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Biomechanics of the Cruciate

Ligaments

Susannah J. Sample

Introduction

Stifle joint kinematics is the result of a complex
mechanical system. Stability is provided by a
combination of static and dynamic structures,
with the cruciate ligaments acting as main sta-
bilizers, providing transitional and rotational
constraint. The canine cranial cruciate ligament
(CrCL) is the most widely studied ligament in
veterinary medicine. Over the past 40 years,
studies of the CrCLhave been primarily focused
on either surgical methods to replace its func-
tion after rupture, or advancing the understand-
ing of histological and mechanical age-related
degeneration. Unlike in humans, little is known
about the specific biomechanical properties of
the CrCL in dogs. More detailed biomechani-
cal descriptions of the CrCL are needed as tech-
nologies for ligament replacement procedures
in dogs evolve.
Biomechanical properties refer to the rela-
tionship between the length and tension of a
given biologic material. This chapter will pro-
vide a short overview of the biomechanical
properties of ligaments, and summarize what
is known about the biomechanics of the canine
CrCL. Biologic factors and disease conditions
that are known to influence the biomechanical
properties of ligament will also be discussed,

as will the biomechanics of graft materials

that have been considered for possible cruciate

repair.

Ligament composition

Ligaments have a hierarchical architecture (Fig-

ure 2.1) and consist of a combination of water

and longitudinally running collagen fibers,

which are mostly type I collagen (70–80% of tis-

sue dry weight and>90% of collagen), with a

small amount of type III collagen (3–10%). Mini-

mal amounts of types V, X, XII and XIV collagen,

elastin, and proteoglycans are also components.

The hierarchical architecture of the ligament

influences its mechanical properties. Ligament

is made up of multiple smaller structures called

fiber bundles (Figures 2.1 and 2.2). Each fiber

bundle, in turn, is comprised of the basic fibers

of the ligament and includes fibroblasts; fibrob-

lasts are the primary cells that make up both lig-

ament and tendon. Ligament fibers have a vary-

ing amount of crimp (Figure 2.2). Crimp is a sort

of wave within collagen fibers (Figures 2.1 and

2.2), and is the primary driving force behind the

nonlinear stress–strain relationship that exists

initially when the ligament undergoes ten-

sile loading. Crimp facilitates a progressive

Advances in the Canine Cranial Cruciate Ligament, Second Edition. Edited by Peter Muir. © 2018 ACVS Foundation.
This Work is a co-publication between the American College of Veterinary Surgeons Foundation and Wiley-Blackwell.

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