Tissue Engineering and Regenerative Medicine Strategies for the Treatment of Osteochondral Lesions - 3D Multiscale Physiological Human - page 29

mineralized matrix (Fig. 2.2 b). They support the structure and metabolism of the
bone [ 25 ]. Osteoclasts are the cells that resorb bone. Bone-lining cells are present
in the non-remodelling surface of the bone. They play an important role in mineral
homeostasis and can be stimulated to proliferate and differentiate into osteogenic
cells [ 26 ].
Bones in the body undergo growth, modelling and remodelling throughout life
to preserve the strength and mineral balance and adapt to new biomechanical con-
ditions. Mature bones in the body can be classified structurally into cortical bone
and trabecular bone. Cortical bone or compact bone covers the outer surface of the
bones and has a dense structure with a porosity less than 5% [ 25 ]. Trabecular bone
is also known as cancellous or spongy bone, and it is composed of trabeculae that
are porous interconnected irregular arrays of lamellar bone plates and rods [ 24 ].
Trabecular bone is present near the ends of long bones, inside the small bones as
well as between the surfaces of flat bones. Trabecular bone and cortical bone show
anisotropic mechanical behaviour due to their structure [ 27 ]. Trabecular bone is not
as dense as the cortical bone. The metabolic activity of trabecular bone is higher than
that of cortical bone [ 25 ].
SB lesions are often related with damage in the articular cartilage. SB affects the
articular cartilage both biomechanically and biochemically [ 28 ]. It absorbs the me-
chanical stress and maintains the shape of the joint [ 29 ]. It forms a transitional layer
between the articular cartilage on the surface and the interior bone volume [ 28 ]. The
calcified layers of the articular cartilage and the SB make contact at a thin interface
called the cement line [ 30 ]. SB is composed of the SB plate and subarticular spon-
giosa, which represent cortical and trabecular bone respectively. However, usually
the term SB is referred to both regions, regardless of their mechanical and physio-
logical differences [ 31 ]. The SB plate is the bony lamella or cortical endplate found
under the calcified layer of the articular cartilage, while the subarticular spongiosa
refers to the trabeculae under the SB plate [ 32 ].
The subchondral region shows large variations in anatomy, for example, in the
contour of the tidemark and cement line, and in composition and thickness [ 33 ].
SB adapts to the applied stress by altering its density and strength [ 29 ]. Long-term
mechanical load distribution within a joint surface affects the density, thickness,
vascularity and biochemical composition of the SB plate [ 32 ]. In patients with OA,
the stiffness, the density and the mineral content of SB are lower as compared to
normal tissue [ 34 ].