Short and long-latency responses in human masseter muscle evoked by chin taps.

Ten healthy subjects received downward, backward and upward chin taps with standardized force and direction. Taps in all three directions elicited short (M1) and long (M2 and M3)-latency excitatory EMG responses in the masseter muscle. M1 (mean latency 8.9 ms, amplitude 150 microV) occurred more frequently and with shorter latency during clenching than during relaxation or opening. Downward taps were more efficient in evoking jaw-jerks (25 per cent) than upward (14 per cent) and backward ones (16 per cent). M2 was a weak biphasic deflection occurring during the silent period (mean latency 42 ms, amplitude 87 microV) and M3 was an EMG burst following the silent period (mean latency 69.3 ms, amplitude 169 microV). No long-latency evoked responses were obtained from the relaxed masseter. Thus, muscle stretch increases M1 response frequency but is not a necessary prerequisite. The hypothesis that vibratory transmission plays an important role was confirmed. Voluntary activation of the stimulated muscle (clenching) increases M1-response frequency and shortens response latency.

Masseter muscle fatigue in man objectively quantified by analysis of myoelectric signals.

Localized muscle fatigue in the masseter muscle was studied with a method based on power spectrum analysis of myoelectric signals. Under the influence of fatiguing contractions, a gradual shift of the spectral curve occurred; the rate of change was taken as a measure of the development of fatigue. The fatigue was dependent on the bite force. The existence of a threshold value of force, below which significant myoelectric fatigue changes do not develop, was shown.

Reflex responses to taps in various directions from the human digastric muscle.

Reflex responses to standardized solenoid chin taps were studied on 21 subjects with electromyographic (EMG) recordings from two jaw muscle antagonists, the masseter and the digastric. Taps were delivered downward and upward as parallel as possible to the masseter fibre direction and also backwards at right angles to these directions. Taps were delivered during isometric masseter and digastric activity as well as during relaxed postural position. Reflex excitation of the digastric muscle with a latency of 25-35 ms was recorded during all three situations after taps in all three directions. When this response was superimposed on ongoing digastric isometric activity after downward and upward taps, it was followed by a period of inhibition (mean duration 31 ms) and directly followed by a second EMG burst (mean latencies 73 and 75 ms, respectively). Responses were significantly (p less than 0.001) more often obtained during digastric background activity than during postural position and clench. Upward and downward taps were equally efficient in evoking the responses, significantly (p less than 0.001) more so than backward taps. The concurrent recordings of the masseter EMG imply the possibility of a reciprocal interplay between the two antagonists. The results accord with reports of the capability of the digastric muscle to produce reflex responses despite lack of anatomically-defined muscle spindles.

Arthroscopy of the temporomandibular joint. A comparative study of arthroscopic and tomographic findings.

Established radiographic criteria of temporomandibular joint osteoarthrosis were evaluated by comparison of arthroscopic and tomographic findings in the superior compartment. 34 joints of 30 patients with long-standing chronic pain and/or functional impairment were investigated according to standardized techniques. Agreement was found between arthroscopy and corrected sagittal tomography regarding diagnosis of osteoarthrosis in all advanced cases. Diagnostic accuracy as regards slight changes was, however, lower. At tomographic examination, the sensitivity was slightly higher than specificity, i.e., pathological changes were more frequently identified than normality. No radiographic sign of osteoarthrosis could be specifically associated with arthroscopic features of osteoarthrosis or synovitis. Predominant location of osteoarthrosis at both arthroscopy and tomography was the posterior slope of the eminence (latero-central part). Osteoarthrosis was more widespread in the fibrocartilage than in the subchondral bone.

Arthroscopy of the temporomandibular joint: occurrence and location of osteoarthrosis and synovitis in a patient material.

The superior compartments of 42 temporomandibular joints of 38 patients with longstanding chronic pain and/or functional impairment were examined arthroscopically. Occurrence and location of osteoarthrosis and synovitis were investigated. Osteoarthrosis occurred in 31 joints (74%). In 17 of them more extensive changes were found. Predominant location of osteoarthrosis was the posterior slope of the eminence. The disk was less frequently affected and most commonly in the latero-central part. Synovitis occurred in 22 joints (52%) and was combined with osteoarthrosis in 17 of them. In 20 of them it was of mild and localised nature. The posterior disk attachment was affected in all of them.

Arthroscopy of the rabbit temporomandibular joint.

Arthroscopy of the rabbit temporomandibular joint was evaluated experimentally on 22 rabbits. A small arthroscope with so-called "Selfoc"-system was employed. The reaction to arthroscopy was analyzed clinically. Macroscopic dissection and histology were then employed to detect possible pathological changes in the joints at periodic intervals. The established criteria for evaluation of postoperative reactions were erythema, suppuration, weight changes and chewing capacity. The results of this study indicate that the rabbit temporomandibular joint may be arthroscoped with subsequent minor reversible changes. It also seems feasible to assume that temporomandibular joint arthroscopy in humans runs a low risk for postoperative complications.

Arthroscopy of the temporomandibular joint. A clinical study.

Temporomandibular joint (TMJ) arthroscopy on 30 joints of 26 patients with different signs and symptoms of TMJ arthropathy was evaluated. Local anaesthesia was used and proved efficient. In 27 cases, the superior, and in 3 cases both, compartments were punctured. Successful examination was performed in 26 superior and 3 attempted inferior compartments. Functional impairment following arthroscopy was minor and rarely lasted more than one week. No postoperative bleeding, nerve damage, or infection was observed. Of the 2 systems tested, the rod-lens arthroscope showed superior optical quality.

Human jaw muscle motor behaviour. II. Reflex and receptor mechanisms.

The stretch reflex is much more complicated than the simple tendon jerk with a monosynaptic reflex pathway. Muscle stretch is not a necessary prerequisite to elicit the short latency EMG excitatory response. Long latency EMG responses are probably of greater functional importance. The silent period (SP) following the jaw jerk is prolonged in patients with symptoms of mandibular dysfunction. In the individual case, however, measurement of the SP may not be of diagnostic value. The Golgi tendon organs are highly sensitive tension receptors which play a much larger role in the proprioceptive regulation of muscle tension than has been believed in the past. So-called type P mechanoreceptors which like a large part of the periodontal receptors have their cells of origin in the mesencephalic nucleus V, have been found in the hard palate. The majority of periodontal mechanoreceptors are slowly adapting and thus able to respond with sustained discharge to long lasting stimulation, as during chenching or bruxing. The postural position has been assigned great clinical significance. It has been assumed that this position is constant throughout life. It has, however, been observed that the postural position rapidly adapts to changes of the vertical occlusal dimension.

Human jaw muscle motor behaviour. I. Motor drive.

Jaw muscle motor behaviour, however complicated, has important implications for the every day dental practice. In recent years the understanding of jaw and other skeletal muscle function has increased considerably. Direct recording of primary afferent discharge in conscious human beings and animals during normal function has caused radical changes of the concepts of muscle receptor function. Central pattern generators at segmental levels and suprasegmental programming centres are important mechanisms behind voluntary and automatic movements of different kinds. The most important proprioceptive function is probably to provide reassurance of correct movement pattern, to adjust the central programming to environmental changes and to directly influence slow movements requiring precision. Muscle spindle receptors contribute to mandibular kinesthesia. Muscle spindles are rarely present in jaw opening muscles. Despite this fact an excitatory reflex similar to the stretch reflex but with longer latency has been demonstrated. Further on a reciprocal organisation with antagonist inhibition has been shown to exist between jaw openers and closers. Motor behaviour of jaw and limb muscles thus seem to have many characteristics in common.

Arthrotic changes and deviation in form of the temporomandibular joint--an autopsy study.

An autopsy study was performed on 84 left human temporomandibular joints. Frequency and location of osteoarthrosis and deviation in form were registered in each joint component. Locations of disc perforations were registered with special emphasis on involvement of the bilaminar zone. The anatomical relationship between the deep masseter muscle and the TMJ capsule was investigated in part of the material. Deviation in form was most frequent in condyles (53%) compared with temporal components (40%) and discs (15%). The relative frequencies of the three joint components were opposite for osteoarthrosis; discs (32%), temporal components (26%) and condyles (21%). Higher frequency of arthrotic changes was found in edentulous specimens than in dentulous ones. Disc perforations were located within disc tissue except for one joint where it also included part of the bilaminar zone. No correlation between sex and arthrosis and/or deviation in form was found. Insertion of the posterior fibres of the deep masseter muscle into the anterior capsule was macroscopically observed in six of ten joints.

Functional adaptation to changes in vertical dimension.

The traditional concept of PP stability defies neurophysiologic explanation. Conversely, the hypothesis presented to explain the rapid adaptation of jaw elevator postural tonus to sudden change of vertical dimension is in accordance with research of automotized motor behavior. Furthermore, there is no logical reason to believe that maladaptive reactions develop after prolonged procedures that increase the vertical dimension of occlusion. Rather, Goldspink reports that within a few weeks complete normalization probably occurs. Jaw muscle motor behavior is more dynamic and adaptable to environmental changes than has been believed.

Distortion of mandibular kinesthesia induced by vibration of human jaw muscles.

Kinesthetic experience accompanying vibration of human jaw muscles was investigated in 12 healthy subjects. Vibration of the masseter muscle with the jaw in a 20-mm opened position caused the subjects either to underestimate jaw closing effects or to experience jaw opening movements depending on whether the mandible was free to move or kept in a constant position. During vibration of the depressor muscles with the mandible in its rest position the subjects underestimated an opening movement, but fixation of the mandible caused no illusions of movement. All kinesthetic illusions perceived during vibration corresponded to elongation of the muscles under study beyond their actual length. Distortion of kinesthesia was independent of the amplitude and frequency of vibration and it persisted during anesthesia of the temporomandibular joints and loading of the mandible. It can be concluded that jaw muscle receptors may contribute to mandibular kinesthesia.

On the regulation of interincisor bite force in man.

The maximum bite force and the ability to perform a requested interincisor bite force was tested on eight healthy subjects with normal dentition and on five full denture wearers under various conditions. Anaesthesia of temporomandibular joint (TMJ) capsular and/or periodontal receptors, with or without simultaneous vibration-induced increase of muscle receptor discharge, did not lead to noticeable changes in these capacities. It is concluded that in maximal voluntary efforts, as well as in trained and prepatterned motor acts, the motor activity may function without significant involvement of peripheral feedback mechanisms. However, all subjects significantly underestimated their bite force in the test series. Another conclusion from the present study is that neither jaw open nor jaw elevator vibration is of any influence upon the maximum bite force. On the other hand, the maximum bite force significantly increased after the performance of the test series.

Arthroscopy of the temporomandibular joint. An autopsy study.

Arthroscopy of the temporomandibular joint (TMJ) is evaluated on an autopsy material. 2 different types of arthroscope--one with a rod-lens system and one with a so-called Selfoc system--were used and compared regarding diagnostic accuracy. On 54 cadavers, it was found that the upper joint compartment may be punctured with accuracy and without damage to vital tissues. Landmarks on the skin facilitating clinical arthroscopy were defined in relation to a guideline from tragus to lateral canthus. A further 49 TMJ specimens were investigated with arthroscopy. The findings are in good agreement with subsequent observations made during dissection. Regarding arthrotic changes, 100% diagnostic accuracy was achieved with both types of arthroscope. Regarding remodelling changes, the diagnostic accuracy was approximately 57%. No significant difference was noticed between the 2 arthroscopes. Photographic documentation with the rod-lens arthroscope was found to be of superior quality.

The masseter muscle: the silent period and its clinical implications.

Standardized solenoid chin taps were delivered downward and upward as parallel as possible to the fibers of the superficial part of the masseter muscle and backward at a right angle to this direction. Each one of the 10 healthy subjects received 10 taps in each direction during isometric masseter muscle activity (clenching the teeth). Taps in all three directions evoked the classic jaw jerk response, M1, a short latency (mean, 8.3 msec) excitation followed by an SP (mean latency, 15.9 msec; mean duration, 42.6 msec). The SP duration varied considerably not only between subjects but also within individuals. Forty-nine percent of the SPs were divided into two parts by an EMG peak, M2, with mean latency 42 msec and mean amplitude 78 microV. Sometimes this peak marked the end of the SP. The SP duration has been claimed to be of diagnostic significance because it is longer in patients with mandibular dysfunction. It is concluded that the large variations of the parameter found in this as well as in other studies make it impossible to establish criteria for a healthy or pathologic SP duration. A carefully obtained history and thorough clinical examination are probably more reliable means to establish a correct diagnosis.

Rotation of synergistic activity during isometric jaw closing muscle contraction in man.

5 healthy subjects were studied during 10-15 min of isometric jaw elevator contraction above fatigue threshold level. Bite force was measured between upper and lower front teeth and electromyographic (EMG) activity recorded from the right temporal and masseter muscles. Of the two muscles only the masseter was active at the start of the test and usually during the whole test. When the test was repeated, however, great relief from the pain in the fatigued masseter was sometimes experienced and the temporalis took over the load. All subjects experienced this "switch" phenomenon after a varying number of tests, one of them already during his first test. The mechanism seemed to be completely out of voluntary control and showed facilitation at repeated tests.

Increase of vertical dimension--consequences for the maxillomandibular relationship. A clinical approach.

The purpose of the present study was to compare the vertical change in the incisor and molar areas caused by insertion of a bite opening appliance and to analyse the occurrence of rotation and eruption during the following treatment period (mean 4.3 months). Eight patients between 15 and 24 years of age with vertical overlaps > or = 4 mm were treated by means of a maxillary fixed lingual arch appliance with an anterior bite plane. The retruded mandibular position was recorded with cold cure acrylic before treatment, immediately after insertion of the fixed appliance, and after removal of it. Vertical overlap and separation between the first molars was measured at each of these occasions. Finally, tooth movements during eruption were measured on the indices in the area of the second molar. The results showed that, independent of the amount of bite raise, the relation between frontal and molar area bite raise was approximately 2:1 for six and 5:3 for two patients. There was a significant difference in thickness of indices from registrations I and III in the second molar area indicating not only eruption during bite opening therapy but also buccolingual rotation.