The Bone–Cartilage Interface - Structural Interfaces and Attachments in Biology - page 91

(Fig. 5.1 ). Within the IVD, the annulus fibrosis is composed of type I collagen that
is organized into lamellae, or highly aligned collagen sheets, of alternating fibril
directions in a manner that is similar to a rotating plywood structure [ 43 ]. The
gelatinous nucleus pulposus consists largely of water and proteoglycans that exist
within a network of collagen fibrils [ 44 ]. Cartilaginous endplates are formed of
hyaline cartilage structures with fibrils that run parallel to the bony endplate, where
a mineralized layer forms within the hyaline cartilage and adjacent to the vertebral
bone. This layered structure enables a physical, mineralized connection between the
bone and the soft tissues [ 42 ].
5.2.1.3 Fluids Within the Joint
A fibrous joint capsule, lined with fluid-producing cells within the synovial
membrane, encapsulates the joint space between adjacent bones. Within the joint
space, viscous fluid bathes the synovial joint's hyaline articular cartilage to lubricate
gliding surfaces and deliver nutrients to the cartilaginous cells (i.e., chondrocytes).
Loading and unloading of cartilage at low strain rates causes the synovial fluid to
exude into the joint space, thus providing additional lubrication, and reabsorb into
the cartilage upon unloading. When impacted at high strain rates, the “shock” is
absorbed by frictional entrapment of synovial fluid within a charged collagen and
proteoglycan matrix [ 45 ]; the resulting high apparent stiffness of the cartilage likely
reduces the modulus mismatch and consequently facilitates transmission of forces
between the bony and cartilaginous tissues.
Within cartilaginous joint tissues, such as within the annulus fibrosus and
nucleus pulposis of the IVD, the water-based interstitial fluid contains little mineral
content. This fluid facilitates removal of waste products and movement of essential
nutrients including oxygen, glucose, and substrates for matrix production [ 44 , 46 ].
Oxygen concentration and pH vary across the disc, with distance from the nearest
blood supply (up to ~7-8 mm), where diffusion drives nutrient and gas exchange
[ 46 , 47 ].
5.2.2 Microscopic Structure of the Bone-Cartilage Interface
Initially, long bones and vertebral bodies lengthen and grow through a process
known as endochondral ossification. In this process, the bone lengthens and the
leading mineralization front progresses as cartilage at the growth plate becomes
calcified to form a secondary center of ossification and the ZCC [ 48 , 49 ]. Bone
remodeling, removal of bone by osteoclasts and replacement with new bone by
osteoblasts, occurs in the SCB plate and underlying subchondral trabecular bone
(STB) to deposit bone during development and maintain the integrity of bone
throughout maturity [ 41 ]. Remodeling cells also may invade the underside of the
ZCC and create a highly interdigitated interface between the ZCC and SCB.