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

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.

Motor control of jaw muscles in chewing and in isometric biting with graded narrowing of jaw gape.

When a certain bite force is applied during unilateral chewing, the combination of jaw elevator muscle activities is different than when a comparable force is applied in unilateral isometric biting, e.g. on a force transducer. Masticatory peak force is generated in a nearly isometric phase of the chewing cycle, with a jaw gape of about 1 mm. In contrast, peak force in isometric biting on force measuring equipment usually induces jaw gapes of 6 mm or even more. Therefore, we tested the hypothesis that the jaw gape influences relative activation of elevator muscles in unilateral isometric biting. We further examined whether such influence could explain the different activity combinations of chewing and isometric biting. In thirty asymptomatic males, masseter and temporalis activities were recorded during intermittent isometric biting with jaw gapes of 6, 5, 3, 2 and 1 mm and during unilateral chewing. Activity combinations were described by working/balancing ratios and by temporalis/masseter ratios. With decreasing jaw gape the working/balancing ratio of the posterior temporalis decreased (P < 0.002) while that of the masseter increased (P < 0.001). Likewise, the temporalis/masseter ratio on the balancing side increased (P < 0.001). With decreasing jaw gape, activity ratios of isometric biting approached ratios of chewing. We conclude that: (i) relative jaw muscle activation in isometric biting depends on the jaw gape, (ii) relative muscle activation in chewing resembles relative activation of isometric biting with a small 'chewing-like' gape. This suggests that characteristic activity combinations in chewing are mainly a result of the approximately isometric contraction during the slow closing phase of the chewing cycle.

Preconditions for estimation of masticatory forces from dynamic EMG and isometric bite force-activity relations of elevator muscles.

PURPOSE: Previous studies proposed to estimate masticatory forces from electromyograms (EMG) by substituting chewing activities of a particular muscle into bite force-activity relations obtained from isometric clenching. For biomechanical reasons, this method requires that ratios between muscle forces in chewing be the same as in clenching. The purpose of this study was to investigate whether this assumption is valid. MATERIALS AND METHODS: In 32 asymptomatic subjects, EMGs of bilateral temporal and masseter muscles were registered in unilateral clenching on a bite fork and in chewing. Ratios of peak activities of different muscles were compared between the two biting tasks. RESULTS: In clenching, the working: balancing ratio of the masseters amounted to 0.92, while in chewing this ratio increased to 1.9. The working:balancing ratio of the temporalis was 1.8 in clenching and dropped to 1.3 in chewing. The temporalis:masseter ratio on the balancing side increased from 0.65 in clenching to 1.1 in chewing. CONCLUSION: The finding that ratios of muscle activities in chewing differed from corresponding ratios in clenching implies different combinations of muscle forces and/or different bite force-activity relations in the two biting tasks. In any of these cases, prediction of chewing force from the EMG of a particular muscle could lead to indefinite overestimations.

Investigation of a possible relationship between kinematic points and condylar attachments of the lateral ligaments.

Kinematic points are proposed as reference for assessment of condyle motion. They are defined by coincidence between open-closing and protrusive trajectories. According to anatomical findings, this feature should also apply to the condylar attachment of the lateral ligament that should guide the condyle on an arc around its tubercular fixation. Our aim was to examine if evidence for a correspondence between the kinematic point and the condylar attachment of the ligament could be derived from condylar dynamics. In 60 asymptomatic volunteers, open-closing and protrusive jaw movements were recorded with 6 d.f. Kinematic points were found 2. 5+/-2.9 mm anterior and 4.3+/-2.9 mm inferior from the terminal-hinge point. Their coinciding traces could be fitted by arcs with radii of 11.7+/-2.2 mm. The centres of the arcs were found 10.4+/-1.6 mm anterior and 4.9+/-2.4 mm superior from the kinematic points. When compared to anatomy, the local distributions of kinematic points and arc centres corresponded well to the condylar and tubercular fixation areas of the ligament. The findings provide circumstantial evidence for a correspondence between kinematic points and the attachment of the lateral ligament. Kinematic points could provide a better means to assess condyle motion than other posterior landmarks, when in addition, movements of adjacent points and rotational properties are considered.

Predicted incidence of excursive occlusal errors in common modes of articulator adjustment.

PURPOSE: Functional relationships between occlusal errors and articulator settings have been the subject of numerous investigations. It is, however, not known how frequently errors of a certain size occur under practical conditions. This study attempted to estimate the frequency of occlusal errors that can be expected with the different levels of registration and articulator adjustment that are currently used. MATERIALS AND METHODS: In 57 asymptomatic subjects, individual articulation parameters, including the condylar and Bennett angles, the spatial relation of dental arches, and the intercondylar distance, were determined experimentally. Tooth-guided lateral movements of the subjects' casts were simulated in a virtual articulator that was programmed to the individual parameters of each person. With respect to this reference setting, occlusal errors that would emerge with different modes of semi-individual adjustment were calculated. The intercondylar distance thereby remained fixed at 110 mm. RESULTS: With complete mean value setting, occlusal errors would exceed 200 microns at the second molar in 16% of the subjects and 300 microns in 6% of the subjects. Individual facebow registration of condylar angles and spatial relations would reduce these rates to 13% (200 microns) and 3% (300 microns). With additional setting of Bennett angles, occlusal errors would exceed the mentioned limits in no more than 1.6% and 0.1% of cases, respectively. This extensive mode of adjustment became, however, increasingly ineffective with higher demands for occlusal accuracy. CONCLUSION: Complete mean value setting is associated with a relatively low risk of occlusal errors exceeding tolerance limits that are widely accepted in practice. Compared to mean value setting, facebow registration of condylar angle and relations yields no profitable improvement of occlusal accuracy.

Differences between traces of adjacent condylar points and their impact on clinical evaluation of condyle motion.

PURPOSE: Recent research revealed that traces of single posterior reference points can depend on the location of the monitored point. The aim of this study was to quantify this dependence and to point out its consequences for clinical application of condylar path registrations. MATERIALS AND METHODS: In 60 asymptomatic volunteers, mandibular motion was recorded during protrusion, lateral excursion, and opening-closing of the jaw. Simultaneous trace-patterns of 10 condylar points including the hinge-axis point and the kinematic-axis point were compared with respect to length, inclination angles, coordination, and shape. RESULTS: In protrusion, traces of the different condylar points were equal and independent of the location of the monitored point. Bennett angles depended on the sagittal position of the reference point and varied by 0.8 degree per 1 mm change of location. Opening-closing traces differed considerably. Their lengths varied by up to 9 mm and inclination angles varied by 40 degrees. Opening-closing patterns also showed irregularities like those observed in temporomandibular disorders. Irregularities were minor for the hinge-axis point and least for the kinematic-axis point. CONCLUSION: In tooth-guided movements "condyle motion" is represented by the traces of any point near the condyle. In opening-closing, however, the trajectory of a single condylar point will not reliably represent condylar motion. For articulator adjustment, condylar angles must not be taken from opening-closing, but only from protrusion. In diagnostic applications, one has to be aware that irregular traces may not only result from dysfunction, but may as well be a result of the choice of reference point.

Reaction of mastication to occlusal changes induced by correction of mandibular prognathism.

Incisor-point masticatory movements of 22 patients were recorded before and after correction of mandibular prognathism. Chewing patterns and movement parameters were compared between preoperative and postoperative states and to corresponding quantities of 85 persons with normal dentitions. The prognathism patients most frequently revealed "chopping-type" patterns characterized by nearly vertical closing strokes, whereas mastication in the reference group was dominated by lateral grinding-type movements. There were no significant differences in the pattern distributions and parameter values between the preoperative and postoperative states. An individual assessment, considering random fluctuations of chewing behavior, showed that only five patients had changed their chewing mode possibly because of the treatment. Only a minor part of these changes could be directly related to the altered occlusion. It is concluded that the characteristic chewing mode of persons with mandibular prognathism does not interfere with the occlusal profile, and hence does not react to alterations.