All-on-4

The All-on-4 concept was consolidated before its name was coined in 2003,^[1] by the work of Per Ingvar Brånemark and colleagues from the late 1960s, who installed four or six implants to support a fixed full-arch prosthesis in the maxilla or mandible.^[3] Brånemark's original protocol required a submerged healing period of three to six months before the implants were loaded.^[4] It was later refined by a same-day immediate-loading protocol using prefabricated components and surgical guides, the Brånemark Novum method,^[5] by Krekmanov and colleagues, who proposed tilting posterior mandibular and maxillary implants to improve prosthesis support,^[6] and by Mattsson and colleagues, who placed four to six maxillary implants with the most posterior on each side angulated.^[7]

Nobel Biocare AB of Gothenburg, Sweden, funded studies by the Portuguese dentist Paulo Maló to test the technique,^[1] the first of which reported on the edentulous mandible before the protocol was extended to the maxilla.^[8] Because it mainly applied existing principles under a new name, reviews have described All-on-4 as an evolution of earlier work rather than a new invention.^[4] Its defining features are four implants supporting a full fixed bridge (documented since 1977^[3]), the use of angulated posterior implants to overcome bony deficiencies (documented since 1990^[9]), and immediate loading (documented since 1990^[10]).

The procedure replaces an entire upper and/or lower set of teeth with a permanent, screw-retained fixed bridge or denture in a single day. It is intended for patients with significant tooth loss or decay, and is used where jaw bone loss would otherwise prevent conventionally oriented vertical implants or require bone grafting. Where only the crowns are missing, a fixed prosthesis replacing the crowns alone (fixed prosthetic 1, or FP1) is sufficient, whereas advanced bone loss requires a prosthesis that also replaces lost soft and hard tissue (fixed prosthetic 3, or FP3).^[11]

The technique uses the denser bone in the front of the jaws and places the two posterior implants at an angle to avoid the sinus cavities in the upper jaw and the nerve canal in the lower jaw. The bone is assessed beforehand, most often by a cone beam computed tomography (CBCT) scan. The back implants are typically angled about 30 to 45 degrees from the occlusal plane, placed in front of the maxillary sinus in the upper jaw and in front of the mental nerve in the lower jaw, with the head emerging at about the second premolar position. This allows a molar to be cantilevered posteriorly, giving a denture or bridge of about twelve teeth.^[4]

The final prosthesis is most commonly made of polymethyl methacrylate (acrylic) denture teeth on a milled titanium bar, or of zirconia.^[12] Acrylic is shock absorbent, which reduces the load transmitted to the implants during chewing, and can be repaired or replaced relatively easily, but it wears and discolours over time.^[4] Zirconia is harder and more resistant to wear and staining, with monolithic zirconia prostheses reporting survival above 95% at five years, though it is more costly.^[12] Hybrid frameworks using polyether ether ketone (PEEK) have also been studied.^[13]

Systematic reviews report high short- and medium-term implant survival, although much of the evidence comes from retrospective studies with limited follow-up and methodological quality.^[4][2] A longitudinal study of the mandible reported implant-related success of about 94.8% and prosthesis survival of 99.2% with up to 10 years of follow-up,^[14] with later follow-up extended to between 10 and 18 years.^[15] Comparable results have been reported for the maxilla over 3 and 5 years^[16] and in an independent study of Japanese patients followed for 3 to 17 years.^[17] These figures are broadly in line with general dental implant survival, which exceeds 90% at ten years.^[18] Much of the long-term data comes from the group that developed the technique, and systematic reviews have called for more independent studies using standardised methods and longer follow-up.^[4]

Studies comparing axial and tilted placement have generally found no clinically significant difference in survival or marginal bone loss, supporting the use of tilted posterior implants to avoid bone grafting,^[19] and immediate and delayed loading have shown comparable outcomes.^[20]

The most frequently reported complication is mechanical, in particular fracture of the acrylic prosthesis, while biological complications such as peri-implantitis are reported less consistently.^[4] Marginal bone loss is generally limited, averaging roughly 1.3 mm over the first one to five years in pooled data, and reviews note that standardised long-term reporting of biological complications remains limited.^[4][2] Mechanical complications are generally more common than biological ones, and many can be managed by repairing or replacing the prosthesis rather than by removing implants.^[4]

A systematic review of oral health-related quality of life after All-on-4 rehabilitation analysed eleven studies covering 693 patients aged 55 to 71, with follow-up from three months to seven years, and reported high satisfaction and improved quality of life as measured by the Oral Health Impact Profile and visual analogue scales.^[21]