"Changes in TMJ by Twin Block Therapy in Skeletal Class II Div 2 Malocclusion in Adolescent Females : MRI Study"

Angle's class II division 2 malocclusion is relatively rare. Its frequency lies between 1.5 and 5% of all occlusion in white western population while in north Indian population it is 5.8% Generally in Class II division 2 malocclusion mandibular denture is more distally placed in relation to maxillary denture, a compensating effect in this type of mandibular retrusion , which offsets some of the disharmony of the facial profile is that the chin point is usually very pronounced3. It has been stated that patients with skeletal class II division 2 malocclusion ,when compared with normal class I or class II division I subject have larger masticatory muscles that in addition are oriented in a more anterior direction. These characteristics results in significant differences regarding their mechanical advantage and magnitude of occlusal forces.5 Several investigators have also stated that the mandible is locked posteriorly in Angle's Class II division 2 malocclusion with deep bite and the treatment which unlocks the bite during an active growth period can allow the mandible to grow or be repositioned forward much more than if treatment of unlocking of a bite were not undertaken. In class II division 2 malocclusion the condyle is positioned posteriorly while the disk is positioned anteriorly in relation to glenoid fossa. The articular disc in Angle's class II Division 2 patients could take a relative protrusive position in comparison with the other Angle's classes of malocclusion .

Successful treatment of many orthodontic problems depends to a great extent on the amount of mandibular growth patients experience during treatment. Functional appliances have been used for nearly 100 years in an attempt to induce mandibular growth by changing muscle function and condyle glenoid fossa relationships. Several studies have shown that the treatment of skeletal Class II patients with functional appliances is more successful when initiated during the adolescent growth spurt.

MRI a multiplanar imaging technique, has the advantage of giving an accurate assessment of both the bony and the soft tissues.It has high sensitivity for visualization of position and configuration of disc . This technique is believed to be non-invasive, radiation free and gives more superior contrast resolution than any other imaging modality. Recent studies have shown that we can have greater confidence in MRI as diagnostic tool because of improved quality.

MATERIALS AND METHODS- It is a prospective descriptive study to evaluate TMJ disc-condyle-fossa relationship using MRI scan following functional appliance therapy in skeletal Class II Division 2 malocclusion in adolescent females.

The present study will be conducted in the Department of Orthodontics and Dentofacial Orthopedics, PGIDS, in association with the Department of Radiology, PGIMS, and Department of Oral Radiology, PGIDS, Pt B. D. Sharma University of Health Sciences, Rohtak.

. Ethical clearance approved by institutional Ethics Committee,Post Graduate Institute of Dental Sciences

Source of data:- The sample size consists of 15 subjects who were selected from the patients attending regular OPD at the Department of Orthodontics and Dentofacial Orthopedics for orthodontic treatment.

TARGET SAMPLE SIZE Sample size on= 13 for present study Calculation based on effect size of 1.23 with standard deviation of 10.5 at 95% confidence interval and 80% power n=2.{Z( 1-α/2)+Z(1-β)} 2/ ∆2 To compansate for 10 % dropout the final sample size will be 15

. INFORMED CONSENT OF THE PATIENT AND AGREEMENT TO BE RANDOMISED A valid, informed written consent of the patient or parent/ guardian and an agreement to be randomized was obtained from the patient before registering the patient in this clinical study. Patient or parent/ guardian was informed about all the theoretical risks and benefits of the intervention under test.

INTERVENTION AND DESIGN OF STUDY The study sample will consist of 15 growing females with skeletal class II and Angle's Class II division 2 malocclusion indicated for functional appliance therapy using Twin Block appliance.

The patients attending the regular OPD at the Department of Orthodontics and Dentofacial Orthopedics will be screened to match the inclusion criteria. Lateral Cephalograms will be taken to assess CVMI stage and growth pattern. Skeletal Class II Div 2 adolescent females with growth phase CVMI stage 2-4 having average to horizontal growth pattern as assessed by SN-MP (Go-Gn) angle will be included in the study.

Following detailed clinical examination, cephalometric analysis and treatment planning , the patients who will be ready to receive treatment will undergo pre-treatment MRI of condyle-glenoid fossa complex,then these patients will be treated with pre-functional therapy to correct retroclined incisors. After this, MRI of the TMJ of all these patients will be taken, that is after unlocking of mandible. Then these patients will undergo functional appliance therapy to achieve class 1 occlusion for an average of 6-8 months. After that, post fuctional MRI of the TMJ of all the patients will be taken.

The MRI protocol will include T1 weighted (T1W) spin echo sequences (TR 450/TE 15/Fov 160*160 mm), Proton density weighted spin echo sequences (TR 1500/TE 30/Fov 150*150) in coronal oblique and sagittal oblique planes of 3 mm slice thickness with no interslice gap. All records, including MRI scans will be collected at three stages and will be traced for various angular and linear measurements to document the alterations within the condyle glenoid fossa complex.

1. Stage- I (pre-treatment),
2. Stage- II (after pre-functional therapy)
3. Stage-III (After 6-8 months of functional appliance therapy that is after correction to Class I molar relation ANGULAR AND LINEAR MEASUREMENTS The eminence angle and the sagittal disc position measured in relation to two reference lines: the posterior condylar line (PC line) and the Frankfurt Horizontal plane (FH Plane). The PC line to be drawn directly on the MRI scan, while the FH plane transferred from the lateral cephalogram to the MRI scan, according to the method given by Nebbe et al.

Transfer of FH plane Determination of long axis of the condyle will be done by two step circle center method by Nebbe et al.

ANGULAR MEASUREMENTS 1-2) The eminence angle and the coronal disc position will be evaluated by the method described by Chintakanon et al.

1. The sagittal disc position will be evaluated using PC line and FH plane as described by Chintakanon et al.
2. Sagittal condylar concentricity will be evaluated using the method described by Pullinger et al.
3. The glenoid fossa angle will be measured on the sagittal films as the angle between the tangents to the anterior and posterior slopes of the glenoid fossa.

LINEAR MEASUREMENTS 6) Condyle and glenoid fossa displacements: The position of the glenoid fossa, and that of the condyle, will be evaluated with respect to the centre of the external auditory meatus (c-EAM). by marking one point at the centre of condyle (c-CH) and the linear distance of the c-CH from the c-EAM will be evaluated as the shortest distance from the constructed FH perpendicular.

1. Distance between c-PGS (crest of the post-glenoid spine ) \& c-EAM : One point will be marked at the crest of the post-glenoid spine (c-PGS) and the linear distance of the c-PGS from the c-EAM will be evaluated as the shortest distance from the constructed FH perpendicular.
2. Superior joint space will be measured from the shortest distance between the most superior point of the condyle and the most superior point of the mandibular fossa.

Statistical analysis:-The data recorded will be processed by standard statistical analysis