A.G. Hannam

Deformation of the Human Mandible During Simulated Tooth Clenching

Localized corpus and dental arch distortions measured directly on human and animal mandibles suggest complex deformation patterns at other mandibular sites during functional loading. To describe these, we simulated selected static bites on a three-dimensional finite element computer model of the human jaw. Five clenching tasks were modeled: intercuspal position, left group function, left group function plus balancing contact, incisal clenching, and right molar clenching. Under conditions of static equilibrium and within the limitations of the current modeling approach, the human jaw deforms elastically during symmetrical and asymmetrical clenching tasks. This deformation is complex, and includes the rotational distortion of the corpora around their axes. In addition, the jaw also deforms parasagittally and transversely. The degree of distortion depended on each clenching task, with actual deformations being relatively small and ranging from 0.46 mm to 1.06 mm for the tasks modeled when all sites were taken into account. The predicted overall narrowing of the dental arch is consistent with clinical reports in the literature during similar, although not identical, static jaw function. The predicted regional deformations of the upper condylar surfaces imply differential loading at their upper surfaces. Although still constrained to forceful static biting conditions, the simulated mandibular and dental arch distortions should be taken into consideration in the design and testing of prosthetic devices in the lower jaw.

Internal Organization in the Human Jaw Muscles

The human jaw muscles are essential to mastication and play an important part in craniofacial growth. They contribute to dental and articular forces, deform the mandible, and, like other tissues, are subject to disorders, often manifested as pain. The literature describes how their contraction is controlled by the nervous system, and how their general structure and function contribute to craniofacial biology, but there has been little appraisal of their internal organization. Most of these muscles are not simple; they are multipennate, complexly layered, and divided by aponeuroses. This arrangement provides substantial means for differential contraction. In many ways, jaw muscle fibers are intrinsically dissimilar from those found in other skeletal muscles, because they are arranged in homogeneous clusters and generally reveal type I or type II histochemical profiles. Most are type I and are distributed preferentially in the anterior and deeper parts of the jaw closers. Additionally, most motor unit (MU) territories are smaller than those in the limbs. There is circumstantial evidence for intramuscular partitioning based in part on innervation by primary muscle nerve branches. During normal function. MU recruitment and the rate coding of MU firing in human jaw muscles follow the general principles established for the limbs, but even here they differ in important respects. Jaw muscle MUs do not have stable force recruitment thresholds and seem to rely more on rate coding than on sequential unit recruitment to grade the amplitude of muscle contraction. Unlike those in the limbs, their twitch tensions correlate weakly with MU fatiguability and contraction speed, probably because there are so few slow, fatigue-resistant MUs in the jaw muscles. Moreover, the type I fibers that are present in such large numbers do not contract as slowly as normally expected. To complicate matters, estimation of jaw MU twitch tensions is extremely difficult, because it is affected by the location used to measure the twitch, the background firing rate, muscle coactivation, and regional, intramuscular mechanics. Finally, there have been very few systematic studies of jaw MU reflex behavior. The most recent have concentrated on exteroceptive suppression and suggest that MU inhibition following intra- and perioral stimulation depends on the location of the MU, its background firing rate, the timing of the stimulus, and the task used to drive the unit. Task dependency is a common feature of human jaw MU behavior, reflecting interaction between peripheral sensory information from orofacial and muscle afferents and corticobulbar drive.(ABSTRACT TRUNCATED AT 400 WORDS)

The influence of altered working-side occlusal guidance on masticatory muscles and related jaw movement

The effect of four different occlusal situations (group function, canine guidance, working side occlusal interference, and hyperbalancing occlusal interference) on EMG activity in jaw elevator muscles and related mandibular movement was investigated on 12 subjects. With a computer-based system, EMG and displacement signals were collected simultaneously during specific functional (unilateral chewing) and parafunctional tasks (mandibular gliding movements and various tooth clenching efforts) and analyzed quantitatively. When a naturally acquired group function was temporarily and artificially changed into a dominant canine guidance, a significant general reduction of elevator muscle activity was observed when subjects exerted full isometric tooth-clenching efforts in a lateral mandibular position. The original muscular coordination pattern (relative contraction from muscle to muscle) remained unaltered during this test. With respect to unilateral chewing, no significant alterations in the activity or coordination of the muscles occurred when an artificial canine guidance was introduced. Introduction of a hyperbalancing occlusal contact caused significant alterations in muscle activity and coordination during maximal tooth clenching in a lateral mandibular position. A marked shift of temporal muscle EMG activity toward the side of the interference and unchanged bilateral activity of the two masseter muscles were observed. The results suggest that canine-protected occlusions do not significantly alter muscle activity during mastication but significantly reduce muscle activity during parafunctional clenching. They also suggest that non-working side contacts dramatically alter the distribution of muscle activity during parafunctional clenching, and that this redistribution may affect the nature of reaction forces at the temporomandibular joints.

The relationship between dental occlusion, muscle activity and associated jaw movement in man.

Abstract Experiments were carried out on adult subjects before and after occlusal adjustment, and during atypical mastication, to study the relationship between occlusion of the teeth, muscle activity and associated jaw movements. A computer-based system was used to record and analyze the electromyographic activity in the right and left anterior temporal, posterior temporal and masseter muscles, as well as the displacement, in 3 planes, of an incisor point on the mandible. Clinical examination of the occlusion was performed by means of a standard procedure, which permitted numerical values to be assigned to variables commonly observed in clinical practice. Unilateral gum-chewing tasks were carried out by each subject. Five subjects were tested both before and two weeks after occlusal adjustment. Two subjects acted as controls. The series also included one subject with a history of bruxism and another who undertook specific chewing tasks. The results indicated a tendency for occlusal adjustment to be associated with an increase in the lateral excursions of the mandible during jaw closure and, in some cases, with a closer approximation of peak muscle activity to the intercuspal position of the teeth. Specific occlusal features showed no clear association with either muscle activity or jaw displacement, although all subjects developed maximum muscle effort very close to, or at, the intercuspal position. Jaw-closing speed during natural chewing appeared to decrease abruptly before maximum bolus resistance was met by the teeth, suggesting the existence of a neuromuscular control mechanism which operates before closing forces become very large.

Relationship between occlusal contacts and jaw-closing muscle activity during tooth clenching: Part II*

Abstract 1. Electromyographic recordings from the anterior temporal muscle fibers bilaterally, the posterior temporal muscle fibers bilaterally, the superficial masseter muscle bilaterally, and the left medial pterygoid muscle were used to study the effects of changing the location, size, and direction of effort on specific contact points during maximal clenching tasks in human subjects. 2. Vertical clenching efforts in the natural or simulated intercuspal position generally showed the highest muscle activities for all the muscles recorded. 3. When the contact point moved posteriorly along the arch from incisors to molars, the activity in the ipsilateral temporal muscles was seen to increase, while the activity in the ipsilateral medial pterygoid and the masseter muscles bilaterally was seen to decrease during vertical clenching tasks. 4. Eccentric efforts on specific contact points generally resulted in lower activity than the corresponding vertical effort. This was usually seen in all muscles, but not all values were significant. 5. The ipsilateral temporal and contralateral pterygoid muscles showed the most activity during maximal clenches in lateral direction with little contribution from the other muscles. 6. The temporal muscles showed the most activity in retrusive clenching, with activity in the other muscles nearly nonexistent. 7. The medial pterygoid and masseter muscles were found to be the most active muscles during protrusive and incisal clenching, while the temporal muscle activity was low. 8. When the size and number of contacts were increased anteriorly, a generalized increase in muscle activity was seen. The same trend occurred posteriorly but was not as consistent or significant. 9. Cross-arch contacts were associated with a slight but significant bilateral increase in masseter muscle activity and an increase in temporal muscle activity ipsilateral to the cross-arch contact when maximum vertical clenches were performed. However, no significant increases were observed when the effort was directed laterally. 10. The findings of this electromyographic study on changes of the contact point, size of contact point, and the direction of effort applied on a contact point confirm their specific associations with the activity of muscle groups. Significant data have also been made available for a biomechanic approach to the investigation of degenerative joint changes.

Relationships Between the Size, Position, and Angulation of Human Jaw Muscles and Unilateral First Molar Bite Force

Human subjects commonly show large variations in bite force produced at the first molar teeth. To evaluate the role of muscle cross-sectional sizes and lever arms in bite-force production, we correlated these variables in 11 healthy adults. Axial and coronal images obtained by magnetic resonance were combined with conventional lateral cephalograms and dental cast data to reconstruct the craniomandibular morphology in each subject. The cross-sectional sizes of the right masseter and medial pterygoid muscles, their lever arms, and the bite-point lever arms were measured directly from these reconstructions. Physiological recordings of bite force were made in the region of the right first molar by means of a customized transducer aligned perpendicular to the functional occlusal plane. The average bite force for the sample as a whole was 189 ± 78 N. The coefficients of variance were greater for bite forces, and for the cross-sectional sizes of the two muscles, than for their respective lever arms. Highly significant Pearson Product Moment correlation coefficients (p<0.005) were found between masseter and medial pterygoid cross-sectional size, and between the cross-sectional size of each muscle and bite force. No significant correlations (p>0.1) were found between muscle or bite-point lever arms and bite force. Despite the fact that craniofacial spatial morphology may differ among subjects, jaw muscle size alone seems to explain most of the variation in bite force reported by ourselves and others.

Dynamic simulation of muscle and articular properties during human wide jaw opening.

Human mandibular function is determined in part by masticatory muscle tensions and morphological restraints within the craniomandibular system. As only limited information about their interactions can be obtained in vivo, mathematical modeling is a useful alternative. It allows simulation of causal relations between structure and function and the demonstration of hypothetical events in functional or dysfunctional systems. Here, the external force required to reach maximum jaw gape was determined in five relaxed participants, and this information used, with other musculoskeletal data, to construct a dynamic, muscle-driven, three-dimensional mathematical model of the craniomandibular system. The model was programmed to express relations between muscle tensions and articular morphology during wide jaw opening. It was found that a downward force of 5 N could produce wide gape in vivo. When the models passive jaw-closing muscle tensions were adjusted to permit this, the jaws resting posture was lower than that normally observed in alert individuals, and low-level active tone was needed in the closer muscles to maintain a typical rest position. Plausible jaw opening to wide gape was possible when activity in the opener muscles increased incrementally over time. When the model was altered structurally by decreasing its angles of condylar guidance, jaw opening required less activity in these muscles. Plausible asymmetrical jaw opening occurred with deactivation of the ipsilateral lateral pterygoid actuator. The models lateral deviation was limited by passive tensions in the ipsilateral medial pterygoid, which forced the jaw to return towards the midline as opening continued. For all motions, the temporomandibular joint (TMJ) components were maintained in continual apposition and displayed stable pathways despite the absence of constraining ligaments. Compressive TMJ forces were presented in all the cases and increased to maximum at wide gape. Dynamic mathematical modeling appears a useful way to study such events, which as yet are unrecordable in the human craniomandibular system.

The Association Among Occlusal Contacts, Clenching Effort, and Bite Force Distribution in Man

The contact area during habitual biting can vary according to the activity of the jaw musculature. Forceful masticatory muscle activity may also induce deformations of the dento-alveolar tissues and the supporting skeleton, yielding various tooth loads despite an apparently even distribution of tooth contacts. To investigate this variability, we measured bite forces simultaneously at multiple dental sites during maximum-effort clenching tasks. In each of four healthy adults with complete natural dentitions, four strain-gauge transducers in the right side of an acrylic maxillary appliance occluded with the lower canine, second premolar, and first and second molars. These, and matching contralateral contacts, were balanced by means of articulating paper and a force monitor (type F appliance). Bite forces were recorded when the subjects, without visual feedback, clenched maximally on the appliance. Similar recordings were made when contralateral molar and all contralateral contacts were removed (type R and type U appliances, respectively). Although the relation between individual forces often changed during the initial increase in force, it was generally constant around the maximum. The maximum forces at the four dental locations varied in distribution between subjects, but were characterized by posteriorly increasing forces. Forces in the anterior region (especially at the canine) significantly increased (up to 10 times) when clenching took place on unilateral contacts only (type U) as compared with fully balanced ones (type F). Bite force distribution thus changed with biting strength and the location of occlusal contacts. Increased force in the canine region during unilateral clenching seems related to the pattern of jaw muscle co-activation and the physical properties of the craniomandibular and dental supporting tissues which induce complex deformations of the lower jaw.

The electromyographic activity of the inferior part of the human lateral pterygoid muscle during clenching and chewing

The nature of activity in the two parts of the muscle is controversial. A reliable technique was developed for recording activity in its inferior part by means of an indwelling needle electrode. This part was most active during anteriorly- or contralaterally-directed intercuspal clenching, vertically-directed clenching with the jaw positioned to the contralateral side or anteriorly, and during jaw opening and least active during vertically-, ipsilaterally- or posteriorly-directed intercuspal clenching, and during vertically-directed clenches with the jaw positioned to the ipsilateral side. During chewing, activity appeared in the late intercuspal phase irrespective of the side used. It commenced earlier when chewing strokes were ipsilateral to the muscle. Activity during both ipsilateral and contralateral chewing strokes continued until maximum opening, when it ceased for the duration of the closing and crushing phases of the cycle. Thus the inferior part, with other muscles, may participate in bracing the condylar head against the articular eminence during vertical-clenching efforts involving condylar displacement, but not in the compressive or crushing phases of the cycle.

A computer-based system for the simultaneous measurement of muscle activity and jaw movement during mastication in man.

Abstract A computer-based system was developed in order to analyze the activity of 6 jaw closing muscles and the associated displacement, in 3 dimensions, of an incisor point on the mandible, during gum-chewing and clenching sequences. Signals were derived by surface electromyography from the right and left anterior temporal, posterior temporal and masseter muscles, and by means of a set of magnetometers which sensed the movement of a small magnet cemented to the lower incisor teeth. Sampling of those signals by the computer was locked in phase to the chewing cycle, and the digitized signals were conditioned and analyzed by software to permit quantitation of a wide variety of parameters. The system proved noninvasive and allowed repeated measurements to be made on different occasions. It is suggested that the technique should be useful in the study of masticatory mechanisms, and in assessing the effects of clinical alterations to the occlusion.