What is the All-on-4 system?

The name All-on-4 is a registered trademark of an implant manufacturer. The principle it rests on — placing four implants, with the two posterior ones angled to obtain more favourable support — is not exclusive to any implant system, and is described in the literature independently of the brand.

Why the posterior implants are angled

In the posterior regions, the height of bone available is frequently limited. In the upper jaw, that limitation results from the resorption that follows the loss of the teeth and from the expansion of the maxillary sinus. In the mandible, resorption can leave little bone height above the canal carrying the inferior alveolar nerve.

Under these conditions, placing vertical implants would call for additional procedures or a different rehabilitation strategy.

Angled placement makes it possible to use bone situated further forward, while keeping the connection point to the prosthesis in a more posterior position. The implant is supported by the existing bone and emerges further back than its base, which makes it possible to work around anatomical limitations and, in many cases, to avoid a bone graft.

There is a second effect: the further back the last point of support lies, the smaller the portion of the prosthesis that extends beyond it — the cantilever.

That extension acts as a lever arm. For the same chewing force, a longer extension increases the leverage on the assembly formed by the prosthesis and the implants. Its length is therefore one of the parameters to be controlled in the planning.

What the evidence says

A review published in 2025, which brought together eight systematic reviews on the subject, reports cumulative survival rates between 94.8% and 99.3% and mean marginal bone loss between 1.1 mm and 1.5 mm ^[1].

The reviews included did not identify statistically significant differences pointing to poorer outcomes for the protocol on four implants compared with approaches using more implants. It is important to be precise about the scope of this conclusion: the absence of statistical differences does not demonstrate that the options are equivalent in all clinical situations ^[1].

As for the patient's experience, a systematic review that brought together 693 patients across eleven studies, with follow-up ranging from three months to seven years, found high levels of satisfaction and of oral health-related quality of life ^[2].

There is also a practical consideration associated with the number of implants. Each implant adds a transition zone between the implant and the prosthesis that has to be cleaned daily. A smaller number of supports means fewer of those zones — and access for hygiene is one of the determining factors for the health of the tissues around the implants in the long term.

Do all the teeth have to be extracted?

This protocol applies in two situations: arches that are already without teeth, and arches in which the teeth present are compromised to the point where keeping them in the mouth is not viable.

When teeth are still present, it is essential to assess the prognosis of each one and its role in a possible rehabilitation. A systematic review on fixed full-arch implant-supported prostheses expressly recommends avoiding the extraction of teeth with a satisfactory prognosis merely in order to make that kind of treatment possible ^[3].

The presence of some compromised teeth does not, in itself, constitute an indication to extract the whole arch. It is necessary to establish which ones can be kept, and with what prospect of stability, function and maintenance.

That is the purpose of the assessment consultation: to determine the viability of the teeth present in the arch, and from there to establish whether this protocol makes sense for the case.

When another approach may be needed

This protocol presupposes bone available to anchor four implants in suitable positions. Fitting a prosthesis on the same day calls, in addition, for sufficient stability and favourable clinical conditions.

In situations of severe bone atrophy, the conventional configuration may not be viable. In the upper jaw, other anchorage strategies may then be assessed — notably zygomatic implants, on their own or in combination with other techniques —, whose indication depends on the anatomy and on the needs of the rehabilitation ^[4].

It may also be appropriate to plan a prosthesis on five, six or more implants. In that case, it is a full-arch rehabilitation with a different configuration.

The number of implants should follow from individual planning. Quantity, on its own, does not replace an appropriate distribution of the supports or a prosthetic design suited to the patient.

How the decision is made

The decision rests on the clinical assessment and on the study of the volume and the distribution of the remaining bone, usually through cone beam computed tomography — CBCT.

I also consider the condition of the teeth present, the gingival tissues, the way the teeth come into contact, the aesthetic needs, the general state of health and the conditions for maintaining hygiene of the future prosthesis.

When a patient comes to me asking about All-on-4, I begin by understanding what they want to achieve: to have fixed teeth again, to improve chewing, to avoid grafts, to shorten the treatment time. The assessment determines whether this protocol answers those aims, or whether another approach offers more favourable conditions for their case.