Exploring the mylohyoid area as a reference for three-dimensional digital mandibular superimposition

INTRODUCTION

the quantification of tooth movements should be obtained at different specific times and compared at different stages for every orthodontic treatment. These movements are generally measured on teleradiographs or casts. The use of Cone Beam Computed tomography (CBCT) for maxillary superimposition is clearly established in the literature, but not well defined for mandibular superimposition. This study aims to explore and evaluate the accuracy of the mylo-hyoid area as a reference for mandibular Digital Dental Cast (DDC) three-dimensional (3D) superimposition.

MATERIALS AND METHODS

the study compared mandibular 3D overlays with profile teleradiographs in 30 patients followed at Nice Saint Roch University Hospital. The molar and incisor coordinates on the 3D superimposition based on the mylo-hyoid area were compared to the ones on the 2D lateral cephalogram. Differences between the two methods of superimposition were assessed using paired t-tests.

RESULTS

No statistically significant difference was observed between the lateral cephalogram-based and mandibular DDC superimposition methods in 3D sagittal and vertical displacements of the lower first molars and central incisors.

CONCLUSION

The study showed the mylo-hyoid area to be an accurate superimposition landmark for the 3D evaluation of mandibular orthodontic tooth displacement. This method is also applicable for patients with conventional orthodontic treatment records. Other studies should be conducted on larger populations, subgroups (malocclusions, therapeutics) and on the use of an intra-oral camera.

[TMJ and orthodontics, "past, present and future"]

In the past, the ATM was mainly associated with the growth of the mandibular condyle. Many studies (on rats) showed the role of condylar cartilage in the growth response following stimulation by orthopedic appliances. From where, Class II dysmorphosis "orthopedic" treatments to grow the mandible; but this concept is discussed in the literature in the absence of fully conclusive results and especially since the contribution of orthognathic surgery. Currently, the operating concept is the mechanical stimulation and therefore the function will shape the ATM during growth and that, from an early age. Prevention of dysmorphoses must go through behavioral counseling to be adopted by parents from the birth of their child: to stimulate mandibular propulsion breastfeeding, then by a hard diet inducing an alternating unilateral chewing. Ignorance of the specificity of temporomandibular dysfunction (TMD) notably among teenagers has, in the past, left a doubt about the positive or negative role that could have orthodontic treatment on the TMJ. Currently, the best knowledge of TMJ and TMD provides a better therapeutic conduct: behavioral counseling especially for the girl hyperdivergente with small condyles, control of the condylar position, occlusal adjustments at the end of orthodontic treatment. The future of TMJ in relation with orthodontics is based on prevention, screening and deepening of our knowledge. The orthodontist will thus not make a treatment in patients at risk or will identify it and finish the treatment perfectly.