DENTAL WAXES - SEMINAR

                                              DENTAL WAXES

 Contents:

Ø Introduction.

Ø Uses of wax in dentistry.

Ø Components of dental waxes.

Ø Properties of waxes.

Ø Classification of waxes.

Ø Description of individual waxes.

Ø Impression procedures with impression waxes.

Ø References

                                                        

Introduction:

 

            Originally applied to natural occurring esters of fatty acids & monohydric alcohols, the term now is used for both naturally occurring & manufactured products resembling esters. They have

•     Dull luster

•     Soapy or greasy texture

•     Soften gradually on heating before forming a liquid

 

Uses in Dentistry:

•     Inlay pattern

•     Boxing of impression

•     Base plate

•     Casting wax

•     Utility wax

•     Sticky wax

•     Corrective impression

•     Bite registration

     Dental waxes are combination of various types of natural & synthetic waxes, gums, fats, fatty acids, oils, resins & pigments compounded to provide desired physical properties.

Components of dental waxes:

Natural Waxes	Synthetic Waxes	Additives
MINERAL     a) Paraffin     b) Montaux     c) Barnsdhal.     d) Ozokerite     e) Microcrystalline	a) Aerosol OT     b) Castor wax     c) Flexowax C     d) Dura wax	FATS     Stearic acid
PLANT a)      Carnauba b)      Ouricury c)      Candelila d)     Japan wax e)      Cocoa butter	NATURAL RESINS a)      Copal b)      Dammar c)      Sandrac d)     Shellac
INSECT         Bees wax	SYNTHETIC RESINS a)      Polyethylene b)      Polysterene
ANIMAL   Spermaciti

Natural Waxes

•     Two main group of organic compounds in waxes are-

•     Hydrocarbons

•     Esters

Some waxes contain free alcohols and acids as well.

 

Synthetic Waxes

•     Complex organic compounds of varied chemical composition

•     Use in dental formulations is limited

•     More refined than natural waxes

Types

•     Polyethylene waxes

•     Polyoxyethylene glycol waxes

•     Halogenated hydrocarbon waxes

•     Hyrogenated waxes

•     Wax esters

Composition

     Polyoxyethylene waxes are polymers of ethylene glycols. They have limited compatibility with other waxes. They have melting temperature. From 37°C to 63°C. but function as plasticizers and toughen films of wax. Others are produced by reaction with natural waxes.

 

Properties

•     Melting range

•     Thermal expansion

•     Mechanical properties

•     Flow

•     Residual stress

•     Ductility

 

Melting Range

     Have a range as they contain several types of molecules, each having a range of molecular weight.

 

Thermal Expansion

     Linear co- efficient of thermal expansion – change in length per unit original length with 1° change in temp. Waxes have the largest co- efficient of thermal expansion among all dental materials. Weak secondary valance forces are easily overcome by thermal energy, more so in mineral waxes than plant waxes. Many waxes exhibit at least 2 rates of thermal expansion. Change in rate occurs at transition points. At these points the internal structural parts becomes become freer to expand. Because the ingredient waxes undergo transition that do not coincide with one another, inlay waxes exhibit more than two changes in rate of expansion.

Mechanical Properties

•     Elastic modulus

•     Proportional limit

•     Compressive strength

All are low when compared to other materials 

 

Elastic Modulus

•     Elastic moduli of carnauba wax is highest

•     Bees wax – lowest

•     Decreases with increase in temperature.

•     Inlay wax (simulates a mixture of 75% paraffin & 25% carnauba wax) – 760 to 48.2 MPa between 23°C & 40°C

 

 

 

Application

•     Modulus ratio for inlay and soft green casting wax is 7:1.

•     To avoid non uniform distortion of the wax pattern during hygroscopic casting procedure use inlay wax (less expansion) for lateral walls and soft green for occlusal surface.

 

Proportional Limit/ Compressive Strength

•     Decrease with increase in temp.

•     E.g. - P.L. for inlay wax decreased from 4.82 to 0.21 from 23°C to 40°C.

•      C.S. – 82.7 to 0.48 MPa

 

Flow

     Result of slippage of molecules over each other. In liquid state of wax it is synonymous with viscosity below melting temperature. It indicates the degree of plastic deformation at a given temp. Flow depends upon::

 

1.    Temp of wax

2.    The force applied

3.    Time for which the force is applied

4.    Flow is greatly increased as melting point is approached

Application

•     A direct inlay wax should have a high flow just a few degrees above the mouth temperature so it is not too hot in workable condition

•     Should have a no flow at mouth temperature so that it does not distort during removal of pattern

•     Yellow beeswax does not flow extensively till it reaches 38°C

•     At 40°C its flow is 7%

Has been used as an impression wax

 

Residual Stress

•     Residual stresses always exist in a prepared wax pattern

•     Presence of such stresses can be demonstrated by comparison of thermal expansion curves of annealed waxes with wax cooled under compression & expansion

•     Extent of change in thermal expansion depends upon

1.    Magnitude of residual stress

2.    Time &

3.    Temp of storage of specimen

 

COMPRESSION

•      When cooled under compression, the atoms & molecules are forced together as compared to when there is no external stress

•     After cooling & upon load removal, motion of molecules is restricted – residual internal stresses

•     On heating the residual internal stresses is added to normal thermal expansion – hence more expansion.

TENSION

•     Cooling under tension results in molecules moving away from one another comparatively

•     On heating, release of these internal stresses work in a direction opposite to thermal expansion

•     Large internal tensile stresses may result even in contraction upon heating

Ductility

•     Like flow, ductility increases with increase in temperature of waxes

•     Lower the melting temperature of a wax, more will be the ductility

•     Waxes made of components having wide melting ranges have more ductility

•     With wide range of melting point of components, the softening point of lowest is approached first on heating

•     On further heating this component liquefies, the softening point of next is approached & so on

•     Entire wax mass is plasticized & ductility increases

 

 

CLASSIFICATION OF DENTAL WAXES:

Pattern Wax	Processing Wax	Impression Wax
1.  Inlay Wax	1. Boxing Wax	1.  Corrective Wax
2.  Casting Wax	2. Utility Wax	2.  Bite registration Wax
3.  Base plate Wax	3.  Sticky Wax

Pattern Waxes

•     Used to form general pre determined size & contour of an artificial restoration

•     Later it is replaced by more durable material such as cast gold, cobalt- chrome- nickel alloys etc

•     They exhibit thermal change in dimension and warpage on standing

INLAY WAXES ADA 4

•     Inlays, crowns & bridge units are formed by a casting process that uses lost wax pattern technique

•     A pattern of wax is constructed that duplicates shape and contour of casting

•     After investing & spruing the wax is eliminated by heating

Typical Composition

Paraffin- 60%

Carnauba- 25%

Ceresin- 10%

Bees wax- 5%

TYPES

BY FLOW

1.    Hard

2.    Regular

3.    Soft

Flow can be reduced by

•     Adding more carnauba wax

•     Using higher melting paraffin wax

 

BY TECHNIQUE

•     (Revised ANSI/ ADA Sp. No. – 4)

•     Type I   - Direct

•     Type II - Indirect

•     Type I - Hard wax used for forming patterns directly in mouth

•     Type II - Softer wax used for indirect technique

TYPE I

•     Lower flow at 37°C to minimize any distortion on removal from mouth

•     Working temperature for registering cavity details is around 45°C

•     Should not be high so as to avoid damage to pulp

•     Maximum linear thermal expansion allowed –

   25°C – 30°C – 0.20%

   25°C – 37°C – 0.60%

   Must be mentioned by the manufacturer so that compensation is made for the shrinkage from mouth temperature to room temperature.

 

Insufficient flow of wax caused by insufficient heating causes

•     Lack of details

•     Excess stress within the pattern

•     Excess flow by over heating makes compression of wax difficult (because of lack of body)

•     From mouth temperature to room temperature (24°C) there is 0.4 % linear contraction

 

TYPE II

•     These waxes are commercially available for crown & bridge work and inlays in the form of

•     Sculpturing / modeling waxes

•     Dipping waxes

Sculpturing Waxes

Properties

•     Low shrinkage due to microcrystalline structure

•     Quick to apply and quick to cool with its optimum melting interval

•     High surface tension

•     Excellent carving properties due to its non-elastic and hard quality

•     Wide assortment of colours

 

Available as

•     Universal

•     Specific purpose like

   -  occlusal

   -  cervical

   -  underlay

UNIVERSAL

•     Ideally suited for quick coverage of large areas with wax

•     Low surface tension

•     Low melting point

•     Small contraction on hardening

•     Excellent flow properties

 

OCCLUSAL

•     High strength - no abrading of contact points

•     High rigidity

•     Can be easily drawn due to its high surface tension

•     Opaque appearance for defined contours and permanent control during modeling

 

Difference between occlusal and universal wax

•     Occlusal wax - greater surface tension and high rigidity

•     The surface tension allows ball-shaped drops to form and harden

•     This eases work considerably in the waxing-up phase

•      The increased strength guarantees precise modeling

•     Universal wax - excellent flooding properties, can be used in many areas

 

CERVICAL

•     Low shrinkage

•     Particularly stable

•     Can be adapted precisely and thinly to the preparation border

•     Excellent carving properties

 

 

UNDERLAY

•     For small undercuts and cavity coverage before inlay modeling

•     Significantly softer than cervical wax

•     Gentle elasticity

•     Good adaptation and carving properties

•     Very low shrinkage

 

Dipping Wax

•     The hotty LED is a wax dipping pot which permits controlled temperature setting and displays the set and the actual value

 

Advantages

•     High precision via low shrinkage

•     Optimum viscosity at 89–91°c (192–196°f)

•     Precise-fitting copings with an even layer thickness

•     High stability and elasticity

•     Contains no acrylic additives

•     Easy to cut off the preparation border

 

Method:

Preparation:

·      Place a thin isolating layer on the die surface.

·      Build-up of bubbles and streaks are thus prevented

 

Dipping:

•     Support your hand comfortably

•     Do a quick dip to just beyond the preparation border

•     Slowly and evenly remove the die from the wax

•     Just before the tip comes out hold it steady in the pot for a short time so that any excess wax can drip down

 

 

 

Warpage

           Patterns distort when allowed to stand unrestrained. It increases with increase in time & temp. of storage. Because of the release of stresses introduced in pattern during formation

 

Minimizing Warpage

1.  Use higher temperature at time of formation – less force to shape – less residual stresses

2.  Soften the wax uniformly at 50°C for 15 min

3.  Warmed carving instruments & die

Casting Wax

•     Used for metallic framework of RPDs

•     Available as sheets (28 – 30 gage; 0.4 – 0.32 mm), readymade shapes & bulk

•     Serve same purpose as inlay wax, differ slightly in physical properties

•     Ingredients similar to inlay waxes 

•     Sheets used to establish minimum thickness in some areas of RPD framework such as palatal & lingual bar

•     No ADA specification but only a federal specification for its properties

 

Flow

•     Are to be used on a cast & not in mouth

•     They have a higher flow as compared to inlay waxes at around 35 – 37°C

•      35°C – 10% min

•      37°C – 60% max

Working Properties

•     Slightly tacky so as to maintain position on cast

•     Pliable & readily adaptable at 40 - 45°C

•     Copy accurately against the surface which it is pressed

•     Shall not be brittle on cooling

•     Must vaporize at 500°C

•     Be bent double on itself without fracture at 23°C

 

Baseplate Wax

ADA 24

•     The basic use is to form occlusal rim on baseplate tray to set teeth for denture

•     Establish VD, contour of the denture after teeth setting

•     Also used for MFPs, patterns for orthodontic appliances

•     Checking articulating relations in mouth & transferring to articulators

•     Supplied as sheets 7.60 x 15 x 0.13 cm, red, pink or orange in colour

•     Three types       

•     Type I – soft wax for contours & veneers

•     Type II – medium wax for temperate climates

•     Type III – hard wax for tropical climates

 

Mainly differ in flow with type III having least

Requirements

•     Linear thermal expansion from 25° to 40°C < 0.8%

•     Softened sheets shall cohere readily without becoming flaky or adhering to fingers

•     No irritation to oral tissues

•     Pigment not to separate on processing

•     No adhesion to other sheets or separating paper on storage

 

PROCESSING WAXES

Boxing Wax

•     Aids in forming a plaster or stone cast from an impression

•     Boxing consists of

•     Adapting a long narrow strip of wax around the impression below peripheral height

•     Followed by a wide strip of wax to form a wax box

•     Also called carding wax: originally used for placing porcelain teeth in packing

 

Federal requirements:

•     Smooth glossy surface on flaming

•     Pliable at 21°C; retains shape at 35°C

   Readily adapt to impression at room temp.

•     Seal easily to plaster with hot spatula

Utility Wax

•     An easily workable, adhesive wax for multiple use such as-

•     Improving the contour of perforated tray for hydrocolloids

•     To stabilize a pontic for a temporary bridge while index is being made

    

FEDERAL REQUIREMENTS:

•     Pliable at 21° to 24°C; workable & easily adaptable at room temp.

•     Flow be between 65% & 80% at 37.5 °C

•     Tacky at 21° to 24°C; adhesion for build up

•     Colour- green or black

Sticky Wax

•     Formulated from a mixture of waxes, resins & other additives

•     Sticky when melted, adheres closely to the surface

•     At room temperature – firm, free from tackiness & brittle

•     Should fracture rather than flow when deformed

 

Uses

•     Used primarily on dental stones and plasters

•     Used in repair for holding the metal or resin parts temporarily

Federal Requirements

•     Have a dark or vivid colour to distinguish from gypsum products

•     Shrinkage - < 0.5 % from 43° to 28°C

 

IMPRESSION WAXES

      Impression waxes, though rarely used to record complete impressions, they can be effectively used to correct small imperfections in other impressions. They are thermoplastic materials, which flow readily at mouth temperature and are relatively soft even at room temperature.

 Impression waxes are classified as:

1) Corrective impression wax

2) Bite registration wax.

There are no ADA federal specifications for impression waxes.

Composition

      Impression waxes consist typically of a mixture of low melting paraffin wax and bees wax in ratio of about 3:1.  In addition it may contain other wax like ceresin and also metal particles e.g. aluminum or copper particles.

Properties

Melting range

      Since waxes have several types of molecules i.e. Crystalline or amorphous of different molecular weights, they have melting ranges rather then a point. Melting range of impression waxes is much lower due to additions like resin. Impression waxes flow at mouth temperature under occlusal load. Heat of fusion is the heat in calories required to convert 1gm of material from the solid to liquid state at the melting temperature.

    Melting temperature:

©  Beeswax is 62.8°C.

©  Paraffin wax is 52°C.

Melting    range:

©  Beeswax is 34-70° C.                                                                              

©  Paraffin wax is 44- 60°C.

Flow

      The flow of corrective temperature wax and bite registration wax is measured at 37°C and is 100% and 2.5%-22% respectively, thus these waxes are susceptible to distortion on removal from the mouth. Flow as tested by compression of cylindrical specimens is 2—85% at 37°C.

Mechanical Properties

The elastic modulus, proportionately limit and compressive strength is low as compared with other materials and are dependent on temperature.

Thermal expansion

      Impression waxes expand when subjected to a rise in temperature and contract as the temperature decreased. Paraffin and beeswax, which are the chief constituents of impression waxes, have different temperature range. Because of the coefficient of thermal expansion is so great, the impressions should be poured immediately to avoid distortion.

      If the impression cannot be poured immediately, they should be stored at or near 0°C. Coefficient of linear thermal expansion is 350-700 x10^-6/ °C.

Advantages

1.      Impression wax can be used in thin layers to record the impression surface of the ridge accurately.

2.      It is relatively easy to manipulate.

3.      It does not need advanced equipments.

Disadvantages

1.      Distortion of the impression wax can occur when the records are stored due to the release of stresses.

2.      It is technique sensitive.

3.      It can be used only to record edentulous surfaces,

Applications

1.      Corrective impression wax is used as wax veneer over an original impression to contact and register the details of soft tissues. It is claimed that it records the mucous membrane and underlying tissues in a functional state in which movable tissue is displaced to such a degree that functional contact with the base of the denture is obtained.

Functional impressions materials with thermoplastic waxes for reline procedures:

      Historically, wax was one of the first materials used to make impressions of the edentulous arch. The first commercially available "mouth temperature" waxes appeared in 1930's they were composed of natural waxes in paraffin base. In late 1950's, thermoplastic resinous materials became available; e.g. adaptol and stalite plastic impression material. These fluid materials were designed to record tissue under an occlusal load and accomplish the same objectives as the waxes.

Technique

When used for a distal extension mandibular removable partial denture that has been in service a long time, provision must me made for correcting the disorientation of the framework that occurs when resorption of the supporting tissue allows the base to settle. This can be accomplish by building up the distal third of the base with red modeling compound and seating framework  with pressure on the rests and indirect retainers with a blunt instrument but no force should be exerted.

Pre impression procedure

Mucosa should be allowed to recover prior to starting the impression procedures. Recovery may be facilitated by adjusting the existing prosthesis, instructing the patient to remove the prosthesis at night and consume a soft diet, surgical intervention or use of a tissue conditioning material.

Impression procedure

The border of the prosthesis must be satisfactory before the impression material is added. The impression will distort too easily if it is not supported 1to 2 mm beyond the border of the prosthesis.  The impression material is placed in a hot water bath or on a hot plate to allow the material to become fluid.  The entire denture base is then coated liberally with the fluid material. A No.2 or larger brush is an acceptable applicator. The prosthesis is placed in the patient's mouth and the patient is instructed to bring the teeth into light occlusal contact. After 4 to 5 minutes, the impression material will have reached mouth temperature. The patient should then bring the teeth into maximum occlusal contact guided by the dentist.  The border areas are moulded by the manipulation of the dentist and the patient.  The prosthesis is then removed and inspected for voids, which can be filled with more fluid impression material. The prosthesis is reinserted in the patient's mouth and again allowed to reach mouth temperature.  The patient is given a stick of chewing gum and instructed to chew the gum on both sides of the mouth for 5 minutes.  After disposing the gum the patient rinses with ice water.  The prosthesis is removed and immediately submerged in an ice water bath.

      An alternative impression material for this technique could be one of the tissue conditioning material. The advantage that the wax or fluid resin has over the tissue conditioning material is that it results in a smooth glass like surface on the finished denture base.  This is of particular importance when a great deal of denture base movement is anticipated and when the tissue is thin, fragile and easily abraded.

      A simple formula for producing & corrective impression wax from paraffin wax and bees wax is described by Mc Crorie  i.e. mixture of yellow bees wax and thermowax or paraffin wax with flow characteristics at 37ºC similar to those of Korecta wax Nos. 2,3 and 4.

Wax Mixture                                                                   (% flow at 37ºC)

75% yellow bees wax: 25%parafiin wax                                      (80)

50% yellow bees wax: 50% paraffin wax                                     (85)

25% yellow bees wax: 75% paraffin wax                                     (89)

      Korecta wax is available in four grades, each with a different degree of plastic deformation at mouth temperature. Each grade is designed for a specific purpose.

1.      Extra hard No.1 (pink):  A reinforcing material used only on the external surface to support -wax extensions beyond tray margins.

2.      Hard No. 2 (yellow): Used in rebasing as a hard foundation for Korecta waxes 3 and 4 when extensive absorption (alveolar resorption) necessitates a bulk of material. Also, used to restore occlusion in a partial denture or a removable partial denture, which has settled due to severe tissue change.

3.      Soft No. 3 (red): Used for minor tray correction and as an initial lining to stabilize the tray.

4.      Extra soft No. 4 (orange): Used to secure a completely adapted impression under natural masticatory pressure. It leaves a finished surface and registers fine tissue details.

   Wax                               %flow at 37ºC

KorectaNo.l (extrahard)               3

Korecta No. 2(hard)                      80       

Korecta No.3 (soft)                       85       

Korecta No.4 (extra soft)              90

2. These  waxes  can  also  be  used  to  produce  a  muco-compressive  impression  of the edentulous saddles for a lower, free-end saddle partial denture. This is called as the Applegate technique.

      Korecta wax No.1 and 4 are no longer available which were originally used for making edentulous impressions. However necessary waxes like extra soft No. 4 -orange are available again and can be used.

Technique

      The wax is melted in a water bath   before being applied to the area of the impression that is faulty or to the impression tray. The impression tray is then returned to the mouth and should be reseated with firm finger pressure. The impression is left in the mouth for sufficient time to raise the wax to oral temperature so it will undergo plastic flow under pressure to record accurately the denture bearing area.

      The results obtained are good but the procedure is difficult. It is hard to stabilize the metal framework on the remnant cast and it becomes necessary to add special stabilizing extensions to the structure. It is also difficult to box and pour the second stage of the techniques and obtain a good union between both parts of the casts.

      Heartwell and Rahn plainly state that wax cannot be used. Henderson, McGivney and Castlebery state that polysulphide rubber or wax can be used for final impression of an edentulous area of a complete or partial denture. In some applications wax may have advantages over polysulphide rubber or other impression materials.

      Holmes J B found that an impression of the edentulous area of a removable partial denture made with fluid wax created the best stability when compared with other materials.

      Mc Cracken stated that in some instances placement of the tissue is necessary to obtain maximum support of mandibular dentures. He advocated the use of fluid wax for this purpose. This is particularly important for older patients with atrophied mandibular residual ridges who may have contra indications for implants or other surgical procedures.

      Levin, Jogleker and Sinkford used a resinous wax for border molding and Carlile used wax for correction of a functional impression.  However, in addition to Heartwell and Rahn, Phillips, Boucher, Renner and Clark do not mention wax as an impression material.

      Wrinkles do not appear when fluid wax is used as the flow properties of wax prevent the distortion. However, wax must flow with minimum pressure and must not overly displace tissue. An advantage of wax is that it may be used with patients in whom saliva is a problem (dementia, hypersialogenous sialorrhea  pancreatica)  and  in  whom  pharmacological  intervention  is contraindicated. Also wax impressions are corrected without redoing the procedure.

Other advantages include:

©  Low cost

©  Easy handling

©  No need for adhesive

©  Odorless

©  Clean material to work with

©  Absence of tissue reactions

Disadvantage:

©  Distortion while removing from the undercut if care is not taken.

PPS Technique

Fluid Wax Technique

©  The anterior and posterior in vibrating lines are marked as for the conventional techniques on the final wash impression.

©  Impressions made with zinc-oxide eugenol or plaster are preferred over the elastic impression materials as they set rigid, are slightly resilient and when reseated in the mouth under pressure, it may distort the relationship between the wax added to the posterior border and the rest of the denture bearing surface. Also, wax will not adhere to elastic materials. Hence, either the material in the seal area must be removed prior to the wax application or laboratory varnish must be applied to the elastic material in the seal area before the wax is placed.

Waxes which can be used are:

• IOWA Wax (white) developed by Dr. Earl S. Smith.

•  Korecta Wax No. 4(orange) developed by Dr. O.C. Applegate.

•  H-L physiologic paste (yellow-white) developed by Dr. C.S. Harkins.

•  Adaptol (green) developed by Nathan G. Kaye.

These waxes are designed to flow at mouth temperature.

Technique

©  The melted wax is painted onto the impression surface within the outline of the seal area.

©  The wax is applied slightly in excess of the estimated depth and allowed to cool to below mouth temperature to increase its consistency and make it more resistant to flow.

©  The impression is carried to the mouth and held in place under gentle pressure for four to six minutes to allow time for the material to flow.

©  Position of the head and tongue.

©  According to Nelson, the soft palate should be impressioned in its most functionally depressed position. The maximum depression (downward and forward position) of the soft palate will be recorded when the Frankfort plane (porion - orbitale) is 30 below the horizontal and the tongue is firmly positioned against the mandibular anterior teeth. The patient should not protrude the tongue beyond the approximated position of the incisal edges, as this will foreshorten the posterior border of the final impression. The head and tongue position translate the mandible anteriorly. The soft palate will then be passively brought downward and forward due to the indirect attachment of the soft palatal tissues to the body of the mandible and the insertion of the palatoglossus muscle into the side of the tongue. Flexion of the head also contributes to moving excess impression material and saliva out of the mouth, rather than progressing down the pharynx.

©  While maintaining the 30° flexion of the head and the anterior tongue position, the patient is asked to periodically rotate the head so that all the functional positions of the soft palate are recorded.

©  After 4-6 minutes, the impression tray is removed from the mouth and the wax examined for uniform contact throughout the PPS area.

      If tissue has been contacted, the wax will have a glossy appearance and if not contacted, it will have a dull surface.

      Addition of wax can be done, if deficient and excess can be trimmed with a hot scalpel if it protrudes from the end of the tray.

3. A technique of impression making whereby an accurate impression of the teeth and correct border extension of the ridges are obtained with the use of a single custom tray in conjunction with one impression material or with a combination of materials of different physical and/or chemical properties provides for seating the tray in a firm and positive manner- in the position that has been previously determined by a spacer.

      The mouth is divided into two zones that are to be registered successively and with one of them always serving as a support for the other. A problem with successive impressions of sections of a partially edentulous mouth is that on making the impression of the second zone, some of the material used slips under the impression of the first zone. To avoid this, it is suggested that before making the impression of zone II, the tray with the impression of zone I be first placed in the mouth, correctly located, and then the impression material for zone III injected through the tray by means of built-in-tubes.

Technique

©  Outline the extension of the custom tray on the diagnostic cast. Determine zone I and zone II of the future impression and mark their limits.

©  Block out undercuts in zone II.

©  Apply the spacer. If wax is being used, cover it with metallic foil to avoid its adhesion to the acrylic resin tray material. If a firmer support is desired, the spacer can be made of acrylic resin.

©  Prepare the custom tray and place the tubes that will permit the injection of the impression material for zone II. Provide adequate venting.

©  Try the tray in the mouth. Verify its easy placement and withdrawal and its correct extension, especially over the distal-extension ridge or ridges.

©  Remove the spacer from zone I and prepare tray with adhesives or perforations for retention of the selected impression material.

©  Make the impression of zone 1.

©  Remove the spacer from zone II and prepare the tray to retain the impression material. With a sharp knife, exactly delineate the impression of zone I because some of the impression material has probably spread over the spacer.

©  Replace the tray in the mouth, with the impression of zone I now acting as a support, and firmly keep it in position. Prepare an elastic impression material, load it in an adequate syringe, and inject it through the tubes embedded in the tray.

Advantages

©  Because a single tray is used, a stable spatial relation is assured between the impressions of the dentulous and edentulous segments. The technique offers an unlimited choice and combination of impression materials to obtain the best possible results.

©  Working with a support ensures that the tray will maintain a stable and predetermined position not only during try-in, but while the impression itself is made.

4. Impression wax can be applied with a brush in small quantities to 'fill' in areas of impression in which insufficient material has been used or in which an 'air blow' or crease has caused a defect.

5. Wax can be used as inter occlusal recording material.  However, because of its properties of having a high coefficient of thermal expansion and high resistance to closure, this material has been graded as most inaccurate among the interocclusal record materials.  Wax plus zinc oxide- eugenol paste results in an increase in a vertical dimension, which is attributed to the distortion of the wax material.

      Bite registration   wax   is   used   to   articulate   accurately   certain   models   of opposing quadrants/obtaining occlusal records e.g. Aluwax.

Technique  

      Aluwax is available in sheets and in arch form, or, two sheets of wax with a cloth from center. Ideally, a maxillary cast is obtained before the wafer to be used for registration is fabricated.  The model is placed on the Aluwax sheet or wafer. A line outlining the maxillary arch form of the teeth is inscribed on the sheet, with 3-4 mm excess left projecting facially, completely around the arch.   Excess that will touch the tuberosity or retro molar area should not be touched when centric registration is recorded. When the size and shape of the wafer desired is obtained, the wafer is reinforced first with Ash-No 7 metal.  A strip of metal approximately 4 inches long and 1½inch width is cut and placed so that center of the wafer between the two pieces of the doubled metal.  The metal is then luted to the wafer with sticky wax on the mandibular and maxillary sides.  The metal should cover as much as of the palatal and tongue areas as possible but should not extend anteriorly past the medial of the mandibular first bicuspid.  Sufficient space must also be left for occlusal surfaces of the posterior teeth.

      After the metal has been luted to the wafer, the anterior portion is reinforced.  Place the wafer over the occlusal and incisal surfaces of the mandibular cast and inscribe a mark at the distal of each cuspid.  Place the wafer on a sheet of base plate wax and inscribe the outline of the anterior portion of the wafer to the distal marks of the cuspids on the base plate wax.  The piece of base plate wax is luted to the mandibular side of the wafer with a hot spatula. The wafer is prevented from bending by the metal, in to the palatal area by the tongue and mandibular anterior teeth during closure.

      The maxillary study cast is moistened or lubricated with vaseline.  The wafer is warmed lightly on a flame or water bath for about 15 seconds. The maxillary side of the wafer is placed on the moistened cast.  With light finger pressure, the wafer is keyed to the cusp tips and incisal edges of the cast. It is then chilled with water/air.

      The wafer is then tried in to the mouth.  The keyed portion of the wafer is placed on the maxillary teeth, held lightly and secured with thumb and finger.  The mandible is manipulated to the terminal hinge position and hinged upwards until the mandibular anterior teeth lightly touch the base plate    reinforcement. Patient is instructed to relax and not to close or bite.   Wafer is checked for fit and made certain that it does not impinge on palatal tissues, retro molar pad area and tuberosity area. Posteriorly, it should extend 1-2 mm distal to the posterior teeth.

      Strips of Tenax wax are luted to the mandibular side of the wafer with a hot spatula.  It extends from the posterior border of the wafer to the edge of the base plate wax reinforcement. The keyed wafer is again placed and the mandible guided into closure.  The mandibular teeth should touch evenly and without pressure.  When the fit is verified and perfected, centric relation position is recorded. The completed wafer is lightly warmed; keyed side placed on maxillary teeth and wafer is supported lightly.  The mandible is manipulated into most retruded and hinged position. The mandible is guided and closed into centric relation, imprinting just the cusp tips of posterior teeth in to Tenax wax.  The mandibular teeth will slightly indent the base plate wax.  As the mandibular teeth close the wafer, it should be left loose to avoid maxillary teeth from imprinting too deeply into the recording wafer.

      The wafer is chilled and removed in a snap to avoid distortion with the wafer still on the mandibular teeth the mandible is guided back into centric closure.  The wafer is then snapped inwards the mandible, removed in one motion and chilled.

      The centric relation record is verified by rechecking.  For greater accuracy, a correction wash is taken with zinc oxide eugenol paste, mixed and placed into the cusp indentation on each side of the wafer. Centric relation record can be easily recorded with aluwax and impression paste in cases with edentulous posterior regions that do not have posterior stops. The wafer is then removed and placed on the cast.  A small pillar or post of wax width ¼ in diameter formed of boxing wax is placed over each I^st and 2^nd molar edentulous area and luted to aluwax wafer with hot spatula.  The wax pillars are shortened with a hot spatula until approximately ¼ inch of space is created between the ridge and end of pillar extending to the wafer.

      The maxillary and mandibular casts are fitted into the Aluwax wafer, held and distance is estimated.  The wafer with wax pillars is checked in the mouth.  A wash impression is taken by placing a mound of paste on each wax pillar.  The centric relation is recorded with wash and a static impression of edentulous ridge areas.  The record is chilled and with a sharp scalpel, carefully trimmed such that impression area of about 4 mm square is present.

6. Wax can be used to make functional wax patterns.

Certain problems of occlusal harmony lend themselves to solution by the use of the functional wax pattern taken in the mouth immediately following cavity preparation.  The manner of gaining this functional pattern is not unlike the initial steps in forming the direct wax pattern. The particular situation may or may not indicate the use of a matrix band and retainer. Should a matrix be indicated in order to confine the wax, it is trimmed and festooned so that soft tissues are not traumatized nor the movements of functional occlusion restricted.   Thoroughly conditioned wax is placed in the matrix band,            the band seated under a heavy sustained force for about ten seconds.  Centric occlusion and lateral excursions are then initiated.  Wax may be added to any occlusal area if under contouring is suspected. Finally the matrix band and retainer, which have been slightly lubricated with petroleum jelly, are removed and the cervical excess is trimmed.  Following removal, the pattern is subsequently refined and the margins are perfected upon the complete die. The result should be excellent harmony of this casting with the occlusal function.

References:

Anusavice, Philipps’: Science of dental materials, 11^th edition, Elsevier.

Craig RG, Powers JH: Restorative dental materials, 11^th edition, Mosby.

William J. O’ brien:  Dental materials and their selection, 2nd edition.

McCabe JF, Walls AWG: Applied dental materials, 8^th edition, Blackwell Science.