Six on top, four underneath: why mixed plans are common

Many people needing a full arch need two: one for each jaw. It is natural to assume the plan will be the same top and bottom, “All-on-4” or “All-on-6” for both. In practice, many careful plans use different numbers, and the reasons tell you a lot about how the four-or-six decision really works.

Two jaws, two different jobs

The upper and lower jaws differ in ways that matter for implants:

Our page on the upper and lower jaw covers the anatomy in more detail.

What the research shows

A 2018 review by Polido and colleagues pooled 93 studies of full-arch bridges and looked at how many implants were used. In the group with five or more implants, the most common choice was six for upper jaws and five for lower jaws. In the group with fewer than five, it was four for upper jaws and three or four for lower jaws. Survival was high in both groups.

The evidence for four implants in the lower jaw is particularly strong. The long-term study by Maló and colleagues followed 471 lower-jaw All-on-4 bridges for 10 to 18 years and reported bridge survival of 98.8%. A review by Papaspyridakos and colleagues of lower-jaw full arches found implant and bridge survival above 96% at ten years.

The upper jaw is where more caution shows up. A 2024 review by Sharaf and colleagues of 53 upper-jaw studies found similar survival with four or six implants, but less bone loss around implants with six. And a meta-analysis of tilted implants found they failed more often than upright ones in studies limited to the upper jaw, though not overall.

Put together, the evidence fits the common clinical habit: more implants on top where bone allows, four underneath.

Typical mixed plans

• Six upper, four lower. The most common mixed plan. Upper bone is good enough for six; lower jaw has dense bone at the front and the nerve limits implants further back.
• Four upper, four lower. Common where the upper sinus has dropped and the patient prefers to avoid a sinus lift.
• Six upper, six lower. Used for heavy grinders, very strong bites, or where both jaws have good bone throughout.
• Four upper, six lower. Less common, but possible if the upper jaw has little bone at the back and the lower jaw has plenty.

Designing two bridges together

When both jaws are treated, the bridges are designed as a pair. The dentist decides tooth positions, the height of the bite and how the teeth will meet before deciding exactly where implants go. The number of implants under each bridge does not show in the bite; what matters is that the teeth meet evenly on both sides and slide smoothly when you move your jaw.

Two fixed bridges biting against each other produce higher forces than a bridge against a denture. That is one reason a dentist treating both jaws may want more support on at least one of them, usually the upper.

One surgery or two?

Both jaws can often be treated in one session. That means one recovery period and one set of temporary bridges, but a longer operation and more swelling at once. Some people prefer to do one jaw first and the other later, which spreads the recovery. If you are treated abroad, doing both jaws in the same trips is usually more practical. Ask your dentist which they recommend for you and how long the surgery would take. Our article on surgery length explains what adds time.

Cost of a mixed plan

A mixed plan costs between the price of four on both jaws and six on both. At clinics that price by package, the difference may be small. At Precious Dental, our top pick, All-on-4 for both jaws is £6,000 and All-on-6 for both jaws is £6,500; a mixed plan is priced according to the implants placed. Always ask for the final price in writing, broken down by jaw.

Only one jaw needs treatment

If you are having a full arch on one jaw and keeping natural teeth, a partial denture or existing bridgework on the other, the full arch is planned around what is opposite. Natural teeth opposite a full arch produce strong forces, which can favour more implants. A denture opposite produces less. If the opposing teeth are likely to need treatment in a few years, mention it: it may change what is sensible now.

Healing in two jaws at once

If both jaws are treated together, both sets of implants heal over the same months, and you wear two temporary bridges. The soft-food rules apply to both. The good news is that you only go through the healing period once. The harder part is that the first week can be more uncomfortable, with swelling above and below. Practical help in the first days, soft meals prepared in advance and patience with speech make it easier.

The final bridges are usually fitted at the same time on the second trip, so the bite can be set for both together. This is one of the main reasons dentists like to treat both jaws in the same timeline: designing two final bridges at once gives the most even result.

If one jaw is done later

Some people have one jaw treated first and the other a year or two later, for cost or health reasons. That works, but the second bridge then has to fit around the first. Tell your dentist at the start if you think you may need the other jaw later. They may plan tooth positions on the first bridge with the second in mind, or suggest a temporary solution on the other jaw so that the bite stays stable.

Reading a mixed-plan quote

• Number of implants for each jaw, stated separately
• Whether any are tilted, and whether grafts or a sinus lift are included
• Implant brand for each jaw
• Material of each final bridge
• Price for each jaw and the total
• Whether both jaws are treated in the same session

Questions to ask

• Why do you recommend this number for my upper jaw, and this number for my lower?
• Would one more pair on the upper jaw need a sinus lift?
• How will the two bridges meet, and who designs the bite?
• Will both jaws be done at once?
• If I grind, does that change the plan for either jaw?

A note on names

A quote reading “All-on-4” for both jaws means eight implants in total; “All-on-6” for both means twelve. A mixed plan is ten. Check per jaw, not the headline.

In short

There is no rule that both jaws need the same number of implants. Six on top and four underneath reflects real differences in bone and anatomy, and fits what the research shows. The best plans decide each jaw on its own scan and design the two bridges together. For the wider decision, see who needs which and four, six or eight.

Common questions

Is it normal to have different numbers on each jaw?

Yes. Each jaw has its own bone, anatomy and loads. Six on top and four underneath is a common combination.

Will my bite be uneven with a mixed plan?

No. The two bridges are designed together so that the teeth meet evenly. The number of implants underneath does not change how the teeth meet.

Could it be four on top and six underneath?

It can, if that is what the bone shows, but it is less common because the upper jaw is usually the one that benefits most from extra support.

Does a mixed plan cost more?

It sits between the price of four on both jaws and six on both. At clinics that price per implant, you pay for the implants actually placed.

Can both jaws be done on the same day?

Often, yes, in one longer session, though some patients and dentists prefer to treat the jaws separately.

Sources

1. Polido et al. (2018), Clinical Oral Implants Research — Number of implants placed for complete-arch fixed prostheses
2. Sharaf et al. (2024), Heliyon — Four or six implants for maxillary fixed prostheses
3. Maló et al. (2019), Clinical Implant Dentistry and Related Research — All-on-4 in the mandible: 10 to 18 years
4. Chrcanovic, Albrektsson, Wennerberg (2015), Journal of Dentistry — Tilted versus axially placed implants
5. Papaspyridakos et al. (2014), Clinical Implant Dentistry and Related Research — Mandibular implant fixed complete prostheses after at least 5 years

Checked October 2026. Where we give our own view as a clinic, we say so.