The tested materials showed different amount of eluted monomers.
Printing device had significant impact on monomer release.
Monomer release is comparable to conventional dental composite materials.
Monomer release of entire surgical guide might reach critical toxic levels.
The aim of this study was to assess the post curing monomer release of resins applicable for 3D printing of surgical implant guides in dependency of printing technique and storing media using high performance liquidchromatography.
Material and methods
Specimens of Nextdent SG, Freeprint Splint, Fotodent Guide, 3Delta Guide, and V-print SG (n = 4) were additively manufactured with the corresponding DLP/SLA printing devices (Rapidshape D20II, Form2, Solflex350). Postprocessing was done according to the manufacturer’s specifications. Subsequently, samples were eluted in methanol and water for 3 days and analyzed with gas chromatography/mass spectrometry (GC/MS).
A total of twelve different substances released from the tested resin materials. The highest eluted concentration for MMA in methanol was 20.27 ± 8.60 μg/mL followed by 12.66 ± 3.38 μg/mL of HPMA. HEMA was found at concentration of 11.17 ± 2.43 μg/mL in methanol and 1.15 ± 0.11 μg/mL in water. TPGDA and TEGDMA reached maximum concentration in methanol of 4.29 ± 0.54 μg/mL and 5.07 ± 0.93 μg/mL and in water of 0.79 ± 0.19 μg/mL and 0.36 ± 0.14 μg/mL, respectively. Significant difference was found for the material Nextdent SG manufactured on SLA and DLP printing device for THFMA (p = 0.041), TEGDMA (p = 0.026), TPGDA (p = 0.05) and EGDMA (p = 0.06). The amount of monomers released into water did not reach the detection threshold for V-print SG.
The study revealed significant influence of the printing technique and resin material on the elution of monomers. The elution in methanol and water was significantly different. While the relative amount of eluted monomers from 3D printed guides is comparable to conventional direct composites and below toxic relevant concentrations, the absolute amount of monomer can rise in a clinic situation due to the size of the guides.
Surgical templates for guided implant surgery have gained increasing acceptance in implant dentistry. Surgical guides allow surgical implant placement based on prosthetic backward planning together with a facilitated surgical procedure which is associated with reduced operative risk of harming critical anatomic structures. Recently the technique of 3D printing has been implemented for cost effective and straightforward manufacturing of surgical implant guides [ ].
3D printing is a generic term for various technical methods for the additive creation of objects. In dentistry the most commonly used method comprises stereolithography (SLA) and the closely related digital light processing (DLP) technique. Irrespective of the particular printing system a curable methacrylate resin is stored in a vat, where a build plate descends in small increments and the liquid polymer is exposed to the curing light. Regarding the SLA approach, the UV laser or laser diode draws a cross-section layer by layer to build the printed object. The DLP approach uses a digital projector screen to transfer a single image of each layer across the entire building plate. Immediately after completion of threedimensional printing the objects are in a temporary state and have to be further processed by cleaning and additional UV polymerisation.
The materials are not only used for surgical templates but also for transposition osteotomies, guided driven apicoectomies and lateral sinus lifting. Irrespective of the individual indication template materials get in close contact with the soft- and hard tissue at the surgical site.
However, less is known on the release of resin monomers and potential hazardous chemical substances from the printed resin materials. Conversion of monomers in 3D printed light-cured resin-composite systems is mostly not complete and ranges from 35 to 77% [ ]. The maximum release of soluble substances commonly occurs during the initial period of soaking and declines substantially within hours following a logarithmic decrement. Ferracane and Condon showed that 75% of the elution of soluble resin components emerges within the first three hours. Depending on the specific chemical properties the rate and total amount of elution is higher in organic media, i.e., 75% ethanol/water, as compared to water [ ].
Numerous studies are available concerning about the possible toxic effects of monomers on the oral environment, i.e., mucosal irritation, inflammation and proliferation of the epithelium [ , ], enhancing apoptosis as a consequence of oxidative stress [ , ], and inducing genotoxic effects in oral mucosa cells [ ].
Yet, only insufficient data are available on the release of monomers and other potentially harmful substances from these surgical guides. This seems even more important since these templates are in close contact with the surgical site during implant placement. Hence, the current study aimed to delineate the release of soluble resin components from 3D printable resin materials used for the creation of surgical guides.
The null hypotheses were as follows: (a) no influence of the 3D printing material was expected (b) nor would the printer have any significant effect on the release of components from the resins.
Material and methods
Preparation of samples
The investigated composites including the manufacturers’ data and used 3D printers are listed in Table 1 and 2 . A computer-assisted STL file was created to design standardized resin test bodies with a diameter of 6 mm and a thickness of 2 mm (surface area 94.2 mm 2 , volume 56.5 mm 2 ) as described previously [ ]. The STL data set was imported into the corresponding nesting software. Specimens for the Form2 printer were nested in CAM Software Preform 3.2.1 (Formlabs, Somerville, USA), specimens for the Rapidspade D20II and Solflex 350 printer were nested in the Netfabb Premium 2019 (Autodesk, Mill Valley, CA, USA) and material-dependent printing parameters were chosen for each material. Slicing was done according to the manufactures settings with corresponding layer thickness. Machining codes were transmitted to the printing devices for the respective resin material and the specimens were manufactured using additive method. Postprocessing was carried out according to manufacturer’s specification and included cleaning of the templates for 3 min in ethanol (96%) activated with ultrasound, and drying. Post-curing was done for the material Nextdent SG by LC-3DPrint Box (Nextdent, Soestberg, Netherlands) for 10 min and for all other materials by Orthoflash G171 (NK Optik,Baierbrunn, Germany) 2 × 2000 flashes.
|3Delta Guide||Deltamed, Friedberg, Germany||385||UDMA, TMPTA, TPO|
|Freeprint Splint||Detax, Ettlingen, Germany||378–388||Acrylated resin, Aliphatic urethane acrylate, TPGDA, THFMA, TPO|
|Fotodent Guide||Dreve, Unna, Germany||385||BIS-EMA, Acrylresin, HEMA, HPMA, Monoester with 1,2-Propandiol, TPO|
|Nextdent SG||Nextdent, Soesterberg, Netherlands||385||Methacrylic oligomers, Phosphine oxide|
|V-Print SG||Voco, Cuxhaven, Germany||385||BIS-EMA, UDMA, TPO|
|Printer device||Manufacturer||λ [nm]||Technology|
|Rapidshape D20 II (RS)||Rapidshape, Heimsheim, Germany||385||DLP|
|Solflex 350 (SF)||W2P, Vienna, Austria||385||DLP|
|Form2 (Form)||Formlabs, Sommerville, USA||405||SLA|
Qualitative and quantitative analysis was performed on a Finnigan Trace GC ultra gas chromatograph connected to a DSQ mass spectrometer (Thermo Electron, Dreieich, Germany). A J&W VF-5 ms GC capillary column was used for gas chromatographic separation (length 30 m, coating 0.25 μm, inner diameter 0.25 mm; Agilent, Böblingen, Germany). Carrier gas was helium 5.0 with a constant flow rate of 1 mL/min. The temperature of the transfer line to the mass spectrometer was 250 °C. Each injection of 1 μl was carried out in the solvent split mode. To evaporate the excess solvent, the inlet for programmable temperature evaporation (PTV) was operated for 1 min at 65 °C in split mode (split flow 30 mL/min) and then heated to 280 °C for 2 min in splitless mode for capillary transfe.
The temperature program for the GC oven was as follows: initial temperature at 50 °C, increased to 280 °C (25 °C/min) and finally holdtime for 5 min at 280 °C. The mass spectrometer was operated in the electron impact mode at 70 eV (ion source temperature 240 °C). Scan mode (m/z 50–600) was used in order to record samples. In order to determine the relative quantities of substances released from the printed specimens, the results were referred to the internal caffeine standard (0.01 mg/mL). Number of analyses for each eluate was four times. The integration of the chromatograms was carried out over the base peak or other characteristic mass peaks of the compounds, and the results were normalized by means of the internal CF standard. The compounds found were identified by comparing their mass spectra with those of reference standards, the NIST/EPA library, literature data, and/or by a chemical analysis of their fragmentation pattern.
In each group, data are given as mean (±SD). Results have been tested within groups for normal distribution using the Kolmogorov-Smirnov test. Homogeneity of variances has been analyzed with Levene test. Analysis of differences has been done separately for each monomer using unpaired sample t-test. P-values <0.05 have been considered significant. All test procedures have been calculated using SPSS 25.0 (SPSS Inc., Chicago, IL, USA).
Substances release depending on material
The results of GC analysis are shown in Table 4 . Considerably different elution rates have been found in methanol and water. A total of twelve substances ( Table 3 ) were detected in methanol and four in water released from resin materials. Fig. 1 presents data on the six monomers showing the highest concentrations in methanol only.
|EGDMA||Ethylene glycol dimethacrylate|
|DMABEE||4-N,N-Dimethylaminobenzoic acid ethyl ester|
|TPGDA||Tri(propylene glycol) diacrylate|