Crowm And Bridgework .


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Fixed/fixed bridge :
definition
A prosthesis where the artificial tooth or teeth tpontic) is supported rigidly on either side by one or more abutment teeth (Figs 1 & 2).
inaicadions
Where missing units are bound by abutment teeth which are capable of supporting the functional I ead of the missing teeth.
Advantages
A fixed/fixed bridge is a strong and retentive restoration for replacing missing teeth. It can be used for single or multiple missing units with the abutment teeth splinted together in the latter case. This can be seen as an advantage as well as a possible disadvantage of this technique as the design of linked abutment units must be considered carefully to allow access for oral hygiene measures.
Di dvantages.
• This technique requires the preparation of the abutment teeth to be parallel to each otherwhich may mean: overpreparation of the teeth, structural weakening of the tooth and endangering the pulpal tissues-
• Teeth do move independently in function and this can lead to cementation failure of a fixedlfixed bridge.
Procedure
The abutment teeth are prepared with parallel taper ( Fig. 3). This can be particularly arduous if the teeth are widely separated, and often means overtapered preparations which are less retentive.
Fig. 1 Fixed/fixed porcelain bridge.
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Fig. 2 Fixed/fixed gold bridge with sanitary pontic design.
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fig. 3 Tooth preparation for crown or bridgework
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Fixed/movable bridge:

Definition
A prosthesis where the artificial tooth or teeth is rigidly supported on one side, usually the distal end by one or more abutment teeth (Figs 4 & 5). One abutment will contain an intracoronal attachment which allows a small degree of movement between the rigid component and the other abutment tooth or teeth (Fig. 6).
Indications
Where abutment teeth are tilted or rotated in relation to each other and the preparation needed to make them parallel would be highly destructive to tooth structure. The construction of large units of bridgework means that the complex task of parallel preparations is increased. The use of
movable joints allows for the separation of large units into several smaller more manageable sections.
Advantages
Divergent abutments can be used in this technique and are more conservative of tooth structure. Such a bridge allows minor movements of abutments in relation to each other. The parts can be cemented separately.
Disadvantages
• This bridge is more demanding of laboratory ti me leading to increased expense.
• The construction of a temporary bridge is more difficult due to the tilting of the abutment teeth.
Procedure
Each abutment tooth can be prepared independently although special consideration should be given to the 
placement of the movable j oint as this is preferably placed intracoronally.


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Fig. 4 Fixed/movable bridge. Fig.5 Fixed/movable bridge.



Fig.6 Occlusal view showing intracoronal attachment for fixed/movable design.

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Cantilever bridge
Definition
A prosthesis where the artificial tooth or teeth are
supported on one side only by one or more
abutment teeth (Figs 7, 8 & 9).
Indications
Where the abutment tooth can carry the occlusal l oad of the artificial tooth and where the occlusion is protected against potentially damaging rotational forces.
Advantages
· This bridge design is generally the most conservative design in terms of tooth preparation ( excluding resin retained designs).
· There is no problem of paralleling abutment teeth during preparation.
Disadvantages
· The size of pontic is limited to one or two units as leverage forces on the neighbouring abutments can be potentially damaging.
· I f a contact point from the pontic to the neighbouring tooth is not placed then potentially rotational forces could be destructive to this type of design.
Procedure
A single tooth preparation is carried out on to the abutment tooth in a similar manner to a conventional crown preparation.



Fig. 7 Anterior cantilever bridge.
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Fig. 8 Posterior cantilever bridge

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Fig. 9 Cantilever bridge: occlusal view

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Spring cantilever bridge
Definition
A prosthesis where the artificial tooth is supported by a connecting bar to the abutment tooth or teeth. This connecting arm can be of various lengths depending on the position in the arch of the abutment teeth in relation to the missing unit/s.
The arm follows the contour of the palate to allow for patient adaptation (Figs 10, 11 & 12).
Indications
This type of restoration is placed where a patient has sound anterior teeth with one missing unit or where diastemas are present around an anterior missing unit.
Advantages
· The pontic does not require support from less favourable adjacent teeth.
· Anterior teeth that are sound and might normally be prepared to support a missing unit do not need to be involved. Posterior teeth are more commonly restored than anterior teeth and therefore their use as abutments is less destructive of sound tooth substance. Diastemas can be preserved.
Disadvantages
· Some patients find the connecting palate uncomfortable.
· The bar may distort if it is too thin or the occlusion on the pontic is excessive.
Procedure
Posterior teeth are prepared for the support of theanterior missing unit. Commonly, the connecting bar does not carry the anterior unit but a core onto which the anterior unit is cemented. This means it can be replaced if the colour needs modification without removing the posterior retainer.


Fig. 10 Laboratory die with spring cantilever bridge.
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Fig. 11 Spring cantilever

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Fig. 12 Linked abutments for spring cantilever bridge.
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Porcelain jacket and porcelain bonded crown
Definitions
· A porcelain jacket crown (PJC) consists of a layer of porcelain which covers the entire crown of the tooth (Fig. 13).
· A porcelain bonded crown (PBC) is one which is constructed in metal alloy with porcelain fused to either all or most of its surfaces (Fig. 13).
Indications
· PJC: When the anterior teeth are heavily restored with composite restorations or where tooth material has been lost as a result of trauma.
· PBC: In situations where a stronger restoration is required, such as the presence of minimal i nterocclusal clearance (Fig. 14).
Advantages
· PJC: Improved appearance. The shade and translucency of adjacent teeth can be recreated i n porcelain work.
· PBC: The strength of this type of restoration is its major advantage.
Disadvantages
· PJC: The brittleness of all-porcelain units and the necessity to remove at least 1 mm of tooth substance are the two main disadvantages of this crown.
· PBC: The necessity to remove at least 1.5 mm of tooth substance buccally to allow for the placement of the alloy and porcelain layers. Unsightliness can result from the difficulty in rendering opaque the alloy layer (Fig. 15).



Fig. 13 PJC on upper right central and PBC on upper left central
and lateral.

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Fig. 14 Post and core preparations for PBCs
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Fig. 15 PBCs in place
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Gold veneer crown
Definition
A gold veneer or gold shell crown (GSC) is a full veneer crown made of a gold alloy (Figs 16, 17& 18).
Indications
For posterior restorations where appearance is not a consideration. In some cultures a full gold veneer crown on an anterior tooth may denote a sign of wealth or be used as a decorative restoration.
Advantages
• Gold can be cast accurately in very thin sections, and can resist repeated loading without distortion.
• Minimal tooth reduction is required when compared to a PBC.
• The incorporation of retention areas for a partial denture, such as rest seats or undercuts, is easily managed with this type of restoration.
• Adhesive gold restorations are now possible by heat treating certain gold alloys to allow adhesive technology to bond the gold to natural tooth structure.
Disadvantages
· There are few if any disadvantages of such a restoration other than cost.
• Some people would find it unsightly and its use i s therefore mainly limited to posterior units.





Fig. 16 Gold crown and simple inlay
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Fig. 17 Gold crown and complex inlay

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Fig. 18 Full veneer gold crowns



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Resin bonded bridge (Maryland

Definition
A prosthesis constructed of a cast metal framework which is luted to the enamel of an abutment tooth by an adhesive composite resin (Figs 19 & 20).
Indications
To replace anterior teeth where the abutment teeth are unrestored and the use of conventional bridgework would cause unnecessary tooth destruction.
Advantages
Minimal preparation of the abutment tooth is required and is all within enamel, as the retainer isattached to the abutment tooth using acid-etchadhesive techniques.
Disadvantages
• These restorations can debond if good isolationis not obtained at the time of cementation.
• If insufficient enamel is present then this type of restoration is unsuitable.
• The restoration is contraindicated where there is evidence of severe tooth wear, parafunction or i nsufficient interocclusal clearance.
Procedure
Cantilevered units are advised (Fig. 21) because if the 'wing retainer' debonds then the bridge will be displaced. Double abutments result in one side debonding but the remaining fixture staying firm. This may lead to caries developing under the debonded retainer. The teeth are prepared with slots or grooves for additional mechanical retention and full lingual coverage to maximise the adhesive bond. The use of a rubber-dam is required to provide the isolation necessary for adhesive bonding techniques.



Fig. 19 Conventional Maryland-upper arch
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Fig. 20 Conventional Maryland-lower arch
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Fig. 21 Conventional cantilevered Maryland
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Rochette bridge
Definition
An older design of bridge similar to a Maryland bridge, in that it derives its attachment to the abutment tooth using adhesive technology ( Figs 22 & 23). The major difference is that the adhesive bond to the metal wing support is mechanical, unlike the chemical and micromechanical adhesion used with a Maryland bridge.
Indications
As for a Maryland bridge; however its use is limited and is most often a temporary solution to a failed Maryland bridge
Advantages
• The retention of the composite resin to the metal alloy is mechanical, by counter sunk holes in the retainer. The risk of debonding at the metal/resin interface is dependent on the strength of the resin and not the bond.
• If the restoration does debond recementation is relatively straightforward
Disadvantages
The use of holes in the retainer requires a thicker cross-section of alloy for strength. This may lead to occlusal problems or may feel bulky to the patient
Comment
In the early days of Maryland bridgework debonding was common, initially due to the unpredictable nature of the bond between the alloy
and the resin. As a result many of the Maryland bridges were converted to a Rochette design by drilling holes in the retainer (Fig. 23). This often was a poor idea as the metal retainer in a Maryland was much thinner than its predecessor and the retainer was significantly weakened by this technique.





Fig. 22 Rochette bridge-occlusal view
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Fig.23 Rochette bridge-lingual view
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Porcelain veneers
Definition
A veneer is a thin tooth-shaped porcelain (acrylic or composite) facing cemented to the underlying tooth structure using a filled resin and acid-etch technique to mask discoloured or malformed teeth
Indications
To mask intrinsic staining or surface defects that result in discolouration of anterior teeth (Figs 24 & 25). To correct malformations of tooth shape, spacing (Figs 26 & 27) or tooth chipping due to trauma. Good oral hygiene is essential when considering this type of restoration. A diagnostic wax up to assess the aesthetic result is alsoadvisable.
Advantages
Minimal tooth preparation is required. They canprovide a superior aesthetic result to full porcelaincoverage, as in a PJC, as they allow for some natural tooth colour to show through if desired.
Disadvantages
I f a substantial amount of natural tooth structure has been lost then a PJC may offer a better alternative as the strength of these restorations is not great. Chipping and cracking of the porcelain because of the thin nature of this restoration can result. This type of restoration is not advised in patients who are bruxists
Procedure
As with all bonding techniques, this procedure is extremely technique sensitive and correct isolation at the time of placement of these restorations is essential.


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Fig. 24 Anterior teeth prepared for  veneers because of tetracycline staining

Fig. 25 Anterior teeth-veneers in place



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Fig. 26Spacing of anterior teeth. Fig. 27Correction with veneers






 Resin-bonded porcelain crowns
Definition
A resin-bonded porcelain crown is a thin section of procelain which encompasses the whole periphery of the tooth unlike a labial or palatal porcelain veneer.
Indications
Resin-bonded crowns are indicated for restoring damaged (Fig. 28) or unaesthetic anterior teeth, where a veneer would be inappropriate but a conventional porcelain jacket crown would be toodestructive of the remaining tissue.
Advantages
These crowns (Figs 29 & 30) require minimal preparation to the tooth. The crown is cemented using adhesive resin technology and, therefore,where diminutive crowns are present there is no advantage to crown lengthening to increase the retention form.
Disadvantages
The crown is thin and therefore cannot withstand high occlusal forces. The procedure like so many of the acid-etch techniques is extremely technique sensitive and success is dependent on bonding being carefully followed
Procedure
A reduction of between 0.5 and 0.75 mm should occur allowing adequate enamel to remain for bonding. The incisal region should be reduced by 1 mm to allow enough thickness of porcelain for strength in this region. Margins are preferably produced using a full chamfer and otherwise general crown preparation principles should apply.These include no undercuts or sharp line or point angles.





Fig.28 Erosion of anterior teeth
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Fig. 29 Placement of resin-bonded crowns

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Fig. 30 Resin-bonded crowns on lower incisors with lower
swinglock denture in place
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Guide surfaces/milled crowns
Definition
Two or more parallel surfaces on abutment teeth which limit the path of insertion of a denture. Guide surfaces may occur naturally on teeth or may require to be prepared in the tooth or within a
restoration such as an amalgam or gold veneer crown.
Advantages
• Increased stability by resisting displacement of the denture.
• Efficient reciprocation of a clasp arm.
• Prevention of clasp deformation during removal of the denture.
• Improvement of the appearance of saddle and tooth (anterior guide surfaces).
Disadvantages
The preparation of guide surfaces in the natural dentition will require the removal of tooth substance. However this disadvantage is overcome by the use of naturally occurring guide surfaces or the incorporation of restored teeth
Procedure
• Guide surfaces are produced by removing a minimal and uniform thickness of enamelusually not more than 0.5 mm-from around the tooth. It should extend vertically for about 3 mm and should be kept as far as possible from the gingival margin.
• The incorporation of a guide surface within a cast restoration may be prepared more accurately with a surveyor in the laboratory ( Figs 31, 32 & 33).






Fig. 31 Porcelain bonded milled crown
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Fig. 32 Close up of crown
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Fig. 33 Guide surface on gold crown
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Telescopic crowns
Definition
A restoration made in two parts: an inner sleeve of hard gold and an outer full crown which covers this inner unit.
Indications
• To overcome differences in the inclination of teeth.
• To provide a removable outer portion for the i nspection of the interdental areas or the abutment tooth itself.
• To splint neighbouring teeth.
Advantages
The abutment teeth do not require to be parallel and therefore the amount of tooth preparation required is reduced. Removal of the outer crown work is relatively straightforward and allows closermonitoring of the abutment units (Figs 34 & 35).
Disadvantages
• Because of the change of the emergence angle of the outer crown from the abutment tooth, particular attention must be paid to plaque removal around the margins.
• The technical stages involved make this more ti me consuming and therefore more costly thanconventional bridgework.
Procedure
• The abutment teeth are prepared usingconventional procedures and the impression isrecorded.
• The inner collars are constructed to allow for aparallel path of insertion for the outer crowns ( Figs 36 & 37).
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Fig. 203 Anterior view of telescopic Fig. 204 Telescopic crowns in place abutments




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Fig. 34 Telescopic abutments in place. Fig. 35 Telescopic superstructure i ncorporated within denture

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