Asymmetric muscle function in patients with developmental mandibular asymmetry

Abstract

The aim was to test the hypothesis that developmental mandibular asymmetry is associated with increased asymmetry in muscle activity. Patients with mandibular condylar and/or ramus hyperplasia having unilateral cross-bite were compared with healthy subjects with normal occlusion. Muscle activity was recorded with surface electrodes in the masseter, suprahyoid, sternocleidomastoid muscle (SCM) and upper trapezius areas during jaw opening-closing-clenching, head-neck flexion-extension, and elevation-lowering of shoulders. Root mean square (RMS) and mean power frequency (MPF) values were calculated and analysed using anova and t-tests with P < 0.05 chosen as significance level. The SCM and masseter muscles showed co-activation during jaw and head movements, significantly more asymmetric in the patients than in the healthy subjects. The RMS and MPF values were higher in the patients than in the controls in the SCM and suprahyoid areas on both sides during jaw opening-closing movement. The results indicate that the ability to perform symmetric jaw and neck muscle activities is disturbed in patients with developmental mandibular asymmetry. This is of clinical interest because asymmetric activity may be an etiologic factor in temporomandibular joint and cervical pain. The results support that co-activation occurs between jaw and neck muscles during voluntary jaw opening and indicate that postural antigravity reflex activity occurs in the masseter area during head extension. Further studies, where EMG recordings are made from the DMA patients at early stages are motivated to verify activity sources and test if the asymmetric activity is associated with muscle and joint pain in the jaw and cervical areas.

The role of orthodontics in temporomandibular disorders

“…Secondly, jaw function alteration including asymmetric pattern of jaw muscles EMG activity (47), lower bite forces (48), reduced thickness of the ipsilateral masseter (49) and a reverse sequence chewing pattern (50, 51) have been reported. Also, these functional alteration normalized after early correction of the crossbite (52–54). Thirdly, it has been hypothesized that the crossbite is a compensatory curvature in the visceral cranium for the transmission of the asymmetry of the body to the skull.…”

Corrective Outcomes after Bimaxillary Orthognathic Surgery for Developmental Facial Asymmetry (Changes of Head Posture and Upper Face)

“…We assume that gradual adaptation of redistribution of jaw muscle activity pattern cause alteration of facial alignment and postural change. 1 …”

TMD symptoms and vertical mandibular symmetry in young adult orthodontic patients in North Sumatra, Indonesia: a cross-sectional study

“…The etiopathogenesis of TMD, which is a common feature in mandibular asymmetry, is poorly understood, because the complexity of biomechanical, neuromuscular, bio-psychosocial and biological factors has contributed to this disorder 6, 35 . Clinicians should note that the complexity of dentofacial variation in orthodontic patients indicates in part why most treatment approaches for malocclusions with TMD are directed to the symptoms rather than to etiology.…”