Is the soft-start polymerisation concept still relevant for modern curing units?

Area of Science:

• Dental Materials Science
• Polymer Chemistry
• Biomaterials Engineering

Background:

• Achieving adequate polymerization in dental composites involves balancing degree of cure, mechanical properties, and shrinkage stress.
• Contradictory relationships between these properties necessitate careful optimization of curing protocols.
• Understanding these relationships is crucial for improving the longevity and performance of dental restorations.

Purpose of the Study:

• To quantify the relationship between degree of cure, shrinkage stress, and mechanical properties in a micro-hybrid composite.
• To evaluate the impact of various light-curing regimes (halogen and LED) on these properties at different depths (2mm and 6mm).
• To assess the efficacy of the soft-start polymerization concept with modern high-power LED units.

Main Methods:

• Real-time analysis of degree of cure, shrinkage stress, and gelation time.
• Micro-mechanical property assessment at varying depths.
• Nano-dynamic mechanical testing on selected curing regimes (Ramp, Pulse, Fast Cure).
• Utilized 13 distinct curing protocols from one halogen and two LED curing units.

Main Results:

• Soft-start polymerization effectively delayed gelation and reduced shrinkage stress at 2mm depth, maintaining degree of cure and mechanical properties.
• At 6mm depth, soft-start regimes showed a slight decrease (<10%) in degree of cure but preserved mechanical properties.
• High-power LED units demonstrated that soft-start polymerization remains beneficial for shallower restorations.

Conclusions:

• The soft-start polymerization strategy is still valid for shallow (2mm) dental composite restorations, even with high-power LED curing units.
• This approach optimizes the balance between low shrinkage stress and adequate material properties.
• While less effective for deeper cavities (6mm) regarding cure degree, mechanical integrity is largely maintained.