Bone Loss & Bone Grafting: All-on-4 vs All-on-6

Why bone quantity matters

Every implant needs enough bone in three dimensions — height, width and length along the ridge — to gain primary stability at the moment it is placed, and to stay firmly anchored afterwards. Quantity is usually described in millimetres of available height and width at each proposed implant site. When teeth are lost, the bone that once supported them starts to remodel and can gradually reduce in volume over months and years — a process sometimes tracked using resorption classifications such as the Cawood & Howell scale, which describes jaw shape changes from a well-rounded ridge through to a flat, resorbed one. The less bone is available at a site, the more implant number, position and angle become part of the conversation, which is why bone quantity — more than personal preference — is usually the starting point for what is even possible.

Why bone quality matters

Bone quantity is only half the picture — density matters too. Bone at the front of the lower jaw is typically the densest in the mouth, while the back of the upper jaw is often the least dense, since it borders the maxillary sinus and tends to be more porous. Systems such as the Lekholm & Zarb bone-quality classification and the Misch bone-density classification are widely used in implant planning to describe this range, from dense, compact bone through to soft, sponge-like bone. Denser bone generally offers better initial (primary) stability for an implant at the time of surgery, which is one reason clinicians favour anterior sites when angling implants in the All-on-4 concept. Poorer-quality bone does not automatically rule out treatment, but it can change the implant length, diameter, surface treatment, or healing time a clinician recommends.

Posterior bone availability

The back of the jaw — the posterior region behind the premolars — is usually where bone is most limited, in both arches. In the upper jaw, this is because the maxillary sinus can expand downward into the space once occupied by tooth roots, a process called pneumatisation, particularly after back teeth have been missing for some time. In the lower jaw, posterior bone height is limited by the position of the mandibular (inferior alveolar) nerve canal, and posterior width can also reduce after long-term tooth loss. Because the posterior region decides how much bone is realistically available for rear implants, it is usually the crux of the All-on-4 vs All-on-6 decision — not the front of the arch, where bone is more often adequate for either configuration.

Angled and tilted implant concepts

The defining feature of the All-on-4 concept, first described in the implant literature by Paulo Maló and colleagues, is that the two rear implants are placed at an angle rather than straight down. Angling the implant lets it travel further forward through the jaw before it exits the bone, so it can engage denser anterior bone and route around structures such as the maxillary sinus or the mental nerve loop, in anatomy where that is possible. It can also shorten the unsupported cantilever at the back of the bridge, compared with placing a straight implant further back. All-on-6 typically places most or all of its implants more upright, spread further along the arch; angled placement is sometimes used for the rearmost implants here too, but the wider footprint is the main structural difference between the two. Neither angling nor implant count alone determines the outcome of a case — placement accuracy, planning and how the bone responds afterwards all contribute. See procedure differences for how this feeds into the wider surgical plan.

Schematic only, not to scale — illustrates typical implant angulation, not a specific patient plan.

When four implants may be considered

• Moderate bone loss where angled placement can make use of the bone that remains, without needing to graft first.
• A preference to reduce or avoid grafting where the anatomy allows it — confirmed on a CBCT scan, not assumed.
• An average bite, without significant grinding or clenching.
• Budget is an important factor, alongside anatomy that supports the option.
• A shorter procedure with fewer implant sites is preferred, where clinically appropriate.

These are illustrative patterns, not a diagnosis — the same list can describe a patient who is ultimately advised to have All-on-6 instead, once their imaging is reviewed. See who needs which for the fuller candidacy picture.

When six implants may be considered

• Posterior bone volume and density are sufficient to support upright implant placement without grafting.
• A heavier bite or confirmed bruxism has been identified, where a clinician judges wider distribution useful.
• The planned bridge is longer, or a clinician judges that spreading load across more fixtures suits the case.
• The patient and clinician agree the additional surgical time and cost, covered on cost comparison, are justified for the anatomy and risk profile involved.
• A patient with adequate bone actively prefers the wider support of six implants, having discussed the trade-offs.

As above, this list describes tendencies, not a formula — it is not diagnostic for any individual reader.

Does All-on-4 always avoid bone grafting?

No. All-on-4's angled placement can reduce or avoid grafting in a meaningful proportion of patients with moderate bone loss, which is part of why the technique became widely used — but it is not a guarantee. Where bone loss is severe, even an angled implant may not find enough anchorage, and grafting, an alternative implant position, or in some cases a different implant protocol altogether may still be discussed. Treat any claim that a specific procedure "avoids grafting" as a generalisation about anatomy that is common, not a promise about your own jaw — only a CBCT scan can settle that.

Does All-on-6 always require bone grafting?

No. Where a patient's posterior bone height and density are already adequate, All-on-6 can sometimes be placed without any grafting at all. Grafting is discussed more often with All-on-6 than with All-on-4 mainly because its rear implants are typically placed more upright, which generally calls for more bone height at the site than an angled implant needs — not because six implants inherently require more surgery. As with All-on-4, the honest answer for any individual patient is anatomy-specific, not procedure-specific.

Sinus anatomy in the upper jaw

The maxillary sinuses are paired air-filled cavities sitting above the back teeth of the upper jaw, separated from the mouth by a thin floor of bone. When upper back teeth are lost, that floor can gradually expand downward into the space the tooth roots once occupied, reducing the bone height available for an implant placed straight down in that area. This is one reason posterior upper-jaw implants are more often discussed alongside grafting than implants elsewhere in the mouth, for either All-on-4 or All-on-6. Where residual bone beneath the sinus floor is limited, a clinician may discuss a sinus lift, an alternative implant angle or position that avoids the sinus, or occasionally a different implant approach; exactly where that line falls is judged from your own CBCT images, not a single fixed number that applies to everyone. See upper jaw for the fuller picture of upper-arch planning.

Bone grafting vs an alternative implant distribution

When posterior bone is limited, a clinician generally has more than one route available, and weighing them is part of the planning conversation rather than a fixed rule. Grafting — including a sinus lift — rebuilds bone so an implant can eventually be placed in something closer to its ideal, upright position, at the cost of additional healing time, often months, before implants go in or are loaded. Angling implants, or repositioning them to use bone that is already present, can sometimes avoid that wait, at the cost of a different cantilever length or prosthetic design. Neither route is universally the "better" one: grafting can offer a more conventional long-term foundation in the right case, while avoiding it can mean a faster overall timeline in the right anatomy. The choice depends on how much bone is missing and where, a patient's priorities around timeline versus predictability, and what the CBCT actually shows.

Severe bone loss

Long-term denture wearers, or patients who lost teeth many years ago, sometimes present with more extensive bone loss than average in one or both jaws. This does not automatically rule out a fixed full-arch solution, but it can change what is realistically on the table. Depending on the anatomy, options a clinician may discuss include more extensive grafting before implants are placed, repositioning or angling implants to use whatever bone remains, or — in more limited cases — alternative implant approaches such as zygomatic implants, which anchor into the cheekbone rather than the jaw itself. None of these should be assumed to apply to any individual reader; they illustrate the range of approaches used in severe cases, and the right path is assessed on imaging and in person, not from a page like this one. See research & evidence for sourced background on outcomes in these more complex cases.

Why a CBCT matters

A cone-beam CT (CBCT) scan gives a clinician a genuinely three-dimensional picture of your jaw — height, width and density at each proposed implant site, plus the exact position of structures such as the sinus and nerve canal. A conventional 2D panoramic X-ray, by contrast, can show general tooth and bone position but cannot reliably measure bone width or show these structures in three dimensions. Guidance on implant treatment planning generally favours 3D imaging ahead of full-arch surgery for this reason: it lets a clinician plan implant number, position and angle around your actual anatomy, rather than working from an approximation. If a clinic proposes a full-arch treatment plan — including whether grafting is needed — without having reviewed a CBCT of your jaw, that is a reasonable point to ask about before proceeding.

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 bone-related factors typically feed into a clinician's thinking — not a formula that outputs "four" or "six" from a checklist.

For the full picture of how these bone factors combine with bite forces, arch length and budget, see who needs which, and try the interactive 4-or-6 decision framework on the comparison homepage.

Frequently asked questions