|Any loss of retention in anterior teeth may be compensated for by providing conservative grooving in the enamel on the mesial and distal aspects of the abutment. A delicate touch is required to produce this well defined "miniature" groove. This groove should not be placed too far labially.|
A direct cantilever resin bonded bridge replacing an upper first premolar. Notice 180 degree wrap around and occlusal bar to increase rigidity.
Resin bonded bridges have been associated with lower retention rate than conventional bridgework. However, recent studies have shown that with improved design, in terms of framework design and tooth preparation, these bridges are lasting longer than the earlier, pioneering, prostheses.
Factors to consider are:
It is recommended that, where possible, retainers for molars be 0.8mm thick or even greater if the retainer is not joined over the occlusal surface.
Maximising the surface area for bonding the framework to enamel is one of the basic premises of good retention and resistance form for resin bonded bridges. Coverage involves extension of the metal work as far occlusogingivally and circumferentially around the tooth as is possible.
This was one of the earliest framework design features and is intended to provide buccolingual stability - especially for posterior prostheses.
Most clinical studies recommend that on anterior teeth the incisal extension finishes 1.0 to 3.0 mm short of the incisal edge to prevent metal from shining through the tooth or possible occlusal interferences. Posteriorly, the extension of metalwork to the occlusal surfaces of posterior abutment teeth has 3 important biomechanical features for prosthesis design: resistance to displacement apically or laterally, increased rigidity of the framework, and greater surface area for bonding.
Significant stresses are applied to the retainers of fixed-fixed resin-bonded bridges because of differential tooth movements between the abutments during functional and parafunctional tooth contacts. These forces stress the cement lute which may lead to de-bonding. Such stress can be reduced by increasing the resistance form of the abutment or by changing the design from fixed-fixed to fixed-moveable or to a 2-unit cantilever design.
Several studies have shown that long-span resin-bonded prostheses have a shorter clinical life. This does not mean that long-span bridges should not be provided but rather that the tooth preparation and framework design should be planned to reduce potential debonding stresses on the retainer.
Axial tooth preparation not only increases the area for bonding but also increases resistance and retention form.
Grooves allow increased resistance form to lateral displacement and may help increase retention form. They also increase the structural rigidity of the metal framework after cementation. Studies have shown that the use of 2 grooves (1mm deep) per abutment, in comparison to no grooves, significantly increases resistance to debonding forces for both anterior and posterior bridges.
These not only allow the transmission of occlusal forces along the long axis of the tooth but also provides resistance form and may, therefore, limit shear forces to the cement lute.
Extension of the metal framework intracoronally into existing restorations or small carious lesions has been advocated to achieve improved resistance form. One modification that is gaining popularity is the joining of the mesial and distal rest seats of the retainer over the occlusal surface to form an occlusal bar to improve the rigidity of the retainer. This is useful because it not only enhances the retainer's resistance to deformation but also improves the resistance form and increases the surface area for bonding.
Recent clinical studies show that 2-unit cantilevered resin-bonded bridges are as retentive or even more retentive than their fixed-fixed counterparts. This type of restoration is indicated when replacing single pontics anterior to the premolars. It is not recommended to cantilever a molar sized pontic because the greater leverage forces from the pontic may cause uncontrolled tooth movement. Abutments requiring cuspal protection or having restorations larger than a Class II are not good candidates for such cantilevered bridges, although in the case of a small restoration being required, this can often be incorporated into the framework design for additional resistance form.
Tooth preparation should be conservative whilst at the same time allowing optimal resistance form.
Ideally, it should be confined within enamel and should maximise the surface area of the abutment for bonding. The framework should be rigid and have optimal resistance form while allowing for good oral hygiene.