Functional Adaptability of Jaw-muscle Spindles after Bite-raising

Myths surrounding vertical dimension of occlusion in restorative dentistry: A scoping review

OBJECTIVE

This scoping review aims to clarify the concept of vertical dimension of occlusion (VDO) in prosthetic dentistry, addressing prevalent myths, and controversies regarding its clinical management and impact on the stomatognathic system.

METHODS

This paper critically examines common beliefs about VDO alteration and its effects on temporomandibular disorders (TMD) through an extensive literature review and an international survey. The survey included 862 general dentists and prosthodontists and explored their approaches to altering VDO and their perceptions of the clinical implications. The literature review provided a thorough analysis of existing research on VDO modification techniques, the adaptability of the stomatognathic system, and the relationship between VDO changes and TMD.

RESULTS

The survey revealed insights into dentists' perspectives on VDO, showing that while many practitioners test a new VDO due to concerns about patient adaptation, a significant majority observed that patients typically adjust well to a new VDO regardless of the technique used. The survey highlighted differing beliefs about the need for a gradual process and trial phase for VDO alteration, as well as the stability of modified VDO. Literature review suggests the stability and adaptability of VDO changes due to muscle adaptability and indicates no direct causal or curative link between VDO changes and TMD. Traditional methods for determining VDO in complete dentures may not be suitable for worn dentition, underscoring the need for tailored approaches.

CONCLUSION

This paper suggests that the traditional belief in the need for a prolonged trial phase for VDO alterations may be reconsidered in light of evidence supporting the adaptability of the stomatognathic system. Moreover, it emphasizes the importance of distinguishing between methods suitable for complete dentures and those for managing cases of worn dentition. The purported link between VDO alterations and TMD is also found to lack scientific evidence, highlighting the need for individualized patient care based on current evidence and clinical judgment.

Impact of occlusal contact pattern on dental stability and oromandibular system after orthodontic tooth movement in rats

How to ensure dental stability in new positions and reduce the likelihood of relapse is a major clinical concern in the orthodontic field. Occlusal contacts between arches may affect the transmission of masticatory forces, thereby influencing the biological response of the periodontal and the oromandibular system. Occlusion factors that may influence the stability after orthodontic tooth movement (OTM) remain largely unknown. Hence, this research was conducted in order to investigate the influence of different occlusal contact patterns on tooth stability and oromandibular system including the masseter muscle and the temporomandibular joint following OTM. By modifying the occlusal surfaces, in vivo animal study models with distinct occlusal patterns corresponding to clinical circumstances were established. The relapse distance of teeth and the level of inflammatory factors in the gingival cervical fluid were analyzed. We also closely observed the histological remodeling of periodontal tissue, masseter tissue, and joint tissue after one week of relapse. Moreover, genes expression in the alveolar bone was analyzed to illustrate the potential biological mechanisms of relapse under the influence of different occlusal contact patterns following OTM. Different occlusal contact patterns after OTM in rats were established. The intercuspation contact between cusp and fossa group exhibited the lowest level of relapse movement, inflammatory factors and osteoclast activity (P < 0.05). On the other hand, groups with interferences or inadequate contacts exhibited more relapse movement, and tend to promote inflammation of periodontal tissue and activate bone resorption (P < 0.05). Adequate occlusal contacts without interference may enhance tooth stability and reduce the likelihood of relapse. After active orthodontic treatment, necessary occlusal adjustment should be made to achieve the desired intercuspation contact relationship and ensure adequate contact between the arches. The elimination of occlusal interferences is crucial to achieving optimal stability and promoting overall healthy condition of the oromandibular system.

Does Orthodontic Treatment Change the Preferred Chewing Side of Patients with Malocclusion?

Whether orthodontic treatment can change the preferred chewing side (PCS) is unknown. This study examined (1) if the PCS changes after orthodontic treatment and (2) which factors contribute to this change. Two hundred fifty patients who visited the orthodontic clinic at Tokyo Medical and Dental University Hospital between 2017 and 2020 were included in the study. Mandibular kinesiograph (MKG) was taken at pre- and post-treatment, and PCS was determined. Patients who showed a change in PCS to the opposite side and those who showed no change in PCS at post-treatment were pooled into the PCS-changed and PCS-unchanged groups, respectively. The demographic, clinical, and cephalometric parameters were compared between the groups. Significant factors associated with changes in were of age < 20 years at the beginning of orthodontic treatment (odds ratio (OR), 2.00), maximum lateral mandibular movement to PCS ≥ 10.0 mm at pre-treatment (OR, 6.51), and change in occlusal canting of ≥1.0° (OR, 2.72). The predicted probability of change in PCS was 13.2%, 36.0%, and 67.5% for no factor, one factor, and two factors associated with PCS change, respectively. Orthodontic treatment may change PCS due to patient age, maximum lateral mandibular movement to PCS, and change in occlusal canting.

Effect of short-term increase in occlusal vertical dimension on masticatory muscle electrical activities and pressure-to-pain threshold: A crossover clinical study

STATEMENT OF PROBLEM

Increasing the occlusal vertical dimension (OVD) is a common procedure in clinical practice. However, few studies have assessed the short-term influence of an increased OVD on the masticatory muscles.

PURPOSE

The purpose of this crossover clinical study was to investigate the effect of increased OVD on the electrical activity of masticatory muscles and the pressure-to-pain threshold (PPT) in asymptomatic participants.

MATERIAL AND METHODS

Twenty asymptomatic participants (22.7 ±2.39 years of age; 10 men, 10 women) were submitted to an increase in OVD by using interocclusal devices of different thicknesses (3- and 6-mm) for 24 hours. Masticatory muscle activity was assessed with electromyography (EMG), expressed in microvolts (μV), while the participants masticated 2 pieces of latex of different consistency (latex S and H) with and without the device. The PPT was measured with a digital algometer before and after the mastication. The results were assessed by using analysis of variance (ANOVA) and the Tukey honestly significant difference (HSD) test (α=.05).

RESULTS

The change of OVD did not significantly affect the electrical activity of the muscles. The latex of hard consistency (latex H) showed higher electrical activity values than the soft texture (latex S). For the working side anterior temporal muscle, a change in the PPT was detected on the day the 6-mm-device was inserted, but for the other muscles, device thickness and latex mastication did not change the PPT.

CONCLUSIONS

For a short time, an increase in occlusal vertical dimension did not affect the electrical activity in the masticatory muscles or pressure-to-pain threshold in asymptomatic participants.

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.

The effects of increasing occlusal vertical dimension on the deep masseter of rat at different ages

OBJECTIVE

To investigate the influence of increasing the occlusal vertical dimension (iOVD) on the fibre-type distribution and ultrastructure of deep masseter of rat at different ages.

DESIGN

A total of forty-eight male Wistar rats were divided into two groups according to age: 'teenage' group (n=24, 1.5 months) and 'young adult' group (n=24, 8 months). Both the teenage and the young adult rats were then randomly divided into the control group (n=12) and the experimental group (n=12). The occlusal vertical dimensions of the rats in the experimental groups were increased by placing composite resin on all maxillary molars. The fibre-type distribution and ultrastructure of the deep masseter were subsequently observed on day 7 and day 14 after iOVD.

RESULTS

In the teenage experimental group, the proportion of type IIa fibres increased, while the proportion of type IIb and type IIx fibres decreased by day 7 after iOVD (P<0.05). However, no significant fibre phenotype transformation was observed in the young adult experimental group until day 14 after iOVD. In addition, the proportion of type IIa in the teenage experimental group was higher than that of the young adult experimental group on day 7 and 14 (P<0.05). Under the transmission electron microscope, muscle fibre reconstruction and the compensatory increase in the number and volume of mitochondria appeared earlier in the teenage experimental group. The cellular traumatic reaction was less than that in the young adult experimental group.

CONCLUSION

The teenage rat alters masseter muscle structure to a slower phenotype earlier and to a greater degree than that of the young adult rat when increasing the occlusal vertical dimension.

Effects of increased occlusal vertical dimension on the jaw-opening reflex in adult rats

OBJECTIVE

Malocclusion with deep overbite and facial esthetics improve when facial height is intentionally increased during orthodontic extrusion of the posterior teeth. Thus, a better understanding of post-treatment stability of increased occlusal vertical dimension (iOVD) in adult patients is important. We focused on the jaw-opening reflex (JOR), which plays an important role in the control of jaw movements during mastication, and investigated the effects of iOVD on the JOR in rats with an electrophysiological technique.

DESIGN

One hundred and twenty 13-week-old male Wistar rats were randomly divided into control and experimental groups. Rats in the experimental group received a 2-mm buildup of composite resin on the maxillary molars at 13 weeks of age. The JOR was induced by low-intensity electrical stimulation of the left inferior alveolar nerve. The electromyographic responses were recorded from the digastric muscle at 13, 14, 15, 17, 19, and 23 weeks of age. JOR properties including latency, duration, and peak-to-peak amplitude were measured and compared between the groups.

RESULTS

The latency of the JOR was significantly longer and the peak-to-peak amplitude was significantly smaller in the experimental group than in the control group from 14 to 19 weeks of age, while the reflex duration was not significantly different. Intra-group comparisons of the latency and peak-to-peak amplitudes among rats 14-19 weeks of age were significantly different between the experimental group and the control group.

CONCLUSIONS

iOVD affected the latency and amplitude of the JOR but not the duration. The JOR adapted after 10 weeks of iOVD.

Markerless three-dimensional tracking of masticatory movement

Conventional methods for measuring mandibular movement are expensive and require headgear and a marker attached to the mandibular incisors. These make assessment of normal chewing difficult. The aim of the present study was to test the validity of a markerless three-dimensional system for tracking masticatory movement by comparing it with a conventional method using an incisal marker. The study investigated 100 chewing cycles in 10 participants. The jaw tracking system consisted of a camera capable of recording depth and red, green, and blue data simultaneously, a laptop computer, and data analysis software. Depth data for each participant's face, tracked in real time, produced a computed 3D mask. The most prominent point of the soft tissue under the lip was defined as the chin point. A dental clasp cemented to the labial surface of the mandibular incisors was defined as the incisal point. The movement of these two measuring points was simultaneously recorded during mastication of chewing gum for 20s. To conduct the same analysis on each cycle from the two measuring points, all cycles were normalized by dividing by the corresponding vertical displacement because of their size variation. The findings showed excellent intramethod correlation for normalized horizontal displacement at every level (>0.9; except for 2 out of 19 levels; 0.896 and 0.898), and a lack of proportional bias. These findings suggest a correlation between the chewing cycles from two measuring points, the incisor and the chin, further suggesting the feasibility of a markerless system for tracking masticatory movement.

Does altering the occlusal vertical dimension produce temporomandibular disorders? A literature review

The purpose of this review was to present a comprehensive review of the scientific evidence available in the literature regarding the effect of altering the occlusal vertical dimens-ion (OVD) on producing temporomandibular disorders. The authors conducted a PubMed search with the following search terms 'temporoman-dibular disorders', 'occlusal vertical dimension', 'stomatognatic system', 'masticatory muscles' and 'skeletal muscle'. Bibliographies of all retrieved articles were consulted for additional publications. Hand-searched publications from 1938 were included. The literature review revealed a lack of well-designed studies. Traditional beliefs have been based on case reports and anecdotal opinions rather than on well-controlled clinical trials. The available evidence is weak and seems to indicate that the stomatognathic system has the ability to adapt rapidly to moderate changes in occlusal vertical dimension (OVD). Nevertheless, it should be taken into consideration that in some patients mild transient symptoms may occur, but they are most often self-limiting and without major consequence. In conclusion, there is no indication that permanent alteration in the OVD will produce long-lasting TMD symptoms. However, additional studies are needed.

The effect of a mandibular advancement appliance on cervical lordosis in patients with TMD and cervical pain

AIMS

A preliminary study to compare cervical lordosis by means of cervical cephalometric analysis, before and after six months of continuous mandibular advancement appliance (MAA) use, and to show how physical therapy posture re-education would improve the cervical lordosis angle.

METHODOLOGY

Twenty-two female patients with temporomandibular disorders (TMD) and cervical pain with lordosis <20° were included. Patients had to have a muscle pain history for at least six months, and with an intensity ⩾6, measured by means of a visual analog scale (a horizontal 0-10 numeric rating scale with 0 labeled as 'no pain' and 10 as 'worst imaginable pain'). Patients had to present the angle formed by the posterior tangents to C2 and C7 of equal or less than 20°. Cephalometric and clinical diagnostics were performed initially (baseline) and at the end of the study period (six months). During the third month with MAA treatment, a physical therapist evaluated the postural deficit and performed a program of postural re-education. Angular and linear dimension data presented a normal distribution (P>0·05; Shapiro Wilk Test), so the paired comparison of the cephalometric measurements was made by t-test for dependent samples.

RESULTS

Angle 1 (OPT/7CVT); angle 3 (CVT/EVT) and angle 4 (2CL/7CL) showed a significant increase in the cervical lordosis. Angle 2 (MGP/OP), angle 5 (HOR/CVT) and the distances C0-C2 and Pt-VER, presented no significant changes.

CONCLUSIONS

The increase in cervical lordosis implies that six months of continuous MAA use, together with a program of postural re-education, promotes the homeostasis of the craniocervical system.

The occlusal appliance effect on myofascial pain

There are limited studies about the effects of occlusal appliance (OA) after three months of use. This study aimed to compare myofascial pain (MP) according to RDC/TMD, craniocervical relationships (CR) and masseter and temporalis bilateral electromyographic (EMG) activity, before and after three months of occlusal appliance use. Nineteen patients participated in this study. Cephalometric and RDC/TMD diagnostics were performed previously (baseline) and at the end of the study period (three months). EMG recordings at clinical mandibular rest position (MRP), during swallowing of saliva (SW) and during maximum voluntary clenching (MVC) were performed as follows: after one hour of use of an OA; after three months of using the OA for a minimum of 16 hours each day; and immediately after removal from the mouth. MP was relieved in all patients at the end of the study period. CR did not change significantly between baseline and after removal of the OA at the end of the study period. EMG activity during MRP, SW, and MVC decreased in both muscles after one hour using the OA and maintained the same level for the three-month period. When comparing baseline versus final EMG activity without OA, a significant decrease was only observed in the masseter muscle. The results observed in the present study are relevant to clinicians because they imply that the therapeutic effect of OA does not significantly affect the homeostasis of the craniocervical system.

Electromyographic activity during awake tooth grinding tasks at different jaw posture in the sagittal plane

OBJECTIVE

The goal of this study was to evaluate the electromyographic (EMG) activity of the anterior temporalis, suprahyoid, infrahyoid and trapezius muscles during tooth grinding at different jaw posture tasks.

MATERIALS AND METHODS

Participants were 30 healthy subjects with natural dentition, bilateral molar support and incisive protrusive guidance. Bipolar surface electrodes were located on the right side of the subject. EMG recordings were performed in the following tasks: (A) Eccentric grinding from intercuspal position to protrusive edge-to-edge contact position; (B) concentric grinding from protrusive edge-to-edge contact position to intercuspal position; (C) eccentric grinding from intercuspal position to the maximum voluntary retrusive position; and (D) concentric grinding from the maximum voluntary retrusive position to intercuspal position. The results were analyzed statistically by Friedman test and Wilcoxon signed rank-sum test.

RESULTS

EMG activity in the anterior temporalis and infrahyoid muscles was significantly higher during task C than the other tasks. EMG activity in the suprahyoid muscles was significantly higher during task C than task D. EMG activity in the trapezius muscle was significantly higher during task C than tasks A and B.

CONCLUSIONS

The higher EMG activity recorded in task C could become important when its frequency, duration and magnitude are enough to exceed the adaptation capability of the individual.

Effect of occlusal splint thickness on electrical masticatory muscle activity during rest and clenching

The extent of separation between the maxillary and mandibular teeth in the fabrication of interocclusal splints designed to achieve efficiency and muscle relaxation is controversial and undefined in the literature. Based on this premise, the aim of this study was to evaluate the effect of interocclusal splint thicknesses of 3 and 6 millimeters on the electrical activity of the anterior temporal and masseter muscles during rest and dental clenching. Twenty asymptomatic individuals (10 males and 10 females) were selected using the Research Diagnostic Criteria (RDC). Electromyography (EMG) was performed both with and without the 3- and 6-mm splints using the Bio EMG software package, which recorded values given in microvolts (µV). The results, which were assessed using analysis of variance (ANOVA) to a 5% significance level (p < 0.05), showed increased electrical activity of the masticatory muscles during dental clenching compared with at rest, with greater activity in the masseter muscle. The electrical activity did not differ according to the thickness of the splints or between males and females. We can conclude that both splint thicknesses are effective in treating muscle hyperactivity given their similar clinical behavior for asymptomatic individuals.

Increased occlusal vertical dimension induces cortical plasticity in the rat face primary motor cortex

Previous studies have demonstrated that functional plasticity in the primary motor cortex (M1) is related to motor-skill learning and changes in the environment. Increased occlusal vertical dimension (iOVD) may modulate mastication, such as in the masticatory cycle, and the firing properties of jaw-muscle spindles. However, little is known about the changes in motor representation within the face primary motor cortex (face-M1) after iOVD. The purpose of the present study was to determine the effect of iOVD on the face-M1 using intracortical microstimulation (ICMS). In an iOVD group, the maxillary molars were built-up by 2mm with acrylic. The electromyographic (EMG) activities from the left (LAD) and right (RAD) anterior digastric (AD), masseter and genioglossus (GG) muscles elicited by ICMS within the right face-M1 were recorded 1, 2 and 8 weeks after iOVD. IOVD was associated with a significant increase in the number of sites within the face-M1 from which ICMS evoked LAD and/or GG EMG activities, as well as a lateral shift in the center of gravity of the RAD and LAD muscles at 1 and 2 weeks, but not at 8 weeks. These findings suggest that a time-dependent neuroplastic change within the rat face-M1 occurs in association with iOVD. This may be related to the animal's ability to adapt to a change in the oral environment.

Masseter length determines muscle spindle reflex excitability during jaw-closing movements

INTRODUCTION

The masticatory muscles are considered to be important determinants of facial form, but little is known of the muscle spindle reflex characteristics and their relationship, if any, to face height. The aim of this study was to determine whether spindle reflexes, evoked by mechanical stimulation of an incisor and recorded on the masseter muscle, correlated with different facial patterns.

METHODS

Twenty-eight adult volunteers (16 women; ages, 19-38 years) underwent 2-N tap stimuli to their maxillary left central incisor during simulated mastication. Reflexes were recorded during local anesthesia of the stimulated tooth to eliminate the contribution from periodontal mechanoreceptors. Surface electromyograms of the reflex responses of the jaw muscles to these taps were recorded via bipolar electrodes on the masseter muscle and interpreted by using spike-triggered averaging of the surface electromyograms. Lateral cephalometric analysis was carried out with software (version 10.5, Dolphin, Los Angeles, Calif; and Mona Lisa, Canberra, Australia).

RESULTS

Two-newton tooth taps produced principally excitatory reflex responses beginning at 17 ms poststimulus. Correlation analysis showed a significant relationship between these muscle spindle reflexes and facial heights: specifically, shorter face heights were associated with stronger spindle reflexes. This correlation was strongest between the derived measure of masseter length and the spindle reflex strength during jaw closure (r = -0.49, P = 0.008).

CONCLUSIONS

These results suggest that a similar muscle spindle stimulus will generate a stronger reflex activation in the jaw muscles of patients with shorter faces compared with those with longer faces. This finding might help to explain the higher incidence of clenching or bruxism in those with short faces and also might, in the future, influence the design of orthodontic appliances and dental prostheses.

Functional Changes of Temporomandibular Joint Mechanoreceptors Induced by Reduced Masseter Muscle Activity in Growing Rats

Objective: To determine the influence of masseter muscle activity during growth on the functional characteristics of temporomandibular joint (TMJ) mechanoreceptors.

Materials and Methods: Sixty-six 3-week-old male Wistar rats were divided into an experimental group, in which the masseter muscles were bilaterally resected at 3 weeks of age, and a control group. Single-unit activities of the TMJ mechanoreceptors were evoked by indirect stimulation of passive jaw movement. Electrophysiologic recordings of TMJ units were made at 5, 7, and 9 weeks of age.

Results: During this period, the firing threshold of the TMJ units was significantly lower and the maximum instantaneous frequency of the TMJ units was significantly higher in the experimental group than in the control group.

Conclusion: Reduced masseter activity during the growth period alters the response properties of TMJ mechanoreceptors.

Effects of a liquid diet on temporomandibular joint mechano-receptors

Load during mastication is an important factor for the development and maintenance of mechano- receptor properties. The purpose of this study was to examine property changes in the rat TMJ mechano-receptors under conditions of liquid diet feeding and low articular load during the growth period. The hypothesis was that alterations in mastication of liquid diet might increase TMJ mechano-receptor sensitivity. Sixty-six two-week-old male Wistar rats were divided into two groups: a control group that was fed on whole pellets, and an experimental group that was fed a liquid diet. Electrophysiological recordings from the TMJ units were obtained from the trigeminal ganglion when the rats were 5, 7, and 9 weeks old. In the experimental group, TMJ mechanoreceptor sensitivity increased, because the firing threshold gradually decreased and the maximum instantaneous frequency gradually increased. In conclusion, functional properties of TMJ mechano-receptors under low articular loading conditions cannot mature normally within the growth period.

Review of clinical EMG studies related to muscle and occlusal factors in healthy and TMD subjects

Several electronic instruments have been developed as adjuncts to objectively record the dysfunctional features of temporomandibular disorders and to study the effectiveness of various treatment interventions. The aim of this review was to assess the value and contribution of clinical electromyographic research in the understanding of asymptomatic and dysfunctional muscle function and the therapeutic effects of interocclusal appliances. For this purpose MedLine and PubMed searches were conducted with the following main keywords alone and in various combinations: electromyography, muscles of mastication, masseter, temporalis, temporomandibular, TMD, utility, validity, repeatability, rest, postural, vertical dimension, occlusal, splint, treatment. The review includes critical evaluation, discussion and conclusions regarding electromyographic studies in asymptomatic and dysfunctional muscles, rest position, occlusal parameters and interocclusal appliances, as well as a critical summary and proposals for further research. Much of earlier critique of many electromyographic studies still applies regarding comparative sample selections, research designs, analyses and conclusions. The areas not well-understood include normal biological variation, capacity for adaptation, fluctuations regarding the clinical course and multidimensional features of temporomandibular disorders and long-term follow-up data, especially in studies that evaluate the effectiveness of therapeutic measures. Considering the required improvements in technical and research designs features and critical appraisal electromyographic research could have value as an adjunct research tool to study features of craniofacial muscle-related dysfunction. Until electromyographic measures are correlated with other multidimensional, especially subjective and pain-related methods, the clinical use of this method for diagnostic purposes of temporomandibular disorders remains in doubt, and is not at present recommended.