SITE SPECIFIC VULNERABILITY TO IMPACT INDUCED DAMAGE
1
1
2
1
Sean Turley , Ashvin Thambyah , Elwyn Firth , Neil Broom
1
Biomaterials Laboratory, Department of Chemical and Materials Engineering, University of Auckland
2
Institute of Veterinary, Animal and Biomedical Sciences, Massey University
In the forelimb of athletic horses, the metacarpophalangeal (MCP) joint
acts as the primary shock absorber1, with the distal end of the third
metacarpal (MC3) bone experiencing high impact in vivo loading. The
palmar aspect of the MC3 is exposed to higher peak forces than the
dorsal2, and is a frequent site of injury and pathology3.
Cartilage-on-bone
section
CARTILAGE BASICS
Articular cartilage consists of an anisotropic matrix of type II collagen
(20% wet weight) which binds hydrophilic proteoglycan molecules (5%
ww). These molecules draw a large amount of water (75% ww) into the
tissue6. The collagen is aligned radially in the deeper zones near the
osteochondral junction, changing to a tangential alignment in the
superficial zone at the articular surface.
Superficial zone
INTRODUCTION
Cartilage thickness
Osteochondral
junction
Metacarpophalangeal
joint
Subchondral bone
MC3
METHODS
Palmar
5
4
1
Dorsal
Athletic horses provide a good model for the human athlete, both of
which suffer from early onset osteoarthritis, which is believed to be
related to loading history within the joint. Additionally, post-traumatic
osteoarthritis is common to both human and equine athletes alike.
We aim to gain new insight into differences that may exist between
the dorsal and palmar surfaces of the MC3, which have different loading
histories, comparing their tissue microstructure, quasi-static mechanical
properties, and microstructural response to impact loading.
2
3
4
Healthy MCP joints were dissected. Each MC3 bone provided four ≈
10×10 mm cartilage on bone blocks i.e. two for each of the palmar
and dorsal groups.
The articular cartilage was creep loaded in uniaxial compression at
0.2 N (0.4MPa) with a cylindrical plane-ended indenter (ø=0.8 mm),
and the elastic modulus determined after 15 s of creep (n=32 per
group) using an elastic single-phase model7.
Samples (n=16 per group) were impacted (energy ≈ 0.5 J) with a 2×5
mm half-cylinder indenter of radius 1mm using a pendulum rig fitted
with a dynamic load cell.
Samples were sectioned through the mechanically tested region and
their morphology and failure mode examined microscopically.
RESULTS
The 15-s modulus and superficial
zone thickness are greater in the
dorsal than the palmar (p=5×10-9
and p=1×10-12 respectively). The
superficial zone thickness had a
positive correlation with 15-s
modulus (R=0.56, p=3×10-6).
Type I = fracture restricted to the
upper third of the cartilage.
The peak reaction forces upon
impact were higher in the palmar
(p=0.001). Conversely, the cartilage
thickness was greatest in the dorsal
(p=0.005). Cartilage thickness had a
negative correlation with peak
reaction force (R= –0.71, p=4×10-6).
TYPE I
TYPE II
TYPE III
Type II = fracture extending into
the second third of the cartilage.
Type III = fracture extending the
entire depth of the cartilage, often
to and along the osteochondral
junction.
FAILURE MODE
DISCUSSION
FREQUENCIES
DORSAL
PALMAR
500 μm
17%
33%
The positive relationship between the 15-s modulus of the cartilage and
the thickness of the superficial zone (a relatively stiff region8 that
functions to distribute loads across the articular surface), suggests that a
thicker superficial zone is more able to redistribute static loads, thus
minimizing axial strain9. Similarly, the significant negative relationship
between peak reaction force and full cartilage thickness indicates thicker
cartilage has a greater ability to dissipate impact energy.
The palmar aspect, which is exposed to higher in vivo loads, has lesser
superficial zone and cartilage thicknesses, making this region more
vulnerable to impact-induced damage. This was reflected by both the
higher peak reaction forces and the tendency towards more extensive
impact-induced damage (50% displaying Type III failure). This may
account for the higher incidence of osteochondral degeneration in the
palmar aspect of the MC33.
67%
17%
17%
50%
CONCLUSIONS
This study shows new microstructural evidence for site specific
differences in vulnerability to impact-induced damage in the equine
MC3, which is of great importance as the cost of MC3 injury and
pathology to the Thoroughbred racing industry is significant. The results
also emphasise the importance of loading history and cartilage
microstructure in the development of early onset and post-traumatic
osteoarthritis, and is as relevant for human athletes as it is for equine.
REFERENCES
1) Back W (2001) in Equine Locomotion, W. Back & H.M. Clayton, Ed.: p. 95-133. 2) Biewener AA, Thomason J, et al. (1983) J Biomech. 16(8): p.
565-576. 3) Riggs CM, Whitehouse GH, et al. (1999) Eq Vet J. 31(2): p. 140-148. 4) Stashak TS & Hill C (2002) in Adams' Lameness in Horses,
T.S. Stashak, Ed.: p. 82. 5) Norrdin RW & Stover SM (2006) J Musculoskeletal Neur Inter. 6(3): p. 251-257. 6) Mankin HJ, Mow VC, et al. (2000)
in Ortho Basic Sci, J.A. Buckwalter, et al., Ed.: p. 443-470. 7) Hayes WC, Keer LM, et al. (1972) J Biomech. 5(5): p. 541-551. 8) Verteramo A &
Seedhom BB (2004) Biorheology. 41(3-4): p. 203-213. 9) Korhonen RK, Wong M, et al. (2002) Med Eng & Phy. 24(2): p. 99-108.
ACKNOWLEDGEMENT
The authors acknowledge the generous funding provided by the Equine Trust.