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

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Abstract
This article takes a closer view of two of the less rare myopathies, myotonic 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 related to the vertical aberration of their craniofacial growth due to the involvement of the masticatory muscles in association with the possibly less affected suprahyoid musculature. Thus, a new situation is established around the teeth transversely. The lowered tongue is not in a position to counterbalance the forces developed during the lowering of the mandible by the stretched facial musculature. This may affect the teeth transversely, decreasing the width of the palate and causing posterior crossbite. The lowered position of the mandible, in combination with the decreased biting forces, may permit an overeruption of the posterior teeth, with increased palatal vault height and development of anterior open bite. The development of the malocclusions in DMD patients also seems to be strongly related to the involvement of the orofacial muscles by the disease. However, the posterior crossbite 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 the transversal expansion of the mandibular arch, possibly because of the decreased tonus of the masseter muscle near the molars, in combination with the enlarged hypotonic tongue and the predominance of the less affected orbicularis oris muscle.

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