Chapter 3 Musculoskeletal Structure and Physiology 63abundant on a molar basis. The functions of
articular cartilage SLRPs are not completely
understood. SLRPs bind many components of
the ECM, including collagen and fibronectin,
and play roles in collagen and elastin fibrillo
genesis as well as serving to sequester growth
factors within the ECM.
Chondrocytes make up a small proportion of
articular cartilage yet they are responsible for
the biosynthesis, maintenance, and turnover of
the entire ECM compartment throughout life.
Collagen within canine articular cartilage is
extremely stable and most lasts for the life of
the individual. In contrast, the rate of turnover
of cartilage PG is more rapid (approximately
300 days; Maroudas, 1980). Under normal con
ditions, chondrocytes maintain a highly regu
lated balance of matrix biosynthesis and
degradation that is precisely adjusted to meet
functional requirements of the tissue. ECM
turnover is influenced by the prevailing
mechanical environment, along with a plethora
of cytokines and growth factors produced by
leukocytes, synovial lining cells, and chondro
cytes. Mechanical load and cytokines elicit the
production of catabolic enzymes that can
degrade ECM components. This degradative
process is normally counterbalanced by pro
duction of enzyme inhibitors and anabolic
growth factors. Imbalance between anabolic
and catabolic functions of the chondrocyte can
result in detrimental changes in ECM composi
tion and loss of mechanical integrity of the
ECM. This basic imbalance underlies most pro
cesses of joint degeneration. Loss of moderate
amounts of PG from cartilage ECM can be
restored by de novo biosynthesis; however,
extensive loss of PG or breakdown of the colla
gen network is irreversible and invariably leads
to progressive joint degeneration.
Articular cartilage is a heterogeneous tissue
the organization of which differs between
joints, as well as between different contact
regions within a given joint. Articular cartilage
is subdivided into five discrete zones
(Figure 3.15B). Zones I through III are the more
superficial and unmineralized zones. Zone III
is separated from the calcified cartilage of
zone IV by the tidemark. The deep limit of the
articular cartilage is the cement line that forms
at the conclusion of endochondral ossification
of the articular portion of the epiphysis.
Zone I (tangential) chondrocytes have flat
tened morphology, and are oriented parallel to
the articular surface. Zone II (transitional)
chondrocytes assume a more globoid shape.
Zone III (radiate) cells become oriented with
their axes perpendicular to the articular sur
face. The chondrocytes within articular carti
lage are arranged in close coordination with
the orientation of the collagen fibrils within
the matrix. Fibril organization varies with dif
ferent regions of the tissue, in accordance with
the local mechanical stresses that are gener
ated by joint loading. In zone I, stresses
develop parallel to the plane of the articular
surface, and collagen fibrils here are largely
parallel or tangential to the articular surface.
Zones II and III experience complex patterns
of shear, tension, and compression, and colla
gen fibrils in these zones are arranged in a
three‐dimensional cross‐linked isotropic web
adapted to resist multidirectional tension.
Compressive stresses prevail in the deeper
zones. Collagen fibrils in zones IV and V are
larger in diameter and, as in bone, mineraliza
tion enhances the ability of the tissue to
withstand compressive stress. Collagen con
centration is highest at the articular surface,
where tensile stresses are greatest, and
decreases within the deeper layers of the carti
lage. In contrast, PG concentration in articular
cartilage is greatest within the deep zones, and
decreases toward the articular surface.
Articular cartilage undergoes significant
deformation during normal activity, but these
strains are elastic and fully reversible. As a
composite tissue, articular cartilage has overall
low modulus of elasticity (stiffness); its
deformability enhances both the congruity and
the contact area of opposing articular surfaces
during joint loading. This results in dynamic
improvements in joint stability and reduced
stress per unit area of the articular surface.
In addition, cartilage surface deformation
drives fluid away from compressed regions
into adjacent regions of matrix, as well as into
the surrounding synovial fluid (Figure 3.16).
This bulk exchange of fluid between the carti
lage matrix and the synovial fluid is a major
route by which nutrients are delivered to the
chondrocyte and underscores the importance
of early joint mobilization and controlled
cartilage loading during rehabilitation from