November 11, 2019

What force should we apply for orthodontic tooth movement?

We all know that we should keep orthodontic forces light for optimum orthodontic tooth movement. Surprisingly, we do not really know how much force we should apply. This new systematic review gives us an answer.

I have never really understood orthodontic tooth movement. I just know that you apply a light force to a tooth and it moves.  I was, therefore, interested to see this new systematic review. A team from the Netherlands and Indonesia did this study. The AJO-DDO published it.

Optimal force magnitude for bodily orthodontic tooth movement with fixed appliances: A systematic review

Christina I. Theodorou

Am J Orthod Dentofacial Orthop 2019;156:582-92

In their introduction, they pointed out we do not fully understand the exact biological mechanisms that determine orthodontic tooth movement. Furthermore, we do not know the optimum force that we need to use with certainty.  As a result, they decided to attempt to find this out by doing a systematic review.

What did they ask?

They wanted to gather information on:

“What is the optimum force range for orthodontic tooth movement in humans who are having orthodontic treatment with fixed appliances”.

What did they do?

They did a systematic review with the following PICOS.

Participants:    Humans in the permanent dentition

Intervention:   Orthodontic treatment with fixed appliances using a measured force applied bodily in a mesiodistal direction.

Control:           No treatment or intervention with a different force

Outcome:        Rate of orthodontic tooth movement (OTM). Secondary outcomes were external root resorption and pain.

Study design: Randomised controlled trials and randomised split-mouth studies.

They did a standard electronic and hand search to make sure that they found as many papers as possible. They used a customised data extraction form and assessed risk of bias with the Cochrane Risk of Bias Tool.

What did they find?

After the searches and usual filters to exclude papers that were not relevant. They identified 12 studies. These were divided into 10 split-mouth studies and 2 RCTs. When they looked at Risk of Bias, they found that 1 RCT was at low risk of bias and all the other studies were at unclear risk.

Unfortunately, they identified high heterogeneity between the studies. This meant that they could not pool the data and carry out a meta-analysis.  The heterogeneity arose from differences in methodology, clinical diversity and poor statistical reporting.

11 of the studies reported on canine tooth movement and one measured 2nd molar movement. In most of the trials Ni-TI springs were used to apply the force.

They reported their data clearly. They did this by dividing the amount of force into 4 groups.  I have put this data into this table.

GroupForce cNOTM mm/week(range)
Very high250-4000.34-0.49

They did not find any clinically significant effects on pain or external root resorption.

The studies that used a high force reported more unwanted side effects such as loss of control or rotation.

Their overall conclusion was:

“There is weak to moderate strength of evidence showing that forces ranging between 50cN and 100cN are optimal for OTM with potentially lower side effects”.

What did I think?

Firstly, I thought that it was great to see a systematic review come up with some positive findings that were clinically relevant. Some readers may feel that these results are rather obvious, however, this is the first systematic review to identify the optimum force for bodily tooth movement.

However, before we all get excited, the authors did point out some issues with the review. Firstly, we need to be cautious because of the risk of bias in the study. Importantly, this introduces a degree of uncertainty into the findings. It was also not possible to take into account the different types of appliances that were used. Nevertheless, we also know that the choice of appliance does not seem to influence tooth movement.  Furthermore, the skill of the operator is likely to affect the outcomes.  While some may feel that this diminishes the value of the finding. I actually think that this could make the results transferable to most clinical settings.

If I were in training and I had exams in the near future, I would look carefully at this paper.

Final comments

I did not understand the cN as a unit of force. I was always taught that we needed to apply a 150gm force to make a tooth move.  A cN is a decimal fraction of a Newton. It is equal to 1.01 gram force.  Therefore, this paper shows that we need to apply between 50 and 100 gram force for optimal tooth movement. Which is less than I thought…and is nice.


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Have your say!

  1. Interesting study and much needed. We must know the relationship between force and tooth movement, or at least base our treatment on the best evidence and logic.

    It is difficult to know what OTM means. Is this the maximum we should use? The maximum rate of tooth movement? Or something more mysterious only known to Angle, Begg and Tweed?

    Does OTM depend on the root surface area of the tooth/teeth being moved? Canine retraction v en masse? We do know that there is no difference in terms of anchorage/retraction. Therefore, force (or more accurately, pressure in the pdl) may really be important. We do know that higher pressure = faster tooth movement. This study suggests that there is no increase in pain or root resorption with higher pressure. The loss of control and rotation are more likely to be due to overpowering the archwires. There cannot be any other explanation, unless there is magic in the brackets and tubes. It should relate to the physics in the system.

    The assumption that we need a light force, is just that, an assumption based on received wisdom and not based on evidence. The real question should be, how can we control teeth with a higher rate of tooth movement, especially as there are no physical side effects?

    Root resorption seems to be related more to the the time in treatment rather that pressure in the pdl. There are poor studies, particularly those out of Sydney, that attempt to show otherwise, largely due to bias and preconceived ideas. Higher pressure = shorter treatment time and less root resorption.

    We know that RAP allows us to move teeth faster. The early application of force across an extraction site certainly results in faster space closure, as we have witnessed when teeth are extracted in treatment. Perhaps we need higher pressure there too?

    Higher controlled pressure appears to be the solution to more efficient treatments. It might be what we have missed because we believed our mentors.

  2. According to Burstone CJ, Choy K. The biomechanical foundation of clinical orthodontics. Quintessence: IL., 2015, pp. 13-14.
    “Traditionally, orthodontists use the gram as the unit of force. In the strict sense as explained, above, this is incorrect because grams are a unit of mass and not force . . . Scientifically, a centi-Newton (cN) is the correct unit of force.”

    • Hey Dan; You will see Kevin actually stated gram-force (gf) which is another unit of force but many incorrectly just call it grams which is different (mass as you pointed out). E.g. 1 gram-force = 0.00980665 Newton = ~1cN

    • Dan, you are right. Gram force (gf) is not part of the International System of Units (SI). That‘s why N should be used. As the numbers for gf and cN are quite similar (from a clinical perspective) it is correct and helpful to use cN.
      Have a nice day

  3. How do we know how much force we are applying in everyday orthodontic practice? We usually apply force by the use if NiTi springs, power chain or elastics, as well as from the light archwires used in the initial alignment stages. I never used to measure it, except when using EOT, I just know that it worked!

  4. Thanks for this post. As good as all others. Gram is not a unit of force. We have some incorrect terms as torque for a root movement…

  5. It appears that this discussion is mired in Newtonian physics and Darwinian genetics. Orthodontic forces applied during treatment are not targeting inanimate objects. These biologic entities have clinical behavior encoded by developmental mechanisms, which are subject to variations in gene expression. If we add information gleaned from molecular genetics and digital technologies to this discussion, a different view of tooth ‘movement’ begins to emerge. Actually, teeth don’t really move (except during mastication, etc. where it’s referred to as tooth support). Even the earliest physicians/dentists that attempted orthodontic tooth movement realized the significance of the “tissue reaction”. The crowns of the teeth, using Newtonian physics, can be viewed as inanimate objects but molecular biology reveals the extent of interactions between the oral microbiome and the surface/sub-surface crystalline lattices, a bit like particle physics in some ways. Teeth don’t move because they have no mechanism by which to do so. Therefore, tooth repositioning relies almost exclusively on bone remodeling, and the crowns of the teeth, which are genetically determined, take up whatever space is left, at least initially. Think of the Archimedes principle. Moreover, if the system is not returned to a level of equilibrium or balance, the ongoing ‘tissue reaction’ will eventually regress to a state of homeostasis, which might be viewed as ‘relapse’ clinically. So, when is a force not a force? When it fails to reach a biologic threshold. Biologic, including craniofacial, thresholds are so precise that patients can detect perturbation of the spatial matrix when a restoration is 10 microns too ‘high’. Thus, the discussion of ‘how much force’ is a bit like using a sledge-hammer to break a peanut shell. Teeth, like other structures in the body, respond to various signals, such as stretch, compression, vibration, pressure changes, spatial relations etc. These signals are detected by mechanoreceptors in the periodontium and, following signal transduction, a cascade of events is initiated that initiates gene transcription and mRNA biosynthesis. Orthodontic mechanics override these sophisticated biologic processes and induce inflammation to forcibly move teeth; an analogy might be that airplanes produce mechanical flight leaving a carbon footprint, while birds deploy physiologic flight mechanisms. In the 21st century, perhaps the question should not be ‘how much force’ but rather, do you want to use protocols that reposition teeth mechanically or biologically?

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