SCAFFOLD CONSIDERATIONS FOR OSTEOCHONDRAL TISSUE ENGINEERING - Intelligent Scaffolds for Tissue Engineering and Regenerative Medicine - page 783

or autografts. Different techniques exist where only one or some-
times several small osteochondral tissue plugs are used to fill a
larger defect (mosaicplasty). This approach has limited success
because of di culties with integration of the graft in the sur-
rounding tissue. In the case of allografts, inflammation can be a
serious problem. However, autografts have the disadvantage of cre-
ating large defects at the harvest site. To overcome these dis-
advantages, tissue engineering approaches are used to generate
osteochondral transplants.
38.4 Functional Properties of Bone and Cartilage and the
Important Differences Between Them
To understand the requirements for successful osteochondral graft
manufacture,itisimportanttounderstandtherolesofboneandcar-
tilage and the differences between them with respect to load bear-
ing,physicalmakeup,etc.,inthejointregion.Articularcartilage,also
referredtoashyalinecartilage,isacomplextissuecomposedmainly
of type II collagen, proteoglycans, water, and cells (chondrocytes).
The interconnected collagen fibrils, composed mainly of collagen
type II butalso types IXand XI, lie parallel to the surface. Withinthe
fiber matrix, aggrecan and hyaluronic acid combine to form highly
negatively charged proteoglycan aggregates. This negative charge
results in a large water retention capacity of the tissue, resulting in
swelling,whichisconstrainedbythecollagenfibers.Itistheseprop-
erties that give cartilage its remarkable mechanical properties both
in compression and in tension. Cartilage is subdivided into three
distinct zones: the superficial, middle, and deep zones. 18 The outer,
superficial zone faces into the joint space and articulates with the
opposing joint surface. At this cartilage-cartilage interface compo-
nents such as hyaluronan, proteoglycan 4, and surface-active phos-
pholipids,producedbysynovium,aswellascartilage,areimportant
to provide boundary lubrication and keep joint homeostasis. 19 The
superficial zone is where cell density is the highest. In the middle
zone, cells are more sparse and rounded with the collagen fibers
less aligned than in the superficial zone. Another change in mor-
phology is seen in the deep zone. Here the fibers are larger and