Takumi Morita

Movement of the mandibular condyle and activity of the masseter and lateral pterygoid muscles during masticatory-like jaw movements induced by electrical stimulation of the cortical masticatory area of rabbits

We examined the functional role of the lateral pterygoid muscle (LP) and the masseter muscle (MS) in the movement of the mandibular condyle in masticatory-like jaw movements induced by electrical stimulation of the cortical masticatory area of urethane-anaesthetised rabbits. EMGs of the LP and MS were recorded along with video images of the mandibular condyle movement filmed with a high-speed CCD camera at a time resolution of 8 ms. The time required for the contractile force of the MS and LP to emerge as bite force or jaw movement was determined by direct electrical stimulation to respective muscles: 32.8+/-1.5 ms for the MS and 34.3+/-2.9 ms for the LP. The LP on the working side showed biphasic activity not only in the jaw-opening phase but also in the middle occlusal phase. It is assumed that the MS on the working side begins to exert actually the maximum mechanical influence on the bite force 32.8 ms after its EMG peak in the early occlusal phase. Such development of the mechanical effect of the MS during middle occlusal phase is appeared to be involved in stabilization of the condyle in the middle occlusal phase. Approximately 25 ms (25.1+/-2.5 ms) after the peak of mechanical influence of the MS, the mechanical influence of the working-side LP activity reached maximum in the late stage of the occlusal phase. This LP mechanical influence seems to be functionally associated with an antero-inferior movement of the working side condyle in the late stage of the occlusal phase that is coincident with a movement of the incisal point towards the balancing side across the midline.

Unusual postero-inferior condylar movements that depend on the position of occlusal contact during fictive mastication in rabbits.

OBJECTIVE The mandible can be modelled as a triangular plate supported at two joints and the point of occlusion. The mandible is stable if the vector of the jaw-closing muscle forces lies within the triangle of support. If this vector lies outside of the triangle of support, one of the three contact points will tend to separate as the mandible rotates around a line connecting the other two points. Here, we examined whether postero-inferior condylar movements (Pi-Cm) due to mandibular rotation may occur during fictive mastication in anaesthetized rabbits. METHODS EMG activities of the masseter (MS) and lateral pterygoid (LP) muscles and movements of the condyle and incisal points were recorded. Condylar movements in the sagittal plane were recorded using a high speed CCD camera. RESULTS Pi-Cm were observed on the working side during occlusal phase in half of the rabbits (altered-movement group), if the biting point was restricted at the posterior most tooth (M3) on the working side using a metal biting plate. Pi-Cm appeared in the period between the estimated maximum force of the MS and the LP during late occlusal phase. The MS EMG ratio between the working and balancing sides in the altered-movement group was significantly less than that in the unaltered-movement group. CONCLUSION Since the space lying between the condyle and the articular eminence expanded during the Pi-Cm, it is likely that the posterior band of the articular disc tended to slip anteriorly. The clinical significance of the Pi-Cm is discussed concerning the anterior dislocation of the disc in patients with TMJ disorder.

Development of medial pterygoid muscle fibers in rabbits fed with a liquid diet

OBJECTIVE This study aimed to investigate the influence of decreased functional load on the medial pterygoid muscle during mastication in rabbits fed with a liquid-diet. MATERIALS AND METHODS Medial pterygoid muscles from 54 rabbits (solid- and liquid-diet groups, n=48; unweaned group, n=6) were histochemically examined at 4, 9, 12, 18, and 33 weeks after birth. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were distinguished via mATPase staining. RESULTS Significant increases in the diameters of all fiber types were seen up to 33 weeks of age in the solid-diet group; however, no significant increase was noted in fiber types I and IC, from 4 to 33 weeks of age, in the liquid-diet group. The proportion of slow fibers increased up to 12 weeks followed by an increase in the number of fast fibers in the solid-diet group, whereas in the liquid-diet group, the number of slow fiber declined after weaning. CONCLUSIONS Liquid-diet consumption caused muscle fiber atrophy and an increase in the number of fast fibers during early developmental stages after weaning. Furthermore, the growth pattern of the medial pterygoid muscle in the liquid-diet group was different from that in the solid-diet group.

Postero-inferior condylar movement induced by artificial occlusal interference on the balancing side during fictive mastication in rabbits.

OBJECTIVE Tooth contact does not occur on the balancing side during mastication. Hence, it is possible that the presence of occlusal interference on the balancing side causes mandibular rotation followed by atypical condylar movement because the jaw-closing muscle activity on the working side is greater than on the balancing side. The aim of the present study was to investigate the relationship between occlusal contact on the balancing side and condylar movement during mastication. METHODS EMG activity of the masseter (MS), lateral pterygoid (LP) and digastric (DG) muscles and jaw movements were recorded. Condylar movements in the sagittal plane were recorded using a high speed charge-coupled device (CCD) camera. Incisal point movements were recorded using a magnet on the mentum and a magnetometric sensor on the nasal bone. A removable biting plate was used to introduce an artificial occlusal interference on the balancing side. RESULTS Nine of the 10 rabbits showed large postero-inferior condylar movement (Pi-Cm) when a biting plate was applied on the balancing side. Five rabbits showed very small Pi-Cm when a biting plate was applied on the balancing side. In the small Pi-Cm group, MS activity decreased markedly and LP and DG transient activity appeared in the early occlusal phase in the presence of the biting plate. CONCLUSION Interference on the balancing side always caused Pi-Cm on the ipsilateral side during mastication. However, the degree of Pi-Cm was often decreased by a jaw opening reflex response.

Changes in movements of the mandibular condyle after articular eminence resection on the working side during fictive mastication in rabbits

The physiological load on the temporomandibular joint (TMJ) plays an important role in maintenance of normal morphological structure and functions of the TMJ. On the other hand, overload on the TMJ can cause temporomandibular disorders. Therefore, it is important to understand the magnitude and direction of load on the TMJ when studying the mechanism for maintenance of normal morphological structure and functions of the TMJ. The authors presumed that compressive force on the TMJ is manifested as movements of the mandibular condyle towards the articular eminence after resection of the articular eminence. Therefore, we estimated the magnitude and direction of load on the TMJ on the basis of changes in movements of the mandibular condyle before and after resection of the articular eminence. EMG activities of the masseter muscle were simultaneously recorded with movements of the condyle as well as the incisor point during fictive mastication induced by electrical stimulation to the cortical masticatory area of anesthetized rabbits. As a result, resection of the articular eminence caused no significant change in EMG activities of the masseter muscle or movements of the incisor point. However, the position of the trajectory of the mandibular condyle shifted in an anterosuperior direction after resection of the articular eminence. In addition, forward projecting movements of the mandibular condyle were seen in the occlusal phase. It was suggested that the TMJ was constantly exposed to a load during masticatory movements, particularly during the occlusal phase. Force holding the articular disc between the articular eminence and the mandibular condyle has an important role in preventing the development of space that would allow the articular disc to move anteriorly.