The effects of myotonic dystrophy and Duchenne muscular dystrophy on the orofacial muscles and dentofacial morphology

This article takes a closer view of two of the less rare myopathies, myoton ic dystrophy (MyD) and Duchenne muscular dystrophy (DMD). A high prevalence of malocclusions was found among the patients affected by these diseases. The development of the malocclusions in MyD patients seems to be strongly r elated to the vertical aberration of their craniofacial growth due to the i nvolvement of the masticatory muscles in association with the possibly less affected suprahyoid musculature. Thus, a new situation is established arou nd the teeth transversely. The lowered tongue is not in a position to count erbalance the forces developed during the lowering of the mandible by the s tretched facial musculature. This may affect the teeth transversely, decrea sing the width of the palate and causing posterior crossbite. The lowered p osition of the mandible, in combination with the decreased biting forces, m ay permit an overeruption of the posterior teeth, with increased palatal va ult height and development of anterior open bite. The development of the ma locclusions in DMD patients also seems to be strongly related to the involv ement of the orofacial muscles by the disease. However, the posterior cross bite is not developed owing to the narrow maxillary arch, as is the case in MyD patients. On the contrary, the posterior crossbite in DMD is due to th e transversal expansion of the mandibular arch, possibly because of the dec reased tonus of the masseter muscle near the molars, in combination with th e enlarged hypotonic tongue and the predominance of the less affected orbic ularis oris muscle.