An evaluation of fit in osseointegrated implant components using torque/turn analysis.

PURPOSE: The accurate and passive fit of dental prostheses supported by endosseous implants is of primary importance in securing long-term restorative success. In the clinical setting, adequate visual and radiographic assessment of joined implant components can be limited. Mechanical engineering principles show a linear relationship between tightening and the degree of rotation of a precision bolted assembly. At a constant torque, with certain variables controlled, a threaded fastener should return to the same rotational end position on repeated tightenings. This study evaluated the terminal screw positions of joined implant components as a potential aid to the clinician in confirming the fit of a fixed and removable prosthesis. There were three areas of experimental inquiry: (1) How reproducible are the various clinical means by which torque is applied to the fastening screws, both in absolute and relative value? (2) How reproducible are the rotational end positions of the gold (attachment) and titanium (center) screws when a controlled torque is applied? (3) Do changes in screw position occur as a function of the magnitude of artificially introduced discrepancies? MATERIALS AND METHODS: Three different torque delivery devices were evaluated: a hand-held screwdriver (DIB 048; NobelpharmaUSA, Chicago, IL), a manual torque wrench (DIA 250; NobelpharmaUSA), and an electronic Torque Controller (DEA 020; NobelpharmaUSA), using a calibrated torque measuring dynamometer (Magtrol, Inc, Buffalo, NY). The reproducibility of turning limits were determined for both the titanium and gold screws contained in five Brånemark implant assemblies. Each assembly was subjected to six trials, tightening to recommended torque. The position of each screw head was recorded with a special scribe on acetate sheets and transferred to graph paper. Five implant assemblies were invested in dental stone within a die form mold. A casting was made supported by three implant analogues. Stainless steel shims of 12.7-microns, 25.4-microns, 38.1-microns, and 50.8-microns thickness were used to create impingement and space discrepancies. Controlled trials were conducted, and changes in rotational limits for each screw were recorded. RESULTS: The following values were measured, intending to achieve a torque of 10 Ncm, based on 10 trials for each implement: hand driver, 6.48 (+/- 0.85) Ncm; torque wrench, 7.77 (+/- 0.56) Ncm; and the Torque Controller, 8.54 (+/- 0.19) Ncm. The electronic Torque Controller proved to be the most reproducible instrument and was selected as the delivery vehicle for the remainder of the study. The titanium center screws had a rotational limit that was reproducible to within 0.6 degrees (+/- 0.2 degrees). For the gold screws, it was found that at least two trials had to be conducted for each assembly before the rotational limits conformed to a reproducible position within 1.85 degrees (+/- 1.87 degrees). A linear relationship of approximately 0.9 degrees/micron was observed between the changes in rotational limit and each subsequent shim thickness. CONCLUSIONS: This study suggests that screw position can be an indicator of fit in dental implant prostheses if the end point of screw rotation is adequately indexed, specific to each assembly and screw.

Development of an in vitro culture system for the study of osteoclast activity and function.

Resorption is a difficult process to study in vivo because of the complex organization of mineralized tissue. The objective of this study was to develop an in vitro culture system for studying osteoclast attachment and activity. Transparent, microporous substrata were prepared using radio frequency-sputtering techniques to apply a thin layer of hydroxyapatite on polycarbonate filters previously coated with Type I collagen. Dentinoclasts isolated from the root surface of rat molar teeth undergoing inflammatory resorption and osteoclasts isolated from the endosteal surfaces of rat long bones were incubated on the experimental substrata. The culture system proved successful, supporting osteoclast viability and activity as demonstrated by cellular attachment and spreading seen by scanning electron microscopy. Activity was further demonstrated by monitoring acid production using a fluorescent probe, acridine orange. Isolated osteoclasts or dentinoclasts, when combined with synthetically prepared substrata, were observed to respond by mechanisms comparable to those that occur in vivo. Therefore, the system developed permits basic observations of osteoclasts in an in vitro system.

Passive fit of implant-retained prosthetic superstructures improved by electric discharge machining.

PURPOSE: An absolutely passive fit at the interface with the superstructure and the abutment cylinders is needed for implant longevity. In this study, a method of cast framework correction using electric discharge machining (EDM) was evaluated. MATERIALS AND METHODS: An original research model incorporating strain gages was devised to measure the fit of implant superstructures in three dimensions as cast and to compare this relationship with the fit following framework machining with EDM. RESULTS: Visually, the fit of the frameworks was excellent, and the results of ANOVA demonstrated that the mean fit of the frameworks significantly improved (P > .04) after machining with EDM. CONCLUSIONS: The EDM process offers a rapid, accurate, and simple technique to correct for casting inaccuracies in the fabrication of hybrid implant prostheses.

The advantages of coated titanium implants prepared by radiofrequency sputtering from hydroxyapatite.

The method used to apply hydroxyapatite to implant surfaces may affect the thickness and ultimately the physical properties of the coating. This study investigated and compared the healing rates of bone around commercially pure titanium implants and titanium implants sputter-coated from a hydroxyapatite target. Forty-five sputter-coated implants and an equal number of noncoated titanium implants were placed into 15 partially edentulated dog mandibles. The implants were removed at three time periods and were evaluated mechanically and histologically. A multiple analysis of variance indicated that the interface bond strength was statistically greater (p less than 0.01) for the sputter-coated implants. Histologic analysis of the bone-implant interface demonstrated that coated implants had nearly twice the percentage of direct bone contact compared with noncoated implants. The results indicate that implants sputter-coated from a hydroxyapatite target will accelerate the healing of bone at the implant interface.

An investigation of dental luting cement solubility as a function of the marginal gap.

The purpose of this study was to investigate the rate of type I zinc phosphate cement solubility as it relates to the degree of marginal opening. Standardized test samples were constructed that would simulate clinically relevant marginal gaps of 25, 50, 75, and 150 microns and their subsequent cement lines. The study was divided into two phases. Phase 1 evaluated the effects of simple diffusion on cement solubility in a static environment, whereas phase 2 investigated the effects of convective forces on cement dissolution in a dynamic environment. Both the phase 1 and phase 2 studies demonstrated no significant difference in the rate of cement dissolution for the 25-, 50-, and 75-micron test groups. The 150-micron test groups for both studies, however, demonstrated an increase in the rate of cement dissolution. The results of the phase 1 and phase 2 studies should not be compared because different methodologies were used.

The influence of condensation method on porosity and shade of body porcelain.

Condensation of porcelain is an important laboratory variable that can affect a variety of physical properties in metal ceramic restorations. This study examined four methods of condensation (brush with vibration, ultrasonic vibration, spatulation, and noncondensation) to evaluate their effects on porosity and color. Standardized 12 x 12 mm acetate plastic squares were invested and cast with Olympia alloy. After oxidation, the metal substructures received one of the following opaque and body porcelains: Ceramco II G Series, Will-Ceram V-Series, Vita VMK-68, or Jelenko. The examination of each sample consisted of three parts: (1) apparent specific gravity analysis (porosity); (2) tristimulus colorimeter analysis; and (3) subjective observer color analysis. No statistical differences (p less than or equal to 0.05) in apparent specific gravity were found between methods of condensation within any of the four porcelains. Statistical differences (p less than or equal to 0.05) in color were noted between methods of condensation within several of the porcelains by both the tristimulus colorimeter and the subjective observers.

Glass-ionomer cement film thickness and working time.

Reports on film thickness for adhesive glass-ionomer cements have not been consistent, and confusion exists relative to proper working times. In this investigation, film thickness was measured over a range of times for three glass-ionomer cements by using a traditional ADA glass plate technique and a simulated clinical cementation technique with castings and dies. The cements were examined from 60 seconds to 255 seconds from the start of mix. Results indicated that the simulated clinical technique generally provided less film thickness. No significant differences in film thickness were found between the brands of cement through 240 seconds with film thickness ranging from 7.24 to 20.5 microns. Greater differences between the cements were observed with the ADA technique beyond 135 seconds from the time of mix. Relative to film thickness, working times would appear to be greater than previously reported for glass-ionomer cements.

The vertical dimensional change resulting from polishing or glazing porcelain.

Studies have shown that polishing porcelain can produce a smooth surface similar to that resulting from a natural glaze. It has been reported that glazing porcelain causes changes in the occlusal vertical dimension. This study evaluated the changes in vertical dimension after natural glazing and polishing procedures. Twenty uniform-size tricuspid porcelain-fused-to-metal crowns were constructed on a mandibular typodont tooth. A inch steel ball was cradled between the three cusps, and the occlusal vertical dimension was measured from the top of the steel ball to the base of the castings. Contact areas were marked and lightly removed. Ten crowns received natural glazing and the remaining 10 were polished with a Shofu polishing kit. Measurements of occlusal vertical dimension were repeated. All of the crowns showed a loss of occlusal vertical dimension. The mean loss for polished crowns was significantly higher than that recorded for the glazed crowns.

Powder enrichment effects on film thickness of zinc phosphate cement.

The effects of powder erichment and subambiant mixing temperatures on the film thickness of three zinc phosphate cements were measured by two methods. A decrease in film thickness accompanied reduced mixing temperatures and standard powder-liquid ratios. Increased powder-liquid ratios with reduced mixing temperatures showed a decreased film thickness when measured according to ADA Material Specification No. 8. Both Fleck's and S. S. White cement showed an increased film thickness when measured by a tapered-die system. Because of the geometry of the preparations, small increases in cement film thickness can be clinically significant when cementing complete veneer crowns.

The use of film thickness to measure working time of zinc phosphate cements.

A method is proposed to quantify the working time of zinc phosphate cement based on sequential measurement of viscosity. The method was used to measure the extension of working time resulting from reduced mixing temperatures. Premixing small amounts of powder prior to standard mix was shown to be ineffective at extending working time. The addition of water to the cement was shown not to affect working time but did decrease the setting time of the cement.

The sporicidal activity of glass bead sterilizers.

Endodontic glass bead sterilizers were evaluated for sporicidal effectiveness. Small metal endodontic instruments contaminated with Bacillus subtilis spores were effectively sterilized when the instruments were exposed for 15 seconds at a temperature of 218 C. Nonmetal objects such as cotton pellets and paper points could not be sterilized because they charred and decomposed under the required temperature conditions.