Do fixed appliances cause more root resorption than Invisalign?

Over the past year, the number of trials involving clear aligners increased. I am pleased to see that this trend has continued with this excellent new study into the effects of clear aligners and fixed appliances on apical root resorption. 

We all know that small amounts of external apical root resorption (EARR) are unavoidable during orthodontic treatment. However, it remains unclear whether there are differences in EARR between aligners and fixed appliances. This could be an important difference between these two treatment methods, and I thought it was great to see this examined in this new study.  

A team from Mato Grosso do Sul, Brazil, did the trial, and Orthodontics and Craniofacial Research published the paper. 

What did they ask?

They did this study to:

“quantify the amount of External Apical Root Resorption (EARR) following non-extraction treatment of Class I malocclusion with moderate crowding, comparing Clear Aligners and Fixed appliances.” 

What did they do? 

They did a parallel randomised control trial with a 1:1 allocation. The PICO was:

Participants

Orthodontic patients with Angle Class I malocclusion and moderate crowding (Little’s index 4-8 mm) treated non-extraction.

Intervention

Group one: Patients were treated with clear aligners (SmartTrack, Invisalign).

Group two: These patients we’re treated with standard fixed appliances with an 022 slot.

Outcomes. 

The amount of external apical root resorption. 

They used a pre-prepared randomisation, They concealed the allocation using sealed envelopes. 

It was not clear in the paper whether one or more operators treated the patient. They prescribed aligner changes every 10 days. The patients were asked to wear their aligners for at least 22 hours per day. 

In the fixed appliance group, patients were treated with conventional fixed appliances bonded to all teeth, following a standardised archwire sequence.

They defined treatment completion as the attainment of the six keys to normal occlusion.

They performed CBCT scans at the start and end of treatment. Importantly, they developed a sophisticated method of superimposing the incisors to calculate the amount of external apical root resorption that had occurred.

A person who was blinded to the allocation analysed the CBCT scans.

They conducted a thorough sample size calculation, which suggested that they needed 12 patients per group.

Finally, they conducted relevant statistical analyses by performing intergroup comparisons using t-tests and then performing linear regression to assess the influence of treatment method, patient age, sex, Little’s Index, treatment duration, and tooth type on the amount of EARR.

 What did they find?

They randomised 20 patients to each intervention group. Of these, five in the aligner group and three in the fixed-appliance group were lost to follow-up.

At the start of treatment, the mean Little’s index was 4.54 millimetres in the aligner group and 4.95 millimetres in the fixed appliance group. The mean treatment duration was 26.2 months (SD 7.84) in the aligner group and 22.4 months (SD 6.0) in the fixed appliance group (p>0.05). There was no statistically significant difference between the groups for any of the pretreatment measurements.

The overall median root resorption for the sample was -0.72 mm. There was no difference between the clear aligners (- -0.71 mm) and the fixed appliances (-0.72 mm). 

The linear regression analysis showed that the amount of EARR was not influenced by treatment type, age, sex, degree of initial crowding, or duration of active orthodontic treatment. The upper lateral incisors had the greatest mean EARR, followed by the lower incisors and the upper central incisors. Importantly, these differences were rather small, in the order of less than 0.5 of a millimetre. 

Their overall conclusion was that

“there was a degree of external apical root resorption for both clear aligners and fixed appliances. There was no difference between the two interventions”,

What did I think? 

This was an excellent small trial that had been done very well. The authors also wrote a clear, succinct paper. 

The strengths of the study were:

• The study method was good, but I was not sure on whether there were multiple or single operators.
• The statistical analysis was entirely relevant and took into account the effect of several co-founders
• Importantly, they also developed a new method of analysing the amount of root resorption using CBCT images. However, this involved CBCT imaging at short intervals.

While the main outcomes in the study were interesting, I also thought it was important to consider other outcomes. These showed that both treatment methods were effective in achieving a good occlusal result. Furthermore, there were no differences in treatment time between the Aligners and the fixed appliances. Another important outcome was the treatment completion rates. In this respect, it was interesting to see that 5 (25%) of the aligner cases did not complete treatment. This is a high discontinuation rate and reflects levels of co-operation with removable appliances.

In the fixed appliance group, 3 patients had concerns about CBCT imaging and did not agree to further imaging. 

When I considered the main outcome measures, it was reassuring to see that, although we must expect some external apical resorption with our treatments, the amount of resorption was low for both treatment interventions, and there were no differences between the treatment groups. 

Finally, it was great to see a well-conducted study comparing the effectiveness and effects of clear aligners and fixed appliances.