Biomedical Engineering Reference
In-Depth Information
Chapter 25
A Micromechanical Viscoelastic Constitutive
Model for Native and Engineered Anterior
Cruciate Ligaments
Jinjin Ma and Ellen M. Arruda
Abstract
Ligaments and tendons are soft tissues that are largely composed of
aligned collagen and elastin. Due to this microstructure, they have nonlinear vis-
coelastic responses. We have developed a micromechanical constitutive model to
capture the inhomogeneous, nonlinear viscoelastic properties of native ACL and of
a tissue engineered ligament graft upon explantation. This constitutive model incor-
porates a viscoelastic collagen network and a nonlinear elastic elastin network. The
model captures the nonlinear viscoelastic responses of these tissues using a lim-
ited number of parameters that can be interpreted in terms of physical properties of
the collagen fibers and elastin. The parameters used to model the tissue engineered
ligament response are similar to those found for the native ACL, indicating that
the microstructure of the tissue engineered ligament graft has developed in vivo to
match that of the native ACL.
25.1 Introduction
Ligaments and tendons are fibrous connective tissues that transmit forces from mus-
cle to bone (tendon) or from bone to bone (ligament). In particular, the anterior cru-
ciate ligament (ACL) is one of four major ligaments that stabilizes the knee. The
ACL assists knee movement in anterior-posterior translation and constrains the tibia
from excessive anterior motion relative to the femur. ACL tears (or ruptures) occur
when the knee experiences a sudden landing with or without direction change. The
incidents of ACL injury are on the rise and an ACL rupture is now one of the most
common knee injuries in the US. Without a functional ACL, a patient's knee will ex-
perience abnormal joint movement and this may induce further knee complications
such as osteoarthritis or multiple ligament tears. Eventually the knee may develop
severe degenerative joint diseases that require a total knee replacement. The current
J. Ma
ยท
E.M. Arruda (
)
University of Michigan, Ann Arbor, MI 48109, USA
e-mail:
arruda@umich.edu
J. Ma
e-mail:
jinjinma@umich.edu
