TEMPOROMANDIBULAR-JOINT'S SENSORY MECHANISMS CONTROLLING ACTIVITIES OF THE JAW MUSCLES

Evaluation of Sensibility Threshold for Interocclusal Thickness of Patients Wearing Complete Dentures

Objective

The aim of this study was to evaluate sensibility threshold for interocclusal thickness in experienced and nonexperienced denture wearers after the insertion of new complete dentures.

Materials and Methods

A total of 88 patients with complete dentures have participated in this study. The research was divided into two experimental groups, compared with the previous experience prosthetic dental treatment. The sensibility threshold for interocclusal thickness was measured with metal foil with 8 μm thickness and width of 8 mm, placed between the upper and lower incisor region. Statistical analysis was performed using standard software package BMDP (biomedical statistical package).

Results

Results suggest that time of measurement affects the average values of the sensibility threshold for interocclusal thickness (F = 242.68, p = 0.0000). Gender appeared to be a significant factor when it interacted with time measurement resulting in differences in sensibility threshold for interocclusal thickness (gender: F = 9.84, p = 0.018; F = 4.83, p = 0.0003).

Conclusion

The sensibility threshold for interocclusal thickness was the most important functional adaptation in patient with complete dentures. A unique trait of this indicator is the progressive reduction of initial values and a tendency to reestablish the stationary state in the fifteenth week after dentures is taken off.

Stomatognathic adaptive motor syndrome is the correct diagnosis for temporomandibular disorders

Temporomandibular disorder is a generic and inadequate conception to be used as a diagnosis. It fails to express the etiology or the pathophysiology and it is mainly associated with the anatomical site. Moreover, the clinical condition presents a mandibular motor problem and not a joint problem. The hypothesis presents the new diagnosis stomatognathic motor adaptive syndrome, which comprehend a motor response and the adaptive processes it induces. Inadequate occlusal contacts cause the mandible to shift in order to reach an ideal intercuspal position. The condylar displacements are proportional to such movements. Temporomandibular joint (TMJ) receptors respond to the capsular mechanical stress and the information reaches the trigeminal sensory nuclei. The mandibular modified position seems to be relevant information and may interfere with catecholaminergic neurotransmission in basal ganglia. The main motor responses comprise increased jaw muscle tone, decreased velocity of movements and incoordination. The overload of muscle function will produce adaptive responses on many stomatognathic structures. The muscle adaptive responses are hypertonia, pain, fatigue and weakness. Temporomandibular joint presents tissue modification, disc alteration and cracking noise. Periodontium show increased periodontal membrane, bone height loss and gingival recession. Teeth manifest increased wear facets, abfraction and non-accidental fractures. The periodontal and teeth adaptive processes are usually identified as occlusal trauma. The altered stomatognathic functions will show loss of velocity during mastication and speech. Fatigue, weakness in jaw muscle and difficulties to chew hard food are related to hypertonia. Incoordination between stomatognathic muscles groups is found, causing involuntary tongue/cheek biting and lateral jaw movements on speech. Otologic complaints, as aural fullness and tinnitus, are related to the tensor tympani muscle, innervated by the trigeminal nerve.

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.

Electromyographic evaluation of mastication and swallowing in elderly individuals with mandibular fixed implant-supported prostheses

This study evaluated the effect of implant-supported oral rehabilitation in the mandible on the electromyographic activity during mastication and swallowing in edentulous elderly individuals. Fifteen patients aged more than 60 years were evaluated, being 10 females and 5 males. All patients were edentulous, wore removable complete dentures on both dental arches, and had the mandibular dentures replaced by implant-supported prostheses. All patients were submitted to electromyographic evaluation of the masseter, superior orbicularis oris muscles, and the submental muscles, before surgery and 3, 6 and 18 months postoperatively, using foods of different textures. The results obtained at the different periods were analyzed statistically by Kruskal-Wallis non-parametric test. Statistical analysis showed that only the masseter muscle had a significant loss in electromyographic activity (p<0.001), with a tendency of similar response for the submental muscles. Moreover, there was an increase in the activity of the orbicularis oris muscle during rubber chewing after treatment, yet without statistically significant difference. Mandibular fixed implant-supported prostheses in elderly individuals revealed a decrease in electromyographic amplitude for the masseter muscles during swallowing, which may indicate adaptation to new conditions of stability provided by fixation of the complete denture in the mandibular arch.

Reflex control of human jaw muscles

The aim of this review is to discuss what is known about the reflex control of the human masticatory system and to propose a method for standardized investigation. Literature regarding the current knowledge of activation of jaw muscles, receptors involved in the feedback control, and reflex pathways is discussed. The reflexes are discussed under the headings of the stimulation conditions. This was deliberately done to remind the reader that under each stimulation condition, several receptor systems are activated, and that it is not yet possible to stimulate only one afferent system in isolation in human mastication experiments. To achieve a method for uniform investigation, we need to set a method for stimulation of the afferent pathway under study with minimal simultaneous activation of other receptor systems. This stimulation should also be done in an efficient and reproducible way. To substantiate our conviction to standardize the stimulus type and parameters, we discuss the advantages and disadvantages of mechanical and electrical stimuli. For mechanical stimulus to be delivered in a reproducible way, the following precautions are suggested: The stimulus delivery system (often a probe attached to a vibrator) should be brought into secure contact with the area of stimulation. To minimize the slack between the probe, the area to be stimulated should be taken up by the application of pre-load, and the delivered force should be recorded in series. Electrical stimulus has advantages in that it can be delivered in a reproducible way, though its physiological relevance can be questioned. It is also necessary to standardize the method for recording and analyzing the responses of the motoneurons to the stimulation. For that, a new technique is introduced, and its advantages over the currently used methods are discussed. The new method can illustrate the synaptic potential that is induced in the motoneurons without the errors that are unavoidable in the current techniques. We believe that once stimulation, recording, and analysis methods are standardized, it will be possible to bring out the real "wiring diagram" that operates in conscious human subjects.

Effects of endodontic instrument handle diameter on electromyographic activity of forearm and hand muscles

AIM

To determine the influence of the handle diameter of endodontic instruments on forearm and hand muscle activity using electromyographic (EMG) recording.

METHODOLOGY

Size 45 K-type files were fitted with four different handle diameters; 3.5, 4.0, 5.0, and 6.0 mm. Seven dentists then attempted to negotiate to the working length acrylic resin root canals with each of the four handle sizes using a reaming motion. EMG activities were recorded from the flexor pollicis brevis muscle (f.p.b.), the flexor carpi radialis muscle (f.c.r.), and the brachioradialis muscle (b) with bipolar surface electrodes. The time taken to negotiate the canals, the area of integrated EMG that corresponded to the amount of EMG activity required during penetration and the maximum amplitude of EMG were measured using the EMG data. Results were analysed statistically using a one-way factorial ANOVA test and multiple comparison tests.

RESULTS

Reaming time and integrated EMG area of each muscle decreased with an increase in handle diameter. The most significant difference in time and area of integrated EMG was detected between handles of 6 mm and 3.5 mm diameter (time: P < 0.01, area of the f.p.b.: P < 0.01, area of the f.c.r. and b: P < 0.05), and between handles of 5 mm and 3.5 mm diameter (P < 0.05). Both 5 mm and 6 mm handles significantly decreased the maximum amplitude of EMG recorded from the f.p.b. compared with 3.5 mm handles (between 3.5 mm and 6 mm: P < 0.01, between 3.5 mm and 5 mm: P < 0.05).

CONCLUSION

The results indicate that handle diameter has an effect on reaming time as well as on muscle activity. As a consequence, handle diameter influenced operator performance during instrumentation.

Examination of the diagnostic value and estimation of the chaos phenomenon in masticatory movement using fractal dimension in patients with temporomandibular dysfunction syndrome

The aim of this study was to estimate the chaos phenomenon (chaos) in masticatory movements using the fractal dimension (FD), and to examine the diagnostic value of the fractal dimension in comparing stomatognathic functional disturbances with normal stomatognathic function. The subjects were all high school students and included nine subjects presenting with acceptable normal occlusion, 18 subjects with TMJ dysfunction syndrome and seven subjects with tooth crowding. Masticatory movements were obtained during free, right side, and left side gum-chewing and were used to calculate the capacity dimension in the FD. Chaos in the masticatory movement was estimated by the FD saturated with some constant value to an increase of embedding dimension (approached a plateau). In the crowding group, the FD was also significantly high on the sagittal plane in comparison with the normal. In the patients with pain, the FD on the sagittal plane was significantly high. In the patients with pain and closed lock, the FD on the frontal plane was significantly high. However, in the patients with pain and with reduction of anterior disc displacement, the FD was significantly low on the horizontal plane. These findings suggest that chaos is present in masticatory movements and the difference in the FD are of diagnostic value in evaluation of the relationship between FD and stomatognathic functional disturbance.

Muscle activity during mandibular movements in normal and mandibular retrognathic subjects

PURPOSE

The masticatory muscles function as a unit during precise mandibular positioning movements that occur during such activities as speech, singing, or playing musical instruments. This investigation was designed to assess jaw muscle recruitment patterns during controlled mandibular movement in normal subjects and in patients with mandibular retrognathism.

PATIENTS AND METHODS

A computer-integrated electromyography (EMG) and movement monitoring (Selspot) system was used to collect data over 7 seconds of a sagittal border movement (Posselt envelope) of the mandible and 4 seconds each of rest position, light tooth contact, and maximum clench. Fine wire bipolar electrodes were placed into the inferior belly of the lateral pterygoid muscles bilaterally and surface electrodes were placed bilaterally over the anterior belly of the temporalis muscles and the masseter muscles. Ten subjects with Class I occlusion, normal cephalometric values, and an absence of temporomandibular joint (TMJ) dysfunction were compared with 12 patients with mandibular retrognathism, Class II malocclusion, and an absence of clinical signs of TMJ internal derangement before and after a bilateral sagittal split and advancement of the mandible.

RESULTS

There was a wide variation in standard deviations of EMG activity for the lateral pterygoid muscles in the retrognathic patients compared with normal controls before surgery (P < .05). In light tooth contact, temporalis muscle activity increased after surgery with respect to both control and the presurgical levels (P < .05, P < .005, respectively). In maximum clench, activity in all muscle groups in the retrognathic patients, both before and after surgery, were below that of control subjects (P < .005). The lateral pterygoid muscles showed late recruitment, with low EMG activity levels during the forward movement phase of the envelope, before surgery compared with controls (P < .001). After surgery, the lateral pterygoid muscle showed early recruitment in the forward movement similar to control levels.

CONCLUSION

The masticatory muscles function as a unit during mandibular positioning movements. Patients with mandibular retrognathism have different muscle recruitment patterns from those of normal subjects with the mandible at rest and during mandibular movement. After orthognathic surgery, adaptation occurs in the phasic timing of jaw muscle activity.

History and evolution of TMD concepts

Historically the field of temporomandibular disorders (TMD) has been based on testimonials, clinical opinion, and blind faith rather than on science. Reparative procedures to the joints, jaws, or occlusal surfaces of the teeth to develop idealized structural relationships that may be required for dental health and function are less likely to be required for the management of chronic musculoskeletal disorders. Because of the concerns of many people today regarding professional credibility and intellectual honesty, the need for a scientific foundation to support the various belief systems is of paramount importance. In fact, therapeutic approaches for TMD are undergoing a major evolution away from the traditional mechanistic dental concepts of the past to the more current biopsychosocial medical concepts that emphasize multidisciplinary approaches. Recent advances in the understanding of pain mechanisms and management of chronic pain have improved long-term treatment outcome. The emphasis is on treatment that involves the patient in the physical and behavioral management of their own problem. The majority of patients with TMD achieve good relief of their symptoms with noninvasive, conservative therapy.

Central connections of trigeminal primary afferent neurons: topographical and functional considerations

This article reviews literature relating to the central projection of primary afferent neurons of the trigeminal nerve. After a brief description of the major nuclei associated with the trigeminal nerve, the presentation reviews several early issues related to theories of trigeminal organization including modality and somatotopic representation. Recent studies directed toward further definition of central projection patterns of single nerve branches or nerves supplying specific oral and facial tissues are considered together with data from intraaxonal and intracellular studies that define the projection patterns of single fibers. A presentation of recent immunocytochemical data related to primary afferent fibers is described. Finally, several insights that recent studies shed on early theories of trigeminal input are assessed.

Influence of lower lip support on recording of vertical dimension in edentulous patients

The purpose of this investigation was to assess the influence of differences in design of the occlusion rim on recorded vertical dimension in complete denture therapy. Edentulous patients were studied with an opto-electronic method when the vertical dimension of rest was established with and without labial support of the occlusion rim. As no significant differences were found, the results did not support the hypothesis that the contour of the lip support will influence the recorded physiological rest position. However, a significant difference between intra and extraoral placement of the point of measurement was found.

Fine structure of calcitonin gene-related peptide-immunoreactive nerve fibres in the rat temporomandibular joint

The distribution and fine structure of these nerve fibres was examined by immunoelectron microscopy. CGRP-immunoreactive fibres were seen in the nerve bundles, blood vessels and periosteum around the condyle as well as in the disc. These nerve fibres were unmyelinated and had diameters varying from 200 to 600 nm. They were completely or partially enclosed by Schwann cell cytoplasm and did not form synaptic contact with any cells. CGRP-immunoreactive nerve fibres may be sensory nature and this peptide could be involved in pain transmission and neurogenic inflammation.

Effect of chewing training on mandibular postural position

The inter-occlusal rest space (free-way space) was studied before and after short and long periods of chewing of a hard resin in 15 healthy subjects. During a month of daily training for one hour of chewing no significant changes in the interocclusal rest space were seen. After 30 min of intense chewing the mean interocclusal distance increased to about twice its original size. This occurred both before and after the training period. In a control group of eight subjects without any chewing exercise no significant changes in the interocclusal rest space were noticed during the observation period.

Influence of balanced occlusion and canine guidance on electromyographic activity of elevator muscles in complete denture wearers

Electromyographic recordings were made from the anterior temporal and masseter muscles during maximal voluntary clenching with complete dentures in the intercuspal position and in the laterotrusive jaw position with balanced occlusion and canine guidance. The different pattern of activity of the two muscles in the laterotrusive occlusal schemes studied suggests that their motoneuron pools receive different inputs. The lower activity in both muscles with canine guidance suggests that canine guidance may be a significant factor for preventing parafunctional activity in edentulous patients.

Peptidergic innervation of the temporomandibular disk in the rat

The peptidergic innervation of the temporomandibular disk was investigated in the postnatal young rat by using an indirect immunofluorescence method. Calcitonin gene-related peptide-containing nerve fibers were located around the blood vessels and terminated as free nerve endings in the disk. These nerve fibers may be of a sensory nature.

Electromyographic recordings of human jaw-jerk reflex characteristics evoked under standardized conditions

A device for imparting reproducible chin taps was employed to evoke monosynaptic jaw-closing reflexes in subjects without (group A) and with (group B) overt muscle-joint pain dysfunction. Latency, duration and amplitude values obtained were consistent within an individual at constant tap force at a single-recording session but varied among subjects. Latency differences between sides were greater in subjects exhibiting mild to moderate dysfunction (group B) than in group A, but there were not corresponding differences in duration and amplitude. Stimuli were delivered in 5 subjects whilst the jaw was firmly held at postural jaw position with the aid of acrylic resin indices secured with adhesive to the maxillary and mandibular teeth. Recordable reflex responses were evoked in the masseter muscles of one subject only, indicating that vibration alone was not an adequate stimulus consistently to evoke a jaw-closing reflex at postural jaw position. Spindle stretch is needed, unless postural motoneurone excitability is at a sufficiently high level. Reproducible jaw-closing reflexes were evoked following standardized stimuli; subtle variations in motoneurone excitability, such as reflected by differences in jaw-jerk latency between sides, may then become apparent.

Maximal bite force with centric and eccentric load

Premolar bite force was measured conventionally with a pressure transducer. During biting the tips of the buccal cusps made contact with the transducer. It was shown that the maximal bite force increased significantly if the eccentric load on the upper premolar was made centric and axial with respect to the transducer by covering the whole occlusal surface of the tooth with a plastic filling. Possible neurophysiological explanations of the difference in bite force are discussed. It is concluded that biting on a pressure transducer with an eccentric load cannot give a fair estimation of the true value of the maximal bite force during clenching in the position of intercuspidation.

Influence of TMJ dysfunction on Bennett movement as recorded by a modified pantograph. Part III: Progress report on the clinical study

The purpose of this investigation was to provide further insight into the characteristics of the induced and voluntary Bennett movement by determining the influence of TMJ dysfunction on the lateral side shift of the mandible as recorded by a modified pantograph. Twelve subjects were examined using pantographic tracings quantitated with the PRI to establish a baseline pattern of dysfunction for each subject. The PRI was used to categorize the subjects as to the degree of dysfunction. Following categorization, eight subjects were used as an experimental group and for subjects as a control group. On all subjects, induced and voluntary Bennett movement registrations were taken at the initial and final adjustment. Only the experimental subjects underwent an occlusal therapy consisting of an occlusal splint and an occlusal adjustment. Success of the therapy was monitored via the PRI. Following occlusal therapy, the reduction in PRI scores and categorization for the experimental group was significant to the 0.0042 level. There was no significant change inthe scores for the control group. Post-treatment Bennett movement registrations (voluntary and induced) indicated a statistically significant increase of the movement at the 0.043 level. The control group showed no significant change. Furthermore, on all subjects studied in this investigation, the voluntary Bennett movement was always within the borders of the induced Bennett movement, suggesting that the subject's neuromusculature was an important component in registering the lateral side shift of the mandible.

Tonic stretch reflexes in lip, tongue and jaw muscles

Despite considerable speculation it remains unclear as to whether stretch reflexes perform a functional role in speech articulator muscles. Recent research, however, has shown that long loop stretch reflex mechanisms are brought into play during voluntary contraction of limb muscles and a functional role in oscillatory damping has been suggested. It was decided, therefore, to use a method and a technique of analysis similar to that used in limb muscles to search for tonic stretch reflex (TSR) responses in lip, tongue and jaw muscles during sustained voluntary contraction. The term 'action TSR' is used to differentiate stretch reflex responses measured from voluntary activity from those measured at rest. Simultaneous electromyogram (EMG) recordings were taken from the lip, tongue and jaw musculature in normal, stutterer and cerebral spastic subjects. Subjects were instructed to hold the appropriate articulator in a fixed position while the experiment applied an irregular, continuously changing, stretching force. The stretch and EMG signals were analyzed using a cross correlation and spectral analysis technique. This provided a sensitive means of detecting any EMG fluctuations which covaried with applied stretch and might therefore be classified as reflex. No suggestion of such action TSR responses could be found in lip or tongue muscles of any of the subjects tested, including the cerebral spastic subjects with dysarthric speech. It is therefore concluded that action TSR mechanisms are not operative in control of lip and tongue muscles in man. Furthermore, dysarthric speech in cerebral spasticity cannot be attributed to exaggerated tone of lip and tongue muscles resulting from hypersensitivity of TSR mechanisms. In contrast, clear action TSR responses were demonstrable in jaw closing muscles while in jaw opening muscles, small amplitude responses were detected but were not substantial in comparison with background activity. Since the action TSR is present in jaw and limb muscles, but absent in lip and tongue muscles, the suggestion of a functional role of this reflex in damping mechanical oscillations associated with inertial loads is further supported.

The absence of proprioceptive nerve endings in the human periodontal ligament: the role of periodontal mechanoreceptors in the reflex control of mastication

A review of the literature was conducted to determine the presence or absence of proprioceptive nerve endings in the human periodontal ligament. A histologic review of the periodontal ligament innervation concluded that nerve endings found were those mediating pain, pressure, or touch and that there is no histologic evidence of any "classic" proprioceptive nerve ending in the periodontal ligament. A summary is given concerning the precise role of nerve endings in the periodontal membrane, their afferent pathways, and the role of masticatory muscle proprioception, jaw reflexes, and the temporomandibular joint in the coordinated control of mastication and mandibular proprioception.

The role of retro-disclusion in the treatment of the TMJ-patient

A clinical procedure is described for treatment of patients with condylar malposition. The method of construction of the occlusal splint used is outlined and its mode of action discussed.

Initiation of reflex activity from the temporomandibular joint of the cat

Afferent nerves from the temporomandibular joint (TMJ) were excited naturally and electrically to study mandibular reflexes initiated from the TMJ. Vertical jaw movements were initiated by electric stimulation of the joint and branches of the auriculotemporal nerve (ATN). Jaw opening was produced in 12 of 14 cats tested by electric stimulation of the ATN or its branches, and in 3 of 5 cats tested by electric stimulation of the interior of the TMJ. Mechanical stimulation rarely produced jaw opening. Neither electric nor mechanical stimulation produced jaw closure.