The Role of Mechanobiology in the Attachment of Tendon to Bone - Structural Interfaces and Attachments in Biology - page 225

critical to bone maintenance, the exact biological mechanisms that transduce
changes in the loading environment to changes in the rate of tissue formation,
which ultimately affects tissue mechanical behavior, are less well understood.
Moreover, extremes of unloading and overloading of musculoskeletal tissues lead
to pathological conditions.
Articular cartilage serves to cushion the ends of bones and to reduce friction
during movement. Mature articular cartilage has a zonal structure through its depth
and is essentially an arrested bone growth front at the ends of long bones. The
morphology is classified into surface, middle, and deep zones, with variations in
structure, composition, and mechanical properties. The tensile modulus of the
superficial zone is higher than the deep zone, while the deep zone has a greater
compressive modulus [ 45 , 46 ]. The two primary loading modes experienced by
chondrocytes in articular cartilage due to joint loading are hydrostatic stress and
shear stress. Chondrocytes subjected to high levels of compressive hydrostatic
stress produce type II collagen and aggrecan while suppressing vascularization;
all characteristics of a stable chondrocyte phenotype [ 47 ]. Shear stresses in cartilage
promote fibrillar collagen production. Tensile stress promotes vascular invasion
and ossification that result in advancement of the growth front [ 48 ]. Cyclic com-
pressive forces induced by joint loading are required to maintain the properties of
articular cartilage. Joint immobilization eliminates cyclic compressive stress,
activating the subchondral growth front and inducing degradation of the cartilage
layer, a characteristic of osteoarthritis [ 49 ]. This also occurs in regions of joints that
experience low levels of loading.
Tendons and ligaments are responsible for transmitting forces from muscle to
bone in the case of tendons and from bone to bone in the case of ligaments. Both are
essential for joint stabilization and movement. Like bone, tendon and ligaments are
remodeled in response to mechanical stimuli. Changes in tendon due to exercise
include an increase in the cross-sectional area and an increase in collagen turnover
[ 50 ]. Immobilization leads to a decrease in collagen synthesis and a decrease in
stiffness and tensile strength [ 51 , 52 ]. Unloading of ligaments results in changes to
the composition and mechanical behavior of the tissue [ 51 - 54 ]. Moreover,
localized changes in the tendon loading mode (e.g., tension vs. compression) due
to wrapping around a bone pulley in a joint induces a more chondrogenic pheno-
type, with increases in the local proteoglycan content and downregulation of
angiogenic factors [ 55 - 60 ]. Based on these observations, it is clear that mechanical
stimuli are critical to the maintenance of skeletal tissues.
11.3.2 Mechanobiology in Fetal and Postnatal Development
Biophysical cues also drive developmentalpatterningandgrowthinthefetaland
postnatal musculoskeletal system [ 61 ]. Bones, tendons, muscles, and joints
are patterned in utero but development continues through postnatal stages. During