Biomedical Engineering Reference
In-Depth Information
1 Introduction
Although the structure and mechanics of collagenous connective tissues have been
studied for decades, a clear understanding of the relationships between hierarchical
organization and material behavior is severely lacking. This can be attributed, at
least in part, to an inability to integrate and couple mechanics between different
physical scales. In the case of ligaments and tendons, this requires integration of
information
on
structure,
organization
and
material
behavior
of
constituents
between the nanoscale, microscale and mesoscale.
In theory, this integration can be accomplished using the theory of homoge-
nization [
37
,
78
,
129
,
203
,
256
]. In homogenization, a microscale boundary value
problem is used to determine the governing behavior at the macroscale. In linear
theory, a homogenization yields the components of the so called ''effective''
elasticity tensor [
172
]. In nonlinear theory, the macroscale boundary value prob-
lem is solved simultaneously using the methods of computational mechanics, and a
nested solution of two boundary value problems is obtained. Homogenization
techniques have been applied to bone [
165
,
172
,
176
,
197
,
223
], cartilage [
203
],
myocardium [
146
,
159
], arteries [
207
], cells [
97
], connective tissues [
4
,
12
,
37
,
42
,
43
,
137
,
149
,
187
] and biomaterials [
43
].
The function of the organizational motifs of collagen in connective tissues, and
their mechanical interactions across scale levels, is of fundamental importance in
understanding normal tissue function and the etiology and treatment of injury and
disease, both acquired and inherited. In the case of injury and disease, changes to
ECM organization are one of the most often observed effects on the tissue. It is
well known that ligaments and tendons primarily heal by scar formation [
10
,
68
,
69
,
106
,
140
,
141
,
238
], and that the healed tissue is inferior to the normal tissue
both in terms of structural organization and material properties [
24
,
54
,
69
,
106
,
112
,
238
]. The reasons for the lack of a more ''regenerative'' healing response
continue to evade us, but an improved understanding of the basic tissue structure
will help to interpret the alterations in structure that are present in healing tissues.
Many heritable diseases directly affect type I collagen structure and fibrillogenesis,
resulting in varied phenotypes that alter the multiscale structure of collagen (e.g.,
osteogenesis imperfecta, Ehlers-Danlos syndrome, [
193
,
219
]). These diseases
cause relatively well-characterized alterations in structure/organization of type I
collagen at the nanoscale (fibril) level, as well as other levels. Additionally, since
collagen fibrillogenesis is regulated by other ECM components, alterations to these
components can directly influence collagen structure. Examples include disorders
that affect decorin, biglycan and elastin/fibrillin such as congenital stromal corneal
dystrophy [
36
], periodontal disease [
92
] and Marfan Syndrome [
161
].
The overall objective of this chapter is to review the state of the art in multi-
scale modeling of ligaments and tendons, while providing sufficient background on
the structure and function of these tissues to allow a reader who is new to the area
to proceed without substantial outside reading.
Section 2
reviews the multiscale
structure and function of ligaments and tendons, including the constituents and
Search WWH ::
Custom Search