Impression materials and techniques for fixed prosthodontic

Impression: is a negative likeness or copy in reverse of the surface of an object.. A variety of impression materials exists in order to capture the surface detail and dimensions of hard and soft tissues for fabrication of prosthesis. Also the impression used for diagnosis (e.g. in orthodontic).

The ideal impression material should possess multiple characteristics: 1- The hydrophilicity of impression material, which is defined as having a high affinity for moisture, providing good surface wetting, and allowing for greater surface detail.

2- The dimensional stability of the material, which is critical for accurate replication of the intraoral structures. Dimensional changes may occur due to (1) contraction from polymerization, (2) liberation of a by-product or accelerator component, (3) water absorption from a wet or humid environment, (4) a change in temperature and (5) incomplete recovery of deformation because of viscoelastic behavior.. Materials with good dimensional stability can remain unchanged for a period of approximately 7 days and resist temperature extremes during shipping.

Overcome dimensional change: 1. Delay pouring the impression after half to one hour until byproduct (hydrogen gas) volatile. 2. Manufactures often add a noble metal, such as platinum or palladium, to act as a scavenger for the released hydrogen gas.

3- Tear resistance and elastic recovery. These are important for preserving the accuracy of the impression during removal from the mouth and after cast separations. Materials with sufficient tear resistance and elastic recovery will withstand multiple pours.

4- Other properties for an ideal impression material would have a pleasant odor, taste, and acceptable color. Absence of toxic or irritant constituents, adequate shelf life for requirements of storage and distribution, setting characteristics that meet clinical requirements, compatibility with cast and die material, readily disinfected without loss of accuracy.

According to the elasticity:

 Elastomers are a group of elastic impression materials. They consist of polymers that bind together in a network of chains that give material a rubber consistency. They are widely used in fixed and removable prosthesis because of their good properties

1- Viscosity  Viscosity describes the flow characteristics of an unset impression material. Materials with low viscosity have high flow and those with high viscosity have low flow. The low viscosity material can be referred to as light body, syringe, or wash material. These lower viscosity materials can flow easily into and record fine details; however, they are usually not used alone. Instead, they are generally used in conjunction with a second more viscous material to push and support the lower viscosity material.

2- Hydrophilicity Impression materials are characterized by their degree of hydrophilicity. Surface wetting describes the relative affinity of a liquid for a solid and can be quantified by measuring the contact angle. A contact angle of zero would indicate complete wetting of the surface; whereas, a high angle would indicate less wetting.

 Hydrophilic materials have a high affinity for moisture (low contact angle), provide good surface wetting, and allow for a high degree of surface detail. Hydrophobic impression materials have a low affinity for moisture (high contact angle), provide poor surface wetting, and a lower degree of surface detail. Hydroactive impression materials are normally hydrophobic and are rendered hydrophilic through the addition of surfactants.

3- Tear Resistance and Elastic Recovery  Tear resistance and elastic recovery are important in preserving the accuracy of the impression during intraoral removal and after cast separation. Materials with sufficient tear resistance and elastic recovery will withstand multiple pours, producing several accurate casts, which is a major advantage in contemporary restorative dentistry.

 4- Working time – a period from the start of mixing to the final time at which the impression can be seated in the mouth without its distorsion.  5- Setting time – a period from the start of mixing till the impression becomes elastic enough to resist deformation during its withdrawal from the mouth.

Advantage Disadvantage 1. Low price. 2. Long working time. 1. Should be poured within 0.5 – 1 hour. 2. Unpleasant mercaptane smell. 3. Long setting time app. to 10 min. 4. Poor elastic recovery – prone to plastic deformation.

AdvantageDisadvantage 1. Naturally hydrophillic. 2. Accurate and high dimensional stability. 3. Good elastic recovery. 4. Low setting contraction. 5. Excellent surface detail reproduction. 6- Highly acceptable to patient. 1-Swells in disinfectant or moist environment. 2- Rather stiff when set material (difficult to remove from mouth). 3- Very expensive.

Advantage disadvantage 1- Easy to disinfect. 2- Highly acceptable to patient. 3- Accurate impressions when poured soon. 4- Good elastic recovery. 5- Lower price. 1- Must pour within one hour. 2- Low tear strength. 2- Hydrophobic. 3- Dimensionally unstable.

AdvantageDisadvantage 1- Good surface detail (dry surface ). 2- Good dimensional accuracy. 3- Low permanent deformation. 4- Highly acceptable to patient. 5- Good storage stability. 1- Hydrophobic ( unless surfactant added. 2- Low tear strength. 3- High cost.

Manual mixing Auto mixing

.The auto mixing result in fewer voids than hand mixes.. Equal mix without lefting any material unmixed.. Low material waste than hand mixing.. automixing was used for all consistencies except putty to be used with this system.. cost.

1- Dual-Viscosity Technique. 2-Single-mix Technique or Monophase Technique. 3- Putty-Wash Technique.

. Dual-viscosity technique also called one step technique in which low-consistency material is injected with a syringe into critical areas and the high-consistency material is mixed and placed in an impression tray. After injecting the low-viscosity material, the tray containing the higher-viscosity material is placed in the mouth. the materials join, bond, and set together.

In the single-viscosity or monophase technique, impressions are often taken with a medium-viscosity impression material. To overcome problems associated with using two different viscosities such as using two mixing guns and any difference between the viscosities, a monophase impression technique with medium viscosity was suggested

The putty-wash technique is also called two- step technique whereby a preliminary impression is taken in high- or putty- consistency material before the cavity preparation is made. Space is provided for a low consistency material by a variety of techniques, and after cavity preparation, a low-consistency material is syringed into the area and the preliminary impression is reinserted.

Gingival retraction is allowed to remain for 5 to 10 minutes, to allow water absorption by the superficial cord and increase crevicular width.(a); Excess moisture is eliminated and the second cord is removed (b) a low viscosity impression material is immediately injected into the sulcus (c).

.All impressions should at least undergo a disinfecting procedure by immersion in 1% sodium hypochlorite for a minimum of ten minutes….Chlorine solution, Aldehyde solutions, iodine and phenol solutions…