Quantitative sensory response of the SCM muscle on sustained low level activation simulating co-contractions during bruxing

OBJECTIVE

Bruxism is discussed as an etiological factor in the pathogenesis of orofacial and cervical pain. As the sternocleidomastoid muscle (SCM) is co-activated during clenching, our aim was to investigate, whether the muscle loading leads to peripheral or central sensitizations.

DESIGN

In twenty-one healthy female volunteers, somatosensory profiles of the SCM were recorded according to the test battery of the German Research Network on Neuropathic Pain (DFNS) prior to and after an isometric muscle exercise. QST comprised thermal and mechanical stimuli. A submaximal activation of the SCM (15% MVC) was kept for 10min in sitting position. In separate test sessions one month apart, one sham and one verum experiment were conducted in randomized order. During the muscle loading, the parameters cold detection threshold (CDT), mechanical pain sensitivity (MPS) and pressure pain treshold (PPT) were tested and experimental pain recorded by visual analogoue scales (VAS). All test sessions were performed during the follicular phase of the menstrual cycle (day 5), to avoid effects on pain perception. Data were analyzed with Repeated Measures ANOVA (SPSS 22.0) RESULTS: No significant changes were found during or after (sham) loading except for stimulus-response-function (SR, P=0.01) and PPT (P=0.02) in the sham test. No effect was observed in the verum experiment (P=0.12 up to 1.0).

CONCLUSION

Prolonged low level contraction of the SCM does not evoke painful sensitization. In contrast, submaximal muscle activation seems to have a protective effect corresponding to a training effect preventing sensitization.

Small vertical changes in jaw relation affect motor unit recruitment in the masseter

Strategies for recruitment of masseter muscle motor units (MUs), provoked by constant bite force, for different vertical jaw relations have not previously been investigated. The objective of this study was to analyse the effect of small changes in vertical jaw relation on MU recruitment behaviour in different regions of the masseter during feedback-controlled submaximum biting tasks. Twenty healthy subjects (mean age: 24·6 ± 2·4 years) were involved in the investigation. Intra-muscular electromyographic (EMG) activity of the right masseter was recorded in different regions of the muscle. MUs were identified by the use of decomposition software, and root-mean-square (RMS) values were calculated for each experimental condition. Six hundred and eleven decomposed MUs with significantly (P < 0·001) different jaw relation-specific recruitment behaviour were organised into localised MU task groups. MUs with different task specificity in seven examined tasks were observed. The RMS EMG values obtained from the different recording sites were also significantly (P < 0·01) different between tasks. Overall MU recruitment was significantly (P < 0·05) greater in the deep masseter than in the superficial muscle. The number of recruited MUs and the RMS EMG values decreased significantly (P < 0·01) with increasing jaw separation. This investigation revealed differential MU recruitment behaviour in discrete subvolumes of the masseter in response to small changes in vertical jaw relations. These fine-motor skills might be responsible for its excellent functional adaptability and might also explain the successful management of temporomandibular disorder patients by somatic intervention, in particular by the use of oral splints.

Static balancing behaviour of the mandible

The objective of this study was to investigate the mechanisms of physiological control of the craniomandibular system during force-controlled biting: in intercuspation, restricted by predetermined anatomic-geometrical conditions [i.e. biting in intercuspation (BIC)]; and on a hydrostatic system [i.e. auto-balanced static equilibrium of the mandible (BAL)], in which the mandible is balanced under unrestricted occlusal conditions. For 20 healthy subjects, the spatial positions of the condyles, the lower molars, and the incisal point were measured, and the electromyographic (EMG) activity of the musculus masseter and musculus temporalis anterior were recorded bilaterally, during force-controlled biting (50, 75, 100 N) on a hydrostatic device. The results were compared with those obtained during BIC. During BAL, the neuromuscular system stabilizes one condyle, so it behaves as a virtual fulcrum, and all available biomechanical degrees of freedom of the opposite side are used to achieve a bilaterally equal vertical distance between the upper and lower dental arches. The variability of the positions of the molars was significantly smaller than for the condyles. The EMG co-contraction ratios calculated for homonymous muscle regions revealed significant differences between BIC and BAL, specifically, greater symmetry during BAL with substantial asymmetry of approximately 25% remaining. In conclusion, the results revealed precise neuromuscular control of the position of the lower dental arch; this information might form the basis for interference-free tracking of the mandible in intercuspation under different conditions.