Screw preload and torque: how implant screws hold, and why they sometimes loosen

A screw is a spring

When you drive a wood screw into a shelf, the thread bites into the wood and friction holds it. An implant screw works differently. It goes into a precisely machined metal thread inside an implant or abutment. When the dentist tightens it to the specified value, the head of the screw presses on the part above, the thread pulls on the part below, and the screw shaft stretches by a tiny amount, a few micrometres. Like a stretched spring, it pulls back, and that pull clamps the two parts together.

That clamping force is called preload. It is what keeps the abutment tight on the implant and the bridge tight on the abutments. As long as the clamping force is greater than the forces trying to separate the parts, the joint stays closed and the screw does not turn.

The torque the dentist applies is not the preload itself. Much of the turning effort is spent overcoming friction in the threads and under the screw head. Only part of it becomes stretch. That is why manufacturers specify both the torque value and, often, which screw and sometimes which lubrication or coating should be used. Change the friction and the same torque gives a different preload.

The numbers on the wrench

Implant torque is measured in newton-centimetres (Ncm). Values vary by manufacturer and by part. An abutment screw into the implant is often tightened higher than a small prosthetic screw that holds the bridge onto a multi-unit abutment. In the laboratory studies reviewed by Pardal-Peláez and colleagues (2017), the screws tested were tightened to between 20 and 45 Ncm. Your dentist uses a calibrated torque wrench or a torque-controlled driver, not a hand screwdriver, to reach the right value.

Why chewing loosens screws

Every bite pushes and rocks the bridge very slightly. Over hundreds of thousands of chewing cycles, microscopic high spots on the mating surfaces flatten out (engineers call this embedment or settling), and tiny movements at the joint gradually let the stretched screw relax. Each relaxation lowers the preload a little. If it drops far enough, the joint can open under load, the screw can start to turn, and the bridge feels loose.

The laboratory evidence shows that some loss is universal. Pardal-Peláez and colleagues found torque losses after cyclic loading of between 16.1% and 39% across the studies they reviewed, which used between 16,667 and 1 million cycles and loads up to 400 N. A 2026 meta-analysis of 21 laboratory studies, Sammour et al., 2026, gave pooled losses by connection type: 23.9% for external hexagons, 19.2% for internal hexagons, 16.6% for Morse tapers and 16.0% for conical connections. Angled abutments increased torque loss, and screw material made a difference.

These are bench tests in a machine, not mouths, and the cycle counts are a stand-in for months or years of chewing. They are still useful because they show the direction of each effect.

What makes loosening more or less likely

A 2018 systematic analysis by Nithyapriya and colleagues listed the factors studied in 36 papers: screw material, how torque is applied, the tightening sequence, the type of abutment connection, lubrication and the abutment collar length. A 2020 review by Vinhas and colleagues concluded that conical abutment connections appear to result in fewer mechanical complications such as loosening or fractures, and better torque preservation. Putting the research together with clinical experience, the main influences are:

• Fit. A framework that does not seat passively is pulled into place by the screws, using up some of their clamping force before you bite. See passive fit.
• Connection design. Conical and tapered joints share load with the cone walls and keep preload better in lab tests (see implant connections).
• Correct torque and re-tightening. Many protocols tighten a screw, wait a few minutes, and tighten it again to the same value, to offset early settling.
• Genuine parts. Screws are made to tolerances matched to each system. Mixing parts can change friction and fit.
• Bite forces. Heavy biting, grinding and clenching increase the load the joint sees. Long overhangs at the back of a bridge act as levers.

We have deliberately written these as separate influences, but in a real mouth they combine. A slightly misfitting bridge on an angled abutment in someone who grinds at night is a very different situation from a passive bridge on straight abutments in a gentle chewer.

What loosening feels like, and what to do

Patients usually notice a faint click, a feeling that the bridge moves when they bite, a new gap, a change in taste or smell from one area, or sometimes nothing at all until a check-up. If you notice movement, stop chewing hard food on that side and contact your dentist. A single loose screw is a simple fix if caught early: the bridge is removed, the connection checked and cleaned, the screw replaced if needed and tightened to the correct torque. Left alone, a loose screw can break, and the remaining screws take extra load.

Provisional teeth, final teeth and screw checks

Temporary full-arch bridges are lighter and more flexible than final ones and are often fitted on the day of surgery, before bone has fully healed around the implants. They have their own problems: in Cercadillo-Ibarguren et al., 2017, 16.8% of provisional full-arch bridges fractured within nine months, more often in people with bruxism. When the final, stiffer bridge is fitted, screws are tightened to the final values and the bite is checked. At later maintenance visits, many dentists remove the bridge periodically to clean underneath and check screws, though how often depends on the person and the protocol.

Why screws are designed to be the weak point

It may sound odd, but engineers generally prefer the prosthetic screw to be the part that gives way first. A screw is cheap, small and replaceable in minutes. If something in the system must yield under an unusual load, such as biting on a stone in a salad, a loosened or broken prosthetic screw is a far better outcome than a cracked implant or a fractured framework. A loose screw is therefore partly a warning signal: it tells you and your dentist that forces on that part of the bridge are higher than they should be, and it is worth looking for the reason, such as a high spot in the bite, a long overhang or night-time grinding.

Access holes and their fillings

On a screw-retained bridge, each prosthetic screw sits at the bottom of a small channel that opens on the biting surface or the tongue side of a tooth. After tightening, the channel is usually sealed with a soft material over the screw head and a tooth-coloured filling on top. If a filling falls out, the screw is still there, but the channel can collect food. Ask your dentist to reseal it; a dentist with the right driver can also check the screw at the same visit.

For travellers

If you have treatment abroad, two things help a dentist at home deal with a screw problem: knowing the implant brand and system, and knowing the screw and torque specification. Your implant passport should give the first; the manufacturer publishes the second. Precious Dental, the publisher of this library, uses MegaGen, Neodent and Straumann implants (named for information; we are not affiliated with any maker) and gives every implant patient an implant passport and copies of their X-rays and scans. For the bigger picture on mechanical problems, see our complication rates data, and for what holds the teeth together, frameworks and bars.

Common questions

What does Ncm mean on a torque wrench?

Newton-centimetres, a unit of turning force. Implant manufacturers specify a value for each screw, and the dentist uses a calibrated torque wrench to reach it. Values tested in the studies reviewed by Pardal-Peláez and colleagues ranged from 20 to 45 Ncm.

Why not just tighten the screw as hard as possible?

Over-tightening can stretch the screw beyond its elastic range, damage the thread or the implant connection, or break the screw. Under-tightening leaves too little clamping force. The specified torque is the balance point for that screw design.

Is a loose bridge an emergency?

It is not usually dangerous, but it should be seen soon. When one screw loosens, the others carry more load and the bridge can rock, which can break screws or chip teeth. Avoid chewing on that side and contact your dentist.

Do screws need replacing?

Sometimes. Some manufacturers recommend a new prosthetic screw when a bridge is finally fitted or after it has been removed a number of times, because threads wear. Follow the protocol for your implant system.

Does grinding my teeth make loosening more likely?

Heavy, repeated forces from grinding or clenching add to the cyclic load that relaxes preload, and grinding is linked to more fractures of provisional full-arch teeth in clinical studies. Tell your dentist if you grind; a night guard may be advised.

Sources

1. Pardal-Peláez B, et al. Preload loss of abutment screws after dynamic fatigue in single implant-supported restorations. A systematic review. J Clin Exp Dent 2017. PMID 29302289
2. Sammour SR, et al. Impact of implant abutment connection designs and cyclic loading on screw stability in dental implants: A systematic review and meta-analysis. J Prosthet Dent 2026. PMID 41102040
3. Nithyapriya S, et al. Systematic analysis of factors that cause loss of preload in dental implants. J Indian Prosthodont Soc 2018. PMID 30111906
4. Vinhas AS, et al. Review of the Mechanical Behavior of Different Implant-Abutment Connections. Int J Environ Res Public Health 2020. PMID 33238476
5. Cercadillo-Ibarguren I, Sánchez-Torres A, Figueiredo R, Valmaseda-Castellón E. Early Complications of Immediate Loading in Edentulous Full-Arch Restorations: A Retrospective Analysis of 88 Cases. Int J Oral Maxillofac Implants 2017. PMID 28403247

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