Das marginale Dimensionsverhalten präfabrizierter Kronen auf Ha-Ti-Implantaten

Transcription

1 - 1 - Original title of the article: Das marginale Dimensionsverhalten präfabrizierter Kronen auf Ha-Ti-Implantaten Published in: Zeitschrift für Zahnärztliche Implantologie 2/97 Carl Hanser Verlag Munich Published by: German Society for Dental and Oral Medicine R. Mathys jun., Ch. Besimo, G. Bigolin, F. Cicoira

2 - 2 - Offprint from Zeitschrift für Zahnärztliche Implantologie Marginal Dimensional Behavior of Prefabricated Crowns on Ha-Ti Implants Influence of laboratory processing and functional loading in vitro The publishers reserve all rights, including those of reprinting, photomechanical reproduction and translation of this offprint. HANSER

3 - 3 - R. Mathys jun. 1, Ch. Besimo 2, G. Bigolin 1, F. Cicoira 1 Marginal Dimensional Behavior of Prefabricated Crowns on Ha-Ti Implants Influence of laboratory processing and functional loading in vitro For implant-mounted superstructures with distinctly submarginal crown margins the demand is for a precision fit that should be devoid of marginal gaps. In an in vitro study a mean marginal fitting accuracy of less than 4 µm was measured after laboratory processing and additional functional continuous loading for prefabricated crowns from the Ha-Ti implant system. Assessment of the dimensional behavior of the same specimens with other parameters confirmed the high precision of the reconstructions. The maximum deviation of the annular seating surfaces from flatness was 1.8 µm on the implants and 2.2 µm on the crowns. The maximum peak-to-valley height of the annular seating surface both on the implant side and on the crown side was 1 µm. The diameter of the pivot of the implants was reduced by 4 µm on average and the hole diameter of the crowns manifested an average increase of 8 µm. Of these parameters only the crown hole showed any significant change during the course of the investigation. 1 Introduction Investigations into the marginal fitting accuracy of cast crowns on natural teeth and implants have demonstrated that under optimum laboratory conditions and clinical experimental conditions gap widths of 20 µm can be achieved (Strub et al. 1983, Belser et al. 1985, Böttger et al. 1988). Clinical studies, on the other hand, produced values which were 100 µm above that figure (Düsterhus 1980, Diedrich and Erpenstein 1985, Spiekermann 1986, Donath and Roth 1987). They confirmed that in a practice-oriented procedure the marginal precision of 50 µm specified by Dreyer-Jørgensen (1958) is unrealistic. However, for implant-mounted superstructures with crown margins, which generally come to rest at a distinctly submarginal position, especially in the two-phase implantation method, precision fit is required to be devoid of any marginal gaps (Tetsch 1991). In an in vitro study the marginal fit of prefabricated single crowns from the Ha-Ti implant system (Mathys AG, 2544 Bettlach, Switzerland) was examined before and after laboratory processing and after continuous loading (Besimo et al. 1995). The crown margins were each displayed at a magnification of 500x over their entire length by means of overlapping scanning electron microscope images. Marginal fit was assessed circularly at the crowns. Measurements were taken every 100 µm. The mean marginal gap width was 1.9 ± 1.8 µm before laboratory processing and 2.7 ± 1.3 µm afterward. After functional loading a marginal fitting accuracy of 3.0 ± 1.5 µm was established. The results of this study made it possible to infer that although a much higher marginal integrity can be achieved with micromechanically prefabricated superstructure elements than with castings, taking commercial factors into account, it is not possible to obtain a fit without a gap. 1 Dr. h.c. Robert Mathys Stiftung, Bettlach, Switzerland. 2 Department of Prosthetics and Mastication Theory at the Institute of Dentistry of the University of Basle (Director: Prof. Dr. C.P. Marinello, M.S.), Switzerland.

4 - 4 - The present study investigates the influence of laboratory processing and functional loading on the marginal dimensional behavior of Ha-Ti implants and prefabricated crowns with other parameters, using the same specimens as in the in vitro investigation just described. 2 Materials and Methods Five prefabricated single crowns for Ha-Ti implants with a diameter of 4.5 mm and a length of 14 mm were subjected to testing. The superstructures consisted of titanium restorations that were screwed onto the implants axially, serving as a connection for attaching the crowns proper with a transverse screw. The definitive shaping of the cylindrical gold crowns by contouring was conducted in a standardized manner so all the frameworks for ceramic veneering had the same dimensions. Burning on the ceramic compounds was performed in a furnace under constant conditions (Vacumat 200; Vita Zahnfabrik GmbH & Co. KG, Bad Säckingen). Then the crowns were mounted on the original implants, whereby the transversal screws, according to a recommendation by the implant manufacturer, were all tightened with a torque of 0.2 Nm. The specimens were subjected to dynamic bending in a servo-hydraulic continuous oscillating system 10 6 times. The cyclic force, which was a maximum of 50 N and a minimum of 10 N, acted on the crowns at an angle of 50 relative to the implant axis. The frequency of load change was 10 Hz. During continuous loading the specimens were kept in Ringer's solution at 37 C. Assessment of the marginal dimensional performance of the Ha-Ti implants and prefabricated crowns took place in parallel with the scanning electron microscope marginal gap analysis before and after laboratory processing and after functional loading. The following parameters were determined in each case (Fig. 1): Flatness of seating surfaces The annular seating surface on the implants and crowns was scanned with a UMM 850 Zeiss instrument at eight points distributed equally around the circumference. Contact force was 0.1 N and measuring accuracy was ± 2 µm. Through the eight measuring points an optimal plane was calculated. As a measure of flatness the mean maximum discrepancy between the lowest and highest points perpendicular to the optimal plane was stated. Fig. 1 The following parts of the Ha-Ti implants and prefabricated crowns were analyzed: (a) ring-shaped seating surface of the implant, (b) diameter of the pivot on the implant, (c) ring-shaped seating surface of the (d) diameter of the pivot hole in the prefabricated crown. Roughness of seating surfaces The mean peak-to-valley height and maximum peak-to-valley height were determined in a radial direction using the T2000 Hommel tester, always at the same point on the seating surface on the implant and crown. Measuring accuracy was ± 5% of the value observed. Diameter of the implant pivot and the crown hole The pivot on the implants and the hole in the crowns was scanned with the UMM 850 Zeiss instrument at four points equally distributed around the circumference. Through the four measuring points an optimal circle was calculated in each case. Twice the mean distance of the measuring points from the centre of the circle was determined as a measure of the diameter. Measuring accuracy was ± 2 µm. The statistical significance of discrepancies in test results before and after laboratory processing and after continuous loading was determined using the Wilcoxon, Mann and Whitney U test. The probability of error p was established as being 0.05.

5 - 5-3 Results Fig. 2. Scanning electron microscopic view of the seating surface K 1, of the implant after dynamic bending, magnification 17x (a), and 100x (b) The mean maximum discrepancy between the lowest point and the highest point perpendicular to the optimal plane, which was determined as a measure of the flatness of the seating surfaces, was 1.6 µm for the factory-new implants. After screwing on and unscrewing the porcelain-veneered crowns and after continuous loading of the specimens a consistent value of 1.8 µm was established. The prefabricated crowns showed a 1.9 µm maximum deviation from the flat plane before laboratory processing, 2.0 µm after porcelain veneering, and 2.2 µm after functional loading. All the results were in the range of the measuring accuracy of ± 2 µm. Measurement of peak-to-valley height and on the seating surface of the implants produced mean values between 0.09 ± 0.01 µm and 0.19 ± 0.04 µm and maximum peak-to-valley heights between 0.80 ± 0.11 µm and 1.18 ± 0.13 µm. The seating surface of the crowns showed mean peak-to-valley heights between 0.11 ± 0.04 µm and 0.16 ± 0.06 µm, and maximum peak-to-valley heights between 1.00 ± 0.52 µm and 1.18 ± 0.42 µm. Statistical analysis of the test data produced no significant discrepancy between implants and crowns (Figs. 2 and 3). Neither with the implants nor with the crowns did laboratory processing and functional loading lead to any distinct change in surface roughness. Fig. 3 Scanning electron microscopic view of the seating surface K k, of the prefabricated crown after dynamic bending, magnification 17x (a), and 100x (b) The mean diameter of the pivot of the factory-new implants was ± mm before and after laboratory processing. Due to continuous loading the diameter was reduced to ± mm. This change in diameter was not significant. The mean hole diameter of the factory-new crowns was ± mm. During laboratory processing it showed a statistically significant increase to ± mm. Only compared with the initial value did continuous loading produce a significant increase to ± mm.

6 - 6-4 Discussion The data obtained for the assessment of the flatness of the seating surfaces of implants and crowns before and after laboratory processing and after functional loading was within the measuring accuracy range of ± 2 µm. Consequently, no significant change in flatness was established during the test procedure. The marginal fitting accuracy of 4 µm to be expected with simultaneous occurrence of 1.8 µm maximum deviation for implants and 2.2 µm for the crowns was comparable with the mean value of 3.0 ± 1.5 µm determined on the same specimens by marginal gap analysis using a scanning electron microscope (Besimo et al. 1995). The contact zones of implants and crowns were characterized by a very smooth surface (Matarasso et al. 1996). The laboratory processing and functional loading had no distinct influence on the roughness of the contact surfaces. For assessment of flatness the peak-tovalley heights had no effect because for this no measuring points were used in valleys. The play between the pivot of the implants and the crown hole increased during the course of testing. The reason for this was particularly the significant enlargement of the crown hole after laboratory processing. Thermal influences during the baking-on of the porcelain were chiefly discussed as being the possible cause of this change. The effect of mechanical continuous loading, on the other hand, was less and only involved the implant pivot. Since measurements at the end of testing were also within the range of production tolerances stated by the manufacturer, 3.70 mm ( mm/ mm) for the implant pivot and 3.70 mm ( mm/ mm) for the crown hole, impairment of crown centering on the implants due to laboratory processing and dynamic bending was ruled out. All in all, this experimental investigation also confirmed the high marginal fitting accuracy of micromechanically prefabricated superstructures from the Ha-Ti implant system already described in the marginal gap analyses using a scanning electron microscope (Besimo et al and 1995). On the basis of other research, however, it was demonstrated that this precision is still far from adequate for a microbiologically tight seal (Besimo et al. 1996). Literature

7 - 7 -