ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

PARAMETRIC ANALYSIS OF AN ENGINEERED ZONE OF
CALCIFICATION AT THE INTERFACE BETWEEN BONE AND
HYDROGEL: MECHANICAL AND NUMERICAL EVALUATION
1. ABSTRACT
J. Hollenstein 1 , A.C. Chen 2 , A. Terrier 3 , R.L. Sah 4 and D.P. Pioletti 5
A zone of calcified cartilage (ZCC) links bone to hyaline cartilage. This zone creates a
gradient of mechanical properties from soft to hard tissues and is essential for the
normal attachment of articular cartilage to bone. The objective of this study was to
evaluate, numerically and experimentally, the potential mechanical advantage of an
engineered zone of calcification (teZCC) for an artificial osteochondral construct. A
numerical push-off test model was then developed to compare the shear stress at the
interface between a bone scaffold and a hydrogel with or without a teZCC. A push-off
of the hydrogel at 15% strain induced an interfacial shear stress of 60 kPa without
teZCC, while this value dropped to 40 kPa with a teZCC at its interface with the
hydrogel. A simulated osteochondral defect treatment also showed a reduction of the
interface shear stress with teZCC at 10% compression. Experimentally, the development
of a teZCC was then performed in an agarose hydrogel at the interface between the
hydrogel and devitalized trabecular bone. The elastic modulus increased from 0.16 MPa
in the hydrogel to 1.5 MPa at the teZCC position. During a push-off test on the
engineered construct, the maximum peak load increased from 0.4 N to 0.8 N with the
presence of the teZCC. Enhanced interfacial mechanical properties between a hydrogel
and a porous rigid structure may therefore be achieved by creating a biomimetic
calcified interface.
2. INTRODUCTION
A zone of calcified cartilage (ZCC) links the interface between bone and hyaline
cartilage. The ZCC creates a gradient of mechanical properties facilitating the transfer
of load across the bone-cartilage interface. The ZCC also allows the attachment of
cartilage to bone, and protects hyaline cartilage from passive mineralization [1]. The
calcified interface functions as a mechanical transition with a mineral content conferring
an intermediate stiffness between that of soft tissue and bone [2, 3]. One of the key
structural components of this interface is hydroxyapatite.
Evaluation of the integration strength between bone and cartilage showed that fractures
occurred into the ZCC in immature bovine tissue. Through maturation, the ZCC creates
a strong interface with the subchondral bone, called the tidemark, at which fractures
occurred in mature tissues [4]. Equally, the formation of a calcified zone increased the
1 Grad Student (MS), Institute of Bioengineering, EPFL, CH-1015 Lausanne, Switzerland and Dept of
Bioengineering, UCSD, 9500 Gilman Dr., La Jolla, CA 92093, USA
2 Research Scientist (PhD), Dept of Bioengineering, UCSD, 9500 Gilman Dr., La Jolla, CA 92093, USA
3 Research Scientist (PhD), Institute of Bioengineering, EPFL, CH-1015 Lausanne, Switzerland
4 Professor (MD ScD), Dept, UCSD, 9500 Gilman Dr., La Jolla, CA 92093, USA
5 Assistant Professor (PhD), Institute of Bioengineering, EPFL, CH-1015 Lausanne, Switzerland