Biomedical Engineering Reference
In-Depth Information
Due to the complex gradient structure, AC shows anisotropic and nonlinear behavior in com-
pression, tension, and shear. Moreover, the mechanical properties of cartilage depend on its height.
The stiffness in axial direction increases with depth from the joint surface, especially in case of
compressed cartilage [5]. A highly hydrated cartilage can be described as a poroviscoelastic mate-
rial with a relatively low compressive stiffness (
E
=
0.1
-
2.0 MPa), Poisson's ratio (
υ
=
0.2), and
10
−
15
m
4
/N s) [6]. Despite the low stiffness, AC is able to transmit high
loads (up to 10 times the body weight in the knee joint). Moreover, AC provides a low coeffi cient of
friction of about 0.008 [5].
The functions of AC can be summarized as follows:
permeability (
k
=
5.0
×
•
•
•
•
•
Distributes high static and repetitive dynamic loads (up to 10 times the body weight)
Protects cancellous bone from high stresses
Provides a smooth, lubricated surface with a fl uid fi lm and a low coeffi cient of friction
Reduces nominal contact pressure
Guarantees joint conformity
21.2 REPAIR OF ARTICULAR CARTILAGE
Arthritis (
arth
infl ammation) is one of the oldest and most common diseases affecting
human joints. It probably affects almost every person more than 60 years of age to some degree [7].
In general, arthritis affects approximately 15% of the total world population, and by the year 2020,
this number is expected to increase to 18.2% due to increased aging. In arthritic joints, the cartilage
is degraded, causing pain, loss of mobility, and joint stiffness. In addition to osteoarthritis, focal
osteochondral injuries arising from sports are another cause for increasing applications of treatment
methods. Due to limited potential for regeneration related to the absence of blood circulation, the
treatment of cartilage defects is one of the most important problems in orthopedic surgery.
Unfortunately, in contrast to bone tissue, AC has a poor intrinsic capacity for repair. This lim-
ited cartilage regeneration is caused by (i) its avascular (and aneural) nature, (ii) relatively low cell
density, (iii) low mitotic activity of the cells, and (iv) structural restriction to free migration of the
cells.
The cartilage tissue response to injury or disease depends largely upon whether it involves
only part of the tissue (partial-thickness defects) or extends to the subchondral bone (full-thickness
defects).
Partial-thickness defects of AC resemble the clefts and fi ssures observed during the initial stages
of osteoarthritis. Defects of this nature in mature tissue do not heal spontaneously. In such a tissue, a
limited repair process takes place in response to the trauma within the tissue immediately adjacent
to the site of the defect (a brief period of chondrocyte proliferation and matrix synthesis).
Full-thickness defects pass through the zone of calcifi ed cartilage and penetrate the subchondral
bone thereby gaining access to the cells that reside in the bone marrow space including the mesen-
chymal stem cells located therein. The repair response of this type of a defect results in the formation
of a fi brocartilaginous tissue in the void. An immediate response to penetration of the subchondral
bone by a full-thickness defect, in some cases, brings about the formation of hyaline-like AC. This
repair tissue is a poor substitute for AC and, with time, there is marked degeneration of the repair
tissue and continued degeneration of the native AC.
Restoring a damaged articular surface is quiet a challenge calling for a multidisciplinary
approach that has to bring together research scientists, clinicians, and patients. Many new AC
repair techniques have emerged in the past decades some of which appear to be very promising.
The currently available treatments can be divided into four basic categories: (i) treatments that
stimulate the bone marrow to form a repair tissue, (ii) transplantation of osteochondral autografts or
allografts, (iii) implantation or transplantation of cultured autologous chondrocytes, and (iv) usage
of resorbable scaffolding with or without cells.
=
joint,
itis
=
