D.C. Smith

A Histological Assessment of the Initial Healing Response Adjacent to Porous-surfaced, Titanium Alloy Dental Implants in Dogs

We report here the results of a histological assessment of the initial healing response following implantation into the dog mandible of a porous-surfaced, titanium alloy endosseous dental implant. Two implants were placed in edentulous areas on each side of the mandible of each dog and covered with a full-thickness mucoperiosteal flap. The implant sites on one side of the mandible were allowed to heal for four weeks, while those on the other side were allowed to heal for eight weeks before the animals were killed. Histological specimens were obtained and assessed both qualitatively and by computer-assisted morphometry. All but one of the 24 implants were well-tolerated and healed with a variable ingrowth of bone into the porous-surface geometry. The histomorphometric measurements revealed that bone ingrowth had reached a plateau by four weeks of initial healing.

Adhesion of Composite Resin to Bleached and Unbleached Bovine Enamel

Cylinders of microfil and small-particle light-cured composite resin were bonded to the flattened labial enamel surface of young bovine incisor teeth which had previously been subjected to four different treatments: (1) teeth immersed in 35% hydrogen peroxide (HP) for 60 min and etched (E) with 37% phosphoric acid gel for 60 sec; (2) teeth immersed in saline (S) for 60 min and E for 60 sec; (3) teeth E for 60 sec and immersed in HP for 60 min; and (4) teeth E for 60 sec, immersed in S for 60 min. Specimens were stored in water at 37°C for one and seven days prior to tension- and shear-testing. A total of 256 teeth was tested— eight teeth in each group for each day, for each resin, and for each test. Statistical analysis of the results indicated that there was a highly significant reduction in adhesive bond strength of the resins when the enamel was exposed to HP as compared with S. SEM examination of randomly selected fractured test specimens indicated that this reduction in adhesive bond strength occurred primarily at the bonding resin-enamel interface. Less significant differences in bond strength were noted in the control specimens, with regard to resin type, time of storage, and the etching order.

Preliminary Surface Analysis of Etched, Bleached, and Normal Bovine Enamel

X-ray photoelectron spectroscopic (XPS) and secondary ion-mass spectroscopic (SIMS) analyses were performed on unground un-pumiced, unground pumiced, and ground labial enamel surfaces of young bovine incisors exposed to four different treatments: (1) immersion in 35% H2O2 for 60 min; (2) immersion in 37% H 3PO4 for 60 s; (3) immersion in 35% H2O2 for 60 min, in distilled water for two min, and in 37% H3PO 4 for 60 s; (4) immersion in 37% H3PO4 for 60 s, in distilled water for two min, and in 35% H2O2 for 60 min. Untreated unground un-pumiced, unground pumiced, and ground enamel surfaces, as well as synthetic hydroxyapatite surfaces, served as controls for intra-tooth evaluations of the effects of different treatments. The analyses indicated that exposure to 35% H2O2 alone, besides increasing the nitrogen content, produced no other significant change in the elemental composition of any of the enamel surfaces investigated. Exposure to 37% H3PO4, however, produced a marked decrease in calcium (Ca) and phosphorus (P) concentrations and an increase in carbon (C) and nitrogen (N) concentrations in unground un-pumiced specimens only, and a decrease in C concentration in ground specimens. These results suggest that the reported decrease in the adhesive bond strength of resin to 35% H 2O2-treated enamel is not caused by a change in the elemental composition of treated enamel surfaces. They also suggest that an organic-rich layer, unaffected by acid-etching, may be present on the unground un-pumiced surface of young bovine incisors. This layer can be removed by thorough pumicing or by grinding. An awareness of its presence is important when young bovine teeth are used in a model system for evaluation of resin adhesiveness.

Fracture Toughness of Dental Composites Determined Using the Short-rod Fracture Toughness Test

Plane strainfracture toughness (KIC) has been evaluated for a number of commercially-available dental composites. A modified Short-rod Fracture Toughness (SRFT) specimen design has been used, enabling small specimens to be tested conveniently. The effect on KIC of aging in water at 37°C for seven days, one month, and six months has been determined for conventional, micro filled, and hybrid (coarse and fine filler particle-containing) composites. Our results suggest that aging for one month or more caused a reduction of KIC for the composites so aged. Comparison of the KIC values determined using the modified SRFT specimen with values obtained using more conventional specimen geometries gave good agreement, thereby suggesting the suitability of the small SRFT specimens for valid KIC determinations.

Adhesion to Bovine Dentin-Surface Characterization

X-ray photo-electron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) were used to characterize the dentin surface, to determine the effects of different pre-conditioning procedures on the elemental composition of the dentin surface, and to investigate the interaction between dentin and a dentin bonding agent (ScotchBond) by studying the changes in the elemental composition of dentin as a result of the interaction. Scanning electron microscopy (SEM) was used to characterize sample surface morphology, which was then correlated with surface elemental composition. The results showed that: (a) the elemental composition of the smear layer was similar to that of the underlying dentin; (b) cleaning with hydrogen-peroxide did not produce any modification in the elemental composition of the dentin surface; and (c) acid-etching led to an almost complete demineralization of the dentin, leaving behind an organic-rich surface. The results suggest that bonding systems that use acid-etching as a pre-conditioning procedure should be based on agents able to interact with the organic components of dentin, since bonding agents that rely on a chelation-to-calcium reaction are unlikely to be successful. The investigation of the interaction between the bonding agent and dentin led to a postulated adhesive-bonding reaction mechanism and suggested a partially cohesive failure in the bonding agent during fracturing of a dentin-bonding-agent-bonded assembly.

Crystalline Interface Formed by Polyacrylic Acid and Tooth Enamel

Several recent insvestigations havse confirmed that the polycarboxylate cements (SrMvITH, Tr Dent J 125: 381, 1968) displayy adhesive bonding to dental enamel that is (rtliable undlei in vivo conditions foi extended periods of time. In studies oIi the mechanism of adhesion of these cements to human enamel, it wsas observed that the interaction of the aqueous polyacrylic acid component of the cement with enamel varied accoirling to the formulation. Purified polyacrylic acit produced only slight etching on clean ctoamel surfaces. Hoissevxe, polyacmylic acid contaiming sulfate ion as a residue from tIme polynierization initiator gave a wshitish appearance to the enamel surface; observation wsith the scanning election microscope (SEM) showed that this was a crystalline deposit (Fig I). Further studies have shown that the crystal formation tlepends mainly on sulfate ion conccntiation in the polyacrylic acid solution and is independetit of the molecular weight or concentration of the solution. Scanning electron microscopy demonstrated that many of the crystals were nucleated at the sosrface of the enamel anti that slight chemical reaction of the surface occussed. Chemical analysis revealed the release of calcium ions from the tooth surface into the supervening polyacrylic acid. The crystals appeared to be bonded finmly to the enamel surface and esistecd mechanical emosval. X-ray diffraction and electron microprobe studieS were carried omt on samples of the crystalline deposit in situ on the enamel surface and also after iemosal by careful scraping. These measuirments revealed that the cmystals in situ mere gypsum CaSO4*R2,O; thme material removetI from the enamel surface suas a mixture of this and hydroxyapatite. Studies of the interaction of synthetic hydroxyapatitc tisks with acids revealed that only the sulfate containing polyacrylic acitl resulted in formation of gypsum crystals, othermiise CaHPO.21HIO was obtained. According to Broswn and Wallacc (Ann NY tcad Sci 57: 690-693, 1958) and Sognnaes (Chemistry of Enamnel Dissolution, 1962, p 66) CaHPO4 2I.O isill precipitate as time solid phase from acidic solution, since it has the los est solubility of the calcium phosphates. At a higher pH, Ca (H2P04) *2H2O is reported to precipitate. Since this does not occur in the polyacrylic acid solution containing sulfate ion, the influence of the polyacrylic acid itself, the

A Clinical and Radiographic Assessment of a Porous-surfaced, Titanium Alloy Dental Implant System in Dogs

A new endosseous dental implant system incorporating a porous-surface geometry with a tapered, truncated-cone shape was placed bilaterally in edentulous areas of dog mandibles in a two-stage procedure. All implants had been stabilized by bone ingrowth by the time of the second procedure (insertion of a transgingival collar and implant-supported bridge). The transgingival collar had a porous-surface geometry on its apical one-third, and was meant to encourage gingival connective tissue ingrowth and attachment, but in fact provided an excellent milieu for bacterial contamination. As a consequence, many of the implants showed clinical and radiographic signs of impending failure by eight months. Only those implants for which the porous coat, including that of the transgingival collar, was completely submerged in bone were observed to be successful. This study reports on the radiographic and clinical assessment of this implant system in dogs during the period of function.

Examination by X-ray Photoelectron Spectroscopy of the Adsorption of Chlorhexidine on Hydroxyapatite

X-ray photoelectron spectroscopy (XPS) was used for determination of the effects of chlorhexidine (CHX) solutions (0.2% and 1% solutions of the digluconate salt) on the elemental composition of hydroxyapatite surfaces. So that the nature of the adsorbed species after they were washed with water could be identified, comparisons were made with reference spectra for CHX obtained from a CHX digluconate film and CHX dichloride powder. The XPS results clearly indicated the retention of CHX moieties, which could be ascertained from the spectra by the presence of N and Cl, features unique to CHX. The spectral envelopes were virtually identical to those obtained from the reference spectra. High-resolution C Is spectra also gave support for the retention of CHX; however, the spectra differed from those of the CHX digluconate film in that no feature attributable to the C-OH of the gluconate anion was present, consistent with the view that the CHX cation remains behind to form an electrostatic bond with the phosphate groups of the hydroxyapatite. The N:Cl ratio for the washed samples was found to be higher than that for the reference samples and may be indicative of partial decomposition of the CHX. Decomposition was also seen to be induced by x-ray exposure. While the high-resolution spectra presented here do not directly address the controversy on the mechanism for the anti-plaque efficacy of CHX, they do provide the necessary basis for the application ofXPS to future in vitro studies on the retention of CHX to dental surfaces.

Materials Science Oscillatory Wear Tests of Dental Composites

The wear behavior of 12 commercial restorative composites (conventional and microfilled) and an unfilled restorative material was evaluated using an in vitro wear test referred to as an oscillatory wear test. The effect of varying test conditions was evaluated in a series of preliminary experiments in order to define test conditions for material evaluation. The materials were tested after being aged in water at 37°C for periods of from seven days to one year. The results indicated consistently lower wear rates for the microfilled materials as compared with those of the conventional composites. Aging did not result in significant differences in wear rates measured by this method. Scanning electron micrography of worn surfaces suggested a possible effect of filler particle shape and bonding to the matrix to explain this difference. Although the relevance of the results to clinical behavior is not yet known, the study indicated the importance of material variables on wear.