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The fetlock of the horse is a high impact weight bearing articulating joint. Its unique
anatomy and kinematics during locomotion lead to site-specific variation of stresses within the joint
and possible adaptations of the joint tissues to such specific loading histories. This study therefore
seeks to determine the variations in cartilage-bone tissue morphology, and the respective tissue
vulnerability to impact-induced microdamage, in different regions within the fetlock joint.
In the forelimb of athletic horses, kinematic data indicates that the metacarpophalangeal (MCP) joint
(commonly known as the fetlock) acts as the primary shock absorber [1], with stresses in the joint
reaching the range of 30 to 35 MPa [2]. The forces acting on the distal end of the third metacarpal
bone (MC3), that forms the MCP joint, are not evenly distributed, with the palmar surfaces being
subject to greater loads than the dorsal surfaces [3-5]. It has been suggested that the palmar aspect of
the MC3 therefore undergoes a number of adaptive changes in response to the high in vivo loads, and
this is evident in its tissue morphology [6]. Linear defects in mineralized articular cartilage and
subchondral bone have been found in this region, accompanied by densification of subchondral bone.
It has been hypothesized that from such defects parasagittal fractures of the MC3 originate [7],
providing the motivation for this biomechanical study.
Specifically, we seek to gain new insight into the mechanical and microstructural response of the
cartilage-bone system of the distal end of the MC3 to impact loading. This is done by both quasistatic
tesing and impact testing the palmar and dorsal sites, and comparing the mechanical response to
subtle differences in tissue morphology. The goal is to assess the relative level of vulnerability to
impact-induced microdamage of the joint system.
Equine MCP joints were collected soon after slaughter and stored at -20°C. Following
dissection general joint condition was documented using matrix uptake of Indian Ink [8] to exclude
those that had obvious injury or damage to the articular cartilage.
Regions of interest on the distal surface of the MC3 were located directly palmar and dorsal of the
transverse ridge, at the centre of the medial and lateral condyles, providing four groups; dorsal-lateral
(DL), dorsal-medial (DM), palmar-lateral (PL), and palmar-medial (PM). A cartilage-on-bone block
with en face dimensions ~10 mm ×10 mm was cut from each of the regions of interest and mounted
in a stainless steel holder using dental cement. The tissue was equilibrated in physiological saline
(0.15 M) for two hours prior to mechanical testing to achieve near-physiological water content and
maintained in this hydrated state throughout testing.
Material properties of the cartilage of 16 MC3 bones were determined using
standard indentation testing protocol [9-12]. The tissue was loaded in uniaxial compression at a
constant load of 0.2 N (0.4 MPa) using a cylindrical plane-ended indenter with a 0.8 mm diameter
Advanced Materials Research Vol. 275 (2011) pp 139-142
Online available since 2011/Jul/04 at www.scientific.net
© (2011) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMR.275.139
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
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