Stable tooth contacts in intercuspal occlusion makes for utilities of the jaw elevators during maximal voluntary clenching

Data are inconsistent concerning whether the level of the surface electromyographic (SEMG) activity of jaw‐closing muscles increases when biting forces elevated during maximal voluntary clenching (MVC). In this study, T‐Scan III system and BioEMG III system were used to record bite force, occlusal contacts and SEMG activity of the anterior temporalis (TA) and of the masseter muscles (MM) simultaneously. Recordings were obtained from 16 healthy young adult males during different conditions: (i) a fast MVC from resting position to intercuspal position (ICP); (ii) mandibular movements from ICP to protrusive or lateral edge‐to‐edge positions with teeth in contact with biting; (iii) a fast MVC in protrusive and lateral edge‐to‐edge positions. A higher level of SEMG activity was associated with a higher bite force during occluding movements (P < 0·05). However, during fast MVC from rest to ICP, the largest number of occlusal contacts was achieved and distributed more symmetrically, the highest level of biting force was obtained, but the SEMG activity of the jaw elevator muscles was reduced compared with its maximum level (P < 0·05). This phenomenon was not observed during the fast MVC in protrusive or lateral edge‐to‐edge positions. The present results that a lower SEMG activity was associated with the largest number of occlusal contacts and the highest level of bite force during centric MVC demonstrated a complex integration of jaw‐closing muscles when a stable occlusion is present.     

Occlusal measurement method can affect SEMG activity during occlusion

Summary Occlusal indicators are widely used in dental treatment to measure tooth contacts that occur during occlusion. However, the presence of an indicator may affect the mechanics of occlusion and lead to invalid tooth contact data. The objective of this study was to determine the effect of four indicators (Parkell, silk, T‐Scan^® sensor and paper) on surface electromyography (SEMG) activity during occlusion. Twenty‐three subjects performed strong bites and maximum clenches onto the four indicators and natural dentition. Surface electromyography recordings of anterior temporalis and superficial masseter activity and the subjects’ perception of each indicator were measured. SEMG activity with the T‐Scan^® sensor and paper was significantly different (higher masseter activity; P < 0·05) compared to that for natural dentition. The Parkell and silk gave no significant differences to natural dentition. Similarly, subjects perceived that T‐Scan^® sensor and paper had the greatest effect on occlusion and were the least comfortable (P < 0·05). Thus, the very plastic T‐Scan^® sensor and very thick articulating paper both affected SEMG activity during occlusion and, therefore, may not provide valid tooth contact information for dental treatment. In conclusion, occlusal indicators can change SEMG activity during occlusion which may affect the validity of the measurements they provide.     

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

Summary Jaw muscle activity ratios in unilateral isometric biting differ from ratios of unilateral chewing but approach the latter if the jaw gape in biting is made as small as the minimum interocclusal distance (MID) of chewing. Especially, the masseter working/balancing side ratio (W/B‐ratio) becomes as asymmetric as in chewing, because of reduction in balancing side (BS) masseter activity. This behaviour of ratios might reflect a ‘chewing‐specific’ motor strategy induced when isometric biting is performed with a ‘chewing‐like’ gape. If this hypothesis applies, activity ratios should be associated with MIDs of sequent chewing strokes in a similar manner as with incremented jaw gapes in isometric biting. To test this prediction, bilateral surface electromyograms of masseter and anterior temporalis muscles and incisor movements were recorded during unilateral chewing in 52 subjects. W/B‐ratios of masseter and temporalis activities and temporalis/masseter‐ratios on both sides were calculated. The ratios were related to MIDs of consecutive chewing cycles. Three of the four ratios were associated with masticatory MID in the same manner as with jaw gape in isometric biting. In particular with decreasing MID, the masseter W/B‐ratio increased from 1·5 to 2·2 (P < 0·01). This increase in asymmetry was attributed to a stronger decrease in masseter activity on the BS than on the working side. We conclude that relative jaw muscle activation is associated with interocclusal distance in a similar way in isometric biting and in chewing. This analogy supports the idea of a common jaw gape‐related neuromuscular strategy facilitated by afferent signalling of interocclusal distance.     

Effect of tongue position on masseter and temporalis electromyographic activity during swallowing and maximal voluntary clenching: a cross‐sectional study

The purpose of this study was to measure and compare the tonic electromyographic (EMG) activity of the temporalis and masseter muscles following placement of the tongue either on the palate or in the floor of the mouth during swallowing and maximal voluntary clenching (MVC). Thirty healthy dental students with natural dentition and bilateral molar support, between the ages of 18 and 22, with no prior history of oro‐facial injury, or current or past pain in the jaw, mouth or tongue participated in the study. Tonic masseter and temporalis EMG activities were recorded using surface electrodes. Subjects were instructed to passively place the tongue either on the anterior hard palate or in the floor of the mouth during swallowing and MVC. At each tongue position, the resulting EMG was recorded. During swallowing, no significant difference in EMG activity was found either for the masseter (P‐value = 0·1592) or the temporalis (P‐value = 0·0546) muscles, regardless of the tongue position. During MVC, there was a statistically significant difference for both the masseter (P‐value = 0·0016) and the temporalis (P‐value = 0·0277) muscles with lower levels recorded with the tongue in the floor of the mouth. This study found that in normal, pain‐free subjects, placing the tongue in the floor of the mouth significantly reduces masticatory muscle activity during MVC. Thus, it may be considered as a possible therapeutic option to decrease masticatory muscle activity; however, further research is needed in patients with oro‐facial pain.     

Neuromuscular function in healthy occlusion

Summary This study aimed to measure neuromuscular function for the masticatory muscles under a range of occlusal conditions in healthy, dentate adults. Forty‐one subjects conducted maximum voluntary clenches under nine different occlusal loading conditions encompassing bilateral posterior teeth contacts with the mandible in different positions, anterior teeth contacts and unilateral posterior teeth contacts. Surface electromyography was recorded bilaterally from the anterior temporalis, superficial masseter, sternocleidomastoid, anterior digastric and trapezius muscles. Clench condition had a significant effect on muscle function (P = 0·0000) with the maximum function obtained for occlusions with bilateral posterior contacts and the mandible in a stable centric position. The remaining contact points and moving the mandible to a protruded position, whilst keeping posterior contacts, resulted in significantly lower muscle activities. Clench condition also had a significant effect on the per cent overlap, anterior–posterior and torque coefficients (P = 0·0000–0·0024), which describe the degree of symmetry in these muscle activities. Bilateral posterior contact conditions had significantly greater symmetry in muscle activities than anterior contact conditions. Activity in the sternocleidomastoid, anterior digastric and trapezius was consistently low for all clench conditions, i.e. <20% of the maximum voluntary contraction level. In conclusion, during maximum voluntary clenches in a healthy population, maximum masticatory muscle activity requires bilateral posterior contacts and the mandible to be in a stable centric position, whilst with anterior teeth contacts, both the muscle activity and the degree of symmetry in muscle activity are significantly reduced.     

Effect of experimental jaw muscle pain on EMG activity and bite force distribution at different level of clenching

Bite force at different levels of clenching and the corresponding electromyographic (EMG) activity in jaw‐closing muscles were recorded in 16 healthy women before, during and after painful stimulation of the left masseter muscle. Experimental pain was induced by infusion of 5·8% hypertonic saline (HS), and 0·9% isotonic saline (IS) was infused as a control. EMG activity was recorded bilaterally from the masseter and temporalis muscles, and static bite force was assessed by pressure‐sensitive films (Dental Pre‐scale) at 5, 50 and 100% of maximal voluntary contraction (MVC) during each session. Visual feedback was applied by showing EMG activity to help the subject perform clenching at 5, 50 and 100% MVC, respectively. EMG activity at 100% MVC in left and right masseter decreased significantly during painful HS infusion (1·7–44·6%; P < 0·05). EMG activity at 5% and 50% MVC was decreased during HS infusion in the painful masseter muscle (4·8–18·6%; P < 0·05); however, EMG activity in the other muscles increased significantly (18·5–128·3%; P < 0·05). There was a significant increase in bite force in the molar regions at 50% MVC during HS infusion and in the post‐infusion condition (P < 0·05). However, there were no significant differences in the distribution of forces at 100% MVC. In conclusion, experimental pain in the masseter muscle has an inhibitory effect on jaw muscle activity at maximal voluntary contraction, and compensatory mechanisms may influence the recruitment pattern at submaximal efforts.     

The effect of tongue position and resulting vertical dimension on masticatory muscle activity. A cross‐sectional study

The purpose of this study was to: (a) compare the tonic electromyographic (EMG) activity of the temporalis and masseter muscles between two tongue positions, (b) compare the vertical dimension (VD) resulting from each tongue position and (c) determine the influence of the VD on the tonic EMG activity for each tongue position. Thirty‐three healthy dental students with natural dentition and bilateral molar support, between the ages of 18 and 22 years, with no prior history of oro‐facial injury, or current or past pain in the jaw, mouth, or tongue participated in the study. Tonic masseteric and temporalis EMG activities were recorded using surface electrodes. Subjects were instructed to passively place the tongue either on the anterior hard palate or in the floor of the mouth. At each tongue position, the resulting EMG and VD were recorded. No significant difference in EMG activity was found for either the masseter (P‐value = 0·5376) or temporalis muscle (P‐value = 0·7410), between the two tongue positions. However, there was a significant difference in the VD resulting from the two different tongue positions, being greater with the tongue placed in the floor of the mouth. There was no statistically significant correlation between VD and EMG activity for both tongue positions. In spite of the lack of difference in the effect of both tongue positions on the masseteric and temporalis EMG activity, an increment of the VD was registered for the floor of mouth–tongue position. However, VD was not correlated with EMG activity for both tongue positions.     

Effects of interocclusal distance on bite force and masseter EMG in healthy participants

The aim of this study was to investigate effects of interocclusal distance (IOD) on bite force and masseter electromyographic (EMG) activity during different isometric contraction tasks. Thirty‐one healthy participants (14 women and 17 men, 21·2 ± 1·8 years) were recruited. Maximal Voluntary Occlusal Bite Force (MVOBF) between the first molars and masseter EMG activity during all the isometric‐biting tasks were measured. The participants were asked to bite at submaximal levels of 20%, 40%, 60% and 80% MVOBF with the use of visual feedback. The thickness of the force transducer was set at 8, 12, 16 and 20 mm (= IOD), and sides were tested in random sequence. MVOBF was significantly higher at 8 mm compared with all other IODs (P < 0·001). Only in women, IOD always had significant influence on the corresponding root‐mean‐square (RMS) value of EMG (P < 0·011). When biting was performed on the ipsilateral side to the dominant hand, the working side consistently showed higher masseter EMG activity compared with the balancing side (P < 0·020). On the contralateral side, there was no difference between the masseter EMG at any IODs. The results replicated the finding that higher occlusal forces can be generated between the first molars at shorter IODs. The new finding in this study was that an effect of hand dominance could be found on masseter muscle activity during isometric biting. This may suggest that there can be a general dominant side effect on human jaw muscles possibly reflecting differences in motor unit recruitment strategies.     

Headache attributed to masticatory myofascial pain: impact on facial pain and pressure pain threshold

There is no clear evidence on how a headache attributed to temporomandibular disorder (TMD) can hinder the improvement of facial pain and masticatory muscle pain. The aim of this study was to measure the impact of a TMD‐attributed headache on masticatory myofascial (MMF) pain management. The sample was comprised of adults with MMF pain measured according to the revised research diagnostic criteria for temporomandibular disorders (RDC/TMD) and additionally diagnosed with (Group 1, n = 17) or without (Group 2, n = 20) a TMD‐attributed headache. Both groups received instructions on how to implement behavioural changes and use a stabilisation appliance for 5 months. The reported facial pain intensity (visual analogue scale – VAS) and pressure pain threshold (PPT – kgf cm^?2) of the anterior temporalis, masseter and right forearm were measured at three assessment time points. Two‐way anova was applied to the data, considering a 5% significance level. All groups had a reduction in their reported facial pain intensity (P < 0·001). Mean and standard deviation (SD) PPT values, from 1·33 (0·54) to 1·96 (1·06) kgf cm^?2 for the anterior temporalis in Group 1 (P = 0·016), and from 1·27 (0·35) to 1·72 (0·60) kgf cm^?2 for the masseter in Group 2 (P = 0·013), had significant improvement considering baseline versus the 5th‐month assessment. However, no differences between the groups were found (P > 0·100). A TMD‐attributed headache in patients with MMF pain does not negatively impact pain management, but does change the pattern for muscle pain improvement.     

Effect of biting speed and jaw separation on force used to incise food

Biting food too quickly might affect the control of jaw-closing muscles and the estimation of bite force. The objectives of this study were to compare the incisal bite forces used to cut food and the activity of masseter (MA) and anterior temporalis (AT) muscles between slow, habitual and fast biting speeds and also between small and large jaw openings. Twenty subjects were asked to use their incisors to cut through a 5 mm thick of chewing gum. In the first experiment, subjects bit at 10-mm incisal separation with slow, habitual and fast biting speeds, and in the second experiment, subjects bit with their habitual speed at 10- and 30-mm incisal separations. The activities in the MA and AT muscles were assessed with surface electromyography, and the bite force was recorded by a force sensor placed beneath the chewing gum. Peak bite forces and associated MA amplitudes were increased significantly as biting speed was increased (P's < .05). Anterior temporalis amplitude was significantly increased during fast biting compared to slow and habitual biting (P's < .001). At 30-mm incisal separation, both peak bite force and AT amplitude were significantly increased, whereas MA amplitude was significantly decreased, compared to those at 10-mm separation (P's < .05). Biting off food quickly with incisor teeth results in larger activities in both MA and AT muscles. In addition, biting a large piece of food resulted in increased activity of AT muscle. Both conditions could be injury stimulator for jaw muscles.     

Does long‐term use of unstable dentures weaken jaw muscles?

Summary Although it is well known that conventional denture wearers have lower maximum bite forces than dentate subjects, no previous studies have compared the strength of the jaw muscles between these two groups. This study compared maximum bite forces, electromyographic (EMG) activity and estimated jaw muscle strength among three groups: (i) 17 edentulous subjects using newly acquired implant‐retained overdentures (seven men, 10 women; mean age 60·3 ± 13·0 years); (ii) 10 age‐matched, fully dentate subjects (five men, five women; mean age 57·9 ± 11·0 years); and (iii) 39 young, fully dentate subjects (19 men, 20 women; mean age 24·4 ± 3·5 years). Electromyographic activity was recorded from subjects’ bilateral superficial masseter and anterior temporalis muscles while they generated maximum voluntary bite forces at the right central incisor, right first premolar and right first molar positions. Jaw muscle strength was estimated as the ratio of average EMG activity for all four muscles to the maximum bite force. At all three bite positions, edentulous subjects produced maximum bite forces that were less than half that of dentate subjects. Edentulous subjects also produced significantly less EMG activity and had significantly lower estimated jaw muscle strength. Our results suggest that weakened jaw muscles are one factor contributing to lower maximum bite forces among users of conventional dentures.     

Effect of the dental arches morphology on the masticatory muscles activities in normal occlusion young adults

Objectives:

The aim of this study was to assess the relationship between the morphology of dental arches and the activity of the masticatory muscles activities in healthy volunteers with full natural dentition.

Methods:

Two-hundred youthful Class I volunteers (113 females, 87 males) were clinically investigated. Alginate impressions of dental arches were taken, and plaster casts were prepared and measured. EMG data from eight masticatory muscles was recorded to assess their activities in central occlusion, lateral and protrusive movements.

Results:

Clinical measurements and plaster casts analyses confirmed normal values of parameters investigated. Most of the arch measurements were significantly larger in the males than in the females. Weak positive correlations were found between overbite and masseter activity in centric occlusion (the right Mm R?=?0·151, P≤0·05; the left Mm R?=?0·191, P≤0·05). Also, the range of protrusive movement positively correlated with masseter activities in central occlusion (the right Mm R?=?0·194, P≤0·05; the left R?=?0·201, P≤0·05).

Conclusions:

The null hypothesis that morphology of dental arches does not affect the masticatory muscles’ activities was rejected. The findings of this investigation indicate that systemic, longitudinal analyses of morphology of occlusion and muscular response, even in normal subjects, are needed.     

The effect of culture on pain sensitivity

Cross‐cultural differences in pain sensitivity have been identified in pain‐free subjects as well as in chronic pain patients. The aim was to assess the impact of culture on psychophysical measures using mechanical and electrical stimuli in patients with temporomandibular disorder (TMD) pain and pain‐free matched controls in three cultures. This case–control study compared 122 female cases of chronic TMD pain (39 Saudis, 41 Swedes and 42 Italians) with equal numbers of age‐ and gender‐matched TMD‐free controls. Pressure pain threshold (PPT) and tolerance (PPTo) were measured over one hand and two masticatory muscles. Electrical perception threshold and electrical pain threshold (EPT) and tolerance (EPTo) were recorded between the thumb and index fingers. Italian females reported significantly lower PPT in the masseter muscle than other cultures (P < 0·001) and in the temporalis muscle than Saudis (P = 0·003). Swedes reported significantly higher PPT in the thenar muscle than other cultures (P = 0·017). Italians reported significantly lower PPTo in all muscles than Swedes (P ≤ 0·006) and in the masseter muscle than Saudis (P < 0·001). Italians reported significantly lower EPTo than other cultures (P = 0·01). Temporomandibular disorder cases, compared to TMD‐free controls, reported lower PPT and PPTo in all the three muscles (P < 0·001). This study found cultural differences between groups in the PPT, PPTo and EPTo. Overall, Italian females reported the highest sensitivity to both mechanical and electrical stimulation, while Swedes reported the lowest sensitivity. Mechanical pain thresholds differed more across cultures than did electrical pain thresholds. Cultural factors may influence response to type of pain test.     

Electromyographic standardized indices in healthy Brazilian young adults and data reproducibility

Summary The determination of normal parameters is an important procedure in the evaluation of the stomatognathic system. We used the surface electromyography standardization protocol described by Ferrario et al. (J Oral Rehabil. 2000;27:33–40, 2006;33:341) to determine reference values of the electromyographic standardized indices for the assessment of muscular symmetry (left and right side, percentage overlapping coefficient, POC), potential lateral displacing components (unbalanced contractile activities of contralateral masseter and temporalis muscles, TC), relative activity (most prevalent pair of masticatory muscles, ATTIV) and total activity (integrated areas of the electromyographic potentials over time, IMPACT) in healthy Brazilian young adults, and the relevant data reproducibility. Electromyography of the right and left masseter and temporalis muscles was performed during maximum teeth clenching in 20 healthy subjects (10 women and 10 men, mean age 23 years, s.d. 3), free from periodontal problems, temporomandibular disorders, oro‐facial myofunctional disorder, and with full permanent dentition (28 teeth at least). Data reproducibility was computed for 75% of the sample. The values obtained were POC Temporal (88·11 ± 1·45%), POC masseter (87·11 ± 1·60%), TC (8·79 ± 1·20%), ATTIV (?0·33 ± 9·65%) and IMPACT (110·40 ± 23·69 μV/μV·s %). There were no statistical differences between test and retest values (P > 0·05). The Technical Errors of Measurement (TEM) for 50% of subjects assessed during the same session were 1·5, 1·39, 1·06, 3·83 and 10·04. For 25% of the subjects assessed after a 6‐month interval, the TEM were 0·80, 1·03, 0·73, 12·70 and 19·10. For all indices, there was good reproducibility. These electromyographic indices could be used in the assessment of patients with stomatognathic dysfunction.     

First molar cross‐bite is more closely associated with a reverse chewing cycle than anterior or pre‐molar cross‐bite during mastication

A posterior cross‐bite is defined as an abnormal bucco‐lingual relationship between opposing molars, pre‐molars or both in centric occlusion. Although it has been reported that patients with unilateral posterior cross‐bite often show unique chewing patterns, the relationship between the form of cross‐bite and masticatory jaw movement remains unclear in adult patients. The objective of this study was to investigate masticatory jaw movement among different forms of cross‐bite. One hundred and one adults were recruited in this study: 27 had unilateral first molar cross‐bite (MC group); 28, unilateral pre‐molar cross‐bite (PC group); 23, anterior cross‐bite (AC group); and 23, normal occlusion (control group). Masticatory jaw movement of the lower incisor point was recorded with six degrees of freedom jaw‐tracking system during unilateral mastication. Our results showed that the reverse chewing ratio during deliberate unilateral mastication was significantly larger in the MC group than in the PA (P < 0·001), AC (P < 0·001) and control (P < 0·001) groups. These findings suggest that compared to the anterior or pre‐molar cross‐bite, the first molar cross‐bite is more closely associated with a higher prevalence of a reverse chewing cycle.     

Association between oral habits and signs/symptoms of temporomandibular disorders in Flemish adolescent girls

The influence of oral habits on the development of signs and symptoms of temporomandibular disorders (TMD) and pain is unclear. The prevalence and nature of such oral habits may also vary geographically. The aim of the present study was to assess the prevalence of parafunctions in Flemish adolescent girls and to study the interrelationship with TMD. A group of 261 school girls (15–16 years of age) participated. A questionnaire used in a similar study ( Gavish et al., 2000 ) was translated and inquired for oral habits (chewing of gum, nails or foreign objects, eating of seeds, crushing of ice or food, continuous leaning on the arm, daytime or night‐time grinding or clenching, jaw play, unilateral chewing) and TMD symptoms (joint noises, catching or locking, joint or muscle pain, tension or fatigue in the muscles). A brief clinical examination was performed by a single examiner: active and passive maximal mouth opening, presence of joint sounds, palpation tenderness of the lateral poles of the joints and of the masseter and anterior temporalis muscles, extent of abrasion of the canines, tongue or cheek imprints. Statistical evaluation used Spearman correlation, chi‐squared analysis and multiple regression analysis. The intra‐examiner reproducibility was moderate to high (κ 0·6–0·8 – Spearman Correlation 0·99 for maximal mouth opening). The frequency of reported oral habits was high: leaning on the arm (98%), gum chewing (89%, mean duration 3 h day^?1), lip‐ (62·1%) and cheek‐biting (41%), ice‐crushing (30%). There was a significant (P < 0·0001) but weak (0·30) positive correlation between the number of oral habits and the number of symptoms. Internal derangements were significantly (P < 0·0001) but weakly (0·25) correlated with jaw play and other oral habits. There was no relation between tooth clenching and myogenous pain (χ², P=0·31, but only 27% power). Multiple regression analysis showed that the number of symptoms increased by 52% if the subject reported ‘jaw play’, by 24% if chewing gum more than 2 h day^?1, by 28% while cheek biting and by 25% if chewing unilaterally.     

Small vertical changes in jaw relation affect motor unit recruitment in the masseter

Strategies for recruitment of masseter muscle motor units (MUs), provoked by constant bite force, for different vertical jaw relations have not previously been investigated. The objective of this study was to analyse the effect of small changes in vertical jaw relation on MU recruitment behaviour in different regions of the masseter during feedback‐controlled submaximum biting tasks. Twenty healthy subjects (mean age: 24·6 ± 2·4 years) were involved in the investigation. Intra‐muscular electromyographic (EMG) activity of the right masseter was recorded in different regions of the muscle. MUs were identified by the use of decomposition software, and root‐mean‐square (RMS) values were calculated for each experimental condition. Six hundred and eleven decomposed MUs with significantly (P < 0·001) different jaw relation‐specific recruitment behaviour were organised into localised MU task groups. MUs with different task specificity in seven examined tasks were observed. The RMS EMG values obtained from the different recording sites were also significantly (P < 0·01) different between tasks. Overall MU recruitment was significantly (P < 0·05) greater in the deep masseter than in the superficial muscle. The number of recruited MUs and the RMS EMG values decreased significantly (P < 0·01) with increasing jaw separation. This investigation revealed differential MU recruitment behaviour in discrete subvolumes of the masseter in response to small changes in vertical jaw relations. These fine‐motor skills might be responsible for its excellent functional adaptability and might also explain the successful management of temporomandibular disorder patients by somatic intervention, in particular by the use of oral splints.     

The effects of manual therapy and exercise directed at the cervical spine on pain and pressure pain sensitivity in patients with myofascial temporomandibular disorders

Summary No studies have investigated the effects of the treatments directed at the cervical spine in patients with temporomandibular disorders (TMD). Our aim was to investigate the effects of joint mobilization and exercise directed at the cervical spine on pain intensity and pressure pain sensitivity in the muscles of mastication in patients with TMD. Nineteen patients (14 females), aged 19–57 years, with myofascial TMD were included. All patients received a total of 10 treatment session over a 5‐week period (twice per week). Treatment included manual therapy techniques and exercise directed at the cervical spine. Outcome measures included bilateral pressure pain threshold (PPT) levels over the masseter and temporalis muscles, active pain‐free mouth opening (mm) and pain (Visual Analogue Scale) and were all assessed pre‐intervention, 48 h after the last treatment (post‐intervention) and at 12‐week follow‐up period. Mixed‐model anovas were used to examine the effects of the intervention on each outcome measure. Within‐group effect sizes were calculated in order to assess clinical effect. The 2 × 3 mixed model anova revealed significant effect for time (F = 77·8; P < 0·001) but not for side (F = 0·2; P = 0·7) for changes in PPT over the masseter muscle and over the temporalis muscle (time: F = 66·8; P < 0·001; side: F = 0·07; P = 0·8). Post hoc revealed significant differences between pre‐intervention and both post‐intervention and follow‐up periods (P < 0·001) but not between post‐intervention and follow‐up period (P = 0·9) for both muscles. Within‐group effect sizes were large (d > 1·0) for both follow‐up periods in both muscles. The anova found a significant effect for time (F = 78·6; P < 0·001) for changes in pain intensity and active pain‐free mouth opening (F = 17·1; P < 0·001). Significant differences were found between pre‐intervention and both post‐intervention and follow‐up periods (P < 0·001) but not between the post‐intervention and follow‐up period (P > 0·7). Within‐group effect sizes were large (d > 0·8) for both post‐intervention and follow‐up periods. The application of treatment directed at the cervical spine may be beneficial in decreasing pain intensity, increasing PPTs over the masticatory muscles and an increasing pain‐free mouth opening in patients with myofascial TMD.     

Posterior scissors‐bite: masticatory jaw movement and muscle activity

Scissors‐bite is a malocclusion characterised by buccal inclination or buccoversion of the maxillary posterior tooth and/or linguoclination or linguoversion of the mandibular posterior tooth. This type of malocclusion causes reduced contact of the occlusal surfaces and can cause excessive vertical overlapping of the posterior teeth. This case–control study is the first to evaluate both masticatory jaw movement and masseter and temporalis muscle activity in patients with unilateral posterior scissors‐bite. Jaw movement variables and surface electromyography data were recorded in 30 adult patients with unilateral posterior scissors‐bite malocclusion and 18 subjects with normal occlusion in a case–control study. The chewing pattern on the scissors‐bite side significantly differed from that of the non‐scissors‐bite side in the patients and of the right side in the normal subjects. These differences included a narrower chewing pattern (closing angle, P < 0·01; cycle width, P < 0·01), a longer closing duration (P < 0·05), a slower closing velocity (P < 0·01) and lower activities of both the temporalis (P < 0·05) and the masseter (P < 0·05) muscles on the working side. In 96% of the patients with unilateral posterior scissors‐bite, the preferred chewing side was the non‐scissors‐bite side (P = 0·005). These findings suggest that scissors‐bite malocclusion is associated with the masticatory chewing pattern and muscle activity, involving the choice of the preferred chewing side in patients with unilateral posterior scissors‐bite.     

Influence of muscle contraction and jaw movement speed on the estimation of chewing force from elevator muscle EMG

Recent studies have shown that an additional muscle activity (AMA) in chewing was responsible for excessive masticatory forces that had been estimated from isometric bite force–activity relations. Our aim was to test the thesis that this AMA could be identical to the speed‐dependent ‘anticipatory activity’ that was found in symmetrical jaw closing (Abbink et al., 1999). In this case, forces estimated from EMG should be realistic for slow mastication, but should strongly increase with faster chewing movements. In 56 dentate volunteers the activities of bilateral masseter and anterior temporal muscles were measured during (a) right‐sided chewing, (b) right‐sided isometric clenching on a bite fork with alternating loads. These biting tasks were carried out with slow, habitual and fast contraction/movement speeds (30, 64, 102 cycles min^?1). Masticatory forces were estimated for each speed by putting activities found in task (a) into bite force/activity relations obtained from task (b). In slow and habitual mastication, activities of the working side masseter were equal and exceeded the corresponding clenching activities by factors of 2·8 and 2·4, respectively (P < 0·001). Consequently, forces estimated from slow (680 N) and habitual (695 N) chewing did not differ significantly and were unrealistically high. These estimated forces and hence the corresponding AMAs did not depend on the speed movement in the same way as the anticipatory activity described in Abbink et al. (1999) . In contrast, forces estimated from fast chewing (811 N) were significantly higher (P < 0·001) and corresponding AMAs qualitatively behaved like anticipatory activities. In conclusion, the AMA in chewing could be explained by anticipatory muscle activity for high movement speeds. In slow and habitual chewing however, the AMA may contain amounts of activity with a different origin.