The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 90

5.2.1 Anatomy of the Bone-Cartilage Interface

The synovial joint is a well-studied region that contains bone and articular cartilage,
has a consistent structure throughout most articular joints, and undergoes significant
damage with aging and arthritis. Similarly, the cartilaginous joint brings together
two adjacent bones via a continuum of fibrocartilage and hyaline cartilage and often
degenerates with age. Despite the prevalent health detriments of disc degeneration,
the microstructural and mechanical makeup of the cartilaginous joint's

bone cartilage interface is less well understood as compared to the interface region

within the synovial joint.

-

5.2.1.1 The Synovial Joint

The synovial joint is diarthrotic, or freely moveable, and permits the relative motion
via sliding of two adjacent bones that are typically covered with ~1-2 mm thick layer
of smooth hyaline articular cartilage (Fig. 5.1 ). The avascular hyaline cartilage is
comprised mainly of Type II collagen and hydrophilic charged proteoglycans that
swell with water to provide cushioning. The cartilaginous structures form the
primary load-bearing surface of the synovial joint, which varies in microstructure
and composition throughout its depth. In superficial layers of cartilage, collagen
fibrils are arranged parallel to the surface such that they distribute loads over a
greater tissue volume and withstand primarily tensile and shear stresses [ 38 ]. As
collagen fibrils progress into the deep radial zones of the cartilage, they bend to a
direction that lies perpendicular to the calcified cartilage surface. The fibrils main-
tain this perpendicular orientation as they penetrate into and through the ZCC
[ 39 , 40 ]. Adjoined to the ZCC is a plate of SCB—a cortical-like region of SCB
that lacks significant porosity yet is well vascularized. The SCB plate and the
adjacent region of the ZCC may undergo remodeling, with ultimate replacement
by bone or infilling of bone within the underlying trabecular compartments [ 41 ]. The
organization of collagen fibrils in synovial joints creates a continuous structure that
extends into the ZCC and serves to transmit primarily compressive and shear loads
between cartilage, SCB, and the extended bony structure.

5.2.1.2 The Cartilaginous Joint

The cartilaginous joint forms a stabilizing and permanent cartilaginous union
between two adjacent bones. These joint structures are slightly moveable, or
amphiarthrotic, and occur at locations that include the pubic symphysis and the
joints between adjacent vertebral segments. The cartilaginous region is formed by
an avascular fibrocartilage disc that attaches to hyaline cartilage-covered bony
ends. As an example, the IVD contains a central nucleus pulposus that is encircled
by the annulus fibrosis and is sandwiched between two cartilaginous endplates