Three-dimensional magnetic resonance image of the mandible and masticatory muscles in a case of juvenile chronic arthritis treated with the Herbst appliance.

The present report documents, in a case of juvenile chronic arthritis (JCA) with mandibular retrognathia, three-dimensional (3D) changes in the mandible and the relationship between the mandible and the masticatory muscles resulting from treatment with the Herbst appliance after cessation of growth. Magnetic resonance scanning of the whole head was carried out before and after treatment. The mandible, the masseter, and the medial and lateral pterygoid muscles were segmented bilaterally and reconstructed in 3D for both stages. Superimposition of the datasets was carried out according to anatomical structures in the brain (cranial base). Mandibular superimposition was performed according to the mandibular symphysis and the lower mandibular border. The mandible moved forward and downward relative to the anterior cranial base. In addition, bone apposition was observed at the superior and posterior surfaces of both mandibular condyles and at the roof of the glenoid fossa. The masticatory muscles remained relatively stable in position in relation to the anterior cranial base. To our knowledge, such information in JCA patients has not previously been published in the literature. Using magnetic resonance imaging (MRI), it was possible to gain improved insight into the 3D morphology including soft tissues without the overlap of the surrounding tissues observed in the conventional radiographs. Accordingly, it is suggested that 3D magnetic resonance analysis is a more useful method for the follow-up of the JCA patients than radiographic techniques.