How accurate is IPR?

After decades of debating the merits of non- extraction / extraction treatment , the ease and enthusiasm with which the orthodontic community has embraced interproximal reduction is remarkable . While John Sheridan has made IPR popular by introducing airotor stripping , its new found unquestioned acceptance is doubtless aligner and market driven. I have often been discomfited at the effect it might have on decay and sensitivity considering the large amounts of IPR that are considered possible,my concerns only partially assuaged at the reassurance that literature give us.

Thus I found the following article very interesting since I believe it to have a bearing not only on efficacy of aligner therapy but also indirectly on enamel soundness.

This study was done by a team from the University of Campania Luigi Vanvitelli, Italy.

De Felice, M., Nucci, L., Fiori, A.,Mir CF,Perillo L.,Grassia V. Accuracy of interproximal enamel reduction during clear aligner treatment. Prog Orthod. 21, 28 (2020).

What did this paper study ?

“To compare the accuracy of the actual space obtained through interproximal enamel reduction (IPR) compared to the amount of IPR planned through the digital setup during clear aligner treatment (CAT)”

How did they study this ?

They chose four cases each from 10 clinicians randomly chosen using the Doctor locator of Align technology making up a total of 40 subjects and potentially 80 dental arches all subjected to IPR with manual stripping. The average difference between the planned IPR and the actual IPR was recorded for each arch individually by comparing mesio-distal measurements from second premolars to second premolars.

What did their results show?

Only 61 arches were studied as opposed to 80 arches mentioned in the abstract.Of these, 29 upper arches and 32 lower arches comprised the sample.Although the quantum of planned IPR was greater in the lower arch the accuracy of IPR on average was 44.95% in the upper arch and 37.02% in the lower arch. Except for 2 cases in the upper arch and 4 cases in the lower arch the IPR performed was less than planned.

What I liked!

I thought this was a simple but important question and rather well executed. Though a prospective clinical study, the random selection of Invisalign providers to some extent does eliminate selection and performance bias. Did the clinicians know what was being evaluated? One assumes so since there appears to be some standardization in the IPR protocol.

The decision to consider only manual stripping of .1 to .5 mm eliminated the question –“Which is the most accurate method of IPR ? “ and focused only on manual stripping.The use of silicone impressions imported into a software system before and after IPR ironed out the possible errors in this method.As an aside… was the choice of impression material the same in all 10 operators? This was not mentioned.

Another positive point is that all measurements were made by a single operator and measurement accuracy on the software was considered.

What I would have liked to know!

The amount of IPR planned on average was not a large amount.( 1.09 mm in the upper arch and 1.43 mm in the lower arch). While the study proved that the average IPR performed was significantly less than that planned with marginally greater accuracy in the maxillary arch, inter-arch comparison was not done. Further , the authors failed to consider whether the discrepancy was more in the anterior or posterior teeth/mesial or distal surfaces? This is an important question that could have been answered considering the authors had data from 1220 teeth and twice as many surfaces .I believe this is valuable information that would have given more clinical insight. A recent published study also from Italy gives some information in this regard.1

Was there a specific reason why the authors used 3Shape and not the Bolton’s tool on the Clinchek software?The latter might have been more in line with the IPR planned for the specific case and the realization of treatment goals .

The 3 Shape software perhaps allow authors to select which part of the tooth is to be measured and ensure standardization before and after ?-or was this left to software intelligence?

Curiously :

The authors state that the” Little’s irregularity index was significantly different pre and post treatment in both upper and lower arches and did not affect the accuracy of IPR.” The Little’s irregularity index (IR) is primarily recommended for the lower arch. The accuracy of IPR was greater in the upper arch whereas the IR was less. Does this mean IPR is more accurate when irregularity is less? Difficult to answer conclusively since the authors did not correlate the IPR to the extent of IR in upper and lower arches .This might have substantiated the above statement better . Moreover the average Little’s IR in both the arches was far greater than the average planned IPR which proves that other methods, possibly expansion helped alleviate the irregularity.

So I believe this point on the Little’s irregularity index is superfluous.

Conclusions and future direction

The study concluded that the IPR performed was less than the IPR planned with greater accuracy in the maxillary arch though the authors opined that though statistically significant and considerable in terms of percentage, the difference between the planned and executed IPR might not be clinically relevant.This was on average about .55 mm in the upper arch (minor mismatch in the SD between abstract and results) and .82 mm in the lower arch. While this might not seem a lot /arch the clinical relevance might be significant if larger amounts of IPR are planned.

Research needs to focus on methods to achieve more reliable IPR outcomes to realize treatment goals and preserve enamel.

1.Zamira Kalemaj,Luca Levrini,

Quantitative evaluation of implemented interproximal enamel reduction during aligner therapy:A prospective observational study.Angle Orthodontist,1 Jan 2021;91(1):61-66.

Contributed by :

Dr.Sridevi Padmanabhan

Prof. Dept. of Orthodontics,SRIHER

Hon. Secretary, IOS

Past Editor, JIOS

Editor ,SRJHS

Assistant Editor , APOS trends