Because of graftless implant placement and immediate loading, the All-on-4™ technique is very successfully being performed by many dentists and has gained high popularity among implant dentists as well as patients. The only limitation with this technique is that prosthesis with only limited number of teeth (10-12 units) can be fixed over these four implants. Further, the loss of any one implant reverts the entire procedure to the initial stage.

To avoid such problems and also for the patients who express the desire for a 14-unit prosthesis, two more implants can be inserted posterior to the posterior wall of the sinus in the maxillary tuberosity and tilted anteriorly at 45° to minimize the length of the unsupported bridge framework between two distal implants. The severely resorbed posterior maxilla with a large volume of posterior expansion of the sinus often does not leave enough bone volume in the tuberosity region to place an implant of an adequate size. In such cases, the implant is inserted in the tuberosity with the apex of the implant at the junction of the pyramidal process of the palatine bone and the pterygoid process of the sphenoid bone. The implant placed would then engage all three bone segments that constitute this region. The implant placement in the tuberosity with its apex engaging the medial pterygoid process of sphenoid bone is the most preferred option because it allows the multicortical engagement of the implant to achieve adequate initial stability for the implant.

To perform the All-on-6 procedure in the mandible, the two straight implants should be inserted usually at the first or second molar site but if inadequate ridge height above the mandibular canal does not allow placement of implants in the molar region, then the short and wide implant can be inserted at the angle of the mandible (into the buccal shelf area) tilted anteriorly at 30-45°.

With the few advantages, there are also several disadvantages with All-on-6 procedures such as increased cost, need of a highly skilled approach to correctly place implants in pterygoid process, difficult approach for the insertion and restoration of posterior implants, need of a skilled technician to fabricate the prosthesis, and problems in oral hygiene maintenance in the back region.

Comparative features of the traditional versus All-on-4™/ All-on-6 approach

Advantages of the All-on-4™/All-on-6 technique

• Vast numbers of the edentulous patients can be treated with the technique.
• Being a graftless and immediate loading technique, it is more acceptable to patients.
• Fewer implants are inserted.
• Fixed teeth can be given on four implants to patients for whom sinus grafting/nerve transpositioning procedures are contraindicated.
• Lower cost of the treatment compared to the traditional implant supported full arch fixed prosthesis which often needs bone grafting, more implants, multiple surgical steps, ete,
• Long tilted distal implant can be maximally stabilized by utilizing high-density bone of the anterior region. Placement of longer implants, enhancement of the area of interaction between bone and implant, and also primary anchorage.
• A greater distance between implants, allowing the elimination of cantilevers in the prosthesis, which results in better load distribution.
• By reducing the number of implants to four, each implant can be placed without interfering with the adjacent implants.
• The placement of implants in residual bone, avoiding more complex techniques of bone graft and/or sinus lift.
• Immediate loading is done in most of the cases so that the patient gets at least provisional fixed teeth on the day of implant placement for aesthetics and function.
• It can be performed by implant surgeons who are not very expert at performing procedures like sinus grafting, block grafting, nerve trans positioning etc.
• Only one surgical step is required (no implant uncovering).
• The high success rate of the procedure (as shown in the various studies).
• Treatment completed in very short period of time (in a few weeks) whereas the traditional technique may however take years to complete treatment. 

Limitations of the All-on-4™/All-on-6 technique

• It cannot be performed in patients presenting with large osseous defects in the anterior region, which needs grafting procedures to regenerate new bone before All-on-4™ implant placement
• Extraction of firm, healthy teeth is mandatory, if any are present in the anterior jaw.
• Reduction of bone crest causes increased soft tissue height which in turn leads to increased pocket depth around the abutment, more chances of bacterial growth, and peri-implantitis.
• With the All-on-4™, only the 10 to 12-unit prosthesis is delivered over the four implants, and often patients request the addition of more posterior teeth to maximize chewing efficiency and improve the overall maxillofacial prosthesis.
• Oral hygiene: Maintenance of the hybrid prosthesis is often difficult for some patients and they need regular visits to the dentist for its cleaning.

Indications

• Edentulous patients who need fixed implant-supported prosthesis – maxillary/mandibular or both.
• Patients with partial maxillary/mandibular edentulism with only few intact natural teeth in the anterior region.
• Patients with worn out dentition which needs extraction and replacement of all teeth.
• Patients with periodontally compromised mobile teeth which need extraction and replacement.
• Edentulous or partially edentulous patients with very limited subantral bone height in the posterior maxilla.
• Edentulous or partially edentulous patients with very limited bone height above the mandibular canals in the posterior mandible.
• Edentulous patients with maxillary sinus pathologies contraindicating the sinus grafting procedure.
• Patients with adequate volume of healthy bone in the maxillary and mandibular anterior region to place implants.
• Implant overdenture cases with severe ridge resorption - tilting posterior implants give more support to the denture and prevent soft tissue abrasion and further bone loss in the posterior region.

Contraindications

• Patients with inadequate bone volume in the maxillary and mandibular anterior region to place implants
• Anterior wall of the sinus is located far anterior to the usual position, contraindicating tilting of the posterior implants to reach the second premolar or first molar position (Fig 22.3).

Key points for successful All-on-4™/All-on-6 implant therapy

• Meticulous treatment planning to see the position and path of anterior, inferior, and posterior wall of maxillary sinus.
• Dental CT planning if possible, to see the possible placement of the implants with desired dimensions and their three-dimensional positioning for the best possible prosthesis.
• Placement of longest possible implants and stabilization in the cortical bone such as nasal floor, basal bone of the anterior mandible, pterygoid process, etc. to achieve high primary stability.
 
• Tilting of the posterior implants using the All-on-4™ guide to avoid extreme tilting which may result in parallelism problems during restoration.
• Selection of multiunit abutments with proper collar height and angulation.
• Sequential radiographs with the drill into the osteotomy during initial osteotomy preparation for posterior implants, to evaluate the direction of the drilling in respect to vital structures such as the sinus wall and the mandibular canal.
• Placement of the implants with minimum diameter of 3.3mm at the anterior positions and 3.75-4.2mm for the posterior positions to avoid problems such as connection screw loosening and implant body fracture.
• Adequate vertical ridge reduction before implant placement to avoid the display of the unaesthetic transition line of prosthesis and ridge tissue when the patient smiles