406 fixed prostho

• An impression is a negative record of the tissues of the oral cavity which constitutes the basal seat of the restoration
• Impression


  A material which has plasticity, and which hardens or sets while in contact with the tissue
• Digital impression


  An impression made using an intra oral scanner
• Conventional impression materials


  • Elastic hydrocolloid (agar, alginate)
  • Elastomeric (polysulphide, polyether, additional silicone, condensation silicone)
  • Rigid (impression plaster, zinc oxide eugenol, impression compound)
• Alginate


  • Does not require any special equipment, being easy to use and inexpensive
  • Reasonable surface detail
  • Relatively hydrophilic and are not displaced from wet surfaces as easily as the elastomers
  • Very poor dimensional stability
  • Low tear resistance
  • The need for impressions to be cast up as soon as possible
• Polysulphides


  • A range of viscosities
  • Long setting times often in excess of 10 minutes
  • Messy to handle and have an objectionable odour
  • Impression shrinkage which is directed towards the impression tray
  • Excellent tear resistance, undergoing considerable tensile strain before tearing
  • Should be removed with a single, swift pull
• Polyethers


  • Heavy, medium and light bodied system
  • Fast setting time of less than 5 minutes
  • No reaction by-product resulting in a material with very good dimensional stability
  • Relative hydrophilicity so, more forgiving of inadequate moisture control
  • Adequate tear resistance and very good elastic properties
  • Relatively rigid when set, hence considerable force may be required to remove the impression
• Condensation silicones


  • Setting reaction produces a volatile by-product (ethyl alcohol)
  • Loss of the by-product leads to measurable weight loss accompanied by shrinkage
  • Dimensional changes slightly greater than polysulphides
  • Should be cast within 6 hours of being recorded
• Addition silicones


  • Most dimensionally stable impression materials
  • No by-product is produced
  • Stable impression allowing impressions to be poured at some days after they were recorded
  • Very hydrophobic, unless the teeth are properly dried
  • Adequate tear resistance and recovery of strain being said to be almost instantaneous
• Latex gloves can inhibit the setting of addition silicone impression materials
• Mechanical decontamination


  Toothbrush cleansing of teeth and gingivae for 30 sec by water, mouth wash, hydrogen peroxide before impression taking
• None of the temporary cements tested was found to have an inhibitory effect on the setting reaction of addition-cured silicones
• Automixing


  • Eliminates problems caused by hand mixing- contamination, void, unmixed portion
• Special impression tray


  • Made to fit the dental arch
  • Carries and confines the impression material to certain difficult areas
  • Uses less impression material
  • Extend 5mm beyond the gingival margin
  • Don't have to cover the palate
  • Avoid undercut areas
  • Thickness of 3mm
  • Spacing to the teeth: 3-4mm (higher clearance for polyether)
• Careful prescription of the design of the special tray by the clinician, and skilled execution of this design by the technician, can lead to significant savings in chairside time and effort, improvement in the accuracy of master impressions, and improvement in the quality of the final prosthesis
• Rigid plastic tray and metal tray produce the least discrepancy of impression, while the use of soft plastic tray gives invisible distortion to the taken impression
• Impression techniques


  • Single step (monophase, two viscosities of silicone used at the same time)
  • Two-step (putty wash technique)
• The optimal technique to use polyvinyl siloxane is to make a custom tray with acrylic resin and impression is made by light body on the tooth surfaces and medium or heavy body material in the tray
• Gingival soft tissue management techniques

  • Mechanical displacement
  • Retraction cord with/without chemicals
  • Retraction paste system
  • Surgical tissue removal (scalpel, electrosurgery, laser, tissue trimming bur)
• Sulcular width of 0.15mm-0.20mm is needed for stability and accuracy of impression materials
• Soft tissue management techniques

  • Mechanical displacement
  • Retraction cord with/without chemicals
  • Retraction paste system
  • Surgical tissue removal
  • Scalpel
  • Electrosurgery
  • Laser
  • Tissue trimming bur
• Sulcular width of 0.15mm-0.20mm is needed for stability and accuracy of impression materials.
• Materials for tissue displacement (retraction)

  • Displacement cords (braided, knitted, hemostatic)
  • Medicaments (epinephrine, astringents, ferric sulfate, aluminium chloride, aluminum sulfate)
  • Cordless displacing material (foam/paste/gel form)
• Single or double cord technique

  • With/without haemostatic agent
  • Cord pressure are unable to stop bleeding without haemostatic agent
  • May cause sulcular inflammation and gingival recession
  • Minimum of 4 minutes
• Aluminium sulphate and ferric sulphate do not cause any inhibition of setting of PVS
• Handling of retraction cord with rubber glove hand will lead to inhibition of setting of PVS.
• The same goes to casual contact of prepared surfaces of tooth before impression is made with PVS
• The single cord technique

  • Indicated in: Small number of abutments, Healthy tissue, Prep is supragingival, No enough space for 2nd cord
  • Place in the sulcus
  • Remove before/after taking impression
• The double cord technique

  • Indicated when: Taking impression for single or multiple abutments, Finish line is located subgingivally, Soft tissue does not maintain lateral tissue displacement
  • 1st cord soaked in medicament and placed
  • 2nd larger cord soaked and placed
  • 2nd cord only removed before taking impression
• Retraction paste


  • Pressure generated by paste retraction significantly lower compared to cord technique and is gentle to periodontium
  • Haemostatic action of aluminium chloride and hygroscopic expansion of kaolin in contact with crevicular fluid provide tissue retraction in 2 minutes time
• Electrosurgery (ES)


  • Used to reduce hyperplastic tissues, expose the gingival margins, and prevent bleeding
  • Widen the gingival sulcus without reducing the height
  • Facilitates removal of the impression without tearing
  • Remove several layers of cells from the inner lining of the gingival sulcus, creating tissue displacement
  • Soft tissues return to their normal appearance between 7 and 10 days
• Rotary gingival curettage (RGC)

  • Alternative tissue displacement to Electro Surgery
  • Removes the inner epithelium of the gingival sulcus along with some of the underlying connective tissue
  • The results after healing are not predictable
• Laser tissue sculpting


  • Alternative surgical technique
  • The use of a laser results in minimal or no postoperative pain and can sometimes be used selectively without anesthesia
  • Less hemorrhage and less inflammation with a faster, painless gingival healing
  • Lasers are more effective than conventional methods in obtaining hemostasis
• Clinical procedures of making an impression

  • Try in tray
  • Adhesive
  • Soft tissue management
  • Impression making
  • Inspection – naked eye, microscope
  • Disinfection
  • Laboratory prescription
• Always ensure dry field, The skillful use of cotton wool rolls, flanged salivary ejector and high-volume aspiration is critical to effective moisture control.
• Allow time for the adhesive's solvent to evaporate and for adequate bond strength to develop at least 7-15min before use.
• Prior to placing the mixing nozzle, a small amount of material should be extruded from the cartridge to ensure no blockage present.
• A good tip is to keep the syringe tip in the expressed material during syringing. Another piece of good advice where access for the tip is restricted is to start syringing from the most difficult area.
• Use a three in one syringe to blow the light body evenly over the preparation.
• Consider blocking out undercuts clinically with cotton wool or waxes.