[ evaluation of the marginal integrity of porcelain fused to base metal crowns after extension of porcelain to the border of the margin]

In PFM restorations, marginal coverage of the metal framework with porcelain, without producing an overcontoured margin is necessary to enhance esthetics. The aim of this study was to evaluate the marginal integrity of a porcelain fused to base metal crown after extension of porcelain to the border of the margin. Thirty brass standard dies were prepared and divided into 3 groups of 10, based on different finishing lines: 90° shoulder, 1.2-mm-wide; 135° slopping shoulder, 1.2-mm-wide; and shoulder, 0.7-mm-wide with a 45° bevel, 0.5 mm-long. For each die a brass cap was fabricated according to the specific finishing line to accommodate a 0.5 mm thickness. Two metal copings were made on each die which were used as test and control samples. The marginal gap of each coping was measured at 4 points under a reflective microscope after casting and oxidation. Porcelain was applied and extended to the border of the margins in the test group and 0.5 mm short of the borders in the control group. Marginal gaps were measured once more at the same points after porcelain firing. Statistical analysis was performed using t-test. The marginal gaps of all copings increased after porcelain firing. Extension of porcelain to the border of the margins did not significantly increase the marginal gaps. These results indicated that porcelain can be extended to the border of the crown's margin without producing a significant increase in the marginal gap

Effect of heat treatment on marginal integrity of metal-ceramic casting in base metal alloy

Increasing PFM restorations longevity is one of the objectives in restorative dentistry and marginal fitness is an important factor to gain this purpose. In this study the effects of heat treatment on marginal fitness using both base metal and precious alloys are surveyed. Forty wax pattern on 40 standard model has been provided. Thirty of them with base alloy and 10 with precious alloy were cast. After divesting, marginal gap for each model have been measured [reflective microscope x 200]. Then samples were divided into 4 groups as follows: Group 1: Cold working- oxidizing- marginal gap measuring. Group 2: Reinvesting the model- heat treating at 1100°C for 20 minutes- ensuing stages same as group 1. Group 3: These samples were made from precious alloy and all procedures were followed as group 2. Group 4: Oxidation- gap measurement - ensuing stages same as group 1. Data were analyzed by ANOVA, T-test and Duncan test. Mean marginal gap in group I was the most and in group III was the least. Mean marginal gap in group II was Lesser than IV and in base alloy was more than precious. Heat treatment of metal framework make marginal adaptation better. If metal framework is reinvested and heat treated before any cold working and oxidization, marginal adaptation will improve

[ SEM evaluation of wax pattern marginal fitness using different time intervals and temperatures]

Introduction and objective: one of the most important laboratory procedures in crown and bridge construction is preparing the wax pattern. A restoration is predictable when its marginal gap is minimum, which depends on the marginal fitness of wax pattern. Since a lot of studies related to marginal distortion of the wax pattern have been done after removing it from the working model [die], this study was designed to evaluate marina1 fitness using different time intervals and temperatures while the wax pattern remains on the die. The purpose of this study was to assess the marginal wax pattern fitness on the working model in different temperatures and time intervals with two types of hard and soft waxes

Materials and Methods: in an experimental study, 30 wax patterns were made on 30 truncated cone shape brass models using dipping wax technique. A brass cap was used to control the thickness of the wax, and marginal fitness. The dimensions of the brass models were 6mm x 6mm with an 8-degree convergence and a shoulder finishing line of l mm width. 15 wax patterns were prepared from hard wax [Ramin] and 15 from soft wax [Renfert]. Each type of wax was divided into third groups: The first group was immersed in the water with 23oc temperature, the second group in water with 32oc temperature and the third group was stored in refrigerator at 8oc. In order to protect the wax patterns from the direct contact with the water, they were located in a brass container. This also helps in properly heat transition. The storage times for each group were 2h, 5h and 22h. The marginal vertical discrepancy was determined by a scanning electron microscope with magnification of 200 at 3 determined points. The data were analyzed with 3 way, 2 way and one way ANOVA and Duncan test

Results: 1] The maximum mean gap for soft wax [Renfert] occurred during 0-2 hours [15.23mm+4.52] and for hard wax [Ramin] during 2-5 hours [1.23mm+/-0.67]. 2] Soft wax [Renfert] showed more marginal gap at the temperature of 32oc and time interval of 0-22h compared with 80oc temperature and the time interval of 0-22 hour. 3] Hard wax [Ramin] showed no significant difference in marginal gap at temperatures of 230G, 32oc and 8oc even after 22h interval

Conclusions: 1] The best storage temperature for sofwax [Renfert] is 8oc. 2] The best wax for marginal fitness is hard wax [Ramin]. 3] The marginal discrepancy in hard wax [Ramin] was not significant clinically even after 22h