The structure and function of the temporomandibular joint

The relationship between pregnancy and temporomandibular disorder (TMD) through diagnostic criteria for temporomandibular disorders (DC/TMD) axis II evaluation: a case-control cross-sectional study

INTRODUCTION

This study focuses on temporomandibular disorders (TMDs), which affect the temporomandibular joint and related muscles and have multiple causes. Recent studies have examined the connection between menstrual cycles, estrogen levels, and TMDs, but results are inconsistent, highlighting the need for more research. The aim is to explore the prevalence of TMDs in pregnant women and consider how hormonal changes during pregnancy might influence these disorders.

METHODS

In this cross-sectional case-control study, we compared 32 pregnant women with 35 non-pregnant women. We evaluated several TMD-related factors such as pain levels, chronic pain classification, scores on the Jaw Functional Limitation Scale-20 and Oral Behaviors Checklist, and psychological health. We used various statistical methods including descriptive statistics, chi-square tests, linear regression, and adjustments for multiple comparisons to analyze the data.

RESULTS

Pregnant women showed different pain perceptions, generally reporting less pain and lower severity. Nonetheless, these differences were not uniform across all TMD-related measures. Linear regression did not find a consistent link between pregnancy and TMD scores, except for chronic pain grade, which was not significant after adjusting for multiple comparisons. There was a significant relationship between depression and TMD severity, emphasizing the need to consider mental health in TMD evaluations.

DISCUSSION

The findings suggest that pregnancy is neither a risk nor a protective factor for TMD. Differences in pain perception, functional status, and psychological health were observed in pregnant women but were not consistent for all TMD-related aspects. The role of estrogen in TMJ health and TMD risk is complex and requires further study. The research highlights the necessity of including mental health, especially depression, in TMD assessments. More comprehensive research with larger sample sizes is essential to better understand the connections between pregnancy, TMD, and hormones, aiming to improve TMD management in pregnant women and others.

Mandibular Jaw Movement and Masticatory Muscle Activity during Dynamic Trunk Exercise

The examination of jaw movement during exercise is essential for an improved understanding of jaw function. Currently, there is no unified view of the mechanism by which the mandible is fixed during physical exercise. We hypothesized that during strong skeletal muscle force exertion in dynamic exercises, the mandible is displaced to a position other than the maximal intercuspal position and that mouth-opening and mouth-closing muscles simultaneously contract to fix the displaced mandible. Therefore, we simultaneously recorded mandibular jaw movements and masticatory muscle activities during dynamic trunk muscle force exertion (deadlift exercise) in 24 healthy adult males (age, 27.3 ± 2.58 years). The deadlift was divided into three steps: Ready (reference), Pull, and Down. During Pull, the mandibular incisal point moved significantly posteriorly (-0.24 mm, p = 0.023) and inferiorly (-0.55 mm, p = 0.019) from the maximal intercuspal position. Additionally, temporal, masseter, and digastric muscles were activated simultaneously and significantly during Pull (18.63 ± 17.13%, 21.21 ± 18.73%, 21.82 ± 19.97% of the maximum voluntary contraction, respectively), with maintained activities during Down (p < 0.001). Thus, during dynamic trunk muscle force exertion, the mandibular incisal point moved to a posteroinferior position without tooth-touch (an open-mouth position). Simultaneously, the activities of the mouth-opening digastric muscles and the mouth-closing temporal and masseter muscles led to mandibular fixation, which is a type of mandible fixing called bracing.

The importance of the posterior joint space for functional mandibular movements: A laboratory cross-sectional study

Background

The search for the ideal, healthy and reproducible position of the condyles is of utmost importance for dental diagnosis and treatment. Thus, the objective of this laboratory cross-sectional study was to verify the relationship between the posterior joint space and the mandibular lateral movements.

Material and Methods

Dental casts from 15 women and 15 men with normal mastication, 28 natural teeth and no history of temporomandibular disorders or pain, were fabricated and mounted on a fully adjustable articulator. From the maximum intercuspal position, condylar displacement was evaluated and measured on the working and nonworking sides during mandibular lateral movement, both to the right and left sides.

Results

The correlation between the measures of interest was assessed with the Pearson correlation coefficient (α=.05). Condylar displacement on the working side and nonworking side condyle was 0.88±0.71 mm and 3.57±1.11 mm (right mandibular lateral movement); and 0.91±0.58 mm and 3.51±0.78mm (left mandibular lateral movement), respectively. No significant correlation in the condylar displacement between the working side condyles on the right and on the left sides was observed (r=.22; P=.248). The condylar poles of the articulator moved posteriorly, simulating the functional movements of the mandible during mastication. In all cases, condylar displacement during mandibular lateral movement both to the right and left occurred posteriorly on the working side condyle.

Conclusions

The condylar poles of the articulator moved posteriorly simulating the functional movements of the mandible during mastication. Moreover, left and right working condyles may require slightly different spaces to function, suggesting minor anatomical asymmetries.

Key words:Mastication, dental occlusion, prosthodontics.

Effects of loading on chondrocyte hypoxia, HIF-1α and VEGF in the mandibular condylar cartilage of young rats

OBJECTIVES

To investigate hypoxia-inducible factor 1-alpha (HIF-1α) and vascular endothelial growth factor (VEGF) expression under altered loading, and to explore the relationship between loading and hypoxia in the mandibular condylar cartilage of young rats.

SETTING AND SAMPLE POPULATION

Eighty Sprague-Dawley rats.

MATERIAL AND METHODS

The reduced loading group was fed soft food, and their incisors were cut to avoid occlusal contact. The increased loading group was fed hard food and had forced jaw-opening. Ten rats from each group (n = 10) were sacrificed at 12, 24, 48, and 96 hours after initiation of the experiment. Pimonidazole hydrochloride (Hypoxyprobe-1, HP-1) was used as a hypoxia marker to confirm the hypoxic state. Hypoxic chondrocytes as indicated by HP-1, HIF-1α and VEGF protein expressions were recognized by immunohistochemical detection. HIF-1α and VEGF mRNA expressions were detected by semi-quantitative RT-PCR.

RESULTS

Hypoxyprobe-1 was confined in the upper layers of cartilage, and was most strongly expressed in the weight-bearing area of TMJ at 12 and 96 hours. Staining of HIF-1α and VEGF was most strongly expressed in the chondrocytes of the fibrous and proliferative layer at all time points. Furthermore, expressions were also displayed in the hypertrophic and calcified layers at 48 and 96 hours. The expressions of HIF-1α and VEGF mRNA were higher in the increased loading group than in the reduced loading group at 48 and 96 hours (P < . 05).

CONCLUSION

Mechanical loading seems to directly induce weight-bearing area hypoxia followed by new vessel formation, which indicates that these factors are related and important for the development of cartilage.

Implications of Vertebrate Craniodental Evo-Devo for Human Oral Health

Highly processed diets eaten by postindustrial modern human populations coincide with higher frequencies of third molar impaction, malocclusion, and temporomandibular joint disorders that affect millions of people worldwide each year. Current treatments address symptoms, not causes, because the multifactorial etiologies of these three concerns mask which factors incline certain people to malocclusion, impaction, and/or joint issues. Deep scientific curiosity about the origins of jaws and dentitions continues to yield rich insights about the developmental genetic mechanisms that underpin healthy craniodental morphogenesis and integration. Mounting evidence from evolution and development (Evo-Devo) studies suggests that function is another mechanism important to healthy craniodental integration and fit. Starting as early as weaning, softer diets and thus lower bite forces appear to relax or disrupt integration of oral tissues, alter development and growth, and catalyze impaction, malocclusion, and jaw joint disorders. How developing oral tissues respond to bite forces remains poorly understood, but biomechanical feedback seems to alter balances of local bone resorption and deposition at the tooth-bone interface as well as affect tempos and amounts of facial outgrowth. Also, behavioral changes in jaw function and parafunction contribute to degeneration and pain in joint articular cartilages and masticatory muscles. The developmental genetic contribution to craniodental misfits and disorders is undeniable but still unclear; however, at present, human diet and jaw function remain important and much more actionable clinical targets. New Evo-Devo studies are needed to explain how function interfaces with craniodental phenotypic plasticity, variation, and evolvability to yield a spectrum of healthy and mismatched dentitions and jaws.

Experimentally Created Nonbalanced Occlusion Effects on the Thickness of the Temporomandibular Joint Disc in Rats

Objectives: To test the hypothesis that experimentally created physiologically nonbalanced occlusion will not affect the thickness of the temporomandibular joint (TMJ) discs in rats.

Material and Methods: Twenty-four 8-week-old Sprague-Dawley rats were equally divided into a control group that was left untreated and an experimental group where a nonbalanced occlusion was created. Elastic rubber bands, 1 mm in diameter, were inserted and 1 week later were replaced by plastic material between the first and the second molars of the left maxillary and the right mandibular dentitions to move the first molars about 0.8 mm mesially. This created and maintained a physiologically nonbalanced occlusion. The animals were euthanized 8 weeks later, and the TMJ disc thickness was measured on histologically prepared slices using an electronic meter. Two-way univariate analysis of variance was used to compare the groups (α level = .05).

Results: The intermediate zone was thicker in the experimental group than in the control group (P = .003), but no differences were found between groups regarding the anterior and posterior bands. There were no significant sex-related effects on this observation.

Conclusion: The hypothesis is rejected. The results indicate that the intermediate zone of rat TMJ disc has the ability to adapt to the alteration of the space between condyle and fossa caused by occlusion changes. Further studies on larger groups that are followed for longer times are needed.

Comparison of altered signal intensity, position, and morphology of the TMJ disc in MR images corrected for variations in surface coil sensitivity

OBJECTIVE

The purpose of this study is to evaluate the corrections of signal intensity of the temporomandibular joint (TMJ) disc caused by variations in sensitivity of the magnetic resonance imaging (MRI) surface coil, to compare the modified signal intensities of the posterior and anterior bands, and then to evaluate the relationship of the signal intensity difference to altered disc position and morphology in a group of TMJ patients.

STUDY DESIGN

MRI was performed on 96 joints. All patients underwent imaging in axial, coronal, and sagittal planes using fast-spin echo sequences (FSE). The images were taken in the closed, partially opened, and maximum opened mouth positions in 2 sequences. Classifications were made according to the position and morphology of the disc. TMJs were divided into normal, anterior disc displacement with reduction (ADDwR), anterior disc displacement without reduction (ADDwoR), and partial anterior disc displacement with reduction (PDDwR). Disc morphology was subdivided as biconcave, lengthened, biconvex, thick posterior band, and others (defined as folded and rounded). The correction of the inhomogeneous sensitivity of the surface coil was done with the original software. The signal intensities (SI) of the posterior band and anterior band of TMJ discs were measured. The correlations among the groups of TMJs and disc morphologies and SI were statistically analyzed by using Bonferroni/Dunn multicomparison method test.

RESULTS

Of the total number of joints studied with the help of MRI, 37 were normal, 12 exhibited ADDwR, 32 ADDwoR, and 9 PDDwR. The corrected MR images indicated that SI of the posterior bands were higher than the anterior band of the discs. It can also be concluded that the SI of the posterior bands increased significantly in the following order: normal, PDDwR, ADDwR, and ADDwoR, while there is no statistical difference in the SI of the anterior band of the discs. In ADDwR and ADDwoR, thick posterior band is the most common shape. In normal TMJ, the biconcave shape is identified as the most frequently encountered shape.

CONCLUSIONS

It was demonstrated that the SI of the posterior bands increase with the progress of internal derangement, and was found to be higher than that of the anterior band of the discs. It appears that disc degeneration starts from the posterior band of the disc.

The postglenoid tubercle: prevalence and growth

AIM

To quantify the prevalence and growth of the postglenoid tubercle in a skull sample and in children.

MATERIAL

a) ninety skulls ranging in age from between 2 years and adulthood, b) sixtyfour corrected lateral tomograms of left and right temporomandibular joints of 32 boys and girls. Their age range was between 9 and 11 years.

METHOD

Impressions of the temporal fossae of the skull material were taken with a silicone type impression material, using a face-bow for leveling the base of the impression parallel to the FH plane. Each impression was divided into two halves along a paramedian plane from the tip of the postglenoid tubercle through the middle of the articular eminence and the surface was photocopied to a 200% scale. Height was measured with an electronic caliper. The presence or absence of a postglenoid tubercle was established on the corrected tomograms.

FINDINGS

Seventy-nine percent of the skulls had a postglenoid tubercle. It steadily enlarged and reached almost its final dimension by the age of 13 years. On corrected tomograms, 66% of the children showed a postglenoid tubercle.

CONCLUSIONS

a) the postglenoid tubercle exists in a high percentage of human temporomandibular joints b) growth is almost completed by the age of 13, and c) there exists a right-left symmetry.

The growth of articular eminence height during craniofacial growth period

This study was done to study the growth of the articular eminence height during the postnatal craniofacial growth period. Ninety (90) skulls were divided into three equal groups, named A, B, and C. The A group belonged to individuals showing deciduous dentition, the B group showed mixed dentition, and the C group permanent dentition. The skulls were stabilized so that the FH plane was really horizontal. This was assured using a common facial bow, inserted into the acoustic meatuses and the infraorbital foramen, and a common water-level device. Using a silicone type impression material, impressions were taken of all temporal fossae. Special care was taken in order that the base of the materials was parallel to the FH plane. After setting, the impression was removed and a common eggslicer was used to cut the material into sections of 2.5 mm each. The three middle sections were removed, photocopied with a common photocopy machine, enlarged to a 200% scale and the height of the eminentia were measured by using an electronic caliper. The mean of the three calculations was reduced to its normal value, and was tabulated for each eminence. The study concluded that the articular eminence height grows at a very high rate until the age of seven (the period of deciduous dentition, development, and function), almost stops until the age of eleven (the period of mixed dentition), and acquires the rest of its height by the age of 20. This growth plan more nearly approximates the somatic or the general growth curve.

Constraints on masticatory system evolution in anthropoid primates

It is well established that some observed patterns of force production in the primate masticatory system match those predicted by a simplified lever model. This model is also commonly invoked in adaptive explanations of craniodental diversity. However, systematic studies of the predictive power of this model are missing, leaving open the possibility that factors not traditionally included in the model alter the function and evolution of the masticatory system. One such factor was proposed for mammals generally by Greaves ([1978] J. Zool. (Lond.) 184:271-285), who argued that the temporomandibular joint (TMJ) was poorly suited to being pulled apart. In this constrained lever model, the avoidance of joint distraction leads to limitations on masticatory system form and function. The goal of the present study was to quantify masticatory system diversity in anthropoid primates for comparison with these predictions. Results indicate that all sampled taxa exhibit a form that is consistent with selection against regular distraction of the TMJ. Also apparent from observed patterns of scaling is a regular interaction among a limited set of cranial and dental dimensions, in accordance with the constrained model. However, the data indicate that specific positional relationships among the muscles, joints, and teeth differ from those predicted by Greaves (1978). The pattern of deviation suggests that selection has favored a conservative masticatory system configuration that safeguards the TMJ from distraction during the dynamic processing of irregular foods. The resulting buffered model leads to alternative hypotheses regarding the response of the masticatory system to dietary selection pressures. It may, therefore, improve our understanding of the adaptive significance of primate craniofacial form.

Responses to conservative treatment in temporomandibular disorders with locking versus with muscle pain

We conducted a follow-up study of 150 patients with temporomandibular disorders, of whom 91 had muscle pain and 59 locking, in order to investigate the response to conservative treatment. The patients with muscle pain showed fairly uniform age distribution, while the percentage over 49 years of age was lower in those with locking. Of the patients with TMJ locking, the percentage who completed treatment was high, and the duration of treatment in this group was longer than that in those with muscle pain. The differences between these patient groups in the response to conservative treatment may be useful in the assessment of individual patients with temporomandibular disorders. Moreover, the greater incidence of completion of conservative treatment in the patients with locking indicates that these patients should be treated by conservative treatment prior to surgical treatment.

Description of mandibular finite helical axis pathways in asymptomatic subjects

Despite wide use of systems to record jaw motion with six degrees of freedom, most studies have analyzed only the movement of a single mandibular point. The finite helical axis (FHA) is a mathematical model which can be used to describe comprehensively the movements of a rigid body. The aim of this investigation was to describe the FHA of the mandible during habitual jaw movements. Thirty subjects (13 females, 17 males; mean age, 26 years; range, 18 to 34 years) without myoarthropathies of the masticatory system participated in the study. Opening and closing movements, performed at 1-Hz frequency, were recorded with the optoelectronic system Jaws-3D. Three opening and closing movements were recorded from the right side and three from the left side of the jaw. The movement data were low-pass-filtered for noise reduction prior to the computation of the finite helical axis by means of a software program developed in our laboratory. The following parameters were calculated: the rotation of the FHA, its spatial orientation, and the translation along it, as well as its position and distance relative to an intracondylar point. In addition, methodological errors of the model were calculated. During opening and closing, the group mean FHA rotation was 24.3 degrees +/- 4.2 degrees. The group mean of the maximum total translation along the FHA was 0.9 +/- 0.7 mm. The group mean distance between the FHA and the intracondylar point was 48.9 +/- 9.9 mm. The FHA pathways were smooth and varied between individuals. Furthermore, the finite helical axes were never localized within the condyle, and often were located outside of the mandible. The analysis of the FHA pathways yields more information on whole mandibular movements than simply the movements of a single condylar point.

A study of the control of disc movement within the temporomandibular joint using the finite element technique

PURPOSE

A two-dimensional finite element model was developed to simulate and study the in vivo biomechanics and mechanisms of the human temperomandibular joint (TMJ) over the range of normal motion.

MATERIALS AND METHODS

A nonlinear model was developed and run using the commercially available ABAQUS software with slide line elements that allowed large displacements and arbitrary contact of surfaces. The three main components of the model were the mandibular condyle, articular disc, and glenoid fossa region of the temporal bone, which were all modeled as deformable bodies. Continuous motion was simulated by doing a static analysis for each of many small steps. A parametric study was performed by determining the maximum stress in each of the three main components as a function of the elasticity of the articular disc.

RESULTS

The articular disc was found to move along with condyle in a lifelike manner, even when there were no attachments to the disc. Stress distribution plots showed relatively high stresses deep in the glenoid fossa for most steps. There was a direct, although nonlinear, relationship between maximum stress for all three components and the stiffness of the disc.

CONCLUSIONS

This model suggests that muscle contraction is not required to maintain proper disc position. Normal motion results in relatively high stresses deep in the glenoid fossa. The elasticity of the in vivo articular disc may be closer to the lower end of the reported values.

Morphology of the lateral ligament in the human temporomandibular joint

The morphology of the lateral ligament of the human temporomandibular joint is of two types: ligamentous and without distinct structure. Under the scanning electron microscope, a sheath-like structure that contained bundles of collagen was mainly found in the posterior region of the lateral ligament. Analysis of macromolecular components revealed that type III collagen was mainly present on the collagenous framework of the sheath-like structure. Type I collagen, laminin, and tenascin were found in the framework of the sheath-like structure. Supported collagenous bundles and the distribution of macromolecular components might be related to the stability of the temporomandibular joint. The sheath-like structure and other components of the lateral ligaments store energy and protect the capsule from stress and tension during movements of the jaw.

Post-imaging temporomandibular joint space analysis

The objective of this study was to develop and test a quantifiable method of temporomandibular (TM) joint space analysis (JSA) unaffected by head rotation. Sixteen female TM joints were imaged using multidirectional axially corrected tomography. Lateral, central and medial image slices were obtained from each joint, traced three times each, and triplicated for drafting. Tracings and draftings were randomized, blinded, and measured manually with digital calipers. Simulated head rotations of +/-10 degrees were applied and the process repeated. Multiple comparisons were done within and between patients by analysis of variance. Based on this study, we found there was no difference for tracing, drafting, and measurement error within and between patients. There were no significant differences between -10, 0, and +10 degrees of head rotation. Our conclusion is that this method for TM-JSA has a high level of reliability as determined by analysis of tracing, drafting, and measurement error for each joint space position and simulated rotation.

Changes in the innervation of rabbit craniomandibular joint tissues associated with experimental induction of anterior disk displacement: histochemical and immunohistochemical studies

We have previously reported that surgical induction of anterior disk displacement (ADD) in a rabbit craniomandibular joint (CMJ) leads to histopathological changes consistent with osteoarthritis. This paper reports the changes that were noted in the innervation of rabbit CMJ tissues following surgical induction of ADD. The right joint of 30 rabbits was exposed surgically and the discal attachments were severed except for the posterior discal attachment (bilaminar zone). The disk was then displaced anteriorly and sutured to the zygomatic arch. The left joints was used as sham-operated control. CMJ tissues were then removed after fixation and processed for histochemical localization of nerve fibers using the silver impregnation technique and immunohistochemical localization of neurofilaments using monoclonal antibodies. The results showed an absence of nerve fibers in the control and experimental disks and their presence in the control and experimental bilaminar zones. The bilaminar zone adhesions to the experimental condyles were also innervated. The spread of nerve fibers into the pathological fibrous adhesions surrounding the arthritic condyles in this animal model of ADD may indicate a possible mechanism of nociception in this disease.

Electromyographic analysis of the masticatory muscles of patients after complete rehabilitation of occlusion with protection by non-working side contacts

Surface electromyography was used to study the function of the anterior temporal, surface masseter, deep masseter and anterior digastric muscles of 14 patients after complete rehabilitation of occlusion with fixed prostheses and/or removable partial prostheses. All patients had a balanced occlusion with protection by non-working side contacts during lateral excursion. Mean resting myoelectric activity was 2.5-3.8 microV, showing no hypertonia or muscular spasm. When patients clenched their teeth (i.e. exerted maximum voluntary occlusal force in centric occlusion), the anterior temporal muscles were the most active, followed by the surface masseters; the digastrics were significantly less active. During lateral displacements, the muscles exhibiting significantly higher potentials than the other muscles monitored were the working side anterior temporal muscle and the contralateral surface masseter. During mastication, the muscle that was most active in relation to its contralateral homologue was the working side surface masseter, the difference between the two being statistically significant.