Bridge (dentistry)

High Translucent zirconium fixed - fixed bridge built with VM9 vita porcelain and stained with luster paste. The upper first premolar is considered the pontic and the teeth prepared are abutments.

Conventional bridges are bridges that are supported by full coverage crowns, three-quarter crowns, post-retained crowns, onlays and inlays on the abutment teeth. In these types of bridges, the abutment teeth require preparation and reduction to support the prosthesis. Conventional bridges are named depending on the way the pontic (false teeth) is attached to the retainer.[5]

A fixed-fixed bridge refers to a pontic which is attached to a retainer at both sides of the space with only one path of insertion. This type of design has a rigid connector at each end which connects the abutment to the pontic. As the abutments are connected together rigidly it is critical that during tooth preparation the proximal surfaces of the abutment teeth must be prepared so that they are parallel to each other.[3]

A cantilever is a bridge where a pontic is attached to a retainer only at one side. The abutment tooth may be mesial or distal to the pontic.[3]

In a spring cantilever, the pontic and retainer are remote from each other and connected by a metal bar. Usually, a missing anterior tooth is replaced and supported by a posterior tooth. This design of bridge has been superseded.[3]

The pontic is firmly attached to a retainer at one end of the span (major retainer) and attached via a movable joint at the other end (minor retainer).

A major advantage of this type of bridge is that the movable joint can accommodate the angulation differences in the abutment teeth in long axis, which enables the path of insertion to be irrespective of the alignment of the abutment tooth.[5] This enables a more conservative approach as the abutments do not need to be prepared to be parallel to one another. Ideally the rigid connector should attach the pontic to the more distal abutment. The movable connector attaches the pontic to the mesial abutment, enabling this abutment tooth limited movement in a vertical direction.[3]

An alternative to the traditional bridge is the adhesive bridge (also called a Maryland bridge). An adhesive bridge utilises "wings" on the sides of the pontic which attach it to the abutment teeth. Abutment teeth require minor or no preparation. They are most often used when the abutment teeth are whole and sound (i.e., no crowns or major fillings).[4]

The incorporation of elements of different conventional bridge designs. A popular combination design is the use of a fixed-fixed design with a cantilever.[3]

Bridges that incorporate elements of both conventional and adhesive bridge designs.[3]

Appropriate case selection is important when considering the provision of fixed bridgework. Patient expectations should be discussed and a thorough patient history should be obtained. Replacement of missing teeth with fixed bridgework may not always be indicated and both patient factors alongside restorative factors should be considered before deciding if providing fixed bridgework is appropriate.[6] The survival rate of bridgework can be affected by the span of bridge needed, the proposed position of the bridge, and the size, shape, number and condition of planned abutment teeth.[7] Furthermore, any active disease including caries or periodontal disease should be treated and followed by a period of maintenance to ensure patient compliance in maintaining appropriate oral hygiene.[8][9]

Study models mounted on a semi-adjustable articulator using a facebow record are a useful aid to study occlusion prior to provision of a fixed prosthesis. They may also be used to practice planned tooth preparation. Subsequently, a diagnostic wax up can be provided to help the patient visualise the final prosthesis and to construct a silicone index. This index can be used to make a temporary prosthesis.[3][10]

Multiple factors influence the selection of appropriate abutment teeth. These include the size of potential abutment tooth, with larger teeth having an increased surface area preferable for retention, using teeth with a stable periodontal status, favourable tooth angulation, favourable tooth position, and an adequate crown-root ratio.[1][11]

Careful abutment selection is critical for the success of bridgework. The prosthesis must be capable of tolerating occlusal forces, which would normally be received by the missing tooth as well as its normal occlusal loading. These forces are transmitted to the abutment(s) via the prosthesis.[3]

Vital teeth are preferred to endodontically treated ones for bridge abutments. Endodontically treated teeth have lost a large amount of tooth structure, weakening them and making them less able to tolerate additional occlusal loading. Post crowns have been shown in some studies to have a higher failure rate.[3]

For resin bonded bridges abutment teeth should ideally be unrestored and have enough enamel to support the metal wing retainer. Additionally there must be sufficient space to accommodate the minimum connector width of 0.7mm and 2mm connector height. It is acceptable for the abutment to be minimally restored with small composite restorations provided they are sound. It is advised to replace old composite restorations prior to cementation to provide optimum bond strength via the oxide layer.[4]

Teeth with active disease such as caries or periodontal disease should not be used as abutments until the disease has been stabilised. Once stable periodontally compromised teeth may be used as abutments, depending on the crown to root ratio described below.[3]

Ante's law, states that the roots of abutment teeth must have a combined periodontal surface area in three dimensions that is more than that of the missing root structures of the teeth replaced with a bridge, is used in bridgework design. This law remains controversial in terms of supporting clinical evidence.[4]

Crown to root ratio is the distance from the occlusal/incisal surface of the tooth to the alveolar crest in relation to the length of root within the bone. The minimum ratio of crown to root is considered to be 1:1, although the most favourable is a crown:root of 2:3. As the proportion of tooth supported by bone decreases, the lever effect increases.[3][4]

Root configuration should be considered when selecting abutment(s). Divergent roots of posteriors provide increased support compared to converging, fused or conical roots. Roots that curve apically provide increased support compared to those which have a fixed taper.[4]

The number of abutments required depends on both the position of the tooth to be replaced and the length of the span. Cantilever designs utilising one abutment is the design of choice for replacing a single tooth anteriorly and can also be used posteriorly.[3]

Occlusion of the pontic with the opposing tooth should be assessed. This may determine which type of design is most appropriate and therefore how many abutments are required.[3] For resin bonded bridges the pontic should have light contact in intercuspal position (ICP) and no contact in lateral excursions.[4][10]

Torquing forces can occur when the pontic lies outside the interabutment axis line as the pontic acts as a lever arm. This is particularly applicable to long span bridges replacing multiple anteriors.[4]

Deflection varies directly with the cube of the length, and inversely with the cube of the occlusogingival thickness of the pontic. The longer the span, the more deflection occurs. The amount of deflection is 8 times greater when the length of the span increases to 2 pontics, and increases to 27 times greater with 3 pontics in comparison to a single pontic. It is likely that increased span length will result in the abutments being subjected to increased torquing forces. The thinner the pontic, the more deflection occurs. If the thickness of the pontic is reduced by 50% this causes an 8 times increase in deflection. Choosing pontics with increased occlusogingival dimension and using high yield strength alloys to construct the prosthesis will help reduce deflection.[4]

1. Replacement of a single tooth or a small spanning space.
2. Good oral health status and motivation of patient to maintain oral health.[10]
3. Periodontal status of remaining dentition at a stable and satisfactory level.[10]
4. Abutment teeth of good quality with minimal restorations and enough surface area and enamel present for adhesion.[11]
5. Splinting of periodontally compromised teeth to improve occlusal stability, comfort and decrease mobility. (Periodontally compromised teeth is also a contraindication).[11]
6. As a way of fixed retention after orthodontic treatment or extraction.[11]
7. Patient unsuitable for implants. This may be due to inadequate bone levels, expense or patient not wanting to receive implants.[11]

1. Size of saddle area too long.[12]
2. Patients with parafunction e.g. bruxism.[12]
3. Tooth mobility increases risk of de-bonding.[11]
4. Malaligned teeth resulting in poor aesthetics and common path of insertion.[11]
5. Abutment tooth quality inadequate for example may have a reduced surface area, reduced enamel or be heavily restored.[11]
6. Increased risk of caries due to increased difficulty in maintaining oral hygiene around the bridgework.[11]
7. Increased risk of loss of vitality.
8. Allergy to base metal alloys e.g. nickel [11]
9. Poor patient motivation. Active dental disease (caries, periodontal disease) and poor oral hygiene.[4]

Unaesthetic display of metal in a porcelain fused to metal restoration

Acrylic resin was the first veneering material used to help restore the aesthetics of crown and bridges, the aim was to maintain a similar colour to natural teeth by attaching it on the labial surface of metal crown / bridges, however, resin veneered dental prosthetics lacked stability and abrasion resistance.[16] Porcelain fused to metal (PFM) was then introduced; the porcelain is composed of two layers (one opaque to cover the metal substructure and another translucent to provide an enamel illusion). Still several researchers consider PFM the gold standard as it has been reported to have 95% success over a 10-year period, a reason why newer types of all-ceramic restorations are usually compared to PFM crowns / bridges to assess its success and durability.[21] However, PFM restorations may show a grey colour at the cervical margins of the tooth showing the metal substructure.

IPs Emax ceramics offer high aesthetic properties, that's why its use has been increasingly popular, however, there's insufficient evidence to determine the longevity of Emax in bridges; some reports found fair short-term survival, but unfavorable medium-term survival.[22] Failures of restorations were most reported in the posterior teeth region. IPs Emax is available as press ingots or as IPs Emax CAD-CAM system.[23] Emax use in crowns or bridges is not recommended for patients who suffer from bruxism.[24]

Zirconia is used in anterior, and posterior fixed bridges, also on implants. Zirconia is fabricated using the dental CAD-CAM technology.[25] It has high mechanical strength and it can withstand high occlusal forces compared to all ceramic materials.[26] in addition it can resist crack propagation in the core material, however, cracks often occur in the veneering material leading to its fracture whether in the tooth supported or implant-supported bridges.[27][28] Reports found that the 3×3 mm designed connectors in zirconia bridges increased the strength to resist fracture by 20%.[29][30]

Although the use of ceramic based fixed prosthesis have been popular as it achieves a lifelike, highly esthetic appearance, a Cochrane review found insufficient evidence to support or refute the effectiveness of ceramic materials for fixed prosthodontic treatment over metal-ceramic.[31]

1. Poor oral hygiene: As with any fixed prosthesis including bridges, maintaining good oral hygiene to prevent plaque formation around the bridge is key. This will ensure prolonged performance. A study examined the gingival health around the fixed bridges after a 14-day - 6 month post insertion found the surfaces were more plaque retentive, causing gingival inflammation regardless the material of fabrication of the bridge, unlike single crowns which did not show the same effect.[35][36]
2. Mechanical failures: These failures can occur due to loss of retention of the bridge due to improper cementation, construction or preparation.[37][38]

Fracture of the metal coating or pontic can also lead to mechanical failures. Fracture in connectors of bridges at the gingival side is a common finding in most all-ceram bridges.[39]

1. Biological failures: These can occur due to caries in the tooth (one of the commonest causes of crown and bridge failures[9]) or due to pulpal injury. Problems with abutment teeth such as tooth fracture, secondary caries or periodontal disease can cause discomfort and put pressure on surrounding soft tissues to also cause a biological failure of the bridge.
2. Aesthetic failures: These can occur at the time of cementation and include; colour mismatch, roughness of margins or improper tooth contour.

Aesthetics failures can also occur over a period of time including through wear of teeth, gingival recession or drifting of teeth.

1. Problems with abutment teeth: Abutment teeth affected by secondary caries, vitality loss or periodontal disease can all lead to bridge failure.[37]

Bridge failures result in clinical complications and patients can present with:

1. Pain in the oral cavity
2. Sensitivity, bleeding and inflammation of the gums [37]
3. Foul breath and taste disturbances

Management of bridge failures depends upon the extent and type of failure and these can be prevented through forming a thorough treatment plan with the patient as well regularly emphasising the importance of maintaining a very good level of oral hygiene after the bridge has been placed. The importance of cleaning underneath the pontic, through the use of interdental cleaning aids, should also be reinforced as plaque control around fixed restorations is more difficult.[9]

Management options include:

1. Keeping the bridge under observation/review
2. Repairing, replacing or removing the fault [9]