Analysis and correlation study of human masseter muscle with EMG, ultrasonography & 3D imaging.

Form and function are inextricably intertwined in orthodontics. Our understanding of the hard tissue relationship has been comprehensive, with extensive clinical trials both cross sectional and longitudinal over the past several decades. The majority of studies have used standardized cephalograms that reflect our current concepts. However, the same cannot be stated for soft tissues and, more specifically, the muscles that envelope the skeletal bases. The aim of the current study was to objectively evaluate the masseter muscle – one of the key elevator muscles in the human masticatory system with innovative diagnostic tools like electromyography (EMG), ultrasonography (US) and 3-D imaging (Mona Lisa®). Standard lateral cephalometrics were used as the gold standard for measuring the vertical facial dimensions and correlating the findings to the above mentioned modalities. This extensive study comprised three major components and involved subjects from the same cohort. Firstly, the masseter muscle spindle reflexes were studied in dynamic chewing and the responses recorded with EMG in 28 human subjects. These were then analysed and correlated to the vertical facial proportion from standardized lateral cephalograms. Although substantial work has been done to evaluate the various trigeminal reflexes¹⁻³, limited research has investigated the reflexes from the muscle spindles in different facial proportions⁴. This is primarily due to the inherent complexity of the neurophysiology compounded by noise within the experimental models. Most often the method used to elicit a muscle spindle reflex in the human jaw involves a brief mechanical depression of the mandible, either by use of a tendon hammer or, if a more controlled stimulus is required, a computer-controlled stretching device⁵⁻⁷. In the current study, a sophisticated apparatus was utilized to deliver precise profile stimulus at predetermined intervals. Significant correlations were observed between the muscle spindle response and selected vertical proportions. The second phase of study was to utilize US for measuring masseter muscle dimensions in the same cohort and once again correlate the various dimensions of the masseter muscle to the facial proportions generated by the lateral cephalometrics. US has been studied in dentistry for over four decades and it has been established as a safe, non-invasive, comfortable and cost-effective diagnostic implement. Also, US produces high resolution images of soft tissues more readily than MRI and CT scans⁸. Among the limitations for US there is the concern about reproducibility and standardization particularly between operators. Hence, it is advisable to have a single operator for data collection in order to preclude interexaminer variability. Future use of US in dentistry will be favoured especially where periodic evaluation is needed and there are concerns about radiation. Finally, for a more holistic assessment, 3D optical scanning was used in the same sample cohort to analyse and correlate masseter muscle dimensions to vertical and transverse facial proportions. Regression equations were generated, albeit from the convenience sample, to explore a mathematical model for deriving variables of interest rather than physically conducting the individual measurements, particularly if that required radiation. Current work shows strong correlations between lateral cephalometric and 3D imaging variables, but relatively weak associations with the ultrasonography. The findings from this study support the use of predictive equations from 3D imaging for lateral cephalometric variables but larger scale studies are required to confirm reproducibility. Moreover, curvilinear measurements from 3D images were significantly different (p < 0.0001) to the linear and should be adopted for more meaningful representation of the soft tissues. These findings are relevant to facial form and function assessment which is used in orthodontic diagnosis and treatment planning; in particular, orthodontic functional appliance therapy. Analyses of the face can be carried out with a myriad of cephalometric packages; however, the role of the elevator muscles is often overlooked and their contribution to changes and/or characteristics that could benefit the orthodontic treatment is not fully appreciated. Thus, the assessment not only of anatomical but also physiological variations in the masseter muscles is important in the management of different facial patterns. The above tools provide a means to evaluate muscle functions which would be of particular interest in growing children and where functional orthopaedic appliances utilize forces from the muscles of mastication. Furthermore, periodic evaluation of such cases is generally desirable and needs to be safe, radiation free and cost-effective.Thesis (Ph.D.) -- University of Adelaide, School of Dentistry, 200

Work-related musculoskeletal disorders in dentists and orthodontists: a review of the literature

Work-related musculoskeletal disorders (WMSD's) are on the rise worldwide. These disorders have numerous repercussions, from serious ill-health effects at the individual level to decreased workplace efficiency and productivity affecting not only quality of life but the economy.This review aims to provide an overview of WMSD's in the dental profession, more specifically, among general dental practitioners and specialist orthodontists.Literature was hand searched from Pubmed with greater emphasis on contemporary literature as the professionals demands have evolved over the years.While there have been numerous papers published relating to the prevalence of ergonomic problems among general dentists, orthodontists have received little investigation.This review highlights the need for research into the risk factors of WMSD's in the dental profession (both general dentistry and orthodontics), as this would provide a more direct approach for prevention which is essential as professionals work for longer years in practice

Work-related musculoskeletal disorders in Australian dentists and orthodontists: Risk assessment and prevention

BACKGROUND: As professionals work longer hours and live longer there have been concerns regarding the Work related Musculoskeletal Disorders (WMSD) affecting both professional and personal lives. Moreover, past decade has seen a surge in interest in all allied health sciences personnel with self reporting cross sectional studies. OBJECTIVE: Health professionals often suffer WMSD due to occupational stress. It is important to assess the problem in order to find ways to prevent it. Hence, the focus of this cross-sectional survey. The aim was to investigate the prevalence and risk factors of WMSD between Australian dentists and orthodontists. MATERIALS AND METHOD: A postal survey was sent to 447 Australian orthodontists and 450 Queensland dentists using the universal Nordic scale previously piloted at UQ and refined for this cross-sectional study. Questions were directed towards individuals, workplace and psychosocial variables and were designed to gather information regarding health, lifestyle, education, awareness of musculoskeletal problems and current preventative strategies. RESULTS:Ahigh prevalence of musculoskeletal problemswas found for both dentists (88.9%) and orthodontists (83.6%) reported in the last 12 months. The main predictor in both groupswas increasedwork stress. Less than a third of those professionals surveyed had received education regarding dental practice ergonomics during their tertiary education. CONCLUSION: Dentists and orthodontists experienced a high rate of musculoskeletal problems which were associated with increased levels of stress at work. Further research should be directed toward interventions aimed at reducing stress in the work environment as well as improving work posture

Authorship characteristics of orthodontic randomized controlled trials, systematic reviews, and meta-analyses in non-orthodontic journals with impact factor

This study was conducted to explore authorship characteristics and publication trends of all orthodontic randomized controlled trials (RCTs), systematic reviews (SRs), and meta-analyses (MAs) published in non-orthodontic journals with impact factor (IF).; Appropriate research strategies were developed to search for all articles published until December 2015, without restrictions regarding language or publication status. The initial search generated 4524 results, but after application of the inclusion criteria, the final number of articles was reduced to 274 (SRs: 152; MAs: 36; and RCTs: 86). Various authorship characteristics were recorded for each article. Frequency distributions for all parameters were explored with Pearson chi-square for independence at the 0.05 level of significance.; More than half of the included publications were SRs (55.5 per cent), followed by RCTs (31.4 per cent) and MAs (13.1 per cent); one hundred seventy-eight (65 per cent) appeared in dental journals and 96 (35 per cent) were published in non-dental journals. The last decade was significantly more productive than the period before 2006, with 236 (86.1 per cent) articles published between 2006 and 2015. European countries produced 51.5 per cent of the total number of publications, followed by Asia (18.6 per cent) and North America (USA and Canada; 16.8 per cent).; Studies published in journals without IF were not included.; Level-1 evidence orthodontic literature published in non-orthodontic journals has significantly increased during 2006-15. This indicates a larger interest of other specialty journals in orthodontic related studies and a trend for orthodontic authors to publish their work in journals with impact in broader fields of dentistry and medicine

Modulation of masseteric reflexes by simulated mastication

It is well-known that limb muscle reflexes are modulated during human movements. However, little is known about the existence of equivalent masticatory muscle reflex modulation. We hypothesized that masticatory reflexes would be modulated during chewing so that smooth masticatory movements occur. To examine this hypothesis, we studied the modulation of inhibitory reflexes evoked by periodontal mechanoreceptor activation and of excitatory reflexes evoked by muscle spindle activation during simulated mastication. In 28 participants, 1- and 2-N mechanical taps were delivered to the incisor. Reflex responses to these taps were examined in the average masseteric electromyogram. To differentiate between periodontal mechanoreceptor- and musclespindle- mediated reflex components, we performed experiments prior to, and in the presence of, periodontal anesthesia. Both periodontal mechanoreceptor and muscle spindle reflexes were reduced during simulated masticatory movements.S. Naser-ud-Din, P.F. Sowman, H. Dang, and K.S. Türke

Reflex control of human mastication

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