Part I: Crystalline Fluorapatite-Coated Hydroxyapatite, Physical Properties

After 20 plus years of steadily improving technology regarding dental implants and numerous scientific advancements as detailed by the National Institutes of Health Consensus Development Conference, we are still faced with the same “age old” issue that continues to plague the success of oral implant procedures, namely the looming threat of superimposed bacterial infection. Mastering the considerable rigors necessary to surgically place dental implants requires that clinicians focus entirely on learning the immense body of developing knowledge and literature specifically related to the field of implant dentistry. Some obvious biologic facts, once understood by all in terms of preventing dental disease, apparently were either not initially learned by younger dentists or were passed over and became ancient history in the rush to understand techniques of placing implants and to keep pace with developing knowledge regarding new materials, and technical aspects of surgical placement. These once venerated microbiologic cornerstones are basic to understanding why oral bacteria, not only have the ability to cause implant failure, but in many cases constitute the principle cause of such failure. Appreciation and attention to this vitally important microbiologic influence is of paramount importance if one is to achieve maximum clinical success of dental implants with regard to both preimplant grafting procedures and treatment of failing and ailing dental implants. It was with this point in mind that we hypothesized that fluoride ion, within the microenvironment of the surgical bed, released from a local reservoir of previously placed fluoridated implant material, could well be efficacious in exerting a bacteriostatic effect and in so doing, contributes to better clinical results through improved implant retention. Thus, in Part I of this investigation (reported here), we sought to introduce the idea of fluoridated grafting material as a potential clinical step, and in so doing, characterize the nature of the reaction product of sodium fluoride (NaF) solution and synthetic hydroxyapatite (HA) (OsteoGen, Impladent, Ltd, Holliswood, NY).

Fluoridation of dental enamel and its efficacy in prevention of dental caries has not only been researched extensively, it is clinically accepted and approved as a therapeutic regimen and has been so for many decades.1 Specific mechanisms of the actions of fluoride have also become more clearly understood in the past 2 decades. Not only does the fluoride ion and subsequent fluorapatite formation physically resist dissolution of mineralized surfaces (ie, dental enamel) in an acid environment, it also exerts a slowing metabolic effect on local bacteria and therefore inhibits bacterial acid production.2 In the case of dental caries, treatment with fluoride achieves a 2-pronged anticariogenic effect in not only leading to less soluble enamel surface but also causing diminished acid production by local bacteria. With regard to implant dentistry, the local availability of fluoride ion would seem to have equivalent benefits. Fluoride within bone graft material (made available by a well-understood mechanism as described in Part II3) would thus serve as an important reactant to be incorporated into local, potentially infective bacteria thus inhibiting and/or drastically reducing acid formation. In diminishing the ability of bacteria ability to lower the pH within the surgical bed of a graft, local fluoride ion helps to maintain the delicate acid-base balance within the surgical microenvironment and thus promotes normal osteogenesis and bone regeneration.

Part II of this series of publications investigated a new augmentation procedure consisting of a method to fluoridate resorbable HA implant material in order to produce the reaction product, fluorapatite (FA)-coated hydroxyapatite (HA).3 In previous dog studies, we compared the histologic microscopic evidence of the osteogenesis potential of FA bone graft material compared to control HA (in preparation). Evaluation of the histology revealed no difference in the bone apposition and integration with use of either microcrystalline surface-coated FA or control HA surfaces. No toxicity was noted in animals in which FA-coated material was used or within new bone surrounding the FA crystals. Further, in the dog model, there was superimposed infection noted in both quadrants of the animal in which nontreated HA implants were employed, whereas no such infection was noted surrounding dental implants in quadrants utilizing previously placed FA. To us, this finding suggested a possible bacteriostatic effect of fluoridated HA,3 thus stimulating our interest to further investigate whether or not fluoridated implant material might be effective in reducing superimposed infection in actual clinical cases.

Accordingly, in Part III in the series, we presented 4 clinical cases utilizing the above- mentioned FA regimen to: (1) treat and graft infected dental implants (with considerable bone loss) that traditionally would require removal of the failing or ailing implant, and (2) graft a preimplant infected site, with massive bone loss, subsequent to extraction of a severely infected molar tooth in preparation for future implant placement. The results in all 4 cases suggested that the FA procedure was efficacious in treating and grafting into infection sites, whether immediately subsequent to extraction in preparation for implant placement or in requisite revisional surgery in an attempt to save ailing and failing implants.4 

In Part I we investigated the physical properties of fluoride-treated HA (FA) using standard chemical and physical property analytic tools, including scanning electron microscopy (SEM), X-ray diffraction (XD), energy dispersive X-ray analysis (EDX), and EDX analysis mapping (EDXM). These are standardized testing methods well accepted in the scientific literature.

Similar chemical and physical analyses have been performed subsequent to fluoridation of human and animal slabs of natural tooth enamel. These studies indicate an uptake of surface fluoride of 3.8%.5–7 It is well established that crystals of biologic apatites are imperfect, with “vacancies” (ie, ions missing) leaving gaps and dislocations and some columns of ions out of alignment with the main bulk. These disruptions increase the potential for reaction with the fluoride ion even at low concentrations within biologic fluid. This increases the exchange of the hydroxyl ion with fluoride ions; in addition, vacant sites can be taken up with fluoride.5 The same can be said of the synthetic resorbable HA (OsteoGen). It is made utilizing a proprietary method that results in star-burst crystalline microstructures of nonsintered, resorbable HA that, similar to biologic apatites, are conducive to fluoride uptake (M. Valen, unpublished data, 2010). The literature8 using electron microprobe supports the fact that tooth surface hydroxyapatite, when exposed to the fluoride ion, converts to fluorapatite thought to be formed by dissolution of surface molecules of hydroxyapatite and recrystallization of fluorapatite in its place.

FA was prepared as previously described and readied for SEM, XD, EDX, and EDXM analysis using standard procedures. The analytic data were tabulated and prepared for interpretation.

The SEM showed only minor evidence of surface change. Changes were characterized as a slightly more distinct crystalline clarity on the surface of the FA sample compared with the HA control (Figure 1). XD patterns showed increased random noise dispersion pattern on the untreated control HA sample compared with the lack of noise patterns in the treated sample FA (Figure 2). The XD graph showed little difference between HA and FA, although in the treated FA sample, monetite peaks were completely missing compared with the obvious presence of such peaks within the nontreated HA sample (Figure 3). EDX analysis of the FA sample revealed a distinct fluoride peak at 0.70 KeV that was missing in the nonfluoridated control (Figure 4). EDX mapping showed an evenly distributed needle-like crystalline-shaped particulate pattern over the entire surface of the FA sample, which was not seen in the HA control sample (Figure 5).

Repetition and characterization of the classic fluoride reaction with crystalline HA surfaces, as reported here, would seem redundant, and perhaps would be so were it not for the fact that this study is performed for a unique and entirely different purpose: its potential application to implant dentistry. Understandably, implant dentists may not be familiar with the volume of fluoride research previously reported in the literature. Further, most prior fluoride studies employed acidulated fluorophosphate, compared with the neutral solution used here. Also, there are no studies in which synthetic crystalline resorbable HA (OsteoGen) was used exclusively as a substrate for fluoride interaction. Therefore, the authors felt it was necessary or, at the very least, useful to perform studies that scientists knowledgeable in the fluoride research literature already recognize and acknowledge since the reagent here is unique (proprietary OsteoGen). Further, the condition under which fluoridation was performed was slightly different (neutral pH).

It was determined that the fluoridation reaction procedure as utilized here, served to purify the initial HA reagent, thus eliminating the more soluble monetite contaminant from the synthetic HA reagent. Also, the fluoride reaction with surface HA, whether it be from a combination of dissolution-reapposition or isomorphic substitution, produces a more uniform and pure crystalline FA product as characterized by a sharper, less cluttered XD pattern. It was concluded that the surface reaction of HA with 4.3% NaF at neutral pH, produces an FA-coated product which, when used as an implant material, serves as a store of sequestered and potentially reactive fluoride ion. The bonded fluoride, ordinarily stable and nonreactive, would only become available to exert a bacteriostatic effect when and if dissolved from fluorapatite under conditions of lowered pH concomitant with superimposed local bacterial presence (infection).

Crystalline fluoridated synthetic hydroxyapatite (fluoridated OsteoGen) is studied using SEM, XD, EDX, EDXM, and infrared. The SEM showed relatively little surface change, which was characterized by a slightly more distinct crystalline clarity on the surface of the FA sample. The XD showed an increased random noise dispersion pattern of the untreated HA sample compared with the lack of noise patterns in the treated FA sample. Monetite was present as a contaminant of untreated HA in that characteristic peaks were noted in control HA samples and similar peaks were totally absent in analysis of the treated FA material, indicating that the fluoridation procedure, in effect, dissolved the monetite component. It was determined that the fluoridation synthesis reaction procedure served as a purification procedure of the initial HA reagent in eliminating the more soluble monetite contaminant. Also, the fluoride reaction with surface HA produced a more crystalline pure FA material characterized by a classic, sharp XD pattern. EDX analysis of the FA sample revealed a fluoride peak at 0.70 KeV that was missing in the nonfluoridated control. Further, EDX mapping showed an evenly distributed, needle-like crystalline-shaped particulate pattern over the entire surface of the FA sample which was lacking in the HA control. It was concluded that the reaction of resorbable synthetic HA with 4.3% NaF at neutral pH, produces an FA-coated HA. The latter material, when used in conjunction with dental implants, may be effective in bacteriostasis and thus lead to improved stability and better prognosis of oral implant procedures.

EDX

energy dispersive X-ray analysis

EDXM

EDX analysis mapping

FA

fluorapatite

HA

hydroxyapatite

NaF

sodium fluoride

SEM

scanning electron microscopy

XD

X-ray diffraction