Acrylic resin has been a staple in dentistry for decades, continuously improving in terms of physical properties and biocompatibility. Among various dental materials, polymethyl methacrylate (PMMA) stands out as the most commonly used denture base resin (DBR) due to its ease of use, adequate strength, low toxicity, and aesthetic appeal. However, the incidence of denture fractures is a concern, with reports suggesting a high rate of up to 25%. This is primarily attributed to mechanical failures. Conventional denture fabrication processes are complex and time-consuming, requiring multiple patient visits and intricate laboratory procedures, which can be costly and inconvenient. However, not all denture fractures necessitate complete refabrication; minor localized fractures can often be repaired using cold-polymerized DBRs, offering a more practical and cost-effective solution.
Heat-polymerized DBRs have traditionally been the go-to choice for definitive removable dentures due to their high flexural strength, biocompatibility, and color stability. However, their fabrication process is labor-intensive and can lead to polymerization shrinkage, presenting certain limitations. On the other hand, cold-polymerized DBRs, commonly used for repairs and relining, offer ease of use and cost-effectiveness but generally exhibit lower mechanical strength and poor color stability.
The advancement of three-dimensional (3D) printing technology has introduced an innovative alternative to conventional denture fabrication. Digital light processing (DLP) and stereolithography (SLA) are commonly employed techniques in denture fabrication, utilizing light polymerization to cure resin layer by layer, resulting in clinically acceptable fits. 3D printing also allows for efficient refabrication using saved computer-aided design (CAD) files, eliminating the need for additional impressions. However, challenges such as interlayer bonding defects and poor color stability persist.
The oral environment poses unique challenges to DBRs, with fluctuating temperatures, mechanical loading, and exposure to various foods impacting their longevity. Among these factors, thermal changes play a crucial role in altering the mechanical and aesthetic properties of DBRs. Repeated exposure to hot and cold foods can induce thermal stress and color changes, warranting preclinical evaluation. Thermocycling, a common research method, simulates temperature fluctuations in a moist oral environment, mimicking the aging process of dental materials.
Studies have reported that the shear bond strength of 3D-printed DBRs is lower than that of conventional DBRs, and it significantly decreases after thermocycling. Similarly, the flexural strength of 3D-printed DBRs is also observed to decrease significantly after thermocycling, although findings on this matter are conflicting. Additionally, 3D-printed DBRs tend to show inferior color stability compared to heat-polymerized DBRs.
Given these findings, this study aimed to evaluate the mechanical properties (shear bond strength and flexural strength) and color stability of heat-polymerized, cold-polymerized, and 3D-printed DBRs before and after thermocycling. The null hypotheses were that there would be no significant differences in mechanical properties or color stability among the three DBR types, and that thermocycling would not significantly affect the mechanical properties of the tested DBRs.
The study utilized heat-polymerized (IvoBase Hybrid), cold-polymerized (PRESS LT), and 3D-printed (Denture 3D+) DBRs. Specimens were designed using commercial CAD software, and sample sizes were calculated based on an A priori power analysis. Heat-polymerized DBR specimens were fabricated using wax discs and a milling machine, while cold-polymerized DBR specimens were fabricated using a negative mold technique. 3D-printed DBR specimens were fabricated using a 3D-printable DBR and a DLP 3D printer.
Shear bond strength, flexural strength, and color stability tests were conducted, and statistical analysis was performed using SPSS. The results revealed significant differences in mechanical properties and color stability among the three types of DBRs, rejecting the null hypotheses. Thermocycling significantly affected the shear bond strength of all DBR types and the flexural strength of 3D-printed specimens.
The bond integrity of DBRs is critical for clinical longevity, especially when repairs are required during the lifespan of the prosthesis. The results of the shear bond strength test highlighted the impact of thermal fatigue on adhesion between DBRs and repair materials. Notably, after thermocycling, the shear bond strength of 3D-printed DBRs was comparable to that of heat-polymerized DBRs, indicating clinically acceptable repairability.
The flexural strength of 3D-printed DBRs consistently showed the highest values across all conditions, supporting its potential as an alternative to conventional heat-polymerized DBRs. However, noticeable color changes were observed in 3D-printed DBRs following thermocycling, suggesting the need for further improvements in their color stability.
This study has limitations, including the in vitro nature of the investigation, which does not fully replicate the complexity of in vivo conditions. Additionally, only one commercial product per material type was tested, limiting the generalizability of the findings. Further research is warranted to explore the effects of surface treatments, extended aging protocols, alternative repair materials, and various 3D-printing parameters to optimize the clinical performance of 3D-printed DBRs.
