a clinical overview of removable prostheses 2. impression making for partial dentures

4 2 2 Dental Update – November 2002

Abstract: This, the second article in a series on the prescription of removable partialdentures, will deal with the issue of primary impressions and primary casts for partialdentures. The principles of definitive impressions and master cast planning will bedescribed.

Dent Update 2002; 29: 422–427

Clinical Relevance: This article outlines how to select a stock tray, how tocustomize a stock tray, if required, how to plan special trays and which impressiontechnique to use.

R E M O V A B L E P R O S T H O D O N T I C S

he keystone of clinical dentistry, in whichever specialty, is sound

diagnosis and treatment planning. Thisoften involves measured contemplationof the case in the absence of the patient,but with clinical data at hand. Thisindirect planning will involve articulatedstudy casts and radiographs. Althoughthe treatment chosen may not involvethe use of partial dentures, in this articlewe discuss the issue of primaryimpressions and casts.

PRIMARY IMPRESSIONMAKINGThe function of all primary impressions

for removable prostheses is to recordthe denture-bearing area; this involvescomprehensive recording of all teeth,the ridges in those spans where thereare missing teeth and the palate.

To achieve this, clinicians tend to usewhat are termed ‘stock trays’, of whicha variety is available. These stock traysvary in quality and price and may bemade of flexible or more rigid plastic ormetal. Most may be used to recordmoderate-sized dentate arches, but forlarge edentulous areas the clinicianmay have to select a specific form ofstock tray or modify the tray to suit theclinical situation.

We deprecate the philosophy held bysome clinicians that ‘it’s just firstimpressions’: high-quality treatmentbegins with a thorough diagnosis andtreatment planning and continues withall stages of prosthodontics. There isno doubt that most technicians are ableto gauge a clinician’s clinical ability onthe basis of their impressions andprimary impressions are no exception.

In this article, we describe threeimpression-making techniques:1. The ‘conventional’ type, where

small saddles exist.2. The technique for dealing with

larger saddles or where, forexample, a large saddle(s) are foundor the patient has a deep palateand the stock tray is of aconventional form.

3. The technique used for a patientwith bilateral free-end saddleswhen a stock tray must be madespecifically for the purpose.

1. Technique for ConventionalImpressionsAny of the stock tray types may beused, on the condition that the trayselected mirrors the width, depth andlength of the arch being replicated. Onevariety we find very useful because ofits range of sizes and variety of forms isthat made by Schreinemaker (Clan,Maarheeze, The Netherlands) (Figure 1).

The impression material used isusually irreversible hydrocolloid,although this is not essential and isentirely a matter of clinician preference.We strongly recommend that theopposing arch is also recorded.Depending on the number and positionof teeth present in both arches, anintermaxillary record may be necessaryto enable articulation of the primarycasts, in order that the clinician mayassess the situation, contemplate thepatient-related factors and decide uponthe design of the prosthesis. (It isassumed that acceptable clinicalpractice such as application ofadhesive, mixing of impression material,impression technique and infectioncontrol practice are carried out).

A Clinical Overview of RemovableProstheses: 2. Impression Making

for Partial DenturesJ. FRASER MCCORD, NICK J.A. GREY, RAYMOND B. WINSTANLEY AND ANTHONY JOHNSON

J. Fraser McCord, BDS, DDS, FDS, DRD RCS(Edin.),FDS RCS(Eng.), CBiol, MIBiol, Professor and Headof Unit of Prosthodontics, University Dental Hospitalof Manchester, Nick J.A. Grey, BDS, MDSc, PhD,FDS, DRD, MRD RCS(Edin.), Consultant/HonorarySenior Lecturer in Restorative Dentistry, EdinburghDental Institute, Raymond B Winstanley, BDS, MDS,FDS RCS(Edin.), Senior Lecturer/HonoraryConsultant in Restorative Dentistry, Charles CliffordDental School, Sheffield, and Anthony Johnson,MMedSci, PhD, Lecturer in Dental Technology,Charles Clifford Dental School, Sheffield.

T


Dental Update – November 2002 4 2 3

2. Technique for LargerSaddles or Deep PalateIn such a situation, the tray selectedmay have sufficient length and width,but the depth tends to be insufficientto guarantee acceptable andcomfortable seating of the tray orpredictable results. For this reason,there is sense in customizing the tray(we prefer in this instance to select ametal tray) with an intermediatematerial; we tend to favour animpression compound material (Figure2). A variety of compound products isavailable: our preference is Cameo(Cottrill Ltd., Feltham, Middlesex, UK),which softens at a lower temperaturethan others and exhibits sufficientelasticity at mouth temperature toenable removal from small dentalundercuts. Once the tray has beencustomized to ensure stable seatingonto the denture-bearing areas, anoverall impression in, for example,irreversible hydrocolloid may berecorded (Figure 2). An additionaladvantage of this technique is that lessimpression material is required and,further, there will in consequence beless likelihood to induce retching bythe patient. In conventionaltechniques, excessive loading of thetray with, for example, irreversiblehydrocolloid to fill the vault of the

palate may result in the impressionmaterial flowing into the oropharynx.

The opposing arch may then berecorded and, if required, theappropriate intermaxillary registrationtaken. If many teeth are missing, itmight not be possible to reproduce thedesired jaw relationship (on the casts)and occlusal rims will be needed beforethe casts can be articulated withaccuracy relative to the intra-oralocclusion. This will be discussed later.

3. Technique for BilateralFree-end Saddles Using aSpecially Designed Stock TrayIn our experience, use of conventional‘box’ trays enclosing irreversiblehydrocolloid impression material alone

rarely results in acceptable impressionsin unilateral and bilateral free-endsaddle cases. The tray may becustomized with, for example, Cameoperfectly satisfactorily (Figure 3) butgood results are also obtained whentrays specifically designed for thisclinical scenario are used.

An example is the tray made by Inox(Schwert, Postfach 69, D78501Tuttlingen, Germany), which may be‘customized’ in tracing compoundbefore recording the entire arch with(e.g.) irreversible hydrocolloid (Figure4). Depending on the number anddistribution of teeth remaining, it maybe possible to articulate the resultantcast and its opposing cast;alternatively, registration rims may berequired.

PRIMARY CAST AND PRE-DEFINITIVE CLINICALPROCEDURESThis area tends to receive scantattention yet it is an important part ofthe design and impression–makingcomponent of partial denture provision.

Clearly, as primary impressionsshould be of acceptable quality torecord the denture-bearing areas and tofacilitate design, the primary castshould also be good enough to enablesubsequent unambiguous articulationof casts. Thus the forms and contours(especially the occlusal contours)should faithfully reproduce the teethbeing recorded. Equally, heels of castsshould not interfere with thearticulation of the upper and lowercasts.

Figure 1. Examples of one of the more reliable makes of maxillary and mandibular metal stock trays.

Figure 2. (a) Cameo compound has been added to the stock tray to provide a more customizedtray, and one that will be more stable during the recording of the impression. (b) The completedprimary impression.

a b


The clinician should ensure that thecasts are mounted on split casts so thatthe primary cast can be removed andexamined thoroughly before designingthe denture(s).

An area of sensible, if not essential,practice is the use of a wax trialinsertion stage before recording thedefinitive impression for a denture witha cobalt-chromium base. This practicehas three principal advantages:

l it allows the patient to visualize,and agree to the appearance of, thedenture at an early stage of thetreatment;

l it enables the clinician to planpaths of insertion and to plan forcrown modification, etc.;

l it enables the technician to planthe wax-up of the framework

without compromising aestheticsand denture stability (Figure 5).

Production of Special TraysRecent studies have indicated thatclinicians are less than proficient inprescribing special tray design.1,2

Details of special tray prescriptions areavailable in standard prosthodontictextbooks and will not be addressedhere, but basically the spacing shouldbe 2–3 mm on dentate areas (the formerfor polyethers, polysulphides andpolyvinylsiloxanes and the latter forirreversible hydrocolloid impressionmaterials) and 1 mm in edentulousareas.

DEFINITIVE IMPRESSIONSOnce the special trays have beenconstructed, spaced appropriately forthe needs of the impression material,the clinician must record the contoursof the remaining teeth, the remainder ofthe arch and the functional depth andwidth of the saddle areas.

Accurate reproduction of theremaining dental component isobtained by recording, precisely, thecoronal and occlusal aspects of theremaining teeth, including any preparedsurfaces. This may be achieved by:

1. Making sure no debris/salivabubbles are present by blowing drythe surfaces with air from the 3-in-1syringe.

2. Placing a controlled amount ofimpression material on the occlusalsurfaces.

3. Placing the loaded tray in themouth.

4. Removing the tray, performingappropriate infection control andensuring the impression is castaccording to the manufacturer’sinstructions. The problems ofsending definitive impressionsrecorded in irreversiblehydrocolloid through the post orby courier are that syneresis/imbibition may result, withconsequent distortion of theresultant cast relative to the mouth.

Where peripheral stability isindicated, for example with unilateral orbilateral free-end saddles, the clinicianis advised to ensure that the intendedsaddle area is moulded appropriately.This is achieved by:

l ensuring that the tray is notoverextended lingually andbuccally;

l moulding the intended saddle areawith tracing compound todetermine the functional width anddepth of the buccal and lingualsulci (Figure 6);

l recording the overall impressionusing an appropriate impressionmaterial.

As no data are available fromscientifically based studies todetermine if one impression material issuperior to another regarding accuracyof fit of dentures, we recommend thecombination of a sound impressiontechnique and appropriate pouring ofthe master cast; the choice of theimpression material is essentially one ofclinician’s preference.

The philosophy of this technique isapplicable to conventional dentures orto implant-supported dentures; theprincipal difference will be that, whereimplants are being used, impression

Figure 4. (a) Examples of the Inox tray,which is designed for the bilateral free-endsaddle case. (b) The edentulous areas havebeen recorded in tracing compound. (c) Thecompleted primary impression.

a b

c

Figure 3. Left: the stock tray customized for thepatient by recording the edentulous areas incompound. Right: the completed primaryimpression.



posts and saddle areas have to berecorded (in addition to any remainingteeth).

SPECIAL MODIFYINGPROCEDURES

Combination of Fixed andRemovable ProsthesesTypically, the denture should bedesigned before construction of thefixed prosthesis. This is good practiceas the fixed prosthesis may well requireguide planes, supporting elements(such as cingulum/occlusal rests ormilled shoulders) or precisionattachment components as essentialcomponents of the denture. Commonsense would therefore dictate that thedenture design is known by thetechnician before construction of thefixed component.

When the fit of the fixedcomponent(s) is clinically acceptable, apick-up impression will be required in

order to make the denture. This maybe performed in a 2 mm spacedspecial tray and will involve the useof any of the common impressionmaterials (we prefer a polyetherimpression material such asImpregum-soft by ESPE (MorleyStreet, Loughborough, Leicestershire,UK), because it is more rigid when setthan other commonly availableimpression materials). We recommendrecording large saddle areas in tracingcompound first to reduce the amountof elastic impression materialrequired. With this pick-up techniquethe fixed and removable componentsmay be incorporated onto one model,thereby satisfying the technicaldemands for denture construction(Figure 7).

Dentures Supported by bothTeeth and MucosaSome dentures are supported by bothteeth and mucosa – for example, theKennedy I, II or IV type denture. Aconsiderable amount of rhetoric hasbeen written (and stated) over whetherimpressions should be mucostatic ormucodisplasive. To date, noscientifically based clinical trial hascompared the two impression types;thus anecdote and clinical preferencetend to reign over fact.

The problems of the differingcapabilities of periodontal membraneand mucosa to support dentures andthe means of addressing them arebeyond the scope of this article;however, we will describe here onetechnique that has been designed toaddress the clinical problems involved,either at the time of prescribing thedenture or perhaps a year or so post-delivery when further alveolar ridgeresorption has occurred.

The Altered Cast Technique

In 1954, Applegate3 described animpression technique which cateredfor both supporting elements, reducedthe potential for occlusal errors andfacilitated maintenance of thecompleted partial denture. He called itthe Altered Cast Technique and it

includes features as follows:l Following the assessment of fit of

the cast framework, a trial base isplaced on the saddle area(s), with1 mm (one thickness of wax)spacing between the resin baseand the soft tissues of the saddle.

l A wash impression is recorded ofthe saddles. Although Applegateused impression waxes to recordthe saddle areas, these are not nowreadily available and light-bodiedimpression materials could be usedinstead of the impression waxes.Some clinicians prefer to use GCIso Functional compound (GCCorporation, Tokyo, Japan). (N.B.In this technique, pressure isplaced only on the occlusal restsor other tooth-supportedcomponents of the framework, noton the edentulous saddles).

l The master cast is sectioned at thedistal end of the last abutmenttooth and the framework placedinto the teeth of the master cast.The cast is ‘altered’ by pouringstone into the saddle areas andprocessing the denture on the newsaddle base (Figure 8).

RELINE IMPRESSIONSEvery partial denture, particularly thosewhich are mucosa borne or tooth andmucosa borne, will inevitably losetissue fit (and possible occlusalcontact) because of residual ridgeresorption. The clinician shouldtherefore plan for this whenconsidering denture design.4

Figure 7. The fixed components were ‘ picked-up’in the polyether impression and a definitivemaster cast poured. This enabled the wax trialdenture to be made appropriately.

Figure 5. The wax trial denture stage permits agood three-dimensional assessment of where theteeth, and thus the framework, ought to beplaced.

Figure 6. The peripheral roll of the denture maybe determined by placing tracing compound onthe buccal and lingual aspects of the special traybefore recording the definitive impression.



Dental Update – November 2002 4 2 7

Basic clinical principles involved forroutine reline procedures:

l Assess the case to confirm that areline is required. Where residualridge resorption is excessive,where the impression surface of thedenture bears no or littleresemblance to the denture-bearingmucosa, or if components of thedenture are missing, a replacementdenture might be a better option.

l Take a reline impression. Thetechnique will depend on whetherthe denture is supported by teethalone, mucosa alone or by bothteeth and mucosa.

For tooth-borne prostheses, thesaddle areas will inevitably, but notexclusively, have a base of dental

casting alloy covered with acrylic resin.This may be recorded via a paste-washimpression or a light or medium-bodiedmaterial (after removing some of theresin overlying the framework) and withthe teeth lightly in occlusion. Whenset, an overall impression should berecorded and the denture picked up inthe impression. In this way, the resinportion of the base may be relinedwithout fear of loss of fit and with nofear of altering the occlusion (Figure 9).

A similar technique applies in thecase of mucosa-borne prostheses.

For the tooth and mucosa-borneprostheses the saddle areas should berelined using a paste-wash impressionmaterial or an impression material oflight-bodied consistency. With thedenture and its reline impression in situan overall impression is recorded as

Figure 9. A functional impression of theimpression surface of the lower removable partialdenture was recorded in Visco-Gel (Dentsply,Germany) and, with the denture in situ, anoverall impression recorded in an irreversiblehydrocolloid impression material.

Figure 8. (a) Outline of how the saddle areasare outlined in light-cured PMMA. (b) Thesaddle areas are recorded in a minimallydisplasive technique, the pressure beingapplied to those aspects of the framework thatare supported by tooth. (c) The framework isadded to a modified master cast and the castaltered by pouring stone into the newlyrecorded saddle areas.

a b

c

described above.Dentures that are tooth and tissue-

borne tend to provide the mainproblems for mandibular removablepartial dentures, and Applegate’sAltered Cast Technique may be usefulin such cases. Some clinicians use amodified Altered Cast Technique andrecord the minimally displaced saddleareas with softened tracing compoundbefore using the overall pick-uptechnique.

REFERENCES

1. Basker RM, Harrison A, Davenport JD, MarshallJL. Partial denture designs in general dentalpractice – 10 years on. Br Dent J 1988; 165: 245–249.

2. Smith PW, Richmond R, McCord JF. The designand use of special trays in prosthodontics:guidelines to improve clinical effectiveness. BrDent J 1999; 187: 423–426.

3. Applegate OC. Essentials of Removable PartialDenture Prosthesis. Philadelphia: WB Saunders,1954; pp.166–174.

4. Davenport JC, Basker RM, Heath JR, Ralph JP.Colour Atlas of Removable Partial Dentures. London:Mosby-Wolfe, 1988.

ABSTRACT

WHY ARE MY CROWNS ‘HIGH’?Assessment of Occlusal Reductionduring Preparation of Teeth for FixedRestorations. C.D. Lynch, R.J.McConnell. Journal of ProstheticDentistry 2002; 87: 110–111.

Most journals include a ‘helpful hint’section from time to time, and this one isextremely simple yet remarkably effective.

As the authors observe, inadequatetooth removal will result in a restorationwhich is thin, poorly contoured and liableto fracture or perforate, whilst excessiveremoval may reduce retention andcompromise the pulp.

It is suggested that from time to timeduring the preparation, the patientoccludes into a small sheet of wax, inboth central and excursive movements.The wax is removed from the mouth andthe resultant indentations measured

with a simple caliper (Iwanson decimalcaliper; ASA Dental SpA, Bouzzano,Italy). The thickness is measuredaccurately and the amount of reductionmodified accordingly.

How many high crowns have Iadjusted in my career, and how muchtime would I have saved, and will I savein the future, by this very simple andsensible procedure?

Peter CarrotteGlasgow Dental School

a clinical overview of removable prostheses 2. impression making for partial dentures

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