Visible Light Activated (VLA) Systems

Question 1

List 5 important features of VLA systems

Answer 1

1. High intensity light used- retinal damage if incorrect usage; orange filters available for protection
2. UV light filtering allows for inc. depth of
   cure
3. Infra-red filtering dec. temp of light protecting from pulpal damage from overheating
4. Composite contact w/ air means incompletely cured surface layer (essential in incremental method of composite placement as successive layers bond together using this O2 inhibited layer)–> removal of this layer achieved in finishing process
5. Soft/pulse cure technique allows for some plastic flow during curing which lessens internal stresses resulting from polymerisation shrinkage

Question 2

Describe the polymerisation process of Siloranes

Answer 2

• Complete polymerisation in presence of oxygen

- No inhibited layer means no bonding between incremental layers, hence restoration done as one big lump

Question 3

Describe soft-start/pulse curing

Answer 3

• Leads to long-chain polymerisation –> higher strength
• Higher intensity systems cause many centres of polymerisation; so many short chain polymers–> decreased strength
• Light must be of the correct wavelength & sufficient intensity for the photo-sensitive initiator (e.g. 470nm for camphochloroquinone)
• Starts low intensity, ends high intensity; drives curing reaction to highest possible conversion only after much stress has been relieved
• Significant stress reduction achieved without increase in exposure time/sacrifice of depth of cure/degree of conversion
• No absolute depth of cure though approx. 2-3mm accepted
• Increased intensity- shorter curing time for given depth cure, or increased depth of cure for given exposure time