The International Journal of Prosthodontics, 03/1995

Twenty-five freshly extracted maxillary first poremolars were divided equally among five modalities for rebuiliding structurally compromised premolars. These modalities were (A) buccal stainless steel Parapost/amalgam core, (B) palatal stainless steel Parapost/amalgam core, (C) two stainless steel Paraposts/amalgam core, (D) two regular Link Plus TMS single shear Minim pins/amalgam core, and (E) palatal cast gold post and core. Loan fatigue was used as the testing protocol, using a 5.2-kg load applied at 72 cycles per minute. Each tooth was prepared with a 1.0-mm ferrule, and an upper limit of 250,000 load cycles was set for the testing. Two failures were exhibited, (1) preliminary failure which involved a crack in the luting cement layer, and (2) catastrophic failure wherein the corea nd crown separated from the root section. Statistical analysis using Kurskal-Wallis test showed three subsets (P.05). The subset with the highest number of load cycles to failure contained the palatal stainless steel post/amalgam core, two stainless steel Paraposts/amalgam core, and palatal cast gold post and core.

Differential scanning calorimetry was used to evaluate the efffects of polymerizing conditions and the addition of pigments for a heat-activated maxillofacial silicone (MDX 4-4515). Unpigmented specimens were examined in the starting condition and after polymerizing at 100 degrees C for 1 hour, 4 hours and 20 hours, and specimens of four different pigmented silicones processed 1 hour at 100 degrees C were also included. The DSC scanning was performed from -150 degrees C to 200 degrees C at 10 degrees C per minute, and a strong melting-recrystallization peak at approximately -40 degrees celcius was found for all eight specimen groups (N=5). Modulated differential scanning calorimetry resolved this peak into separate melting and recrystallization peaks, and confirmed the existence of a very weak glass transition peak in this silicone at approximately -125 degrees celcius. The mechanical properties of the MDX 4-4515 silicone appear to be dominated by melting and recrystallization, rather than the glass transition, and differences may exist in the viscoelastic and creep behaviour for the unpigmented and pigmented materials.

The release of elements from a series of palladium-copper alloys into cell-culture medium was measured using atomic absorption spectroscopy. The effect of the alloys on the succinyl dehydrogenase activity of Balb/c 3T3 fibroblasts was also measured. Palladium inhibited the release of Cu in alloys up to 70 atomic weight percent Cu. Below 70 at. % Cu, Cu release was 0.8 ug/cm2 at 72 hours. Palladium release was lower from purebPd (0.08 ug/cm2 at 72 hours from the Pd-Cu alloys containing >70 at.% Cu, where it increased to 1.2 ug/cm2. The cytotoxicity of the alloys correlated closely with the realease of Cu, becoming markedly cytotoxic above 70 at.% Cu, when Cu release was greater than 2.2 ug/cm2.

The tensile bond strength ofinlay materials to dentin was evaluated. Five materials, two direct resin composite inlay products, one hybrid resin composite, a preformed ceramic block, and a conventional porcelain material were bonded to bovine dentin and bonded together using a dual-polymerizing resin luting agent. Specimens were tested with and without thermocycling. Some bar-to-bar bonding groups fractured through the bar. Significantly higher (P.05) bar-to-bar bonds were recorded than the bar-dentin bonds, irrespective of the inlay materials. VitaDur N Showed the lowest bar-to-bar failure values (7.6 MPa) compared to the four other inlay materials (14.4 MPa to 22.2 MPa), but had the highest dentin bond strength (4.5 MPa). Unexpectedly, thermal loading increased the bond strength to bovine dentin, although this was only statistically significant for VitaDur N. The dentin bond strength of Charisma (1.4 MPa) was inferior (P.02) to all othe materials (2.6 MPa to 4.5 MPa) when thermocycled before testing.

The flecural strength of six recently introduced dental ceramic materials was measured using a three-point-bend test. Conventional feldspathic porcelain and soda-lime glass were used as controls. All six of the new materials had significantly greater breaking strength than the controls. The alumina-based crystalline-reinforced materials exhibited the highest breaking strengths. The silica-based crystalline-reinforced materials resulted in ceramic materials with more moderate strength but still with significantly greater strength than the controls. Scanning electron microscopic analysis of the fractured surfaces indicated crack deflection appeared to be the principal strengthening mechanism in the highly crystalline materials.

The purpose of this study was to determine if loading post-restored teeth at different angles would affect their resistance. Thirty maxillary central incisors were treated endodontically to within 5.0 mm of the apex. Standardized cast post and cores were fabricated using plastic #4 Parapost patterns. Post and cores were cemented using zinc phosphate cement followed by crowns of average dimensions. Ten teeth each were loaded at 110, 130, and 150 degrees to the long axis of the tooth. Mean failure loads were:110 degrees = 372.4 N± 140.8 (SD); 130 degrees = 597.6 N±138.5 (SD); 150 degrees = 1274.3 N±429.9 (SD). There were significant differences in fracture resistance between teeth tested at the following angles: 110 degrees versus 150 degrees (P.0001), and 130 degrees versus 150 degrees (P.0001), but not for 110 degrees versus 130 degrees (P=.2016). Mean failure loads increased as load angle approached parallelism to the long axis of the teeth. Significant differences in fracture resistance of post restored teeth can occur as a result of load angle.

The development of a computer-aided design system for a clinical crown restoration is reported. Initially, th shape of the standard crown was deformed and adapted on the die epending on the shape of the margin. Then, the measured occlusal record data were taken in the computer graphics, and the adapted crown was adjusted with these data to obtain using a functionally generated path record and the occlusal interferences eliminated. The ends of the adapted crown were connected with the margin. Thus, the entire crown was efficiently designed using the computer program developed.

Four groups of autopolymerizing acrylic resins used for various purposes in current dental practice were investigated by monitoring their temperature changes during the polymerization process. Resins were compared in terms of characteristic parameters (such as the time interval needed to reach mechanical stability and the maximum temperature generated during the polymerization process) and interpretations were made of such temporal temperature profiles.

The strength and mode of failure of three different designs of custom-made all-ceramic implant abutments fabricated by milling of In-Ceram crowns were fabricated for placement on the all-ceramic abutments, and in one test group, a veneer porcelain was filed directly on the abutment; crowns in the control group were fabricated using the CeraOne System. Tem-mm-long Branemark implants were placed into a brass block that allowed loading at a 30-degree angle to the long axis. The test group in which the veneer porcelain was fired directly on the all ceramic abutments was the weakest, and it showed fractures at a mean value of 236 N. The fracture strength of the three other test groups was dependent on the extension of the crown margin relative to the location of the screw head. The test group that had the screw on the top compressing the entire ceramic abutment showed a mean value of 422 N that was similar to the results that were achieved with the CeraOne system (427N). The weakest link in the all-ceramic single implant restorations was the abutment screw in which the bending began at approximately 190 N.

In this retrospective study reviewing 30 months of treatment, 53 kpartially or completely edentulous patients were provided with implant-supported restorations. The restorative aspects of the therapy were undertaken by an experienced prosthodontist who had just begun involvement with implant rebahilitation techniques. Patients were treated in two centres, 21 in a major teaching institution (Group A) and 32 in a private practice (Group B). In Group A, the implants were placed by experienced oral and maxillofacial surgeons just beginning involvement in implant techniques. Teh implants of the Group B patients were placed by surgeons with a minimum of 2 years implant experience. It appeared that the surgeon's experience had a major impact on the failure probability of unloaded implants. Loading conditions and the design of the prosthesis may be the decisive determinants for the probability of success with loaded implants. The hazards of extensive cantilevered extensions were demonstrated. The results suggest that those entering implant prosthodontics should not expect their early work to match the results obtained from established centres.

The effects of experimental lithium-containing ion exchange pastes on dental porcelain were compared with a commercial ion stregthening paste. Maximum biaxial flexure strength using the experimental pastes was obtained by firing lithium paste containing 200 g per L lithium sulphate dihydrate for 45 minutes at 750 degress celcius onto the surface of the porcelain. However, the commercial paste produced the greatest strengthening overall.

This in vitro study evaluated the viscoelastic properties of Permaflex compared to other soft lining materials. The surface condition of this material was also investigated under both laboratory and simulated clinical conditions and with and without the application of a varnish. The tests provided practical instructions for the use of Permaflex, which showed good adaptive properties to strexss and surface condition initially and after adjustment.