Chapter 6 - master casts and occlusion rims Flashcards
Boxing the final impression
Why do it? (3):
- preserve contour of the border of the impression,
- control thickness of land area of the master cast to protect its border,
- to confine the stone into the impression when its poured
Boxing the Mx impression:
steps
- cut off handle so ridges are parallel to glass slab
- draw a line 3-4mm from vertical height of impression border-this is a guide for strip seal
- put strip seal on (width: 5-6mm anteriorly, +/-10mm posteriorly)
- add another rope of material from one disto-buccal corner to the other (this creates additional land area to prevent fracture)
- put caulking under tray in tuberosity areas on each side and press on glass to make mean ridge plane parallel with glass slab
- duct tape around impression and join at the anterior midline, then mark a line 15mm above the longest flange of the impression, this is a guide for pouring the stone
Boxing the Md impression:
steps
- draw line 3mm from border, but don’t draw it around around hamular notches
- play doh in to simulate tongue (2mm away from border and 7mm extended posteriorly)
- strip seal (5-6mm wide), it should be 2mm away from border to prevent excessive undercut in the corresponding part of the master cast
- put some caulking under anterior area of handle and press against glass slab so that the mean ridge is parallel to the glass slab
- put duct tape around and mark 15mm above highest part of the impression border on inner surface of duct tape
Pouring the final impression
steps
- mix 250gm stone with 62cc water in vacuum mixer, vibrate carefully into impression
- final impression is hydrophobic so air can be trapped in fine areas, for this reason use a camel hair brush to work stone into rugae of mx impression
- pour stone to the levels marked on the inner surface of the duct tape
Separate master cast from the impression
- remove boxing material from cast (best done 30 min after pouring to ensure easy and clean separation, it heats up when curing so chill the caulking with cold water first, quick pull gives cleaner separation)
- stone should set for a minimum of 45 minutes, then carefully remove the impression along its path of insertion, there is undercut in the retromyohyoid fossae, because of this soften the disto-lingual flanges of mandibular tray with an alcohol torch and release the flanges from the undercut before separating the impression from the cast (alternatively you can section the tray at the mid line and lift the halves medially. The cast should be moist when applying flame to the flange of the tray so the stone will not be damaged by h eat)
- remove wax and any remnant of material from cast surface
- save the final impressions. There are valuable references when you adjust the final dentures
Trim the master casts
- draw a line parallel to mean ridge plane that gives a 12mm thickness at the thinnest part (the deepest part of the palate on the mx cast and the lingual land area for the mandibular cast) —now reduce the base to this level
- reduce height so vestibule is 2-3mm
- land area parallel to base (except for areas distal to retromolar pad and mx tuberosity, these areas should be trimmed so they are sloped 15 degrees towards the base posteriorly)
- *lingual of vestibule of md cast should be 2mm in depth for easier access in lab work
- *posterior land area of mx cast should be reduced to 2.5mm above the post dam area so the palate of the finished denture won’t be too thick
- reduce width of all land areas to 3-4mm (min of 3mm is needed to protect border of the cast)
- *leave land area 5mm wide at distal and disto-buccal corner of md cast in order to prevent breakage of cast in this area
Index the casts
- this is done prior to mounting in an articulator. Why? Because it permits accurate repositioning of the casts back on the mounting plaster. It is particularly helpful during remounting procedures or when the cast separates from the mount plaster accidentally
- place grooves, notches, or special remounting plates on the case of the casts
- to avoid weakening the casts place the grooves in the thickest area of the cast and parallel to the ridges (use a fast-kut wheel at 45 degree angle to make a V-shaped groove), also make sure they are not undercut to form mechanical locks (they should be 3-4mm deep)
Record bases and occlusion rims
- record bases used from now in every future procedure until the dentures are completed
- occlusion rims are used to place the master cast in the articular in the same relation that the mx and md have in the mouth (centric relation at an acceptable occlusal vertical dimension), then they are used to carry the artificial teeth back to the mouth to verify the jaw relation, speech, and aesthetics, finally they will form a portion of the mold in which the dentures will be processed
Fabricate the auto polymerizing acrylic resin record base
-block out undercuts with baseplate wax (keep to a minimum to maintain a stable base, if too much on labial of mx anterior then a poorly adapted labial flange and excessive lip support, creating difficultly in judging lip contour in subsequent procedures)
Mx cast:
-baseplate can usually be withdrawn in labial direction, this minimizes wax needed to block out labial undercuts, but this necessitates the block out of posterior undercuts (ie undercuts distal to the rugae and tuberosities)
Md cast:
-undercuts in retromylyhyoid fossae need careful blocking out (md baseplate need sto be extended at distal end but not beyond halfway the height of the retromolar pad), it does not usually result in an overhanging flange in this area
-I think this is now for both casts…
-apply foil substitute to casts (or Vaseline and put it also on land area and side of the casts)
-mix acrylic resin tray material in same manner as was done for fabrication of impression trays
-put Vaseline on fingers and work the mixed material into a wafer as soon as it begins to lose its stickiness, then place the wafer of acrylic on cast and adapt it to cast the same was as in making the impression trays (quite important to make sure that the vestibules are completely filled so the flanges of the record base will be fully extended)
-even thickness in palate of mx base (it should be no more than 3mm)
-when ridge is full the thickness of resin over the ridge crest and to facial aspect of ridge can be thinned to 1mm while resin is being adapted
-*weakest point on md record base is midline of md, if this isn’t rigid it will distort or break so make sure the resin on the anterior lingual flange and ridge crest is thick enough (3mm) to provide rigidity
-acrylic resin will take 30 min to set then you can remove from cast, might need to pry a bit to get it off (if you do then do it distobuccally and alternate on both sides)
-trim off excess using fast-kut wheel and carbide burr, inner edge of land area should be visible on internal surface of baseplate (use this as a guide for trimming
-the resin on the labial aspect of the ridge crest can be thinned to 0.5mm to allow positioning of artificial teeth, palate should be an even thickness of 2mm
Fabricate the wax occlusion rims
point of it and info for the mx one
-whats the point of this thing? It is to transfer important info to the dental lab (such as jaw relationships, occlusal plane, lip line, midline, cuspid line, and lip support)
-usually they are made in predetermined average dimension, and usually made larger than they will ultimately be because its easier to reduce the wax rim by removing wax with a warm instrument than to add contour to it
Mx Occlusion Rim
-warm 1 and ½ sheets of baseplate wax over flame, starting with ½ sheet
-form it into roll approx. 10cm in length and adapt the soft wax roll to baseplate and then seal it with a hot spatula
-mold wax rim and conform to the shape of the mx arch
-fill in depressions apical to the rim on labial and buccal surfaces with softened wax, ensure to have a continuous contour from the occlusal edge of the rim to the periphery of the record base
-the labial aspect of the mx rim should incline slightly towards the labial (use the larger end of the ruler to establish this inclination), the buccal contour should go straight from the occlusal edge of rim wax to buccal border of the base
-contour the wax rim to the desired dimensions with a hot plate (heat up the plate and stick it on and then let it drip off)
-smooth wax and check dimensions
Dimensions of mx occlusal rim:
Length from highest point of the flange to the occlusal surface of the rim: Central incisor region: 25mm
First molar region: 20mm (the actual height here is determined by a plane parallel to the base of the cast)
*build the rim oversized to allow easy adjustment after
Width of occlusal surface Incisor region: 6mm
Molar region: 10mm
Distal extension of wax rim Ends at a 45 degree angle about 10mm anterior to the hamular notches
Labial extension of the wax rim: Labio-incisal edge is 7-8mm anterior to center of the depression (which is formed by the incisive papilla), this usually created a slight labial inclination (within 5 degrees) on the labial surface of the occlusal rim
Md occlusion rim
- similar to mx but a diff shape
- only extends from cuspid to cuspid (and therefore only requires half a sheet of baseplate wax)
- the occlusal plane should in line with the middle of the height of the retromolar pad
- distance between the vestibular border of the base and the incisal edge of the rim should be 18mm
