Statement of problem
Computer-aided design and computer-aided manufacturing (CAD-CAM) milling and 3-dimensional printing are readily available for the fabrication of interim restorations. However, studies comparing the marginal gap after a long period of function are lacking.
The purpose of this in vitro study was to evaluate the marginal gap of interim crowns fabricated from different materials and with different techniques before and after receiving simulated oral stress.
Material and methods
Two conventional resins, a polymethyl methacrylate resin (Unifast Trad) and a bis-acryl resin (Protemp 4), a milled polymethyl methacrylate resin (Brylic Solid), and a 3-dimensionally printed bis-acrylate resin (Freeprint Temp) were evaluated. Interim crowns (n=10/group) were fabricated by using the conventional direct technique for Unifast Trad and Protemp 4, with a maxillary molar Dentiform tooth as a template and by using CAD-CAM for Brylic Solid and Freeprint Temp. After finishing and cementation, the marginal gap was measured at the middle of all surfaces and line angles. The average value from all 8 sites was used to represent the marginal gap of each specimen. Subsequently, all interim crowns were thermocycled (5000 cycles of 5 °C and 55 °C), and cyclic occlusal load with a 5-mm steel ball at central pit (100 000 at 100 N) and the marginal gap were remeasured. The effects of different material types and aging on marginal gap were analyzed with 2-way ANOVA. The difference in marginal gap before and after aging was analyzed by using the paired t test, and the increased marginal gap was analyzed with 1-way ANOVA (α=.05).
Both the material types and the aging regimen had a significant effect on marginal gap ( P <.001). The Unifast Trad group and the Protemp 4 group had a significantly larger marginal gap than the Brylic Solid group and the Freeprint Temp group, both before and after aging ( P <.01). Each group had a significantly larger marginal gap after the aging regimen ( P <.001). The increased marginal gap was greatest in the Unifast Trad group, which differed statistically from the Freeprint Temp group ( P =.004) but not from the other groups ( P >.05).
The interim crowns fabricated with the Brylic Solid and the Freeprint Temp had a smaller marginal gap than those fabricated with Unifast Trad and Protemp 4, both before and after aging. The Unifast Trad group showed a significantly larger increase in marginal gap after the aging regimen than the Freeprint Temp group.
In clinical practice, the marginal gap formed between the interim restoration and tooth should be as low as possible. For long-term application, the CAD-CAM interim restoration might be more suitable than the conventionally fabricated interim restoration.
Interim restorations are an essential part of fixed prosthesis treatment and must satisfy interrelated biologic and biomechanical requirements until the definitive restorations are delivered. Computer-aided design and computer-aided manufacturing (CAD-CAM) is a recently introduced method that can be either subtractive or additive with 3-dimensional (3D) printing. Each of these allows the interim resin to polymerize in well-controlled conditions, which improves mechanical properties, reduces discoloration, and increases precision, especially for complex treatments.
Minimizing marginal discrepancy results in a better biologic response of soft tissue surrounding the restoration and reduces the chance of the patient encountering postoperative sensitivity or secondary caries. , The clinically acceptable marginal opening has been suggested to be less than 120 μm, although marginal gap values of up to 160 μm or 172 μm have been reported to be acceptable. Material properties, including polymerization shrinkage, , thermal expansion and contraction, , water sorption, , , and plastic deformation play roles in maintaining dimensional stability and reducing the marginal gap of an interim restoration. Marginal gaps can increase when interim restorations are stressed by thermal changes and repeated occlusal load in an oral environment. , ,
The marginal gap of interim restorations fabricated from different materials (monomethacrytate-based resin or composite resin) and different fabrication processes (conventional direct process or CAD-CAM process) has been evaluated. , , Some of those studies have evaluated the marginal gap after thermocycling, , , but only a few have included repeated mechanical loading in the experimental protocol. Moreover, only a few have compared the marginal gap of conventional interim restorations with CAD-CAM milled interim restorations, and these reports reported conflicting results. Several studies reported that CAD-CAM milling interim restorations had superior marginal integrity, , , , whereas others did not find any difference. , Studies of the adaptability of interim crowns from the 3D printing process are still relatively scarce. Lee et al compared the marginal gap of interim restorations fabricated from 2 printing systems and 1 milling system and reported that the milling interim restoration group exhibited a significantly wider internal gap than the others. Mai et al also demonstrated comparable results of marginal gap between the printing and milling method, both of which performed significantly better than conventionally fabricated interim restorations. However, both studies measured the marginal gap of 3D printing interim restoration by the silicone replica method without simulated aging. An analysis of the effect of aging on the marginal gap of interim restorations made from different materials and with different fabrication processes is lacking.
The purpose of this in vitro study was to investigate the effect of simulated aging processes on the marginal gap of interim crowns made with different materials and fabrication methods. The null hypotheses tested were that type of material and simulated aging would not affect the marginal gap.
Material and methods
A maxillary right first molar Dentiform tooth (Nissin Dental Products) was fixed in a polyvinyl chloride tube with autopolymerizing clear polymethyl methacrylate (PMMA) resin (Ortho-Jet; Lang Dental). Three indexes were placed to accommodate the positioning of the silicone index in the fabrication of direct interim restorations, as well as to help with superimposing during the scanning and designing process of CAD-CAM interim crowns. Another Dentiform tooth with the same positioning was prepared for a ceramic crown with a 2-mm occlusal reduction and a 1-mm chamfer finishing line at 1 mm above the cementoenamel junction with 6 degrees of convergence.
Scoring marks were placed under the finishing line to identify the position of the marginal gap measurement at the middle of all surfaces and line angles. This prepared Dentiform tooth was then replicated with a 1:1 mixture of bisphenol-A epoxy resin (Epotec YD128; Aditya Birla Chemicals) reinforced with 50-μm alumina particles (Loxley) and poured into a silicone mold (RA-320; Rungart). Any die with a surface defect, incomplete margin, or any other defect detected under ×2.5 magnification loupes was excluded. A total of 40 dies were assigned to 4 experimental groups (n=10/group) by using block randomization with online software (seal envelop).
Different types of material and fabrication methods of interim restorations were assigned to each group, as shown in Table 1 . For the conventional direct method, each die was painted with petroleum jelly. Autopolymerizing PMMA resin (Unifast Trad; GC chemicals) and autopolymerizing bis-acryl resin (Protemp 4; 3M ESPE) were mixed as per the manufacturer's recommendations and loaded into the silicone mold (Provil novo; Kulzer GmbH), which mimicked the external surface of an unprepared tooth. The excess material at the margin was removed under ×2.5 magnification loupes by using a tungsten carbide bur, followed by an aluminum oxide polishing disk (Sof-lex disc; 3M ESPE) and a pumice slurry ( Fig. 1 A).
|Unifast Trad||GC chemicals||Polymethyl methacrylate resin||Conventional Direct|
|Protemp 4||3M ESPE||Bis-acryl resin (or dimethacrylates)||Conventional Direct|
|Brylic Solid||Sagemax Bioceramics||Highly polymerized polymethyl methacrylate resin||CAD-CAM milling|
|Freeprint Temp||DETAX GmbH||Light-polymerized bis-acrylate resin||CAD-CAM printing|
For CAD-CAM–fabricated interim crowns, each die was scanned with an intraoral scanner (TRIOS 3; 3Shape A/S) to make a 3D cast of the prepared tooth. The prepreparation scan of the model was performed the same die by using the conventional interim crown template. Minor adjustments were made as necessary to achieve the required dimensions. The cement space for each specimen was standardized at 50 μm. For the fabrication of the CAD-CAM milled interim crowns, the prepolymerized PMMA resin block (Brylic Solid; Sagemax Bioceramics) was used with a 5-axis milling machine (CORiTEC 250i; imes-icore GmbH) ( Fig. 1 B). No further adjustment was made, except for removal of the supporting struts. For the fabrication of the CAD-CAM 3D-printed interim crowns, a light-polymerized resin (Freeprint Temp; DETAX GmbH) was used with a digital light processing printer (Asiga Max; Asiga) ( Fig. 1 C). After removing the supporting struts, another thin layer of printing resin was applied over the printed crowns and light polymerized to glaze the external surface.
After cementation with interim cement (Kerr Corp) under a 50-N constant load for 6 minutes, the marginal gap was measured by using a traveling stereomicroscope (Measurescope 20; Nikon) equipped with an electronic micrometer counter (SC-102; Nikon) at 8 predetermined sites on the experimental die ( Fig. 2 ). Each site was measured 3 times, and the mean of the measurements was used to represent the marginal gap of each specimen. The interim crowns were then subjected to 5000 cycles of 5 °C to 55 °C, with dwell times of 60 seconds and transfer times of 15 seconds, by using the thermocycling machine (TC301; KMITL) to represent 6 months in the oral environment. One hundred thousand cycles of occlusal load of 100 N at 4 Hz frequency was then applied with a 5-mm-diameter metal ball mounted on a dynamic testing instrument (Electroplus E1000; Instron) at the central fossa along the long axis of each tooth in a 37 °C electric water bath, representing oral conditions for 6 months. Then, the marginal gap was remeasured by using the method previously described.
All measurements were analyzed by using a statistical software program (IBM SPSS Statistics, v20.0; IBM Corp). Two-way ANOVA was used to analyze the influence of material types and thermomechanical aging on marginal gap, and Bonferroni post hoc was then used for pairwise comparison (α=.05). A paired t test was used to investigate the difference in marginal gap between before and after aging in each group (α=.05). Comparison of the amount of increased marginal gap that each material received from the aging process was made by using 1-way ANOVA with the Tukey post hoc test (α=.05).