Tissue engineering - Orthopaedic Biomaterials in Research and Practice - page 233

into the defect to induce and direct the growth of new bone. The goal
is for the cells to attach to the scaffold, multiply, differentiate (i.e.,
transform from a nonspecific or primitive state into cells exhibiting the
bone-specific functions), and organize into normal, healthy bone as the
scaffold degrades.
Interest in using components from bone to provide a more “natu-
ral” alternative led to the development of bone morphogenetic proteins
(BMPs), which were initially discovered by Marshall Urist in 1965.
These are growth factors that are capable of inducing the formation of
new bone. As an alternative to autologous bone graft, which is associated
with significant donor site morbidity, recombinant versions of BMPs,
rhBMP-2 and rhBMP-7, have been shown clinically to be beneficial in
treating a variety of bone-related conditions including delayed union
and non-union. Although BMP has demonstrated extensive popularity,
growing substantially in use after its clinical introduction, recent years
have brought controversy with its use. For example, there are concerns
with swelling after its use in anterior cervical fusion. The development
and use of BMPs are further discussed in Chapter 10.
Other areas of development pertain to treating injuries from combat
situations. Although the current gold standard for replacing bone is auto-
graft material, the intent is to provide off-the-shelf materials containing
biological factors (e.g., hemostatic agents, analgesics, antibiotics, drugs,
hormones, and cells) and biodegradable scaffolds to stabilize, protect,
minimize tissue damage, and promote tissue repair. This avoids the need
to harvest autograft bone from a wounded individual or to remove exces-
sive bone when the supply may be limited because of extensive bone
injuries. Off-the-shelf calcium aluminate materials have been investi-
gated on the basis of hydratable formulations that are engineered with
antimicrobial and bone regeneration-enhancing activities. These grafts
are intended for use as an easily castable/moldable bone implant mate-
rial for a variety of reconstructive orthopaedic (e.g., implants for critical-
sized fractures, hip and knee replacements).

Cartilage Cells from cultured periosteum have the ability to form new bone, as

well as cartilage. However, articular cartilage has limited capacity to
regenerate itself compared to bone. Even minor lesions or injuries to
cartilage may lead to progressive damage, and in case of articular carti-
lage, these can lead to subsequent joint degeneration. Autologous chon-
drocyte implantation for cartilage repair was first performed in 1987.
Damaged knee cartilage has been replaced by harvesting autologous
chondrocytes, growing them in a biodegradable matrix, and then trans-
planting them in place of the damaged tissue. However, some concerns
with the formation of fibrocartilage and progression of degenerative
changes in the joint have been raised. Cartilage repair and restora-
tion is a relatively new advancement and is becoming an increasingly
important part of orthopaedic care. Knowledge and understanding of
the available surgical techniques are critical to the appropriate use of
these interventions. Further discussion of cartilage materials is included
in Chapter 10.