Bond Strength of a GPDM-Based Self-Adhesive Cement Compared to Conventional Resin Cements on 3D-Printed Crown Material

Technical Summary: Bond Strength of a GPDM-Based Self-Adhesive Cement Compared to Conventional Resin Cements on 3D-Printed Crown Material

Problem Statement
The clinical success of 3D-printed permanent restorations relies heavily on the durability of the bond between the restorative material and the resin cement. While resin-based luting cements are known for their efficacy with 3D-printed materials, comparative data regarding the performance of conventional versus self-adhesive cements on these specific substrates after aging remains limited. This study addresses the need to evaluate and compare the shear bond strength (SBS) and microtensile bond strength (µTBS) of four different resin cements applied to a 3D-printed permanent crown material.

Methodology
The study utilized an in vitro design with a robust statistical power analysis (target effect size Cohen's f = 0.40, power = 0.82).

• Specimen Fabrication: 3D-printed permanent crown resin (Formlabs Inc.) was used to create square specimens (20 mm x 20 mm x 2 mm) for SBS testing and cylindrical specimens (1 mm diameter x 10 mm length) for µTBS testing using ExoCAD software and a Formlabs 3D printer.
• Cement Groups: Four dual-cure resin cement groups were tested:
  • Conventional (C1, C2): i-Fix Duo (i-dental) and GC-LinkForce (GC Corp.), both requiring a bonding agent (G-Premio Bond).
  • Self-Adhesive (S1, S2): G-Cem ONE (GC Corp.) and Nova Resin (Imıcrly), applied without a bonding agent.
• Surface Preparation: Specimens were sandblasted with 600-grit silicon carbide and cleaned with isopropyl alcohol.
• Aging: All samples underwent thermal cycling equivalent to one year of clinical service (5–55°C for 10,000 cycles).
• Testing Protocols:
  • SBS: Tested at 1 mm/s using a universal testing machine.
  • µTBS: Tested at 0.5 mm/s using a microtensile tester.
• Analysis: Fracture surfaces were examined via electron microscopy and stereomicroscopy (x40) and classified as adhesive, cohesive, or mixed. Statistical analysis was performed using One-way ANOVA with Tukey's HSD post-hoc tests.

Key Results

• Shear Bond Strength (SBS): Significant differences were found among the groups (p < 0.05). Nova Resin (S2) and GC-LinkForce (C2) demonstrated the highest bond strength values, with no statistical difference between them. Both were significantly stronger than i-Fix Duo (C1) and G-Cem ONE (S1). Notably, there was no overall statistical difference in bond strength between the self-adhesive and conventional resin cement systems as a category.
• Microtensile Bond Strength (µTBS): Significant differences were also observed (p < 0.05). Nova Resin (S2) exhibited the lowest µTBS values compared to the other three groups, indicating lower retention resistance under tensile load.
• Fracture Analysis:
  • SBS: Cohesive fractures were the most prevalent type.
  • µTBS: Mixed fractures were statistically significantly less frequent than adhesive and cohesive types.
  • Nova Resin (S2): This group showed a high frequency of cohesive fractures, suggesting the failure occurred within the cement itself rather than at the interface.

Key Contributions and Claims
The study rejects the null hypothesis, confirming that bond strength varies significantly depending on the specific resin cement used on 3D-printed substrates.

• Chemical Mechanism: The high SBS of Nova Resin (S2) is attributed to its chemical composition, specifically the presence of GPDM (glycerol phosphate dimethacrylate) and 4-META monomers. The authors suggest these acidic phosphate monomers form strong chemical bonds with the resin substrate, similar to the mechanism of 10-MDP on zirconia.
• Material Compatibility: The high performance of GC-LinkForce (C2) is linked to its structural similarity (Bis-EMA) to the 3D-printed crown material, which may preserve bond strength better after thermal cycling.
• Fracture Behavior vs. Strength: The study highlights a complex relationship between fracture type and strength values. While Nova Resin (S2) achieved the highest SBS, it recorded the lowest µTBS. The authors propose that the high cohesive fracture rate in the S2 group is related to the physical properties of the cement (specifically its high filler ratio of 70% and hydrophilic dimethacrylate content) rather than a weak interface. The cohesive failure suggests the cement's internal physical strength may be the limiting factor under tensile stress.

Significance and Limitations
The paper concludes that both the GPDM-based self-adhesive cement (Nova Resin) and the conventional cement (GC-LinkForce) with a universal adhesive offer superior resistance to shear forces on 3D-printed crowns compared to the other tested cements. However, clinicians should exercise caution regarding the µTBS performance of Nova Resin, particularly in restorations with short crown lengths where resistance to tensile forces is critical.

The authors maintain a modest stance regarding the clinical applicability of these findings. They explicitly acknowledge limitations, including the inability to fully replicate the complex biological oral environment (enzymatic activity, bacterial byproducts) and the use of only a single 3D-printed resin material. Consequently, the study emphasizes that while the in vitro results are promising, long-term clinical trials are necessary to definitively determine the performance of these materials in practice.