Effects of sustained unilateral molar clenching on the temporomandibular joint space

Temporomandibular joint stress analysis of patients with different mandibular deformities during unilateral molar occlusion

Mandibular prognathism, retrusion and deviation are common mandibular deformities. They can lead to functional and aesthetic problems due to their important role in the oral system. Different from other occlusions, unilateral molar occlusion often occurs during mastication, which has a deep impact on the functions of temporomandibular joints (TMJs). Therefore, the study of unilateral molar occlusion is of great importance and significance to the daily life of patients with mandibular deformities. A total of 35 individuals were involved in this study, including 11 asymptomatic subjects, 10 patients with mandibular prognathism, 5 patients with mandibular retrusion and 9 patients with mandibular deviation. Finite element (FE) models corresponding to the unilateral molar occlusion were constructed. During unilateral molar occlusion, mandibular deformity increases the pressure on the condyle and articular disc. Compared with mandibular protrusion or retraction, facial asymmetry will significantly increase the stress of TMJ. Chewing on the non-deviated side also will lead to higher stress in the TMJ of patients with mandibular deviation. Therefore, patients with mandibular deviation have the highest risk of temporomandibular disorder (TMD), and it is recommended that patients with mandibular deviation chew bilaterally or with the deviated side.

The Dental stick test - Useful to reveal intra-articular conditions in persons with temporomandibular disorder?

BACKGROUND

There is a need for clinical tests that can reveal intra-articular conditions in the temporomandibular joint to be able to give proper treatment. The Dental stick test, where participants bite with their molars on a wooden stick, is a pain provocation test for the temporomandibular joint.

OBJECTIVE

The purpose of this study was to explore if the Dental stick test can be used to reveal intra-articular conditions in the temporomandibular joint in persons with long-lasting painful disorders.

DESIGN

Cross-sectional.

MATERIALS AND METHODS

Forty persons with pain in one or both temporomandibular areas were examined using the Dental stick test. The participants were between 18 and 70 years, and had 65 symptomatic joints. Sensitivity, specificity, false positive and false negative responses, and area under the curve were calculated for the Dental stick test with MRI and/or CT as reference standards. The phi-correlation coefficient (phi) was used to measure the correlation between the Dental stick test and intra-articular findings.

RESULTS

The sensitivity and specificity for the Dental stick test, revealing intra-articular findings, were 0.62 and1.0 respectively. False positive and false negative responses were 0 and 0.41 respectively, and the area under the curve 81% with p < 0.001. The correlation between a positive Dental stick test and intra-articular findings was not significant (phi = 0.60).

CONCLUSIONS

When the Dental stick test is positive, intra-articular findings will most probably be present on MRI/CT, but the test alone cannot reveal intra-articular conditions in persons with long-lasting painful temporomandibular disorder.

Immediate effects of temporary bite-raising with light-cured orthodontic band cement on the electromyographic response of masticatory muscles

To assess the immediate effects of temporary bite-raising using light-cured orthodontic band cement on the superficial masseter and anterior temporalis electromyography (EMG) activity in healthy adults. Surface EMG signals were recorded bilaterally from the superficial masseter and anterior temporalis muscles of 30 volunteers with a normal occlusion, before and after having temporary bite-raising. The bite-raising was done by adding light-cured orthodontic band cement (3x5x2 mm WxLxH) on the lingual cusps of both upper first molars. The measurements were recorded (i) at rest, (ii) while clenching in centric occluding position and (iii) while chewing on an artificial test food. The EMG activity at rest and during clenching, the maximum voltage, and the duration of the identified EMG signal burst while chewing the artificial test food before and after temporary bite-raising were statistically compared using the paired t-test or the Wilcoxon signed-rank test based on the normality of the variables. The significance level was set at 5%. After temporary bite-raising, we found no significant change in integral EMG activity at rest position for the superficial masseter (mean difference (MD)=7.5 μVs) and for the anterior temporalis muscle (MD=36.8 μVs); however, the integral EMG activity during clenching was significantly reduced for the superficial masseter (MD=201.2 μVs) and for the anterior temporalis muscle (MD=151.8 μVs). During mastication, the maximum voltage of the identified burst was significantly reduced on the preferred chewing side of the superficial masseter and anterior temporalis muscles (MD=127.9 and 47.7 μV, respectively), while no significant change was found for the duration of the identified burst (MD=-34.1 and 3.4 ms, respectively) after temporary bite-raising. The results point to an altered neuromuscular behavior during clenching and chewing immediately after temporary bite-raising with light-cured orthodontic band cement. This information is relevant for orthodontists to inform their patients what will happen to their masticatory muscle activity when this bite-raising method is used.

Distraction of the temporomandibular joint condyle in patients with unilateral non-reducing disc displacement: Fact or fiction?

OBJECTIVES

This study investigated the distractive effect of a unilateral pivot splint on patients with unilateral disc displacement without reduction.

METHODS

The study group was comprised of 18 patients who had no history of treatment with removable prosthetic restorations of molars, premolars, or canine teeth, and no previous treatment for temporomandibular disorder. Joint spaces measurements made on magnetic resonance images indicated the affected side to be narrower than the healthy side. Unilateral distraction splints were made for all patients. An ultrasonic motion analyzer was used to measure the vertical shift occurring on the affected side as patients closed their mouths with maximal force with the splint in their mouths.

RESULTS

Closing with maximal force on the unilateral distraction splint led to a noticeable downward movement of the affected condyle.

DISCUSSION

The findings of this study indicate that the TMJ condyle of patients with unilateral disc displacement without reduction may be unilaterally distracted if the articular space is narrowed.

Impact of lateral occlusion schemes: A systematic review

STATEMENT OF PROBLEM

Although several lateral occlusion philosophies have been proposed in the literature, there is a lack of compelling evidence supporting any scheme.

PURPOSE

The purpose of this systematic review was to investigate the clinical implications of different lateral occlusion schemes.

MATERIAL AND METHODS

A literature search was completed through PubMed (MEDLINE), Google Scholar, and Cochrane Library, up to January 2014. The literature search aimed to retrieve 2 study categories: group 1: comparative studies; group 2: clinical outcome studies. The inclusion criteria were peer-reviewed human clinical studies published in English. The search was further supplemented by manual searching through the reference lists of the selected studies.

RESULTS

The initial search revealed a total of 680 studies; however, after applying the inclusion criteria, 26 studies were found suitable for the analysis (13 for group 1 and 13 for group 2). The most commonly evaluated lateral occlusion schemes were canine-guided occlusion (CGO) and group function occlusion (GFO). Group 1 studies evaluated the impact of lateral occlusion schemes on muscular electromyographic (EMG) activity, condylar displacement, mastication, and mandibular movement. Group 2 studies evaluated the impact of restored occlusion on longevity, patient comfort, and pathologic consequences. CGO was associated with narrower mastication and less EMG activity of the masticatory muscles during clenching. GFO was associated with wider mandibular movement and quicker mastication. During mastication, there was no difference in EMG activity between the 2 lateral occlusion schemes. Furthermore, the long-term studies indicated that there is no difference between the 2 schemes in patient comfort and restoration longevity.

CONCLUSION

Although there are immediate differences between the different lateral occlusion schemes, patients have the capability to successfully adapt to CGO or GFO.

Finite element analysis of the human mastication cycle

The aim of this paper is to propose a biomechanical model that could serve as a tool to overcome some difficulties encountered in experimental studies of the mandible. One of these difficulties is the inaccessibility of the temporomandibular joint (TMJ) and the lateral pterygoid muscle. The focus of this model is to study the stresses in the joint and the influence of the lateral pterygoid muscle on the mandible movement. A finite element model of the mandible, including the TMJ, was built to simulate the process of unilateral mastication. Different activation patterns of the left and right pterygoid muscles were tried. The maximum stresses in the articular disc and in the whole mandible during a complete mastication cycle were reached during the instant of centric occlusion. The simulations show a great influence of the coordination of the right and left lateral pterygoid muscles on the movement of the jaw during mastication. An asynchronous activation of the lateral pterygoid muscles is needed to achieve a normal movement of the jaw during mastication.

A study of the temporomandibular joint during bruxism

A finite element model of the temporomandibular joint (TMJ) and the human mandible was fabricated to study the effect of abnormal loading, such as awake and asleep bruxism, on the articular disc. A quasilinear viscoelastic model was used to simulate the behaviour of the disc. The viscoelastic nature of this tissue is shown to be an important factor when sustained (awake bruxism) or cyclic loading (sleep bruxism) is simulated. From the comparison of the two types of bruxism, it was seen that sustained clenching is the most detrimental activity for the TMJ disc, producing an overload that could lead to severe damage of this tissue.

Afferent sensory mechanisms involved in jaw gape-related muscle activation in unilateral biting

OBJECTIVES

In unilateral biting or chewing, the working/balancing-side ratio (W/B-ratio) of masseter activities is inversely proportional to the jaw gape which was interpreted as a neuromuscular strategy to protect occlusion. This suggests that jaw separation is afferently perceived, raising the question how this perception might work. In related studies, isometric biting was exerted on rubber pieces that slightly yielded similar to compressed food in chewing. We hypothesized that minor jaw movements associated with this yielding are necessary to elicit a jaw gape-related control of relative activation in isometric biting.

MATERIALS AND METHODS

Surface electromyograms of masseter muscles were recorded bilaterally in 20 males during (a) unilateral chewing, (b) isometric biting on rubber pieces inducing jaw gapes of 5, 3, 2, 1, and 0.5 mm, and (c) isometric biting with teeth embedded in rigid splints causing gapes of 5 and 1 mm.

RESULTS

With rubber, the masseter W/B-ratio increased from 100 % (5 mm) to 166 % (1 mm) (p = 0.0003) whereas with the splint it increased just slightly to 112 % (p = 0.005). With 1 mm gape, W/B-ratios in splint biting were significantly smaller than in rubber biting or in chewing (p = 0.01).

CONCLUSIONS

We conclude that minor jaw motion preceding peak force in unilateral biting is necessary to create afferent sensory information that could elicit jaw gape-related activation of masseter muscles.

CLINICAL RELEVANCE

Demonstrating a condition under which jaw gape-related activation can lose its occlusion protecting effect, these findings might contribute to disclose the causes of craniomandibular disorders.

Shear mechanics of the TMJ disc: relationship to common clinical observations

The temporomandibular joint (TMJ) is a complex hinge and gliding joint that induces significant shear loads onto the fibrocartilage TMJ disc during jaw motion. The purpose of this study was to assess regional variation in the disc's shear loading characteristics under physiologically relevant loads and to associate those mechanical findings with common clinical observations of disc fatigue and damage. Porcine TMJ discs were compressed between an axially translating bottom platen and a 2.5-cm-diameter indenter within a hydrated testing chamber. Discs were cyclically sheared at 0.5, 1, or 5 Hz to 1, 3, or 5% shear strain. Within the anterior and intermediate regions of the disc when sheared in the anteroposterior direction, both shear and compressive moduli experienced a significant decrease from instantaneous to steady state, while the posterior region's compressive modulus decreased approximately 5%, and no significant loss of shear modulus was noted. All regions retained their shear modulus within 0.5% of instantaneous values when shear was applied in the mediolateral direction. The results of the disc's regional shear mechanics suggest an observable and predictable link with the common clinical observation that the posterior region of the disc is most often the zone in which fatigue occurs, which may lead to disc damage and perforation.

Risk factors associated with incidence and persistence of signs and symptoms of temporomandibular disorders

OBJECTIVE

To analyze whether gender, self-reported bruxism, and variations in dental occlusion predicted incidence and persistence of temporomandibular disorder (TMD) during a 2-year period.

MATERIAL AND METHODS

The study population comprised 280 dental students at Umeå University in Sweden. The study design was that of a case-control study within a 2-year prospective cohort. The investigation comprised a questionnaire and a clinical examination at enrolment and at 12 and 24 months. Cases (incidence) and controls (no incidence) were identified among those without signs and symptoms of TMD at the start of the study. Cases with 2-year persistence of signs and symptoms of TMD were those with such signs and symptoms at all three examinations. Clinical registrations of baseline variables were used as independent variables. Odds ratio estimates and 95% confidence intervals of the relative risks of being a case or control in relation to baseline registrations were calculated using logistic regression analyses.

RESULTS

The analyses revealed that self-reported bruxism and crossbite, respectively increased the risk of the 2-year cumulative incidence and duration of temporomandibular joint (TMJ) signs or symptoms. Female gender was related to an increased risk of developing and maintaining myofascial pain. Signs of mandibular instability increased the risk of maintained TMD signs and symptoms during the observation period.

CONCLUSION

This 2-year prospective observational study indicated that self-reported bruxism and variations in dental occlusion were linked to the incidence and persistence of TMJ signs and symptoms to a higher extent than to myofascial pain.

Indications for jaw gape-related control of relative muscle activation in sequent chewing strokes

Jaw muscle activity ratios in unilateral isometric biting differ from ratios of unilateral chewing but approach the latter if the jaw gape in biting is made as small as the minimum interocclusal distance (MID) of chewing. Especially, the masseter working/balancing side ratio (W/B-ratio) becomes as asymmetric as in chewing, because of reduction in balancing side (BS) masseter activity. This behaviour of ratios might reflect a 'chewing-specific' motor strategy induced when isometric biting is performed with a 'chewing-like' gape. If this hypothesis applies, activity ratios should be associated with MIDs of sequent chewing strokes in a similar manner as with incremented jaw gapes in isometric biting. To test this prediction, bilateral surface electromyograms of masseter and anterior temporalis muscles and incisor movements were recorded during unilateral chewing in 52 subjects. W/B-ratios of masseter and temporalis activities and temporalis/masseter-ratios on both sides were calculated. The ratios were related to MIDs of consecutive chewing cycles. Three of the four ratios were associated with masticatory MID in the same manner as with jaw gape in isometric biting. In particular with decreasing MID, the masseter W/B-ratio increased from 1.5 to 2.2 (P < 0.01). This increase in asymmetry was attributed to a stronger decrease in masseter activity on the BS than on the working side. We conclude that relative jaw muscle activation is associated with interocclusal distance in a similar way in isometric biting and in chewing. This analogy supports the idea of a common jaw gape-related neuromuscular strategy facilitated by afferent signalling of interocclusal distance.

Organization and function of the collagen fiber system in the human temporomandibular joint disk and its attachments

The collagen fiber organization in the anterior band (AB), intermediate zone (IZ) and posterior band (PB) of the disk of the human jaw joint disk and in its attachments was studied under the polarizing microscope. Observations were made on serially sectioned joints (n = 6) and three sets of disk samples sectioned along mutually perpendicular planes (n = 21). The collagen fiber bundles in all disk regions branch and join or are decussated with other bundles. The fibers of the IZ were oriented generally parallel to the disk surfaces. Most of these fibers extend into the AB and PB where they either join with transverse or vertically oriented fiber groups or pass through the bands into the disk attachments. The lateral branching/decussation angles of fibers in the IZ and adjacent regions of the AB and PB were measured in the central region of four disks. The mean angle for all regions was 28 +/- 13 degrees . The mean angles in the region transitional between the IZ and AB and in the anterior region of the IZ were significantly greater than those in the posterior part of the IZ (p = 0.001 and 0.050 for the two comparisons). The large caliber, vertically oriented fibers in the AB, IZ and PB were counted in nine specimens. Disk dimensions were also measured in these specimens. The vertically and transversely oriented bundles were more frequent in the band regions. The number of vertically oriented fiber bundles varied within and between the disk bands. Vertical fiber number in the AB was greater laterally than medially (p = 0.04). In the PB the fiber number was greater than in either the AB or IZ (p = 0.000 for both comparisons) and within the PB itself the fiber number was greater in its thicker, medial half (p = 0.014). The fiber number in the AB and IZ was not different. The thickness of the AB, IZ and PB and disk length was measured in sections located laterally, centrally and medially. No difference in disk length was found across these planes. Statistically significant differences were found in regional disk thickness. In all of the mediolateral planes, the AB and PB were thicker than the IZ and the PB was thicker than the AB (p = 0.000 for all comparisons). The IZ was thicker medially than laterally (p = 0.034). The PB was thicker centrally and medially than laterally (p = 0.002 and 0.001, respectively). PB thickness in its central and medial regions was not different. The findings, combined with other evidence, suggest that the fiber system of the disk serves a stress distribution function and that within the AB and PB, the regions containing the greatest number of vertical fibers may also be the sites of greatest compressive stress during jaw function.

Biomechanics of the human temporomandibular joint during chewing

Experimental data on the loading of the human temporomandibular joint during chewing are scarce. Coincidence of the opening and closing chewing strokes of the condyles probably indicates compression in the joint during chewing. Using this indication, we studied the loading of the joint during chewing and chopping of a latex-packed food bolus on the left or right side of the mouth. Mandibular movements of ten healthy subjects were recorded. Distances traveled by the condylar kinematic centers were normalized with respect to the distances traveled during maximum opening. We judged coincidence of the opening and closing condylar movement traces without knowing their origin. When subjects chewed, the ipsilateral condyles traveled shorter distances than did the contralateral condyles. During chewing and chopping, all contralateral condyles showed a coincident movement pattern, while a significantly smaller number of ipsilateral condyles did. These results suggest that the ipsilateral joints were less heavily loaded during chewing and chopping than were the contralateral joints.

Maxillomandibular relationship in TMD patients before and after short-term flat plane bite plate therapy

This study assessed the maxillomandibular relationship in temporomandibular disorders (TMD) patients, before and after short-term, flat plane bite plate therapy. It was of interest to determine the incidence and degree of mandibular deviation in a group of TMD patients and whether the mandible would shift to the midline and consequently affect reported symptoms. Seventeen female and three male subjects (age range 19-60) were included in the study. Thirteen subjects were diagnosed with myofascial pain while seven were diagnosed as exhibiting disk displacement with reduction (Research Diagnostic Criteria). After taking impressions for these subjects, casts were fabricated and mounted. Maxillomandibular relationship was evaluated by the Denar Centric Check system (Anaheim, CA). The maxillary and mandibular labial frena were used as a reference to evaluate mandibular shift. Symptom questionnaires were used to assess temporomandibular joint pain and clicking. All subjects exhibited deviation (12 subjects to the right and 8 subjects to the left) prior to bite plate therapy. After flat plane bite plate therapy, the mandibular position of all subjects shifted toward the labial frenum midline position. Based on the Binomial test, the shift was significant (p < 0.001). Measurements on the Centric Check system showed a significant movement of both condyles in the anterio-posterior plane as well as the vertical plane. There was also significant reduction in TMJ pain and clicking (p < 0.01). The results support the hypothesis that the balanced position of the mandible is with frena aligned. When occlusal obstructions are eliminated, the mandible will drift to this position.

Microscopic analysis of the temporomandibular joint in rabbits (Oryctolagus cuniculus L.) using an occlusal interference

The purpose of this study was to assess the tissue alterations in the temporomandibular joint (TMJ) of the New Zealand White rabbit (Oryctolagus cuniculus L.), after a unilateral occlusal interference insertion on the animal's right side back teeth. A total of 36 animals were used, thirty of which belonged to the experimental group and six to the control group. We established three experimental periods: 24 hours, three days and seven days. The control group animals were divided two by two; each pair followed the same experimental periods of the former one. The experimental group animals were submitted to the use of a 0.3 mm thick metallic cap with a visor. All animals were euthanized, and the TMJs were removed. Using a microscope for examination we observed, in all experimental periods, the presence of intra-articular hemorrhage in the supra- and infra-disk compartments as well as in the retro-disk zone. There were no inflammatory cells detected. The thickness of the condylar fibrocartilage presented significant alterations among the animals of the three experimental groups. In the left TMJs no inflammatory cells were detected. The results suggest that the insertion of a unilateral occlusal interference in rabbit back teeth does not cause any inflammatory intra-articular process within seven days; however, it does cause bilateral intra-articular hemorrhage and a larger compression of the condylar fibrocartilage in the joint opposite the side where the interference is placed. We also concluded that, in order to do research on the temporomandibular joint using animals, it is necessary have an independent (or separate) group of animals as controls.

Viscoelastic properties and residual strain in a tensile creep test on bovine temporomandibular articular discs

This study was designed to evaluate the creep characteristics and residual strain of bovine temporomandibular joint (TMJ) discs in tension. Twenty discs were divided into three specimens each: central, lateral and medial regions. Tension of 1.0 MPa was applied and sustained for 20 min to the specimens from 10 right-side discs, and tension of 1.5 MPa to specimens from 10 left-side discs. After the period of tension for creep, the specimens were removed from the tension devices and restoration observed for 20 min. Time-dependent creep curves showed a marked change in strain during the initial 5s. The essential time delay in strain ceased after 2 min, and strain reached an almost steady level after 3 min. At a tensile stress of 1.5 MPa, a strain of 14.5% on average was produced after 20 min creep in the central specimens; peripheral specimens showed strains of 12.4% on average. There were significant differences in strain between the central and peripheral specimens. The residual strain after 20 min restoration was 0.93% on average and there were no significant regional differences. This creep feature could be well represented by a generalized linear viscoelastic model. It was concluded that the regional differences in viscoelasticity might be caused by the complicated articulating functions of the TMJ, and that the residual strain caused by sustained stress could be an important factor in disc deformation.

The effect of altering jaw position on the transmission of vibration between the skull and teeth in humans

The forces that develop in the human temporomandibular joint during function have never been directly measured, yet many patients exhibit excessive localized wear and tear of the joint, suggesting that at times these forces exceed tissue tolerance. The purpose here was to gain insight into vibration transmission between the cranium and mandible in healthy humans during variations of jaw position. In 13 normal healthy adult men (age 25-40 years) with normal dentitions, vibration (around 400 Hz, sine wave) was applied to the skull vertex and changes in the intensity of the vibration signal were measured using several small linear accelerometers cemented to the buccal surfaces of mandibular first molars and the labial surfaces of maxillary central incisors. The jaw was opened, protruded and moved laterally by the individual participant and vibration signal intensities were tested (lower first molar signal/upper incisor signal) for change at each mandibular position by ANOVA. The results showed the vibration signal changed significantly with opening and less so on protrusion and laterotrusion. The vibration signal during opening could be categorized into three types: (1) signal increased continuously with increased opening; (2) signal increased, then reached a plateau during last third of opening; (3) signal increased, then decreased when the individual moved from 40 mm to maximum opening. These findings show that as the jaw moves laterally, and especially as it opens, the magnitude of the vibration signal increases substantially compared with the near-closed position.

Viscoelastic properties of the pig temporomandibular joint articular soft tissues of the condyle and disc

It has been suggested that a sustained loading condition such as clenching could compress the temporomandibular joint (TMJ) articular soft tissues. However, there is still no clear understanding of how the TM joint articular tissues respond under compression. To answer this question, we performed in vitro indentation tests on fresh articular discs and cartilage-bone systems of the condyles of 10 Yorkshire pigs (aged 7 months) using a self-developed indentation tester. The indenter was 5 mm in diameter and was controlled by means of a computer-aided feedback mechanism. Bilateral condyles from the same mandible were uniformly prepared; one was used for measurements under sustained compression (SC) and the other for measurements under intermittent compression (IC). The displacements of the indenter induced by a SC of 10, 20, and 30 Newtons (N, units of force) for 10 min and by an IC, also of 10, 20, and 30 N, with one-second duration and two-second intervals for 10 min were measured by means of a displacement sensor with a resolution of 0.001 mm. From these data, the indentation curves of the articular discs and the cartilage-bone systems were calculated. Both the disc and the articular cartilage showed characteristic displacement vs. time curves-namely, an instantaneous deformation upon load application, followed by a time-dependent creep phase of asymptotically increasing deformation under constant load. However, the indentation curves of the two tissues were not identical: The deformation of the articular cartilage was dose-dependent, but that of the disc was not. Moreover, the articular cartilage deformed significantly less under IC than under SC. This difference was not found in the disc. It can be concluded that both the disc and the articular cartilage of the pig temporomandibular joint have viscoelastic properties against compression; however, the disc is stiffer than the articular cartilage.