T. M. Lee

In vitro and in vivo mechanical evaluations of plasma-sprayed hydroxyapatite coatings on titanium implants: The effect of coating characteristics

This study was undertaken to evaluate the effect of coating characteristics on the mechanical strengths of the plasma-sprayed HA-coated Ti-6Al-4V implant system both in vitro and in vivo. Two types of HA coatings (HACs) with quite different microstructures, concentrations of impurity-phases, and indices-of-crystallinity were used. In vitro testings were done by measuring the bonding-strength at the Ti-6Al-4V-HAC interface, with HACs that had and had not been immersed in a pH-buffered, serum-added simulated body fluid (SBF). The shear-strength at the HAC-bone interface was investigated in a canine transcortical femoral model after 12 and 24 weeks of implantation. The results showed a bonding degradation of approximately 32% or higher of the original strength after 4 weeks of immersion in SBF, and this predominantly depended on the constructed microstructure of the HACs. After the push-out measurements, it was demonstrated that the HACs with higher bonding-strength in vitro would correspondingly result in significantly higher shear-strength at each implant period in vivo. Nevertheless, there were no substantial histological variations between the two types of HACs evaluated. The most important point elucidated in this study was that, among coating characteristics, the microstructure was the key factor in influencing the mechanical stability of the HACs both in vitro and in vivo. As a consequence, a denser HAC was needed to ensure mechanical stability at both interfaces.

Intramedullary implant of plasma‐sprayed hydroxyapatite coating: An interface study

An intramedullary implant model in the canine femora was developed to evaluate the mechanical and histological responses between cancellous bone and plasma-sprayed hydroxyapatite coatings (HACs) on ti-6A1-4V implants, with 12- and 24-week follow-ups. HACs of different thicknesses were investigated. Results of the mechanical testings revealed that after 24 weeks of implantation, the mean shear strength (2.49 +/- 0.12 MPa) of the 50 microns HACs was significantly higher (p < 0.05) than that of the 200 microns HACs (1.44 +/- 0.19 MPa). However, using backscattered electron images (BEIs) throughout all the implant periods, no substantial histological variations in the extent of new bone apposition between the two HACs were observed. Occasionally, solution-mediated disintegration of the 50 microns HAC was found 24 weeks postimplantation. Histomorphometric studies from the BEIs demonstrated that for both HACs the percentage of the direct HAC-cancellous bone contact was approximately 50% at 12 weeks and 75% at 24 weeks. After the mechanical tests, the 200 microns HACs had fracture sites either inside the coating layers or at the HAC-titanium interfaces, which might explain why the mechanical performance of the 200 microns HACs was inferior to that of the 50 microns HACs even though both HACs had the same histological behaviors.

Attachment and proliferation of neonatal rat calvarial osteoblasts on Ti6Al4V: effect of surface chemistries of the alloy.

This study examined the cell attachment and proliferation of neonatal rat calvarial osteoblasts on Ti6Al4V alloy as affected by the surface modifications. The modifications could alter simultaneously the surface chemistries of the alloy (elemental difference of Ti, Al, V, Cu and Ni about 300-600mum thick examined by EDS) as well as the XPS nano-surface characteristics of oxides on the metal surface (chemistries of oxides, amphoteric OH group adsorbed on oxides, and oxide thickness). Three materials including two from modifications and a control were examined. It is argued that a slight change of the nano-surface characteristics of oxides as a result of the modifications neither alters the in vitro capability of Ca and P ion adsorption nor affects the metal ion dissolution behavior of the alloy. This implies that any influence on the cytocompatibility of the materials should only be correlated to the effect of surface chemistries of the alloy and the associated metal ion dissolution behavior of the alloy. The experimental results suggest that the cell response of neonatal rat calvarial osteoblasts on the Ti6Al4V alloy should neither be affected by the variation of surface chemistries of the alloy in a range studied.

Comparison of plasma‐sprayed hydroxyapatite coatings and hydroxyapatite/tricalcium phosphate composite coatings: In vivo study

This study aimed to compare biological properties, including osteoconduction, osseointegration, and shear strength, between plasma-sprayed hydroxyapatite (HA) and HA/tricalcium phosphate (TCP) coatings, using a transcortical implant model in the femora of canines. After 3 and 12 weeks of implantation, the implants with surrounding bone were assessed histologically in undecalcified sections in backscattered electron images (BEIs) under a scanning electron microscope (SEM). After short-term (3 week) follow-up, both coatings conducted new bone formation and revealed direct bone-to-coating contact. The HA/TCP coating could not enhance early host-to-coating responses. At 12 weeks, serious dissolution of the HA/TCP coatings evidently occurred. By the new bone healing index (NBHI) and apposition index (AI), we found no significant difference between HA/TCP-coated implants and HA-coated implants throughout all implant periods. At 12 weeks of implantation, some particles dissociated from the HA/TCP coating were found within the remodeling canal. After push-out measurements, the shear strength and failure mode of HA/TCP-coated implants were similar to those of HA-coated implants, and no statistical differences were found between either coating. Consequently, this study indicates that HA/TCP coatings have excellent biological response and may be considered suitable bioactive ceramic coatings for short-term clinical use.

Characteristics of plasma-sprayed bioactive glass coatings on Ti-6A1-4V alloy: an in vitro study

Abstract Conventional bioactive glasses, in bulk form, are being considered as biomaterials in prosthetic applications. In this study, a new attempt was made to coat bioactive glasses on Ti-6A1-4V by plasma spraying. This method will coat the bioactive glass coatings (BGCs) onto metal substrate, potentially combining the excellent mechanical strength of metal and biocompatibility of bioactive glass. Analysis by X-ray diffractometry (XRD) of the BGCs, revealed that the amorphous structure of glass was preserved. BGCs were soaked in simulated body fluid (SBF) to evaluate their properties in vitro. After soaking in SBF for 1 day, precipitation of fiber structure was observed on the surface of the BGCs. After 2 and more days, the surface of the BGCs was completely covered with precipitates. The precipitates, identified as the apatite phase by XRD, contained carbonate and hydroxyl functional groups detected by Fourier transform IR reflection (FTIR) spectroscopy. After soaking for 16 days, a thin layer of about 10 μm, rich in calcium and phosphorus but poor in silicon, was observed on the surface of the BGCs. The composition of the CaP rich layer was consistent with the apatite structure identified by various methods, but the apatite layer was significantly thicker than reported in bulk form. The formation of an apatite phase surface has been suggested to be indicative of biocompatibility. All findings in this study indicated the formation of apatite on the surface of plasma-sprayed BGCs, and this material is expected to be biocompatible in vivo.

A comparison of the surface characteristics and ion release of Ti6Al4V and heat-treated Ti6Al4V.

This work seeks to investigate the nanosurface characteristics and ion release for a Ti6Al4V alloy prepared by various methods (as received and heat treated at 1300 degrees C for 2 h) with three different passivation treatments (34% nitric acid passivation, 400 degrees C heating in air, and aging in 100 degrees C deionized water). The surface and nanosurface composition are not related to the surface passivation treatments and experimental materials as evaluated by energy dispersive spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses. After passivation and autoclaving treatments, the specimens were immersed in 8.0 mM ethylenediaminetetraacetic acid (EDTA) in Hanks solution and maintained at 37 degrees C for periods of time up to 16 days. The 400 degrees C treated specimens exhibit a substantial reduction in constituent release, which may be attributed to the thicker thickness and rutile structure of the surface oxides. After soaking in Hanks-EDTA solution, a significant time-related decrease in constituent release rate is observed for all kinds of specimens throughout the 0-16 day experimental period. The thicker oxides may be a factor in the improved dissolution resistance. Upon immersion, nonelemental Ca and P are both detected on the surfaces of all kinds of specimens by XPS analysis, and this could be explained by the existence of two types of hydroxyl groups (acidic and basic OH groups) on the oxide surface of the specimens.

SURFACE CHARACTERISTICS OF TI6AL4V ALLOY : EFFECT OF MATERIALS, PASSIVATION AND AUTOCLAVING

The properties of passivated films for Ti6Al4V alloy prepared by various methods (as-polished, brazed at 970 °C for 2 h and brazed at 970 °C for 8 h) were investigated. Four passivations (non-treated, nitric acid passivation, 400 °C-treated in air, and aged in boiling water), with or without autoclaving treatments, were adopted for evaluating the changes of surface properties, including chemical composition, chemical structure, and oxide thickness. From X-ray photoelectron spectroscopic (XPS) analyses, surface elements of copper and nickel in brazed samples were undetected for non-treated, acid-passivated and boiling water-aged specimens, while they were found in the 400 °C-treated specimen. The relative contents of Ti2++Ti3+ to Ti4+ were determined by passivation treatments, but were not related to the experimental materials and autoclaving treatment. Passivation and autoclaving decreased the Ti to Ti4+ ratio by virtue of an increase in oxide thickness. Of the four types of passivation treatment, the 400 °C thermal treatment exhibited the lowest content of suboxides and metallic elements and the thickest oxide by XPS analysis; however, this treatment may cause a desorption of the basic OH group in the hydration layer on the surface of titanium alloy.

The cell attachment and morphology of neonatal rat calvarial osteoblasts on the surface of Ti-6Al-4V and plasma-sprayed HA coating: Effect of surface roughness and serum contents

The biocompatibility of material plays an important role in the bone–implant interface for the prosthetic implant fixation. The biocompatibility of implants is associated with the chemical composition, surface topography, surface energy and surface roughness of biomaterials. The effects of two factors, surface roughness and serum contents, on osteoblast behavior at the surface of Ti-6Al-4V and plasma sprayed HA coating were investigated in the experiment. The osteoblasts derived from neonatal rat calvarial were cultured in Dulbecco’s modified Eagle medium (DMEM) with fetal bovine serum (FBS) on the surface of polished Ti-6Al-4V (Ti-p), grit-blasted Ti-6Al-4V (Ti-b), polished HA coating (HAC-p), and as-sprayed HA coating (HAC). Under culture medium containing 4% FBS, the level of cell attachment to the polished surface is significantly higher than the rough surface of the same experimental materials during all culture periods. Increasing the contents of FBS up to 10%, the difference of osteoblast attachment is not found between Ti-p and Ti-b. Under 4% serum condition, the cell morphology attached to smooth surfaces (Ti-p and HAC-p) is spread faster and are more flattened than the one to rough surface of the same experimental materials by SEM. After 24 h culture, the corroded cracks are easily observed at the surface of polished HA coatings, and the cell morphology on HAC-p coatings are elongated and less flattened compared with Ti-p. The result is consistent with statistical difference of cell attachment between Ti-p and HAC-p under 4% serum condition.

Biological responses of neonatal rat calvarial osteoblasts on plasma-sprayed HA/ZrO2 composite coating

Plasma-sprayed hydroxyapatite (HA) coating, applied to metal substrates, can induce a direct chemical bond with bone and hence achieve a biological fixation of the implant. However, the poor bonding strength between the HA coating and the substrate has been a concern for the orthopedists. In a previous study, the zirconia-reinforced hydroxyapatite composite coatings (HA/ZrO2) could significantly improve the mechanical strength before and after soaking in simulated body fluid. This study aims to investigate the biological responses of osteoblasts on plasma-sprayed HA/ZrO2 coating. The osteoblasts derived from neonatal rat calvarial were cultured in Dulbecco’s modified Eagle medium (DMEM) with fetal bovine serum (FBS) on the surface of plasma-sprayed HA coating, HA/ZrO2 coating, and ZrO2 coating, respectively. The biological responses were investigated by the cell growth (1, 3, 5, and 10 days) and the cell morphology under scanning electron microscopy (SEM) (3, 6, 12, 24 and 48 h). Examination by SEM revealed that osteoblasts on HA coatings exhibit less spreading during the medium phase (6 and 12 h), while, better morphologies were observed at the latter phases (24 and 48 h). This should be derived by the dissolution of HA coating in the culture medium. On HA/ZrO2 coating, the cells showed the poor morphologies at the latter phases (24 and 48 h). This could be explained by the no apatite formed at the surface HA/ZrO2 coating after soaking in simulated body fluid. The lower contents of ZrO2 coating in HA coating and the addition of other solid solution (ZrO2–MgO, CaO–ZrO2, ZrO2–CeO2) in HA coating are the two possible methods to improve the cytocompatibility of HA/ZrO2 coating.

Effect of passivation on the dissolution behavior of Ti6A14V and vacuum-brazed Ti6A14V in Hank's ethylene diamine tetra-acetic acid solution Part I Ion release

This work aims to investigate the effects of three factors, namely: (1) two differently prepared materials (as-polished Ti6A14V and 2 h brazed Ti6A14V); (2) three different surface passivation treatments (34% nitric acid passivation, 400°C heated in air, and aged in 100°C de-ionized water); and (3) periods of immersion time (up to 32 days), on trace element release in Hanks ethylene diamine tetra-acetic acid (EDTA) solution. After passivation and autoclaving treatment, the specimens were immersed in 8.0 mM EDTA in Hanks solution and maintained at 37°C for periods of time up to 32 days. The 400°C -treated specimens exhibit a substantial reduction in constituent release, which may be attributed to the higher thickness and rutile structure of the surface oxides. For acid-passivated and water-aged treatments, a highly significant decrease in the trace levels of Ti, A1, and V is detected from the brazed Ti6A14V compared to those obtained from the Ti6A14V specimens. It is hypothesized that an anatase–rutile transformation of surface TiO2 is likely to occur, accelerated by the elements of copper and nickel in the brazed specimens. In addition, a significant time-related decrease in constituent release rate is observed for all kinds of specimens throughout the 0–8 day experimental period. The implication of the results is discussed.