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

The challenge of ecological validity in temporomandibular disorders research

OBJECTIVE

To review the ecological validity of outcomes from current research involving temporomandibular disorders (TMDs), with an emphasis on chronic myofascial pain and the precocious development of degenerative disease of the temporomandibular joint (TMJ).

MATERIALS AND METHODS

Current approaches used to study TMDs in terms of neuromechanics, masticatory muscle behaviours, and the dynamics of the autonomic nervous system (ANS) were assessed for ecological validity in this review. In particular, the available literature was scrutinized regarding the effects of sampling, environmental and psychophysiological constraints and averaging data across biological rhythms.

RESULTS

Validated computer-assisted numerical modelling of the neuromechanics used biological objective functions to accurately predict muscle activation patterns for jaw-loading tasks that were individual-specific. With respect to masticatory muscle behaviour, current findings refute the premise that sustained bruxing and clenching at high jaw-loading magnitudes were associated with painful TMDs such as myofascial pain. Concerning the role of the ANS in TMDs, there remains the need for personalized assessments based on biorhythms, and where the detection of dysregulated physiologic oscillators may inform interventions to relieve pain and restore normal function.

CONCLUSIONS

Future human research which focuses on TMD myofascial pain or the precocious development and progression of TMJ degenerative joint disease requires experimental designs with ecological validity that capture objectively measured data which meaningfully reflect circadian and ultradian states.

Can hard and/or soft occlusal splints reduce the bite force transmitted to the teeth and temporomandibular joint discs? A finite element method analysis

Objective: To test whether two different materials used for occlusal splints would reduce the stress to the temporomandibular joint discs.Methods: Geometric data from a young-adult male patient were obtained from computed tomography and magnetic resonance imaging. 3D biomodels and the finite element analyses were performed using specific software.Results: The mandibular model presented the highest bone stress areas in the mandibular rami and insertion of the temporalis muscles. Regardless the material, the highest stress in the occlusal splints was located in the second molar regions on the occlusal splint surface and in the opposing mandibular second molars. Stress reduction was only observed in the internal surface of the occlusal splints embracing the maxillary teeth. No differences between occlusal splints were found in the stress intensity and distribution in either left or right TMJ discs, being concentrated more in the anterior portion of the disc.Discussion: Hard acrylic OS should be preferred over soft EVA OS in the majority of cases, soft OS only for temporary use. Thinner OS (2-3 mm anterior thickness) should be preferred over thick ones (3-4 mm) in order to keep the stress concentrations in the center of the TMJ discs. Lighter contacts over heavier contacts should be preferred in the second molar OS contact surface area to prevent stress concentrations and fractures. Maxillary occlusal splints should be chosen if the teeth or implant are in the maxilla, and vice-versa.

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 (T_20Nave ). 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%T_20Nave . 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.

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.

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.

Is There an Association Between Temporomandibular Joint Effusion and Arthralgia?

PURPOSE

The literature on joint effusion (JE) and its association with clinical and radiologic variables in patients with temporomandibular disorders (TMDs) is inconsistent and is characterized by multiple methodologic limitations. The primary aim of this investigation was to evaluate the association between magnetic resonance imaging (MRI) identified JE and temporomandibular joint (TMJ) arthralgia. The secondary aim of this investigation was to determine the association between JE and other clinical and MRI-identified soft tissue characteristics.

MATERIALS AND METHODS

A retrospective cohort study was conducted. Clinical and soft tissue imaging assessments were carried out according to the Diagnostic Criteria for Temporomandibular Disorders guidelines. The dependent variable was JE and the primary independent variable was arthralgia. The secondary independent variables were TMJ pain-associated characteristics and MRI-identified variables. When applicable, Pearson χ² or t test was used to determine the statistical associations between JE and clinical characteristics and between JE and MRI-identified variables. Furthermore, generalized estimating equation (GEE) modeling was conducted to determine which of the independent clinical and MRI-identified variables were associated with JE.

RESULTS

Data for 158 participants, representing 312 joints, were extracted. The mean age of the female sample (59.4%) was 31 ± 11.1 years and that of the male sample (40.6%) was 29.8 ± 9.7 years. No association was found between JE and arthralgia. However, statistically significant associations were found between JE and lateral disc rotation (P = .001) and between JE and disc position in the coronal and sagittal planes (P = .001). The GEE model suggested that disc displacement with reduction (odds ratio = 2.5) was a statistically relevant contributing factor for JE in the absence of degenerative joint disease.

CONCLUSION

Results associated JE with the position of the disc in the sagittal plane. No association was found between JE and arthralgia or TMJ pain-associated clinical characteristics in patients with TMDs.

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.