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.
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.
|Group||Force cN||OTM mm/week(range)|
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.
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.
Emeritus Professor of Orthodontics, University of Manchester, UK.