This study evaluates a new, individualized treatment for adolescents with temporomandibular joint disorders (TMD) caused by bilateral, reducible disc displacement. Participants will undergo clinical and imaging examinations, including cone-beam CT (CBCT), MRI, and intraoral digital scanning to create three-dimensional models of the teeth and jaw. Based on these data, the study team will design and fabricate a personalized repositioning digital occlusal splint for each participant. Patients will be instructed on how to wear the splint and will return for regular follow-up visits to monitor symptoms and adjust the splint as needed. The study aims to improve pain, chewing function, and overall quality of life. All research-related imaging, digital scans, splint fabrication, and follow-up visits will be provided at no cost to participants. Possible risks include temporary discomfort or pressure when first wearing the splint, and the possibility that symptoms may not improve as expected. Participant safety, privacy, and data confidentiality will be strictly protected, and participants may withdraw from the study at any time without affecting their future medical care.

Detailed Description This study evaluates four different methods for mandibular repositioning in patients with bilateral reducible temporomandibular joint (TMJ) disc displacement requiring occlusal reconstruction.

Methods of Mandibular Repositioning:

1. Intraoral Direct Positioning Group: The therapeutic jaw position is determined clinically using the clicking elimination method, where patients open wide until clicking occurs, then gradually retract the mandible while performing opening-closing movements to find the minimal protrusion position without clicking.
2. Articulator Positioning Group: Uses mechanical facebow transfer and semi-adjustable articulator (Artex CR) with condylar guidance parameters obtained from protrusive and lateral excursion records. The therapeutic position is determined by analyzing CBCT joint space measurements and MRI disc displacement data.
3. Digital Anatomical Positioning Group: Combines intraoral scanning data with CBCT-reconstructed 3D jaw models. The therapeutic position is determined by referencing MRI disc reduction phase, optimal joint space ratios, and anatomical landmarks using CAD software (Exocad).
4. Digital Jaw Motion Tracking Group: Integrates 3D jaw models with electronic jaw tracking data (Zebris). The therapeutic position is determined by analyzing condylar movement trajectories (identifying the "figure-8" pattern crossover points indicating disc displacement) combined with dynamic MRI and optimal joint space parameters.

Treatment Protocol: All groups receive digitally designed and manufactured repositioning splints. Patients wear splints 24 hours daily (except during meals) for 3 months, with follow-up adjustments at 2 weeks, 1 month, 2 months, and 3 months.

Imaging Protocol:

• CBCT (I-CAT FLX): 120 kV, 5 mA, 0.2 mm resolution
• MRI (GE Discovery MR750w 3.0T): Bilateral TMJ surface coils, oblique sagittal and coronal views in closed-mouth position, oblique sagittal in maximum opening

Assessment Methods:

• Joint space measurements using Ikeda's method on registered pre- and post-treatment CBCT images
• Condyle position analysis using Gelb 4/7 position criteria
• Disc-condyle angle measurement on MRI (>15° defined as disc displacement)

Inclusion Criteria:

1. Diagnosis of reducible temporomandibular joint (TMJ) disc displacement according to the 2014 revised criteria of the International Association for Dental Research;
2. Bilateral reducible disc displacement confirmed by MRI;
3. Reproducible joint clicking during mandibular opening and closing;
4. Permanent dentition with full cognitive and communicative ability;
5. Signed informed consent with good treatment compliance.

Exclusion Criteria:

1. Non-reducible disc displacement, osteoarthrosis, isolated myofascial pain, or other conditions not meeting diagnostic criteria;
2. Receipt of other TMJ disorder treatments within the past 3 months;
3. Presence of systemic diseases, psychiatric disorders, or impaired consciousness.

Blinding and Masking Procedures:

1. Outcome Assessor Blinding

   • All clinical and radiographic outcome measurements will be performed by independent assessors who are unaware of the participant's group allocation.
   • Participants will be assigned unique, randomly generated identification codes that do not reveal group assignment.
   • Radiographic images (CBCT and MRI) will be anonymized and presented in a randomized order to blinded radiologists.

2. Limitations of Masking

   o Complete blinding is not possible due to the distinct nature of interventional approaches: a) Participants will be aware of their specific mandibular repositioning technique b) Clinicians performing interventions cannot be blinded due to different positioning methods

3. Blinding Process for Outcome Assessment o Clinical outcome measures (Craniomandibular Index, Joint Movement Assessment) will be evaluated by a research assistant not involved in the intervention.