Changes in mandibular incisor position and arch form resulting from Invisalign correction of the crowded dentition treated nonextraction

Kesling¹  introduced the concept of tooth movement using sequential removable appliances in 1945. In 1998, Align Technology released Invisalign, which uses impressions or scans that are converted through stereolithographic technology (.stl) to a virtual study model. These.stl files are transported into the AlignTech proprietary software, ClinCheck. This three-dimensional (3D) modeling program allows for virtual simulation of ideal tooth alignment. Each aligner is ideally worn for 14 days, moving the teeth at a maximum rate of 0.33 mm per aligner.

Anterior crowding is a key reason people seek orthodontic treatment.²  Resolution of crowding requires either reduction in tooth mass with interproximal reduction (IPR), extraction, or an increase in arch perimeter with distalization, buccal arch expansion, or incisor protrusion. To date, two studies have investigated anterior tooth movement with Invisalign.^3,4  However, these studies focused on the position change of the lower incisors using superimposition of the 3D models. Our study used cephalometric analysis to determine the change in incisor position and angulation.

There are limited studies on the reliability of Invisalign in the treatment of crowding. Wax et al.⁵  found a 91.4% reduction in mandibular incisor crowding in a cohort of Invisalign-treated patients.

From a methodology perspective, Krieger et al.⁴  found that, when assessing Invisalign treatment results, 48% of maxillary crowding was resolved using interproximal reduction, and 58% of the patients had their mandibular crowding resolved by IPR and incisor protrusion. The term protrusion refers to the anteroposterior position of an incisal edge of an incisor.⁴  In this study, there was no discussion of incisor proclination. Proclination refers to the long axis angulation of an incisor, which is typically measured on a lateral cephalometric radiograph. Incisor proclination is a widely accepted treatment method, and consequence, when correcting anterior crowding.⁴ 

Kravitz et al.³  looked at the predictability of the movement of anterior teeth comparing the 3D digital models of ClinCheck vs actual treatment results. The tooth movements studied included the expansion, constriction, intrusion, extrusion, mesiodistal tip, rotation, and labiolingual tip. They found that the mean accuracy of the anterior tooth movement with the Invisalign appliance was low at 41%.³ 

Through analysis of the literature we can conclude that lower incisor proclination and protrusion are critical treatment modalities in helping with crowding resolution. The lower incisor position can be measured with a reasonable means of accuracy from digital cephalometric radiographs. The initial incisor position should be a key diagnostic factor when planning treatment for Invisalign cases. If there is an intention to push the lower incisor forward in the basal bone of the mandibular symphysis, facial aesthetics, periodontal health, arch perimeter, intercanine width, and stability must be considered.

A sample of 61 White, mixed-gender, nongrowing patients were selected from a single specialist’s orthodontic practice. They were treated nonextraction with or without IPR using only Invisalign. Patients were excluded for poor compliance or if interarch elastics were used. This retrospective chart review received ethics approval from the University of Manitoba Bannatyne Campus research ethics board.

Patients were distributed into three groups depending on severity of mandibular crowding: mild (2–3.9 mm), moderate (4–5.9 mm), or severe (>6 mm). The degree of mandibular crowding was measured using Carey’s analysis.^6,7  Gender was not equally distributed, with 44 women and 17 men. Patients were distributed into one of three groups as mild (20), moderate (22), or severe (19).

Pre- and posttreatment records were collected for each of the 61 participants and consisted of the following:

• digital study models created from.stl files of the dentition from iTero scans (iTero, software version 4.0, Cadent Inc., Carlstadt, N.J.),

• digital lateral cephalometric radiographs taken by the Planmeca Proline CC digital radiographic machine (Planmeca Inc., Helsinki, Finland), and

• digital study models of the dentition provided as.stl files from Align Technology and converted from polyvinyl siloxane (PVS) impressions (Align Technology, San Jose, Calif).

An intraclass correlation (ICC) test was completed on 20% of the sample to examine measurement reliability. Twelve digital study models and 12 digital cephalometric radiographs were randomly selected and measured by two independent investigators at a 4-week time interval to identify possible measurement error.

The iTero scans taken pretreatment and posttreatment were uploaded into Geomagic Qualify software (Qualify, version 12:0, Geomagic, Rockhill, S.C.). The Geomagic digital software was used to measure tooth width, arch perimeter, and arch width. Digital measurement software has been shown to be more accurate, reproducible, and time efficient than measuring traditional plaster casts.⁸ 

Each of the pre- and posttreatment lateral cephalometric radiographs were exported as Joint Photographic Experts Group (.jpg) digital files into Dolphin imaging software. Radiographic landmarks consistent with the measurement of lower incisor position and angulation were completed.^9,10  Landmark identification was undertaken by a single investigator to minimize measurement error.

SPSS software (SPSS, software version 20.0, IBM Corp, Armonk, NY) was used to analyze the data. An analysis of covariance (ANCOVA) test statistic was used to determine if there was a significant difference in the cephalometric variables for a change in lower incisor position and angulation following treatment with Invisalign. The P value was considered significant at α <.05.

Intercanine width was measured from cusp tip to cusp tip, interpremolar width was measured from the palatal cusp of the first premolars, and intermolar width was measured from the mesiolingual cusp tip of the first molars.¹¹  In cases of attrition or restorative work the estimated cusp tip was used.¹² 

An ANCOVA test statistic was used to determine if there was a statistically significant difference in the arch width following treatment with Invisalign. The P value was considered significant at α <.05.

IPR was identified from the ClinCheck simulation for each of the samples of the total amount of tooth mass reduction that should have been completed for the entire treatment duration. This IPR value was assumed to be completed as prescribed by the software.

To quantify the intrarater reliability an ICC was used. The ICC values for both the lateral cephalometric radiographs and iTero 3D models were in excess of 0.98, indicating extremely high reliability.

The treatment time was determined by the total number of lower aligners worn by each patient for 2 weeks. The mean treatment time in weeks for each group was mild (53.6 ± 21.12), moderate (63.73 ± 20.69), and severe (71.68 ± 16.31).

The cephalometric values were analyzed for each group using a paired t-test to determine if there was a statistically significant change between T₀ and T₁, and the means were calculated for each group. Differences in these means established if there was a statistically significant change between the outcomes from T₀ to T₁.

In the mild crowding group (n  =  20), the only statistically significant change (P <.05) was overjet. In the moderate crowding group (n  =  22), statistically significant differences were only seen in overbite and overjet (P >.05). This shows that the lower incisor position did not change with statistical significance if the crowding was between 2.0 mm and 5.9 mm (Table 1).

In the severe crowding group (n  =  19), the majority of the dependent variables, excluding overbite, showed a statistically significant change between T₀ and T₁. Lower incisor angulation and anteroposterior position increased with a P ≤.05. The results indicate that when crowding was greater than 6.0 mm, the lower incisors proclined and protruded (Table 1).

An ANCOVA test was used to analyze differences between groups using a group P value to determine if the change in the lower incisor position was statistically significant between the three different groups. The groups were then paired together to determine if the different groups were statistically significant to each other. This is reflected in the pairwise P values (P ≤.05) (Table 2).

The mild and moderate groups had no statistical significance in lower incisor position or angulation between them (P >.05). The severe crowding group was statistically significant from both the mild and moderate crowding groups (P <.05). When there was a statistically significant difference in the lower incisor position between the groups (ie, when the group P value was <.05), the pairwise P values that included the severe crowding group were also statistically significant at P <.05.

A Pearson correlation test was completed to compare the landmarks that used both linear and angular measurements; these included L1-NB and L1-APog. There is a statistically significant relationship between the position of the lower incisor in millimeters and the angulation in degrees for the two planes that have both of the descriptive analyses. The Pearson correlation test reported 0.887 for L1-NB and 0.897 for L1-APog. Thus, when there is a change in the anteroposterior position of the lower incisor there is also a change in its angulation.

The difference from T₀ to T₁ showed a highly statistically significant change in the intercanine, interpremolar, and intermolar widths for all three groups. The arch width increase is greater in the premolar region for the moderate and severe crowding groups (Table 3).

Although there was a statistically significant increase in the buccal arch expansion in each of the groups, there were no statistically significant differences in the intercanine, intermolar, and intercanine widths between the groups at T₁. This shows that the change in arch width is correlated to the pretreatment arch form because there is no statistically significant difference in arch width posttreatment (Table 4).

When the prescribed vs actual IPR are compared as a linear regression relationship (r²  =  0.44), it shows a poor correlation (Figure 1).

There are very few studies investigating the relationship between Invisalign treatment and crowding resolution from both reliability and methodology perspectives. Wax et al.⁵  reported a reliability index of 91.4% in the correction of mandibular incisor crowding. Krieger et al.⁴  reported that 58% of patients had lower incisor crowding resolved with a combination of IPR and incisor protrusion, but they did not report on incisor proclination, with protrusion referring to the anterior movement of the incisal edge. Kravitz et al.³  found that predictability of tooth movement when resolving <5 mm of crowding or spacing with incisor protrusion was only 41%. Cephalometric evaluation increases the accuracy of measuring the proclination and protrusion of incisors; however, consideration must be made when planning treatment for protrusion. The most accurate treatment methodology for crowding resolution is to increase the amount of IPR.

Our study showed that in the mild and moderate crowding groups, aside from overbite and overjet, there were no statistically significant changes between T₀ and T₁. In the severe crowding group, all of the variables, including lower incisor position and angulation, showed a statistically significant change from T₀ to T₁. Therefore, we can conclude that, in a more severely crowded dentition, the Invisalign treatment caused the lower incisor to procline and protrude.

Our study is the first to report on buccal arch expansion and its relationship to crowding resolution. The results revealed buccal arch expansion played a significant role in crowding management. The mean increase in intermolar width was 1.65 mm in the mild crowding group, 1.86 mm in the moderate group, and 2.65 mm in the severe group. Interpremolar widths increased 1.57 mm, 2.52 mm, and 3.19 mm, respectively, and intercanine widths increased 1.28 mm, 1.77 mm, and 1.74 mm, respectively. It was noted that there was no statistical difference between the three groups in final arch width, suggesting there may be a correlation between crowding and pretreatment arch width. Ricketts et al.¹³  calculated a formula between arch width and arch perimeter and reported that for every 1 mm of molar expansion, 0.25 mm of space is produced, and for every 1 mm of premolar expansion, 0.7 mm of space is created.

The inclusion criteria for our study required that IPR be done as programmed by the ClinCheck software. Studies have shown that, even when IPR is completed clinically as prescribed, it might not be accurate. Chudasama and Sheridan¹⁴  and Johner et al.¹⁵  highlighted that the amount of completed IPR achieved was consistently less than prescribed. In our study, when we compared the pretreatment with the posttreatment sum of the mesiodistal tooth widths, we found similar results. When removing the outliers, the percentage of crowding correction with IPR reduced to a mean of 35% between the three groups.

Of interest when considering IPR is the difference in mean values of IPR in the mild vs severe group of less than 1.0 mm. An increase in crowding did not correlate with a significant increase in IPR (P>.05). Our results clearly showed that buccal arch expansion and anterior proclination were the predominate contributors to crowding resolution in the severe crowding cohort. This indicates that if more IPR is prescribed, a reduction in proclination could be expected.

Reducing incisor proclination has benefits from a periodontal and stability perspective. Periodontally, moving roots through the thin labial bone increases the risk of bone dehiscence and recession that can occur in certain gingival phenotypes.¹⁶  Maintaining lower incisors in their original position can reduce relapse.^17,18  Broadening contacts with IPR is still controversial in the literature but has shown that flattening the interproximal contacts through IPR may increase long-term stability.^19,20 

McNamara et al.²¹  reported that patients with more than 6 mm of crowding in the mandibular arch typically require extraction. In cases treated with Invisalign, buccal arch expansion was insufficient to prevent lower incisor proclination. Due to the periodontal strain and increased relapse potential reported from lower incisor proclination, the clinician must consider the potential for extraction in crowded cases of greater than 6 mm or increase the amount of IPR in the treatment plan.¹⁶ 

• In mild to moderate lower anterior crowding cases, there is no change in the lower incisor position or angulation.

• Where crowding exceeds 6 mm treated nonextraction, lower incisors tend to procline and protrude.

• Buccal arch expansion and interproximal reduction have a statistically significant effect on the resolution of crowding in mild, moderate, and severe cases.

• When treating crowded dentitions, buccal expansion and IPR are important clinical tools to negate or minimize lower incisor proclination.

• Intercanine, interpremolar, and intermolar widths are not significant factors in the mild, moderate, and severe cases. Thus, pretreatment crowded arches are narrower than noncrowded arches.

• Prescribed IPR is not accurately applied.