Chronic Pain-Related Jaw Muscle Motor Load and Sensory Processing.

Chronic pain associated with temporomandibular disorders (TMDs) may reflect muscle mechanoreceptor afferent barrage and dysregulated sensory processing. This observational study tested for associations between Characteristic Pain Intensity (CPI), physical symptoms (Patient Health Questionnaire-15 [PHQ-15]), and cumulative jaw muscle motor load (mV*s). In accordance with institutional review board oversight and Strengthening the Reporting of Observational Studies in Epidemiology guidelines, adult subjects gave informed consent and were identified via Diagnostic Criteria for TMD (DC-TMD) examination and research protocols. Subjects were assigned to ±Pain groups using DC-TMD criteria for myalgia. CPI scores characterized pain intensity. PHQ-15 scores were surrogate measures of dysregulated sensory processing. Laboratory tests were performed to quantify masseter and temporalis muscle activities (mV) per bite force (N) for each subject. In their natural environments, subjects recorded day- and nighttime electromyography from which cumulative jaw muscle motor loads (mV*s) were determined for activities consistent with bite forces of >1 to ≤2 and >2 to ≤5 N. Data were assessed using univariate analysis of variance, simple effects tests, K-means cluster classification, and 3-dimensional regression analyses. Of 242 individuals screened, 144 enrolled, and 125 with complete data from study protocols, there were 35 females and 15 males for +Pain and 35 females and 40 males for -Pain. Subjects produced 324 daytime and 341 nighttime recordings of average duration 6.9 ± 1.7 and 7.6 ± 1.7 h, respectively. Overall, +Pain compared to -Pain subjects had significantly higher (all P ≤ 0.002) CPI and PHQ-15 scores. Cumulative jaw muscle motor loads showed significant between-subject effects for time, diagnostic group, and sex (all P < 0.003), where motor loads tended to be higher for daytime versus nighttime, +Pain versus -Pain groups, and males versus females. Two clusters were identified, and regression relations showed associations of low-magnitude daytime masseter motor load, PHQ-15, and CPI scores for cluster 1 (n = 105, R2 = 0.44) and cluster 2 (n = 18, R2 = 0.80). Furthermore, these regression relations showed thresholds of motor load and PHQ-15 scores, above which there were nonlinear increases in reported pain.

Effect of Sustained Joint Loading on TMJ Disc Nutrient Environment.

The temporomandibular joint (TMJ) disc nutrient environment profoundly affects cell energy metabolism, proliferation, and biosynthesis. Due to technical challenges of in vivo measurements, the human TMJ disc extracellular nutrient environment under load, which depends on metabolic rates, solute diffusion, and disc morphometry, remains unknown. Therefore, the study objective was to predict the TMJ disc nutrient environment under loading conditions using combined experimental and computational modeling approaches. Specifically, glucose consumption and lactate production rates of porcine TMJ discs were measured under varying tissue culture conditions (n = 40 discs), and mechanical strain-dependent glucose and lactate diffusivities were measured using a custom diffusion chamber (n = 6 discs). TMJ anatomy and loading area were obtained from magnetic resonance imaging of healthy human volunteers (n = 11, male, 30 ± 9 y). Using experimentally determined nutrient metabolic rates, solute diffusivities, TMJ anatomy, and loading areas, subject-specific finite element (FE) models were developed to predict the 3-dimensional nutrient profiles in unloaded and loaded TMJ discs (unloaded, 0% strain, 20% strain). From the FE models, glucose, lactate, and oxygen concentration ranges for unloaded healthy human TMJ discs were 0.6 to 4.0 mM, 0.9 to 5.0 mM, and 0% to 6%, respectively, with steep gradients in the anterior and posterior bands. Sustained mechanical loading significantly reduced nutrient levels (P < 0.001), with a critical zone in which cells may die representing approximately 13.5% of the total disc volume. In conclusion, this study experimentally determined TMJ disc metabolic rates, solute diffusivities, and disc morphometry, and through subject-specific FE modeling, revealed critical interactions between mechanical loading and nutrient supply and metabolism for the in vivo human TMJ disc. The results suggest that TMJ disc homeostasis may be vulnerable to pathological loading (e.g., clenching, bruxism), which impedes nutrient supply. Given difficulties associated with direct in vivo measurements, this study provides a new approach to systematically investigate homeostatic and degenerative mechanisms associated with the TMJ disc.

Mechanobehavior and Ontogenesis of the Temporomandibular Joint.

Craniofacial secondary cartilages of the mandibular condyle and temporomandibular joint (TMJ) eminence grow in response to the local mechanical environment. The intervening TMJ disc distributes normal loads over the cartilage surfaces and provides lubrication. A better understanding of the mechanical environment and its effects on growth, development, and degeneration of the TMJ may improve treatments aimed at modifying jaw growth and preventing or reversing degenerative joint disease (DJD). This review highlights data recorded in human subjects and from computer modeling that elucidate the role of mechanics in TMJ ontogeny. Presented data provide an approximation of the age-related changes in jaw-loading behaviors and TMJ contact mechanics. The cells of the mandibular condyle, eminence, and disc respond to the mechanical environment associated with behaviors and ultimately determine the TMJ components' mature morphologies and susceptibility to precocious development of DJD compared to postcranial joints. The TMJ disc may be especially prone to degenerative change due to its avascularity and steep oxygen and glucose gradients consequent to high cell density and rate of nutrient consumption, as well as low solute diffusivities. The combined effects of strain-related hypoxia and limited glucose concentrations dramatically affect synthesis of the extracellular matrix (ECM), which limit repair capabilities. Magnitude and frequency of jaw loading influence this localized in situ environment, including stem and fibrocartilage cell chemistry, as well as the rate of ECM mechanical fatigue. Key in vivo measurements to characterize the mechanical environment include the concentration of work input to articulating tissues, known as energy density, and the percentage of time that muscles are used to load the jaws out of a total recording time, known as duty factor. Combining these measurements into a mechanobehavioral score and linking these to results of computer models of strain-regulated biochemical events may elucidate the mechanisms responsible for growth, maintenance, and deterioration of TMJ tissues.

Jaw closing movement and sex differences in temporomandibular joint energy densities.

Energy densities (ED, mJ/mm3 ) quantify mechanical work imposed on articular cartilages during function. This cross-sectional study examined differences in temporomandibular joint (TMJ) ED during asymmetric versus symmetric jaw closing in healthy females versus males. ED component variables were tested for differences between and within sexes for two types of jaw closing. Seventeen female and 17 male subjects gave informed consent to participate. Diagnostic criteria for temporomandibular disorders and images (magnetic resonance (MR), computed tomography) were used to confirm healthy TMJ status. Numerical modelling predicted TMJ loads (Fnormal ) consequent to unilateral canine biting. Dynamic stereometry combined MR imaging and jaw-tracking data to measure ED component variables during 10 trials of each type of jaw closing in each subject's TMJs. These data were then used to calculate TMJ ED during jaw closing asymmetrically and symmetrically. Paired and Student's t tests assessed ED between jaw closing movements and sexes, respectively. Multivariate data analyses assessed ED component variable differences between jaw closing movements and sexes (α = 0.05). Contralateral TMJ ED were 3.6-fold and significantly larger (P < .0001) during asymmetric versus symmetric jaw closing, due to significantly larger (P ≤ .001) distances of TMJ stress-field translation in asymmetric versus symmetric movement. During asymmetric jaw closing, contralateral TMJ ED were twofold and significantly larger (P = .036) in females versus males, due to 1.5-fold and significantly smaller (P ≤ .010) TMJ disc cartilage volumes under stress fields in females versus males. These results suggest that in healthy individuals, asymmetric compared to symmetric jaw closure in females compared to males has higher TMJ mechanical fatigue liabilities.

Jaw mechanics in dolichofacial and brachyfacial phenotypes: A longitudinal cephalometric-based study.

OBJECTIVES: To determine whether dolichofacial (Frankfort horizontal mandibular plane angle (FHMPA) ≥30°) vs brachyfacial (FHMPA ≤22°) phenotypes differ in temporomandibular joint (TMJ) loads and whether these differences correlate longitudinally with mandibular ramus height (Condylion-Gonion, Co-Go). SETTING AND SAMPLE POPULATION: Lateral and posteroanterior cephalographs from ten dolichofacial and ten brachyfacial individuals made at average ages of 6 (T1), 12 (T2) and 18 (T3) years and available online (http://www.aaoflegacycollection.org/aaof_home.html) were used. MATERIALS AND METHODS: Three-dimensional anatomical data were derived from cephalographs and used in numerical models to predict TMJ loads for a range of biting angles on incisors, canines and first molars. Two criteria were used to define clinically important between-group TMJ load differences: statistical significance was defined with a two-group t-test, and where differences were also ≥20%. A statistical approach called response surface analysis was used to assess correlation between TMJ loads and its predictors considered in this study. RESULTS: The two phenotypes had significantly different FHMPA at all ages (P<.05). No differences in TMJ loads were found at T1. Ipsilateral and contralateral TMJ loads at T2 and T3 were significant and ≥20% larger in dolichofacial than brachyfacial phenotypes for specific biting angles (all adjusted P<.05). Regression analysis indicated age and ramus height contribute 53% of the variability in normalized values of TMJ loads. At higher ages, dolichofacial phenotypes had significantly higher TMJ loads which were correlated with shorter ramus heights compared to brachyfacial phenotypes. CONCLUSIONS: Craniofacial mechanics may explain, in part, mandibular growth differences between dolichofacial and brachyfacial phenotypes.

Preface to COAST 2016 innovators' workshop on personalized and precision orthodontic therapy.

OBJECTIVE: A second focused workshop explored how to transfer novel findings into clinical orthodontic practice. SETTING AND SAMPLE POPULATION: Participants met in West Palm Beach (Florida, USA), on 9-11 September 2016 for the Consortium for Orthodontic Advances in Science and Technology 2016 Innovators' Workshop (COAST). Approximately 65 registered attendees considered and discussed information from 27 to 34 speakers, 8 to 15 poster presenters and four lunch-hour focus group leaders. MATERIAL AND METHODS: The innovators' workshops were organized according to five themed sessions. The aims of the discussion sessions were to identify the following: i) the strength and impact of the evidenced-based discoveries, ii) required steps to enable further development and iii) required steps to translate these new discoveries into orthodontic practice. RESULTS: The role of gene-environment interactions that underlie complex craniofacial traits was the focus of several sessions. It was agreed that diverse approaches are called for, such as (i) large-scale collaborative efforts for future genetic studies of complex traits; (ii) deep genome sequencing to address the issues of isolated mutations; (iii) quantifying epigenetic-environmental variables in diverse areas myofascial pain, alveolar remodelling and mandibular growth. Common needs identified from the themed sessions were multiscale/multispecies modelling and experimentation using controlled and quantified mechanics and translation of the findings in bone biology between species. Panel discussions led to the consensus that a consortium approach to establish standards for intra-oral scanning and 3D imaging should be initiated. CONCLUSIONS: Current and emerging technologies still require supported research to translate new findings from the laboratory to orthodontic practice.

Mechanobehaviour in dolichofacial and brachyfacial adolescents.

OBJECTIVES: To test whether mechanobehaviour (temporomandibular joint (TMJ) loads, jaw muscle use) was different between facial types and correlated with ramus height (Condylion-Gonion, mm). SETTING AND SAMPLE POPULATION: University of Missouri-Kansas City (UMKC) Orthodontic Clinic. Ten dolichofacial and ten brachyfacial adolescents (Sella-Nasion-Gonion-Gnathion (SN-GoGn) angles ≥37° and ≤27°, respectively) consented to participate. MATERIALS AND METHODS: Numerical models calculated TMJ loads for a range of static biting based on subjects' three-dimensional anatomy. Subjects were trained to record jaw muscle electromyography (EMG) over 2 days and 2 nights in their natural environments. Laboratory EMG/bite-force calibrations determined subject-specific EMG for 20 N bite-force (T20Nave ). Jaw muscle use via duty factors (DF=muscle activity duration/total recording time, %) was determined from day and night recordings for muscle-specific thresholds from ≥5% to ≥80%T20Nave . ANOVA and Tukey's HSD post hoc tests assessed for group differences in mechanobehaviour (TMJ loads, DFs). Regression modelling correlated subjects' normalized TMJ loads, DFs and ramus height. RESULTS: Dolichofacial compared to brachyfacial subjects produced significantly higher (P<.05) TMJ loads, where ipsilateral loads were ≥20% larger for some biting angles, but had significantly less (all P<.05) masseter (day, night) and temporalis (night) DFs. Regression analysis showed a significant relationship amongst normalized TMJ loads, masseter DF and ramus height (R2 =.49). CONCLUSIONS: Mechanobehaviour showed significant differences between facial types and was correlated with ramus height.

Speed of human tooth movement in growers and non-growers: Selection of applied stress matters.

OBJECTIVES: To test that the speed of tooth translation is not affected by stress magnitude and growth status. SETTING AND SAMPLE POPULATION: Advanced Education Orthodontic clinics at the Universities of Nebraska Medical Center and Missouri-Kansas City. Forty-six consenting subjects with orthodontic treatment plans involving maxillary first premolar extractions. MATERIALS AND METHODS: This randomized split-mouth study used segmental mechanics with definitive posterior anchorage and individual vertical-loop maxillary canine retraction appliances and measured three-dimensional tooth movements. Height and cephalometric superimposition changes determined growing (G) and non-growing (NG) subjects. Subjects were appointed for 9-11 visits over 84 days for maxillary dental impressions to measure three-dimensional tooth movement and to ensure retraction forces were continuously applied via calibrated nitinol coil springs. Springs were custom selected to apply two different stresses of 4, 13, 26, 52 or 78 kPa to maxillary canines in each subject. Statistical analyses (α=0.050) included ANOVA, effect size (partial η2 ) and Tukey's Honest Significant Difference (HSD) and two-group t tests. RESULTS: Distolateral translation speeds were 0.034±0.015, 0.047±0.019, 0.066±0.025, 0.068±0.016 and 0.079±0.030 mm/d for 4, 13, 26, 52 and 78 kPa, respectively. Stress significantly affected speed and partial η2 =0.376. Overall, more distopalatal rotation was shown by teeth moved by 78 kPa (18.03±9.50º) compared to other stresses (3.86±6.83º), and speeds were significantly higher (P=.001) in G (0.062±0.026 mm/d) than NG subjects (0.041±0.019 mm/d). CONCLUSIONS: Stress magnitude and growth status significantly affected the speed of tooth translation. Optimal applied stresses were 26-52 kPa, and overall speeds were 1.5-fold faster in G compared to NG subjects.

Fluid pressurization and tractional forces during TMJ disc loading: A biphasic finite element analysis.

OBJECTIVES: To investigate the ploughing mechanism associated with tractional force formation on the temporomandibular joint (TMJ) disc surface. SETTING AND SAMPLE POPULATION: Ten left TMJ discs were harvested from 6- to 8-month-old male Yorkshire pigs. MATERIALS AND METHODS: Confined compression tests characterized mechanical TMJ disc properties, which were incorporated into a biphasic finite element model (FEM). The FEM was established to investigate load carriage within the extracellular matrix (ECM) and the ploughing mechanism during tractional force formation by simulating previous in vitro plough experiments. RESULTS: Biphasic mechanical properties were determined in five TMJ disc regions (average±standard deviation for aggregate modulus: 0.077±0.040 MPa; hydraulic permeability: 0.88±0.37×10-3 mm4 /Ns). FE simulation results demonstrated that interstitial fluid pressurization is a dominant loading support mechanism in the TMJ disc. Increased contact load and duration led to increased solid ECM strain and stress within, and increased ploughing force on the surface of the disc. CONCLUSION: Sustained mechanical loading may play a role in load carriage within the ECM and ploughing force formation during stress-field translation at the condyle-disc interface. This study further elucidated the mechanism of ploughing on tractional force formation and provided a baseline for future analysis of TMJ mechanics, cartilage fatigue and early TMJ degeneration.

A pilot study of nocturnal temporalis muscle activity in TMD diagnostic groups of women.

Temporomandibular disorder (TMD) incidences are believed to be related to parafunctional behaviours like teeth clenching. This pilot study aimed to (i) develop an automated clench-detection algorithm, and (ii) apply the algorithm to test for differences in nocturnal clenching in women with and without TMD. Subjects gave informed consent to participate. Adult women were categorised using Diagnostic Criteria for TMD according to presence/absence (+/-) of both TM joint disc placement (DD) and chronic pain (P) into two groups (+DD+P, -DD-P) with 12 subjects each. Surface temporalis electromyography was recorded during oral tasks performed by subjects at two laboratory sessions. The data were used to characterise muscle activity per N of bite force (µV/N) for each subject, develop the clench-detection algorithm and test its accuracy. Ambulatory surface temporalis electromyography was self-recorded by each subject over three nights and analysed using the algorithm and bite force (N) versus muscle activity µV/N calibrations. Bonferroni-adjusted homoscedastic t-tests assessed for significant between-group differences in clenching (P < 0·05). Sensitivity, specificity and accuracy of algorithm-detected laboratory clenches were all ≥96%. During self-recordings 95% of clenches had durations of <4 s and peak forces of <10 N in both groups. Mean clench durations were significantly longer (P = 0·042) in +DD+P (1·9 ± 0·8 s) than -DD-P subjects (1·4 ± 0·4 s). Mean temporalis duty factors (%clench time/total recording time) were significantly larger (P = 0·041) in +DD+P (0·47 ± 0·34%) than -DD-P (0·26 ±0·22%) subjects. Nocturnal temporalis muscle activities detected by a validated algorithm were longer per clench and recording time in +DD+P compared to -DD-P women.

Mechanobehavioral Scores in Women with and without TMJ Disc Displacement.

Cartilage fatigue may be a factor in the precocious development of degenerative changes in the temporomandibular joint (TMJ). This cross-sectional study estimated potential for cartilage fatigue via TMJ energy densities (ED) and jaw muscle duty factors (DF), which were combined to calculate mechanobehavioral scores (MBS) in women with (+) and without (-) bilateral TMJ disc displacement (DD). All subjects gave informed consent to participate and were examined using Diagnostic Criteria (DC) for Temporomandibular Disorders (TMD) and magnetic resonance (MR) and computed tomography (CT) images. Forty-seven subjects were categorized into +DD ( n = 29) and -DD ( n = 18) groups. Dynamic stereometry (MR images combined with jaw-tracking data) characterized individual-specific data of TMJ stress-field mechanics to determine ED (ED = W/ Q mJ/mm3, where W = work done, Q = volume of cartilage) during 10 symmetrical jaw-closing cycles with a 20-N mandibular right canine load. Subjects were trained to record masseter and temporalis electromyography over 3 days and 3 nights. Root mean square electromyography/bite-force calibrations determined subject-specific masseter and temporalis muscle activities per 20-N bite-force (T20 N, µV), which defined thresholds. Muscle DF (DF = % duration of muscle activity/total recording time) were determined for a range of thresholds, and MBS (ED2 × DF) were calculated. Intergroup differences in ED, DF, and MBS were assessed via analyses of variance with Bonferroni and Tukey honest significant difference post hoc tests. Average ED for contralateral TMJs was significantly larger ( P = 0.012) by 1.4-fold in +DD compared to -DD subjects. Average DF were significantly larger (all P < 0.01) for +DD compared to -DD subjects by 1.7-, 2.5-, and 1.9-fold for day, night, and overall, respectively. Daytime MBS were significantly larger (all P < 0.04) by up to 8.5-fold in +DD compared to -DD subjects. Significantly larger ED, DF, and MBS were shown in women with compared to women without bilateral TMJ DD.

TMJ energy densities in healthy men and women.

OBJECTIVE: Cartilage fatigue, due to mechanical work, may account for the early development of degenerative joint disease (DJD) in the temporomandibular joint (TMJ), and why women are three times more likely to be afflicted. This study tested for gender differences in mechanical energy densities in women and men with healthy TMJs. DESIGN: Eighteen women and eighteen men gave informed consent. Research diagnostic criteria including imaging were used to ensure that subjects' TMJs were normal, without disc displacement or signs of DJD. Numerical modeling determined TMJ loads (Fnormal). Jaw tracking and three-dimensional dynamic stereometry characterized individual-specific data of stress-field dynamic mechanics during 10 symmetrical jaw closing cycles. These data were used to estimate tractional forces (Ftraction). Energy densities were then calculated, where: Energy Density = W/Q (W = work done or mechanical energy input = Ftraction*distance of stress-field translation, Q = volume of cartilage). Two-way analysis of variance (ANOVA) and follow-up two-group comparisons tested mean energy densities for ipsilateral and contralateral TMJs in women vs men. RESULTS: Mean energy densities ± standard deviations in ipsilateral and contralateral TMJs in women were 9.0 ± 9.7 and 8.4 ± 5.5 mJ/mm3, respectively, and were significantly larger (P = 0.004 and 0.001, respectively) compared to ipsilateral and contralateral TMJs in men, which were 5.6 ± 4.2 and 6.3 ± 4.2 mJ/mm3, respectively. CONCLUSIONS: Energy densities were significantly larger in healthy TMJs of women than men. Larger TMJ energy densities during normal jaw functions could predispose earlier mechanical fatigue of the TMJ disc.

Personalized and precision orthodontic therapy.

OBJECTIVE: To bring together orthodontic stakeholders from academics, industry, and private practice for a series of thematically focused workshops to explore and develop the transfer of novel approaches into clinical orthodontic practice. SETTING AND SAMPLE POPULATION: Twenty-seven invited speakers, eight poster presenters, and participants of the Consortium for Orthodontic Advances in Science and Technology (COAST) 2014 Innovators' Workshop at the Eaglewood Resort and Spa, Itasca, Illinois, September 11-14, 2014. MATERIAL AND METHODS: Five themed sessions involving between 4-7 presentations followed by panel discussions were organized. The aims of the discussion sessions were to highlight important findings and consider the strength of evidence for these, indicate next steps and needed research or technological developments to move forward, and to weigh the expected benefits from these findings and steps to implement in clinical practice. RESULTS: Among important areas for attention identified were need for multiscale and multispecies modeling and experimentation for interspecies translation of results; large-scale collaborative efforts within the profession to address the need for adequate sample sizes for future genetic studies of complex traits such as malocclusion; a consortium approach to improve new technologies such as intra-oral scanning and 3D imaging by establishing standards; and harnessing the growing body of knowledge about bone biology for application in orthodontics. CONCLUSIONS: With increased awareness of the potential of current and emerging technologies, translation of personalized and precision approaches in the field of orthodontics holds ever-increasing promise.

A pilot study of ambulatory masticatory muscle activities in temporomandibular joint disorders diagnostic groups.

OBJECTIVE: To determine differences in masticatory muscle usage between temporomandibular joint disorders diagnostic groups. SETTING AND SAMPLE POPULATION: Seventy-one informed and consented subjects (27 men; 44 women) participated at the University at Buffalo. MATERIAL AND METHODS: Research diagnostic criteria and imaging data were used to categorize subjects according to the presence/absence +/- of TMJ disc placement (DD) and chronic pain (P) (+DD+P, n=18; +DD-P, n=14; -DD-P, n=39). Electromyographic (EMG)/bite-force calibrations determined subject-specific masseter and temporalis muscle activities per 20 N bite-force (T20N , µV). Over 3 days and nights, subjects collected EMG recordings. Duty factors (DFs, % of recording time) were determined based on threshold intervals (5-9, 10-24, 25-49, 50-79, ≥80% T20N ). anova and Tukey-Kramer post hoc tests identified 1) diagnostic group differences in T20N and 2) the effects of diagnostic group, gender, time and interval on muscle DFs. RESULTS: Mean (±SE) temporalis T20N in +DD+P subjects was significantly higher (71.4±8.8 µV) than masseter T20N in these subjects (19.6±8.8 µV; p=0.001) and in -DD-P subjects (25.3±6.0 µV, p=0.0007). Masseter DFs at 5-9% T20N were significantly higher in +DD-P women (3.48%) than +DD-P men (0.85%) and women and men in both other diagnostic groups (all p<0.03), and in +DD+P women (2.00%) compared to -DD-P men (0.83%; p=0.029). Night-time DFs at 5-9% T20N in +DD-P women (1.97%) were significantly higher than in -DD-P men (0.47%) and women (0.24%; all p<0.01). CONCLUSIONS: Between-group differences were found in masticatory muscle activities in both laboratory and natural environmental settings.

Infield masticatory muscle activity in subjects with pain-related temporomandibular disorders diagnoses.

OBJECTIVES: Pain-related temporomandibular disorders (TMDs) are the most prevalent conditions among TMDs. There is contrasting evidence available for association of pain-related TMD and masticatory muscle activity (MMA). The present investigation assesses the associations between MMA levels of masseter and temporalis muscles during awake and sleep among pain-related TMD diagnostic groups. SETTING AND SAMPLE POPULATION: The department of Oral Diagnostic Sciences, University at Buffalo. Twenty females and six males participated in this study. MATERIAL AND METHODS: Using the diagnostic criteria for temporomandibular disorders (DC-TMDs), participants were diagnostically categorized. Subjects used a custom monitoring system, which recorded infield muscle activities. A factorial model tested for association between independent variable (muscle, time period, MMA level, and diagnostic group) effects and the logarithm of MMA. Greenhouse-Geisser test was used to determine any statistically significant associations (p≤0.003). RESULTS: No statistically significant association was found between four-way, three-way, and two-way analyses. However, among the main effects, range of magnitudes was the only variable to be statistically significant. Although the data suggest a trend of increased masseter MMA in the pain-related TMD diagnoses group both during awake and sleep time periods, such observation is not maintained for the temporalis muscle. In addition, temporalis MMA was found to be higher in the pain-related TMD diagnoses group only at extreme activity levels (<25 and ≥80% ranges). CONCLUSION: This data support the association between masticatory muscle hyperactivity and painful TMD conditions.

Diagnostic group differences in temporomandibular joint energy densities.

OBJECTIVES: Cartilage fatigue, due to mechanical work, may account for precocious development of degenerative joint disease in the temporomandibular joint (TMJ). This study compared energy densities (mJ/mm³) in TMJs of three diagnostic groups. SETTING AND SAMPLE POPULATION: Sixty-eight subjects (44 women, 24 men) gave informed consent. Diagnostic criteria for temporomandibular disorders (DC/TMD) and imaging were used to group subjects according to presence of jaw muscle or joint pain (+P) and bilateral disk displacement (+DD). MATERIAL AND METHODS: Subjects (+P+DD, n=16; -P+DD, n=16; and -P-DD, n=36) provided cone-beam computed tomography and magnetic resonance images, and jaw-tracking data. Numerical modeling was used to determine TMJ loads (Fnormal). Dynamic stereometry was used to characterize individual-specific data of stress-field dynamics during 10 symmetrical jaw-closing cycles. These data were used to estimate tractional forces (Ftraction). Energy densities were then calculated as W/Q (W=work done or mechanical energy input=tractional force×distance of stress-field translation, Q=volume of cartilage). anova and Tukey-Kramer post hoc analyses tested for intergroup differences. RESULTS: Mean±standard error energy density for the +P+DD group was 12.7±1.5 mJ/mm³ and significantly greater (all adjusted p<0.04) when compared to -P+DD (7.4±1.4 mJ/mm³) and -P-DD (5.8±0.9 mJ/mm³) groups. Energy densities in -P+DD and -P-DD groups were not significantly different. CONCLUSION: Diagnostic group differences in energy densities suggest that mechanical work may be a unique mechanism, which contributes to cartilage fatigue in subjects with pain and disk displacement.

Modeling of muscle forces in humans with and without temporomandibular joint disorders.

OBJECTIVES: Subjects with/without temporomandibular joint disorders (TMJD) were tested for differences in muscle forces. SETTING AND SAMPLE POPULATION: School of Dental Medicine, University at Buffalo. Ninety-one subjects were classified in four groups based on the presence/absence (±) of chronic myofascial and/or TMJ pain (P) and bilateral disc displacement (DD). MATERIAL AND METHODS: Validated numerical models employed an organizational objective and subjects' anatomy to calculate masticatory muscle forces during static biting. anova and Holm's step-down procedure post hoc tests assessed group differences. Theoretical geometries, representing the range of subjects' muscle orientations, were surveyed via numerical models to identify key combinations resulting in high muscle forces. Effect size (Cohen's d) and anova/post hoc tests assessed group differences in key muscle orientations. RESULTS: +P-DD subjects had significantly higher muscle forces, especially for lateral pterygoid muscles, compared to the other groups (p<0.01) for bite forces that were directed posteromedially or posterolaterally on mandibular molars and posteriorly and slightly medially on mandibular incisors. Key muscle orientations for peak lateral pterygoid muscle forces were identified, and group comparisons showed mean orientation in +P-DD compared to other diagnostic groups was ≥5° more upright for masseter and ≥3° more posteriorly directed for temporalis muscles (all Cohen's d≥0.8). CONCLUSION: Predicted lateral pterygoid muscle forces were significantly higher in +P-DD compared to other groups for specific biting conditions and were attributable, in part, to differences in masseter and temporalis muscle orientations.

Association between waking-state oral parafunctional behaviours and bio-psychosocial characteristics.

The term 'oral parafunctional behaviours' encompasses behaviours that are different from those required for, or associated with, physiological functional needs such as mastication, communication, swallowing or breathing. Previous reports have associated waking-state oral parafunctional behaviours with biopsychosocial characteristics such as female gender, presence of psychological symptoms, intensity of pain and pain-related temporomandibular disorders (TMD) diagnosis. However, the findings have been inconsistent, possibly due to methodological limitations and differences. In the present investigation, we aim to determine whether any association is present between waking-state oral parafunctional behaviours and biopsychosocial characteristics. All participants were investigated using a set of standardised and validated self-reporting questionnaires and diagnostic criteria for temporomandibular disorders (DC/TMD) examination protocol for clinical characterisation. Univariate analysis found that self-reported waking-state oral parafunctional behaviours were statistically significantly associated with presence of anxiety, depression and physical symptoms, pain intensity and TMD diagnosis. However, forward model multiple linear regression analysis indicated that only self-reported presence of physical and depression symptoms could explain statistically significant portions of the variance in self-reported waking-state oral parafunctional behaviours.

Muscle organization in individuals with and without pain and joint dysfunction.

Central nervous system organization of masticatory muscles determines the magnitude of joint and muscle forces. Validated computer-assisted models of neuromuscular organization during biting were used to determine organization in individuals with and without temporomandibular disorders (TMD). Ninety-one individuals (47 women, 44 men) were assigned to one of four diagnostic groups based on the presence (+) or absence (-) of pain (P) and bilateral temporomandibular joint disc displacement (DD). Electromyography and bite-forces were measured during right and left incisor and molar biting. Two three-dimensional models employing neuromuscular objectives of minimization of joint loads (MJL) or muscle effort (MME) simulated biting tasks. Evaluations of diagnostic group and gender effects on choice of best-fit model were by analysis of variance (ANOVA) and Tukey-Kramer post hoc tests, evaluations of right-left symmetry were by Chi-square and Fisher's exact statistics, and evaluations of model accuracy were by within-subject linear regressions. MME was the best-fit during left molar biting in +DD individuals and incisor biting in men (all p < 0.03). Incisor biting symmetry in muscle organization was significantly higher (p < 0.03) in healthy individuals compared with those with TMD. Within-subject regressions showed that best-fit model errors were similar among groups: 8 to 15% (0.68 ≤ R(2) ≤ 0.74). These computer-assisted models predicted muscle organization during static biting in humans with and without TMDs.

Reliability of electromyographic activity vs. bite-force from human masticatory muscles.

The reproducibility of electromyographic (EMG) activity in relation to static bite-force from masticatory muscles for a given biting situation is largely unknown. Our aim was to evaluate the reliability of EMG activity in relation to static bite-force in humans. Eighty-four subjects produced five unilateral static bites of different forces at different biting positions on molars and incisors, at two separate sessions, and the surface EMG activities were recorded from temporalis, masseter, and suprahyoid muscles bilaterally. Intraclass correlation coefficients (ICCs) were determined, and an ICC of ≥ 0.60 indicated good reliability of these slopes. The ICCs for jaw-closing muscles during molar biting were: temporalis muscles, ipsilateral 0.58-0.93 and contralateral 0.88-0.91; and masseter muscles, ipsilateral 0.75-0.86 and contralateral 0.69-0.88. The ICCs for jaw-closing muscles during incisor biting were: temporalis muscles, ipsilateral 0.56-0.81 and contralateral 0.34-0.86; and masseter muscles, ipsilateral 0.65-0.78 and contralateral 0.59-0.80. For the suprahyoid muscles the 95% CIs were mostly wide and most included zero. The slopes of the EMG activity vs. bite-force for a given biting situation were reliable for temporalis and masseter muscles. These results support the use of these outcome measurements for the estimation and validation of mechanical models of the masticatory system.