When Biomechanics Catches Up

Some cases don’t fail early. They wait… until biology meets physics again.

Resume :

Full-arch implant rehabilitation success depends on the balance between surgical placement, prosthetic design, and biomechanical principles. This clinical case or "this real live situation " illustrates a delayed prosthetic failure occurring more than a decade after mandibular rehabilitation. The initial configuration—characterized by limited anterior–posterior (AP) spread and unfavorable cantilever—remained stable under reduced functional load. However, subsequent maxillary fixed rehabilitation increased occlusal forces, leading to overload at the prosthetic interface. The failure manifested as fracture of multi-unit abutments and prosthetic screws. This case highlights the long-term biomechanical consequences of compromised implant distribution and reinforces the importance of fundamental prosthetic principles in full-arch rehabilitation.

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Good morning, world. Good morning Lisbon.

We’ve just returned from an amazing ski holiday — skiing, snowboarding with the kids — energy fully recharged for what’s ahead.

Granada… oh Granada… what a beautiful city! I love it… and I love even more the slopes and the après-ski in Sierra Nevada!

But before leaving, I had one of those cases that remind us why we can never take things lightly.

A patient of mine from over 10 years ago. A referral case from a colleague — the great Pedro from the south bank of lisbon (Almada), a classmate from the old ISCS-Sul ( now called Egas Moniz), more than 25 years ago.

A severely compromised mandible, with fractured implants and a high level of complexity — aggravated by extreme dental anxiety.

At the time, we treated her under general anesthesia.

I remember it well: limited experience, limited tools. Implant extractors didn’t exist like they do today. Removal was difficult, demanding, almost “manual.”

Liliana was sweating… I was sweating…Everybody was sweting ...

We worked with extreme care around the mental nerves, together with Dr. Marta.

One of those cases that stays with you.

Especially because, at that time, these cases were usually handled by the clinic owner — and I wasn’t supposed to do them.

But this one… had to be mine.

And of course — no real help....actually no help at all.....

Building resilience ( we leave that for a different refletion).

Let’s go.

After removing the fractured implants, we performed bone regularization and placed four implants in the interforaminal region.

And here comes the critical point:

implants placed almost in a straight line, with very limited AP spread.

Sometimes this happens due to anatomical limitations.

But other times — it’s fear.

Fear of getting too close to the mental nerve (always with Solar’s paper and Sonick’s anterior loop in the back of your mind…).

And we end up here.

At the time, it worked.

In fact… it worked for 10 years.

Osteointegration and function — more than a decade.

And it worked for a simple reason:

the opposing arch was a complete removable denture → low masticatory forces → the system was “forgiving.”

We restored with a 12-unit metal-acrylic hybrid prosthesis.

Biomechanically, far from ideal:

• Limited AP spread

• Unfavorable cantilever

• Inefficient load distribution

But clinically… stable.

For years.

Until everything changed.

Around 10 years later, the patient decided to rehabilitate the maxilla.

We placed six implants in a very narrow ridge, under sedation. With difficulty, we achieved a fixed upper rehabilitation.

(this case deserves its own Docs in Dentistry article…)

And at that moment… everything changed.

Function increased.

Masticatory forces increased.

And the lower system was no longer “forgiven.”

This week, the patient came in with a simple complaint:

“Doctor, I feel a click.”

On examination:

👉 All multi-units fractured

👉 Screws broken inside the implants

What we would call… a proper mess.

Notably:

• Osseointegration remained intact

• Implants did not fracture

• Failure localized at the prosthetic level

This aligns with the principle that overload is often expressed at the most vulnerable component of the system.

The case reinforces the importance of:

• Adequate implant distribution

• Minimizing cantilever

• Anticipating future changes in occlusion

• Designing restorations not only for present conditions but also for potential functional evolution

Complete removal of fragments (about 2 hours), replacement of components, and re-tightening of the prosthesis.

This system did not fail by chance.

It failed exactly where it was most predictable:

at the prosthetic interface.

Interestingly, Margarida observed everything… but didn’t really see it.

She didn’t ask the key question:

why?

Typical of someone who hasn’t yet internalized the value of a true proof of principle.

Indifference in these cases is dangerous:

it prevents understanding that replacing the multi-units only treats the consequence — not the cause.

And more importantly, it prevents better decision-making in future cases, where attention to the fundamental pillars of full-arch rehabilitation is critical.

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Takeaway

This case shows something very clear:

When two of the fundamental pillars of full-arch rehabilitation are compromised (AP spread and cantilever), the system will hold… until it doesn’t.

The 5 pillars remain unchanged:

1. AP spread

2. Cantilever

3. Occlusion

4. Interocclusal space

5. Implant-related factors

Here, at least two were outside acceptable limits.

And biomechanics, sooner or later, always collects its debt.

It wasn’t the implant.

It wasn’t osseointegration.

It was the weakest link:

👉 the prosthetic components.

And fortunately… it failed there.

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We now enter the second quarter of the year — with energy, focus, and a new clinic ahead of us.

The game continues.

But the rules… they don’t change.

Happy spring to everyone.

Chenão 🚀

Clinical Implications

• Long-term success of full-arch rehabilitation is dynamic, not static

• “Acceptable” biomechanics under low load may fail under increased function

• Opposing dentition must be considered a critical variable

• Prosthetic complications may be early indicators of biomechanical overload

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Conclusion

This case highlights that biomechanical compromises may remain silent for years before manifesting clinically.

When AP spread and cantilever are not within optimal parameters, the system may function temporarily but remains vulnerable. Changes in occlusal load can trigger failure, most commonly at the prosthetic interface.

Ultimately, biomechanics is unforgiving.

It does not fail immediately.

But it never forgets.

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Keywords

Full-arch rehabilitation; AP spread; cantilever; prosthetic complications; implant