Biomedical Engineering Reference
In-Depth Information
have increased roughness and surface particles boundaries. Therefore, similar to
ceramics, these nano properties are promising for bone formation.
However, despite these promises there may be some problems with certain
titanium alloys. For example, Hong et al. reported that Ti surfaces are excessively
highly thrombogenic [83], leading to more rapid collagenous encapsulation and
faster remineralization. Some researchers have discussed the potential cytotoxic-
ity associated with the vanadium element in Ti6Al4V [77]. When Ti6Al4V was
compared with commercially pure Ti [84], gingival fi broblasts demonstrated a
rounded cell shape and a reduced area of spreading on the alloy, presumably be-
cause of a minor toxicity to vanadium or aluminum. In this light, it is important to
mention that fi broblast adhesion has been shown to be lower on nano compared
to conventional metals.
7.3.3 Nanostructured Polymers
Polymers are a class of synthetic materials characterized by their high versatility
[85] and, thus, have been widely applied in orthopedics. They can mechanically
secure components of hip replacements into bone—as in bone cements such as
injectable polymethylmethacrylate (PMMA)—and can line acetabular cups for
smooth articulation with metallic femoral head components (such as in UHM-
WPE). However, there also exists many disadvantages related to traditional
polymers used in orthopedic implants. For example, PMMA suffers from fatigue-
related cracking and impact-induced breakage due to its poor setting and fi xation
[86]. Furthermore, PMMA can also elicit a host response due to release of toxic
monomers and necrosis of the surrounding tissue caused by exothermic polymer-
ization
in vivo
[87]. Increased articulation-induced wear of UHMWPE frequently
leads to osteolysis (see section 7.2.2.2) [88].
Therefore, further development of various polymer alternatives that could
improve osseointegration is necessary, not only for fracture fi xation but also as
bone tissue engineering scaffolds. For this reason, polymers—such as polylactic
acid (PLA), polyglycolic acid (PGA), poly(lactic-co-glycolic acid) (PLGA) and
various hydrogels—currently generate more interest from scientists to investi-
gate their potentials as bone tissue engineering scaffolds to repair bone defects
due to their excellent biocompatibility, suitable mechanical properties, bio-
degradability and ease of modifi cation for different applications.
An ideal bone repair scaffold should be biocompatible to minimize local
tissue response but maximize osseointegration, and be biodegradable after new
bone formation. As will be described, an important consideration in scaffold
design is providing a polymer nanoscale framework for cellular interactions.
7.3.3.1 Nanostructured Biodegradable Polymers as Bone Tissue Engi-
neering Scaffolds.
One of the most common polymers used as a biodegradable
biomaterial has been PLGA [85]. It has high biocompatibility, the ability to de-
grade into harmless monomer units, and a useful range of mechanical properties,
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