Three-Dimensional Evaluation of Pharyngeal Airway and Facial Dimension Changes in Skeletal Class II Patients Treated With Twin Block Versus Myobrace Appliances

This study is a multicenter randomized comparative clinical trial, employing non-stratified randomization in two-parallel arms with equal allocation ratio of 1:1. Patients will be recruited from Orthodontic Clinic at the College of Dentistry, University of Kufa, and from specialized dental centers affiliated to the Iraqi Ministry of Health, in addition to some private clinics. Initially, the patients will be assessed for eligibility to participate in the study according to the inclusion criteria. Then, the parents will be informed verbally about the purpose of the study and will be provided with the informed consent form to be signed. Patients will be treated using two types of functional appliances ( Twin Block and Myobrace) for six months. Regular follow-ups will be conducted during the course of the treatment and progress will be charted out at 4-weeks intervals.

Participants will be randomly assigned to one of two groups, each receiving one of the specified treatments over a six-month period.

Group I: Twin Block Appliance

• Design: Comprises maxillary and mandibular components anchored using Adams' clasps on the first permanent molars and lower first premolars. A passive labial bow will be incorporated into the upper arch, with ball-end clasps for additional retention.
• Bite Registration: Achieved through a symmetric protrusive bite using modeling wax, ensuring an edge-to-edge incisal relationship with a 2-3 mm bite opening between central incisors.
• Inclined Bite Blocks: Positioned at a 70° angle to facilitate forward mandibular displacement.
• Patient Instructions: Continuous wear is mandated (24 hours/day), excluding periods of oral hygiene. Adjustments using self-cured acrylic may be made for further mandibular advancement.

Group II: Myobrace Appliance

• Selection: Appropriate sizing determined by measuring the inter-canine distance using a specialized ruler.
• Fitting: Ensures alignment of upper canines within designated slots, matching the appliance's midline with the dental midline.
• Wear Protocol:
• Week 1: Gradual increase in daytime wear duration.
• Week 2 Onwards: Overnight wear targeting at least 8 hours.
• Post Week 4: Combination of 1-2 hours daytime wear plus overnight usage.
• Supplementary Activities: Incorporates exercises targeting breathing habits, tongue positioning, swallowing patterns, and lip function.

Imaging Protocols:

• Cone Beam Computed Tomography (CBCT):
• Timing: Conducted pre-treatment and at end of study .
• Positioning: Subjects seated upright with the Frankfort horizontal plane parallel to the floor, maintaining natural occlusion, relaxed lips, and resting tongue posture.
• Parameters: Scans performed using the Morita Veraview X 800 device (100 kV tube voltage, 3 mA tube current, 14 x 15 cm field of view, 0.08 mm voxel size, 18-second scan time).

three dimensional (3D) Facial Scanning:

• Equipment: Utilizes an iPhone Pro 14 equipped with the 3D Scanner App™.
• Landmark Identification: Seventeen specific facial landmarks will be marked using black eyeliner to ensure consistency.
• Participant Positioning: Subjects will be seated in a relaxed posture with a natural head position under standardized lighting conditions.
• Pre-Scan Preparations: Removal of glasses, jewelry, and use of hairbands to minimize obstructions.
• Data Processing: Captured three dimensional (3D)facial models will be exported as Standard Tessellation Language (STL) files and analyzed using Blender 3.6 LTS software for linear and angular measurements.

Data Analysis:

• three dimensional (3D) Model Reconstruction: CBCT data will be processed using ProPlan CMF 3.0 (Materialise) software.
• Segmentation: Hard tissues and upper airway structures will be delineated based on specific Hounsfield Unit (HU) thresholds.
• Pharyngeal Airway Segmentation: Divided into nasopharynx, velopharynx, glossopharynx, and hypopharynx, with precise anatomical boundaries defined for each segment.

Measurements:

• Airway Analysis: Assessment of minimum cross-sectional area, total airway volume, and pharyngeal airway length.
• Soft Tissue Evaluation: Linear and angular anthropometric measurements derived from 3D facial scans.

Inclusion criteria

• Age: 10-14 years, at growth spurt phase as determined by the cervical vertebrae maturation stage (C3- C4 ) assessed by lateral cephalograms.
• Skeletal relationship: skeletal class II (ANB angle > 4°) with normal maxilla (SNA angle 78°- 84°), and retrognathic mandible (SNB angle < 77 °).
• Dental relationship: Moderate to severe dental class II division 1 malocclusion with overjet 6 -10 mm.
• Convex facial profile.
• Well aligned/mild crowding (< 4 mm) of dental arches.
• Good to fair oral hygiene status as based on a simplified oral hygiene index (OHI-S).
• Normal or decreased lower facial height (SN-MP angle ≤ 32).

Exclusion criteria

• Previous orthodontic/orthopedic treatment.
• Patients with dental open bite
• Patients with obvious facial asymmetry.
• Patients with orofacial cleft.
• Previous extractions.
• Presence of mandibular shifts.