A bridge is a fixed dental restoration (a fixed dental prosthesis) used to replace one or more missing teeth by joining an artificial tooth definitively to adjacent teeth or dental implants.
Definitions Fixed bridge: A dental prosthesis that is definitively attached to natural teeth and replaces missing teeth.
Abutment: The tooth that supports and retains a dental prosthesis.
Pontic: The artificial tooth that replaces a missing natural tooth.
Retainer: The component attached to the abutment for retention of the prosthesis. Retainers can be major or minor.
Unit: Pontics and abutment teeth are referred to as units. The total number of units in a bridge is equal to the number of pontics plus the number of abutment teeth.
Saddle: The area on the alveolar ridge which is edentulous where at least one missing tooth is to be reinstated.
Connector: Joins the pontic to the retainer or two retainers together. Connectors may be fixed or movable.
Span: The length of the alveolar ridge between the natural teeth where the bridge will be placed.
Resin-bonded bridge: A dental prostheses where the pontic is connected to the surface of natural teeth which are either unprepared or minimally prepared.
Advantages and disadvantages of tooth replacement +AdvantagesDisadvantages Restore aesthetics (particularly important for anterior edentulous areas)Loss of tooth tissue (tooth preparation of abutments often requires significant tooth preparation)Restore function (mastication, speech)Pulpal damage (tooth preparation opens dentinal tubules providing a connection between bacteria in the mouth and the pulp. Deep preparations can cause pulpal injury) Occlusal stability (prevent tilting, drifting, rotation and over-eruption of adjacent/opposing teeth)Secondary caries (around crown margins, under debonded retainer wings)Patient preference
Types of bridge Conventional bridge
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.
Fixed–fixed bridges 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.
Cantilever 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.
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.
Fixed–movable 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. 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.
Resin-bonded bridge
An alternative to the traditional bridge is the resin-bonded or adhesive bridge (also called a Maryland bridge). A resin-bonded bridge utilises retainer "wings" on the sides of the pontic which attach it to the etched enamel of 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). Single retainer cantilever resin-bonded bridges have less complications than end-to-end resin-bonded bridges with retainer wings on both sides.
Combination designs The incorporation of elements of different conventional bridge designs. A popular combination design is the use of a fixed-fixed design with a cantilever.
Hybrid designs Bridges that incorporate elements of both conventional and adhesive bridge designs.
Advantages and disadvantages of conventional bridge designs AdvantagesDisadvantagesFixed–fixed Maximum strength Very retentive Most accepted design for longer spans Construction is relatively straightforward Preparations must be parallel which may require extensive tooth preparation. Weakens the tooth and risks damaging the pulp Achieving parallel preparations is challenging. It is important to avoid undercuts. Abutments are major retainers requiring extensive preparation Cemented as one unit which may be challenging Fixed-movable Non-parallel abutments may be used which avoids extensive tooth preparation Minor retainers are used so the preparations are more conservative of tooth tissue Enables small movements of teeth Cementation is more straightforward as components can be cemented separately Not appropriate for long spans Complex design for the laboratory to construct Temporary bridge is complex to make Cantilever Only one abutment is required therefore conservative Destructive parallel preparations not required Most appropriate design when replacing anteriors Maintenance and oral hygiene is easier for the patient Span length limited to one pontic due to leverage on the abutment Rigid design to prevent distortion Tilting of abutment teeth due to occlusal forces
Advantages and disadvantages of resin-bonded bridge designs AdvantagesDisadvantagesFixed-fixed Large surface area providing good retention Simple to construct in the laboratory Debonding which can allow secondary caries to develop under the failed wing Tilted abutments may require extensive preparation to achieve parallelism and sufficient retention Ideally abutments should be equally retentive which is difficult to achieve, especially posteriorly when the retainers are a molar and premolarFixed-movable Abutment teeth are able to move independently The retention of the two retainers does not need to be equal Minor retainer can be minimally retentive Stops tilting of the posterior abutment Design not advised for anterior bridges Design not appropriate for long spans More complex to construct in the laboratoryCantilever Most conservative of tooth tissue as minimal (if any) preparation required on one abutment only. Design of choice for replacing single anteriors Can be used for short posterior spans Straightforward to construct in the laboratory Patients are able to maintain good oral hygiene as it is easy to clean Limited retention due to small surface area Risk of debonding due to torquing forces Appropriate positioning on cementation may be complex
Case selection and treatment planning Case selection 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. 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. 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.
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.
Selection and evaluation of abutment teeth 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.
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.
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.
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.
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.
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.
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.
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.
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.
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. For resin bonded bridges the pontic should have light contact in intercuspal position (ICP) and no contact in lateral excursions.
Biomechanical considerations