The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 92

Thus, hyaline cartilage, calcified cartilage, and SCB form a morphologically
continuous transition that is thought to play a major role in preventing large
cartilage deformations and facilitating efficient load transfer [ 50 ].
5.2.2.1 Hyaline Cartilage
The composition and structure of the hyaline articular cartilage in the synovial joint
changes with depth—from the articular surface to its interface with bone [ 49 ,
51 - 57 ]. Articular cartilage is classified into four zones: the superficial tangential
zone (STZ), the middle or radial zone (RZ), the deep zone (DZ), and the ZCC
(Fig. 5.1 ). The STZ, ~10% of the cartilage thickness, contains fine, densely packed
and parallel-aligned collagen fibrils that bend to form the outermost articular
gliding surface [ 54 , 58 , 59 ]. This zone contains little proteoglycan and may serve
to resist shear forces via Type IX collagen bundles that sit perpendicular to and thus
reinforce the collagen type II fibers. The middle RZ layer contains a greater
proteoglycan component [ 38 ]. In this layer, randomly oriented fibers transition
shearing forces at the surface into compressive forces within the DZ. In the DZ,
proteoglycans are constrained to less than 20% of their free solution volume and
trapped within a collagen fibril mesh. The resulting substantial osmotic pressure
within the hydrated cartilage creates a high-energy state and loads collagen fibrils in
tension [ 37 , 60 , 61 ]. These same DZ fibrils form bundles and assume an orientation
that is perpendicular to the mineralized tissue layer as they penetrate the synovial
joint's ZCC [ 62 ]. Cartilage's microstructural organization thus directs compressive
and shear loads from the articulating surface into the ZCC and underlying SCB.
Similarly, collagen fibers within the cartilaginous IVD are primarily loaded in
tension [ 42 , 63 ]. The IVD's cartilaginous endplate forms a barrier between the disc
and the subchondral plate and borders the cranial and caudal surfaces of each vertebral
body. Unlike the normally penetrating fibrils in the synovial joint, the collagen fibrils
in hyaline cartilage lie parallel to the mineralized tissue surface (Fig. 5.1 ).
The orientation of these fibrils as they penetrate into the ZCC, and thus how the soft
tissue within the IVD is anchored into the cartilaginous endplate, is not well described
in the literature. Further, the cartilage endplate is known to increase in mineral density
with aging and disc degeneration, thus exacerbating the property differences between
the soft IVD and the mineralized endplate region. This mismatch, in combination with
a possible lack of fibrillar anchors into the mineralized tissues, could explain the
high failure rate observed at the IVD osteochondral interface.
5.2.2.2 The Zone of Calcified Cartilage
Stresses are thought to be dissipated through the hydrated proteoglycans and
collagen fibrils that extend from the hyaline cartilage into the ZCC [ 64 ]. The ZCC
is formed when mineral-containing fluid exudes from pore spaces in the bone.