Page 155 - Materials Chemistry, Second Edition

142 2 Solid-State Chemistry

(iv) The surrounding tissue will replace the implant if the material is nontoxic and
soluble
The encapsulation of soft tissue (ii above), causes deleterious wear and abrasion in
metallic implants. Other contributing factors in decreasing the lifetime of an implant
include infection, inflammation, and lack of prolonged bonding between the implant
and surrounding bone. Consequently, the current average lifetime of a titanium
orthopedic implant is on the order of 10–15 years. However, there are many efforts
devoted to improving the longevity of implants. One such approach features mod-
ifying the titanium surface via anodization (see Chapter 3), to increase the surface
area and mimic the roughness of natural bone, which facilitates the adsorption of a
larger number of bone-forming cells known as osteoblasts. To address the other
factors, this approach also features coating the implant with anti-infection and anti-
inflammation drugs (penicillin/streptomycin and dexamethasome, respectively). [90]
There are three primary methods used for bone substitution, required for appli-
cations such as spinal deformities and “non-unions” (fractures that do not heal within
9months). Autografting consists of transplanting a bone from one region of the
patient’s body (usually the pelvic region) to the desired location. This procedure is
often preferred, as it precludes immunogenicity problems; however, there may be
complications and additional pain at the harvesting site, as well as the additional

surgical costs for the combined harvesting/transplanting procedures. In contrast, allo-
grafting consists of harvesting bone from a live or deceased donor for the transplanting
procedure. These implants are much less successful than autograft implants due to
immuogenicity, transmitted diseases, and the absence of viable osteoblasts. Due to
these limitations, it is becoming increasingly more popular to use synthetic materials as
bone substitutes. Whereas refractory ceramics such as alumina (Al 2 O 3 ) and zirconia
(ZrO 2 ) are used in high-wear applications such as joint replacements, calcium phos-
phate/sulfate based ceramics are used for bone regeneration applications.
The most common synthetic bone grafting ceramic is Ca 10 (PO 4 ) 6 (OH) 2 , known as
hydroxyapatite (Figure 2.98). Unlike other calcium phosphates, hydroxyapatite is
stable under physiological conditions, which features a pH range of 5.5–7.2.
Hydroxyapatite serves as an osteoconductive scaffold to which proteins and cells
may nucleate, and within which bone-forming cells known as osteoblasts are
generated (Figure 2.99). In addition to using hydroxyapatite as a filler material, it
may also be used as a coating material. That is, while its mechanical properties are
too brittle to withstand load-bearing applications, hydroxyapatite may be used as a
coating on metallic alloys to impart a greater biocompatibility of joint replacements,
while reducing the release of metallic ions from the implant.
Interestingly, the composition, phase, morphology, and placement of hydroxyapa-
tite will influence the speed and extent of bone growth. Since the resorption process is
surface-driven by the adsorption of osteoblasts, the ultimate solubility of a ceramic
will be directly related to its surface area – i.e., crystal size and density. In addition,
careful control of processing parameters is necessary to prevent thermal decomposi-
tion of hydroxyapatite into other soluble calcium phosphate phases (e.g., tricalcium
phosphate, Ca 3 (PO 4 ) 2 , tetracalcium phosphate, Ca 4 (PO 4 ) 2 O, and CaO), which