The aim of this study was to evaluate and compare the stress transmission of different prosthetic designs for the two and four implants model, and the analysis of the photoelastic stress. For evaluating the stress transmission and photoelastic stress, the two and four implants models were constituted. For all the models, the implants were placed parallel to each other and their midline. The stress distribution around the implants were evaluated using the photoelastic stress analysis. During installation, the stress lines (Fringes) were observed with a polariscope and determined by taking photos of the models. For the two implants models, the conventional locater attachment overdenture, crown design-retained removable prosthesis with clasp retention, bridge design-retained removable prosthesis with clasp retention, and bridge design-retained removable prosthesis with precision attachment retention, and for the four implants model, the conventional bar attachment overdenture, fixed bridge design-retained removable prosthesis with clasp retention, and fixed-retained removable prosthesis with precision attachment retention were evaluated.
First, six heat polymerized acrylic resin models (Kulzer Dental) were prepared to model an edentulous patient’s mandible. Cylindrical implants (4.1×12 mm tissue level, Straumann, Institut Straumann AG) were embedded in the parasymphyseal area of each wax model by a parallelometer (Orthofex, Fogászat Gyártơ KFT). For the two-implant models, the distance between the implants were set to 22 mm and the implants were located in the lateral-canine region.11 For the four-implant models, the distance was set to 11 mm. The acrylic models with implants were duplicated with a C type silicone impression material by the putty-wash technique (Zetaplus, Zhermack). Silicone molds were cured for 1 week at 25 °C to eliminate the tensions that can occur in photoelastic models depending on the shrinkage of the silicone. The curing was performed to avoid the inaccurate and redundant fringes arising from polymerization shrinkage. The models were tested for residual stresses after setting and after the curing period of 1 week. The Photoelastic resin (PL-2; 35 g) and the catalyzer (PLH-2; 35 g) (Measurements Group Inc.) were used. Manufacturer recommended formula was used to calculate the ratio of resin.
Both resin and silicone molds were placed into an incubator for 1 hour and heated to 46–52 °C. This heating process provided the lower viscosity and produced a more homogeneous mixture. After incubation, the catalyzer and resin were mixed with a glass pipette avoiding excessive movements and bubbles. The mixing resin filled the entire silicone molds. Mixtures in the silicone molds were polymerized at 25 °C for 24 hours. Following this, all resin models were prepared and polished, and the prosthetic laboratory processes were started. For all prosthetic designs, silicone molds were used for standardization of settings for all steps like wax modelation, porcelain application, and artificial teeth settings. For light transmission, transparent acrylic resin (Akribel Hot Acryl, Atlas Enta Dental) was used according to the manufacturer’s recommendations12 for all types of prosthesis. For gingival stimulation, the posterior areas of the prosthesis were removed at 3 mm thickness using a drill that marked 3 mm in advance to guide the removal. After removal of the prosthesis, the areas were filled with an artificial gingival mask (Zhermack Gingifast Elastic) and placed on the photoelastic models, and the outflows were removed for all types of prosthesis.
Preparation of the implant-supported removable partial dentures
For the fixed components of implant-supported removable partial dentures (IRPD), 5.5 mm solid abutments were placed into the two- and four-implant models and torqued with 35 Ncm. For the laboratory stages, laboratory models were duplicated from the photoelastic models with vinylpolysiloxane impression material (Virtual, Ivoclar Vivadent). First, two and four implant-supported bridge design restorations were modeled, and then a silicone index was used to standardize the bridge precision attachment and crown design models. An extracoronal resilient precision attachment was used (Probolte Attachments, Kargı Dental). With small dimensions (2.0 mm height and 4.2 mm length), these attachments are used particularly in anterior regions with two abutment teeth. Good retention, lack of milling requirements, and simplicity of use are the most important properties of this system.7 The casting procedures were performed according to the manufacturers’ recommendations with a Ni-Cr alloy (61% Ni, 25.60% Cr, 10.94% Mo, 1.51% SI, 0.01% Mn, and 0.01% C).
After the laying procedure, metals were sandblasted with 25–70 µm Al2O3 particles (Star Dental) and ultrasonically cleaned for 15 minutes before porcelain application. Porcelain (Shofu Vintage Halo, Shofu Inc., PN 6507) was applied and then checked on the master models.
For the removable prosthesis with clasps, I bar clasps were planned. All models were cast with a Cr-Co alloy; 64% Co, 28.5% Cr, 8% Fe, and 3% Mo (Wironium Plus, Bego, PN 50190) After the artificial teeth were set for all models. After checking the setting on the master models, the finishing procedures were made with standard methods. Laying and polishing procedures were then conducted.
Preparation of the implant-supported overdentures
Initially, for the four IO models, SynOcta abutments (Straumann, Institut Straumann AG) and gold copings were screwed on. Bar attachments were cut according to the distance between the copings and blocked out with utility wax in order to set them 5 mm away from the tissue (3 mm gingival height, 2 mm distance from the tissue). The prepared bars were adjusted by millimeters, a pattern resin was used for splitting the bars, and the gold copings were unscrewed for the soldering procedures.
For the two-implant models, locater abutments were fitted and torqued with a 35 N/cm force, and an acrylic base plaque was prepared and fixed to the locater matrix. After the positioning of the attachments, models were duplicated for the laboratory procedures. Artificial teeth settings were made according to the silicone indexes. All finishing procedures were completed according to the standard methods on the duplicate models. After the laying and polishing procedures, the prostheses were checked on the master models. A blue-colored retentive rubber with a retention force of 1.5 lbs = 66.7 N was used.
A 300 N vertical load was applied bilaterally to the central fossa of the first premolar occlusal surface by a universal test machine (TSTM 02500, Elista Ltd.). The models were placed a special clamp-like device for stabilization so that there was no movement when the load was applied. The resulting stresses of the models were observed and recorded photographically (Powershot G3, Canon) in the field of a circular polariscope (Sharples 2026). The polariscope was placed into the universal test machine; the placement order from back to front was as follows: white light, source polaroid plates, the model, quarter-wave plates, and the photographic machine (Fig. 1).
The stress fringes showed some properties such as color, frequency, and distribution that state the amount of strain. Fringes were composed of different color bands (isochromatic), and the border of these isochromatic fringes was determined by the amount of strain. The characteristic properties of isochromatic fringes are expressed numerically in Table 1.13,14