In vitro stability of a highly crystalline hydroxylapatite coating in a saturated citric acid solution.

A novel pressurized hydrothermal post-plasma-spray process has been developed to convert the crystalline non-HA and amorphous components of plasma-sprayed hydroxylapatite coatings back into crystalline HA. The process, known commercially as MP-1, was used to produce coatings comprising approximately 96% crystalline HA. The in vitro solubility of the coating in saturated citric acid solution has been measured to simulate the effect of implant detoxification procedures, which use citric acid as a cleaning medium. The MP-1 coating solubility in saturated citric acid solution (pH = 1) was compared to that of coatings with crystalline HA contents ranging from 37.5-82%. All coatings showed an initial sharp rise in coating dissolution, which correlated with crystalline HA content, followed by a steady state dissolution rate. After 60 s at 25 degrees C, the MP-1 coating showed a 65% decrease in solubility compared to a highly amorphous coating (AM-2). All coatings showed very similar steady state dissolution rates, except for AM-2, which was significantly higher. SEM analysis showed that the AM-2 coating surface was degraded substantially more than the other coatings, resulting in partial coating exfoliation. A mechanism of coating dissolution is proposed, in which the initial rapid leaching of soluble phases from the coating leaves behind a porous layer of highly crystalline HA at the coating surface. The stability of this porous crystalline layer leads to steady state, diffusion-limited dissolution of the remainder of the coating. The observed two-regime dissolution profile can be accurately represented by a 2-parameter model, which predicts the initial sharp rise in coating dissolution followed by a slower, steady state loss in coating mass. Model parameters were determined from experimental solubility data, and were shown to correlate with the percentage of crystalline HA in the coatings. The present data suggest that the treated coating is significantly more resistant to degradation from aggressive detoxification procedures such as citric acid burnishing.

Highly crystalline MP-1 hydroxylapatite coating. Part I: In vitro characterization and comparison to other plasma-sprayed hydroxylapatite coatings.

A novel pressurized hydrothermal post-plasma-spray process, referred to as MP-1, has been developed to convert the crystalline non-HA and amorphous components of plasma-sprayed hydroxylapatite coating back into crystalline HA. No detrimental effects are observed on the strength of either the base metal or the coating. X-ray diffraction (XRD) and FTIR analysis, surface roughness, shear adhesion strength and calcium solubility testing were conducted on Calcitite coated samples before and after treatment with this process. Other commercially available coatings were also studied using XRD and solubility testing. Quantitative XRD data show that the MP-1 treatment increases the average crystalline HA content of the Calcitite coating from 77% to 96%, while the amorphous content decreases from 21% to 4%. Other commercially available dental implant coatings ranged in crystalline HA content from 45% to 73%, with amorphous phase content ranging from 29% to 62%. FTIR spectra for treated coatings were significantly more well defined, and showed an increase in peak separation and intensity. Surface roughness and shear adhesion strength were not affected by the treatment. In vitro solubility testing revealed that for all coatings there is an initial introduction of calcium into solution over the first 2 h of testing; however, the amount of calcium dissolved was significantly lower for the MP-1 coating. Under a pH and temperature representative of normal physiologic conditions, the rate of calcium dissolution for the MP-1 coating was significantly lower than that of the other commercial HA coatings. In increasingly acidic conditions, the MP-1 coating was compared to the Calcitite coating and was found to have a significantly slower rate of calcium release. The MP-1 treatment enhances typical HA coatings by increasing the crystalline HA content at the expense of the plasma-spray-induced soluble phases without a reduction in the strength of the coating. The resulting coatings exhibit significantly decreased in vitro solubility over a wide range of pH. The results of this solubility testing suggest that the treated coating may show significantly enhanced in vivo stability, even under the extreme conditions encountered during periods of infection or rigorous detoxification procedures. The significant differences between plasma-sprayed HA coatings reported here underscore the need for industry and academic researchers to raise the level of discourse and understanding of HA coatings. By offering consistent and accurate descriptions of coating compositions and methods of analysis, meaningful comparisons between different HA coatings can be made.

Highly crystalline MP-1 hydroxylapatite coating. Part II: In vivo performance on endosseous root implants in dogs.

The in vivo integration strength and degree of bone apposition were compared for oral endosseous implants with different plasma-sprayed hydroxylapatite (HA) coatings. Pullout strength measurements and histological analysis were used to compare two different commercially available coating from the same manufacturer. One coating does not receive a post-plasma-spray treatment and contains about 75% crystalline HA. The other coating is treated with the MP-1 process, a pressurized hydrothermal post-plasma-spray process, which increases the coating composition to approximately 95% crystalline HA without changing the coating's adhesive or cohesive strength. Comparisons were made in dogs after healing times of 3 and 15 weeks in the mandible. No significant differences were found in either case between the two coatings. Two different methods were used to determine the degree of bone apposition at 15 weeks. Both methods confirmed that the MP-1 process does not affect the osseointegration rate of plasma-sprayed HA coatings. Qualitative histology data suggest that the treated coating is more stable than the control coating, especially in cases of direct soft tissue attachment to the implant. The present data suggest that extensive dissolution of calcium phosphate components into surrounding tissue is not a necessary precursor for direct apposition of bone to HA-coated implants.

Evaluation of three different dental implants in ligature-induced peri-implantitis in the beagle dog. Part II. Histology and microbiology.

The purpose of this study was to evaluate experimental peri-implant breakdown microbiologically, radiographically and histologically. Hydroxyapatite-coated, titanium plasma-sprayed, and titanium alloy surfaces were investigated. Eighty-four implants were placed in 14 beagle dogs. Standardized radiographs and microbiologic samples (DNA) were obtained. Dogs were sacrificed at 3 and 6 months. Undecalcified histologic sections were prepared. Thickness of hydroxyapatite coating, changes in crestal bone height, and marginal changes in osseointegration were measured. Vertical bone loss was computed. Radiographs were analyzed using computer-assisted densitometric image analysis (CADIA). Microbial analysis (DNA) did not clearly favor any of the examined surfaces. CADIA did not show differences among implant surfaces. No significant differences among the three implants were noted for histometry, except the experimental titanium plasma-sprayed surface showed an increase in vertical bone loss 6 months (P < .05). Thickness of hydroxyapatite was decreased in active peri-implantitis sites (P < .05). Clinical attachment level was shown to be the most sensitive clinical parameter for detecting histologic changes. All implants were equally susceptible to peri-implantitis.

Evaluation of three different dental implants in ligature-induced peri-implantitis in the beagle dog. Part I. Clinical evaluation.

The purpose of this study was to clinically evaluate experimental peri-implant breakdown. Hydroxyapatite-coated, titanium plasma-sprayed, and machined titanium-alloy surfaces were investigated. Eighty-four implants were placed in 14 beagle dogs. Pocket probing depths and clinical attachment level and mobility measurements were made. Dogs were sacrificed at 3 and 6 months. All experimental implants showed a significant loss in clinical attachment level (P < .05). Increased pocket probing depths for experimental implants occurred during the first 2 months, after which a plateau was reached. At the 3- and 6-month evaluation, pocket probing depths at experimental implants were significantly increased (P < .05). No differences among the three implant types were noted for clinical attachment levels and pocket probing depths. In general, greater mobility was found with the titanium-alloy implants than with hydroxyapatite-coated and titanium plasma-sprayed implants (P < .025). In addition, mobility measurements were significantly greater for experimental titanium-alloy implants during the first 3 months (P < .05). Clinical attachment level measurements were most sensitive to peri-implant status. All implants were equally susceptible to ligature-induced peri-implant breakdown. Consequently, meticulous oral hygiene and regular maintenance care are prerequisites for successful implantology.