Structure and integrity of a plasma sprayed hydroxylapatite coating on titanium

Nanofeatured surfaces in dental implants: contemporary insights and impending challenges

Dental implant therapy, established as standard-of-care nearly three decades ago with the advent of microrough titanium surfaces, revolutionized clinical outcomes through enhanced osseointegration. However, despite this pivotal advancement, challenges persist, including prolonged healing times, restricted clinical indications, plateauing success rates, and a notable incidence of peri-implantitis. This review explores the biological merits and constraints of microrough surfaces and evaluates the current landscape of nanofeatured dental implant surfaces, aiming to illuminate strategies for addressing existing impediments in implant therapy. Currently available nanofeatured dental implants incorporated nano-structures onto their predecessor microrough surfaces. While nanofeature integration into microrough surfaces demonstrates potential for enhancing early-stage osseointegration, it falls short of surpassing its predecessors in terms of osseointegration capacity. This discrepancy may be attributed, in part, to the inherent "dichotomy kinetics" of osteoblasts, wherein increased surface roughness by nanofeatures enhances osteoblast differentiation but concomitantly impedes cell attachment and proliferation. We also showcase a controllable, hybrid micro-nano titanium model surface and contrast it with commercially-available nanofeatured surfaces. Unlike the commercial nanofeatured surfaces, the controllable micro-nano hybrid surface exhibits superior potential for enhancing both cell differentiation and proliferation. Hence, present nanofeatured dental implants represent an evolutionary step from conventional microrough implants, yet they presently lack transformative capacity to surmount existing limitations. Further research and development endeavors are imperative to devise optimized surfaces rooted in fundamental science, thereby propelling technological progress in the field.

Nanostructural control of transparent hydroxyapatite nanoparticle films using a citric acid coordination technique

Hydroxyapatite (HA), as the main mineral component in hard tissues, has good biocompatibility. In particular, HA films are widely used as bioactive coatings for artificial bones and dental implants in biomedical fields. However, it is currently difficult to prepare a nanostructure-controlled HA film by a wet process for further applications. Herein, we report the synthesis of HA nanoparticles coordinated by citric acid (Cit/HA) based on the interactions between carboxylate and calcium ions to control the sizes and shapes of the hybrid nanoparticles, to improve their dispersibility in water and to eventually form uniform transparent films with nanospaces, and investigated the film formation mechanism. As compared with the well-known rod-like HA nanoparticles (size: 48 × 15 nm²), we successfully synthesized spherical and negatively charged Cit/HA nanoparticles (size: 25 × 23 nm²) to achieve highly transparent Cit/HA films using the spin-coating technique. The Cit/HA films had uniform and crack-free appearance. About the nanostructures, we found that the Cit/HA film surfaces had meso-scaled nanospaces with a diameter of 4.2 nm based on the regular arrangement of spherical nanoparticles, instead of the HA film with a nanospace diameter of 24.5 nm formed by non-uniform accumulation. Therefore, we successfully achieved the control of the nanospace sizes of the films with the nanoparticle arrangement and realized transparent nanoparticle film formation in a very simple way, which will provide more convenient bioceramic films for biomedical applications.

In vivo cytotoxicity of MgO-doped nanobioactive glass particles and their anticorrosive coating on Ti–6Al–4V and SS304 implants for high load-bearing applications

Magnesium-doped NBG composites (SiO₂–CaO–P₂O₅–MgO) coated implant is found to be a potential nanocomposite for high load-bearing applications with better anticorrosive property and long-term stability.

A review of the effect of various ions on the properties and the clinical applications of novel bioactive glasses in medicine and dentistry

Bioactive glass is a novel material that dissolves and forms a bond with bone when exposed to body fluids. Bioactive glasses are silicate-based, with calcium and phosphate in identical proportions to those of natural bone; therefore, they have high biocompatibility. Bioactive glasses have wide-ranging clinical applications, including the use as bone grafts, scaffolds, and coating materials for dental implants. This review will discuss the effects of ions on the various compositions of bioactive glasses, as well as the clinical applications of bioactive glasses in medicine and dentistry.

Micro-Raman spectroscopy shows how the coating process affects the characteristics of hydroxylapatite

The diversity in the structural and chemical state of apatites allows implant manufacturers to fine-tune implant properties. This requires suitable manufacturing processes and characterization tools to adjust the amorphous phase and hydroxyl content from the source hydroxylapatite. Hydroxylapatite was processed by high-velocity oxy-fuel spraying, plasma spraying and flame spraying, and primarily analyzed by Raman spectroscopy. Investigation of rounded splats, the building blocks of thermal spray coatings, allowed correlation between the visual identity of the splat surface and the Raman spectra. Splats were heat-treated to crystallize any remaining amorphous phase. The ν1 PO4 stretching peak at 950-970 cm(-1) displayed the crystalline order, but the hydroxyl peak at 3572 cm(-1) followed the degree of dehydroxylation. Hydroxyl loss was greatest for flame-sprayed particles, which create the longest residence time for the melted particle. Higher-frequency hydroxyl peaks in flame- and plasma-sprayed splats indicated a lower structural order for the recrystallized hydroxylapatite within the splats. Crystallization at 700 °C has shown potential for revealing hydroxyl ions previously trapped in amorphous calcium phosphate. This work compares Fourier transform infrared and Raman spectroscopy to measure the hydroxyl content in rapidly solidified apatites and shows that Raman spectroscopy is more suitable.

Thickness and surface structure of a ceramic layer created on three indirect resin composites with aerosol deposition

PURPOSE

Aerosol deposition is a technology for coating ceramics with impact consolidation at room temperature. The aim of the present study was to investigate the thickness and the microstructure of the aluminium oxide layer on different three dental resin composite materials created by means of aerosol deposition.

METHODS

Disk-shaped specimens were fabricated with three resin composites (Estenia C&B, Targis, and Gradia). The specimens were ground flat, and then subjected to aerosol deposition using aluminium oxide submicron particles without inducing a localized temperature rise. The average thickness (AVH) and maximum thickness (Hmax) of the aluminium oxide layer deposited on the resin composite material were measured using a profilometer. Data were analyzed by ANOVA and post hoc Tukey compromise test at α=0.05. The specimen surfaces were also observed using a scanning electron microscope.

RESULTS

The aluminium oxide layer formed on Estenia C&B (AVH 8.1 μm, Hmax 9.1 μm) and Targis (AVH 7.7 μm, Hmax 8.9 μm) were significantly thicker than that on Gradia (AVH 4.2 μm, Hmax 5.4 μm). The micrograph showed that the aluminium oxide layer on Estenia C&B was similar to that on Targis. However, the aerosol deposition area of Gradia was seen relatively rough and partly caved.

CONCLUSIONS

The type of resin composite affected the microstructure of the deposited aluminium oxide layer. The highly filled light- and heat-cured resin composites are advantageous as a target material rather than the lower filled light-cured resin composite.

Deposition and investigation of functionally graded calcium phosphate coatings on titanium

A series of calcium phosphate coatings with graded crystallinity were deposited onto heated titanium substrates using ion beam assisted deposition. The microstructure of the coating was examined using transmission electron microscopy (TEM). The coating thickness was observed to be in a range of 594-694 nm. The degree of crystallinity and microstructural grain size of the coating showed a clear decrease with increasing distance from the substrate-coating interface. Fourier transform infrared spectroscopy (FTIR) confirmed the presence of PO(4)(3-), and X-ray photoelectron spectroscopy (XPS) analysis on the coating top surface showed that the atomic Ca/P ratio was in the range of 1.52+/-0.15 to 1.61+/-0.07. The biological response to the coatings was also evaluated using an osteoblast precursor cell culture test. More cells and a higher integrin expression of cell attachment sites were observed on the coating surface when compared to the control group (blank titanium surface). The pull-off test showed average adhesion strengths at the coating-substrate interface to be higher than 85.12+/-5.37 MPa. Nanoindentation tests indicated that the Young's moduli of all coatings are higher than 91.747+/-3.641 GPa and microhardness values are higher than 5.275+/-0.315 GPa. While the adhesion strength results helped us to identify the best setup for substrate temperature and processing parameters to begin the deposition, the culture test and XPS results helped identifying the optimum parameters for the last stage of deposition. TEM, X-ray diffraction, FTIR and nanoidentation results were used to further evaluate the quality of the coating and optimization of its processing parameters.

Surface nanocrystallization of hydroxyapatite coating

Nanocharactered biomaterials, such as nanopowders, nanocrystalline compacts and nanostructured films, as well as materials with nanoscale roughness, have attracted much attention recently, due to their clear effects on cell response. Surface nanocrystallization of plasma-sprayed hydroxyapatite (HA) coating can be realized by conventional post-heat treatment. This study reveals that 20-30nm nanocrystals formed on HA coatings post-heat treated at 650 degrees C, and the increase in holding time increased the number of surface nanocrystals and intensified their aggregation. Hard aggregation occurred when HA coatings were repetitively post-heat treated. This indicates that the surface nanocrystallization is controllable. Cell experiments were carried out with rat calvarial osteoblasts. The post-heat treated HA coatings exhibit an obviously better osteoblast response than the as-sprayed coatings. Well-flattened cells attached themselves to the coating surfaces, with a good interaction between their filopodia and the nanocrystallized region. It is proposed that the surface nanocrystallization should be taken into account when the post-heat treatment process is introduced for the fabrication of HA coatings.

Functionally graded bioactive glass coating on magnesia partially stabilized zirconia (Mg-PSZ) for enhanced biocompatibility

The coating of magnesia partially stabilized zirconia (Mg-PSZ) with a bioactive glass was investigated for enhancing the bioactivity and bone-bonding ability of Mg-PSZ orthopedic implants. Individual coatings of three different bioactive glasses were prepared by depositing a concentrated suspension of the glass particles on Mg-PSZ substrates, followed by sintering at temperatures between 750 degrees C and 850 degrees C. Two silicate-based glass compositions (designated 13-93 and 6P68), and a borosilicate glass composition (H12) were investigated. The microstructure and adhesive strength of the coatings were characterized, and the in vitro bioactivity of the glasses was compared by measuring their conversion kinetics to hydroxyapatite in an aqueous phosphate solution at 37 degrees C. The 6P68 glass provided the highest adhesive strength (40 +/- 2 MPa) but showed very limited bioactivity, whereas the H12 glass had lower adhesive strength (18 +/- 2 MPa) but the highest bioactivity. A functionally graded coating, consisting of a 6P68 interfacial layer and an H12 surface layer, was developed to provide a coating with high adhesive strength coupled with rapid in vitro bioactivity.

Structural evolution and adhesion of titanium oxide film containing phosphorus and calcium on titanium by anodic oxidation

This study investigated the microstructure evolution and defects of the titanium oxide layer containing calcium (Ca) and phosphorus (P) formed by anodic oxidation in a solution containing Ca and P compounds. Results show that the anodic film exhibited a two-layer structure: a pore-containing amorphous titanium oxide layer dispersed with nano-sized crystallites formed prior to sparking, and a porous overlay dotted with craters formed after sparking. Ca and P were predominantly incorporated in the porous overlay, in which the amorphous region contained more Ca and P than the crystalline region regardless of the anodizing voltages. Moreover, the ratio of amorphous to crystalline regions in the porous overlay changed insignificantly with anodizing voltage. Increasing anodizing voltage enhanced the incorporation of Ca and P in the anodic film, but deteriorated the adhesion of the anodic film to the substrate. This deterioration was related to two inherent adhesive weaknesses: the aligned pores in the titanium oxide layer and the craters in the major overlay, signifying that a new anodic oxidation process that can produce high Ca- and P-containing oxide film at relatively-low anodizing voltages, i.e. approximately 200 V, is a necessity.

Effects of anodic oxidation parameters on a modified titanium surface

Anodic oxidation is an electrochemical treatment that can be used to control the thickness of an oxide layer formed on a titanium surface. This procedure has the advantage of allowing the ions contained in an electrolyte to deposit onto the oxide layer. The characteristics of a layer treated with anodic oxidation can vary according to the type and concentration of the electrolytes as well as the processing variables used during anodic oxidation. In this study, the constant electrolyte for anodic oxidation was a mixed solution containing 0.02 M DL-alpha-glycerophosphate disodium salt and 0.2M calcium acetate. Anodic oxidation was carried out at different voltages, current densities, and duration of anodic oxidation. The results showed that the current density and variation in the duration of anodic oxidation did not have a large effect on the change in the characteristics of the layer. On the other hand, the size of the micropores was increased with increasing voltage of anodic oxidation, and anatase and rutile phases were found to co-exist in the porous titanium dioxide layer. In addition, the thickness of the oxide layer on titanium and the characteristic of corrosion resistance increased with increasing voltage. The MTT test showed that the cell viability was increased considerably as a result of anodic oxidation. The anodizing voltage is an important parameter that determines the characteristics of the anodic oxide layer of titanium.

Hydrothermal crystallization of carbonate-containing hydroxyapatite coatings prepared by radiofrequency-magnetron sputtering method

Carbonate-containing hydroxyapatite (HA) films were prepared by low-temperature hydrothermal annealing from carbonate-containing calcium phosphate amorphous coatings on titanium substrates. The biocompatibility of the carbonate-containing HA layers was estimated by in vitro tests using simulated body fluid (SBF). Precursory amorphous coatings were deposited with rf-magnetron sputtering apparatus, using calcium phosphate glass target in Ar/CO2 atmosphere. The carbonate-containing HA coatings were successfully formed by the annealing at above 130 degrees C for 20 h. On the basis of SEM observation, about 2-microm thickness films coated rigidly were durable enough for the hydrothermal treatment. The coating layer was revealed to consist of single phase of PO4(-) and OH- partially carbonated HA by XRD and IR analyses. Overgrowing of bone-like apatite layers on the carbonate-containing HA surfaces in the SBF implied that the obtained films acquired a sufficient osteoconductivity, while it was still unclear that activity was enhanced, compared to pure HA coatings. The low-temperature hydrothermal annealing method was effective for preparation of rigid HA coatings on titanium as well as modification of their chemical compositions.

Effect of Ceramic Coating by Aerosol Deposition on Abrasion Resistance of a Resin Composite Material

Aerosol deposition (AD coating) is a novel technique to coat solid substances with a ceramic film. The purpose of the present study was to investigate the effect of AD coating on abrasion resistance of a resin composite material. A 5-microm-thick aluminum oxide layer was created on the polymerized resin composite. The specimen was cyclically abraded using a toothbrush abrasion simulator for 100,000 cycles. Abraded surface was then measured with a profilometer to determine the average roughness (Ra) and maximum roughness (Rmax). It was found that abrasion cycling increased the Ra value of the No-AD-coating group, but decreased the Ra and Rmax values of the AD coating group. Moreover, the AD coating group showed significantly smaller Ra and Rmax values after 100,000 abrasion cycles as compared to the No-coating control group. Microscopic observation supported these findings. In conclusion, the resistance of the resin composite against toothbrush abrasion was improved by AD coating.

Osteointegration of femoral stem prostheses with a bilayered calcium phosphate coating

Our purpose was to evaluate the osteointegration of bilayered calcium phosphate (CaP)-coated femoral hip stems in a canine model. A first layer of hydroxyapatite (HA) 20 microm thick and a superficial layer of Biphasic Calcium Phosphate (BCP) 30 microm thick were plasma-sprayed on to the proximal region of sandblasted Ti6Al4V prostheses. Bilayered CaP-coated and non-coated canine femoral stems were implanted bilaterally under general anesthesia in 6 adult female Beagle dogs. After 6 and 12 months, a significant degradation of the bilayered coating occurred with a remainder of 33.1+/-12.4 and 23.6+/-9.2 microm in thickness, respectively. Lamellar bone apposition was observed on bilayered coated implants while fibrous tissue encapsulation was observed on non-coated femoral stems. The bone-implant contacts (BIC) were 91+/-3% and 81+/-8% for coated and 7+/-8% and 8+/-12% for non-coated implants, at 6 and 12 months, respectively. Our study supports the concept of a direct relationship between the biodegradation of CaP coating and the enhanced osteointegration of titanium prostheses. A bilayered CaP coating might therefore enhance bone apposition in the early stages because of the superior bioactivity of the BCP layer while the more stable HA layer might sustain bone bonding over long periods.

An Investigation of Residual Stress of Porous Titania Layer by Micro-Arc Oxidation under Different Voltages

The surface modification of titanium by micro-arc oxidation under different voltages was processed to achieve good direct oseointegration. The new technique of two-dimensional X-ray diffraction was used to measure the residual stress of the layer. The results show that a porous titania layer containing Ca and P is obtained by micro-arc oxidation. The pore size and Ca/P of the layer are affected by the voltage. The high voltage can induce forming CaTiO3. The residual stress under different voltage is compressive stress and increases with the improvement of the voltage.

Adherent apatite coating on titanium substrate using chemical deposition

Plasma-sprayed "HA" coatings on commercial orthopedic and dental implants consist of mixtures of calcium phosphate phases, predominantly a crystalline calcium phosphate phase, hydroxyapatite (HA) and an amorphous calcium phosphate (ACP) with varying HA/ACP ratios. Alternatives to the plasma-spray method are being explored because of some of its disadvantages. The purpose of this study was to deposit an adherent apatite coating on titanium substrate using a two-step method. First, titanium substrates were immersed in acidic solution of calcium phosphate resulting in the deposition of a monetite (CaHPO4) coating. Second, the monetite crystals were transformed to apatite by hydrolysis in NaOH solution. Composition and morphology of the initial and final coatings were identified using X-ray diffraction (XRD), Scanning Electron Microscopy, and Energy Dispersive Spectroscopy (EDS). The final coating was porous and the apatite crystals were agglomerated and followed the outline of the large monetite crystals. EDS revealed the presence of calcium and phosphorous elements on the titanium substrate after removing the coating using tensile or scratching tests. The average tensile bond of the coating was 5.2 MPa and cohesion failures were observed more frequently than adhesion failures. The coating adhesion measured using scratch test with a 200-microm-radius stylus was 13.1N. Images from the scratch tracks demonstrated that the coating materials were squashed without fracturing inside and/or at the border of the tracks until the failure point of the coating. In conclusion, this study showed the potential of a chemical deposition method for depositing a coating consisting of either monetite or apatite. This method has the advantage of producing a coating with homogenous composition on even implants of complex geometry or porosity. This method involves low temperatures and, therefore, can allow the incorporation of growth factors or biogenic molecules.

Surface modification of titanium implant by microarc oxidation and hydrothermal treatment

Surface modification of titanium implant is processed by microarc oxidation and hydrothermal treatment. A porous surface with a biologically active bone-like apatite layer was formed. The apatite layer consists of very fine crystals and high crystallinity and is integrated with the titanium alloy substrate with a graded structure without a distinct interface. Such a bioactive layer is expected not only to enhance the bony ingrowth into the porous structure, but also to improve the interlocking between implant and bone.

A low-temperature biomimetic calcium phosphate surface enhances early implant fixation in a rat model

The present study demonstrates increased early mechanical fixation of titanium implants coated with a new biomimetic apatite surface in a rat model. Male Sprague-Dawley rats received unilateral femoral medullary implants for periods of 1-4 weeks. The strength of fixation of the implant to the host bone increased more rapidly in the group receiving apatite-treated implants compared with the control group as evidenced by the apatite group's 21-fold greater fixation strength at 1 week (p = 0.009), 4-fold greater fixation strength at 2 weeks (p = 0.041), and 2-fold greater fixation strength at 4 weeks (p = 0.093) compared with the control. Fixation strength was correlated with bone-implant contact as determined from micro computed tomography assessment of the specimens (r2 = 0.338, p = 0.011 in the control group and r2 = 0.543, p < 0.001 in the apatite group). Furthermore, for a given amount of bone-implant contact, the fixation strength was higher in the apatite group than in the control group (p = 0.011), suggesting that the bone formed a stronger bond to the apatite coating than to the titanium. This difference in bonding strength accounted for the difference in mechanical behavior.

Preparation of different forms of titanium oxide on titanium surface: Effects on apatite deposition

Methods of preparing different types of titanium oxide (TiO(2)) and their effects on apatite deposition and adhesion on titanium surfaces were investigated. Forty-eight commercially pure titanium (Ti) discs were divided into four groups (12 per group) and each group was subjected to the following treatments: Group 1, heat treatment at 750 degrees C; Group 2, oxidation in H(2)O(2) solution followed by heat treatment; Group 3, dipping in rutile/gelatin slurry; and Group 4, dipping in anatase/gelatin slurry. Surface-treated Ti discs were immersed in a supersaturated calcium phosphate solution to allow apatite deposition. Results showed that the percentage of area covered by deposited apatite was highest in Group 4 compared to the other groups. Apatite deposited on Ti discs pretreated in H(2)O(2) solution (Group 2) demonstrated the highest adhesion to the titanium substrate. Results from this study indicated that surface treatment method affects the type of TiO(2) layer formed (anatase or rutile) and affects apatite deposition and adhesion on the Ti surface.

Long-term changes of hydroxyapatite-coated dental implants

There are many controversies about the long-term prognosis of hydroxyapatite (HA)-coated implants. Failure may be related to compositional and structural changes of the coating occurring during implantation. Two retrieved and two unused HA-coated blade-type implants were examined by stereomicroscopy, secondary electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and electron probe microanalysis. The objective was to investigate the HA morphology, composition, and structure, and to characterize the changes that occurred in the retrieved implant coatings. Retrieved implants presented partial loss of the coating, especially at the apical and mesiodistal edges. Remaining HA was thick and flattened in the cervical and central areas and gradually thinner and rougher towards the apical and mesiodistal edges. Increase of Cl and Mg, decrease of OH, and X-ray diffraction peak broadening were found in the retrieved implant coatings, in comparison with the unused implants. Morphological changes of the retrieved implants seem to depend on stress values in the surrounding bone and on implant mobility. Compositional changes and increased amount of lattice imperfections appeared in the retrieved implant coatings, as a result of ion substitutions in the apatite lattice. However, the present study could not confirm the influence of these changes on implant failure.

Electrophoretic coating of multilayered apatite composite on alumina ceramics

By means of an electrophoretic deposition technique followed by sintering, alumina and zirconia ceramics were coated with apatitic composites composed of porous surface and intermediate layers of hydroxyapatite and an adhesive calcium phosphate layer. The electrophoretic deposition of these layers was attained by the use of a mixed solvent of acetylacetone and alcohol as well as the mixed powders of the calcium phosphates and alumina. The adhesive layer was formed by the codeposition of calcium phosphate glass powders (Ca/P = 1/2) with hydroxyapatite, while the open porosity of the surface layer was increased with the addition of alumina to the hydroxyapatite layers. The resultant phases of sintered composite layers were tricalcium phosphate and alumina with a small amount of hydroxyapatite.

Structural and morphological study of pulsed laser deposited calcium phosphate bioceramic coatings: influence of deposition conditions, laser parameters, and target properties

Calcium phosphate (CaP) bioceramics, especially hydroxyapatite (HA), have been used as coatings on implants owing to their biocompatible properties. The commercial practice for applying HA coating, plasma spraying, has some disadvantages which limit the long-term stability of the implants. Pulsed laser deposition (PLD) is being investigated as an alternative technique. The purpose of this research was to systematically study the effect of various parameters of the PLD process on the properties of CaP coatings. In this study, three types of HA targets and two laser wavelengths were used to make six categories of coatings. Predominantly crystalline HA coatings were produced under all six categories at optimum conditions, although small amounts of minor phases sometimes were found. Sufficient coating/substrate bond strength was also obtained. A wide variety of coating morphologies was obtained, from rather dense and uniform to rough and porous. The important factors that affected the morphology included target properties, vacuum level, deposition temperature, and laser wavelength and energy density. PLD's ability to produce both amorphous and crystalline, and both smooth/dense and rough/porous coatings may be a unique advantage.

Analytical and mechanical testing of high velocity oxy-fuel thermal sprayed and plasma sprayed calcium phosphate coatings

Plasma spraying (PS) is the most frequently used coating technique for implants; however, in other industries a cheaper, more efficient process, high-velocity oxy-fuel thermal spraying (HVOF), is in use. This process provides higher purity, denser, more adherent coatings than plasma spraying. The primary objective of this work was to determine if the use of HVOF could improve the mechanical properties of calcium phosphate coatings. Previous studies have shown that HVOF calcium phosphate coatings are more crystalline than plasma sprayed coatings. In addition, because the coatings are exposed to more complex loading profiles in vivo than standard ASTM tensile tests provide, a secondary objective of this study was to determine the applicability of four-point bend testing for these coatings. Coatings produced by HVOF and PS were analyzed by profilometry, diffuse reflectance Fourier transform infrared spectroscopy, X-ray diffraction, four-point bend, and ASTM C633 tensile testing. HVOF coatings were found to have lower amorphous calcium phosphate content, higher roughness values, and lower ASTM C633 bond strengths than PS coatings; however, both coatings had similar crystal unit cell sizes, phases present (including hydroxyapatite, beta-tricalcium phosphate, and tetracalcium phosphate), and four-point bend bond strengths. Thus, the chemical, structural, and mechanical results of this study, in general, indicate that the use of HVOF to produce calcium phosphate coatings is equivalent to those produced by plasma spraying.

Fatigue behavior of calcium phosphate coatings with different stability under dry and wet conditions

To obtain stable plasma sprayed calcium phosphate coatings, coatings with a high crystallinity and low solubility were developed. However, stability of ceramic coatings is also influenced by their fatigue resistance. Recently, fatigue failure was proposed to explain coating detachment from implants under loaded conditions. Therefore, plasma-sprayed calcium phosphate coatings with different crystallinity were investigated in vitro for fatigue failure. An amorphous and a crystalline hydroxylapatite coating (AHA and CHA) and a highly crystalline fluorapatite coating (FA) were subjected to cyclic load tests, both in dry conditions and in simulated body fluid (SBF). The results in SBF revealed that the crystalline CHA and FA coating detached completely at the highest stressed middle section of the bar. The FA coating delaminated earlier than the CHA coating. The amorphous AHA coating showed only partial coating loss at the completion of the test. Tests in dry conditions did not reveal any change in the coatings tested. These results suggest a relation between crystallinity of apatite coatings and their failure due to fatigue: high crystallinity coatings demonstrate earlier and more complete fatigue failure than the amorphous apatite coatings. It can be concluded that coating stability is not determined solely by static dissolution, but by fatigue failure as well.

Effects of cyclic bending and physiological solution on plasma-sprayed hydroxylapatite coatings of varying crystallinity

This study investigated the effects of cyclic bending stress levels and testing in simulated physiological solutions or air on the integrity of plasma-sprayed hydroxylapatite (HA) coatings of two different crystallinities. Hydroxylapatite-coated commercially pure (CP) Ti rods were evaluated by immersion testing in Hank's Balanced Salt Solution (HBSS) and by rotating bending in air and HBSS. Static immersion testing of nonstressed specimens resulted in significant microcracking of coating surfaces after 42 days. Specimens cyclically tested at bending stresses above the yield strength of Ti experienced low cycle fatigue failure of the Ti substrates prior to spallation of the HA coatings. Coatings tested at 1 x 10(6) cycles with interface bending stresses of 180 MPa displayed increased surface microcracking, but no bulk coating spallation. Coatings cycled in HBSS displayed greater amounts of microcracking and surface alteration than samples cycled in air. There was no apparent relation between HA crystallinity and mechanical integrity under cyclic bending stresses.

The structural characteristics and mechanical behaviors of nonstoichiometric apatite coatings sintered in air atmosphere

Two types of nonstoichiometric apatite coatings on Ti6Al4V substrates were prepared in our laboratory by electrodeposition and hydrothermal synthesis. One was composed of highly pure needle-like calcium-deficient hydroxyapatite, and the other consisted of needle-like calcium-deficient hydroxyapatite and plate-like brushite. In this paper, the morphology, phase transition, and bonding strength of these coatings sintered at 450( degrees ) approximately 980 degrees C in an air atmosphere were studied. In addition, the residual stresses in pure calcium-deficient hydroxyapatite coatings and coatings sintered at 800 degrees C were measured by a hole-drilling method. The results show that after being sintered at a higher temperature, the coating/Ti6Al4V samples exhibited structural characteristics such as biphasic [hydroxyapatite + beta-Ca3(PO4)2] coatings and dense TiO2 film/Ti6Al4V in which the residual stresses in the coatings slightly increased. In addition, the results indicate that bonding strength was improved.

Calcium phosphate coatings for medical implants

In surgical disciplines where bone has to be repaired, augmented or improved, bone substitutes are essential. Although bone banks, such as Eurotransplant, are founded to supply such substitutes, natural bone is not always adequate. For example, frequently these so-called bone grafts resorb after implantation (1). Further, they cannot be used for joint and tooth replacement, and recently worries have been raised about the transfer of infectious diseases. Therefore, interest has dramatically increased in the use of synthetic materials for replacement of lost or damaged bone tissue. The generic name of these tissue alternatives is biomaterials. A special class of these biomaterials is composed of metallic devices with coatings to improve bone bonding. These specialized coatings used to improve the metallic implant are the topic of this paper.

Healing of large (2 mm) gaps around calcium phosphate-coated bone implants: a study in goats with a follow-up of 6 months

Plasma-sprayed hydroxylapatite (HA) coatings are known for their ability to demonstrate osseointegration with bone. Recently it was found that the amount of bone apposition was strongly reduced 6 weeks after implantation in a goat model if gaps of two millimeters between bone and apatite coating existed. Stability of the apatite coatings examined did not influence the gap-healing ability. This study investigated whether a longer follow-up period of 24 weeks would be sufficient for the restoration of bone apposition on apatite coatings in an identical surgical model with 2 mm gaps, and whether bone apposition on the apatite coatings is influenced by the coating stability. Three coatings were investigated: 25-30% crystalline HA (aHA), 60-63% crystalline HA (cHA), and 85-90% crystalline fluorapatite (FA). Uncoated Ti-6A1-4V implants were used as controls. Implants were inserted in the femoral condyles of both femora of eight goats. Each goat received four implants. Histology revealed that bone formation on each of the apatite coatings remained low and did not increase with an extended follow-up period of 24 weeks. The coatings showed significantly (P < 0.01) more bone contact than the uncoated control implants. The three different coatings did not show significant differences in bone apposition. The aHA coating in most cases had disappeared completely after 24 weeks. Despite the disappearance of the aHA coating, bone contact was seen on the substrate surface without fibrous tissue interposition. The cHA coating showed minor signs of degradation while the FA coatings showed no visible degradation. It is concluded that non-press-fit implantation of apatite-coated implants leads to more bone apposition as compared to uncoated Ti-6A1-4V implants. However, it is suggested by these results that the upper limit of gaps around apatite implants is 2 millimeters in a non-weight-bearing model in goats. Bone apposition will not increase by extending the follow-up period more than six weeks, nor will it be altering the stability of the apatite coatings used.

Healing of gaps around calcium phosphate-coated implants in trabecular bone of the goat

Hydroxylapatite coatings are under clinical investigation in orthopaedics and dentistry. Bone formation on apatite coatings in the presence of gaps is important for clinical applications. The importance of the stability of the coating is not known at present. By varying the plasma-spray parameters, and by the addition of fluoride, the crystallinity and stability of calcium phosphates can be changed. It is suggested that bone formation is enhanced by dissolution of the apatite coating. We studied apatite coatings of varying stability with regard to their gap-healing characteristics, and we examined what the maximum gap would be that can be bridged if a coating is applied. Ti-6A1-4V implants coated with 62% crystalline hydroxylapatite, 30% crystalline hydroxylapatite or fluorapatite, or noncoated Ti-6A1-4V were implanted in 16 goats. The implants were surrounded by gaps of 1 or 2 mm, and the follow-up period was 6 weeks. Histological examination and histometry revealed that gaps of 1 mm can be bridged by bone if an apatite coating is applied. However, only a minimal amount of bone contact was seen on the apatite coatings with 2 mm gaps. Uncoated implants demonstrated no bone contact at all. Among the three different coatings there were no differences in gap healing. It can be concluded that in the goat, gaps of 2 or more mm between coated implants and host bone tissue inhibit bone deposition on the coating (p < 0.05), but the stability of the coating does not influence gap-healing characteristics.

Histomorphometric evaluation of the thin hydroxyapatite layer formed through anodization followed by hydrothermal treatment

By the previously described method of electrochemical and hydrothermal reaction, a thin hydroxyapatite (HA) layer of 1 microm thickness was formed on machined, grit-blasted, and titanium plasma-sprayed implants, the surfaces of which were equipped with a gap zone of 0.15 mm in depth. These implants, together with HA and titanium plasma-sprayed implants as control materials, were placed in dog mandibles for 4 weeks. Histomorphometrical comparison was performed to examine the effects of the thin HA layer and the surface topography on bone formation. The roughened implants, especially the grit-blasted implants, were surrounded with thin bone newly formed along the rough surfaces and showed higher bone apposition than the smooth implants. The gap zone of the HA plasma-sprayed implant was repaired with new bone that had vertically extended from the surrounding bone. The thin HA layer had as much osteoconduction as a plasma-sprayed HA coating but showed significantly different bone response. The results suggest that bone formation on an HA film is affected by degradation in living tissue that is related to the crystallinity and the chemical composition of the HA film itself.

Effects of Ca and H2O2 added to RPMI on the fretting corrosion of Ti6Al4V

Titanium and its alloys have demonstrated considerable success in various surgical procedures including orthopedic, dental, and cardiovascular surgery. However, particulate debris from corrosion and wear is present in a considerable quantity in tissue local to the implant. This study evaluated the effect of Ca, since it is present in both serum and bone, and H2O2, since it is produced through local inflammation, on the amount of titanium release. Four sets of Ti6Al4V plates and Ti6Al4V screws were used. Each set was designated to one of four solutions: RPMI (cell culture growth media), RPMI with CaCl2, RPMI with CaCO3, and RPMI with H2O2. A fretter was used to cause corrosion by creating micromotion between two screws and a two-hole plate of Ti6Al4V. After fretting for 72 h, weight loss of the plate and screws and the amount of Ti and vanadium (V) in solution was used to assess the amount of fretting corrosion which had occurred. Results of weight loss and Ti in solution indicated that the presence of H2O2 increased the amount of particulate debris produced in RPMI as compared with RPMI alone. The addition of CaCl2 to RPMI also increased both weight loss and Ti in solution compared with RPMI alone. The addition of CaCO2, however, did not give values significantly different from RPMI alone. Comparison of weight loss and Ti in solution indicated that the increase in fretting corrosion was not different between RPMI with CaCl2 and RPMI with H2O2. The particulate wear debris from the four solutions was black in color and the size of the particulate produced was compared using a Coulter Multisizer. The results indicated that particles produced in the four solutions were not different, with mean values between 1.324 and 1.100 microns, and they were similar in size to the particulate found in tissues surrounding failed total hip replacements. In order to better understand the role of Ca in the fretting corrosion of Ti6Al4V, energy dispersive x-ray analysis (EDXA) using SEM was used to determine elemental composition of one countersink surface of a plate which had been run four times in RPMI with CaCl2. The presence of Ca in the bulk was not significant (% composition < 0.5%). However, Ca was present in two surface particles which were examined at a magnification of 55,000, with a Ca% composition of 63.2% and 19.2%. While results from this study indicate that both soluble Ca(CaCl2) and H2O2 increase the fretting corrosion of Ti6Al4V, the insoluble form of Ca, which would be found in bone and hydroxyapatite, has no effect. These data indicate that it is important to specify the media used in corrosion, dissolution, and elution experiments.

Interfacial shear strength and histology of plasma sprayed and sintered hydroxyapatite implants in vivo

The interfaces of bone with sintered hydroxyapatite (SHA) and plasma sprayed hydroxyapatite-coated (HAC) implants in the femora of six dogs were examined by light microscopy, scanning electron microscopy, energy dispersive X-ray analysis, and push-out tests. The results demonstrated that there was no significant difference at 12 and 24 weeks after insertion between the interfacial shear strengths with bone for the two types of implants, however, the histological characteristics of the bone around the plasma sprayed HA could be distinguished from that of the sintered HA. The HAC implants showed an early surface biodegradation as compared with the SHA implants. The observed differences in the interfacial zones may be attributed to different bone cell activities and variations in the dynamics of bone formation, possibly resulting from a higher level of dissolution/reprecipitation along the plasma sprayed HA surface.

Hydroxyapatite/metal composite coatings formed by electrocodeposition

Early bone infusion by cementless fixation of composite orthopedic and dental implants consisting of metallic substrates and bioceramics is well documented. Calcium phosphate ceramics in general and hydroxyapatite (HA) in particular have been the most popular of the bioceramics used for coating metals. Here, a non-line of sight coating procedure by electrocodeposition is reported for mechanically fixing HA particles in a metal matrix. Analyses of the coating showed excellent adhesion to the substrate and no structural transformation in either crystallinity or stoichiometry. Adhesion and surface coverage of HA depended upon the particle size. As a demonstration of the coating procedure's non-line of sight applicability, it was successfully used to coat titanium rods sintered with small titanium spheres.

Mechanical and histological investigation of hydrothermally treated and untreated anodic titanium oxide films containing Ca and P

In a previous study a new method for forming thin hydroxyapatite (HA) layers on titanium was described. Titanium was anodized at 350 V in an electrolyte solution containing sodium beta-glycerophosphate and calcium acetate, and an anodic titanium oxide film containing Ca and P (AOFCP) was formed on the surface. Then numerous HA crystals were precipitated on the AOFCP during hydrothermal treatment in high-pressure steam at 300 degrees C. In this study three types of hydrothermally treated films differing in amounts of precipitated HA crystals and tensile adhesive strength, and untreated films were histologically and mechanically investigated in a transcortical rabbit femoral model for 8 weeks of implantation using light microscopy, scanning electron microscopy (SEM), and push-out tests. Machined titanium and HA ceramics served as control materials. The push-out shear strength and bone apposition of the AOFCP significantly increased after hydrothermal treatment, and were equivalent to those of HA ceramics, although the HA layer on the AOFCP was thin at 1-2 microns. From SEM observation of the pushed-out specimen, it was found that the thin HA layer had directly bonded to bone but the AOFCP had not. The push-out strength of the hydrothermally treated film resulted from the chemical bonding of the bone-HA layer interface, while that of the untreated film resulted from mechanical interlocking force between bone and the microprojections. There was a small difference in bone apposition but no significant difference in push-out strength with the amount of precipitated HA crystals on the treated films. Among the treated films, the film formed at the lowest electrolyte concentration showed the lowest bone apposition because of incomplete covering by the HA crystals, and showed the highest stability against mechanical failure because the adhesive strength was very high at about 38 mPa. Also, the hydrothermally untreated anodic oxide films, whose surfaces were rough as a result of the large microprojections, showed much higher push-out strength and bone apposition than titanium. The good hard-tissue compatibility may be attributed to the surface roughness and the possible inhibition of titanium ion release from the specimen.

Characterization of thin hydroxyapatite layers formed on anodic titanium oxide films containing Ca and P by hydrothermal treatment

An anodic titanium oxide film containing Ca and P (AOFCP) was formed on commercially pure titanium which was anodized in an electrolytic solution of dissolved beta-glycerophosphate (beta-GP) and calcium acetate (CA). Hydroxyapatite (HA) crystals were precipitated by hydrothermally heating the AOFCP at 300 degrees C. After hydrothermal treatment, the film was characterized by scanning electron microscopy (SEM), energy-dispersive X-ray microanalysis (EDX), and tensile tests. The morphology, composition, and amount of HA crystals precipitated were significantly affected by the composition of the electrolytes. Near-stoichiometric HA crystals with high crystallinity were precipitated completely covering the AOFCP surface at specific electrolyte concentrations. The HA layers were thin at 1-2 microns in thickness. The adhesive strength of the film increased with decreasing electrolyte concentration and the maximum value was about 40 MPa. In vitro tests for 300 days suggested that the stability of the film was high. The high adhesive strength may result from the AOFCP existing as an intermediate layer between the HA layer and a titanium substrate. The intervention of the AOFCP may have prevented abrupt changes in Ca and P content at an HA coating-titanium interface as seen in a plasma-sprayed one. The porous TiO2 matrix of the AOFCP may be suitable for nucleation sites of HA crystals, as well as SiO2 matrix of silicate bioactive glasses or glass ceramics.

Comparison of bone-implant interface shear strength of hydroxyapatite-coated and alumina-coated metal implants

We performed a transcortical push-out test to determine the effect of surface roughness of hydroxyapatite (HA)-coated implants on bone-implant shear strength in a canine model. Hydroxyapatite- and alumina-coated SUS316L with the same surface roughness (roughness average: Ra = 5 microns) and HA-coated Ti-6A1-4V (Ra = 8.4 microns), sintered HA (Ra = 0.9 micron), and dense alumina (Ra = 1.3 microns) were inserted into the dog's femur. The interface shear strength of the dense alumina was significantly lower than that of other implants at both 4 and 12 weeks after implantation. At 4 weeks after implantation, the interface shear strength of the alumina-coated SUS316L was significantly lower than that of other implants (P < .05) except the dense alumina, but at 12 weeks, there was no significant difference between the implant types except the dense alumina. This indicates that the surface roughness of the HA coating affects the enhancement of the bone-implant interface shear strength at the early period after implantation, and that a surface roughness of several micrometers does not influence the bond strength between bone and HA. A scanning electron microscopic study indicated that in almost all cases at 12 weeks, the failure site after push-out testing was the coating-substrate interface, not the coating-bone interface. Therefore, protection of the coating-substrate interface from direct shear loading is needed.