. Between surface pressures of 4.8 MPa and 6.0 MPa, fiber orientation above the tide mark rotated 10 deg from the radial direction, with an overall loss of alignment. With further increase in load, the fibers “crimped” as shown by the appearance of subsidiary peaks approximately $±10 deg$ either side of the principal fiber orientation direction. Failure at higher loads was characterized by a radial split in the deep cartilage, which propagated along the tide mark while the surface zone remained intact. In lesions, the fiber organization was disrupted and the initial response to load was consistent with early rupture of fibers, but the matrix relaxed to an organization very similar to that of the unloaded tissue. Tracer measurements revealed anisotropic solid and fluid displacement, which depended strongly on depth within the tissue.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

(a) A schematic of the loading apparatus and (b) a photograph of the loading apparatus

Figure 2

Examples of microinjections of (A) Evans blue and (B) Rhodamine B base injected into articular cartilage. The dotted line represents the lines along which intensity profiles were analyzed: (a) the upper front, (b) as the lower front, and (c) and (d) the lateral boundaries of the microdot.

Figure 3

The orientation of collagen with depth relative to the cartilage surface for increasing load determined from the angular variation in the intensity of 223 Å band. (a) 0 Pa, (b) 0.6 MPa (±0.06 MPa), (c) 1.1 MPa (±0.11 MPa), and (d) 1.8 MPa (±0.18 MPa). The depth, 0 mm, is the tide mark. Articular cartilage is the region in the negative direction consisting of the tangential (T), transitional (Tr), and radial (R) zones indicated by the dotted lines. The positive direction moves into the mineralized cartilage (MC) toward the subchondral bone (SB) as marked in (a). The white dotted line indicates the movement of the cartilage surface with applied load.

Figure 4

Changes in orientation of collagen (for the 223 Å diffraction maxima) fibers relative to the articular surface with increased surface pressure, 0.05 mm above the tide mark. All pressures are given with an error of (±10%).

Figure 5

Polarized light images under crossed polarizers of articular cartilage under increasing surface pressure: (a) 0 Pa, (b) 1.2 MPa (±0.12 MPa), and (c) 2.2 MPa (±0.22 MPa). The depth, 0 mm, is the tide mark (TM). Articular cartilage is the region in the negative direction with the tangential (T), transitional (Tr), and radial zones (R) marked. The positive direction is the tide mark through the mineralized cartilage (MC) toward the bone. The dotted line indicates the indenter front.

Figure 6

Averaged intensity profiles of polarized light images shown in Fig. 5. The depth, 0 mm, is the tide mark. Articular cartilage is the region in the negative direction and in the positive direction moving into the calcified cartilage toward the bone.

Figure 7

(a) A micrograph of the fracture of articular cartilage, mineralized cartilage (MC), and subchondral bone (SB) after a surface pressure of 30 MPa (±3.0 MPa) has been applied under normal microscopy. (b) A schematic overlay of the same image showing the position of the fracture line in the micrograph.

Figure 8

Color maps showing collagen orientation at different lateral locations through a lesion, in the apex region, in the unloaded state. The depth, 0 mm, is the tide mark. Articular cartilage is the region in the negative direction and in the positive direction moving into the mineralized cartilage toward the bone.

Figure 9

Color maps showing collagen orientation at each of the locations in the lesion shown in Fig. 8 under a load of 0.4 MPa (±0.04 MPa).

Figure 10

Intensity profiles of articular cartilage viewed under crossed polarizers parallel and perpendicular to the articular surface as a function of time after initial loading of 4.5 MPa (±0.45 MPa). The light regions represent regions of fibers aligned parallel and perpendicular to the surface and the dark regions show regions with no preferred orientation. The tangential zone (T), the transitional zone (Tr), radial zone (R), and mineralized cartilage regions (MC) have been marked.

Figure 11

(a) Displacement of Evans blue microdots injected at different depths beneath the articular surface after a load has been applied for 2 s and after 30 min and (b) the ratio of width to height. These data results are means from seven different samples.

Figure 12

(a) Displacement of Rhodamine B base microdots injected at different depths beneath the articular surface after a load has been applied for 2 s and after 30 min and (b) the ratio of width to height. These data results are means from five different samples.

Figure 13

A schematic showing the collagen orientation in the tangential (T) zone, transitional region (Tr), radial zone (R), and mineralized cartilage (MC): (a) unloaded, (b) loaded up to approximately 1.5 MPa, and (c) loaded above 5.8 MPa

Figure 14

Schematic representation of the time response of collagen fiber orientation in cartilage under applied load

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