A fixed dental bridge is a permanent restoration designed to replace one or more missing teeth by spanning the gap and attaching to existing structures. This restoration is cemented securely into the mouth, offering a stable solution for restoring function, appearance, and the integrity of the dental arch. The success of this appliance relies entirely on the strength and health of the teeth chosen to bear the load of the replacement structure.
Identifying the Supporting Teeth
The fixed bridge is composed of three main parts that work together to replace the missing tooth unit. The natural teeth that anchor the entire structure are known as the abutment teeth, which must be prepared to receive the restoration. These abutment teeth are positioned on either side of the empty space, acting as the foundation for the appliance.
The artificial tooth or teeth that fill the gap are called the pontic, which is suspended between the abutments. The retainers, typically crowns, are fabricated to fit precisely over the prepared abutment teeth. The retainers are permanently cemented onto the abutments, holding the pontic securely in place and completing the bridge unit.
In the most common design, the teeth immediately adjacent to the missing space are selected as the abutments. When multiple teeth are missing, creating a longer span, it may be necessary to involve more than one tooth at each end for adequate support.
Biomechanical Criteria for Abutment Selection
The selection of an abutment tooth is guided by strict biomechanical principles to ensure the longevity of the restoration. The overall health of the tooth’s supporting structures is a primary consideration. The surrounding bone and gum tissue must be free of active disease, and any existing periodontal disease must be treated and controlled before a tooth can be considered a viable anchor.
A fundamental concept in assessing support is Ante’s Law. This law states that the combined root surface area of the abutment teeth must be equal to or greater than the root surface area of the teeth being replaced. This principle ensures the periodontal ligament can withstand the additional chewing forces transferred by the bridge. Ignoring this rule can lead to excessive stress, resulting in the abutment teeth becoming mobile or failing over time.
The crown-to-root ratio is another significant measurement, comparing the length of the tooth visible above the bone (the clinical crown) to the length of the root embedded in the bone. An ideal ratio for an abutment tooth is approximately 2:3, meaning the root is significantly longer than the crown. This provides greater stability against rotational and leveraging forces. A ratio where the clinical crown is equal to or longer than the root substantially increases the risk of mechanical failure.
Root configuration also impacts the tooth’s ability to withstand the forces transferred from the bridge. Teeth with longer, wider roots, especially those that are broad labio-lingually, are preferred over short, conical, or rounded roots. Multi-rooted posterior teeth with widely separated roots offer superior stability and anchorage compared to teeth with fused or converging roots.
The overall length of the space being bridged significantly affects the stress placed on the abutments. Biomechanical studies demonstrate that the deflection of a bridge under a chewing load increases by the cube of the span’s length. For instance, a bridge replacing two missing teeth will experience approximately eight times the deflection of a bridge replacing a single tooth. This necessitates stronger abutments or the inclusion of additional support to manage the increased leverage.
When Natural Teeth Cannot Provide Support
There are scenarios where the natural teeth adjacent to the gap are missing, structurally compromised, or lack the necessary bone support to serve as bridge abutments. When this occurs, the most common alternative for a permanent fixed solution is an implant-supported bridge. In this method, a titanium post is surgically placed directly into the jawbone, acting as an artificial root.
The dental implant serves as the artificial abutment, providing the necessary support and stability for the pontic. An advantage of this approach is that it avoids placing stress on the remaining natural teeth. This is beneficial if the adjacent teeth are already weakened or should not be altered. The implant integrates with the bone, offering a foundation that often rivals the strength of a natural tooth root.
A more conservative option, typically used for replacing a single anterior tooth, is the resin-bonded or Maryland bridge. This design utilizes a pontic attached to small metal or porcelain wings, which are bonded directly to the back surface of the adjacent teeth. Because this method relies on adhesive bonding rather than full crown coverage, it requires minimal alteration to the supporting teeth. While less invasive, the Maryland bridge offers significantly less retention than a traditional crown-supported bridge and is reserved for situations with lighter biting forces.