Masticatory function before and after orthognathic surgery

Abstract

Deficiency of mandibular length is a frequently occurring developmental dentofacial deformity. This deformity is also known as mandibular retrognathism. As a consequence of this deficiency, the lower dental arch is positioned posterior with respect to the upper dental arch. When treatment is indicated, it commonly includes three phases, i.e. presurgical orthodontic treatment, surgical lengthening of the mandible by means of a bilateral sagittal split osteotomy and postsurgical orthodontic treatment. Dental occlusion and skeletal relationships are abnormal before treatment and change as a result of the treatment. It is likely that this will be reflected in the function of the masticatory system. The aim of this thesis was to analyze masticatory function during the various phases of treatment in patients with a mandibular retrognathism who are undergoing mandibular advancement surgery. In order to quantify masticatory function, the following parameters were measured. After a fixed number of chewing cycles on cubes of a silicone rubber, the median size of the fragmented particles was determined with a sieving procedure as a measure of masticatory performance. The breakdown of food particles can be considered as the composite result of selection and breakage. Selection is the chance that a food particle is at least damaged during a chew. Breakage refers to the extent to which a particle, once selected, is fragmented. For three particle sizes, both parameters were determined in one-chew experiments. Maximum bite force was measured bilaterally at the level of the first molars using a bite fork with 2 force transducers. Electromyographic activity (EMG) of the anterior temporal muscle and the masseter muscle was recorded bilaterally during isometric clenching at maximal and submaximal bite force levels and during the chewing experiments. EMG was expressed as a function of bite force which gives an indication of muscle efficiency. Before any treatment, compared with controls, in patients masticatory performance, selection and breakage are impaired and maximum bite force is lower. As a result of presurgical orthodontics, masticatory performance, selection, breakage, and maximum bite force did not change. After surgical advancement of the mandible and after completion of postsurgical orthodontics (1-1.5 years after surgery), masticatory performance, selection and breakage, maximum bite force, EMG during maximal clenching and peak EMG during chewing were essentially not changed and values were below those found in controls. Treatment did not change EMG/bite force relationships. The EMG/bite-force ratio determined at 10% - 40% of the maximum bite force was higher in patients, indicating lower efficiency for patients when less than half of the maximum bite force is applied. This finding is interesting, since muscle activities up to approximately 50% of the maximum EMG were used during chewing. Five years after surgery, masticatory performance did improve, particularly in patients with a poor masticatory performance before surgery. However, control values were not reached. Maximum bite force did not change and was still lower than in controls. In this study we found that in retrognathic patients before treatment, masticatory function was impaired. Treatment did hardly influence masticatory function. Nevertheless, years after surgery, improvement of the chewing performance may occur, mainly in patients with a poor masticatory performance before treatment.