http://cro.sagepub.com/Critical Reviews in Oral Biology & Medicinehttp://cro.sagepub.com/content/13/4/366The online version of this article can be found at:DOI: 10.1177/154411130201300406CROBM 2002 13: 366J.H. KoolstraDynamics of the Human Masticatory SystemPublished by:http://www.sagepublications.comOn behalf of:International and American Associations for Dental ResearchAdditional services and information for Critical Reviews in Oral Biology & Medicine can be found at:Email Alerts: http://cro.sagepub.com/cgi/alertsSubscriptions: http://cro.sagepub.com/subscriptionsReprints: http://www.sagepub.com/journalsReprints.navPermissions: http://www.sagepub.com/journalsPermissions.navDownloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010366 Crit Rev Oral Biol Med 13(4):366-376 (2002)IntroductionThere is a large body of literature describing the dynamics of thehuman musculoskeletal system (for a review, see, for instance,Nigg and Herzog, 1999). In this large and mostly well-developedfield, the human masticatory system occupies a relatively smallplace. One of the reasons for this underexposure is probably its relativecomplexity, which makes it more difficult to analyze than, forinstance, the system of the shoulder, arm, hip, knee, or leg.There are several reasons why masticatory dynamics are difficultto analyze. First, the masticatory system consists of a largenumber of muscles of various shapes and sizes, making it impossibleto determine, unambiguously, how they might cooperate toperform a certain task (Wood, 1987; Lund, 1991; Hannam andMcMillan, 1994). Second, they have a complex architecture(Schumacher, 1961; van Eijden et al., 1997), and their actions cannotbe determined from their overall orientation only (van der Helmand Veenbaas, 1991). Third, the upper and lower jaws articulatethrough two very complexly shaped incongruent temporomandibularjoints (Werner et al., 1991; Wish-Baratz et al., 1996). Anysimplification of these joints based on concepts usually used forother joints (like hinges or balls-and-sockets) leads to considerableloss of functionality (van Loon et al., 1999). Furthermore, the articularsurfaces are separated by a cartilaginous articular disc whichis able to move more or less freely between these surfaces (Bade etal., 1994; Schmolke, 1994) and are influenced by, and affect, themovements of the jaw (Rees, 1954; Isberg and Westesson, 1998).Apart from the intrinsic complexity of the system, there areseveral limitations to the collection of experimental data onmasticatory function. For instance, some of the masticatory

muscles run deep and are partially hidden behind bony structures,which prevents easy access for electromyographic (EMG)measurements (Wood et al., 1986; Koole et al., 1990; Murray etal., 1999a). Furthermore, many jaw movements are relativelysmall, posing stiff challenges to experimental systems designedto record relevant properties adequately (Naeije et al., 1996).In summary, there are many factors that impede assessmentof the mutual contributions of the relevant active and passivestructures to jaw movements. Recently, the application of biomechanicalmodels has provided an adequate experimental frameworkto explore masticatory dynamics without several of thedrawbacks that accompany experiments with human subjects.They are powerful tools for establishing causal relationships inthis field and have led to updates of or new formulations on variousinsights into the function of the masticatory system.Generally, monographs on jaw movement (Griffin andMalor, 1974; Brown, 1975; Goodson and Johansen, 1975) havebeen written from a more clinical perspective, and have providedinformation mainly about the position, and positionalchanges, of the lower jaw. The dynamic aspects and their consequenceswere rarely taken into account. Recent developments inthese areas have provided new insights. The present review isbased upon a selection of studies on jaw movement analysis,from both clinical and basic perspectives. Its purpose is to establishan updated overview of the fundamentals of jaw movement,and it focuses on the contributions of muscles and the influenceof passive constraints. It has been written from a biomechanicalperspective and with an emphasis on masticatory dynamics.Since relevant topics such as neural control and feedback as wellas properties of masticatory motor units have been reviewed relativelyrecently (Lund, 1991; Hannam and McMillan, 1994; vanEijden and Turkawski, 2001), they will not be discussed here.In section 1, the relevant anatomical properties of thehuman masticatory system are reviewed briefly. They serve asa basis for all aspects of jaw movement and its determinants.Jaw movement is discussed in section 2, where its physics andits properties are reviewed. Interactions among anatomicalstructures and jaw movement properties are reviewed in section3. An attempt is made to analyze the causal relationshipsbetween the two components. Finally, relevant issues that havenot yet been resolved, but are assumed to be of critical importancefor jaw movement analysis, are discussed in section 4.(1) The Human Masticatory SystemThe human masticatory system consists of a mandible which isable to move in relationship to the skull and is guided by twotemporomandibular joints through contractions of the masticatorymuscles. To establish the contribution of each individualstructure to jaw movements, one must explore the constructionof the joints and the muscular system as well as the mechanical

consequences of this construction. The morphology of thehuman masticatory system will be summarized very briefly.While there is a large quantity of literature in this area, the listof relevant citations in this article is far from complete.DYNAMICS OF THE HUMAN MASTICATORY SYSTEMJ.H. KoolstraDepartment of Functional Anatomy, Academic Centre for Dentistry Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, the Netherlands; j.h.koolstra@amc.uva.nlABSTRACT: In this review, the movement characteristics of the human masticatory system are discussed from a biomechanicalperspective. The discussion is based upon the three fundamental laws of mechanics applied to the various anatomical structuresthat are part of the masticatory system. An analysis of the forces and torques applied to the mandible by muscles, joints,articular capsules, and teeth is used to assess the determinants of jaw movement. The principle of relating the interplay of forcesto the center of gravity of the lower jaw, in contrast to a hinge axis near its joints, is introduced. It is evident that the musclesare the dominant determinants of jaw movement. The contributions of the individual muscles to jaw movements can be derivedfrom the orientation of their lines of action with respect to the center of gravity of the lower jaw. They cause the jaw to acceleratewith six degrees of freedom. The ratio between linear and angular accelerations is subtly dependent on the mass andmoments of inertia of the jaw, and the structures that are more or less rigidly attached to it. The effects of articular forces mustbe taken into account, especially if the joints are loaded asymmetrically. The muscles not only move the jaw but also maintainarticular stability during midline movements. Passive structures, such as the ligaments, become dominant only when the jawreaches its movement boundaries. These ligaments are assumed to prevent joint dislocation during non-midline movements.Key words. Jaw movement, masticatory muscles, biomechanics.Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010JOINTSMorphologyThe anatomy of the temporomandibular joint has beendescribed thoroughly (Rees, 1954). Mandibular movementsare guided by its articular surfaces (Brown, 1975; Williams etal., 1995). These surfaces reside on the temporal bone of theskull, involving an articular eminence and a mandibularfossa, and on the roughly ovoid condylar head of themandible. They are both irregularly shaped (Wish-Baratz etal., 1996), covered with fibrocartilage, and inaccessible fordirect measurements in vivo.The articular surfaces are separated by a cartilaginous articulardisc with non-uniform thickness (Bade et al., 1994;

Schmolke, 1994). This disc is able to move together with themandibular condyle along the articular eminence while simultaneouslyrotating on the condyle (Rees, 1954). Disc movementsgenerally run smoothly with respect to the articular surfaces.The articular disc is connected superiorly to the temporalbone and inferiorly to the mandible by relatively loose fibrousstructures. Together, these structures make up the articular capsule.It is reinforced laterally by the temporomandibular ligament,and is the only capsular structure that runs directlybetween the temporal bone and the mandible (Schmolke, 1994).Cadaver material reveals that the articular capsule is slack(Rees, 1954; Schumacher, 1983). There are two accessory ligaments:the sphenomandibular ligament, which runs mediallyfrom the mandibular ramus; and the stylomandibular ligament,which attaches to the mandibular angle from behind.Mechanical consequencesThe articular surfaces are highly incongruent, which meansthat the shapes of the upper and lower surfaces differ considerably.This allows for a large amount of motion at the cost ofa lessened joint stability and relatively small areas of joint contact.The articular disc is supposed to reduce joint incongruencyand increase joint stability by enlarging the contact area(Williams et al., 1995).A second consequence of the incongruency of the joint, incombination with the slackness of its capsule, is that the movementsin the joint are not restricted to rotations about more orless fixed joint axes, as in classic joints (Andrews and Hay,1983). The condyle and temporal bone can be regarded as twoseparate bodies in space, usually held in appositional contactwhen the jaw moves. As a consequence, the mandible may beable to move with six degrees of freedom. Theoretically, it mayrotate about an axis through, for instance, its incisor point.Therefore, the motion of this point per se bears no relationshipto condylar motion. This property is known as kinematicredundancy. Furthermore, if the incisor moves from one pointto another its path is not necessarily defined a priori. It maychoose to move along a straight path or along a detour. In principle,the number of possible paths is infinite.MUSCULAR SYSTEMMorphologyFrom a classic anatomical perspective, the masticatory musclesare divided into elevator and depressor groups. The elevatorgroup consists of the masseter and temporalis muscles, whichare located more or less superficially, and the medial pterygoidmuscle, which is located more deeply. The muscles of the depressorgroup are located in the floor of the mouth. This group consists(from superior to inferior) of the geniohyoid, the mylohyoid,and digastric muscles. The geniohyoid and mylohyoid musclesconnect the hyoid bone with the body of the mandible. The

digastric muscle connects the mastoid process of the skull withthe body of the mandible and is attached to the hyoid bone via afibrous loop which runs around its intermediate tendon. The lateralpterygoid muscle completes the muscular system. It consistsof a superior and inferior head running from the mandibularneck in forward and medial directions. Since both heads are consideredto have different actions, they cannot be regarded exclusivelyas elevator or depressor (Juniper, 1981).The elevator muscles are heavily pennate (Hannam andMcMillan, 1994; van Eijden et al., 1996, 1997). They have relativelylarge physiological cross-sectional areas and are suitablefor the generation of large forces. The fibers are short, whichlimits their capacity for active shortening during contraction.The depressor muscles and the lateral pterygoid have more orless parallel fibers and are therefore able to contract over alonger distance with less force.Mechanical consequencesThe human masticatory system contains more muscles than areapparently necessary to accomplish its tasks. This seems to beunnecessary from a mechanical perspective, but it must benoted that there are also spatial requirements to the constructionof the muscular system. For instance, a muscular systemthat is mechanically optimal probably violates spatial requirementswith respect to the adjacent airway and alimentary tract.The muscles can perform almost any task in various ways.Although the system is able to generate cyclic movements controlledby a central pattern generator (Lund, 1991; Ottenhoff etal., 1993), its muscles cannot be lumped into a limited numberof alternating muscle groups. One of the reasons for this is thatthey have to adapt constantly to the texture of the food betweenthe teeth (Thexton, 1992). The system is mechanically redundant,which means that there is an infinite number of musclecontraction patterns which can cause the same movement.It has been demonstrated that various masticatory muscleshave the capacity to deploy regionally different portionsfor different tasks (Møller, 1966; Wood, 1986; Miller, 1991;Blanksma and van Eijden, 1995; Blanksma et al., 1997; Murrayet al., 1999b). Such functional heterogeneity, in combinationwith a relatively large attachment area, may cause the directionof the line of action of such a muscle to vary as well (vander Helm and Veenbaas, 1991). While there is no a priori evidenceof co-activation between different muscle portions, thesystem is capable of fine-tuning, to a large extent, the orientationof the required muscle force (van Eijden et al., 1988) byselective activation of motor units (for reviews, see Hannamand McMillan, 1994; van Eijden and Turkawski, 2001). The relativelylarge extensive nature of some muscles may also causespatially distant fibers within a muscle to shorten to variousdegrees during mandibular movements (van Eijden and

Raadsheer, 1992; Hannam and McMillan, 1994; van Eijden etal., 1996, 1997). This may cause shifts in muscle lines of actionwhich are not caused by the nervous system.The depressors are directly or indirectly attached to thehyoid bone. When this bone moves downward through actionof the infrahyoid muscles during wide jaw-opening (Muto andKanazawa, 1994), the jaw depressors are stretched, which, inturn, lengthens their possibilities for active shortening. Thismay help in obtaining wider jaw gapes.(2) Jaw Movement BasicsDEGREES OF FREEDOM FOR JAW MOVEMENTIn three-dimensional space, a body able to move freely may performtranslations and/or rotations. This applies to the lowerjaw, although the degrees of the various movements are limited.Translations can be performed along, and rotations about, three13(4):366-376 (2002) Crit Rev Oral Biol Med 367Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010independent axes. The translation axes and the rotation axes arenot necessarily the same, but usually the three orthogonal axesof some Cartesian systems aligned to anatomical planes areused for this purpose. Translations can be described along axeswhich are, for instance, anteroposterior or X, mediolateral or Y,and supero-inferior or Z. Rotations can be defined by termssuch as azimuth (about the Z-axis), elevation (about the Y-axis),and roll (about the X-axis) (Fig. 1) or yaw, pitch, and roll(Baragar and Osborn, 1984). It must be noted that there aremany other conventions about sets of axes which are applicable.Independent of the applied set of axes, every movement can beexpressed by a unique combination of the six independent fundamentalmovements (which are known as the six degrees offreedom of movement). The lower jaw does not move freely butis guided by its joints. These structures, therefore, may reducethe number of degrees of freedom (vide infra). Although translationsand rotations relative to any of the three independent axesremain possible, they are no longer independent. For instance,if the joints should restrict one degree of freedom, the movementis completely determined by the other five.If the movement of a point is studied—for example, theincisor point of the lower jaw or a point representing thecondyle—it should be recognized that a point does not have anorientation. Rotations about axes through this point are thusmeaningless, and it should be recognized that movementsaccomplished by rotations about distant axes can also be performedby translations. A point, therefore, can move with, atmost, three degrees of freedom. The consequence is that themovement of any point on the jaw can be reconstructed fromthe movement of the jaw, but not the other way around.It is not easy to recognize functional aspects of jaw movement

from a combination of translations and rotations withrespect to pre-defined axes. An alternative way to describe amovement is by a rotation about and a translation along a socalledhelical axis or screw axis (Kinzel et al., 1972). A movementcan be described by subsequent (six degrees of freedom)small displacements. The orientation and location of the helicalaxis related to such a displacement (Fig. 2) provide informationas to how it took place, while the moving body translates alongand rotates about this axis. Generally, the helical axis is not stationaryand may itself undergo displacements during movement.Therefore, each instant of movement is connected to aunique instantaneous helical axis. For sagittal movements, thisaxis is directly analogous to the instantaneous center of rotationfor plane motion (Wu et al., 1988; Lindauer et al., 1995; Chen,1998). It should be noted that the location and inclination of thehelical axis, and the amounts of rotation about and translationalong this axis, contain six independent variables according tothe six degrees of freedom for movement.Instantaneous helical axes thus provide a completeoverview of jaw movement (Gallo et al., 1997, 2000; Koolstra andvan Eijden, 1997b; Chen and Katona, 1999; Gal et al., 2000). Themovement of teeth and condyles can be derived from them (Fig.2). The relative contributions of rotations and translations of themandibular condyle, for instance, can be determined from thedistance to the axis. If, at a certain instant, the condylar movementis characterized primarily by a rotation, the helical axis willbe situated close to the joint. If, in contrast, the translation componentis dominant, the helical axis will be located at a distantlocation. These differences were demonstrated for jaw-openingmovements performed with different muscle recruitment patternsshowing no clear visual differences from normal movementsin terms of displacement of teeth and condyles, but greatdifferences in terms of helical axis positions (Koolstra and vanEijden, 1997b). This emphasizes that, from a clinical perspectiveas well, this approach can be relevant, for example, to anenhanced possibility of discrimination among different translationsof the same type of movement by quantification of the inclinationand remoteness of the helical axis. Furthermore, it enableson to discriminate between and among apparently similarmovements caused by different muscle contraction patterns.PHYSICS OF JAW MOVEMENT: NEWTON’S LAWSThe dynamics of a moving lower jaw are expressed by its position,its velocity, and its acceleration. According to the six degreesof freedom for movement, each of these three properties also consistsof six independent variables. In a Cartesian system, the positionis not defined only by the (X, Y, and Z) position of the centerof gravity with respect to the origin of this system, but also by theorientation (azimuth, elevation, and roll) of the jaw. The velocity

and the acceleration also have three linear and three angular components.For each of the six components, velocity is the (time)derivative of position and acceleration the derivative of velocity.Every moving body, including the lower jaw, obeysNewton’s laws. Movements are caused by forces acting on thejaw. They may be active muscle forces and also passive (reaction)forces generated by joints, ligaments, and dental elements.The forces also have six components. Each linear force (Fx, Fy,Fz) is accompanied by a moment (angular) or torque (Mazimuth,Melevation, Mroll). The resultant forces and torques generate accelerationsaccording to Newton’s second law (acceleration equalsforce divided by mass) (Fig. 1). This accounts for each degree offreedom, emphasizing the fact that the mass of the jaw also consistsof three linear and three angular components. The three linearcomponents of the mass of the lower jaw are equal to thecommon mass. The three angular masses (moments of inertia)are dependent on the distribution of mass about the axis underconsideration and therefore on the shape of the lower jaw andadhering structures. The moment of inertia about an axis isdefined as the sum of the mass of each particle multiplied by thedistance between this particle and the relevant axis to the powerof two (Nigg, 1999). For a lower jaw of about 0.44 kg, themoments of inertia have been estimated as 8.6 kg.cm2, 2.9kg.cm2, and 6.1 kg.cm2 for Iazimuth (about the z-axis), Ielevation(about the y-axis), and Iroll (about the x-axis), respectively(Koolstra and van Eijden, 1995). This means that it requiresabout three times less muscle torque to accelerate the jaw foropen-close movements than for latero-deviations. The accelerationscause changes in (linear and angular) velocity, and thevelocities cause changes in (linear and angular) jaw position.Figure 1. Six degrees of freedom for jaw movement. Dashed lines:principal axes. a: (linear) accelerations. F: (linear) forces. m: mass. a:angular accelerations. M: torques. I: moments of inertia.368 Crit Rev Oral Biol Med 13(4):366-376 (2002)Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010INFLUENCE OF JOINTSThe degrees of freedom of articulating bones are often reduceddue to the construction of the connecting joint. An ideal balland-socket joint, for instance, does not allow for translations.Therefore, such a joint allows for movements with a maximumof three degrees of freedom. In contrast, the degrees of freedomin the temporomandibular joint are not reduced by its construction.While its articular capsule is relatively slack and its articulatingsurfaces incongruent, the mandibular condyle is able tomove almost freely in the three-dimensional half-space boundedsuperiorly by the articular surface of the temporal bone.

Articular contact is not necessarily maintained, although thedistance along which the condyle is able to move perpendicularto the articular surfaces is relatively short. Furthermore, thearticular surfaces are not rigid. The articular cartilage and thearticular disc are deformable such that the distance between thebony surfaces will be proportional to the joint load (HuddlestonSlater et al., 1999). Consequently, the mandible is also able tomove with six degrees of freedom. If the joints are assumed tomaintain articular contact all the time, and the joint contact isassumed to be rigid, a translation of the condyle in a directionperpendicular to the articular surface of the temporal bone isrestricted, and the number of degrees of freedom for condylarmovement is reduced to five. Furthermore, if both joints areassumed to be connected rigidly through the mandibular symphysis,the rotation of the lower jaw about an antero-posterioraxis is restricted. In this (simplified) situation, it is able to movewith four degrees of freedom (Schumacher, 1961).INFLUENCE OF MUSCLESThe jaw moves through contractions of the masticatory muscles.Each muscle contraction is associated with a force whichis expressed by three independent variables: its magnitude, itspoint of application, and its orientation. The latter two aredetermined by the muscle’s line of action, defined by thegeometry of the system. Each muscle can produce a translationof the lower jaw along its line of action, and a rotation aboutan axis perpendicular to it and running through the jaw’s centerof gravity (Stern, 1974; Koolstra and van Eijden, 1995), asillustrated in Fig. 3. The translation and rotation caused by amuscle are not independent, and they express only one degreeof freedom. Therefore, if such a muscle is activated homogeneously,the nervous system is able to influence only onedegree of freedom through the magnitude of its force. If themuscle can be activated heterogeneously, and is representedby more than one independent line of action, it can influencemore than one degree of freedom. Conversely, if separate musclesor muscle portions cannot be activated independently,then, together, they are able to influence only one degree offreedom. A system of muscles, therefore, is represented by anumber of degrees of freedom equal to the number of independentlines of action. The masticatory system contains atleast 20 muscle portions which may be activated independently(Fig. 4). The number of degrees of freedom of the muscularsystem, therefore, is considerably larger than the (maximum)six degrees of freedom of jaw movement. This causes amechanical redundancy in the masticatory system.(3) Determinants of Jaw MovementJaw movements caused by the masticatory muscles are guidedby passive structures; thus, both passive and active elementsgenerate forces and torques which accelerate the jaw (Fig. 5).

Because of the large number of these forces, their changes duringjaw movements, and their strong interdependency, it is difficultto separate these influences.ACTIVE ELEMENTSAlthough passive structures in the masticatory system may actas constraints for jaw movements and guide the mandiblealong its path, active masticatory muscles are the primemovers in this system. Therefore, it can be expected that theywill be the dominant determinants of jaw motion.Muscle lines of actionWhile muscle lines of action differ considerably between muscles,each contributes to masticatory movements in a uniquemanner. Furthermore, the lines of action depend on the positionof the lower jaw with respect to the skull. This causes continuouschanges in the interplay of muscle forces and torques.Investigators have estimated the lines of action of the masticatorymuscles, in vitro, by connecting the centers of theattachment areas (centroids) on the skull and the mandible(Baron and Debussy, 1979). However, functionally differentmuscle portions that share attachment areas cannot be discriminatedwith this approach. This drawback can be overcome bymeasurement of the orientation of fiber bundles and incorporationof the influence of tendinous sheets (van Eijden et al.,1997). Furthermore, this method facilitates the estimation offiber length as a function of jaw position (van Eijden et al., 1996;Koolstra and van Eijden, 1997b). The drawback of all in vitromethods is that results are not necessarily applicable to individualsubjects. Therefore, muscle lines of action have beenestimated in vivo by a determination of putative muscle attachmentpoints (Goto et al., 1995, 2001) or by fitting the long axisthrough “Magnetic Resonance Imaging” sections (Koolstra etal., 1990). Although this enables individual characteristics to beincorporated, the estimates of the lines of action remain coarse.Generally, the contribution of a muscle to jaw movementscan be established by the direction of its line of action and theposition of this line with respect to the center of gravity of thelower jaw. It accelerates the jaw in the direction of the line ofaction according to: a = F/m, where a is the linear accelerationvector, F the muscle force vector, and m the mass of the jaw. Also,an angular acceleration about the center of gravity occursaccording to: a = M/I, where a is the angular acceleration vector,M the muscle torque vector about the center of gravity, andI the moment of inertia vector. The moment of inertia is dependenton the related axis, whereas the mass is not (Koolstra andvan Eijden, 1995). The actual movement, then, is determined bythe resultant instantaneous linear and angular accelerations initiatedby the forces of all active and passive structures. For a singlemuscle, the ratio between angular and linear acceleration isproportional to the length of the moment arm of the muscle with

respect to the center of gravity. Furthermore, it is proportional tothe ratio between the mass of the lower jaw and its moment of13(4):366-376 (2002) Crit Rev Oral Biol Med 369Figure 2. Helical axis. (A) Rotations about and translations along a hypotheticalhelical axis during a non-midline jaw movement (dashed line). (B)Subsequent helical axes during jaw closure (after Gallo et al., 2000).Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010inertia relative to the axis of the muscle torque. These combinedfactors determine the effect of muscle contraction and, consequently,the contribution of each single muscle to jaw movement.It must be emphasized that these equations represent simplifieddynamics. To avoid excessive complexity, the terms relating to,for instance, inertial coupling, centripetal forces, and coriolisforces have been neglected (Nigg and Herzog, 1999).In a sagittal plane analysis, the lines of action of most jawclosersare directed upward, and those of the jaw-openers,downward and backward (Fig. 5). However, in both cases, eachline of action has a similarly directed moment with respect tothe sagittal axis through the center of gravity of the lower jaw.Jaw-closers and -openers are able to produce a similarly directedtorque about this axis which leads to an angular accelerationin the “negative elevation” (opening) direction. Consequently,almost every muscle pair that is activated symmetricallyattempts, aside from its specific action, to perform an openingrotation about the center of gravity. It is through this mechanismthat both jaw-closers and -openers, despite their differencein orientation, are able to maintain articular contact whileperforming unloaded (symmetrical) jaw movements.Muscle dynamicsThe optimum isometric force produced by a muscle Fopt is proportionalto its physiological cross-section S (in cm2) and its activationA (in %), as denoted in the equation Fopt = 37 x S x A (Weijsand Hillen, 1985). Due to the dynamic muscle properties (Fig. 6),the instantaneous force of a concentrically contracting muscle isless than Fopt, and an eccentrically contracting muscle may producean instantaneous force larger than Fopt. Due to these properties,the forces produced by masticatory muscles may change constantlyduring a movement, even though their activation levelsremain constant (Koolstra and van Eijden, 1997a). The forcelengthrelationship quantifies the property that enables a muscleto produce a force when its sarcomeres are not shortened below,or elongated beyond, certain lengths, and this property has beendemonstrated to be an important limiting factor for masticatorymuscle force development. For instance, the limited amount ofmaximum shortening of the fibers of the lateral pterygoid musclesprevent protrusion of the jaw beyond its normal limits (Koolstraand van Eijden, 1996). Furthermore, maximum jaw-opening is

limited by the maximum shortening of the jaw-openers, which iscounteracted by the passive forces of the elevators (Koolstra andvan Eijden, 1997b; Langenbach and Hannam, 1999). The lattereffect would be even more dramatic if the instantaneous center ofrotation remained close to the joint (Weijs et al., 1989).Muscle force is also dependent on the shortening velocitythrough the force-velocity relationship (Fig. 6). For jaw openclosemovements, where all jaw-opening or jaw-closing muscleswere activated simultaneously, it was demonstrated that the trajectoryof movement is not very dependent on the speed ofmovement (Koolstra and van Eijden, 1997b). While this trajectorydepends on mutual muscle forces, the force-velocity relationshipdoes not considerably affect the mutual ratio of instantaneousmuscle forces. Consequently, the possibilities for forceproduction are affected similarly in all contributing muscles(Koolstra and van Eijden, 1997a). In contrast, the force-velocityrelationship does appear to assist in deceleration of the lowerjaw after a sudden disappearance of resistance during forcefulbiting (Slager et al., 1997). When such an event occurs, the forceof the closing muscles can disappear instantly through the suddenlylarge shortening velocity, which may be considered profitablewhen the teeth are near occlusion, and there is little timeto activate the jaw openers to decelerate the jaw.Contribution of muscle action to jaw movementsJaw movements are performed through co-contraction of variousmuscles. Electromyographic (EMG) measurement of the masticatorymuscles during various jaw movement tasks (Carlsöö, 1952,1956a,b; Møller, 1966; Wood, 1987), therefore, cannot be used toestablish the individual contributions of the various muscles to amovement. For instance, it does not provide a means to decidewhether two or more active muscles assist each other to performa certain movement or work against each other to stabilize thesystem. EMG measurement has been used to detect functionalheterogeneity in the activation of different muscle portions (e.g.,Møller, 1966; Wood, 1986; Blanksma and van Eijden, 1990; Miller,1991; Blanksma et al., 1992; Murray et al., 1999b). For example, ithas been shown that the temporalis muscle shows a gradual heterogeneityin the antero-posterior direction, and that the massetermuscle can be subdivided functionally into a superficial, an anteriordeep, and a posterior deep portion.EMG registrations of masticatory muscles correlate muscle370 Crit Rev Oral Biol Med 13(4):366-376 (2002)Figure 3. Force and torque generated by a muscle (arrow) withrespect to the center of gravity of the lower jaw.Figure 4. Overview of the masticatory system. Ventro-lateral view.Continuous lines: muscle lines of action. Cross-bar: muscle origin.Circle: muscle insertion. MAS_S: superficial masseter. MAS_P: deepmasseter. MPT: medial pterygoid. TEM_A: anterior temporalis. TEM_P:

posterior temporalis. LPT_S: superior lateral pterygoid. LPT_I: inferiorlateral pterygoid. DIG: digastric. GEH: geniohyoid. MYH: mylohyoid.Dots: position of centers of right and left condyle and incisor point.Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010activity with jaw movements or static bite tasks. These correlations,however, do not necessarily reflect causal relationships.For causal relationships between muscle contraction and jawmovement to be established, the influence of the passive constraintsmust also be taken into account. Generally, this influencecan be simplified to a screw displacement axis defined by themovement constraints in the joint (Andrews and Hay, 1983). Thefunction of a muscle can then be defined by its moment withrespect to this pre-determined axis. This concept was adopted byGrant (1973) and applied to the instantaneous center of rotationduring jaw movements. Unfortunately, the method applies tojoints influenced by only one degree of freedom by any muscle.Since the temporomandibular joint allows for movements withat least four degrees of freedom, this kind of analysis is irrelevantboth for static situations (when the influence of the joint force isomitted) and for dynamic situations (when torques areexpressed with respect to the center of gravity; Stern, 1974). Incontrast, masticatory muscle function is not dependent on thelocation of the center of rotation. Conversely, the location of theinstantaneous center of rotation (or instantaneous helical axis fornon-midline movements) is dependent on the actions of the masticatorymuscles. For instance, it has been demonstrated thatjaw-open movements can be performed with different musclerecruitment patterns. Despite the relatively similar appearanceof these movements, the trajectories of the instantaneous centersof rotation are very different (Koolstra and van Eijden, 1997b). Ifone wishes to estimate the work done by a muscle force duringa movement, then its position with respect to the instantaneoushelical axis is relevant (Gal et al., 2000).A causal relationship between masticatory muscle contractionand jaw movement can be demonstrated experimentallyonly when muscles are activated independently. This is not anoption in a regular experimental setting, nor is it likely evenwith intensive training, due to the central organization of motorcontrol in the jaw muscles (Lund, 1991). An effective method,however, is registration of jaw movements evoked by electricalstimulation of isolated muscles. This can be done with indwellingelectrodes (Zwijnenburg et al., 1996, 1999) or by electromagneticstimulation of selected portions of the motor cortex(McMillan et al., 1998). The amplitude of such stimulation is necessarilyrestricted, to protect the experimental subject. Thus, theevoked jaw movements are small and require a very sensitivejaw-tracking device to be recorded. When the masticatory musclesare not fully relaxed, the other muscles may easily disturb

the measurements. Even if complete rest can be obtained (forinstance, in the unconscious state), the passive muscle forcesmay be relatively large compared with the evoked force andcannot be ignored. Consequently, direct muscle stimulation hasnot yet convincingly demonstrated the contributions of individualmasticatory muscles to jaw movements.Though direct experimental methods have failed to disclosethe full functional potential of the masticatory muscles,biomechanical modeling approaches can help to develop reliablepredictions on this subject. Such models are always simplifications;care must be taken that all relevant properties beincluded, and correct assumptions regarding model behaviormust be made. In one model, for instance, it has been predictedthat the jaw-opening muscles have the tendency to dislocate thetemporomandibular joint (Osborn, 1993), while in another, ithas been suggested that they have the tendency to stabilize thejoint (Koolstra and van Eijden, 1997b). In the former model,torques were computed not with respect to the center of gravity(Stern, 1974) but with respect to the joint, thereby neglectingthe law of conservation of angular momentum (Nigg andHerzog, 1999). In contrast, in the latter model, all relevant propertiesof (Newtonian) rigid body dynamics were implemented,strengthening the reliability of its predictions. According tothese rigid body dynamics, the temporomandibular ligamentsplay an insignificant role during symmetrical jaw movements. Itwas demonstrated that activation of all jaw-closing musclessimultaneously leads to natural-looking jaw-closing movements,including a condylar movement similar to that observedexperimentally (Koolstra and van Eijden, 1995). Conversely,activation of all jaw-opening muscles resulted in normal jawopeningmovements (Koolstra and van Eijden, 1997b). It is possiblethat the varying instantaneous center of rotation can beused as a means for the assessment of different muscle recruitmentpatterns during apparently similar jaw movements.With the exception of the anterior temporalis and the superficialmasseter, the principle that muscles, when activated unilaterally,generate a translation along their line of action and a rotationabout the center of gravity has been confirmed. When theirtorque about the vertical axis through the center of gravity isconsidered, a contralateral latero-deviation would be expected,but instead, an ipsilateral latero-deviation occurs (Zwijnenburget al., 1996, 1999). This apparent paradox can be resolved if onetakes into account that joint loads caused by muscle forces alsocontribute to jaw movement (Fig. 7). Both the anterior temporalisand superficial masseter tend to tilt the contralateral condylefrom the articular eminence because of a large ipsilateral jointload. The ipsilateral joint reaction force results in an ipsilaterallydirected joint torque about the vertical axis through the center ofgravity (Koolstra and van Eijden, 1999), which overcomes the

muscle torque to cause an ipsilateral latero-deviation.PASSIVE STRUCTURESPassive structures contribute to jaw motion because they havethe ability to resist its movements along one or more degrees13(4):366-376 (2002) Crit Rev Oral Biol Med 371Figure 5. Forces acting on the lower jaw in the sagittal plane. Crosshairs:center of gravity. Fclosers: mean force of the jaw-closing muscles.Fopeners: mean force of jaw-opening muscles. Fjoint: joint force. Fbite:bite force. a: moment arm of the different forces.Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010of freedom. This resistance is expressed by the structure’s abilityto generate a reaction force and/or torque. Furthermore,the jaw itself contributes to its movements through its inertialproperties. The ratio between linear and angular accelerationseffected by a muscle is subtly dependent on the mass andmoments of inertia of the jaw, and all structures that are moreor less rigidly attached to it. This attachment may include thepart of the masticatory muscles attached to the mandible, thetongue, skin, and other soft tissues. It has been demonstrated,however, that the influence of these inertial properties on thefinal movement is small (Koolstra and van Eijden, 1995).Articular surfacesWhen the temporomandibular joints are loaded, reaction forcesare transferred to the mandibular condyles. The cartilaginousstructures in the joint have very low friction, and their deformationis fairly independent on loading direction (Beek et al., 2000),so it is probable that these reaction forces are directed perpendicularto the contacting articular surfaces. In a sagittal plane,they are directed inferiorly and pass posteriorly to the center ofgravity of the lower jaw. These forces, therefore, have the tendencyto lower the condyle and separate the articular surfaces(Fig. 5). The joint reaction forces also apply torques with respectto the center of gravity of the lower jaw. While the line of actionof the reaction forces passes posteriorly to this center, the jointreaction torques lead to an angular acceleration about the sagittalaxis through the center of gravity, which is bound to producean elevation movement in the positive direction. Consequently,the reaction forces attempt to perform a closing jaw rotationabout this axis (Koolstra and van Eijden, 1995).The cartilaginous temporomandibular joint disc and thecartilage lining of the articular surfaces are deformable, with afinite and non-linear elasticity (Beek et al., 2001). This elasticitycauses the joint reaction force to be dependent on the deformation.The more both bony articular surfaces move toward eachother, the more the cartilaginous structures between these surfacesare compressed, and the larger themagnitude of the reaction force.Consequently, even if articular contact is

maintained during static and dynamic situations,some movement perpendicular to thearticular surfaces occurs. This is supportedindirectly by the finding that the condylarpath during an unloaded opening lies superiorto the path during an unloaded closingmovement (Yatabe et al., 1997; HuddlestonSlater et al., 1999). The consequence of thisobservation is that joint loading during(unloaded) jaw opening is most likely largerthan that during unloaded jaw closing.Furthermore, it supports the suggestion thatthe joints are loaded by the torque of the jawopeningmuscles (Koolstra and van Eijden,1997b).The movement range of the condyle isnot limited to any great extent by the articularsurfaces of the skull. Only when thejoints are compressed in an upward orobliquely backward direction by manipulationcan the fossa restrain jaw movements inthe region of the condyles. The articular surfacesdo not restrict protrusive and mediolateraltranslations or rotations about any ofthe three axes. If the mandible performs alatero-deviation, the contralateral condylehas to move forward relative to the ipsilateralone, due to the interconnection of both condyles. Duringthis movement, it is forced to move downward along the articulareminence. Consequently, this movement includes notonly an azimuth rotation, but also a roll rotation (Fig. 8). Whilethese rotations are interdependent, they nevertheless implyonly one degree of freedom.Articular capsule and ligamentsThe bony parts of the temporomandibular joint are connectedby an articular capsule composed of relatively loose collagenousfiber bundles organized in a more or less parallel fashion(Schmolke, 1994). On the lateral side, it is reinforced by a temporomandibularligament. The fibers are able to withstandsome stretching. They deform according to their (non-linear)elastic properties (Woo et al., 1993) and, in doing so, generatetensile forces. These forces may decelerate the attachedcondyle when it moves away from the articular surface of thetemporal bone. A similar mechanical function can be attributedto the accessory ligaments, though these are very thin(Williams et al., 1995) and their function is probably negligible.For quite a while, the temporomandibular ligaments havebeen considered a dominant constraint for condylar movement

and, therefore, for jaw movement. This has been illustrated bythe construction of more or less fixed axes for mandibular rotations,especially in the final phase of jaw closure, and the ideahas been applied widely in the areas of prosthetic dentistry(Ramfjord and Ash, 1966) and temporomandibular dysfunctionrehabilitation (Crawford, 1999). Presently, there is a considerableamount of evidence that does not support the presence of afixed hinge axis (Lindauer et al., 1995; Chen and Katona, 1999)in normal functional movements. Another dominant role for thetemporomandibular ligaments has been proposed for symmetricaljaw-opening movements. Here, it has been assumed thatthe temporomandibular ligament is always taut and forces thecondyle to slide down the articular eminence (Osborn, 1993).However, if the temporomandibular ligament is taut, it cannot372 Crit Rev Oral Biol Med 13(4):366-376 (2002)Figure 6. Dynamic muscle properties. Total force is the sum of the forces produced by thesarcomeres (Fsarcomeres) The active force (Factive) is dependent on the activation through theactivation dynamics, the instantaneous sarcomere length, and contraction velocity. Theparallel elastic force (Fpassive) is dependent on the instantaneous sarcomere length.Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 201013(4):366-376 (2002) Crit Rev Oral Biol Med 373allow for habitual latero-deviation movements unless there is amechanism for tightening the ligaments during midline movementsand for slackening them during non-midline movements.It is very unlikely that such a mechanism exists. These considerationsfavor a less dominant role for the temporomandibularligaments in controlling habitual jaw movements.The temporomandibular ligament may play a dominantrole in preventing the mandibular condyle from movingbeyond the limits of the articular surface of the temporal bone.It has been shown that the masticatory muscles could pull thecondyles a few mm beyond these limits if the temporomandibularligaments were not present (Koolstra and vanEijden, 1999; Koolstra et al., 2001). Due to the interconnectionof left and right condyles through the mandibular symphysis,the two ligaments are sufficient to limit both antero-posteriorand medio-lateral condylar movements, during protrusiveand retrusive mandibular movements, and during latero-deviations,respectively. Furthermore, when the mandible reachesthese positions, the contracting masticatory muscles havealmost reached the length where they become insufficient forforce production (Koolstra and van Eijden, 1999). The forcesapplied to the ligaments will then probably not be large.Teeth and food

The direct influence of teeth and food on jaw movements is dueto the reaction forces occurring when the upper and lower teethcome into direct contact with each other or with a bolus of foodin between. Through the interplay of muscle and joint forces,these reaction forces will be predominantly directed downward(Koolstra et al., 1988) and will be accompanied by an openingtorque with respect to the center of gravity (Fig. 5), thus causinga joint loading. There is also an indirect influence, since the centralnervous system is able to detect forces on the dental elementsthrough mechanoreceptors in the periodontal ligament(Lund, 1991). This system, therefore, is able to adapt muscleactivation as required by the presence of food. Furthermore, thenervous system is able to react even faster through reflexes.The reaction forces are due to the resistance to deformationby the underlying structures. The dental elements are very hard,and should they come into contact with each other with somevelocity, even a small deformation could result in a very largereaction force. Fortunately, the dentition is connected to themandible via a deformable, collagenous periodontium. When atooth of the lower dentition comes into contact with its upperneighbor with a certain velocity, it undergoes little deformationbut is pushed into its socket. Due to the elasticity of the periodontium,a reaction force occurs which becomes larger as thetooth is pushed further into its socket. This reaction force acts onthe mandible and causes deceleration until the movement stops.If food is compressed between the teeth, the reaction forceincreases more slowly, while the bolus itself is deformable.The direct influence of the teeth on jaw movements isreflected by the superior portion of the Posselt envelope ofincisal point motion, but the dentition can also have an indirectinfluence on jaw movements. It has been demonstratedthat subjects with malocclusion have a more irregular chewingpattern than normally found (Gibbs et al., 1971; Lewin, 1985).It is not clear whether these aberrant patterns are due to toothcontacts themselves or to external factors.During mastication, food is compressed and/or fracturedbetween the dentition to reduce the particle size and facilitateswallowing. This compression and fracturing take place in theslow-closing phase of the masticatory cycle. It is apparent that themovement in this phase will be dependent on the mechanicalproperties of the food. For tough foods, the compression will beslower than for soft foods. Notably, the muscles are able to generatelarger forces when contracting slowly. The peak velocity thatfollows the fracturing of hard, brittle food is much greater thanthat for soft food (Peyron et al., 1997). This peak velocity, however,is less than might be expected, possibly due to a decrease in muscleforce as a consequence of the force-velocity relationship (Slageret al., 1997). Also, the size of the food affects mandibular movements,since the mandible has to be opened wider for larger pieces

of food to be chewed. In the frontal plane, it has been observedthat subjects chewing hard food tended to perform larger lateralexcursions than when chewing soft food (Lewin, 1985).Impact loads on the dentition may have consequences forthe joints, since they transfer to the joints via the mandible. Ahealthy periodontium partially absorbs impact loads, and thus,it may prevent peak loads on the joints. This property does notexist if the dentition is connected with the mandible through anartificial implant. The mandible itself, however, is deformable(van Eijden, 2000), so it is possible that the transfer of impactloads of the teeth to the joints may be reduced by its elasticity.At present, there are no quantitative data on this subject.MusclesWhen inactive, the masticatory muscles generate passive forceswhich are dependent on the instantaneous length of their sarcomeres(Epstein and Herzog, 1998). When the sarcomeres areat or below optimum length, estimated at 2.73 μm (van Ruijvenand Weijs, 1990), they are negligible, but increase exponentiallyif they are stretched beyond this length. Apart from thesepassive forces, muscle stretch can, indirectly, cause reflexes,because it is detected by muscle receptors (Lund, 1991).The passive forces of the jaw-closing muscles are believedto decelerate the jaw at the end of jaw opening during mastication(Ostry and Flanagan, 1989) and become significant whenthe jaw nears its maximum opening (Koolstra and van Eijden,1997a). It has been proposed that they are a determinant of maximumjaw opening (Langenbach and Hannam, 1999; Peck et al.,2000; Koolstra et al., 2001). Mathematical models applied to thestudy of the passive forces of the masticatory muscles have beenunable to open the jaw more than about 3 cm, whereas an openingof 6 cm is frequently observed in vivo (Posselt, 1962; Brown,1975). Therefore, the quantitative nature of these predictions isdisputable. Due to the proposed exponential relationshipbetween the passive muscle forces and their sarcomere lengths,small errors in the constants that determine this relationshipmay lead to relatively large errors in the projected passiveFigure 7. Schematic overview of the possible actions generated by forceFm of the superficial masseter or anterior temporalis muscle viewed inthe horizontal plane. (A) Possible rotations: Rg, about center of gravity;Rr, about a vertical axis behind the right joint for a latero-deviation tothe right; and Rl, about a vertical axis behind the left joint for a laterodeviationto the left. (B) Influence of joint forces: am, moment arm ofmuscle force; Frj, right joint force; arj, moment arm of right joint force;Fjl, left joint force; and ajl, moment arm of left joint force.Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 2010374 Crit Rev Oral Biol Med 13(4):366-376 (2002)forces. As long as there are no accurate quantitative data on the

relationship between sarcomere length and passive force of thehuman masticatory muscles, this issue remains uncertain.INTERPLAY OF PASSIVE AND ACTIVE STRUCTURESThe most dominant determinants for jaw movement are theforces generated by active muscles. Passive forces may modulatejaw movements, but become dominant as the jaw reachesits movement boundaries. Axes of rotation of the jaw duringfree jaw movements are not primarily related to passive structuresof the masticatory system, but are determined by muscleuse. Nevertheless, in clinical practice, axes of rotation whichwere assumed to be connected to the temporomandibular jointhave been measured and applied successfully for the diagnosisand treatment planning of masticatory dysfunction (Roth andWilliams, 1996). This indicates that conclusions drawn fromstudies which were performed to search for practical solutionsto a clinical problem cannot be automatically extrapolated toother problems in particular or to jaw movements in general.The influence of the passive constraints appears to be moredominant as jaw movement deviates from the midline.Dynamic biomechanical analysis has demonstrated that themasticatory muscles are capable of maintaining the integrity ofthe masticatory system, in most cases, without the need for anarticular capsule with ligaments to maintain articular apposition(Koolstra and van Eijden, 1997b). In contrast, they appearto play a role in reducing the medio-lateral movements of themandibular condyle during latero-deviation (Koolstra and vanEijden, 1999). If the joints are loaded asymmetrically, the influenceof their reaction forces on jaw movement has to be considered.When a muscle is activated unilaterally, the condylarreaction forces may produce a reverse movement comparedwith the one expected from the muscle’s line of action (videante). In practice, however, the muscles contract as groupsrather than in isolation. For both midline and non-midline jawmovements, dynamic muscle properties should be taken intoaccount, since they limit the force-producing capacities of themuscles, thereby restricting jaw movement possibilities.(4) Final RemarksJaw movement analysis has evolved from early observation(Ulrich, 1896; Bennett, 1908) to experiments designed to formulateand validate or falsify testable hypotheses. In particular,the availability of dynamic biomechanical modeling methodshas accelerated our understanding of jaw movements andthe masticatory system (Koolstra and van Eijden, 1995, 1997b,1999; Hannam et al., 1997; Langenbach and Hannam, 1999;Peck et al., 2000). It is now possible to predict the actions of thedifferent muscles in this complex system by applyingNewton’s laws. It has become clear that the masticatory musclesnot only control jaw movements, but also maintain thephysical integrity of the masticatory system. However, during

relatively large medio-lateral excursions, the muscles may failto keep the articular components in apposition, at which timethe articular ligaments may be presumed to perform this role.Although these developments have improved our knowledgeof the working of the human masticatory system, thepersistence of some outdated theories is striking. A prominentexample is related to the articulation (Gysi, 1910), where thereis a need to find a simple, reliable method for describing jawmovements near dental occlusion in anatomically differentpatients. Often, researchers and clinicians have attempted touse hinge axes to describe this movement. However, it hasbeen known, since the end of the 19th century, that such axesare non-existent during habitual jaw movements (Ulrich,1896). The concept of a hinge axis may have been revived bythe demonstration that rotary jaw movement near occlusioncan be accomplished through manipulation on cadaverousmaterial (Rees, 1954). This concept has survived in clinicalpractice, where, despite a lack of a scientific basis (Mohl et al.,1990; Lindauer et al., 1995), it has been applied successfully fora long period (Roth and Williams, 1996).While, for instance, with the use of dynamic biomechanicalmodels, hypotheses regarding muscle actions in the functioningmasticatory system are being validated (Koolstra and vanEijden, 1999), joint load predictions have not yet been verifiedsatisfactorily. This is a consequence of the fact that direct measurementof temporomandibular joint loading without disturbingarticular integrity has remained impossible. Unfortunately,this parameter is considered as a major influence on the developmentof wear and degeneration of the cartilaginous and bonystructures of this joint. Insight on temporomandibular joint loadingis, therefore, still limited to model predictions, and the reliabilityof these predictions is directly related to the assumptionsand parameters built in such models. These include joint morphologyand the material properties of its deformable structuresthat contribute to load distribution. Furthermore, muscle tensionsapplied during joint loading are required. In each subject,these parameters may be different, leading to the need for in vivomeasurement. Progress has been made in reconstructing the relevantmuscle lines of action (Koolstra et al., 1990, 1992) and bonyparts of the joints (Krebs et al., 1994) in vivo, but, to date, no reliablemethod is available to create reconstructions of the cartilaginoustissues in the joints. For a complete overview of theapplied muscle tensions to be acquired, their physiological crosssections,architecture, and degree of activation must be estimated.Physiological cross-sections have been estimated fromanatomic cross-sections (Weijs and Hillen, 1984; Koolstra andvan Eijden, 1992) and muscle activation from EMG recordings(Carlsöö, 1952, 1956a,b; Møller, 1966; Lehr et al., 1971; Widmalmet al., 1988; Murray et al., 1999a,b), but it is questionable whether

such methods can be applied routinely to all muscles involved.Masticatory muscle architecture has been studied in vitro (vanFigure 8. Rotations about vertical and horizontal axes during a latero-deviation movement.Downloaded from cro.sagepub.com at Queen Mary, University of London on October 31, 201013(4):366-376 (2002) Crit Rev Oral Biol Med 375Eijden et al., 1997), but the influence of individual variations onmodel predictions and the possibility of applying relevant correctionshave not been established.A start has been made on assessing the dynamic materialproperties of the cartilaginous structures in the human temporomandibularjoint (Beek et al., 2001), but the nature andinfluence of individual variations are subject to speculation.Consequently, although much is known qualitatively, quantificationof joint forces that incorporate individual variation stillcannot be performed unambiguously.The ultimate limiting factor for reliable masticatory functionanalysis incorporating biological variation is the lack of knowledgeabout masticatory muscle recruitment patterns. Themechanical redundancy of the masticatory system prevents theirunambiguous prediction (Koolstra and van Eijden, 2001). Theforces generated by the active muscles are the most dominantdeterminants of jaw movement and joint loading. The search fora rational way to predict muscle recruitment patterns remains adominant challenge in the field of jaw movement analysis.AcknowledgmentsThis research was supported by the Interuniversity Research School ofDentistry, through the Academic Center of Dentistry Amsterdam. I am gratefulto Prof. T.M.G.J. van Eijden for his critical suggestions.REFERENCESAndrews JG, Hay JG (1983). Biomechanical considerations in the modelingof muscle function. Acta Morph Neerl-Scand 21:199-223.Bade H, Schenck C, Koebke J (1994). The function of discomuscularrelationships in the human temporomandibular joint. Acta Anat151:258-267.Baragar FA, Osborn JW (1984). A model relating patterns of human jawmovement to biomechanical constraints. J Biomechan 17:757-767.Baron P, Debussy T (1979). A biomechanical functional analysis of themasticatory muscles in man. Arch Oral Biol 24:547-553.Beek M, Koolstra JH, van Ruijven LJ, van Eijden TMGJ (2000). Threedimensionalfinite element analysis of the human temporomandibularjoint disc. J Biomechan 33:307-316.Beek M, Aarnts MP, Koolstra JH, Feilzer AJ, van Eijden TMGJ (2001).Dynamical properties of the human temporomandibular joint disc.J Dent Res 80:876-880.

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