Interaction ofCartilage andCeramic Matrix - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 298

1 Introduction
Articular cartilage defects have only a limited potential to heal spontaneously,
which results in joint pain and restricted functioning [ 1 , 2 ]. Small defects of the
articular cartilage surface lead to progressive loss of proteoglycans and disruption
of the collagen network. Often cartilage damage proceeds to a full-thickness
defect, which also affects the subchondral bone [ 3 ]. Consequently, regeneration of
the underlying bone has to be incorporated into cartilage repair. Additionally, as
the fixation of in vitro generated cartilage often causes problems during and after
implantation because of high shear stresses, e.g. of up to 1.7 times body weight in
the knee joint, a simultaneous replacement of cartilage and bone is advisable even
if the bone is not affected [ 4 - 6 ].
Tissue engineering approaches therefore focus on the generation of osteo-
chondral implants [ 7 ]. Instead of transplanting a cartilage-bone cylinder from a
non-load-bearing area of the joint into the defective site, as done during autologous
osteochondral transplantation (AOT), cells are cultivated in combination with
biomaterials in vitro. These biphasic constructs are designed to reconstruct carti-
lage tissue as well as the underlying bone after implantation [ 7 , 8 ]. Integration of
the carrier into the bone provides an anchorage for the cartilage constructs in the
joint. According to Martin et al. [ 5 ], approaches can be divided into four strategies:
(a) The bone phase is grown with a scaffold, but the cartilage is cultivated without
a scaffold on top of the bone. (b) Different scaffolds are used for the bone and the
cartilage phase and cultivated separately. Scaffolds are connected during
implantation. (c) One scaffold is used for both phases. This scaffold has different
structures or compositions for the bone and the cartilage tissue. (d) One homog-
enous scaffold is used for both phases.
Bioceramics such as bioactive glasses, hydroxyapatite and other calcium
phosphates are often used for these approaches as they offer bioactive,
osteoconductive and partly osteoinductive properties [ 9 ]. Calcium phosphate
ceramics in particular are favoured as bone substitute or coating materials because
of their natural occurrence in the human body [ 10 , 11 ]. Additionally, bioceramics
contribute good mechanical (compression strength, stiffness, wear resistance) and
chemical stability. Against this, their disadvantages are their brittleness and low
biodegradability.
During generation of osteochondral implants, biomaterials provide a scaffold
both for bone ingrowth as well as for cartilage formation. Thus, the applicability of
the bone equivalent to cartilage tissue has to be proven. As cultivation principles
often include a proliferation step of chondrocytes which is connected with a
dedifferentiation, as additional requirements biomaterials should exhibit not only a
carrier for cell growth but also have to support chondrocyte differentiation and
cartilage development.