Make sure the condition of the soft tissue is optimal: rested and free of inflammation. If inflammation persists, the final impression should be delayed, and more aggressive measures attempted to return the tissue to a healthy condition (consider anti-fungal agents and complete denture rest).
Make sure all denture adhesive is removed from the mouth.
Place the tray in the mouth and evaluate the extension of the tray. In visible areas, lift and pull on the lip/cheek to observe proximity between the tray periphery and vestibular reflections. Clinical experience is useful at this point, to adjust the tray where it is overextended, and to plan extensive border molding strategies where it is under-extended. A properly extended tray is 2 mm short of the functional vestibule, except the upper posterior area where it ends at the vibrating line.
The purpose of border molding is to capture the vestibule of an edentulous arch under functional conditions. This requires the clinician to optimize a number of variables, including tray fit, extension and placement, border molding material flow, border movement, and judgment about the appropriateness of the result.
It is typical to border mold the upper tray in 3 sections and the lower in 4 sections. Make sure the area of the tray to be molded is dry before placing compound.
Warm the tip of the Isofunctional compound over the Bunsen burner and brush the warm material on the edge of the tray. The thickness of material should be 4mm, about 1/2 the volume of the original compound stick.
Warm the compound on the tray thoroughly with the Hanau torch. There should be no voids, wrinkles or gaps in the compound; it should be shiny and smooth; it should be free-flowing, and the wrist should be rotated to keep the softened compound in position on the tray.
Temper in the water bath for a count of 5.
Place in the mouth so that no tissue contact deforms the warm compound until it is completely seated.
Immediately initiate border movements, either patient derived (smile, purse lips, move jaw horizontally, tongue up to and along the upper lip) or clinician derived (pulling lips and cheeks).
Carefully remove after 10 seconds (without distorting the warm compound).
Place in cold water.
When chilled, remove from the water, dry the compound and inspect the result. The compound should have a smooth and dull surface, with identifiable features such as frena. It is useful if there is some excess material inside or outside the tray, as evidence that the compound fully reproduced the vestibule. The compound is trimmed with a sharp knife to remove internal excess and to thin or shape the outer contour of the compound to appropriate facial support.
In areas that are incompletely filled, add more compound over the previous material, repeating the steps for warming, tempering, placing and molding until the result is correct.
In areas where the tray shows through very thin compound sections, the tray may be overextended and should be shortened > 2mm, then the border molding repeated with new compound.
In the posterior aspect of the upper tray, compound is placed inside the tray, following the vibrating line. The intent of the border molding in this area is to create pressure in the posterior palatal tissue, as a functional posterior palatal seal. It is not desirable to extend the compound posteriorly past the tray, unless the tray is short; under no circumstances should the compound extend onto the movable soft palate.
After completing border molding, use a bur to relieve the interior of the tray in areas of displaceable tissue and undercuts. Place a vent hole in the palatal vault with a large round bur.
Paint a thin layer of adhesive on the tray and compound. Make sure to use adhesive specific for the impression material.
Apply a thin layer of Vaseline to the patient's lips and skin around the mouth, to make removalof impression material from the face easier after the appointment is concluded.
Make sure the patient's clothing is protected from dripping impression material by barriers; the elastomers we use are nearly impossible to remove or launder from fabric.
There are currently 3 impression materials used by the department for edentulous final impressions: polyvnyl siloxane, polysulfide and polyether. We strongly advise against using polyvinyl siloxane (Aquasil) for maxillary impressions because of poor retention in the final denture or relIne.
Polysulfide is provided as base and catalyst in squeeze tubes - equal lengths are extruded onto a paper mixing pad and the components are hand mixed. Place 4" lengths of each material (the length of the paper mixing pad) for a lower impression; use 50% more (one full length plus 1/2length) of each material, for upper impressions. Mix the 2 components forcefully in circles and figure eights with a wooden or metal spatula; occasionally scrape up the material from the surface of the pad to thoroughly mix the material.
The material is adequately mixed when there are no streaks in the mix, normally within 1 minute of start of mix.
Polyether medium body is provided in an electric mixing device (do not use light body in a gun mix - it is too runny for a full arch impression). Before placing the mixing tip, make sure there in material in the device, and bleed the material to verify that blue material extrudes from the large orifice and red material extrudes from the small orifice. Place the mixing tip and extrude mixed material onto a paper pad. Make sure the material is mixed (color will be purple); 6 seconds is enough for a lower impression, 10 seconds is adequate for upper impressions.
For polyvinyl siloxane impressions, we recommend either the light or mono phase materials for mandibular impressions. The material is injected directly from the mixing gun onto the impression tray.
Using a clean instrument, convey the impression material to the custom tray and butter the material as a monolayer or wash into all internal aspects of the tray and over the periphery to cover all of the border molding material. DO NOT OVERFILL THE TRAY! The impression surface should be smooth and free-flowing before placing in the mouth.
Quickly dry the mucosa with 2x2 gauze. We advise against packing the mouth with gauze prior to mixing the impression material, because this often stimulates excess salivation.
For the upper impression, place a small amount of the impression material in the palatal vault with the tip of a finger, then seat the impression tray anteriorly first, then posteriorly last.
Typically maxillary impressions require moderate to high force to fully seat. Swipe excess material from the posterior palate with a mirror or cotton swabs.
For the lower impression, place the impression uniformly down on the ridge, using light seating force.
Immediately begin vigorous border molding while holding the tray absolutely still. It is advisable that the clinician perform the lip and cheek manipulations, since patient smiling and pursing may move the tray. The patient must move the tongue for the lower impression. Repeat theborder molding thoroughly every 30 seconds until the material gels.
Polysulfide impressions take 8 minutes from start of mix for full polymerization at mouth temperature and humidity; polyether and pvs materials take 5 minutes. Always confirm set by observing full indentation recovery after pressing the material in the mouth with a fingernail; do not perform this test on the counter-top since room temperature setting occurs more slowly.
Remove the impression by breaking the peripheral seal.
Wash the impression, dry and inspect the result. If acceptable, spray with disinfectant and place in a Ziploc baggie. If not acceptable, strip the impression material from the tray and repeat the above steps with attention to correcting the previous errors.
Then impression(s) is submitted to the laboratory with a prescription signed by the instructor master cast, record base and wax rim fabrication.
Master Cast Lab Work (FYI only)
The final impressions should be boxed prior to mixing and pouring dental stone. There are 2 methods for boxing:
Wax box - A bead of sticky wax is applied with a hot spatula around the entire periphery of the impression 5mm below the peripheral roll. Rope wax is applied on the sticky wax, creating a ledge 3-5mm below the edge of the impression. The wax rope and sticky wax are sealed with a hot spatula. For the mandibular impression, the tongue space is closed with a triangular piece of boxing wax, and this section is sealed to the rope-sticky wax complex with a hot spatula. Finally the entire impression is surrounded with boxing wax to create a vertical cylinder that is sealed to the rope wax. Pour water into the boxed impression to confirm that it is water-tight.
Pumice/plaster technique - Place a full bag of lab plaster and a similar volume of medium grit pumice in a mixing bowl and mix with water to a creamy but viscous consistency. Make a circular pad of the mix on a plastic model plate, then insert the occlusal or outer surface of the impression into the pad (it may be necessary to model grind the handle to reduce the elevation of the tray above the plate). Position the impression so that the residual ridge is horizontal. As the plaster sets, move it up and around the periphery of the impression - the plaster investment around the impressionshould be 10mm thick, and just below the peripheral roll as the plaster begins to set. A wet finger is used to smooth the plaster to a horizontal shelf around the entire periphery of the impression, and to smooth the tongue space in the mandibular arch. When the plaster is hard, the invested impression can be removed from the model plate and model trimmed within 5mm of the impression. Use a sharp knife and toothbrush to create a smooth and horizontal plaster shelf. Vaseline the shelf (so the dental stone model doesn't stick to the plaster investment) then place boxing wax around the plaster to form a vertical cylinder, securing the wax with a rubber band. Pour water into the wax cylinder to confirm that it is water tight.
Dental stone is vacuum-mixed and vibrated into the impression, filling to a depth of at least 15mm.
After the dental stone sets, the wax is chilled in cold running water and then the wax is removed from the impression. If there is plaster investment, this is removed by prying pieces away with a green-handle knife. Finally, to remove the model from the impression, it should be immersed in warm water for 5 minutes to soften the border molding compound, and then the impression should lift easily away from the model without stressing the gypsum material.
Note that once the border molding of the final impression has been disturbed by removal of the model from the cast, the impression cannot be poured a second time.
Inspect the model for adequacy, then remove blebs, model grind and trim the cast. Place 3 triangular grooves in the base of the cast, as index features for later mounting. Refine the land area of the master cast to be horizontal, 3-5mm width, and 3-5mm depth.
Show the master cast to an instructor to have the removable laboratory prescription signed, authorizing laboratory fabrication of record base/wax rim (5 days). The department does not prohibit students from "do-it-yourself" rims, but strongly prefers professional craftsmanship for this item.