A model relating patterns of human jaw movement to biomechanical constraints

After testing the effects of the ways in which several different ligaments and bony constraints would influence movements of the human mandible in three dimensions, a mathematical model based on constraints due to the articular eminences, temporomandibular ligaments and sphenomandibular ligaments has been constructed. The effects of these constraints on jaw movements during opening and lateral movements are analysed. The model predicts the observed translation of the human condyle during jaw opening and Bennett shift during lateral jaw movements. The model is refined to account for observed irregular movements of the condyle during opening and to predict a locus for the instantaneous centre of rotation. The model can also be used to predict the new position taken up by any point on the mandible after the jaw has been opened and/or moved laterally a given amount.     

Differences in loading of the temporomandibular joint during opening and closing of the jaw

Kinematics of the human masticatory system during opening and closing of the jaw have been reported widely. Evidence has been provided that the opening and closing movement of the jaw differ from one another. However, different approaches of movement registration yield divergent expectations with regard to a difference in loading of the temporomandibular joint between these movements. Because of these diverging expectations, it was hypothesized that joint loading is equal during opening and closing. This hypothesis was tested by predicting loading of the temporomandibular joint during an unloaded opening and closing movement of the jaw by means of a three-dimensional biomechanical model of the human masticatory system. Model predictions showed that the joint reaction forces were markedly higher during opening than during closing. The predicted opening trace of the centre of the mandibular condyle was located cranially of the closing trace, with a maximum difference between the traces of 0.45 mm. The hypothesis, postulating similarity of joint loading during unloaded opening and closing of the jaw, therefore, was rejected. Sensitivity analysis showed that the reported differences were not affected in a qualitative sense by muscular activation levels, the thickness of the cartilaginous layers within the temporomandibular joint or the gross morphology of the model. Our predictions indicate that the TMJ is loaded more heavily during unloaded jaw opening than during unloaded jaw closing.     

Anterior displacement of the TMJ disk: repositioning of the disk using a Mitek system. A 3D finite element study

In this paper the behaviors of the temporomandibular joint (TMJ) with an anteriorly displaced disk without reduction and with a surgically repositioned one were compared with the response of a healthy disk during jaw opening. The movement of each joint was obtained imposing the same opening path between incisors and assuming that the movement of the condyle is determined by the passive action of the masticatory muscles and the restrictions imposed by the articulating surfaces and the ligaments. A fiber-reinforced porohyperelastic model was used to simulate the behavior of the articular disk. The influence of the friction coefficient in the diseased joint was also analyzed, finding that the final displacement of the complex condyle-disk was smaller as the friction coefficient increased. On the other hand, its displacement in the repositioned joint was different than in the healthy case because the artificial sutures used in the surgery do not fully stabilize the disk posteriorly as the retrodiscal tissue does. The stress response of the disk changed in both pathologic cases: in the displaced joint the highest stresses moved from the intermediate zone (healthy case) to the posterior band, and in the reconstructed one the most loaded zone moved posteriorly at total opening. Besides, local stress concentrations appeared in the neighborhood of the artificial sutures and therefore damage of the disk and releasing of the sutures might be possible postoperatively.     

Superficial,suprahyoid, and infrahyoid neck musculature in naked mole-rats (Heterocephalus glaber): Relative size and potential contributions to independent movement of the lower incisors

Naked mole-rats (Heterocephalus glaber) are fossorial, eusocial rodents that exhibit the unusual capability of moving their lower incisors independently in lateral and rostroventral directions. The evolution of this trait would presumably also involve concurrent alterations in neck musculature to support and control movements of the lower incisors. In order to assess morphological adaptations that might facilitate these movements, we performed detailed dissections of the neck musculature of adult naked mole-rats. In addition to characterizing attachment sites of superficial, suprahyoid, and infrahyoid musculature, we also quantified muscle mass and mandibular features thought to be associated with gape (condyle height, condyle length, and jaw length). Based on muscle attachment sites, the platysma myoides may contribute to lateral movement of the lower incisor and hemi-mandible in naked mole-rats. The large digastric muscle is likely to be a main contributor to rostroventral movement of each lower incisor. The geniohyoid and mylohyoid muscles also likely contribute to rostroventral movements of the lower incisors, and the mylohyoid may also produce lateral spreading of the hemi-mandibles. The transverse mandibular (intermandibularis) muscle likely serves to reposition the lower incisors back to a midline orientation following a movement.     

Biplanar X-ray motion analysis of the lower jaw movement during incisor interaction and mastication in the beaver (Castor fiber L. 1758)

The first biplanar X-ray motion analysis of mastication and food processing for Castor fiber is presented. While particles are chipped off interaction of incisors involves variable movements of the lower mandible and thus incisors. After jaw opening the tip of the lower incisors can reach different positions anteriorly of the upper incisors. Then the mandible moves upwards and backwards and brings the tips of the incisors into contact. The lower incisors slide along the wear facet of the upper to the ledge when the cheek teeth occlude. The glenoid fossa and lower jaw condyle are in close contact during incisor contact and no transverse movements are observed. Mastication involves interaction of the cheek teeth with no contact of the incisors. When the cheek teeth are in occlusal contact the mandible is moved forward and transverse, or mediolateral. In consecutive power strokes the jaw is moved alternately to the right and left side. When the jaw opens it is brought into a more central but not totally centred position. During mastication the condyles are positioned posteriorly to the glenoid allowing lateral movement of the mandible. The lateral movement is particularly noticeable in the anterior part of the mandible. With the lateral movements of the incisors one glenoid has to move posteriorly, the other anteriorly.     

Numerical simulation (FEM) of the human mandible: validation of the function of the masticatory muscles]

The article describes part of a research project aiming to develop a new modular software tool for the individual dynamic numerical simulation of the human mandible using the finite element method (FEM). Its planned use in the clinical setting makes it very important to validate the results of the simulations. Here, the function of the masticatory muscles is to be tested. On the basis of biomechanical data from the literature, standard movements, such as closing the mouth, forward movement, lateral movement or backward movement, were dynamically simulated. Apart from muscle activity, the movements of the mandible are defined by the temporomandibular joint. At present, translating the condylar dynamics to the simulation still poses problems. For this reason, therefore, simulations of the two extreme cases "fixed" and "force-free" condyles are compared. While in the case of fixed condyles, some of the movements could be reproduced either not at all or only weakly, in the case of force-free condyles, all standard movements were reproduced qualitatively, albeit without the guiding effect of the joint capsule or the articular disc.     

Mastication in springhares,Pedetes capensis: A cineradiographic study

Analysis of lateral and dorsoventral radiographic films shows that ingestion, transport, and mastication in Pedetes capensis (Rodentia) are cyclic and their movement patterns are essentially similar for the three food types offered. During the ingestion cycle, closing of the mouth is accompanied by a backward translation of the condyles, so that movement is predominantly orthal. During the opening stage, the extent of the anterior condylar translation is smaller. As a result the mandibular incisors move ventrally and posteriorly. During the ingestion cycles, food is transported to the back of the tongue, with the transverse rugae and the folds of the upper lip playing important roles. Springhares show a bilateral masticatory pattern; food is chewed on both sides simultaneously. During chewing, the condyles lie in their most forward position at maximum opening of the mouth. The mouth is closed by rotation of the lower jaw around the temporomandibular joint coupled with posterior condylar translation. At the beginning of the slow-closing stage, the upward rotation of the mandible slows and the jaw slowly shifts forward. During the grinding stage, the mandible is shifted forward with both toothrows in occlusion. During the opening stage, the jaw returns to its starting position. Comparison of kinematic and anatomical data on rodent mastication suggests that some dental characteristics form the most important factors regulating the masticatory pattern and consequently allow reasonably reliable prediction of rodent masticatory patterns.     

Electromyography and mechanics of mastication in the albino rat.

The masticatory apparatus in the albino rat was studied by means of electromyography and subsequent estimation of muscular forces. The activity patterns of the trigeminal and suprahyoid musculature and the mandibular movements were recorded simultaneously during feeding. The relative forces of the individual muscles in the different stages of chewing cycles and biting were estimated on the basis of their physiological cross sections and their activity levels, as measured from integrated electromyograms. Workinglines and moment arms of these muscles were determined for different jaw positions. In the anteriorly directed masticatory grinding stroke the resultants of the muscle forces at each side are identical; they direct anteriorly, dorsally and slightly lingually and pass along the lateral side of the second molar. Almost the entire muscular resultant force is transmitted to the molars while the temporo-mandibular joint remains unloaded. A small transverse force, produced by the tense symphyseal cruciate ligaments balances the couple of muscle resultant and molar reaction force in the transverse plane. After each grinding stroke the mandible is repositioned for the next stroke by the overlapping actions of three muscle groups: the pterygoids and suprahyoids produce depression and forward shift, the suprahyoids and temporal backward shift and elevation of the mandible while the subsequent co-operation of the temporal and masseter causes final closure of the mouth and starting of the forward grinding movement. All muscles act in a bilaterally symmetrical fashion. The pterygoids contract more strongly, the masseter more weakly during biting than during chewing. The wide gape shifts the resultant of the muscle forces more vertically and moreposteriorly. The joint then becomes strongly loaded because the reaction forces are applied far anteriorly on the incisors. The charateristic angle between the almost horizontal biting force and the surface of the food pellet indicates that the lower incisors produce a chisel-like action. Tooth structure reflects chewing and biting forces. The transverse molar lamellae lie about parallel to the chewing forces whereas perpendicular loading of the occlusal surfaces is achieved by their inclination in the transverse plane. The incisors are loaded approximately parallel to their longitudinal axis, placement that avoids bending forces during biting. It is suggested that a predominantly protrusive musculature favors the effective force transmission to the lower incisors, required for gnawing. By grinding food across transversely oriented molar ridges the protrusive components of the muscles would be utilized best. From the relative weights of the masticatory muscles in their topographical relations with joints, molars and incisors it may be concluded that the masticatory apparatus is a construction adapted to optimal transmission of force from muscles to teeth.     

Aspects of masticatory form and function in common tree shrews,Tupaia glis

Tree shrews have relatively primitive tribosphenic molars that are apparently similar to those of basal eutherians; thus, these animals have been used as a model to describe mastication in early mammals. In this study the gross morphology of the bony skull, joints, dentition, and muscles of mastication are related to potential jaw movements and cuspal relationships. Potential for complex mandibular movements is indicated by a mobile mandibular symphysis, shallow mandibular fossa that is large compared to its resident condyle, and relatively loose temporomandibular joint ligaments. Abrasive tooth wear is noticeable, and is most marked at the first molars and buccal aspects of the upper cheek teeth distal to P². Muscle morphology is basically similar to that previously described for Tupaia minor and Ptilocercus lowii. However, in T. glis, an intraorbital part of deep temporalis has the potential for inducing lingual translation of its dentary, and the large medial pterygoid has extended its origin anteriorly to the floor of the orbit, which would enhance protrusion. The importance of the tongue and hyoid muscles during mastication is suggested by broadly expanded anterior bellies of digastrics, which may assist mylohyoids in tensing the floor of the mouth during forceful tongue actions, and by preliminary electromyography, which suggests that masticatory muscles alone cannot fully account for jaw movements in this species.     

Analysis of the tendinous structure in human masticatory muscles.

In the masticatory muscles, the development of bundles of the tendon was examined: they were composed of many collagen fibers and a few elastic fibers. In the masseter muscle, the property of the tendon differs in the distribution and size of collagen fibers and elastic fibers in comparison with those of other masticatory muscles. This difference is concerned with the kinetic force for the stress or the stretch of each tendon and muscle during jaw movement.     

A Three-Dimensional Musculoskeletal Model of the Human Knee Joint. Part 1: Theoretical Construct

A three-dimensional model of the knee is developed to study the interactions between the muscles, ligaments, and bones during activity. The geometry of the distal femur, proximal tibia, and patella is based on cadaver data reported for an average-size knee. The shapes of the femoral condyles are represented by high-order polynomials; the tibial plateaux and patellar facets are approximated as flat surfaces. The contacting surfaces of the femur and tibia are modeled as deformable, while those of the femur and patella are assumed to be rigid. Interpenetration of the femur and tibia is taken into account by modeling cartilage as a thin, linear, elastic layer, mounted on rigid bone. Twelve elastic elements describe the geometry and mechanical properties of the cruciate ligaments, the collateral ligaments, and the posterior capsule. The model is actuated by thirteen musculotendinous units, each unit modeled as a three-element muscle in series with tendon. The path of each muscle is approximated as a straight line, except where it contacts and wraps around bone and other muscles; changes in muscle paths are taken into account using data obtained from MRI. In the first part of this paper, the model is used to simulate passive knee flexion. Quantitative comparisons of the model results with experimental data reported in the literature indicate that the relative movements of the bones and the geometry of the ligaments and muscles in the model are similar to those evident in the real knee. In Part II, the model is used to describe knee-ligament function during anterior-posterior draw, axial rotation, and isometric knee-extension and knee-flexion exercises.     

A Three-Dimensional Musculoskeletal Model of the Human Knee Joint. Part 1: Theoretical Construction

A three-dimensional model of the knee is developed to study the interactions between the muscles, ligaments, and bones during activity. The geometry of the distal femur, proximal tibia, and patella is based on cadaver data reported for an average-size knee. The shapes of the femoral condyles are represented by high-order polynomials: the tibial plateaux and patellar facets are approximated as flat surfaces. The contacting surfaces of the femur and tibia are modeled as deformable, while those of the femur and patella are assumed to be rigid. Interpenetration of the femur and tibia is taken into account by modeling cartilage as a thin, linear, elastic layer, mounted on rigid bone. Twelve elastic elements describe the geometry and mechanical properties of the cruciate ligaments, the collateral ligaments, and the posterior capsule. The model is actuated by thirteen musculotendinous units, each unit modeled as a three-element muscle in series with tendon. The path of each muscle is approximated as a straight line, except where it contacts and wraps around bone and other muscles; changes in muscle paths are taken into account using data obtained from MRI. In the first part of this paper, the model is used to simulate passive knee flexion. Quantitative comparisons of the model results with experimental data reported in the literature indicate that the relative movements of the bones and the geometry of the ligaments and muscles in the model are similar to those evident in the real knee. In Part II, the model is used to describe knee-ligament function during anterior-posterior draw, axial rotation, and isometric knee-extension and knee-flexion exercises.     

Trends in the Actions of Mammalian Masticatory Muscles

The actions of the masticatory muscles of a variety of mammalsin which feeding behavior and the configuration of the masticatoryapparatus differ have been reported. The most common approachused in these studies involves (1) obtaining a good anatomicalperception of the musculature, (2) deriving a theoretical modelof the actions of these muscles during jaw movement, and (3)testing this model by recording muscle activity and jaw movementssimultaneously. A catalogue of the activity patterns in eleven species of mammalsduring food reduction reveals certain trends in the actionsof the masticatory muscles. Horizontal jaw movements are generatedprimarily by differential activities of the deep temporalis,superficial masseter, and medial pterygoid. Vertical movementsand the maintenance of tooth to food contact apparently areproduced by action of the superficial temporalis, deep masseter,and zygomaticomandibularis. Thus, horizontal movements are seeminglygenerated by muscles having fibers arranged in marked anteroposteriordirection, whereas vertical movements are generated by muscleshaving more or less vertically arranged fibers. The asymmetry of jaw movement and the muscular activity generatingit suggest that mastication involves an interactionbetween anunbalanced and flexible functional unit (muscles) and a balancedand stable structural unit (skull and teeth). Thus, any unbalancingof the structural unit results in a further unbalancing of themasticatory process.     

Muscle-driven finite element simulation of human foot movements

This paper describes a finite element scheme for realistic muscle-driven simulation of human foot movements. The scheme is used to simulate human ankle plantar flexion. A three-dimensional anatomically detailed finite element model of human foot and lower leg is developed and the idea of generating natural foot movement based entirely on the contraction of the plantar flexor muscles is used. The bones, ligaments, articular cartilage, muscles, tendons, as well as the rest soft tissues of human foot and lower leg are included in the model. A realistic three-dimensional continuum constitutive model that describes the biomechanical behaviour of muscles and tendons is used. Both the active and passive properties of muscle tissue are accounted for. The materials for bones and ligaments are considered as homogeneous, isotropic and linearly elastic, whereas the articular cartilage and the rest soft tissues (mainly fat) are defined as hyperelastic materials. The model is used to estimate muscle tissue deformations as well as stresses and strains that develop in the lower leg muscles during plantar flexion of the ankle. Stresses and strains that develop in Achilles tendon during such a movement are also investigated.     

Muscle-driven finite element simulation of human foot movements

This paper describes a finite element scheme for realistic muscle-driven simulation of human foot movements. The scheme is used to simulate human ankle plantar flexion. A three-dimensional anatomically detailed finite element model of human foot and lower leg is developed and the idea of generating natural foot movement based entirely on the contraction of the plantar flexor muscles is used. The bones, ligaments, articular cartilage, muscles, tendons, as well as the rest soft tissues of human foot and lower leg are included in the model. A realistic three-dimensional continuum constitutive model that describes the biomechanical behaviour of muscles and tendons is used. Both the active and passive properties of muscle tissue are accounted for. The materials for bones and ligaments are considered as homogeneous, isotropic and linearly elastic, whereas the articular cartilage and the rest soft tissues (mainly fat) are defined as hyperelastic materials. The model is used to estimate muscle tissue deformations as well as stresses and strains that develop in the lower leg muscles during plantar flexion of the ankle. Stresses and strains that develop in Achilles tendon during such a movement are also investigated.     

Ultrastructural abnormalities of muscle spindles in the rat masseter muscle with malocclusion-induced damage

Human temporomandibular disorders due to disturbed occlusal mechanics are characterized by sensory, motor and autonomic symptoms, possibly related to muscle overwork and fatigue. Our previous study in rats with experimentally-induced malocclusion due to unilateral molar cusp amputation showed that the ipsilateral masseter muscles undergo morphological and biochemical changes consistent with muscle hypercontraction and ischemia. In the present study, the masseter muscle spindles of the same malocclusion-bearing rats were examined by electron microscopy. Sham-operated rats were used as controls. In the treated rats, clear-cut alterations of the muscle spindles were observed 26 days after surgery, when the extrafusal muscle showed the more severe damage. The fusal alterations affected predominantly capsular cells, intrafusal muscle fibers and sensory nerve endings. These results suggest that in the malocclusion-bearing rats, an abnormal reflex regulation of the motor activity of the masticatory muscles may take place. They also allow us to hypothesize that muscle spindle alterations might be involved in the pathogenesis of human temporomandibular disorders.     

Investigation of influencing variables on the computer-aided simulation of contacts in dynamic occlusion based on optically digitized plaster casts]

In dentistry, mechanical articulators with which mandibular movements can be reproduced in dentals casts play a major role. Commonly used semiadjustable articulators, however, have major limitations: On the one hand, the movement of the mandible is not reproduced exactly, on the other, they do not provide time-related information on jaw movement. Both problems can be solved by replacing the mechanical articulator by a digital simulation ("virtual articulator") based on digitized plaster casts and electronically recorded masticatory movements. We present a system for the 3D measurement of plaster casts in a skull-related, anatomical coordinate system using the fringe projection technique, and electronically recorded condylar movements. Using numerical algorithms, the contacts between upper and low jaw, and the angle of rotation of the temporomandibular joint can be computed for each movement in dynamic occlusion. Taking the data recorded from a patient as an example, the influence of the accuracy of the digitization of plaster casts on the computation of the rotation of the temporomandibular joint is discussed in relation to the anatomy of the masticatory apparatus.     

Mandibular movements of the albino rat during feeding

Jaw movements of albino rats during biting and mastication of relatively hard food were recorded by means of conventional and X-ray cinematography. Mandibular kinetics have been analysed in the context of passive mechanical limits imposed by jaw morphology, particularly of the joints, and by the food itself. Movements have been described in terms of degrees of gape, condylar translation and horizontal rotation of the rami about the symphysis. During biting the condyle remains in the anterior two-thirds of the fossa, moves forward as the jaw opens and the converse. The rami usually spread well apart; the lower incisors are usually approximated. Incised food particles are transported toward the molars by means of coordinated jaw and tongue movements. The prominent palatal rugae of the diastemal region abet this process. In the power stroke of mastication, the mandible shifts forward as the lower toothrows move a little inward; the condyles occupy the posterior two-thirds of the fossa. All movements seen were bilaterally symmetrical. Simultaneous chewing occurred on both sides. It is suggested that the lingual components in the primarily anterior power stroke enhance grinding efficiency. A movable symphysis appears to be of critical importance in facilitating this type of mastication.     

Occurrence and distribution of muscle spindles in masticatory and suprahyoid muscles of the rat.

The occurrence and distribution of muscle spindles was studied in histochemically and conventionally stained serial cross sections of 6-week-old and adult rat masticatory and suprahyoid muscles. Spindles were present in moderate to large numbers in jaw closers, but they were absent in jaw openers and two of four muscles of an accessory suprahyoid group. In jaw closers, 67% or more of the total spindle population was concentrated relatively distant from the temporomandibular joint, in muscle portions which contained large numbers of extrafusal fibers reacting strongly for oxidative enzymes. Because of their location, spindles in these portions should be stretched more and, subsequently, should respond with a greater afferent discharge at any given muscle length than spindles situated nearer to the joint. Spindles in jaw closers, especially the medial pterygoid and deep masseter, often occurred in clusters and complex forms near the terminal branching of intramuscular nerve trunks. No such concentrations were seen in the two muscles of the accessory suprahyoid group that had spindles. The association in jaw closers of spindles with extrafusal fibers high in oxidative enzyme activity is consistent with the view that spindles are the sensory component of a reflex system that recruits these fibers for finely-graded contractions in response to small internal length-changes of the muscle (Botterman et al., '78); however, in jaw openers and two muscles of the accessory suprahyoid group, the absence of spindles, coupled with the presence of large populations of extrafusal fibers high in oxidative enzyme activity, is not easily reconciled with this concept.     

Arch form,tooth size,and occlusomandibular kinesis in the Ceboidea

Correlations between dental morphology, arch configuration, and jaw movement patterns were quantitatively investigated in 23 ceboid species to elucidate integrative aspects of occlusal functional anatomy in an adaptive and evolutionary context. Differential maxillary-mandibular arch widths are primary in guiding lateral jaw movements. These movements are characterized according to their associated condylar shifts as either predominantly translatory or rotational. Predominantly translatory movements result from peripheral contact relationships between maxillary arches which are considerably wider posteriorly than their opposing mandibular arches. The greatest degree of mandibular movement is in the molar region in functional association with wide “primitive” maxillary molars, narrow mandibular molars, constricted maxillary intercanine widths, and narrow maxillary incisors. In contrast, predominantly rotational masticatory jaw movements result from differential arch widths which are greatest in the maxillary canine and incisor regions. Here most jaw movement is in the anterior segment and this is reflected in small maxillary-mandibular molar width differences, a high degree of premolarization, wide-set maxillary canine teeth, and wide maxillary incisors. Possible selectional factors in the putative evolution of rotational predominance in mastication from the more primitive translatory pattern are discussed.