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Background: Disc displacement with reduction (DDwR) therapy should be performed only when pain or discomfort is presented. Data on treatment options for painful DDwR are very limited. Objective: The aim was to investigate whether isometric training of the lateral pterygoid muscle (LPM) is equivalent to stabilisation appliance therapy with regard to the treatment of painful DDwR. The training program is based on the science of Janda. Methods: This was a prospective, randomised study with a comparative treatment group. Sixty patients (≥18 years) with DDwR and pain were randomly assigned to two groups: (1) muscle training and (2) stabilisation appliance. The following variables were recorded at baseline examination and after 2, 4 and 6 months: changes in orofacial pain, clicking sounds of the temporomandibular joint (TMJ), force degrees for the lateral movement of the mandible and interincisal opening distance. p values of <.05 were considered statistically significant but 95% confidence intervals were also presented. Results: A decrease in orofacial pain intensity was seen in both groups (p <.0001). Registered TMJ clicking disappeared after 6 months of treatment in 37% (n = 11) of the patients in the training group and in 27% (n = 8) of the appliance group (p =.0009 and p =.0047). Muscle training showed 27 improvements in Janda force degrees at the end of the study (p <.0001). Conclusion: Muscle training and appliance therapy improved mouth opening and reduced pain intensity in both patient groups. Muscle training might be a promising option in the treatment of patients suffering from painful DDwR. [ABSTRACT FROM AUTHOR]

Background: Clinical use of botulinum neurotoxins (BoNTs) in masticatory muscles is usually bilateral, but most studies on the functional consequences of BoNT treatment have used unilaterally treated animals. Objectives: To test the hypothesis that bilateral BoNT treatment of the rabbit masseter hampers mastication and to assess its effects on bone density of the mandibular condyles. Methods: Ten 5‐month‐old female rabbits received injections of BoNT into both masseter muscles and nine sham animals received saline. Body weight, incisor bite force during masseter tetany, and surface and fine‐wire electromyography (EMG) of the masseter and medial pterygoid muscles were assessed at regular intervals. Half the sample was terminated after 4 weeks and the remainder after 12 weeks. Muscles were weighed and mandibular condyles were scanned with microCT to analyse bone density. Results: BoNT rabbits lost weight and required a soft‐food diet. Incisor occlusal force plummeted after BoNT injection and remained lower than the shams. The duration of masticatory cycles was increased in the BoNT rabbits for 5 weeks, with most of the increase due to the adductor burst. Masseteric EMG amplitude began to improve at Week 5, but remained low on the working side throughout the experiment. At the 12‐week endpoint, masseter muscles were smaller in the BoNT rabbits. Medial pterygoid muscles did not compensate. Condylar bone density was reduced. Conclusion: Bilateral treatment of the rabbit masseter by BoNT severely affected chewing performance. Even after a 3‐month recovery period, deficits remained in bite force, muscle size and condylar bone density. [ABSTRACT FROM AUTHOR]

Background: Speaking depends on refined control of jaw opening and closing movements. The medial pterygoid muscle (MPT), involved in jaw closing, and the lateral pterygoid muscle (LPT), involved in jaw opening, are two key mandibular muscles in mastication and are likely to be recruited for controlled movements in speech. Objectives: Three hypotheses were investigated, that during speech the MPT and LPT: (1) were both active, (2) but exhibited different patterns of activity, (3) which fluctuated with the vowels and consonants in speech. Methods: Intramuscular EMG recordings were made from the right inferior head of the LPT and/or the right MPT in five participants during production of 40 target nonsense words (NWs) consisting of three syllables in the form /V1C1V2 C2ə/ (V = vowel; C = consonant; ə = unstressed, reduced vowel), spoken by each participant 10 times per NW; analysis focussed on the target syllable, C1V2. Results: Both MPT and LPT exhibited robust increases in EMG activity during utterance of most NWs, relative to rest. Peak LPT activation was time‐locked to the final part of the target consonant (C1) interval when the jaw begins opening for the target vowel (V2), whereas peak MPT activation occurred around the temporal midpoint of V2, when the jaw begins closing for C2. EMG amplitude peaks differed in magnitude between "high" vowels, i.e., for which the tongue/jaw are high (e.g., in SEEK), and "low" vowels, i.e., for which the tongue/jaw are low (e.g., in SOCK). Conclusions: These novel findings suggest a key role for the LPT and MPT in the fine control of speech production. They imply that speech may impose major synergistic demands on the activities of the MPT and the LPT, and thereby provide insights into the possible interactions between speech activities and orofacial activities (e.g. mastication) and conditions (e.g. Temporomandibular Disorders) that involve the masticatory muscles. [ABSTRACT FROM AUTHOR]

Objectives: To test the hypotheses that (a) the force thresholds at onset of medial pterygoid muscle single motor unit (SMU) activity do not decrease with an increase in the rate of force generation in standardised vertical or horizontal jaw‐force tasks, and (b) there is evidence for functional heterogeneity within the medial pterygoid muscle. Methods: In 14 healthy participants, electromyographic recordings of the right medial pterygoid muscle were performed with intramuscular fine‐wire electrodes during four isometric force tasks: vertical, horizontal contralateral, horizontal protrusion and horizontal ipsilateral, performed at two rates of force development (slow ramp, fast ramp). Computer tomography scans confirmed electrode location within the muscle, which was divided into medial and lateral parts. Force thresholds of onset of discriminated SMUs were compared between rates in each task; significance accepted at p < 0.05. Results: Of 45 SMU force thresholds studied in one or more tasks, there was no significant difference between slow and fast ramp within each force task, except slow ramp thresholds from the lateral part during the vertical force task were significantly higher than fast ramp thresholds. Reversals of recruitment order between tasks provided evidence for functional heterogeneity within the muscle. Force thresholds of the vertical tasks (range: 1–292.6 N) were mostly higher than for the horizontal tasks (range: 0.1–12.5 N). Conclusion: The data are consistent with the proposal that the medial pterygoid muscle stabilises the jaw in the vertical plane during isometric force generation in the jaw closing, as well as horizontal directions. [ABSTRACT FROM AUTHOR]

Background: Little is known regarding the functional properties of single motor units (SMUs) in the medial pterygoid muscle (MPt) during jaw movements. Objectives: The aims are (a) to report the thresholds of onset of MPt SMUs during 4 goal‐directed jaw movement tasks, and (b) to determine whether the threshold of onset of SMU activation varies with the velocity of jaw movement and the location within the muscle. Methods: Intra‐muscular electrodes were inserted in the right MPt of 18 participants performing ipsilateral (right), contralateral, protrusive and opening‐closing jaw movements recorded at 2 velocities. Task phases were as follows: BEFORE, OUT, HOLDING, RETURN and AFTER. SMU onset thresholds were determined from the displacement (mm) of the lower mid‐incisor point. Electrode location within 4 arbitrary muscle divisions was determined with computer tomography. Statistical tests: Spearman's correlations, Kruskal‐Wallis tests; significance accepted at P <.05. Results: A significant inverse relation occurred between velocity and threshold for the RETURN of the ipsilateral movement (n = 62 SMU thresholds), while a significant positive relation occurred for the OUT of the contralateral movement (n = 208); there were no significant associations for the protrusive (n = 131) and opening‐closing (n = 58) tasks. Significant threshold differences occurred across the 4 muscle divisions only during the OUT of the contralateral and protrusive movements. Some evidence was provided for gender differences in MPt SMU properties. Conclusions: The absence of a significant inverse relation between velocity and SMU threshold for most recorded movements suggests the MPt acts as a stabilizer of the jaw in horizontal and opening‐closing jaw movements. [ABSTRACT FROM AUTHOR]

Whiplash injury is an initiating or aggravating factor of temporomandibular disorder (TMD). Although there are sex‐related differences in the mechanism of pain perception and pain control, there is a lack of research on differences in TMD after whiplash injury. We aimed to evaluate sex‐related differences in the clinical symptoms and magnetic resonance imaging (MRI) findings of patients with TMD attributed to whiplash injury. This retrospective, cross‐sectional study included 100 patients (50 women; 50 men; mean age, 37.60 years) who visited our oro‐facial pain clinic with symptoms of TMD after whiplash injury. All patients underwent detailed evaluations for history of trauma, and their clinical and MRI findings were comprehensively assessed. Women with TMD after whiplash injury perceived more pain and presented more tenderness upon palpation than did men with TMD. In addition, women showed higher volume (58% vs 26%) and signal changes (54% vs 20%) in the lateral pterygoid muscle (LPM) and more anterior disc displacement without reduction (ADDWoR) (40% vs 20%) than did men. The presence of ADDWoR (odds ratio, 10.58; P = 0.007) and condylar degeneration (odds ratio, 9.30; P = 0.015) predicted LPM volume; stressful conditions (beta = 1.34; P = 0.011) correlated with increased visual analogue scale scores, and sleep problem was associated with an increased palpation index (PI) (beta = 0.42; P < 0.001) and neck PI (beta = 0.49; P < 0.001) scores only in women. Our results showed sex‐specific differences in pain intensity, distribution of clinical and abnormal MRI findings, and their relationships, and these differences should be considered when treating patients with TMD. [ABSTRACT FROM AUTHOR]

Objectives: To determine (a) whether the medial pterygoid muscle is active in an isometric vertical force task and in isometric horizontal force tasks in the contralateral, protrusion and ipsilateral directions; (b) whether the same single motor units (SMUs) could be active across different directions of isometric force generation; and (c) whether different regions of the medial pterygoid muscle exhibit different patterns of SMU activation during the generation of any one direction of isometric force. Methods: Intramuscular electromyographic (EMG) recordings were made from the right medial pterygoid muscle in 15 healthy participants during isometric force tasks: vertical and horizontal contralateral, protrusion and ipsilateral. A computed tomography scan divided the EMG recording site into a medial or lateral part in each participant. Single motor units were discriminated in each task. Results: Medial pterygoid SMU activity was recorded in 100% of participants for the vertical biting tasks, 86% of participants for the horizontal contralateral and horizontal protrusion tasks and 57% of the horizontal ipsilateral tasks. Of the 72 SMUs that were discriminated, 36% were active in all tasks; 18% were active only in the vertical tasks and 17% were active in the vertical, horizontal contralateral and horizontal protrusion tasks. The proportion of SMUs that was active in at least 1 horizontal task in the lateral part (33/39) was significantly higher than the proportion (21/33) in the medial part (Chi‐Square, P < 0.05). Conclusion: The data are consistent with a stabilisation role for the medial pterygoid muscle in isometric jaw forces in the vertical and horizontal planes. [ABSTRACT FROM AUTHOR]

In a previous study, we used muscle functional magnetic resonance imaging to show that the anterior movement of the occlusal point increased the activity of the superior head of the ipsilateral lateral pterygoid muscle (ipsilateral SHLP) during unilateral single-tooth clenching. The purpose of this study was to verify the hypothesis that the increased activity of the ipsilateral SHLP described above serves to antagonise the occlusal force acting on the condyle. In total, 9 healthy volunteers were requested to perform left unilateral clenching at the first molar or first premolar region for 1 minute at 20% or 40% maximum voluntary clenching force. Changes in the mean proton transverse relaxation time (∆T2) were examined from the magnetic resonance images obtained before and after each clenching act as an index of the activity in all masticatory muscles. Correlation analyses of the mean ΔT2 for each volume of interest were performed with the first molar or premolar clenches to analyse the correlation between the activities in each muscle. A statistically significant correlation was exhibited between the ipsilateral temporal and ipsilateral SHLP ( r = .651, P = .003) during first premolar clenching. However, no significant correlations were observed in the ipsilateral SHLP during first molar clenching. The results of this study suggest that the ipsilateral SHLP may contribute to the pulling of the mandibular condyle forward against the occlusal force generated by the ipsilateral temporal muscle. [ABSTRACT FROM AUTHOR]

There is limited information of the normal function of the human medial pterygoid muscle ( MPt). The aims were to determine whether (i) the MPt is active throughout horizontal jaw movements with the teeth apart and (ii) whether single motor units ( SMUs) are active during horizontal and opening-closing jaw movements. Intramuscular electrodes were placed in the right MPt of 18 participants who performed five teeth-apart tasks: (i) postural position, (ii) ipsilateral (i.e. right) jaw movement, (iii) contralateral movement, (iv) protrusive movement and (v) opening-closing movement. Movement tasks were guided by a target and were divided into BEFORE, OUT, HOLDING, RETURN and AFTER phases according to the movement trajectories recorded by a jaw tracking system. Increased EMG activity was consistently found in the OUT, HOLDING and RETURN phases of the contralateral and protrusive movement tasks. An increased RETURN phase activity in the ipsilateral task indicates an important role for the MPt in the contralateral force vector. Of the 14 SMUs active in the opening-closing task, 64% were also active in at least one horizontal task. There were tonically active SMUs at the postural jaw position in 44% of participants. These new data point to an important role for the MPt in the fine control of low forces as required for stabilisation of vertical mandibular position not only to maintain postural position, but also throughout horizontal jaw movements with the teeth apart. These findings provide baseline information for future investigations of the possible role of this muscle in oro-facial pain conditions. [ABSTRACT FROM AUTHOR]

Masticatory muscle activity during teeth clenching is affected by occlusal pattern. However, few studies have performed simultaneous evaluation of all masticatory activities during teeth clenching under various occlusal conditions. The aim of this study was to use muscle functional magnetic resonance imaging (mfMRI) to evaluate the effects of changes in occlusal point on masticatory activity during single tooth clenching. Changes in mean proton transverse relaxation time (∆T2) as an index of activity in all masticatory muscles during left unilateral clenching at the first molar or first premolar for 1 min were examined in nine healthy volunteers. Bite force was maintained at 40% of the maximum voluntary clenching force. The ∆T2 values of the masseter and lateral pterygoid muscles were analysed separately for superficial and deep layers, and for superior and inferior heads. The ∆T2 values for the ipsilateral deep masseter were significantly lower, and for the superior head of the ipsilateral lateral pterygoid muscles were significantly higher, after left first premolar clenching compared to left first molar clenching. These results quantitatively demonstrate a significant increase in activity of the superior head of the ipsilateral lateral pterygoid muscle and a significant decrease in activity of the ipsilateral deep masseter muscle with forward displacement of the occlusal contact point during unilateral tooth clenching. [ABSTRACT FROM AUTHOR]

In 1921, the German dentist Hans Wertheim reported that more individuals were able to shift the mandible more towards the left than to the right. This study analyses the deviation from symmetrical mobility of the lower jaw in either direction. Using a millimetre ruler, maximum jaw opening (MJO), maximum left laterotrusion (MLL), and maximum right laterotrusion (MRL) were recorded in 141 healthy individuals and in 141 patients with temporomandibular disorders (TMDs). For both sexes, the mean maximum movements to the left and to the right were greater in the healthy group as compared with the TMD group. Healthy subjects as well as patients were able to move the mandible more to one side. Only a minority had identical values for MLL and MRL. The majority of healthy individuals and TMD patients could move more to the left (P < 0·001). In the healthy group, the mean ratio between MJO and MLL was 5·0, and 5·5 between MJO and MRL. In the TMD group, the corresponding values were 4·6 and 6.1. The mean absolute difference between MLL and MLR (in mm) was 1·24 [95% confidence interval (CI): 0·99; 1·49] among healthy females, and 2·09 (95% CI: 1·52; 2·66) among healthy males. In the TMD group, the corresponding values were 2·62 (95% CI: 2·21; 3·04) and 2·83 (95% CI: 1·67; 4·00), respectively. From the results of our study we conclude that moderate deviations from symmetric movements (mean: 1·2 mm for women, 2·1 mm for men) appear to be the norm even in healthy individuals. [ABSTRACT FROM AUTHOR]

The aim of this study is to compare phosphocreatine (PCr) concentrations of human masseter and medial pterygoid muscles by a recently developed localized magnetic resonance spectroscopy (MRS) method, chemical shift imaging (CSI). The characteristic spectra of phosphorous metabolites including PCr and β-ATP from the superficial part of the masseter (SM) and the deep part of the masseter (DM) and the medial pterygoid muscles (MPt) from 11 volunteers, 20–27-year-old were obtained. The study clearly demonstrated higher PCr/β-ATP in the SM and MPt than in the DM both in mean values (P < 0·01) and in individual subjects. The results indicate that SM and MPt are power producers. There were no significant differences in the mean values of the PCr/β-ATP ratios in SM and MPt, however, the PCr/β-ATP ratios varied individually and the subjects could be divided into three distinct groups: values of MPt higher than SM (group A, 4 subjects); values of MPt almost equal to SM (group B, 3 subjects); and values of MPt lower than SM (group C, 4 subjects). There appears to be a close relationship between the PCr content as determined in the groups here and occlusal guidance. [ABSTRACT FROM AUTHOR]

To grasp the documented evidences available regarding the cause of the anteriorly displaced human temporomandibular joint (TMJ) disc. With 20 specimens of human TMJ from cadavers, at first the number of heads of lateral pterygoid muscle (LPM) bundles were analysed with macroanatomical method. Secondary histological procedure with haematoxyline and eosine staining and orceine staining were carried out for the same specimens. Macroscopically, in 13 specimens an upper and a lower head of the LPM were present. In the other seven specimens, not only an upper and a lower head, but also a third inner head of the LPM covered with a separate fascia was observed. Histochemically, the specific localization of certain bundles of thick elastic fibres in the antero-medial parts of the anteriorly displaced deformed TMJ disc were observed. [ABSTRACT FROM AUTHOR]

The aim of the present study was to compare the activity levels within the two bellies of the lateral pterygoid muscle between different jaw positions to test the hypothesis that the upper head is primarily a stabilizer. Electromyographic (EMG) recordings, using monopolar concentric needle electrodes, were made from 14 healthy subjects during mandibular rest position (RP), clenching in intercuspal position and jaw opening, first about 10 mm and then about 25 mm. Both bellies had very little activity during RP. The activity level of the superior belly was high during clenching and large opening (LO) with a dip during low opening degree. This pattern differed from that of the inferior belly where the activity was relatively low during clenching and then gradually increased to its highest level during LO. The results support that the lower belly is primarily a jaw opener while the superior belly acts as a stabilizer keeping the disc and condyle in a functionally stable position during clenching and jaw movements. [ABSTRACT FROM AUTHOR]

Disc displacement is accepted as one of major findings in temporomandibular disorders (TMD). However, the associations of disc positions with morphological and positional changes of temporomandibular joint (TMJ) components and lateral pterygoid (LP), TMD clinical symptoms, and occlusion have rarely been discussed quantitatively. In this study, the morphological and positional changes of TMJ components and LP were assessed by means of magnetic resonance imaging (MRI) and tomography of the TMJ in 41 TMD and nine control (CN) subjects. Disc positions in TMD subjects were divided into normal position (NP) and anterior displacement with and without reduction (ADR+ and ADR-, respectively). From MRI scans and tomograms, the morphological and positional changes of TMJ components and LP were measured and compared among CN, NP, ADR+ and ADR- groups. Correlations between these measurements and the scored clinical symptoms and occlusal factors were analysed in TMD subjects. The results indicated that: (1) TMJ osseous structures and LP showed no significant difference among CN and the three TMD groups, except for a posterior seat of condyle and shorter/steeper condylar movement during jaw opening; (2) disc length and inclination were significantly shorter and steeper, respectively, in ADR+ and ADR-; (3) disc positions were not specified by clinical symptoms and occlusal factors, except for the dominant TMJ sounds in ADR+; (4) an uncoordinated movement of the condyle/disc complex was found in ADR+ and/or ADR-; (5) TMJ osseous structures and the disc were weakly associated with clinical symptoms and occlusal factors. However, the LP showed negative associations with palpable pain for both the TMJ and jaw muscles and the static occlusal factors. These findings suggest that TMJ internal derangements are more related to the positional changes or spatial relationships of TMJ components but less to the individual morphologies of TMJ osseous structures, disc and LP, as well as specific clinical symptoms and occlusal factors, which might be in disagreement with a large body of previous statements. [ABSTRACT FROM AUTHOR]

The aim of this study was to evaluate masticatory muscle responses during food crushing. Electromyograms were recorded simultaneously with mandibular movement and the sound of food fracture in eight healthy adult males at the superior and inferior heads of the lateral pterygoid muscles, the anterior belly of the digastric muscle, the temporal muscle, and the masseter muscle. After fracture of the food, silent periods of the elevator muscles and excitatory responses of the depressor muscles were observed. The latency and duration of the silent periods increased significantly with increasing incisal velocity after fracture. The mean latency of the excitatory responses of the depressor muscle was stable. After the application of anaesthetic into the temporomandibular joint capsule, the excitatory response disappeared, whereas the silent period was partly maintained. These findings suggest that the excitatory response of the depressor muscle occurs via a polysynaptic reflex arc, and that several reflex arcs are involved in the development of the silent period during food crushing. An important role for the temporomandibular joint receptors is implied. [ABSTRACT FROM AUTHOR]

Patients with chronic periodontal disease sometimes complain of difficulty in masticating hard foods. Computed tomography (CT) was used to investigate the cross-sectional area (CSA) and density of the masseter and medial pterygoid muscles in a pilot study of 12 healthy female subjects aged 30-39 years. Six subjects (the ‘diseased’ group) had suffered from chronic periodontal disease and had, on average, 40% bone loss. Their disease had been treated by scaling and root planing and had remained clinically stable for at least 6 months before their inclusion in the study. The six age-matched ‘control’ subjects had no history of periodontal disease. All bone-loss measurements were evaluated as a percentage of root length. At defined scan planes, sections of both muscles were readily identified. Results from the 'diseased' group as compared with the 'control' group revealed that in the former the CSA of masseter and medial pterygoid was reduced significantly by 25% and 10%, respectively. There were no significant differences in the X-ray density of both muscles between the two groups, which indicated a similar fibro-fatty content. These findings indicate a reduction in the masticatory force that may be utilized by patients with reduced periodontal support and may help explain their masticatory difficulties. [ABSTRACT FROM AUTHOR]

Mastication is controlled by central rhythm and burst generators, but whether this control is similar under automatic and voluntary chewing is not clear. Our first aim was to determine whether increased chewing frequency led to a proportional decrease in both the burst duration (the time of muscle activity) and the interburst interval (time of inactivity) of each muscle or to a minimal decrease in the burst duration and a substantial decrease in the interburst interval. Our second aim was to determine whether these two manifestations of control differed between the opening and closing muscles. Electromyograms from digastric, medial pterygoid, masseter and anterior temporalis muscles were obtained during gum chewing by five subjects at four frequencies. A nearly fourfold decrease in total cycle duratin (TCD) with increased chewing frequencies was accompanied by minimal changes in burst duration in the digastric muscle but proportional changes in the jaw closing muscles. The onset latency of the digastric muscle as a percent- age of TCD decreased and became negative (i.e. occurred before opening started) as the TCD decreased. The onset latencies of the jaw closing muscles remained roughly 10-20% of the TCD for the three lower chewing frequencies. Control of the opening muscle appears to differ from control of the closing muscles. [ABSTRACT FROM AUTHOR]

Maxillary full-arch splints in the retruded position (RP) and in a right lateral occlusion (1.0-1.5 mm to the right of the retruded contact position) were fabricated for ten subjects. Surface electromyography of the masseter and anterior temporal muscles was performed during submaximal clenching in order to investigate the immediate effects of the splints on the activity patterns of these muscles relative to the patterns found with the subjects occlusion in the inter-cuspal position. The splints in the RP were found to have no effect on the asymmetry of the activity of the masseter and the anterior temporal muscles, while the splints in a right lateral occlusion resulted in relative increases in right anterior temporal muscle activity (P<0.005). [ABSTRACT FROM AUTHOR]

An objective parameter for the electromyographic evaluation of the masticatory function of edentulous patients was studied. The normal ranges of the parameters for such subjects were also established. This study focused on the time parameter of electromyography, which is one of the most reproducible parameters. The variation coefficient of the time parameters (interval and duration) were revealed to be reliable and reproducible for the evaluation of masticatory functions. [ABSTRACT FROM AUTHOR]

The electromyographical (EMG) activity of masticatory muscles during full clenching in the retruded contact position (RCP) and intercuspal position (IP) with and without a posterior stabilizing splint was studied. The linear envelope EMG signal from three bilateral muscles was recorded (masseter, anterior temporal and posterior temporal). Thirty-seven subjects were evaluated. Clenching in the RCP without the splint and with the presence of an unstable occlusal contact inhibited the masseter muscle activity and reduced anterior temporal and posterior temporal muscle activity. The masticatory muscle activity returned to normal when clenching in the RCP with a splint that permitted stability in the dentition. EMG activity was the same in the IP with and without the splint. The results indicate that the determinant of maximal masseter isometric muscle contraction is the amount of stability in the dentition rather than the jaw position. If the dentition takes the major role of stabilizing the mandible, i.e., there is good intercuspation, the masseter muscle can exert maximal isometric contraction. If the stability is not provided by the dentition, i.e., there is a premature contact, the jaw muscles must contribute to the stabilization and reduce the magnitude of the maximal contraction to avoid damage to the structures involved in the compensatory stabilization. [ABSTRACT FROM AUTHOR]

The relationships of the lateral pterygoid muscle within the infratemporal fossa were observed by conventional dissections and by examination of specimens sectioned in the horizontal and frontal planes. The following less well-known features were noted. At the origins of the superior and inferior heads there are regions in which the fibres are interlaced or closely overlapped by fibres of either the temporalis muscle or the medial pterygoid muscle. Fibres of the superior head insert not only into the meniscus of the temporomandibular joint, but also into the pterygoid fovea at the neck of the mandibular condyle. Specimens sectioned through the origin of the inferior head of the muscle show internal tendon lamellae consistent with a pennate structure. Electromyographic (EMG) activity was recorded in five healthy subjects using concentric needle and fine-wire electrodes. Strong to very strong activity was consistently observed in the superior head during clenching and tooth gnashing. The inferior heads were silent or had negligible to slight activity most of the time during ipsilateral movements or clenching, but were co-activated bilaterally, with strong to very strong activity during jaw opening, protrusion, swallowing, tooth gnashing and during passive retrusion. They showed marked activity unilaterally during contralateral movements. [ABSTRACT FROM AUTHOR]

A three-dimensional photoelastic model was reproduced from a human skull. The insertions of the superficial and deep portions of the masseter muscle, the temporalis muscle, the medial pterygoid muscle and the temporalis fascia were simulated with leather bonded to the appropriate areas. The following loading conditions were employed: (i) all three muscles were bilaterally loaded without occlusal loads; (ii) all three muscles were bilaterally loaded while occlusal loads were simultaneously applied on the maxillary teeth. The stress freezing technique was used to lock the resulting stress patterns into the model skull. The stress trajectories were observed and photographed in a circular polariscope. The mandibular closing muscles and the occlusal loads produced stresses which progressed through the maxilla, following a nasal, a zygomatic and a pterygoid route, while stress concentrations were seen in the frontonasal, zygomaticomaxillary and the pterygopalatine sutures. In the malar bone area the stresses were seen to branch superiorly to the frontozygomatic suture and posteriorly along the zygomatic arch to the zygomaticotemporal suture and the temporal bones Generally, the stresses were concentrated in those areas of the skull where architectural reinforcement had been demonstrated by other methods. [ABSTRACT FROM AUTHOR]

This study was prompted by the hypothesis that the contractile power of the masseter and medial pterygoid muscles influences not only the shape of the gonial angle of the mandible into which these muscles insert, hut also that they play a major part in determining the amount of free-way space present in any individual. Tracings were made from the lateral cephalograms of 130 dentate subjects and the gonial angle measured in each case. Free-way space was also measured, using a dakometer, and the paired values subjected to statistical analysis. A strong inverse correlation was found to exist, indicating that throughout the sample a decrease (or sharpening) of the gonial angle is accompanied by a predictable increase in the free-way space. By applying the processes of linear regression to the paired results, a formula was obtained which may be used to predict the best value of one variable for any given value of the other. Thus, in an edentulous case, where the value of the free-way space measurement is unknown, a method is provided for predicting its best value by the determination of the patients gonial angle through cephalometric analysis. [ABSTRACT FROM AUTHOR]

Oromandibular dystonia (OD) is a movement disorder characterized by involuntary activity of the craniofacial muscles. The present study deals with dystonic activity in the lateral pterygoid muscles (LP) of two women causing mandibular overbite, tiredness in the face and chewing and speech difficulties in one (HN, 61 years). In the other (BS, 37 years) OD was characterized by irregular lateral and protrusive jaw movements with and without tooth contact and frequent bilateral headache. By intra-oral approach and without attempt to distinguish between the two heads the activity of LP was assessed by quantitative electromyography (EMG). Concentric needle electrodes were used for diagnosis and follow-up, monopolar cannula electrodes for the EMG-guided injection of botulinum toxin (Botox® , Allergan Inc., USA). In addition, activity in the temporal, masseter and digastric muscles was recorded with bipolar surface electrodes and mandibular movements were assessed graphically. Compared with reference values of postural activity, OD in both patients was associated with a marked increase in LP. Botox injections resulted in transient reduction of spontaneous and maximal activity, but it took two to three treatments with intervals of 3–4 months to obtain distinct subjective and clinical improvement. However, in HN followed for 18 months after breakthrough of treatment, the effect of the two latest injections lasted 8–9 months or three to four times longer than typical. Using quantitative EMG to locate the abnormal activity and to guide injections, treatment of LP dystonia with botulinum toxin is effective and safe . [ABSTRACT FROM AUTHOR]