Hydrothermal Crystallization with Microstructural Self-Healing Effect on Mechanical and Failure Behaviors of Plasma-Sprayed Hydroxyapatite Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 238

, and (2) increased uniform bone ingrowth and ongrow the bone-implant inter-
face. Therefore, HA has been generally acknowledged as an excellent bone substitute [1,2]
because of its ability to reduce the healing time after reconstructive surgery and extend the
functional life of the prosthesis.
During various clinical dental and orthopedic applications for the repair of bone defects
and immediate tooth root replacement, crystalline HA obtained through hot-pressing or
other conventional sintering processes were used as bulk implants. The bulk HA implants
generally show the maximum porosity less than 5% by volume with the micropores size
less than 1 μm in diameter [8,9], and they are also called dense HA. Although the crystal-
line dense HA exhibits higher flexural strength (about 115-200 MPa) [10] than the human
cortical bone (about 15-150 MPa) [11], the fracture toughness of the dense HA sintering
bulks (about 1.0 MPa m 1/2 ) is much lower than the human cortical bone (about 2-12 MPa
m 1/2 ) [11]. Dense HA is brittle and relatively weak compared with common implant metals,
alloys, and high-strength ceramics. In spite of the good biocompatibility and osteocon-
ductivity of HA, the limitations for the usage of the dense HA sintering bulks for bone
replacement are their low fracture toughness and bending strength under load-bearing
situations [12-14]. In the biomedical fields, bioactive ceramics have often been used as coat-
ings to modify the surface of bioinert metallic implants and in some cases to create an
entirely new surface that gives the implant properties quite different from the uncoated
implants. Figure 6.1 illustrates the artificial Ti-6Al-4V stem and the acetabular cup with
an HA-coated surface for the joint replacement of total hip prosthesis.
The concept to design a surface biological fixation for orthopedic joint implants such
as a total hip prosthesis has been achieved by the bone ingrowth and bone apposition
methods. Investigations for calcium phosphate ceramic coatings on metallic implants
started with the observation that HA in the pores of a metal implant with a porous coating
would significantly affect the rate and vitality of bone ingrowth into the pores [15]. HA is
extensively applied for the purpose of improving the bioactivity of these bioinert metal
implants including the stem and the acetabular cup. Therefore, the combination of high-
strength metallic substrates with osteoconductive properties of bioceramic ceramics makes
HA-coated titanium implants attractive for the load-bearing situations in orthopedic and
dental surgery. In addition to enabling earlier stabilization of the implant in surrounding
bone, using an HA coating extends the functional life of the prosthesis and improves the
adhesion of the prosthesis to the bone. Animal studies have indicated that HA-coated
titanium implants showed higher push-out strength compared to uncoated titanium
implants [16-19], and postmortem studies reported direct bone contact with implants
without a fibrous tissue interface in patients who had successful HA-coated total hip
arthroplasties [20,21]. Moreover, the bone bonding capacity of the HA coatings can help
cementless fixation of orthopedic prostheses. It has been shown that the skeletal bonding
is enhanced immediately after implantation [22-24].
There are many coating techniques available for applying HA onto metallic substrates,
including plasma spraying, chemical vapor deposition (CVD), RF sputtering coating, sol-
gel coating, electrochemical deposition, electrophoresis, and biomimetic coating methods
[25-32]. Compared with these techniques, plasma-sprayed HA coatings (HACs) deposited
on metallic implants, which is a state-of-the-art and extensively used process in commer-
cial products, can exhibit enhanced interfacial strength and tend to avoid the inherent
mechanical property limitations of HA without any significant loss in biocompatibility
[33,34]. Reports on the short-term to medium-term results of implantation of HA-coated
femoral stems have been encouraging [17,23,35-37]. Among the various biocompatible
metals, Ti-6Al-4V alloy used as a metallic substrate is more favorable compared to other

[4,6,7], and (2) increased uniform bone ingrowth and ongrow the bone-implant inter-