An electromyographic study on the sequential recruitment of bilateral sternocleidomastoid and masseter muscle activity during gum chewing.

Mandibular functions are associated with electromyographic activity of the jaw muscles and also the sternocleidomastoid muscle (SCM). The precise spatiotemporal relation of SCM and masticatory muscles activities during chewing is worthy of investigation. To analyse the sequential recruitment of SCM and masseter activities during chewing as indicated by the spatiotemporal locations of their activity peaks. Jaw movements and bilateral surface electromyographic activity of SCM and masseter were recorded during gum chewing in 20 healthy subjects. The timing order was decided by comparing the length of time from the time when the opening started to the time when the surface electromyographic activity reached its peak value. Spatial order was analysed by locating the peak electromyographic activity onto a standard chewing cycle which was created based on 15 unilateral chewing cycles. Paired t-test, one-way ANOVA and Student-Newman-Keuls post-test were used for comparisons. Although the Time to Peak for the balancing side SCM appeared shorter than for the other three tested muscles, most often it did not reach a level of significance. However, the location of the balancing side SCM's peak activity was further from the terminal chewing position (TCP) than the working side SCM and bilateral masseters (P < 0·05). The balancing side SCM activity reached its peak significantly further away from TCP than the other three tested muscles during chewing. Further studies with spatiotemporal variables included should be helpful to understand the roles of the head, neck and jaw muscles in orofacial and cervical dysfunctional problems.

Unbalanced lateral mandibular deviation associated with TMJ sound as a sign in TMJ disc dysfunction diagnosis.

The aim was to study the characteristics of lateral mandibular horizontal deviations during opening-closing movements and their association with TMJ sounds of the clicking type. Subjects were 28 healthy volunteers and 38 patients diagnosed with MRI imaging as having TMJ disc dysfunction, 22 with disc displacement without (DD) and 16 as having disc displacement with reduction (DDR). TMJ sounds were recorded with miniature microphones placed in the ear canals, and jaw movements were documented with a kinesiograph. A sign, unbalanced lateral deviation (ubd) was defined as a rapid, short duration, change in jaw movement direction from, and back to, a smooth deviation path in the horizontal plane. The hypotheses were that degrees of maximal deviations, proportions of unbalanced deviation (ubd) and such deviation associated with TMJ sounds (ubdS), differ between healthy subjects and patients with DD or DDR. Comparisons between groups were made using one-way anova and chi-square analysis, as appropriate. No differences were found between groups regarding degree of lateral deviation per se. The proportions of ubd and ubdS were significantly higher in patients with DDR than in healthy subjects and than in patients with DD (P < 0·001), but no such differences were found between healthy subjects and patients with DD. For prediction of DDR, the sensitivity and specificity of the sign ubdS were found to be 68·8% and 89·3%, respectively. For the sign ubd, they were 100·0% and 64·3%. This indicates that the sign ubdS has diagnostic value in screening for DDR.

SEMG activity of jaw-closing muscles during biting with different unilateral occlusal supports.

The aim of this study was to test the hypothesis that experimental and reversible changes of occlusion affect the levels of surface electromyographic (SEMG) activity in the anterior temporalis and masseter areas during unilateral maximal voluntary biting (MVB) in centric and eccentric position. Changes were achieved by letting 21 healthy subjects bite with and without a cotton roll between the teeth. The placement alternated between sides and between premolar and molar areas. The SEMG activity level was lower when biting in eccentric position without than with a cotton roll between teeth (P < 0.043). It was always lower with premolar than with molar support when biting with a cotton roll (P < 0.013). In the anterior temporalis areas, the SEMG activity was always lower on the balancing than on the working side (P < 0.001). Such a difference was also found in the masseter areas but only during molar-supported centric biting (P = 0.024). No differences were found when comparing the SEMG levels in masseter areas between centric and eccentric biting (P > 0.05). In the anterior temporalis area, the balancing side SEMG activity was lower in eccentric than in centric but only in molar-supported biting (P = 0.026). These results support that the masseter and anterior temporalis muscles have different roles in keeping the mandible in balance during unilateral supported MVB. Changes in occlusal stability achieved by biting with versus without a cotton roll were found to affect the SEMG activity levels.

Trend analysis of jaw jerk action potential-amplitude data.

The amplitude of the jaw jerk action potential (MSP) is a parameter with potential diagnostic value. It has apparently a large and irregular variation which has not yet been subjected to a thorough analysis. It is important to know if samples of MSP contain trends. The occurrence of trends indicates that the average value is influenced in a principally different way by changes in the number of observations per sample than if trends do not occur. If, for instance, there is a positive linear trend in the data, this means that the average value is larger if the number of observations is small. Samples of MSPs therefore were recorded, using surface EMG electrodes in the masseter and the anterior temporal muscles of young and elderly subjects, and subjected to trend analysis. Trends of the 1--4th grade occurred in 51% of the samples. Both positive and negative linear trends were found and had about the same frequency. The trend components were, however, randomly distributed. Comparisons between groups may therefore be made without the claim of trend-free samples. This significantly facilitates the use of the parameter MSP-amplitude, since it is difficult to obtain samples which have a normal and identical distribution.

Standard values of the amplitude of jaw jerk action potential and differences due to age, sex and L-dopa treatment.

The amplitude of the jaw jerk action potential (MSP) is a parameter with possible diagnostic value. Standard values have, however, not yet been established. MSPs were recorded using surface EMG electrodes in the masseter and the anterior temporal muscles of young and elderly healthy males and females. The group means ranged from 0.26 to 2.11 mV in the masseter and from 0.17 to 0.81 mV in the anterior temporalis. The amplitude of MSP was higher in females than in males, decreased in elderly subjects and lower in the temporalis than in the masseter. The jaw jerk action potentials also were recorded in four subjects before and after L-Dopa administration. The amplitude increased by about 90%. It is concluded that the parameter MSP amplitude can be useful in the studies of muscle spindle sensitivity and the influence of various physiological, pathophysiological and pharmacological factors on the nervous regulation and muscle function in the masticatory system.

Computerized classification of temporomandibular joint sounds.

Sounds, such as clicking and/or crepitation, evoked in the temporomandibular (jaw) joint during function may indicate pathology. Analysis of the reduced interference time-frequency distribution of these sounds is of diagnostic value. However, visual evaluation is expensive and error prone, and there is, thus, a need for automated analysis. The aim of this study was to find the optimal signal representation and pattern recognition method for computerized classification of temporomandibular joint sounds. Concepts of time-shift invariance with and without scale invariance were employed and mutually compared. The automated analysis methods provided classification results that were similar to previous visual classification of the sounds. It was found that the classifier performance was significantly improved when scale invariance was omitted. This behavior occurred because scale invariance interfered with the frequency content of the signal. Therefore, scale invariance should not be pursued in the classification scheme employed in this study.

Race and gender as TMD risk factors in children.

The aim of this study was to record the prevalence in preschool children of oral/facial pain symptoms of clinical interest in the diagnoses of temporomandibular disorders (TMD) and to analyze the association with the race and gender factors. Children, 525 4-6 year olds, mean age 5.1 +/- 0.65 (SD), 326 Caucasian and 199 African American, from a preschool and kindergarten program in a low income industrial area, who participated in a voluntary oral health examination, were examined. Comparisons were made using Chi-Square test. An alpha-level of 5% was chosen, and the effect of making multiple comparisons was compensated for by Bonferroni correction. No gender differences were found, but racial differences were observed regarding six of the 10 variables. Twenty-five percent of the children had recurrent (at least one to two times per week) headache. Thirteen percent had recurrent earache, African-American children more often than Caucasian children (p approximately 0.0038). Thirteen percent had recurrent temporomandibular joint (TMJ) pain, and 11% had recurrent neck pain. Pain or tiredness in the jaws during chewing was reported by 29% of the children, more often by African-American than by Caucasian (p < 0.00001). Pain at jaw opening occurred in 13% of the children, more often in the African-American than in the Caucasian children (p approximately 0.00004). Palpation pain was found in the posterior TMJ area in 28%, in the lateral TMJ area in 22%, in the masseter area in 19%, in the anterior temporalis area in 15% and was found more often in all of those regions in the African-American than in the Caucasian children (p approximately 0.00001), except for the temporalis area. In conclusion, this study showed that mild, but distinct, TMD-related oral/facial pain symptoms occur already by ages 4-6 with significant differences in distribution observed between the African-American and the Caucasian races. While gender seems to play a negligible role in this age group, this does not necessarily mean that race is a causative factor. The pain symptoms may be caused by other factors with different distribution in the two racial subgroups.

Oral parafunctions as temporomandibular disorder risk factors in children.

Oral parafunctions are generally considered to be important factors in the etiology of temporomandibular disorders (TMDs) and many reports have been published about their prevalence in adults and schoolchildren. However, few have included significant numbers of children below the age of 7. The aim of this study was to examine the association between parafunctions and oral/facial TMD-related pain in preschool children. Bruxism, nail biting, and thumb sucking were found to be significantly associated with important oral/facial pain symptoms of clinical interest in the diagnoses of TMD indicating that those parafunctions are risk factors. The study included 525 4- to 6-year-old African-American and Caucasian children, mean age 5.1 +/- 0.65 (SD). An alpha level of 5% was chosen for comparison with a Pearson Chi-Square test. Bonferroni correction was made and a p-value of < 0.005 was accepted as significance level. Only 28% of the children had no history of any parafunction. More girls (82%) than boys (63%) in the Caucasian subgroup had at least one parafunction (p approximately 0.00017). No such difference was found in the African-American subgroup where the corresponding figures were 71% for girls and 73% for boys. Thumb sucking was reported by 57% of the children, more often by Caucasian girls (69%) than by Caucasian boys (43%) (p < 0.00001). Thirty percent still had the habit. Forty-one percent had a history of nail biting. Bruxism was noted in 20% of the children, but occurred mostly in combination with other parafunctions and was seldom (in 3.4%) the only parafunction. Of the 10 pain variables, bruxism was significantly associated with eight, thumb sucking with three, and nail biting with two. Analysis with logistic regression confirmed the results. Association does not, however, tell if a parafunction is the cause or the consequence of pain, or if a third factor is causing both pain and increased prevalence of oral parafunctions. Further prospective longitudinal studies including higher age groups are needed to clarify those relations and to determine if there are long-term effects of childhood parafunctions.

Crepitation and clicking as signs of TMD in preschool children.

Children [N = 540, age 5.1 +/- 0.72 (SD)], were tested for association between temporomandibular (TM) joint sounds and symptoms of TM disorder (TMD). The prevalence of TMJ sounds as found by auscultation and confirmed by self-report was 16.7%. There was significant association after Bonferroni correction between the presence of TM joint sounds, as reported by the children, and all but one of the eleven pain/dysfunction variables. There was significant association also between crepitation as heard at auscultation and palpation tenderness in the TMJ and masseter areas (p < 0.001), but not between clicking and any of the TMD variables. Agreement between subjects and examiners regarding the presence of TMJ sounds was poor (kappa = 0.097). The results indicate that joint sounds and TMD symptoms are common already in small children and thus demonstrate a possible early onset of TMD. Patients' own reports of TMJ sounds may have more clinical relevance than auscultation findings.

Use and abuse of bite splints.

Bite splints are often used in the treatment of patients with oral parafunctions, temporomandibular joint (TMJ) dysfunction, or temporomandibular disorders. The most common reasons for prescribing a bite splint are to protect the teeth in patients with bruxism, to improve jaw-muscle and TMJ function, and to relieve related pain. The risk for negative side effects is small in conservative bite splint treatment. Complications from long-term use of splints, however, can be severe and irreversible. The risks are especially high when mandibular advancement splints, or splints that make contact only with parts of the opposing dentition, are used for more than 4 to 6 weeks without appropriate supervision. As a general rule, a dentist should not encourage a patient to use any type of splint for more than a few months except for cases in which the teeth need to be protected because of persistent oral parafunctions. Appropriate record keeping, including signed consent forms, is necessary; when neglected, it becomes difficult for the dentist to defend himself from false accusations of malpractice.

Asymmetric muscle function in patients with developmental mandibular asymmetry.

The aim was to test the hypothesis that developmental mandibular asymmetry is associated with increased asymmetry in muscle activity. Patients with mandibular condylar and/or ramus hyperplasia having unilateral cross-bite were compared with healthy subjects with normal occlusion. Muscle activity was recorded with surface electrodes in the masseter, suprahyoid, sternocleidomastoid muscle (SCM) and upper trapezius areas during jaw opening-closing-clenching, head-neck flexion-extension, and elevation-lowering of shoulders. Root mean square (RMS) and mean power frequency (MPF) values were calculated and analysed using anova and t-tests with P < 0.05 chosen as significance level. The SCM and masseter muscles showed co-activation during jaw and head movements, significantly more asymmetric in the patients than in the healthy subjects. The RMS and MPF values were higher in the patients than in the controls in the SCM and suprahyoid areas on both sides during jaw opening-closing movement. The results indicate that the ability to perform symmetric jaw and neck muscle activities is disturbed in patients with developmental mandibular asymmetry. This is of clinical interest because asymmetric activity may be an etiologic factor in temporomandibular joint and cervical pain. The results support that co-activation occurs between jaw and neck muscles during voluntary jaw opening and indicate that postural antigravity reflex activity occurs in the masseter area during head extension. Further studies, where EMG recordings are made from the DMA patients at early stages are motivated to verify activity sources and test if the asymmetric activity is associated with muscle and joint pain in the jaw and cervical areas.

Limitation of the diagnostic value of MR images for diagnosing temporomandibular joint disorders.

OBJECTIVES: Many studies have shown that MRI findings are reliable when experienced calibrated observers work as a group. The hypothesis was that MRI findings can be used as the gold standard also when evaluation is made by single expert observers. STUDY DESIGN: Temporomandibular joint (TMJ) MRIs of 34 patients were evaluated independently by four reviewers with expert knowledge of radiology for the presence of 13 specified pathologic entities, as well as the quality of the images, on a 5-step scale from "Sure Yes" to "Sure No". Intraclass correlation coefficients were calculated to estimate the rating reliability of the examiners. A coefficient of at least 0.8 was deemed good, between 0.60 and 0.80 was deemed acceptable, and less than 0.60 was considered poor. Additionally, weighted kappa statistics were used for pair-wise comparisons across all four reviewers. RESULTS: The hypothesis was not supported by the results. None of the 13 correlation coefficients for comparisons between single examiner evaluations of pathologic entities was good and 8 were poor. CONCLUSION: A diagnosis of TMD based on MRI examination protocols made by a single examiner should not be accepted as a gold standard with regard to TMJ disorders.

Long latency components of the jaw jerk and blink reflex in man.

In the present study, the electromyographic (EMG) activity in the masseter and temporal muscles during about 200 ms after chin taps (jaw jerk response) was compared with the EMG activity in the orbicularis oculi after glabella tap (blink reflex). In two of ten subjects components with long latency and duration, analogous to blink reflex components BR2 and BR3, were found in the masseter and temporal muscles. They may represent the mechanical contraction which results in jaw closure. In two of the subjects chin taps were found to elicit late responses in the orbicularis oculi with the same latencies (25-30 and 80-100 ms) as BR2 and BR3 associated with glabella tap.

Tooth tapping ability of young and elderly males and females.

The ability to perform high frequency tooth tapping movements was studied in young dentulous, elderly dentulous and elderly edentulous subjects of both sexes as a possible test of the overall efficiency of the masticatory motor apparatus and related structures. Tapping frequency was measured during a test period of 90s by counting the number of taps per 5 s intervals. It was found that observed values of tapping frequency could be fitted to an exponential equation with a high degree of fit for 90% of the young subjects and for 60% of the elderly subjects. The values of the remaining elderly subjects were best fitted to straight lines. The elderly subjects could thus be regarded as belonging to two different populations which were differently affected by advanced age and loss of teeth regarding their tooth tapping ability. Young dentulous subjects had the highest starting frequency (6-1 Hz) followed by elderly dentulous (5-5 Hz) and elderly edentulous subjects (4-9 Hz). The lower limits were 4-6 Hz for young dentulous subjects, 4-1 Hz for elderly dentulous and 3-7 Hz for elderly edentulous subjects. The average group means pointed all in the same direction that increased age and loss of teeth change the functional ability by deteriorating it.

The silent period in the masseter muscle of patients with TMJ dysfunction.

The silent period (SP) following a jaw jerk elicited during sustained contraction in the masseter muscles has been studied in two groups of subjects, one with and one without, acute and distinct symptoms of TMJ dysfunction. The subjects with acute TMJ dysfunction symptoms showed significantly shorter latency and longer duration of the silent period and the period of their depressed activity (DA) was also significantly longer than in the group without TMJ dysfunction. The duration of the SP of both muscles was symmetrical in about 60% of subjects in each group. This study validates that the duration of the silent period may be a useful diagnostic tool of clinical interest.

The refractory period of the masseteric cutaneous silent period.

Silent periods were elicited in the electromyographic activity of the masseter and temporalis muscles from six normal subjects, by electrical stimulation of the skin over the contralateral masseter during clenching. Pairs of suprathreshold square pulses with varied time intervals were delivered using surface electrodes. It was found that the silent periods, which were mostly of the long latency (about 53 ms) type, had a refractory period, during which a new silent period could be evoked only by increasing the strength of the second stimulus. This refractory period could last for up to 1.5 s. During the first 200 ms after the first stimulus a second silent period was almost impossible to evoke with the moderately painful stimuli used in the present experiments. It is suggested that the refractory period should be considered as significant for the methodology when recording the silent period duration. It is also suggested that the long latency silent period should be recorded as a separate parameter and differentiated from the short latency (10-12 ms) silent period. This may be of special importance to recognize when a short latency silent period and a long latency silent period merge.

The influence of eye closure on muscle activity in the anterior temporal region.

In the experiments reported here the influence of eye closure upon the muscle tone in the region of the anterior temporal muscles was studied during mandibular rest in four patients with TMJ dysfunction and in four healthy subjects. The muscle activity as well as the cortical activity (EEG) of the patients was influenced by changes in visual stimulation. EMG (the anterior temporal region) and EEG (parieto-occipital region) were recorded with needle electrodes. Ongoing EMG activity was present in all patients when their eyes were open. Closure of both eyes reduced the muscle activity by 50-100%. Closure of one eye could also abolish all activity but the degree of this influence was variable. Opening of the eyes blocked the alpha rhythm in the EEG. Simultaneously, increased activity was observed in the region of the temporal muscle. The activity level in the temporal region of the healthy subjects was very low and could easily be reduced to noise level using ordinary relaxation manoeuvres without eye closure. The results reported here indicate that visual stimulation may increase muscle tone in the temporal region probably as part of an arousal reaction. It is, however, possible that the activity might stem not from the temporalis muscle but from the temporoparietalis.

Kinematics of the tooth tapping movement.

Electrical activity in the masseter muscles and tooth contact vibrations were recorded simultaneously from subjects tapping their teeth slowly into maximal intercuspidation and again with maximal frequency. High speed cinephotography was also used with four of the ten subjects. Three main parts could be distinguished on the obtained graphical representation of the tooth tapping movement: tooth contact phase (TCP), opening phase (OP) and closing phase (CP). Tapping frequency was increased by decreasing the jaw opening degree and the durations of TCP, OP and CP. The jaw velocity immediately before tooth contact, which may be of significance for the reflex response, was however not increased. The average jaw speed was nevertheless increased from 10 to 15 cm/s since the turning from OP to CP was more abrupt in high than in low frequency tapping. The duration of electrical activity after tooth contact was significantly shorter at tapping with high than with low frequency. The teeth maintained contact without detectable rebound between each open-close cycle. The OP started about 100 ms after the cessation of electrical activity both at low and high tapping frequency. The time between end of electrical activity and the start of a new OP was supposed to be dependent upon the relaxation time of the masseter muscles.

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.