Roberta Lione; Francesca Chiara De Razza; Francesca Gazzani; Paola Cozza; Chiara Pavoni
Abstract
Objectives: To evaluate leveling of the Curve of Wilson (COW) by two different treatment appliances (clear aligners [CA] and continuous archwire fixed appliances [FA]) in a permanent dentition sample of patients.
Trial design: This is a prospective two-arm parallel group randomized clinical trial with 1:1 allocation ratio.
Materials and Methods: Digital casts of 40 patients (CA group 1⁄4 20 patients; FA group 1⁄4 20 patients) were collected. Angular values for COW, right and left vertical height difference of lower first molars, and linear distance between lower teeth and the WALA ridge were analyzed for pre- (T1), posttreatment (T2) and on final virtual (ClinCheck) models (T2-CC) of the CA group. An unpaired t-test was used to evaluate significant intergroup differences (P , .05), while a paired t-test was used for posttreatment CA intragroup comparison.
Results: FA group showed better control of second molar crown positions compared to CA group (47-WALA 1⁄4 0.2 6 0.1 mm, 37-WALA 1⁄4 0.6 6 0.3 mm). No significant difference was detected for linear distance of lower first molars and the WALA ridge or for vertical height difference. CA group showed a greater reduction of distance between lower premolars and the WALA ridge (mean differ- ence: –0.5 mm for both 45-WALA and 35-WALA; mean difference: 0.5 mm for 44-WALA, 0.6 mm for 34-WALA). Predictability for the CA group was high for every measurement (87% Right COW, 89% Left COW, 88% 46 Vertical Diff, 87% 36 Vertical Diff).
Conclusions: Clear aligner and continuous archwire mechanics were effective in leveling COW. FA was more effective in changing crown position of lower second molars with respect to the WALA ridge, while CA provided a greater distance reduction between lower premolars and WALA ridges compared to FA. (Angle Orthod. 2024;00:000–000.)
Keywords: Orthodontics; Biomechanics; Digital cast analysis
Introduction
An ideal and functional occlusal plane shows a cur- vilinear shape, modulated by two physiological curves: the Spee curve in the sagittal plane and the curve of Wilson in the frontal plane.
The curve of Wilson (COW) ideally passes through the first permanent molar buccal and palatal/lingual cusps. It can be seen as a concave curve in the lower arch and as a convex curve in the upper arch.2
The COW allows lateral dynamic jaw movements without occlusal interferences.3 The mandibular dental arch curvature increases from adolescence to adulthood, especially in the molar region.4 An enhanced lower COW displays lateral-posterior negative crown torque. This mandibular adaptation is a common response to a maxillary transverse deficit.4,5 The American Board of Orthodontics (ABO) reported that a func- tional COW can be assessed when the lingual/palatal cusps are less than, or 1 mm lower, than the first per- manent molar buccal cusps.3 Therefore, the main goal of orthodontic therapy is to achieve proper coordination between dental arches, which improves occlusion and masticatory effectiveness, while positioning roots over supporting bones.6,7
Flattening of the COW with conventional fixed appli- ances has been extensively investigated,8–10 but few studies focused on COW leveling using Clear Aligners (CA). A recent retrospective study by Goh et al. analyzed predictability of the COW correction with CA. The authors reported low predictability of COW flattening, especially in the mandibular first molar region, while significant cusp tip expansion and buccolingual crown inclination were found in lower premolars.11
Lim et al.12 recently evaluated the predictability of CA in leveling only the maxillary COW and found an under- expression of upper arch expansion, except for second molars. These results were also supported by Bowman,13 who measured the buccolingual inclination of the maxil- lary arch in patients treated with CA.
In addition to buccolingual crown inclination, WALA ridges are considered a good landmark for establishing arch morphology and detecting the orthodontic effects on COW depth.14 WALA ridges are a soft-tissue band immediately superior to the mucogingival junction in the mandible, representing a stable anatomical structure that determines the ideal contour for the roots of teeth in the basal bone.15
The current retrospective study aimed to investigate the lower COW posttreatment modifications in patients with permanent dentition treated with CA, and to compare the lower COW leveling produced by CA with continuous archwire fixed appliance therapy (FA). Therefore, the null hypothesis was that COW leveling for patients in the per- manent dentition who underwent treatment with clear aligners would not differ from patients with the same occlusal characteristics who underwent treatment with conventional fixed appliances.
Materials and Methods
This project was approved by the ethical committee of the University of Rome “Tor Vergata” (Protocol Number 48/23). Each patient gave informed consent.
Two groups were selected retrospectively from the Department of Orthodontics of the Hospital of Rome “Tor Vergata.” The first group (CA group) of 20 patients (8F, 12M, mean age of 14.5 6 0.7 years) was treated with clear aligners (Invisalign System, Align Technology Inc., Santa Clara, CA, USA), while the second group (FA group) of 20 patients (15F, 5M, mean age: 14.8 6 0.6 years) was treated with conventional fixed appli- ances (McLaughlin Bennet 5.0, Forestadent, Pforzheim, Germany).
Inclusion Criteria
The inclusion criteria were: Caucasian ancestry, Angle Class I molar or Class II edge-to-edge molar relation- ship, permanent dentition with fully-erupted second molars, increased COW, moderate or slight transverse discrepancy, and moderate dental crowding. A pre- treatment evaluation of the molars according to ABO standards was also performed by measuring the height differences between buccal and lingual cusps on each lower first molar, grouped as ABO-nonconforming when the vertical cusp height difference was .1.0 mm to 2.0 mm.
Exclusion Criteria
Exclusion criteria were: incomplete second molar distobuccal cusp registration, crossbite, supernu- merary teeth or tooth agenesis, need for extractions, cleft lip and/or palate history, any dental/periodontal disease.
The CA group was treated with a mean number of 20–40 aligners for each arch, with a maximum of three revision aligner sets. There were no limitations about the number/type of attachments. COW flattening was digi- tally planned following ABO guidelines: lower molar buccal cusps were positioned , 1 mm higher than the lingual cusps on the digital grid, according to crown anat- omy.3 COW overcorrection was never prescribed, but a cusp-fossa relationship was digitally planned. Each patient was seen every 6 weeks and changed their aligners every week. Patient compliance was evaluated by a three-point Likert-type scale: compliance was good if patients wore aligners full time, moderate for 16–20 hours, and poor for less than 16 hours.16
The FA group was treated with full-fixed, conventional, preadjusted edgewise brackets and MBT prescription, with the following lower arch torque values: 12° on first-premolars, 17° on second premolars, 20° on first molars, and 10° on second molars. A tapered-form standard archwire sequence was used (0.016-inch round, 0.017 3 0.025-inch rectangular, 0.019 3 0.025- inch rectangular martensitic active nickel-titanium alloys, and 0.019 3 0.025-inch stainless steel, 0.021 3 0.025- inch beta-titanium). The complete arch sequence allowed for wire play reduction in the bracket slot.17 Follow-up checks for each patient were performed before the fin- ishing stage and the last archwire was inserted for about 4 months to allow for proper torque expression.
For the CA group, pretreatment (T1) and posttreatment (T2) lower digital dental casts were made using an intra- oral scanner iTero Orthodontic (version 5.2.1.290, Align Technology). For the FA group, pr-treatment (T1) and posttreatment (T2) lower digital dental casts were obtained using an extra-oral scanner (OrthoXScan, Dentaurum GmbH and Co, Ispringen, Germany).
Lower digital casts were exported in stereolithogra- phy file format and analyzed using Viewbox4.0 (dHAL Software, Kifissia, Greece). A trained examiner (FCDR) performed all measurements.
On each lower digital-cast, three reference planes were established:
- Occlusal Plane (OP): to construct this plane, 26 points were digitized on the digital casts (buccal and lingual cusp tips of first and second molars, buccal cusps of first premolars, second premolars, and canines, and incisal edges of lateral and central incisors). This defined the orientation and position of a three- dimensional best-fit occlusal plane including as many points as possible, as shown in Figure 1;
- Wilson right plane (WRP): plane passing through the lower right first molar buccal and lingual cusps;
- Wilson left plane (WLP): plane passing through the lower left first molar buccal and lingual cusps (Figure 2).
