Biomedical Engineering Reference
In-Depth Information
within the matrix. Therefore, strategies for the treatment of cartilage lesions have
been developed in which a new cell population are brought to the lesion site [
13
].
One such strategy is enhancing the cartilage healing by migration of stem cells from
the bone marrow to the injury site. Some examples of methods based on this strategy
are drilling of SB [
14
], joint debridement [
15
] and spongialization [
16
]. It has been
reported that none of these methods is actually helping cartilage to regenerate the
original tissue since the new formed tissue is a fibrous tissue. It has been attempted to
treat OC lesions with auto- and allografts as well [
10
,
11
]. The eventual fibrous tissue
formation caused biomechanical drawbacks, such as instability and reduced mechan-
ical strength and congruency of articular surfaces [
12
]. In addition, low availability
of material and donor site morbidity are also important limitations [
17
].
The goal of tissue engineering and regenerative medicine (TERM) is to regener-
ate tissues by preferably using patient's own cells, biodegradable biomaterials, and
relevant growth factors, alone or in a combination to increase the effectiveness. Con-
ceptually, in a tissue regeneration process, the cells can be obtained by means of
biopsy from the patient, grown in vitro and seeded into a porous scaffold, followed
by the cultivation of the scaffold-cell construct for some time in vitro in a cell culture
medium. This construct is implanted into the defect, and after the implantation, even-
tually, cells synthesize their extracellular matrix (ECM) and the scaffold degrades
gradually. However, the whole process is challenging. The challenge is bigger when
regeneration of more than one type of tissue is required. For example, in the case of
OC lesions, both articular cartilage and SB need to be treated.
2.2 Articular Cartilage
Articular cartilage plays important roles in the body. It covers the articulating ends
of the bones inside the synovial joints to form a low-friction gliding surface. The
cartilage reduces the peak stresses on the SB and also serves as a shock absorber [
8
].
Articular cartilage is an avascular, aneural and alymphatic tissue with a generally
anaerobic metabolism [
18
]. There are two main components in the cartilage: cells
called chondrocytes and the ECMwhich surrounds the cells [
7
]. Figure
2.2
a presents
a histological image of an articular cartilage. The chondrocytes and the ECM are
interdependent. The chondrocytes are responsible for the synthesis and the break-
down of the ECM. In return, the ECM surrounds the cells and protects them from
mechanical impacts while transmitting signals to the chondrocytes upon loading of
the cartilage [
19
].
Chondrocytes take up only 1-10% of the cartilage volume [
12
]. They synthesize
many enzymes, cytokines and growth factors that affect the anabolic and catabolic
activities [
3
,
6
]. Mechanical loading affects the functions of chondrocytes [
20
]. Some
chondrocytes have short cilia that reach into the matrix, which may have a function
in detecting changes in the matrix. Even though individual chondrocytes have ac-
tive metabolism, the articular cartilage still has slow metabolic activity due to low
cellularity [
19
].
