ickness of the cement layer may affect its aisso~~~Qrn~ manm~act~F~rsattempt to rn~n~rn~z~ film thickining a suitable cemen cement line will prov gth. This study ~ete~rnin~~ the effects of various film thickmesses om the bomd s~~~ng~~of Panavia
used to provide a 2.78 mm ~~tai~i~g hoie at one end of the he patterms were sprued on the cut swface retaining bole, invested in ~ig~-$~~~ investmemt (J, F. delenko Co. Armor&, NJ?.), cast in an induction casting machime ( Chicago, Ill.), and then air cooled prior to aev~stim~. The
Fig. 3. Tensile testing apparatus.
Fig. 4. Mean bond strengths. cast specimens were refined an a machinist’s lathe to a 90degree angle, smoothed with 400-grit silicon carbide paper, and air fired in a calibrated porcelain furnace (Flagship VPF, J.F. Jelenko Co) at 1010’ C. Each casting was abraded with 50 pm AlsOa for 15 seconds and ultrasonically cleaned in distilled water for 10 minutes (Fig. 1). The 120 specimens were then randomly divided into six groups and pairs of cylinders within a group were bonded with Panavia Opaque adhesive. The film thicknesses were: group 1,20 pm; group 2,50 pm; group 3,80 pm; group 4,110 Mm; group 5, 140 Km; and group 6, 200 pm. A micrometer device (Fig. 2) secured the specimens in a rigid position during bonding, and assured a controlled film thickness for each group. The combined length of two rods was measured with the micrometer apparatus prior to bonding and the adhesive agent was mixed according to the manufacturer’s directions Excess adhesive was applied to the test ends of both cylinders and the specimens were repositioned within the micrometer device. The micrometer was then adjusted to provide the desired cement space, and excesscement was removed prior to the surface application of the Oxyguard (Kuraray Co., Ltd., Osaka, Japan) material. The specimens were retained in place by the micrometer apparatus for 5 minutes. The specimens were stored at 37” C in distilled deionized water for 1 day and then the bonded specimens were ther-
mocycled for 1 day in distilled water baths maintained at 5’ and 60’ C. The cycle rate was one cycle every 80 seconds with a 30-second dwell time in each bath, and 1080 cycles were completed. After thermocycling, the specimens were stored at 37’ C in distilled deionized water for an additional 28 days. After the storage period, the samples were positioned in an Instron testing machine (Instron Corp., Canton, Mass.) (Fig. 3) and a tensile load was applied until bond failure, using a 500 kg load cell with an 0.5 cm/min crosshead speed. The tensile force required to sever the bond was used to measure the adhesive bond strength (in megaPascals), and a stereomicroscope (A0 Scientific Instruments, WarnerLambert, Buffalo, N.Y.) was used to evaluate the mode of failure.
RESULTS The means, standard deviations, range, standard error of the mean, and the coefficients of variation for the six groups are depicted in Table I, while Fig. 4 illustrates the mean bond strengths. The results were subjected to a one-way analysis of variance and to Duncan’s multiple range test at the 95 % confidence level (Tables II and III). The specimens bonded with an 80 pm cement film thickness demonstrated substantially higher tensile bond strengths (Table III).
joint.14 The faim thickness with. a specific cement depends on the particle size of the powder, the cQ~ce~tratjQ~ of the force applied to the casting durmanner in which the force is app!ied, and the fit of the restoratioa.15 permitted varying the film tbickadhesive cement to measure the ength. A pPevious stuap used a similar ~~~~~rne~t apparatus to investigate the ~e~~t~~~sbi~ her the film thicknesses between 20 and 140 pm of zinc ~bos~~ate cement on the retention of veneer crowns. T at variation in the film thickness of the minimal effect on crown retenti~n.~~ icated that a film thickness of 80 pm cre~a~-to-com~osit~ resin bon is theorized that with a ce less than 80 pm, irregularities within the metal specimens mjlections on the metal surface mg. 6) and form surface flaws within the luting media &wing bonding. Tlae
Fig. 5. Adhesive failure with 20 pm luting agent film thickness.
subsequent application of a tensile force resulted in crack propagation of those flaws, with subsequent fai1ure.l’ The 80 iurn film thickness group also recorded the smallest coefficient of variation. Greater coefficients of variation were evident with the larger cement film thicknesses, but the bond strengths exceeded the accepted strengths for enamel-to-composite resin bonds.18 Difficulties in specimen alignment were encountered at elevated film thicknesses, which could account for the extreme coefficients of variation in those groups. Stereomicroscopic evaluation of the samples revealed voids in all specimens, and larger voids were observed in specimens with greater film thicknesses (Fig. 7). The voids may also account for the elevated coefficients of variation within those groups. The results in this study pertained only to a specific luting agent. The Panavia Opaque adhesive system consists of a filled BIS-GMA composite resin, methacrylates, and a phosphate ester added to the mon0mer.r This cement exhibited a greater compressive strength with a lower degradation rate than traditional cements such as zinc phosphate and polycarboxylate.
This study suggested that the film thickness of the luting agent influenced the bond strengths; therefore future in vitro bond strength studies should be instituted to control this variable. Numerous cementation materials are available to dentists, but the ramifications of the physical properties of a specific dental material should be acknowledged prior to its clinical use.
CONCLUSIONS This study determined the effects of cement film thickness on the tensile bond strength of a prosthodontic adhesive (Panavia Opaque) with these results: Il. All film thicknesses demonstrated tensile bond
6. A, Lathed and sandblasted specimen for bonding. (Original magnification ~35.) B, Surface irregularity at higher magnification. (Original magnification X500.)
7. Voids in specimen with 200 pm luting agent film thickness.
strengths exceeding the accepted values for bonding enamel to composite resins. 2. The elevated cement film thicknesses demonstrated greater coefficients of variation. 3. A film thickness of 80 pm recorded a statistically
10. il. 12. 13.
14. 45. 16.
Adhesive cements and techniques. Proceedings of the International Sympo&m on Adhesive Prosthodontics. Academy of Dental Mate.ria!s. Nijmegen, The Netherlands 1966;16. Christensen GJ. Marginai fit of gold inlay castings. J PROSTHET DENT 1966;16:297-305. Dedmon HW. Disparity in expert opinions on size of acceptable margin openings, Oper Dent 1982;7:97-101. Dedmon HW. Ability to evaluate nonvisible margins with an explores. Oper Dent 1985;10:6-11. Dedmon HW. The relationship between open margins and margin designs on full cast cunwns made by commercial dental laboratories. J PROSTHETDENT 1985;53:463-6. Phillips R&V. Skinner’s science of dental materials. 8th ed. Philadelphia: WB Saunders Co, 1982:463. Craig RG. Restorative dental materials. 8th ed. St Louis: CV Mosby Co. 1985:195-6. Jorgensen KD, Esbensen AL. The relationship between the film tbickness of zinc phosphate cement and the retention of veneer crowns. Acta Odontol Sand 1968;26:169-75. Dunn 8, Levy MN, Reisbick MH. Improving the fracture resistance of dentaf ceramic. J Dent Res 1977:56:1209-13. Livaditis Gd, Thompson VP. Etched castings: an improved retentive mechanism for resin bonded retainers. -4 PRQSTHET DENT 1982;47:62-8.
DR..&4 M.DM-&%NOLD [email protected]
UNIVERSITY ok: IOWA IOWA CITY, IA 52242