Biomedical Engineering Reference
In-Depth Information
A scaffold should provide an open porous network structure, allowing easier
vascularization, which is important for the maintenance of penetrating cells from
surrounding tissues and the development of new bone in vivo. The higher the macro-
porosity, the easier the vascularization of implant. The failure to develop an adequate
vascular network means that only peripheral cells may survive or differentiate,
supported by diffusion. Chang et al.
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proposed that the degree of interconnectivity
rather than the actual pore size has a greater influence on osteoconduction. Inter-
connectivity is a physical characteristic that aids in the delivery of nutrients and
removal of metabolic waste products. Some studies have shown that bone normally
forms in the outer 300
m periphery of scaffolds and that this may be explained by the
lack of nutrient delivery and waste removal.
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When the pore size is too small, pore
occlusion can occur by cells, preventing further cell penetration and bone formation.
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Pore size distributions for an ideal scaffold in bone tissue engineering applications are
summarized in Table 4.1. It is pertinent to note that much higher rate of mass transfer
exists at the periphery of a scaffold and that these higher rate promote mineralization,
further limiting the mass transfer of nutrients to the core of a scaffold.
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It is essential
that a scaffold possess a high degree of interconnectivity in conjunction with a suitable
pore size to minimize pore occlusion.
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4.5 AN EXCULPATION OF POROUS SCAFFOLDS
The concept behind nearly inert, microporous bioceramics is the ingrowths of tissue
into pores on the surface or throughout the implant. The porosity is a critical factor
for growth and integration of a tissue into the bioceramic implant. In particular, the
open porosity, which is connected to the outside surface, is critical to the integration
of tissue into the ceramic, especially if the bioceramic is inert. The increased
interfacial area between the implant and the tissues results in an increased inertial
resistance to movement of the device in the tissue. The interface is established by the
living tissue in the pores. This method of attachment is often termed biological
fixation. The limitation associated with porous implants is that for tissue to remain
viable and healthy, it is necessary for the pores to be greater than 100-150
min
diameter. The large interfacial area required for the porosity is because of the need to
provide blood supply to the ingrown connective tissue. Vascular tissue does not
appear in pores, which measure less than 100
m
m. If micromovement occurs at the
interface of a porous implant, tissue is damaged, the blood supply may be cut off,
tissue dies, inflammation ensues, and the interfacial stability can be destroyed.
The potential advantage offered by a porous ceramic is the inertness combined
with the mechanical stability of the highly convoluted interface when the bone grows
into the pores of a ceramic. The mechanical requirements of prostheses, however,
severely restrict the use of low strength porous ceramics to low-load or nonload-
bearing applications. Studies show that when load bearing is not a primary
requirement, nearly inert porous ceramics can provide a functional implant. Apart
from biological aspects, the mechanical requirement should also be fulfilled by the
engineered implant.
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