Different designs of

bridges

Yasmin Mohammed Group E2

Fixed Partial Dentures

Fixed partial dentures (FPDs) are "dental prostheses that are luted, screwed, or mechanically attached or otherwise securely retained to natural teeth, tooth roots, and/or dental implant abutments."1 During the past decades, many types of FPDs or "bridges" have been used to replace missing teeth. With the introduction and widespread use of osseointegrated implants, many missing teeth are now being replaced in this manner rather than with FPDs. Dental bridges can, of course, still be used successfully, and this article will briefly review the many methods of bridge construction and relate them to their applicability and current acceptance of the practicing dentist and the treated patient. These will include: cast-gold, stress-broken bridges; resin-bonded, etched retainers; porcelain-fused-to-metal (PFM) bridges; and all-ceramic bridges, including zirconia.

THE BRIDGE CONSISTS OF :

1. Retainer : it is the part of the bridge which is cemented to the abutment

teeth .it could be full metal ,full veneer with facing , partial veneer ,

post crown or inlay … etc

2.Pontic : Is the part of the bridge which represent the missing tooth and it connected to the retainer by a connector

3. connector : is the part of the bridge which connect the pontic and the retainer , it maybe rigid (solid joint ) or movable joint (precision attachment , key and key way )

4. abutment : Is the natural tooth which support the bridge and on which the retainer is cemented (tooth or root )

.

Bridge compnents

Types of connectors : Connectors are those parts of a fixed partial denture (FPD) or splint that join the individual retainers andpontics together. Usually this is accomplished with rigid connectors , although nonrigid connectors are used occasionally. The latter are usually indicated when it is impossible to prepare a common path of insertion for the abutment preparations for an FPD.

The main types

•1 .Rigid

•2 .non rigid •_tenon mortise conectors

•_loop connectors •_split pontic connectors

•_cross pin and wing connectors

Rigid connectors

Rigid connections in metal can be made by casting, soldering, or welding. Cast connectors are shaped in wax as part of a multiunit wax pattern. Cast connectors are convenient and minimize the number of steps involved in the laboratory fabrication. However, the fit of the individual retainers may be adversely affected because distortion more easily results when a multiunit wax pattern is removed from the die system. Soldered connectors involve the use of an intermediate metal alloy whose melting temperature is lower than that of the parent metal .The parts being joined are not melted during soldering but must be thoroughly wettable by liquefied solder.' Dirt or surface oxides on the connector surfaces can reduce wetting and impede successful soldering; for example, the solder may melt but will not flow into the soldering gap. Welding is another method of rigidly joining metal parts. Here the connection is created by melting adjacent surfaces with heat or pressure. A filler

metal whose melting temperature is about the same as that of the parent metal can be used during welding. In industrial metalworking, a distinction is made between soldering, in which the filler metal has a melting point below 450° C (842° F), and brazing, in which the filler has a melting point above 450° C.2 Rigid connections in dentistry are generally fabricated above 450° C, but the process has almost always been referred to in the dental literature as soldering.However, a proposed international standard uses the term brazing. With time the latter term may become more generally accepted. In this text, however,the term soldering will be used.

NONRIGID CONNECTORSNonrigid connectors are indicated when it is not possible to prepare two abutments for an FPD with a common path of placement. Segmenting the design of large, complex FPDs into shorter components that are easier to

replace or repair individually is advisable. This can be helpful if there is uncertainty about an abutment's prognosis. If the abutment fails, only a portion of the FPD may need to be remade. In the mandibular arch, nonrigid connectors are indicated when a complex FPD consists of anterior and posterior segments. During themandibular opening and closing stroke, th mandible flexesmediolaterally.3 Rigid fixed partial dentures have been shown to inhibit mandibular flexure, and extensive splints have been shown to flex during forced opening. The associated stresses can cause dislodgment of complex FPDs. Segmenting complex mandibular FPDs can minimize this risk Nonrigid connectors are generated through incorporation of prefabricated inserts in the wax patternor through custom milling procedures after the first casting has been obtained.

The second part is then custom-fitted to the milled retainer and cast. They are often made with prefabricated plastic patterns. The retainers are then cast separately and fitted to each other in metal.

connector design The size, shape, and position of connectors all influence

the success of the prosthesis. Connectors must be sufficiently large to prevent distortion or fracture during function but not too large; otherwise, they will

interfere with effective plaque control and contribute to periodontal breakdown over time. Adequate access (i.e., embrasure space) must be available for oral hygiene aids cervical to the connector. If a connector is too large incisocervically, hygiene is impeded, and over time periodontal failure will occur . For esthetic FPDs, a large connector or inappropriate shaping of the individual retainers

may result in display of the metal connector, which may compromise the appearance of the restoration and lead to patient dissatisfaction In addition to being highly polished, the tissue surface of connectors is curved faciolingually to facilitate cleansing. Mesiodistally, it is shaped to create a smooth transition from one FPD component to the next. A properly shaped connector has a configuration similar to a meniscus

formed between the two parts of the prosthesis. In a buccolingual cross section, most connectors have a somewhat elliptical shape. Such an elliptical

connector is strongest if the major axis of the ellipse parallels the direction of the applied force. Unfortunately, because of anatomic considerations, this can not always be achieved. In fact, due to space constraints, most connectors have their greatest dimension perpendicular to the direction of applied force, which tends to result in a weaker connector. For ease of plaque control, the connectors should occupy the normal anatomic interproximal contact areas because encroaching on the buccal, gingival, or lingual embrasure restricts access. However, to improve

appearance without significantly affecting

plaque control, anterior connectors are normally placed toward the lingual.

depicts typical locations for connectors on selected teeth. Pulp size and clinical crown height can be limiting factors in the design of nonrigid connectors. Most prefabricated patterns require the preparation of a fairly sizable box. Fhis allows incorporation of the mortise in the cast restoration without overcontouring

of the interproximal emergence profile.

Short clinical crowns do not provide adequate occlusocervical space to ensure adequate strength. Most manufacturers recommend 3 to 4 mm of vertical height

RIGID CONNECTORS

Rigid connectors must be shaped and incorporated into the wax pattern after the individual retainers and pontics have been completed to final contour but before reflowing of the margins for investing

Cast Connectors. Connectors to be cast are also waxed on the master cast before reflowing and investing of the pattern. The presence of a cast connector makes the latter somewhat more awkward. Access to the proximal margin is impeded, and the

pattern cannot be held proximally during removal from the die. Restricting cast connectors to complete coverage restorations is therefore advisable, which

can be gripped buccolingually. Partial-coverage wax patterns are easily distorted when they are part of a single-cast FPD. One-piece castings often appear to

simplify fabrication but tend to create more problems than do soldered connectors, especially as pattern

complexity increases.

Soldered Connectors. As with cast connectors, connectors to be soldered are waxed to final shape but are then sectioned with a thin ribbon saw

therefore, when the components are

cast, the surfaces to be joined will be flat, parallel, and a controlled distance apart. This allows accurate soldering with a minimum of distortion . Molten solder will flow toward the location where

the temperature is highest. In metal, the two flat surfaces previously created in wax retain heat, ensuring that the highest temperature is in the connector

area.

Soldering Gap Width. As gap width increases, soldering accuracy decreases. Extremely small gap widths can prevent proper solder flow and lead to an incomplete or weak joint.' An even soldering gap of about 0.25 mm is recommended. If a connector

area has an uneven soldering gap width,

obtaining a connector of adequate cross-sectional dimension without resulting distortion is more difficult.

Loop Connectors . Although they are

rarely used, loop connectors are sometimes required when an existing diastema is to be maintained in a planned fixed prosthesis. The connector consists

of a loop on the lingual aspect of the prosthesis that connects adjacent retainers and/or pontics. The loop may be cast from sprue wax that is circular in

cross section or shaped from a platinum-goldpalladium (Pt-Au-Pd) alloy wire. Meticulous design is important so that plaque control will not be impeded.

NONRIGID CONNECTORS

The design of nonrigid connectors that are incorporated in the wax pattern stage consists of a mortise (also referred to as the female component) prepared

within the contours of the retainer and a tenon (male) attached to the pontic The mortise is usually placed on the distal aspect of the anterior retainer. Accurate alignment of the dovetail or cylindrically shaped mortise is critical; it must

parallel the path of withdrawal of the distal retainer. Paralleling is normally accomplished with a dental surveyor. When aligning the cast, the path of placement of the retainer that will be

contiguous with the tenon is identified The mortise in the other retainer is then shaped so its path of insertion permits concurrent seating of the tenon and its corresponding retainer.

The mortise can be prepared freehand in the wax pattern or with a precision milling machine. Another approach is to use prefabricated plastic components for the mortise and tenon of a nonrigid connector. As an alternative, a special mandrel can be embedded in the wax pattern and the abutment retainer can be

cast, with refinement of the female component as necessary; the male key is then fabricated of autopolymerizing

acrylic resin and attached to the

pontic.

Tenon mortise

Loop connector

split pontic

Cross pin and wing connector

Crown_ root ratioIs the measurement of the length of tooth occlusal to the alveolar crest of the bone compare with the length of root embedded in the bone . The ideal crown root ratio for a tooth to be used as a bridge abutment is 1:2 , and the minimum ratio acceptable for prospective abutment under normal circumstances is 1:1 .

•

• The root surface area (pericemental area )of the abutment teeth should be more or at least equal to the root suface area (pericemental area )of the missing teeth being replace .

Types of bridges• There are 4 main type of bridges

• 1. fixed _ fixed bridge

• 2. fixed movable bridge

• 3.cantilever bridge

4.resin bonded bridge (conservative bridge )

Fixed _fixed bridge

In this type the pontic is attached to the retainers (mesial and distal)by rigid connector(solid joint) so they should have one path of insertion . This is the most commonly used FPD

.

Advantages

• 1. maximum retention and support .

• 2. abutment teeth are splinted together .

• 3. the design is most practical for larger bridges .

disadvantages

1.require preparation to be parallel

2. All the retainers are major retainers and require extensive ,destructive preparation of the abutment teeth .

3.Has to be cemented in one piece . Fixed movable bridge

• In this type the pontic is attached to one distal major retainer (distal end of the pontic )by fixed connector while the other end is attached to the minor retainer (in front of pontic )by movable joint .it s indicated in case of

drifted abutment teeth and difficulty

to obtaining parallel abutments. Advantages:

• 1. preparations do not need to be parallel to each other .

• 2. more conservative of tooth tissue because preparations for minor retainers are less destructive .

• 3. parts can be cemented separately .

Disadvantages

• 1.more complicated to construct in laboratory than fixed fixed bridge .

• 2.difficult to make temporary bridge .

Cantilever bridgen areas of your mouth that are under less stress, such as your front teeth, a cantilever dental bridge may be used. A cantilever dental bridge is when the “false tooth” is only supported on one side. The result is just as aesthetic but has the benefit that fewer teeth have to be treated.

resin bonded bridge

A resin-bonded fixed partial denture is a prosthetic construction which can replace I or several teeth in an occlusal system and which comprises a pontic element which is adhesively attached to 1 or more abutment teeth. To compensate for the limited shear strength of the adhesive

layer, the Jixed partial denture is occlusally supported by the abutment(s). A direct resin-bonded fixed partial denture is made of composite, reinforced or not by a frame of flexible metal or fiber material. For an indirect resin-bonded fixed partial denture, a metal, fibre-reinforced composite or ceramic substructure is fabricated in a dental laboratory. The basic principle of a resin-bonded fixed partial denture is minimal invasiveness. However, a restoration in an abutment tooth requires a certain occlusal space which is realized by tooth preparation. Resistance preparations may be performed to improve the longevity of resin-bonded fixed partial dentures. Both financially and biologically, a resin-bonded bridge is a cost-effective prosthetic construction. The longevity is limited, but when the construction fails the negative consequences for the

abutments are generally limited, which leaves open several types of other treatments.

Spring Cantilever It is a tooth and tissue supported bridge. A pontic is supported at some distance from the retainer (). It is a type of cantilever bridge. Strong retention is required as for all cantilever bridges and double abutments are usually necessary. The retention of a spring bridge is severely tested when force is exerted in apicoincisal direction as seen on biting sticky food as the retainer is subjected to detrimental stresses. To avoid this, two retainers in adjacent teeth are used together to give added strength. This bridge design is used while replacing anteriors with diastema or in case of existing existing endodontically treated tooth posteriorly. This design cannot be

used in lower arch because of lack of suitable tissue support. The bar should follow the natural contours of the rugae in the palate, so that it lies obscurely in the valleys and its lateral margins do not represent an attraction to the tongue. The cross – section of the bar should be a flat, oval or a rounded – T shape[8]. The model should be lightly scraped to ensure firm seating on the soft tissue and minimize food trapping. High platinized gold or spring metal is used. Class IV casting gold is ideal. The Achilles heel with this design is the junction of retainer and the bar and the leverage on the abutment.

Resin retained bridges 1. Bonded pontic2. Rochette bridge 3. Viginia bridge 4. Maryland bridge 5. Adhesive bridge

1. Bonded pontic Introduced by Ibsen and Portnoy in 1973, these are the earliest resin retained prosthesis[9]. They are the resin tooth or patient’s natural tooth bonded directly to the etched enamel. These are meant for short term replacements. The limiting factor was the weakness of the composite resin connector.

2.Rochette bridge CAST PERFORATED RESIN-RETAINED FPDS (MECHANICAL RETENTIO In 1973, Rochette introduced the concept of bondingmetal to teeth using flared perforations of the metal casting to provide mechanical retention. He used the technique principally for periodontalsplinting but also included pontics in his design. Howe and Denehy recognized

the metal framework's improved retention (as compared to bondedpontics) and began using FPDS with cast-perforated metal retainers bonded to abutment teeth and metal-ceramic pontics to replace missing anteriorteeth. Their design recommendation, extending the framework to cover a maximum area of the lingual surface, suggested little or no tooth preparation. Patient selection limited these FPDs to mandibular teeth or situations with an open occlusal relationship.The restorations were bonded with a heavily filled composite resin as a luting medium. This concept was expanded to replacement of posterior teeth by Livaditis. Perforated retainerswere used to increase resistance and retention. The castings were extended interproximally into the edentulous areas and onto occlusal surfaces. Thedesign included a defined occlusogingival path of insertion by tooth modification, which involved

lowering the proximal and lingual height of contour of the enamel on the abutment teeth. These restorationswere placed in normal occlusion; many have survived and have been seen on recall for up to 13 years Despite this success, the perforation technique presents the following limitations:

•Weakening of the metal retainer by the perforation Exposure to wear of the resin at the perforations Limited adhesion of the metal provided by thePerforations Clinical results with the perforated technique were followed for 15 years in a study at the University of Iowa.'-' The results from this well-controlled study suggest that for anterior fixed partial dentures, 63% of the perforated retainer prostheses fail in about 130 months.'6 Later data'-'

indicate that 50% fail in about 110

months.

ETCHED CAST RESIN-RETAINED FPDS

(MICROMECHANICAL RETENTION-"MARYLAND

BRIDGE")

Based on the work of Tanaka et al" on pitting corrosion for retaining acrylic resin facings and the metal etching studies of Dunn and Reisbick," Thompson and Livaditis at the University of Maryland

developed a technique for the electrolytic

etching of Ni-Cr and Cr-Co alloys. Etched castm retainers have definite advantages

over the castperforated restorations: • Retention is improved because the resin-toetched metal bond can be substantially

stronger than the resin-to-etched enamel.

The retainers can be thinner and still resist

flexing. The oral surface of the cast retainers is highly polished and resists plaque accumulation. During the course of this work, the need for a composite resin with a low film thickness for luting the casting became apparent. This led to the first generation of resin cements, which permitted micromechanical bonding into the undercuts in the metal casting created by etching while providing adequate strength and allowing complete seating of

the cast retainers. Comspan,* the first of these cements, was moderately filled (60% by weight) with a film thickness of approximately 20 um.21 Such cements are not chemically adhesive to the metal.

Electrolytic etching of base metal alloys proved to be critically dependent on the base metal alloy and attention to detail in the laboratory. Initial etching methods were developed for a Ni-Cr alloy* and a Ni-Cr-Mo-Al-Be alloy. These methods were followed by simplified techniques, chemical etching, 23 or attempts at gel etching. 24 They all yield similar results, provided the technique is optimized for a specific alloy. Proper etching requires evaluation of the alloy surface with a scanning electron microscope. The degree of undercut created by this etching process can be seen in Figure 26-3. Lack of attention to detail can result in electropolishing or surface contamination .26 With time, both severely degrade bond strengths in a moist environment.

Highly variable results were reported for dental laboratories when etching the same alloy . Etchingand bonding techniques were adopted based on

bond strength testing of specimens only subjected to 24 hours or 7 days of water exposure. When resin-tometal test specimens were aged for 6 months in water and then thermally stressed by 10,000 or more thermal cycles, large reductions in bond strengths were recorded . Therefore, data from specimens that have not been aged and thermally stressed should be viewed skeptically. Even particle abrasion

will provide initially high resin-to-metal bonds, which can degrade to almost zero with time . Well-researched and tested resin systems for direct adhesion to metal surfaces have now completely supplanted

metal etching as retention mechanisms.

MACROSCOPIC MECHANICAL RETENTION RESINRETAINED FPDS ("VIRGINIA BRIDGE")

As a result of concerns about etching base metal and the desire to use alternative alloys, several methods have been developed to provide visible macroscopic

mechanical undercuts on the inner surface of FPD retainers. The first was developed at the Virginia Commonwealth University School of Dentistry and is known as the "Virginia Bridge. It involves a "lost

salt crystal" technique. On the working cast, the abutments are coated with a model spray, and a lubricant is then applied. Within the outlines of the retainers, specially sized salt crystals* (150 to 250 um) are sprinkled over the surface in a uniform monolayer, leaving a 0.5-mm border without crystals at the periphery of the pattern. This is followed by application

of a resin pattern. After pattern investment, the salt crystals are dissolved from the surface of the pattern. Adequate bond strengths are possible with this method, but the thickness of the casting must be increased to allow for the undercut thickness. Although no long-term results have been reported with this technique, it does permit the use of almost any metal-ceramic alloy An alternative technique for macroscopic retention is the use of a cast mesh pattern

on the internal surface of the retainers. The mesh, usually made of nylon,* should be adapted to the lingual and proximal surfaces of the abutments. The mesh is

then covered by wax or resin; this must be done carefully to prevent occluding the mesh with the pattern material. Investing and casting then followThis method is technique sensitive but

can provide adequate retention with a resulting thick lingual casting. The cast mesh and the lost salt crystal method have been supplanted by direct adhesion

with resin, which is possible for most casting alloys if the correct surface treatment is provided .

Cast mesh fixed bridge

Virginia bridge

Adhesive Bridges 5.

Inspite of electrochemical etching being very popular in America, alloy etching and macroscopic retention is obsolete in Japan since 1980s . As a result of extensive research chemically active adhesive cements were developed for direct bonding to metal . These cements rely on chemical adhesion to the metal and not on microretention in the surface of the metal for bond strength. Etching was no longer necessary[19] Adhesive bridge shows chemical bonding between the metal and the resin luting agent. Direct bonding involves the chair side and lab systems. Metabond is first of these resin systems. It is based on formulation of Methylmetha acrylate (MMA) polymer powder and MMA liquid modified with adhesion

promoter 4- META (4-methacryloxyethyl trimellitate anhydride). Unique tributyl borane catalyst is added to liquid. On base metal alloys, Superbond has highest initial bond strengths of any adhesive resin systems[20]. But, it gives weak bond with high gold alloys and the bond shows hydrolytic instability. Introduction of Metabond was followed by Panavia which can be used both with high gold ( after tin plating) and base metal alloy. Tin plating can be done in lab, chair side or intraorally. Intraoral tin plating is done by tin amide solution. Adhesive monomer used in Panavia is MDP(10- methacrylolyloxydecyl dihydrogen phosphate). The phosphate end reacts with Calcium of tooth and with the metal oxide. Bond strength to etched base metal is greatly exceeded to that of tooth. Lab system for adhesive bonding have been developed. Silicoater Classical ( Tiller et. al , 1984) is based on the need for an intermediate layer containing silica as this provides sufficient bonding of the resin via a silane bonding agent. New version of Silicoater MD was introduced in 1998. This uses a special oven that burns a

chrome endowing silica layer onto the surface. Pyrosil Pen Technology (1998) is the chairside version of silicoater. Rocatec System is a novel acrylic and metal bonding system which uses a tribochemical and thermal embedding of a silica layer by means of sand blasting on the metal surface[22]. Metal is thus rendered more reactive to resin via silane. It is unfortunate that this treatment modality is not very popular amongst dentist but if the case selection is proper it offers outstanding conservatism with tremendous bond strength .