Quantification of MyHC mRNA in masticatory muscles during orthodontic treatment in pigs

Orthodontic treatment of retrognathism is aimed at correcting the position of the jaws and at rectifying functional problems. Changes induced in the bone morphology of the mandible and in the dentoalveolar region by different appliances are well documented. Less is known and measurable with respect to the functional status and muscular capacity of orofacial muscles induced by the appliance within the framework of the training effect in Class II malocclusion cases. The aim of the present investigation was to analyze and quantify the change in fiber type levels of type I (slow Myosin heavy chain (MyHC)) and type IIb (fast b MyHC) MyHC RNA transcripts, using the competitive reverse polymerase chain reaction (cRT-PCR) during sagittal advancement of the mandible in pigs. The experiments were carried out on 10-week-old piglets (six test animals, six controls) over a period of 28 days. Six piglets received acrylic build-ups for permanent sagittal advancement of the mandible. Muscle tissue was taken from seven different regions of the masseter (m1-3), temporal (tp1, 2), medial pterygoid (pm), and geniohyoid muscles (gh). A marked increase in the absolute amount of slow MyHC RNA transcripts was detected after treatment in the anterior region and in the medial region of the masseter muscle, and in the anterior region of the temporal muscle. For fast b MyHC fibers no significant changes were registered. This can be seen as a shift directed to slow MyHC, confirming results of the training effect in sport and rehabilitation medicine.