| Owing to excellent biocompatibility, Hydroxyapatite (HA) has been widely used as bone substitute. It is able to form the chemical bonding between implants and biological bone. The interface between implants and biological bone exhibits the continuity in chemical composition and mechanical properties, which result in the function continuity. However, bulk HA ceramics can not been used in load-bearing situations because of their poor mechanical properties. Titanium and its alloy implants have good mechanical properties and are used widely in orthopedics as prostheses for joint replacement and dental implants. But, this kind of material has no ability to bond to bone chemically. There exists a layer of the fibrous tissue between the implant and the surrounding bone. Hence, HA coatings on metal or alloy implants have been developed. Among the coating preparation methods, plasma-spraying technique has received the most extensive application. However, the interfacial tensile bond strength of the plasma-sprayed HA coating to the metallic substrate remains great concerns although successful applications of HA plasma sprayed Ti implants have been reported. Using the 'bridge-like binding' of HA coating and bioactivity of titanium oxide film, the new implant is introduced in this thesis, which have strong bone-bonding strength between host bone and implant as well as the 'bridge-like binding' ability. The main results is described as fellows: 1.The as-received plasma sprayed HA coating were treated in water-vapor at 125℃, with a pressure of 0.15Mpa for 6 hours; and most of the amorphous phase in the coating was converted into crystalline HA, resulting in achieving good crystallinity. Meanwhile, the α-tricalcium phosphate, tetracalcium phosphate and CaO formed from HA decomposition during the plasma spraying process were also completely transformed into crystalline HA. The dissolution experiment in distilled water at room temperature showed that the post-water vapor–treated treatment gave more stable coating, compared with the extensively used post-heat treatment method at present 2.The effect of structure and chemistry of plasma sprayed HA coating on mechanical behavior was further studied. Two kind of post-treatment methods were been used: (1) Heated in air at 650℃ for 30 min, (2) Heated in water vapor at 125℃, 0.15MPa for 6 hours. XRD showed that the nanocrystals of HA coating increased after water vapor treatment. The interfacial tensile bond strengths between HA and Ti substrate were 45.0±1.82MPa, 39.1±1.27MPa and 30.3±1.61MPa for as-received coatings, water vapor treated coatings and heated in air coatings, respectively. After 3 months implantation in dogs limbs, the pushed-out strengths between implants and bone were 11.27±2.71 MPa, 11.63±3.11MPa, 23.92± 2.01MPa and 18.8± 1.82 MPa for Ti implants, as-received coating implants, water vapor treated implants and heated in air implants, respectively. The results showed that the existence of the amorphous phase and nanocrystals inhibited crack formation and led to better fatigue properties than the more crystalline coating, Therefore, the presence of some amorphous phase and nanocrystals in the coating isimportant for the achieviement of better mechanical properties. 3.The healing of gaps around implants, called 'bridge-like binding', is one of the major advantages of HA coatings over pure metal implants. To evaluate this property, both pure titanium and plasma sprayed hydroxyapatite Ti implants were implanted in dogs. The gap of the implant and its surrounding bone was 2mm and the follow-up period was set 12 weeks. Histological examination revealed that gaps could be bridged by bone when the hydroxyapatite coating was applied. In contrast, a layer of fibrous tissue was observed around titanium implants and no bone ingrowth occurred at all. The bridge-like binding characteristic of HA coating was further validated by the clinical trial case with HA coated Ti hip joint implant, where 2mm gap existed between femur bone tissue and the distal part of the implant. After 1, 6, 12, 24 months, the hip joint implant was evaluated by radiological examination. The result showed that one month after operation, a part of the gap had been filled with new bone. After 6 months, new bone occupied about 80% of gap, and this expanded over to 90% after 12 and 24 months. 4.The biological properties of titanium implant depend on its surface oxide film. Because the nature oxide film is bioinert, titanium is generally bioactivated via surface modification of oxide film. Ti-OH groups were reported to be able to induce apatite nucleation and the crystal structure on their surface also plays an important role for inducing the formation of apatite. However, the relevant mechanism of apatite formation is unclear. The effect of microporous structure on bone-bonding ability of titanium was also studied in this thesis. The three different structures of titanium oxide film were prepared: (1) The commercial pure titanium was treated with heating in air at 700℃ for half hour and a dense rutile film was obtained on titanium (HS Samples); (2) The commercial pure titanium was treated by chemically treating and a layer of amorphous titania gelwas realized on the Ti surface (TS Samples); (3) After chemically treating, the samples were further heated in air at 700 ℃ for half hour, and nano-particles coalesced microporous titanium oxide (rutile) film was achieved on titanium surface (XS Samples). The dense rutile and amorphous titania gel did not induce apatite formation on their surfaces in SBF solution for 48 hours, whereas the nano-particles coalesced microporous rutile structure induced apatite formation on their surfaces. Mechanical test and histological examination were investigated after the samples implanted in dogs limbs for 3 months. The results of push-out test were 12.96, 29.48 and 35.83 MPa for HS, TS and XS sample, respectively. Histological results showed that TS sample and XS sample contacted the bone directly, without any intervening fibrous tissue. In contrast, there was a fibrous tissue layer between the bone and HS samples. 5.The gaps around implants can been healed by plasma sprayed HA coating, called 'bridge-like binding'. But the adhersion strength between the metallic substrate and plasma sprayed HA coating is low. Using the 'bridge-like binding' of HA coating and bioactivity of titanium oxide film, a new implant is proposed, which have strong bone-bonding strength between host bone and implant as well as the 'bridge-like binding' ability. The thin HA coating (about 30μm) was prepared by plasma-spraying technique on titanium which had been treated by chemically treating and heated in air at 700℃ for half an hour. The as-received coating was kept in water vapor at 125℃, with a pressure of 0.15MPa, for 6 hours. After 3 months implant in dogs limbs, Histological examination revealed that most HA coating disappeared, and the bioactivated titanium surface contact with host bone directly. The push-out strength between implants and bone was 46.65MPa. |
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