Jaw movement responses to electrical stimulation of different parts of the human temporalis muscle.

Previous EMG studies have provided indications for the differential activation of the human temporalis muscle. However, in these studies the contribution of different parts of the temporalis muscle could not be separated from the contributions of other muscles, since contraction was performed by voluntary activation. Therefore, the aim of this study was to examine functional differences among various parts of the human temporalis muscle by means of recording the incisal point movement response to electrical stimulation of parts of the muscle. Five healthy male subjects participated in this study. Three locations (anterior, middle, and posterior temporalis muscle) were stimulated, by means of monopolar wire electrodes and rectangular pulses. The insertion depths of the stimulation electrodes were determined by means of magnetic resonance images. Stimulation was performed in four jaw positions (resting position, 50% maximum mouth opening, 1 cm to the left, and 1 cm to the right). Movement responses to stimulation of the different muscle parts were recorded with the OKAS-3D jaw movement analysis system. The movement responses were expressed in polar coordinates. The variation in the direction of the jaw movement response was partly explained by the factors 'stimulation location' and 'jaw position' (ANOVA, p < 0.001). When the stimulation location shifted in an antero-posterior direction, the response changed from a vertical-lateral incisal point movement to a lateral-posterior movement with a smaller vertical component. The jaw position during stimulation also influenced the movement response. A functional subdivision of the temporalis muscle into at least three parts is favored.

Mandibular movements in response to electrical stimulation of superficial and deep parts of the human masseter muscle at different jaw positions.

Anatomical and electromyographical evidence suggests a compartmentalized function of the human jaw-closing muscles during both static and dynamic motor tasks. However, the voluntary nature of these tasks hampers unequivocal interpretation of this evidence, because it is impossible to activate voluntarily a single part of a muscle exclusively. Activation of discrete, localized regions can be accomplished with electrical stimulation. A previous study confirmed a functional subdivision of the temporalis muscle into at least three parts. Here, differences in the direction of the lower incisal-point (IP) movement in response to electrical stimulation of four different parts of the masseter muscle were examined in five healthy men. The deep masseter muscle and the anterior, middle, and posterior parts of the superficial masseter muscle were stimulated with monopolar wire electrodes in four different jaw positions (resting position; 50% maximum mouth opening; and 10-mm right and left lateral excursions, both with respect to resting position). Electrode-insertion depth was measured from magnetic resonance images. Movement responses to stimulation were recorded with the OKAS-3D jaw-movement analysis system. The variation in the direction of the IP movement in response to stimulation of parts of the masseter was partly explained by the effects of stimulus location and jaw position. The response to stimulation of the deep masseter was mainly laterovertically directed, whereas the response to stimulation of each of the superficial parts had a mainly anterovertical direction, the responses being most pronounced with the mandible in its resting position. These results provide further evidence for a functional subdivision of the masseter into a superficial part and a deep part, but not for a further subdivision of the superficial part into an anterior, middle, and posterior part.

Interobserver variability in assessment of signs of TMD.

PURPOSE: The aim of this report was to study the ability of examiners to measure reliably the clinical signs of temporomandibular disorders (TMD). Four examiners participated in this study of 11 TMD patients and 25 nonpatients. MATERIALS AND METHODS: Vertical and lateral excursions of the jaw were measured using a millimeter ruler. Joint sounds during vertical jaw movements were assessed using digital palpation. The reliability of delivering appropriate degrees of digital pressure to assess masticatory muscle pain was assessed using a manometer after training examiners to exert specified pressures. RESULTS: Intraclass correlation coefficients for the measurement of vertical and protrusive jaw movements were > or = 0.87, which was considered excellent. The intraclass correlation coefficient for measurements of left and right lateral jaw excursions varied between 0.73 and 0.85, which was considered acceptable. The interobserver agreement for detecting the joint sounds showed overall agreement across examiners of 78%. Kappa for every possible pair of examiners varied between .52 and .86 (median .75, interquartile range .18). Reliability for diagnostic categories from the Helkimo index and Research Diagnostic Criteria for Temporomandibular Disorders involving joint noises showed modest reliability. CONCLUSION: Point estimates and measures of spread for reliability measures of single clinical TMD signs as well as combinations of signs into diagnostic categories from the Helkimo index and Research Diagnostic Criteria for Temporomandibular Disorders involving joint noises were sufficient in a group of four examiners.

Functional subdivision of the human masseter and temporalis muscles as shown by the condylar movement response to electrical muscle stimulation.

In previous studies from our laboratory, a functional subdivision of the human temporalis and masseter muscles was demonstrated by means of opto-electronic recordings of the lower incisal point movement responses to electrical muscle stimulation. In the present study, it was examined whether this subdivision was also reflected in different movement responses of the mandibular condyle. To that end, the condylar movement responses to unilateral stimulation of four masseter muscle parts and three temporalis muscle parts were studied in four different jaw positions. The kinematic centre was used for condylar reference point. For both the amplitude and the direction of the movement responses, the effects of stimulation location and jaw position were studied using multivariate ANOVA and contrast analyses. It was found that for both outcome variables, the functional subdivision of the masseter and temporalis muscles was also reflected in some, but not all, of the movement responses of the mandibular condyles. The deep masseter muscle part and the (anterior) temporalis muscle part responded similarly to electrical stimulation.

Relationship between condylar and incisor point displacement during habitual maximum open-close movements.

The relationship between condylar movements and incisor point movements during habitual maximum open-close movements were studied in 10 healthy male and 10 healthy female subjects. Jaw movements were recorded by means of an opto-electronic jaw movement recording system, OKAS-3D, capable of recording the six degrees of freedom at a sample frequency of 300 Hz. The lower jaw position of the lateral pole of the condyles was found by means of palpation. In order to analyse the movements, the opening and closing path of the incisor point were divided into ten equal intervals and the corresponding condylar displacement in each interval was calculated. A displacement index was obtained by normalizing the condylar displacement with respect to the maximum condylar displacement. Due to the normalization, the displacement index is not sensitive to possible errors in the location of the lateral condylar point. A clear condylar displacement was already recorded in the first movement interval, right at the start of opening (average displacement index in the first opening interval was significantly greater than zero, P < 0.0005). The condylar displacements in the start and the end interval of opening and closing were smaller than in the intermediate movement intervals (P < 0.00005).