Introduction

The conventional full-arch fixed implant-supported prosthesis often needs placement of several implants (8-12) to support a 12- to 14-unit fixed prosthesis. Moreover, the patients who come to the implant dentist for the full-arch implant-supported prosthesis usually present the loss of ridge volume especially in the maxillary and mandibular posterior regions, which limits the insertion of the implant with adequate dimensions without prior bone augmentation.

The long-term edentulism of the posterior maxilla or replacement of lost maxillary molars and premolars other than the implant prosthesis, may result in lowering of the sinus floor because of the pneumatization of the sinus. In several cases, it results in the presence of inadequate bone height to insert the implant. Thus, in several cases that need the full-arch implant prosthesis, the sinus augmentation procedure needs to be performed to regenerate the desired bone dimensions. However, this procedure may take 6-8 months before implants can be inserted. Moreover, the density of the bone in this region is usually poor and may require subgingival healing of the inserted implants for a further 6-8 months before they are loaded to support fixed prostheses.

Thus full arch implant supported prosthesis in mandible also requires 6-10 implants to be placed as the presence of the mental foramina and often very thin ridge or inadequate bone availability over the mandibular canals in the posterior region limits insertion of adequate length of implants. This leaves us with possibility of implants in the anterior region with long cantilevered extensions which have their own limitations and complications. Onlay bone grafting in the posterior region has its own limitations and requires multiple surgical interventions which adds to complications and cost of the treatment.

Thus full arch implant supported prosthesis in maxilla and mandible offers several limitations like

• Need for sinus grafting in upper arch and/or onlay grafting in lower arch.
• Delayed implant placement as it may require time for the grafts to take up and achieve the desired bone dimensions and density. Implants may again require further 6-8 months of subgingival healing before they can be loaded to support any fixed prostheisis.
• Multiple surgical procedures and increased cost.

To overcome these above limitations in 1993 Dr. Paulo Malo at the Malo clinic, Lisbon, Portugal invented the 'All on 4' Implant procedure for restoring edentulous jaw by tilting posterior implants for utilizing the highest amount of bone and stabilizing the implants in highest possible bone density.

The tilting of the posterior implants reduces the length of the posterior cantilevering of the prosthesis. Moreover, it allows long implant placement and the insertion of the implant apex in the high-density anterior maxilla or anterior mandible, to achieve adequate primary stability for immediate loading on the implants, using a provisional splinted acrylic prosthesis. Hence, this facilitates optimal support for an acrylic prosthesis that can be immediately fixed over the inserted implants to restore aesthetics and function within a few hours of implant insertion surgery. All-on-4™ is based on Nobel Biocare’s pioneering ‘Immediate Function’ capability.

To perform this technique, a total of four implants are inserted with the back implants tilted up to 45°, often in close approximation to the inferior and anterior wall of maxillary sinus in the upper jaw and superior and anterior to the inferior alveolar nerve and mental foramina in the mandible, to take maximum advantage of existing bone by inserting long implants and firmly stabilizing their apex in high-density anterior bone. A fixed standardized surgical guide is used to correct implant placement. Both flap and flapless (guided) approaches are compatible with the technique. Special components are developed to correct the prosthetic angulation of the tilted implants as well as to immediately restore the implants in function. If necessary, a cantilever can also be added to the final prosthesis.