Dental Impression is a negative replica of the oral cavity, many impression materials are present which are capable of getting the perfect impression if the proper guidelines are followed. Impression Compound is used to take a negative replica or an Impression of the Edentulous Oral cavity in a process called PRimary Impression. The material is a Reversible type material which means, it can be reused or softened using heat for a limited period of time to capture the perfect impression.
It is a rigid, reversible impression material which sets by physical change.
According to ADA Specification Number 3:
- High fusing compound – tray compound.
- sticks, cakes, & cones.
- olors – green, brown, red.
- For making primary impressions.
- For individual tooth impression
- For peripheral tracing or border moulding.
- To check undercuts in inlay preparation.
Requirements of an ideal impression compound:
- It will harden after heat treatment when brought back to mouth temperature and be stable after hardening and removal from mouth.
- When softened it should have uniform consistency to help reproduce fine details of oral tissue.
- Should become hard uniformly throughout without discrepancies and distortion.
- It should not be harmful to Oral tissues when it is elastic in plastic state.
- Should not be harmful and not have toxic or Irritating materials to Oral tissue.
- Material should have cohesive property and not adhesive property.
- When exposed to flaming after removal from mouth should exhibit a smooth glossy surface.
- After hardening flaking and chipping should not be seen during trimming and reshaping using lacron carver.
- Rosin 30
- Copal resin 30
- Carbuna wax 10
- Stearic acid 5
- Talc 25
- Coloring agentapp.
- During softening of the material , the outside will soften first & inside last. So to ensure uniform softening the material should be kept immersed for a long time in water bath.
- The layer adjacent to the mouth tissue swill remain soft. Therefore Its imp to cool the compound thoroughly before removing the impression.
i) Thermal conductivity:
- has low thermal conductivity.
ii) Coefficient of linear expansion. (CTE)
- high CTE , 0.3% acceptable.
- high fusing compounds – < 2% at 37 .C & < 70-85 % at 45.C
- low fusing compounds – < 3% at 37 C& < 80-85% at 45 C
iv) Glass transition temp:
- It is the temp. at which the material loses its hardness or brittleness on heating or forms a rigid mass.
- App. 39.CMaterial removed from the mouth after glass transition temp. is reached.
v) Fusion Temp.
- It’s the temp. at which the impression compound becomes plastic.
- Its 43.5.C
vi) Dimensional stability.
- Poor dimensional stability.
- To prevent distortion pour the cast within 1 hr.
vii) surface detail reproduction is less because of high viscosity & low flow.
- Sticks:Smallamount softened over flame
- Cakes: Softened in a thermostatically controlled waterbath.
- Prolonged immersion in a water bath is not indicated overheating
- In water makes the compound sticky & difficult to handle.
- Avoid incorporating water while kneading.
- Cheap reusable
- Does not produce irritation to the patient impression
- Can be remodified & resoftned again till an accurate impression is obtained
- Difficult to record details accurately.
- Soft tissues are compressed due to pressure applied while taking the impression.
- Difficult to remove undercuts.
- Does not have an pleasant taste.
- Can be uncomfortable in patients because of the rigidity.