Unconventional pontics in fixed partial dentures Mansi Manish Oswal1, Manish Sohan Oswal2 1 Assistant Professor, D.Y. Patil Dental School, Lohegoan, Pune, Maharashtra, India2 M.D.S, Prosthodontics, D.Y. Patil Dental School, Lohegoan, Pune, Maharashtra, India Date of Web Publication25-Oct-2016
The fixed-removable prosthesis resembles a flangeless denture that is retained solely by several osseointegrated implants. There is no contact between the prosthesis and the tissues of the alveolar ridge.
The original design of the fixed-removable prosthesis was developed by Swedish investigators using the two-stage endosseous implant system developed by Brånemark. The prosthesis consisted of a gold alloy framework attached to the copings of the implant. Acrylic resin denture teeth were arranged on the framework and secured with acrylic resin . The fixed-removable prosthesis represented a unique aspect of prosthodontics reconstruction for edentulous arches, since implants were situated in the anterior region and the posterior sections of the framework were cantilevered from the anterior portion of the framework (Fig. 1). The length, height, and width of the cantilever are crucial in minimizing the amount of deformation of the prosthesis (Fig. 2). According to Glantz, the mount of deformation of the cantilever is directly proportional to the cube of the length and inversely proportional of the width and the cube of the height of the cantilever . In addition, there is a direct relation between the amount of deformation and the force of the occlusion (loading force) as well as an inverse relation with the modulus of elasticity of the material to be used for the framework. This relationship can be expressed in the following equation:
According to Zarb and Jansson, frameworks in fixed prostheses could be designed in one of the two ways: (1) where metal frameworks comprised the bulk of the prostheses, and artificial teeth and minimal denture bases were the only non-metallic components. (2) Implant fixed prostheses consisting mostly of acrylic resin denture bases (wraparound design) and artificial teeth, with minimally sized metal frameworks .
Implant treatment was based on basic prosthodontics principles that included preliminary and definitive impressions, jaw relation records, wax try-in, metal framework try-in, and insertion of definitive prostheses. Frameworks were fabricated according to the following criteria: bulk for strength, adequate access for oral hygiene procedures, minimal display of metal on the facial and occlusal surfaces, and strategic thinning of implant frameworks to allow for retention of acrylic resin denture teeth and denture basses .
A well-accepted principle of restorative dentistry is that the final restoration can be only as accurate and well adapted as the final impression. The clinical challenge is to provide an accurate final impression of the intraoral condition to the laboratory if the impression materials are prone to dimensional changes due to on-going chemical reactions  and stone will show expansion due to secondary reactions whilst setting . The misfit of fixed partial dentures on natural teeth will result in forces on the underlying teeth. Natural teeth however can move 25-100 µm in axial direction and 56-108 µm in lateral direction [35, 36] and adapt to a slightly different position in the bone due to the periodontal ligament should there be a slight misfit of the prosthetic work. Implants on the other hand will only show a range of motion of 3-5 µm in axial direction and 10-50 µm in lateral direction after osseointegration due to compression of the bone . Ill-fitting frameworks will generate stress on the implants which may have a biological effect on the bone-implant interface [37, 38]. It has been also shown that stresses introduced by misfit were comparable with that related to occlusal forces . Prosthetic complications as screw loosening or fracture may be also related to ill-fitting framework fit . The aforementioned factors have resulted in the paradigm that passive fit of the framework is one of the key factors for long-term success in implant dentistry [41, 42] stressing the importance of a reliable and precise impression procedure. Several strategies have been developed to ascertain passive fit [35, 43]. An intraoral scanner could overcome some of the errors associated with traditional impression taking  and cast production , since digital output data can be fed directly into a digital workflow.
The introduction of computer-aided design/computer-aided manufacturing (CAD/CAM) has facilitated the use of new dental ceramic materials. Zirconium oxide, known as zirconia is currently used as a core material for fabrication of frameworks for tooth and implant-supported fixed partial dentures (Fig. 11). Esthetics is optimized with zirconia restorations due to the natural shade of the substrate, thus eliminating the problem of the gray effect, especially at the cervical area, of implant prostheses with metal alloy substructures (Fig. 12) . Zirconia stabilized with yttrium oxide possesses good chemical and physical properties such as low corrosion potential, low thermal conductivity, high flexural strength (900-1200 MPa), and hardness (1200 Vickers) [48, 49]. In addition, zirconia is considered more biocompatible than other ceramics, titanium, and metal alloys, which may facilitate soft tissue response in terms of heath . 153554b96e