The Effects of the Chin Cup on Temporomandibular Joint and Mandibular Dimensions

This in vivo comparative study will evaluate the changes in the mandibular dimensions and the glenoid fossa after skeletal class III subjects' therapy by chincup appliance and compare it with an untreated class III control group. Pre and post-treatment low-dose computed tomography images will be taken before and after achieving positive overjet and undergoing 16 months of active treatment/ observation. Dimensional and volumetric changes in the mandible, condyles, and glenoid fossa will be calculated and compared to those observed in the control group.

Chincup is a usual treatment strategy for growing subjects with skeletal Class III malocclusions due to the mandibular overgrowth. However, the review of the literature reveals controversies and contradictions regarding the efficacy of chincup therapy in the retardation of mandibular growth. Moreover, the adaption of the temporomandibular joint (TMJ) following this treatment isn't clear yet. Two recent systematic reviews recommended the need for high-quality studies.

Computed tomography has been considered an ideal tool for evaluating the TMJ with the possibility of determining the real dimensions of the structures under study. Low-dose computed tomography was introduced as an alternative procedure with an effective dose proximately equal to traditional radiographs. Moreover, the volumetric analysis afforded by 3D imaging has proven reliable and accurate in the evaluation of mandibular and condylar sizes. Therefore, the objective of this randomized control trial is to evaluate the dimensional changes in the mandible, condyles, and glenoid fossa after chin cup therapy for children with mandibular prognathism in comparison with untreated Class III patients

Methods: patients ages between 6 and 8 with anterior crossbite, Class III relationships of the permanent first molars or mesial step relationships of the primary second molars, and short-faced pattern, will be selected from the orthodontic department in the faculty of dentistry at Damascus university. Thirty-eight subjects will be enrolled on the study, who will meet the following radiological inclusion criteria based on cephalometric images: Mild to moderate skeletal class III (4- \<the sagittal skeletal angle\< 0 degree), due to mandibular protrusion (SNB \> 80°), and normal or horizontal growth pattern (Bjork's sum ≤396° ±5°).

Patients in the experimental group will receive an individual fabricated bonded bite block and occipital chincup. A 400-500g/side retroactive force will be applied in the direction of the condyles. Patients or panthers will be instructed to wear the appliance /14-16/ hours a day. Pre-(T1) and post-treatment (T2) low-dose computed tomography images will be taken after a positive overjet of 2-4 mm is obtained and undergoing 16 months of active treatment/ observation. The changes in condyle-mandibular volumes and superficial areas and relative positions of the condyles and glenoid fossa will be calculated and compared with those of untreated. Statistical analysis will be done by using the Statistical Package for Social Sciences, Windows version 26.0 (SPSS Inc., Chicago, Illinois, USA) Paired t-test and Wilcoxon signed-rank tests will be used for intragroup comparisons. For the intergroup comparisons, independent t-tests and Mann-Whitney U tests will be carried out.The confidence level is 95% (P\< 0.05).

Eligibility Criteria:
Inclusion Criteria:

Clinical Inclusion criteria:

1. Children ages 6-8 years with anterior crossbite or edge-to-edge incisor relationship.
2. Class III relationships of the permanent first molars or mesial step relationships of the primary second molars.
3. the absence of centric relation and maximum intercuspation discrepancy
4. short-face pattern, adequate overbite.

Radiological Inclusion criteria:

1. Mild to moderate skeletal class III (4- \<the sagittal skeletal angle (ANB)\< 0 degree), due to mandibular protrusion (SNB \> 80°).
2. normal or horizontal growth pattern (Bjork's sum ≤396° ±5°)

Exclusion Criteria:

1. temporomandibular joint disorders
2. craniofacial anomalies
3. history of previous orthodontic treatment