Biomedical Engineering Reference
In-Depth Information
4.2.2 Cartilage
encourage force transfer in load-bearing
defects. In this way, mechanical signals are
gradually transmitted to the resident cells,
thus encouraging tissue remodeling via
exposure to dynamic loading conditions [
Cartilage is an avascular tissue composed of
chondrocytes embedded in an extracellular
matrix consisting of water and a solid matrix.
The solid matrix consists of proteoglycans and
collagens, as well as glycoproteins in lesser
amounts. Three types of cartilage have been
described, which differ in composition: hyaline
cartilage, elastic cartilage, and fi brocartilage
[
2
,
106
].
Over several decades, a number of bio-
materials for orthopedic applications have
been investigated and developed. In this
chapter, applications, important properties,
and different types of biodegradable materials
will be discussed in order to provide an over-
view of the state of the art in orthopedic
biomaterials.
].
Hyaline cartilage is a glassy and homoge-
neous cartilage composed primarily of type II
collagen fi bers and proteoglycans. This unique
combination of collagen fi bers and hydrophilic
proteoglycans gives cartilage important visco-
elastic properties that allow it to disperse forces
while acting as a lubricator. Elastic cartilage is
similar to hyaline cartilage; however, it also
contains elastic fi bers and an interconnecting
sheet of elastic material. It is often found in the
external ears and the walls of the acoustic
meatus. Fibrocartilage possesses properties
that are intermediate between those of dense
connective tissue and hyaline cartilage and
contains both type I and II collagen. Fibrocar-
tilage is the main constituent of tissues such as
the meniscus of the knee [
36
,
45
4.2 Background
Before developing biomaterials for a particular
orthopedic tissue-engineering application, it is
important fi rst to understand the basic proper-
ties of the different musculoskeletal tissues
such as bone, cartilage, ligament, and tendon.
This basic information allows developing mate-
rials and strategies that are specifi cally tailored
for each type of tissue defect.
36
,
45
].
4.2.1 Bone
4.2.3 Tendon
The main function of bone tissue is to support
the body. Bone tissue is maintained by the
balance in activity between bone-forming and
bone-resorbing cells. The collagen fi bers impart
tensile strength, and the mineral salts, a form
of calcium phosphate (hydroxyapatite), increase
the toughness and hardness of the tissue [
Tendons are dense tissues that connect muscle
to bone. Tendon tissue consists of fi broblasts
surrounded by type I collagen, a small amount
of type III collagen, and small quantities of
proteoglycans (dermatan sulfate and hyal-
uronic acid). Triple-helical collagen molecules
are assembled into fi brils that are cross-linked
through aldol or Schiff base adducts between
aldehydes on one or more of the
].
Three types of cells coexist in bone: osteoblasts,
osteoclasts, and osteocytes. Osteoblasts are
bone-forming cells responsible for the forma-
tion of the hard extracellular matrix, whereas
osteocytes are fully mature embedded bone
cells that maintain the tissue structure. Osteo-
clasts selectively resorb bone in certain areas
in response to a biochemical or biomechanical
stimulus [
7
-chains of
collagen molecules and aldehydes or amino
groups on adjacent chains. This cross-linking
imparts the high tensile strength needed for
proper tendon function [
α
5
,
31
].
4.2.4 Ligament
].
Human bones are described as compact
(cortical) or spongy (cancellous), depending on
their density. Compact bone consists of central
canals and perforating canals surrounded by
concentric rings of matrix. Spongy bone is
much less dense, having irregular lattice struc-
tures where spaces are fi lled with bone marrow
[
21
Ligaments are made up of closely packed
fi bers and are in many respects similar to
tendons. However, the relative amounts of the
various extracellular matrix (ECM) compo-
nents are not the same as in tendons. Specifi -
cally, ligaments have less total collagen and
more proteoglycans than tendons. Ligaments
101
].
