MATERIALS FOR ORTHOPEDIC APPLICATIONS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 86

weeks soaking in SBF and the subsequent bonelike apatite crystal precipita-
tion. These coatings showed superior mechanical stability to the pure HAp
coatings, indicating much better long-term stability of the composite coatings
in physiological environment. With the interposition of a composite bond coat
composed of 50 vol% HAp and 50 vol% TiO 2 , a composite coating on titanium
substrate 122 was fabricated by plasma spraying and no chemical reaction was
observed between HAp and TiO 2 . Toughness increases with the addition
of TiO 2 .
Godley and co - workers 123 argued that the essential condition for a biomate-
rial to bond with living bone is the formation of a biologically active bonelike
apatite on its surface. In their work, it was demonstrated that chemical treatment
can be used to create a calcium phosphate (CaP) surface layer, which might pro-
vide the alkali-treated Nb metal with bone-bonding capability. The formation of
a similar CaP layer upon implantation of alkali-treated Nb into the human body
should promote the bonding of the implant to the surrounding bone. This bone
bonding capability could make Nb metal an attractive material for hard tissue
replacements. This possibility requires further investigation.
Pajamaki et al. 124 studied the effect of glass-ceramics coating on Titanium
implants. They exhibited the results with uncoated Ti, and showed that after 52
weeks, the coated metal showed 78% bone ingrowth, whereas in the case of un-
coated metal, the bone coverage was only 37%.
Munting 125 explained the merit and demerit of HAp coating on metal im-
plants. The limitation of hydroxyapatite coatings to implant fi xation were dis-
cussed following a fi ve-year histomorphological study of the bony incorporation
of macroporous stemless hemiarthroplasties in dogs. The results, obtained with
implants without coating, with a pure titanium coating and with a hydroxyapatite
coating were compared. Munting showed that HAp coating suffers from limited
strength and poor fatigue strength 125 . Also, HAp can be dissolved in vivo in an
acidic environment, created by macrophages. Bone ingrowths can be observed in
contact with resection surface. To this end, the coating thickness is the most im-
portant parameter as thicker coating has a chance to delaminate and thinner
coating has a shorter life. A coating with thickness of 40-60

μ m has been reported

to be ideal one.
Wang et al. 126 compared the in vivo bone apposition on plasma-sprayed and
electrochemically deposited hydroxyapatite coatings on Ti6Al4V alloy with un-
coated Ti6Al4V alloy. It was revealed that plasma-sprayed HAp coatings had a
higher bone apposition ratio than those exhibited by bare Ti-6Al-4V and elec-
trochemically deposited HAp coatings after seven days. However, after 14 days
of implantation, both the coated materials exhibited similar bone apposition ra-
tios, much higher than that for uncoated Ti-6Al-4V. Figure 3.14 shows the inter-
action of tissue with plasma-sprayed (3.14a) and electrochemically deposited
(3.14b) coatings.
In their work, Chen et al. 127 converted the surface of bioinert NiTi shape
memory alloy into a bioactive surface by immersing the implant alloy into simu-