Interproximal Reduction: Splitting hairs?
Many of us use interproximal reduction (IPR) daily as a conservative means of space creation. We all seem to have our way of doing this. However, few can claim to know how precise we are at undertaking the procedure.
Inter-proximal reduction is also often prescribed as part of aligner therapy producing conservative amounts of space to promote the achievement of occlusal objectives. But how accurately do we mirror our planning in respect of space creation with IPR? Do we generate excessive or inadequate space? In which regions of the mouth or tooth might we be more accurate? Are any of the bewildering array of IPR options more or less accurate?
A team from Milan looked at these questions. The Angle Orthodontist published this study. It is therefore freely available online.
Authors:Zamira Kalemaj and Luca Levrini
Angle Orthod. 2021. doi:10.2319/040920-272.1
What did they do?
They conducted a prospective cohort study involving 50 participants.
Patients undergoing Invisalign therapy in one or two arches. They were periodontally stable, treated non-extraction, and not undergoing dental restorative work.
Six orthodontists treating between 5 and 10 participants each were involved. Two of these were very experienced, with four having ‘moderate’ experience. They used a range of mechanical approaches to interproximal reduction, including A combination of manual strips and burs, burs only, and contra-angle mounted strips. Four of the practitioners used measuring gauges. They obtained impressions at the beginning of the treatment and after the first set of aligners. The latter impressions may have corresponded either with the end of active treatment or the refinement stage.
Correlation between the interproximal planned and performed based on the mesiodistal widths of the teeth on the consecutive digital models
The effect of the approach to IPR, use of gauge for clinical measurement of IPR, type of tooth, and IPR timing on the accuracy of the procedure
They did a sample size calculation, which showed that 50 participants were required overall to demonstrate a difference of 0.15mm between planned and performed IPR at a single location.
What did they find?
They planned IPR in the maxillary arch in 43 patients (Mean: 0.25 mm) and in the mandibular arch in 38 patients (Mean: 0.28 mm). They did the IPR both before (24 cases) and following (26 cases) alignment.
The practitioners were conservative in their removal of enamel. On average, the mean difference between planned and implemented IPR was 0.14mm. This difference represents around 50% of the planned amount. The use of burs was associated with a minor discrepancy while measuring gauges also helped somewhat. They did even less removal on the distal surfaces of the teeth than on the mesial. Finally, prior alignment followed by IPR (round-tripping) was associated with more precise IPR than undertaking IPR at the outset.
What did I think?
I thought that this was a fascinating study. I particularly liked it because it was simple and ‘didn’t try to do too much. The authors posed a fundamental question that is undoubtedly worthy of answering- how accurately do we perform IPR? The results are interesting and instructive- we may be less good than we think, and some approaches might help us to improve. The authors wrote the paper very nicely, with a clear description of the study’s rationale and a detailed interpretation of the results.
In terms of design, the authors carried out a prospective cohort study. This allowed for longitudinal assessment of planned versus performed IPR. Moreover, they included several practitioners and a range of techniques. As such, there are some confounding factors, including operator, the mechanical approach used, the timing of IPR, and the use of gauges. Arguably, however, this is also a strength of the study. It gives us an indication of the relative merits of each approach and indicates that experience may not be the discriminator that we reassure ourselves it is.
The magnitude of IPR overall was minor, and one would have to question how sensitive ClinCheck is to these minor differences. Nevertheless, they found that the repeatability of the measurements was acceptable.
The key message is that we may perform less IPR than we initially plan. This, of course, may be a lesser evil than performing too much. The use of burs and gauges may help to minimise the discrepancy between planned and executed IPR. It would be interesting to see how this shortcoming influences the progression of aligner therapy and, indeed, the outcome of treatment. This could be assessed in greater detail. However, I can’t help feeling that doing so would have muddied the waters a little and detracted from the simple message.
The finding that IPR is more accurate following the prior alignment of contact points is intuitive. It is worth highlighting that this finding is based on a limited sample. We should counterbalance this possible benefit with the relative merits of postponing IPR from clinical and patient management viewpoints.
What can we conclude?
Given the relatively small sample and range of variables, we cannot make concrete inferences. However, it should certainly help us reflect on our practice and evaluate a little more critically our performance of interproximal reduction. It appears that we may undertake much less IPR than we plan to. Our precision may be influenced by the technique that we deploy, and gauges may help us carry out IPR more accurately. Simple conclusions from a simple and relevant study.
Professor of Orthodontics, Queen Mary University of London, UK