kianoosh torabi

Statement of the Problem: Marginal fitness is the most important criteria for evaluation of the clinical acceptability of a cast restoration. Marginal gap which is due to cement solubility and plaque retention is potentially detrimental to both tooth and periodontal tissues.
This in vitro study aimed to evaluate the marginal and internal fit of cobalt- chromium (Co-Cr) copings fabricated by two different CAD/CAM systems: (CAD/ milling and CAD/ Ceramill Sintron).
We prepared one machined standard stainless steel master model with following dimensions: 7 mm height, 5mm diameter, 90˚ shoulder marginal finish line with 1 mm width, 10˚ convergence angle and anti-rotational surface on the buccal aspect of the die. There were 10 copings produced from hard presintered Co-Cr blocks according to CAD/ Milling technique and ten copings from soft non- presintered Co-Cr blocks according to CAD/ Ceramill Sintron technique. Marginal and internal accuracies of copings were documented by the replica technique. Replicas were examined at ten reference points under a digital microscope (230X). The Student's t-test was used for statistical analysis. p Statistically significant differences existed between the groups (p Hard presintered Co-Cr copings had significantly higher marginal and internal accuracies compared to the soft non-presintered copings.

A precise impression is mandatory to obtain passive fi t in implant-supported prostheses. The aim of this study was to compare the accuracy of three impression materials in both parallel and nonparallel implant positions. In this experimental study, two partial dentate maxillary acrylic models with four implant analogues in canines and lateral incisors areas were used. One model wassimulating the parallel condition and the other nonparallel one, in which implants were tilted 30° bucally and 20° in either mesial or distal directions. Thirty stone casts were made from each model using polyether (Impregum), additional silicone (Monopren) and vinyl siloxanether (Identium), with open tray technique. The distortion values in three-dimensions (X, Y and Z-axis) were measured by coordinate measuring machine. Two-way analysis of variance (ANOVA), one-way ANOVA and Tukey tests were used for data analysis (α = 0.05). Under parallel condition, all the materials showed comparable, accurate casts (P = 0.74). In the presence of angulated implants, while Monopren showed more accurate results compared to Impregum (P = 0.01), Identium yielded almost similar results to those produced by Impregum (P = 0.27) and Monopren (P = 0.26). Within the limitations of this study, in parallel conditions, the type of impression material cannot affect the accuracy of the implant impressions; however, in nonparallel conditions, polyvinyl siloxane is shown to be a better choice, followed by vinyl siloxanether and polyether respectively.

Because of compromised angulations of implants, the abutments are sometimes prepared. The purpose of this study was to investigate the effect of removing one wall of the implant abutment on the retention of cement-retained crowns. Four prefabricated abutments were attached to analogues and embedded in acrylic resin blocks. The first abutment was left intact. Axial walls were partially removed from the remaining abutments to produce abutments with three walls. The screw access channel for the first and second abutments were completely filled with composite resin. For the third and fourth abutments, only partial filling was done. Wax-up models were made by CAD/CAM. Ten cast copings were fabricated for each abutment. The copings of fourth abutment had an extension into the screw access channel. Copings were cemented with Temp Bond. The castings were removed from the abutment using an Instron machine, and the peak removal force was recorded. A one-way ANOVA was used to test for a significant difference followed by the pairwise comparisons. The abutments with opened screw access channel had a significantly higher retention than the two other abutments. The abutment with removed wall and no engagement into the hole by the castings exhibited the highest retention. Preserving the opening of screw access channel significantly increases the retention where one of the axial walls of implant abutments for cement-retained restorations is removed during preparation.

The early computer-aided design/computer-aided manufacturing (CAD/CAM) systems were relied exclusively on subtractive methods. In recent years, additive methods by employing rapid prototyping (RP) have progressed rapidly in various fields of dentistry as they have the potential to overcome known drawbacks of subtractive techniques such as fit problems. RP techniques have been exploited to build complex 3D models in medicine since the 1990s. RP has recently proposed successful applications in various dental fields, such as fabrication of implant surgical guides, frameworks for fixed and removable partial dentures, wax patterns for the dental prosthesis, zirconia prosthesis and molds for metal castings, and maxillofacial prosthesis and finally, complete dentures. This paper aimed to offer a comprehensive literature review of various RP methods, particularly in dentistry, that are expected to bring many improvements to the field. A search was made through MEDLINE database and Google scholar search engine. The keywords; ‘rapid prototyping’ and ‘dentistry’ were searched in title/abstract of publications; limited to 2003 to 2013, concerning past decade. The inclusion criterion was the technical researches that predominately included laboratory procedures. The exclusion criterion was meticulous clinical and excessive technical procedures. A total of 106 articles were retrieved, recited by authors and only 50 met the specified inclusion criteria for this review. Selected articles had used rapid prototyping techniques in various fields in dentistry through different techniques. This review depicted the different laboratory procedures employed in this method and confirmed that RP technique have been substantially feasible in dentistry. With advancement in various RP systems, it is possible to benefit from this technique in different dental practices, particularly in implementing dental prostheses for different applications.

Marginal adaptation is the most critical item in long-term prognosis of single crowns. This study aimed to assess the marginal quality as well asthe discrepancies in marginal integrity of some PFM single crowns of posterior teeth by employing parallel radiography in Shiraz Dental School, Shiraz, Iran. In this descriptive study, parallel radiographies were taken from 200 fabricated PFM single crowns of posterior teeth after cementation and before discharging the patient. To calculate the magnification of the images, a metallic sphere with the thickness of 4 mm was placed in the direction of the crown margin on the occlusal surface. Thereafter, the horizontal and vertical space between the crown margins, the margin of preparations and also the vertical space between the crown margin and the bone crest were measured by using digital radiological software. Analysis of data by descriptive statistics revealed that 75.5% and 60% of the cases had more than the acceptable space (50µm) in the vertical (130±20µm) and horizontal (90±15µm) dimensions, respectively. Moreover, 85% of patients were found to have either horizontal or vertical gap. In 77% of cases, the margins of crowns invaded the biologic width in the mesial and 70% in distal surfaces. Parallel radiography can be expedient in the stage of framework try-in to yield some important information that cannot be obtained by routine clinical evaluations and may improve the treatment prognosis

Provisional cements are commonly used to facilitate retrievability of cement-retained fixed implant restorations; but compromised abutment preparation may affect the retention of implant-retained crowns.The purpose of this study was to investigate the effect of abutment design and type of luting agent on the retentive strength of cement-retained implant restorations. Two prefabricated abutments were attached to their corresponding analogs and embedded in an acrylic resin block. The first abutment (control group) was left intact without any modifications. The screw access channel for the first abutment was completely filled with composite resin. In the second abutment, (test group) the axial wall was partially removed to form an abutment with 3 walls. Wax models were made by CAD/CAM. Ten cast copings were fabricated for each abutment. The prepared copings were cemented on the abutments by Temp Bond luting agent under standardized conditions (n=20). The assemblies were stored in 100% humidity for one day at 370C prior to testing. The cast crown was removed from the abutment using an Instron machine, and the peak removal force was recorded.Coping/abutment specimens were cleaned after testing, and the testing procedure was repeated for Dycal luting agent (n=20). Data were analyzed with two- way ANOVA (α=0.05). There was no significant difference in the mean transformed retention (Ln-R) between intact abutments (4.90±0.37) and the abutments with 3 walls (4.83±0.25) using Dycal luting agent. However, in TempBond group, the mean transformed retention (Ln-R) was significantly lower in the intact abutment (3.9±0.23) compared to the abutment with 3 walls (4.13±0.33, P=0.027). The retention of cement-retained implant restoration can be improved by the type of temporary cement used. The retention of cast crowns cemented to implant abutments with TempBond is influenced by the wall removal.

Although logbook is a useful tool in learning and assessment of the student, its use in the education of undergraduate dentistry students is not well-established. The present study was conducted to assess the effect of logbook as a study guide and an effective method for assessment of the students in the fixed prosthesis course. This quasi-experimental study was performed in Shiraz Dental School. The subjects of this study consisted of 60 students categorized into two experimental and control groups. In the control group, the students underwent the current format of Fixed Prosthesis curriculum. In the experimental group, the intervention was carried out using the Logbook. Pre and post tests were done using MCQ. The instructors and students had to evaluate the students’ daily activities. The data were analyzed using SPSS software. T-test, Paired T-test and Mann Whitney test were used for statistical analysis. The logbooks were effective in cognitive and psychomotor domain (knowledge and practice) of dental education and the mean difference between the two groups was significant (P<0.01). The use of logbook provided a more objective evaluation and led to further student satisfaction. Logbook is a useful tool for teaching and learning as an interactive study guide and assessment tool. Using logbook in dentistry education necessitates more studies to be conducted in this regard and also the revision of dental curricula.