- Most commonly used framework materials include precious alloys, Co-Cr alloys, titanium and zirconia
- The framework volume and design depends on the amount of loss of vertical space and must follow the final design of the prosthesis to provide an even surface for the layering material
- CAD/CAM milling of the frameworks is the option of choice, due to high precision, passive fit and high material quality
Material selection and restoration design considerations
A commonly used option for fixed restorations is a metal or zirconia framework with ceramic or acrylic veneering. The amount of alveolar bone resorption determines whether teeth or teeth and soft tissue must be replaced. In cases presenting minimal bone resorption, a ceramo-metallic restoration is preferred in order to accommodate for interarch space limitations. When advanced or severe bone resorption has occured, the increased vertical space is compensated for by increasing the framework volume and by adding pink veneering to avoid unfavourable tooth morphology (teeth appearing too long). In patients with a high lip line the result is esthetically unacceptable and the large spaces also compromise phonetics. Bone grafting procedures can be a necessary pre-treatment solution to build up the missing tissue volume. As a prosthetic solution a removable prosthesis design with buccal flanges can be considered to esthetically cover the smile and lip line.
Frameworks can be made of zirconia, titanium, precious alloys and Co-Cr base alloys and they can be cement-retained or screw-retained. Since these materials vary in their physical properties they present different advantages and disadvantages in terms of stability, esthetics, technical processing, required framework dimensions and patient comfort.
With traditional casting techniques using precious alloys, cast-on gold cylinders are included in the casting and subsequently the veneering is performed.
For manufacturing a titanium or Co-Cr framework different processing techniques are available:
- The framework is cast and then connected to titanium cylinders by e.g. laser welding or bonding with adhesive resin cements
- Prefabricated titanium framework segments are connected to each other
- The framework is milled in one piece using the CAD/CAM technology (ComputerAidedDesign/ComputeAidedManufacturing technology).
Using CAD/CAM, a resin framework pattern is fabricated on the master cast and both the framework pattern and the position of the implant abutment replica are then laser-scanned. In the production unit, based on the scan, an identical copy of the resin framework pattern is milled in one piece in titanium, Co-Cr or zirconia. The milled framework is returned to the dental laboratory where the veneering is performed using denture teeth and acrylic resin or ceramic.
Nowadays, CAD/CAM milling of frameworks is the option of choice due due the high precision, passive fit, design flexibility, high material quality and reduced laboratory effort and time. Considering the relatively poor precision of casted frameworks, precious alloys loose acceptance and relevance from both a technical and economic point of view.
Potential complication aspects
Due to highly homogenous and strainfree material characteristics of CAD/CAM milled frameworks, fractures are less frequent than in cast frameworks. However, adequate connector size and design are crucial. Several techniques have been developed to address potential chipping of the veneering ceramic, such as e.g. an anatomically designed framework with appropriate ceramic thickness. In case of repairs the easy retrievability of a screw-retained restoration is a major advantage.