Material and methods

This investigation was a retrospective experiment with pretreatment and posttreatment CBCT data from patients who had clear aligner therapy from 1 practitioner (D.G.). The study protocol was reviewed and approved by the Institutional Review Board Committee of the University of Louisville (number 16.0395).

The records of 458 patients who had consecutively finished clear aligner therapy by 1 orthodontic provider were reviewed for inclusion in the study. Inclusion criteria consisted of the records of patients who had diagnostically acceptable pretreatment and posttreatment records including photographs and initial and final CBCT data and who had comprehensive treatment with clear aligners. The sample consisted largely of nonextraction patients, with only 6 having extractions as part of their comprehensive treatment. Exclusion criteria included patients who had received previous orthodontic treatment, whether comprehensive or early interceptive treatment, and those treated with adjunctive appliances or a combination of clear aligners and fixed appliances. Of the 160 patients who met the criteria, all had pretreatment CBCT (T1) volumes taken within 6 months of beginning treatment. Most patients had posttreatment CBCT (T2) images made on the day that active treatment ceased; however, to limit patient exposure to unnecessary radiation, the orthodontic provider used the progress CBCT as the final image if that assessment was performed within 9 months of treatment completion.

CBCT images were acquired using an i-CAT device (Imaging Sciences International, Hatfield, Pa). It was operated at 3.8 mA (pulse mode) and 120 kV using a high frequency generator with fixed anode and a 0.5-mm nominal focal spot size. The anterior symphyseal region of the mandible of each patient was stabilized by a chin holder, and vertical and horizontal lasers were used to position the face. The patient’s head was oriented by adjustment of the chin support until the midsagittal plane was perpendicular to the floor, and the horizontal laser reference coincided with the base of the alar of the nose. Lateral scout images were made, and small adjustments to head position were carried out so that discrepancies between bilateral structures (eg, posterior and inferior borders of the mandibular rami and zygomatic arches) were less than 5 mm. A single 360° rotation, 20-second scan, comprising 306 basis projections was then made for each patient with a 17.0 cm (diameter) × 13.2 cm (height) field of view using the i-CAT acquisition software (version 1.7.7). Control of acquisition parameters, mA and kV(p), was automated. Primary reconstruction of the data was automatically performed immediately after acquisition and took approximately 60 seconds. The DICOM data from the CBCT scan was uploaded into 3-dimensional imaging software (Dolphin Imaging and Management Solutions, Chatsworth, Calif) for storage and interpretation.

DICOM data were exported anonymized from the Dolphin imaging software and codified according to the clinical and treatment parameters defined above. The DICOM data were imported into another dental software package (inVivo, version 5.3; Anatomage, San Jose, Calif) installed on an Alienware P18E laptop computer (Dell, Round Rock, Tex). All images were viewed on an 18.4-in WLED TrueLife display monitor screen (Dell) having a resolution of 1920 × 1080 pixels.

The CBCT volume was reoriented so that each of the orthogonal planes (axial, coronal, and sagittal) were perpendicular to the long axes of each maxillary central and lateral incisor under investigation ( Fig 1 ). A reference line was drawn using the proprietary software connecting the buccal and palatal cementoenamel junctions. The principal investigator (C.A.) measured the perpendicular distance between the most apical point of the tooth and the reference line at the cementoenamel junction ( Fig 2 ). The root lengths of the teeth in all pretreatment images were measured. After a minimum 3-week waiting period, posttreatment root length was assessed without access to the pretreatment data.

Examples of 3-dimensional orientation protocol for CBCT images: A, preorientation; B, postorientation.

Measurement technique using the sagittal slice of the tooth imaged. The perpendicular distance between the most apical point of the tooth and the reference lines at the cementoenamel junction was measured.

A minimum of 3 weeks later, the investigator repeated the reorientation protocol and root length measurements for T1 and T2 CBCT images for 10% of the sample by selecting every tenth patient in consecutive order. The rater was blinded to previous measurements.

Additionally, the pretreatment and posttreatment positions of the teeth relative to the orofacial cortical plates were qualitatively assessed according to the rating scale reported by Kan et al. The only modification was the addition of a fifth category that identified teeth outside the labial cortical plate. Each sagittal root projection in relationship to its orofacial cortical plates was classified as follows: Class I, the root is positioned against the labial cortical plate ( Fig 3 , A ); Class II, the root is centered in the middle of the alveolar housing without engaging either the labial or the palatal cortical plates at the apical third of the root ( Fig 3 , B ); Class III, the root is positioned against the palatal cortical plate ( Fig 3 , C ); Class IV, at least two thirds of the root is engaging both the labial and palatal cortical plates ( Fig 3 , D ); and Class V, the root is positioned outside the labial cortical plate ( Fig 3 , E ).

Modified rating scale used to assess root position relative to the orofacial cortical plates: A, Class I sagittal root position; B, Class II sagittal root position; C, Class III sagittal root position; D, Class IV sagittal root position; E, Class V sagittal root position.

Results

The records of 458 patients whose treatment with clear aligners was consecutively completed between July 2013 and August 2016 were reviewed; 160 records met the inclusion criteria. Table I shows the demographics of the subjects, and Table II shows their characteristics in regard to age at the start of treatment, treatment duration, and time interval between the T1 and T2 CBCT examinations. The mean age of the population studied was 34 ± 16 years. The mean total treatment time and mean time between the T1 and T2 CBCT evaluations were 2.19 ± 0.81 and 2.11 ± 0.85 years, respectively. Table III provides the frequency distribution for the dental characteristics of the sample and treatment details used to facilitate clear aligner therapy.

Table IV shows the ICC used as an index of reliability of the rater for the 2 repeated measurements. The rater was highly reliable between the repeated measurements (ICC, 0.99; P < 0.01).

Table V gives the mean reductions in root length between T1 and T2. Mean absolute reduction among maxillary incisors varied between 0.47 ± 0.61 mm (right lateral incisor) and 0.55 ± 0.70 mm (left central incisor). The range of change in root length was –5.24 to 0.23 mm. The mean percentage of reduction varied from 3.60% ± 4.50% to 4.30% ± 5.31%. Table VI shows a comparison between maxillary lateral and central incisor changes in root length. No differences were proven for root length reduction between central and lateral incisors when comparing either absolute differences in root length ( P = 0.55) or percentages of change ( P = 0.35). Table VII shows mean absolute and percentage changes and ranges in root length between T1 and T2 for each incisor. Applying a clinical threshold indicated that of the entire sample, only 2 or 3 subjects had greater than a 25% reduction in root length ( Fig 4 ).

Table VII

Number of patients experiencing >25% resorption

Maxillary incisor	≤25%	>25%
Right lateral	155 (99.36%)	1 (0.64%)
Right central	158 (98.75%)	2 (1.25%)
Left central	157 (98.13%)	3 (1.9%)
Left lateral	155 (99.36%)	1 (0.64%)
Both centrals	158 (98.75%)	2 (1.25%)

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