Dental impression materials Libyan International Medical University
What is a dental impression? It’s a negative likeness of the teeth and/or other tissues of the oral cavity. The impression materials must be in a plastic or fluid status which become hard or set while in contact with oral tissues. Impression tray is a device used to carry and control impression material while making an impression.
Ideal requirements of impression materials • Biocompatible, non toxic and non irritant. • Should have acceptable taste, colour and smell. • Adequate shelf life for storage. • Easy to manipulate. • Suitable working and setting time. • Should be elastic to allow removal from undercuts without permanent deformation. 7. Adequate strength so that will not tear or break on removal from the mouth.
8. Should give accurate reproduction of tissue details. 9. Compatible with cast or die materials. 10. Adhere to the impression tray. 11. Should have good flow. 12. Dimensionally stable. 13. Can be disinfected without any changes of properties. 14. Economic or not expensive.
Classification of impression materials • Non-elastic imp. Compound, imp. Plaster, zinc oxide eugenol and imp. Wax. Must not have undercuts
2. Elastic……can be used in undercut • Hydrocolloids Reversible (agar)……sol.↔ gel (temp) Irreversible (alginate)…..sol.→ gel b. Elastomers Polysulfide (PS)............set by polymerization Silicon rubber (AS and CS) Polyethers (PE)
Impression techniques • Full dentures • Partial dentures • Fixed prosthodontics
What's important? Patient • Neutral taste and odour • Short setting time • Small tray • Easily removed • No retakes • Non-toxic
What's important? Dentists • Easily mixed • Short working time • Good quality impression • Low cost • Easily disinfected • Simple procedure
Ease of mixing • Low viscosity less viscous is easier silicones are easiest to mix followed by polyether and polysulphides • Automatic mixing reduces mixing time and bubbles use correct mixing tip
Working and setting time • Dentist's time requirements must be balanced with patient's comfort. A total working and setting time of 4 min is adequate for most crown and bridge procedures. • Using auto-mixing device for direct injection reduce working time to one minute. • A fast setting material can be removed from the mouth in about 3 min.
Easy of removal • Flexible. depends on the elastic modulus • Minimal distortion. depends on degree of permanent deformation. • Sufficient tear strength.
Flexibility of Impression materials Easy Moderate Hard
Resistance to Tearing High Medium Low
Accurate reproduction of surface details • Viscosity High- muco-compressive Low- muco-static • Wettability…..ability of liquid to cover the surface of the substrate completely Hydrophobic Hydrophilic
Why do we want good wetting? • Fewer voids • Less entrapment of oral fluids • Bubble-free dies and models • Fewer retakes
Wettability of Impression materials Excellent Good Fair
Wettability of Impression materials • Displace water and other oral fluids from the surfaces in the mouth so as to prevent voids in the impression and consequent bubbles in the casts. • Improve adaptation of gypsum so as to prevent voids in the casts. • Disinfection procedure!
Dimensional accuracy and stability • Choice and use of tray • Setting shrinkage • Permanent set • Storage stability
Choice and use of trays • Avoid flexible trays • Use perforated trays • Make sure the trays is clean • Apply only a thin layer of adhesive • Wait appropriate time before taking impression • Do not mix adhesives and impression materials
Dimensional change of impressions High Medium Low
a. Impression in place before removal, b. during removal- the impression is subjected to both compressive and tensile stresses.
View of a material being used to record an impression of a crown with subgingival shoulders. The thin areas of impression material, which are most prone to fracture are arrowed.
Impression plaster • Rarely used these days as they are brittle and fractures very easily. • Mucostatic impression of edentulous ridges • High W/P ratio • Can be combined with impression compound
Impression plaster advantages • Easy to mix • Working time 2-3 min • Setting time 2-3 min • Low viscosity-mucostatic • Good dimensional stability and accuracy • Cheap
Disadvantages • Low strength • Rough surface finish • Poor abrasion resistance • Rigid once set • Dry sensation in the mouth • We need separator
Impression compound • Sheet primary impression of edentulous mouth. close fitting special tray used with zinc oxide eugenol wash. • Stick single crown impression (copper ring technique). Border molding.
Composition • Thermoplastic materials, natural or synthetic resin and waxes….47% characterizes the softening temp. • Fillers…..e.g. Talc or shalk 50%..gives body increasing viscosity of the softened materials, reduce thermal contraction. • Lubricant or acids 3% e.g. stearic acid to improve flow properties (plasticizer).
Properties • Softening temperature 55-60ºC….↑ flow by Temp. • High viscosity-mucocompressive…37ºC solid. • High coefficient of thermal expansion…. • Do not over heat because it loose oils and altered characteristics • Rigid….↑ with rigid tray • Poor thermal conductivity….take time during heating or cooling. • Poured as soon as possible
Since pressure is applied during the formation of the impression, the residual stress occurs in the material storage in warm environments or extended storage results in warpage. • Contraction of imp. Compound from oral temp. to room temp. is 0.3%
Zinc oxide eugenol • Mucostatic impression of edentulous ridge. • Initial ST 3-6 min. • Final ST 10 min. • Two pastes, different colors, equal length, different size opening…. • Not elastic. • Temp. and moisture ↓ ST….accelerate reaction…drop of water accelerate setting. • Stick to dry surfaces. • Eugenol irritant, cause burning sensation of lips. • Pour any time.
No separator needs. • Protect unsupported edges. • Put Vaseline on patient lips. • If patient irritant to eugenol use rubbers imp. or eugenol free (phenols) zinc oxide impression. • Mucostatic • Hydrophilic….accurate reproduction of surface details • Dimensionally stable…. 0.1% • Compatible with dental stone….separated by softening the set paste in water at 60ºC.
Eugenol react with zinc-oxide to form salt of zinc eugenolate and unreacted zinc oxide. • Normally used with close-fitting special tray. • The thickness of paste used is normally around 1 mm. • This thin section ↓ the dimensional changes. • Low viscosity of the mixed paste gives accurate recorded details. • Defect sometimes arise on the surface of the impression but can be corrected by imp. Wax.
The major restriction is their lack of elasticity. • The set material easily distort or fracture when removed from undercut. • Available in two types identified as hard set and soft set. • The mixed materials has adequate adhesion to acrylic or tray material, therefore adhesives do not need.
Hydrocolloids • Hydro=water kola=glue Oid=like • A solution is a homogenous mixture consisting of single phase. • A suspension is a mixture of two phases • A colloid is a heterogeneous mixture of two phases, where the two phases are not easily differentiated. • Agar and alginate impression materials.
Reversible hydrocolloids …..agar Gel → Sol → Gel Irreversible hydrocolloids…..alginate Sol → Gel heating cooling Chemical reaction
Agar • Sea weed • Uses…..crown/bridge imp, cast duplicate-lab. • Composition; Agar 15%, Borax .2%, sodium sulphate 1%, anti fungal and water 85% • Good storage • Strengthens gel • Affect gypsum setting against? Why!!! • Accurate details
Largely replaced by alginate and rubber impression materials. • The preference of alginate as result of minimum equipment required and rubber imp due to greater dimensional stability, high quality gypsum model and option of obtain metal or epoxy die. • It supplies as a gel in tooth paste like tube or syringe.
Manipulation: Agar is normally conditioned before use, using a specially designed conditioned bath. This consists of three rooms each contain water. The tube or syringe of gel is first placed in the 100ºC bath. This rapidly convert gel to sol and content of tube become very fluid (10 min). The tube then transferred to the 65ºC bath where it is stored until required for use.
A few minutes before the impression is recorded, the contents are cooled to 45ºC, prevent burn and pulp death. • If the material is maintained in 45ºC for long, it slowly convert to gel. • The conversion from sol to gel takes place slowly (many minutes) at mouth temp and it could be accelerator by cooled water spray onto the imp tray or by using water-cooled imp tray. • Agar is very sensitive to water uptake and loss (syneresis and imbibition).