Statement of problem
How processing by computer-aided design and computer-aided manufacturing (CAD-CAM) or traditional chairside fabrication techniques affects the presence of defects and the mechanical properties of interim dental prostheses is unclear.
The purpose of this in vitro study was to compare the effects of CAD-CAM versus traditional chairside material processing on the fracture and biomechanical behavior of 4-unit interim prostheses with and without a cantilever.
Material and methods
Two types of 4-unit interim prostheses were fabricated with abutments on the first premolar and first mandibular molar, one from a prefabricated CAD-CAM block and one with a traditional chairside polymer-monomer autopolymerizing acrylic resin (n=10). Both groups were assessed by compressive strength testing and additionally with or without a cantilevered second molar by using a universal testing machine with a 5-kN load cell. A finite element model (FEM) was built by scanning both prosthesis designs. Finite element analysis (FEA) replicated the experimental conditions to evaluate the stress distribution through the prostheses.
Interim fixed prostheses manufactured by CAD-CAM showed significantly higher mean fracture loading values (3126 N to 3136 N) than for conventionally made interim fixed prostheses (1287 N to 1390 N) ( P =.001). The presence of a cantilever decreased the fracture loading mean values for CAD-CAM (1954 N to 2649 N), although the cantilever did not influence the traditional prostheses (1268 N to 1634 N). The highest von Mises stresses were recorded by FEA on the occlusal surface, with the cantilever design, and at the transition region (connector) between the prosthetic teeth.
Interim partial prostheses produced by CAD-CAM had a higher strength than those manufactured traditionally. The presence of a cantilever negatively affected the strength of the prostheses, although the structures manufactured by CAD-CAM still revealed high strength and homogenous stress distribution on occlusal loading.
CAD-CAM processing produces high-strength interim dental prostheses with favorable occlusal stress distribution that should provide suitable clinical performance.
The traditional chairside fabrication of interim fixed prostheses (IFPs) involves the autopolymerization of a polymer powder and liquid monomer or of 2-part composite resin pastes that lead to defects in the material’s microstructure. Such defects provide stress concentrations that can lead to mechanical failure. Computer-aided design and computer-aided manufacturing (CAD-CAM) is widely available, and industrial polymerization under controlled conditions has been reported to improve the microstructure and performance of the prosthesis. , The fracture strength of polymethyl methacrylate (PMMA) prosthetic structures prepared by CAD-CAM has been reported to be higher than that of those prepared with the traditional chairside polymerization technique. , High strength minimizes the risk of early fractures during normal mastication, especially with extensive multiunit prostheses. , , ,
Among several methods of mechanical assessment, finite element model (FEM) and finite element analysis (FEA) have been applied to predict the stress distribution through different prosthetic design and materials. Thus, factors related to loading, design, and materials can be evaluated before experimental mechanical assessment. However, previous studies on IFPs have been separately focused on only mechanical testing or on biomechanical analysis.
The purpose of this in vitro study was to compare CAD-CAM and traditional chairside material processing on the fracture and biomechanical behavior of 4-unit interim prostheses with and without cantilevers. The research hypothesis was that the use of CAD-CAM processing can improve the mechanical performance of interim prostheses.
Material and methods
A nickel-chromium model simulating a 2-tooth edentulous space (second premolar and first mandibular molar) with abutments on the first premolar and second mandibular molar was used to fabricate 4-unit interim prostheses (N=40) ( Fig. 1 ). A second similar nickel-chromium mandibular model with missing second premolar and second molar with abutments on the first premolar and first molar was used to fabricate 4-unit cantilever prostheses (N=40) with a cantilevered second mandibular molar pontic. The abutment tooth preparations were 5-mm high with a 6-degree convergence and a 1-mm-wide chamfer finish line. The models were representative of common clinical situations and similar to those used in previous studies. ,
For the CAD-CAM interim prostheses, high-density polymer blocks (inLab MC XL; Dentsply Sirona) were used, while chemically polymerized resin-based materials were used for the traditional chairside processing method ( Table 1 ). The models were scanned by using an intraoral scanner (Bluecam; Dentsply Sirona), and the data set was transferred to a CAD-CAM unit (Cerec 3D, Serial nº 2866, Ref. 63748559 D 3534; Dentsply Sirona). The anatomy of the interim prostheses was designed by using information from a dental design software program (Cerec 3D; Dentsply Sirona) ( Fig. 1 ). , The interim premolar had a height and width of 8 mm, and the width of the molar was 10 mm. The cross-sectional area of the connectors was 14 mm 2 . In the 4-unit cantilever prostheses, the pontic was approximately 0.5 mm from the model. For the autopolymerized interim prostheses, a silicone (Zetalabor; Zhermack) mold was used to ensure that all the prostheses had the same design as the CAD-CAM prostheses. The material (Protemp 4 or Dentalon Plus) was injected into the silicone mold, which was then seated in the model and allowed to polymerize. As in previous studies, , the prostheses were not cemented to avoid an additional variable. The prostheses were stored in a distilled water bath at 37 °C for 30 days to simulate contact with the oral fluids.
|Preparation||Materials||Chemical Composition||Commercial Presentation||Manufacturer||Batch Number|
|CAD-CAM method||Vita CAD-Temp||PMMA within 14% (wt) microfiller||High-density polymerized blocks||Vita Zahnfabrik||38810|
|Telio CAD||99.5% PMMA||Ivoclar Vivadent AG||S30125|
|Automixture dispensing||Protemp 4||Bis-acrylic||Autopolymerizing||3M ESPE||530950|
|Powder/liquid mixture||Dentalon Plus||PMMA||Kulzer GmbH||Powder (10501) Liquid (10214)|
After storage, the resin-based prosthetic structures were immediately placed without adhesive luting on the Ni-Cr abutments and then submitted to compressive loading tests at 1 mm/min until fracture by using a universal testing machine with a 5-kN load cell (TIRAtest 2705; TIRA GmbH). The universal testing machine had previously been calibrated with a digital caster and camber gauge device (AccuLevel v5; Longacre Racing Products), and the mechanical tests followed DIN EN ISO/IEC 17025:2005 for calibration of material testing machines (DKD-K-16401; TIRA GmbH). A 1.5-mm-thick polytetrafluoroethylene plate was placed between the occlusal surface and stainless-steel ball to provide a homogeneous loading distribution, as seen in Figure 2 . The load was applied with a Ø6-mm stainless-steel ball on the occlusal surface of the conventional prostheses between the second premolar and first molar. On the prostheses with a cantilever, the loading occurred in the region between the first molar and second molar. The data were analyzed by Student t test and 2-way analysis of variance (ANOVA), followed by the Tukey post hoc test by using a statistical software package (IBM SPSS Statistics, v22; IBM Corp) (α=.05).
After the compressive tests, the fracture surfaces (n=10) were photographed at ×40 magnification by using magnifying lenses (Leica Wild; Leica) coupled to a camera (Leica DFC295; Leica). The fracture surfaces of specimens (n=3) were sputter-coated with a 20-nm Au-Pd film and inspected by field emission gun electron scanning microscopy (FEGSEM) (FEI Nova 200; FEI) coupled to energy dispersive spectroscopy (EDS). FEGSEM was used to inspect the fracture surfaces and correlation with the loading spot, failure pathways, design features, and defects such as cracks and pores. FEGSEM images were gathered by secondary electron (SE) or backscattered electron (BSE) mode at 10 or 15 kV.
Finite element analysis (FEA) was performed considering the mechanical properties of materials and design of each model built from the geometric model after CAM. The anatomy of the 3D models was reconstructed from STL files by using the CAD software program (Cerec 3D; Dentsply Sirona), and then a 3D FEM was developed by using a nonlinear software package (Marc; MSC Software Corp) ( Fig. 3 ) replicating the experimental apparatus. The mesh was performed to achieve convergence, and the 3D models had 138 000 tetrahedral quadratic elements for the nickel-chromium model and 207 600 elements for the prostheses. The contact interaction between 2 components was considered as a touching contact with a coefficient of friction at 0.3 and nonsliding between plate and the occlusal surface. The boundary conditions assumed in the FEM were equivalent to the experimental ones. The teeth and abutments were considered rigid (without periodontal ligament) in the model. The properties of the materials were assigned from similar previous studies on mechanical testing ( Table 2 ), and the linear and isotropic model behavior was assumed until a 1-kN load was applied. Vertical asymmetric loading was applied on the polymeric prosthetic crowns perpendicularly to the occlusal plane as mentioned in the experimental testing, and the load displacement was assessed. A numerical analysis was carried out to evaluate geometric influences on the von Mises stress distribution at the joint and cantilever regions.
|Material||Young Modulus (GPa)||Poisson Ratio||3-Point Bend Strength (MPa)||Author, Year|
|Protemp 4||2.5||0.21||91-116||Twal and Chadwick, 2012; Yao et al, 2014; manufacturer|
|Telio CAD||2.9||0.24||115-130||Niem et al, 2019; Yao et al, 2014; manufacturer|
|Dentalon Plus||2.4||0.34||58-75||Valittu, 1999; Akova et al, 2006; manufacturer|
|Vita CAD Temp||2.8||0.29||80-97||Yao et al, 2014; manufacturer|
The mean values of maximum force required to fracture the conventional and cantilever interim prostheses are shown in Table 3 . The interim prostheses processed by CAD-CAM showed the highest maximum force values: VITA CAD-Temp (3136 ±7.4 N) and Telio CAD (3126 ±165 N) ( P =1.0). Dentalon Plus had a mean maximum force value approximately 60% lower (1390 ±82 N) than the highest maximum force values. The bis-acrylic showed the lowest mean of maximum load (1287 ±214 N), which was approximately 29% less than the materials processed by CAD-CAM. No significant statistical differences were found between the conventional and cantilever prostheses when using an autopolymerized PMMA (Dentalon Plus), although statistical differences were recorded for the bis-acrylic group (Protemp 4), as seen in Table 3 . Regarding loading displacement for each type of material and prosthetic design, the Dentalon Plus and ProTemp4 groups showed similar mean values of maximum displacement, as seen in Table 4 . VITA-CAD Temp and Telio CAD showed similar maximum displacement regarding the design free of cantilever. Results for the cantilever interim prosthesis group were less variable among the different test materials. The highest mean values of maximum load (2649 ±686 N) were recorded for the Telio CAD group, followed by the ProTemp4 group (1954 ±144 N) and VITA-CAD group (1634 ±152 N). The lowest values were recorded for the Dentalon Plus group (1268 ±151 N). Within the cantilever interim prosthetic group, statistically significant differences were detected between the Dentalon Plus and Protemp 4 materials ( P =.021), while no significant differences were found between the remaining materials ( P >.05), as shown in Table 5 .
|P a||Vita b||Telio b||Protemp b||P a||Vita b||Telio b||Protemp b|
The macroscale view of the fracture zone by photographic images at ×40 revealed catastrophic and bending failures along the vertical axis of the prosthetic structure ( Figs. 4-7 ). Fractures occurred on the prosthetic teeth out of the abutment region and at the connection regions in the cantilever ( Figs. 4-6 ). A variation of fracture regions was noted for the PMMA-based materials composed of acrylic resin powder and liquid. Results of the von Mises stresses recorded by FEM (equivalent stress) on the interim prostheses are seen in Figure 8 . On both prostheses, the highest values of the von Mises stress of around 6 MPa to 7 MPa were detected on the occlusal contacting regions, at the connectors, and within the transition region between the teeth. The decrease in prosthetic thickness promoted stress concentrations for crack propagation on loading. No significant differences in von Mises stresses were found regarding the materials on the 4-unit prostheses manufactured by CAD-CAM or traditional chairside processing method. Regarding the influence of design, the cantilever model revealed the highest von Mises stresses in the nickel-chromium framework and pontics. Also, the loading displacement was higher in the prostheses with the cantilever (ranging from 3.2 to 4 mm) than that recorded for the groups without cantilever (ranging from 2.2 to 2.9 mm), as seen in Table 4 .