All-on-4 vs All-on-6 for the Lower Jaw

How the mandible differs from the maxilla

The lower jaw (mandible) and upper jaw (maxilla) are not interchangeable when it comes to implant planning, and the differences run in the opposite direction to what people sometimes expect. The mandible is a single, dense, horseshoe-shaped block of bone that tends to hold its density well even after tooth loss, which is one reason implants placed in the lower jaw often achieve strong primary (initial) stability at surgery. It has no equivalent to the maxillary sinus. What it does have, and the maxilla does not, is the inferior alveolar nerve — a branch of the trigeminal nerve running through a bony canal inside the body of the mandible before exiting at the mental foramen, usually near the second premolar, to supply sensation to the lower lip and chin. That nerve, not a sinus, is the anatomical structure that most shapes lower-jaw implant planning.

Bone density in the mandible

Bone-quality classification systems such as Lekholm & Zarb and the Misch bone-density classification consistently place anterior and much of posterior mandibular bone toward the denser end of the scale, in contrast with the softer, more porous bone typical of the posterior maxilla. Denser bone generally offers better primary stability at the moment of implant placement, which is part of why lower-jaw implants are, on average, associated with fewer stability-related complications at surgery than upper-jaw implants in the same patient. This is a tendency described in the literature, not a guarantee for any individual site — bone density is still assessed per site, per patient, on imaging.

The inferior alveolar nerve and mental foramen

The inferior alveolar nerve enters the mandible near the back of the jaw, runs forward within a bony canal, and exits through the mental foramen — an opening usually located beneath or near the second premolar — before continuing forward as the mental nerve, which supplies the lower lip and chin. Implants placed too close to this canal, or that damage it during drilling, carry a risk of temporary or, less commonly, lasting altered sensation (paraesthesia) in the lip or chin. This is the defining anatomical constraint of the lower jaw, and it is why posterior mandibular implant planning relies heavily on 3D imaging (CBCT) to map the canal's exact position and depth before any drilling begins, with a safety margin built into the plan.

The interforaminal region and posterior limitations

The area of the mandible between the two mental foramina — often called the interforaminal region — is generally considered the most straightforward zone for implant placement, since it sits above where the nerve canal runs and carries comparatively low nerve risk. Moving further back along the arch, toward and behind the mental foramen, the canal typically runs closer to the crest of the bone, which is why posterior lower-jaw implants are planned with more caution and, in All-on-4 style plans, often angled forward to make fuller use of the safer interforaminal bone rather than being placed straight down close to the canal. See bone loss & bone grafting for how bone volume in this region is assessed more broadly.

Schematic only, not to scale — illustrates typical implant angulation in a full arch, not a specific patient's lower-jaw plan.

Bite forces and why the mandible matters mechanically

The mandible is the jaw that moves. It is the lower jaw's musculature that generates the closing force during biting and chewing, which means the forces acting on a lower-arch prosthesis and its supporting implants can be substantial, particularly at the back of the mouth where mechanical advantage is greatest. This is one reason implant distribution and prosthetic design are treated as seriously in the lower jaw as bone density and nerve position — a plan has to withstand functional loading over years, not just achieve initial stability at surgery.

Implant distribution and cantilever considerations

Distribution — where implants sit along the arch, not simply how many there are — is central to lower-jaw planning in the same way it is for the upper jaw, but constrained differently. In the All-on-4 concept, the two posterior implants are angled forward, which both engages bone further from the nerve canal and shortens the unsupported cantilever compared with a straight implant placed directly above the canal's exit point. All-on-6 spreads implants across a wider footprint, which can shorten the cantilever further and add supporting points, but only where the additional posterior sites offer adequate distance from the nerve canal. As in the upper jaw, a well-distributed four-implant plan can outperform a poorly distributed six-implant one in the same mouth; implant count on its own is a limited way to compare two plans.

Opposing dentition and bruxism

What sits opposite the lower arch — natural upper teeth, an upper implant-supported arch, or an upper denture — affects the loading pattern the lower prosthesis experiences, and is weighed alongside bone and nerve position when a clinician discusses implant number and distribution. Confirmed bruxism (teeth grinding or clenching) is often raised specifically in lower-jaw planning, since the mandible is the moving jaw that generates the grinding force; where it is identified, a clinician may discuss wider implant distribution, a particular prosthetic material, occlusal adjustments, or a night guard. As with the upper jaw, this is one factor feeding into several decisions together, not a rule applied on its own.

Why six is not automatically mandatory

It isn't. Because mandibular bone is, on average, denser and more forgiving than maxillary bone, All-on-6 is discussed for the lower arch somewhat less often than for the upper arch in the literature and in clinical practice — but "less often discussed" is not the same as "never needed". Where bite forces are heavy, bone volume is compromised by long-term tooth loss, or the interforaminal bone available is narrow, additional implants and a wider footprint may still be part of the plan. Treat any claim that the lower jaw "never needs six" as a generalisation, not a rule for any specific reader.

Why four can still be appropriate

All-on-4 was originally described, in the implant literature by Paulo Maló and colleagues, as an approach suited to both jaws, with its angled posterior implants designed to make fuller use of available bone while shortening the cantilever. In the lower jaw, where bone is typically dense and the interforaminal region offers a reliable zone for anterior implants, four well-placed and well-distributed implants — angled to work around the nerve canal rather than close to it — can support a stable, fixed prosthesis without additional grafting. This is why All-on-4 remains a frequently used, legitimate option for the lower arch, not a compromise chosen only when six is unavailable.

Why treatment plans differ between patients

Two patients can present with what looks, superficially, like a similar lower jaw — similar tooth loss, a similar bite — and still be advised different implant numbers or configurations once a CBCT scan is reviewed. Bone density is not uniform across the arch or between patients; the nerve canal's depth and course vary considerably from one person to the next; opposing dentition, bruxism history and the planned prosthesis design all differ too. A treatment plan is built around what the imaging shows for that individual jaw, weighed against the patient's own priorities around timeline, cost and risk tolerance — not applied from a general rule about "the lower jaw" as a category. See who needs which for the fuller candidacy framework that brings bone, bite and budget together, and procedure differences for how the surgical steps themselves compare.

Decision framework (non-diagnostic)

None of the sections above are a diagnosis, and none replace a clinical examination. The table below summarises, at a glance, how the main lower-jaw-specific factors typically feed into a clinician's thinking — not a formula that outputs "four" or "six" from a checklist.

For the upper-jaw picture, which differs in several of these respects, see upper jaw. For what can go wrong and how it is managed — including nerve-related risk specifically — see failure & complications, and for how bone loss and grafting are assessed more broadly, see bone loss & bone grafting. Return to the All-on-4 vs All-on-6 home page for the full comparison overview.

Frequently asked questions