DEFINITIVE IMPRESSIONS

what are other terms for definitive impression

• secondary impression

• working impression

outline the magnitude of alveolar resorption

• the magnitude of alveolar resorption is approx. x4 greater in the mandible than the maxilla

outline the site and pattern of mandibular resorption

the residual ridge becomes more:

• lingually placed anteriorly compared to the natural dentition because the labial bony plate is weaker

• buccally placed posteriorly compared to the natural dentition because the lingual palate is weaker

» in at the front and out at the back

what is the relevance of mandibular resorption to the position of the denture

• because the residual ridge becomes more lingually placed anteriorly, the lower anteriors must be set on or slightly ahead of the residual ridge

• because the residual ridge becomes more buccally placed posteriorly, the 6 must be set on or just inside the residual ridge

—

• the canines and premolars must be set directly over the residual ridge

• the 7 must be set on or just outside of the residual ridge

outline the site and pattern of maxillary resorption

• the residual ridge is more palatal in all regions than the position of the natural dentition

what is the relevance of maxillary resorption to the position of the denture

• all the maxillary teeth can be set slightly labially/ buccally rather than directly over the residual ridge

describe the trend of bone resorption

• rate of bone resorption is rapid when the teeth are first taken out

• rate of resorption slows after 12 months

what factors influence the rate of resorption

• systemic factors: osteoporosis increases rate of resorption

• local factors: retained roots and biocompatible implants preserve alveolar bone

• denture induced: limited evidence that denture wearing contributes to resorption

what are immediate dentures

• temporary prosthetics custom made to be fitted right after teeth have been extracted

• this provides an immediate replacement

what issues can arise due to having dentures

• denture insecurity

• occlusal problems

• pain and pain caused by irregular resorption

• appearance

what occlusal problems can denture patients encounter

• if the patient has a cross bite or a class III malocclusion (underbite) the placement of denture teeth will be influenced by the skeletal classification of the patient

what is a molar crossbite

• when the buccal cusps of the upper molars sit inside of the buccal cusps of the lower molars

how do issues of pain come about in denture patients

• as the mandibular ridge resorbs, the mental foramina becomes closer to the surface, making it more likely to be affected by compression of the overlying mucosa by the denture

  • this can lead to numbness and paresthesia

• irregular resorption of the ridges can also cause pain when the denture is compressed against sharp bony spicules

how do aesthetic issues come about with dentures

• the lack of support of the soft tissues lead to a loss in face height overtime which can age the patient significantly

• this sagging can contribute to angular cheilitis (fungal infection in oral commissures)

what are requirements of taking a working impression of a ridge without undercuts

a ridge without undercuts or minimal undercuts

• rigid impression material in a close fitting special tray

what are requirements of taking a working impression of a ridge with large undercuts

a ridge with large undercuts

• elastic impression material in a spaced special tray

what is an undercut

undercut: the area between the maximum bulbosity of the ridge and the deepest part of the sulcus beneath it

diagrams of undercuts

outline ZOE

ZOE: zinc oxide eugenol

• rigid impression material

• usually used with a mucostatic technique in patients with little/ no undercuts

• ZOE can become mucocompressive if used in a close fitting special tray

• sometimes used with one spacer or two

what are advantages of ZOE

• cheap

• easy to modify

• accurate

outline a close fitting special tray

close fitting special tray

• extends 1-2mm short of where the final denture border should be

• used in minor/ moderate undercuts

• takes a mucocompressive impression

• ZOE used

outline a spaced special tray

spaced special tray

• extends 2-3mm short of where the final denture should extend to

• used in moderate to severe undercuts

• takes a mucostatic impression

• alginate used (tray should be perforated if alginate is used to ensure mechanical retention)

outline tissue stops

• all impression materials work at an optimum thickness

• tissue stops ensure a uniform thickness of the impression material by preventing the tray from being fully seated against the oral tissues

  • i.e. stops the tray showing through if excessive pressure is placed

different tissue stop depths for different impression materials

what should be done before taking a secondary impression

check the tray!

if the tray is overextended what should be done

• trim back using a burr so the frenae and junction of fixed and mobile mucosa are not impeded

if a tray is underextended what should be done

• add green stick compound or self curing acrylic

how to obtain Zinc Oxide Eugenol

• when mixing, the ratio of the materials MUST be 1:1 otherwise setting times will change

when is ZOE contraindicated

• if patients have an elastoplast allergy, do NOT use ZOE

how should patients be informed before a ZOE impression is taken

• ZOE has a strong taste and may give a burning sensation - tell the patient it is normal to feel some warmth

• the material is also sticky

• it sets reasonably quick, after which we will take it out

how should a xerostomic patient be prepared for a ZOE impression

• if a patient has xerostomia, they should rinse their mouth beforehand

how should a ZOE impression be taken

• ensure border moulding is done well and the patient moves their tongue - wait a little after putting the material in their mouth to ensure it is not too runny before BM

• always keep fingers on the tray in premolar region (finger rests can be incorporated into the special tray)

how can a ZOE impression be adjusted if the tray penetrates through

• can add some more ZOE onto the tray and take the impression again if there are imperfections

• rather than retaking the whole impression

if fibrous ridges are present, how can an impression be taken

• use different materials for different areas of the mouth for a selective pressure impression

outline flabby/ fibrous ridges

• a ridge that becomes displaceable due to fibrous tissue deposition

• most frequently seen in upper anterior region

• can occur when natural teeth oppose an edentulous ridge

what issue can flabby/ fibrous ridges cause in relation to dentures

• denture instability due to the lack of underlying bone

flabby/ fibrous ridge statistics

• found in 1 in 4 upper ridges

• found in 1 in 20 lower ridges

how should flabby/ fibrous ridges be recorded during impression taking

• record this area with a mucostatic technique

what kind of special tray can be made to accommodate flabby/ fibrous ridges

• window box trays

window box tray technique for recording flabby/ fibrous ridges

• posterior region recorded in ZnO using a mucocompressive technique

• anterior region will be recorded in a mucostatic way with a material such as light bodied silicone

outline the Alma Gauge

Alma Gauge

• a measurement tool that gives the lab a vertical and horizontal reading so bite blocks come back at approx. the correct dimensions

• can be used if patients have an old set of dentures they like

image of the Alma Gauge

place plunger in indentation where the incisive papilla is

instructions to give to the lab

• please make upper and lower registration rims

• Alma gauge readings of old upper denture using incisive papilla as fixed point V12 H7

if the patient has no previous dentures, what measurements should be given to the lab

• 22mm upper

• 18mm lower

» these measurements are from the deepest point of the labial sulcus, not the incisive papilla