Evaluation of the consistency between the pre-planned ClinCheck® IPR, which is often adjusted by the clinician according to the clinical case requirement, and that implemented clinically has received very little scientific evaluation, especially for teeth in each quadrant. A very careful space analysis is required prior to any IPR-based treatment; additionally, the accurate implementation of the desired IPR is likely essential to achieving desired goals and treatment objectives. The evaluation of 75 STL ClinCheck® initial and final models has revealed that IPR was often carried out in the lower arch with a total of 362/566 (64%) teeth being slenderized to resolve lower crowding compared to 204/566 (36%) teeth slenderized in the upper arch. This agrees with previous findings that assessed Invisalign® treatment results and found that 58% and 48% of mandibular and maxillary crowding, respectively, were resolved by IPR . Furthermore, our sample primarily included adult patients, aged 38 ± 15 years on average; therefore, it is also expected to see a more crowded lower dentition than on the upper arch, attributed to what is known as late incisors’ crowding, which is exhibited in individuals over 20 years of age  and considered to be multifactorial [20, 21].
Data assembled from the total of 566 upper and lower teeth have shown that the overall average amount of I-IPR for the upper and lower teeth was significantly smaller than that programmed in the ClinCheck®, even when teeth were analyzed individually, except for the lower left second premolar (Table 1). Two previous studies reported reduced amount of the I-IPR in clear aligner therapy compared to that initially programmed in the ClinCheck®[14, 16]. De Felice et al.  investigated differences between planned and performed IPR in 25 cases (total arch measurements) and found that the difference was on average 0.55 ± 0.64 mm (p < 0.05) in the upper arch and 0.82 ± 0.84 mm (p < 0.05) in the lower arch; they had an accuracy of 44.95% for the IPR in the upper and 37.02% for the lower arch. In our study, using their same accuracy formula, we found approximately 45% and 46% accuracy in the mandible and maxilla, respectively. Our study is the first to estimate the difference between the average P-IPR vs I-IPR per tooth (right and left) in both upper and lower arches and compare this difference between similar teeth in different regions (Table 2). It revealed a statistically highly significant difference between the average P-IPR and I-IPR per tooth for the lower (0.14 ± 0.19 mm, p < 0.0001) and upper (0.13 ± 0.17 mm, p < 0.0001) teeth. This may not only indicate a statistically significant level but such a high level of inadequate I-IPR might be considered clinically significant, especially considering that when we analyzed teeth individually the statistically significant differences were evident for almost all teeth (Table 1).
Similar findings to our study were reported by another observational study . In that study, the overall difference between implemented and programmed IPR for 464 teeth was on average 0.15 ± 0.14 mm (p = 0.0001) compared to our findings for 566 teeth (0.14 ± 0.18 mm, p < 0.0001) with the implemented IPR per tooth being less than that digitally programmed. This study had a smaller sample size and did not evaluate the average IPR difference per tooth for each quadrant (right or left) but rather grouped the teeth into incisors, canines and premolars. On the other hand, our findings disagree with a recent study by Lagana et al.  whom reported that the amount of IPR performed in vivo correlates with that planned by the orthodontists in ClinCheck®. They retrospectively studied digital models for 30 subjects and measured the widest mesiodistal diameter for each tooth pre- and post-treatment using the OrthoCAD® software and calculated the average P-IPR vs I-IPR for the upper and lower arches. In this study, each tooth was 2D sectioned from its mesial to its distal height of contour using the Ortho Analyzer software cross-sectional tool; this helped improve the accuracy of visualizing the maximum width of the teeth between the mesial and distal maximum contact point contours. Therefore, the different measurement tools used for assessing the amount of IPR and the smaller sample size in their study might have attributed to these differences in outcomes.
The precision of implementing the planned IPR clinically might be possibly related to three factors: technical-, operator- and patient-related factors. Manual and mechanical techniques (technical factors) are usually undertaken in clinical orthodontics for precise IPR implementation: the traditional hand pulled strips, oscillating segmented disks and motor-driven abrasive strips. Accuracy of these procedures has been previously investigated; the results seem to be controversial [16, 22, 23]. One study reported that upon quantitative evaluation of the stripped enamel between these three commonly used stripping procedures, great variability was noticed, with all of these techniques delivering less IPR than intended . On the other hand, Danish et al.  reported smaller amount of removed enamel using Ortho-Strips, compared to metal strip and air rotor IPR. In our study, operators used a combination of all these techniques for slenderizing, which might have contributed to having an overall average I-IPR per tooth less than what is programmed in the ClinCheck®.
As for operator-related factors, orthodontists are often conservative in initiating the stripping process. And while performing IPR, minimal enamel amounts are often slenderized symmetrically from the prescribed contact areas before the maximal (planned) amount is reached. To avoid over-reduction and side effects related to sensitivity and pulpal irritation, especially in narrow and crowded teeth, clinicians subconsciously reduce conservative amounts and less than what is prescribed. This is often seen if the amount of crowding is significant, where the amount of programmed IPR is increased, which makes it challenging to break the contact between teeth and perform symmetrical IPR. Therefore, in such cases, clinicians might reduce less amount of enamel than what is programmed. This aligns with the finding that the mean P-IPR is consistently greater than the mean I-IPR regardless of P-IPR, and the mean discrepancy increases by 0.64 mm (p < 0.0001) per unit (mm) increase in P-IPR (Fig. 3).
As for patient-related factors, there is also a possibility that the location, anatomy, periodontal condition of the teeth play a role in the precision of implementing the planned IPR, especially considering that interproximal enamel thickness varies among individuals and teeth [24, 25]. Therefore, our study also aimed to assess the correspondence between digitally planned and implemented IPR for individual teeth (second premolar to second premolar) in the upper and lower arches. Most of the teeth displayed a statistically significant difference between the amount of the programmed and implemented IPR. In the lower arch, the highest discrepancy was exhibited for all anterior teeth (canine to canine, p < 0.0001) with a tendency toward inadequate amount of I-IPR (Table 1). Greater precision was observed for the lower premolars, with the lower left second premolar (LL5) showing the greatest correspondence between the planned and implemented IPR (p = 0.0724). It can be assumed that the majority of clinicians performing IPR in this study were right-handed whom had better access to the left quadrant of the jaw than the right. It was also noticed that P- IPR for the lower anterior teeth (0.33 ± 0.16 mm) is on average greater than that prescribed for the lower posteriors (0.24 ± 0.15 mm) and that prescribed for the upper anteriors (0.26 ± 0.15 mm, Table 2). This is probably due to the greater amount of crowding often encountered in the lower anterior region, especially for adults . On the other hand, more P-IPR was prescribed for the upper posterior teeth (0.32 ± 0.18 mm) compared to lower posteriors (0.24 ± 0.15 mm). This is possibly due to the need for provision of posterior spacing to achieve Class I canine relationship in Class II malocclusion cases . Moreover, our results indicated that IPR difference (P-IPR-I-IPR) was more evident for lower anterior teeth than for upper anterior teeth (p = 0.0302) and on the contrary had a larger discrepancy in the upper posterior teeth than lower posterior teeth (p = 0.0059) (Table 2). This can be explained by the greater prescribed P-IPR in both mandibular anterior and maxillary posterior regions (Table 2). Another explanation may be related to the accessibility of the interproximal surface to reduce. The mandibular anterior region is often more crowded than any other jaw segment, especially in adults , resulting in tipping, distortions, and very tight interproximal contacts between these teeth, which eventually hinder the smoothness of performing IPR in this region. Accessibility of the upper posterior segments compared to the lower posterior segments is more challenging for the clinician to control the IPR procedure while using indirect visualization technique. This premise is supported by Kalemaj et al.  who reported lesser discrepancy between planned and implemented IPR for lower premolars and higher discrepancy for the mandibular canines. Finally, this observed imprecision might be associated with the stretching of the periodontal ligament while performing IPR and using the measuring gauge in a crowded area, that it might falsely appear to the clinician that the desired amount of enamel reduction has been achieved [28, 29].
Even though ClinCheck ®offers an IPR timing that is automatically staged when access to interproximal contacts is feasible . The amount of IPR performed clinically even in clear aligner therapy is still under the influence of enamel hardness, tooth morphology, pressure applied, size of the abrasive tool and polishing procedures . As mentioned previously, technical-, operator- and patient (teeth)-related factors play a role in the accuracy of implementing the planned IPR for any desired treatment plan. Therefore, precision in implementing the digitally programmed IPR remains challenging, and clinicians should pay extra attention to attain the planned IPR as requested. Otherwise, the tracking of the aligners could be compromised, and case refinement ClinCheck® may be required. Increasing the precision of the IPR can be achieved by using the predetermined thickness disks and the measuring gauges, the progressive reduction of enamel and the use of wedges for teeth separation .
Finally, the cross-sectional technique used for measurements in this study proved to be superior to what has been previously utilized to assess IPR; it provided an advantage over measuring the mesiodistal teeth from contact point to contact point on digital or plaster casts. Each tooth was individually isolated and cross-sectioned along its long axis; this allowed accurate measurement for the data. The accuracy of the calibrated digital model measuring tool of the Ortho Analyzer software has been investigated, and its reliability and precision were confirmed . More recently, intraoral direct measurements taken in individuals’ oral cavity with a 0.01-mm accuracy digital caliper were compared to measurements using the 3 Shape Ortho Analyzer cross-sectional tool; results indicated that these measurements are accurate replica and as reliable as direct measurements .
The main limitation of this study is that it was a retrospective evaluation for STL models from ClinCheck® for patients treated or undergoing treatment with clear aligners. Multiple potential confounding factors were present due to patients being treated by several providers with various clinical experiences who utilized different IPR procedures. Intrarater reliability was not assessed, since the accuracy and reliability of the Ortho Analyzer software have been studied, and its calibrated digital measuring tool has proved to be accurate as indicated previously .