Simulation of dental collisions and occlusal dynamics in the virtual environment.

Semi-adjustable articulators have often been used to simulate occlusal dynamics, but advances in intra-oral scanning and computer software now enable dynamics to be modelled mathematically. Computer simulation of occlusal dynamics requires accurate virtual casts, records to register them and methods to handle mesh collisions during movement. Here, physical casts in a semi-adjustable articulator were scanned with a conventional clinical intra-oral scanner. A coordinate measuring machine was used to index their positions in intercuspation, protrusion, right and left laterotrusion, and to model features of the articulator. Penetrations between the indexed meshes were identified and resolved using restitution forces, and the final registrations were verified by distance measurements between dental landmarks at multiple sites. These sites were confirmed as closely approximating via measurements made from homologous transilluminated vinylpolysiloxane interocclusal impressions in the mounted casts. Movements between the indexed positions were simulated with two models in a custom biomechanical software platform. In model DENTAL, 6 degree-of-freedom movements were made to minimise deviation from a straight line path and also shaped by dynamic mesh collisions detected and resolved mathematically. In model ARTIC, the paths were further constrained by surfaces matching the control settings of the articulator. Despite these differences, the lower mid-incisor point paths were very similar in both models. The study suggests that mathematical simulation utilising interocclusal 'bite' registrations can closely replicate the primary movements of casts mounted in a semi-adjustable articulator. Additional indexing positions and appropriate software could, in some situations, replace the need for mechanical semi-adjustable articulation and/or its virtual representation.

Current computational modelling trends in craniomandibular biomechanics and their clinical implications.

Computational models of interactions in the craniomandibular apparatus are used with increasing frequency to study biomechanics in normal and abnormal masticatory systems. Methods and assumptions in these models can be difficult to assess by those unfamiliar with current practices in this field; health professionals are often faced with evaluating the appropriateness, validity and significance of models which are perhaps more familiar to the engineering community. This selective review offers a foundation for assessing the strength and implications of a craniomandibular modelling study. It explores different models used in general science and engineering and focuses on current best practices in biomechanics. The problem of validation is considered at some length, because this is not always fully realisable in living subjects. Rigid-body, finite element and combined approaches are discussed, with examples of their application to basic and clinically relevant problems. Some advanced software platforms currently available for modelling craniomandibular systems are mentioned. Recent studies of the face, masticatory muscles, tongue, craniomandibular skeleton, temporomandibular joint, dentition and dental implants are reviewed, and the significance of non-linear and non-isotropic material properties is emphasised. The unique challenges in clinical application are discussed, and the review concludes by posing some questions which one might reasonably expect to find answered in plausible modelling studies of the masticatory apparatus.

A dynamic model of jaw and hyoid biomechanics during chewing.

Our understanding of human jaw biomechanics has been enhanced by computational modelling, but comparatively few studies have addressed the dynamics of chewing. Consequently, ambiguities remain regarding predicted jaw-gapes and forces on the mandibular condyles. Here, we used a new platform to simulate unilateral chewing. The model, based on a previous study, included curvilinear articular guidance, a mobile hyoid apparatus, and a compressible food bolus. Muscles were represented by Hill-type actuators with drive profiles tuned to produce target jaw and hyoid movements. The cycle duration was 732 ms. At maximum gape, the lower incisor-point was 20.1mm down, 5.8mm posterior, and 2.3mm lateral to its initial, tooth-contact position. Its maximum laterodeviation to the working-side during closing was 6.1mm, at which time the bolus was struck. The hyoid's movement, completed by the end of jaw-opening, was 3.4mm upward and 1.6mm forward. The mandibular condyles moved asymmetrically. Their compressive loads were low during opening, slightly higher on the working-side at bolus-collapse, and highest bilaterally when the teeth contacted. The model's movements and the directions of its condylar forces were consistent with experimental observations, resolving seeming discordances in previous simulations. Its inclusion of hyoid dynamics is a step towards modelling mastication.

Dynamic mechanics in the pig mandibular symphysis.

During mastication, various biomechanical events occur at the mammalian jaw symphysis. Previously, these events have been studied in the static environment, or by direct recording of surface bone strains. Thus far, however, it has not been possible to demonstrate directly the forces and torques passing through the symphysis in association with dynamically changing muscle tensions. Therefore, we modified a previously published dynamic pig jaw model to predict the forces and torques at the symphysis, and related these to simulated masticatory muscle tensions, and bite, joint and food bolus forces. An artificial rigid joint was modelled at the symphysis, allowing measurements of the tri-axial forces and torques passing through it. The model successfully confirmed three previously postulated loading patterns at the symphysis. Dorsoventral shear occurred when the lower teeth hit the artificial food bolus. It was associated with balancing-side jaw adductor forces, and reaction forces from the working-side bite point. Medial transverse bending occurred during jaw opening, and was associated with bilateral tensions in the lateral pterygoid. Lateral transverse bending (wishboning) occurred at the late stage of the power stroke, and was associated with the actions of the deep and superficial masseters. The largest predicted force was dorsoventral shear force, and the largest torque was a 'wishboning' torque about the superoinferior axis. We suggest that dynamic modelling offers a new and powerful method for studying jaw biomechanics, especially when the parameters involved are difficult or impossible to measure in vivo.

Dynamic modeling and jaw biomechanics.

Bioengineered simulations of dynamic events in the human masticatory system are relatively new. A primary advantage is their ability to integrate structure and function in cause-and-effect scenarios. By permitting detailed analyses of these interactions, and the prototyping of prosthetic additions, the models generate working hypotheses. Significant issues in their use include the importation and measurement of structural geometry, the choice of parameters affecting dynamics (e.g. inertial properties and viscoelasticities) and the nature of the modeling process (e.g. whether models are kinetically driven by muscle contraction, or kinematically defined by movement channels). Presently, there are few accepted standards or conventions for managing these computational data in the human jaws, and the data used are often derived from multiple and disparate sources. This review focuses on the approaches, assumptions, and key applications of dynamic modeling in the human masticatory system. It considers the role of imaging, the restrictions imposed by assumptions of unknown or unverifiable data, and how modeling can be a useful research technique despite these hurdles. The review concludes with a comment on creating virtual models for educational purposes.

Modelling the masticatory biomechanics of a pig.

The relationships between muscle tensions, jaw motions, bite and joint forces, and craniofacial morphology are not fully understood. Three-dimensional (3-D) computer models are able to combine anatomical and functional data to examine these complex relationships. In this paper we describe the construction of a 3-D dynamic model using the anatomical (skeletal and muscle form) and the functional (muscle activation patterns) features of an individual pig. It is hypothesized that the model would produce functional jaw movements similar to those recordable in vivo. Anatomical data were obtained by CT scanning (skeletal elements) and MR imaging (muscles). Functional data (muscle activities) of the same animal were obtained during chewing by bipolar intramuscular electrodes in six masticatory muscles and combined with previously published EMG data. The model was driven by the functional data to predict the jaw motions and forces within the masticatory system. The study showed that it is feasible to reconstruct the complex 3-D gross anatomy of an individual's masticatory system in vivo. Anatomical data derived from the 3-D reconstructions were in agreement with published standards. The model produced jaw motions, alternating in chewing side, typical for the pig. The amplitude of the jaw excursions and the timing of the different phases within the chewing cycle were also in agreement with previously published data. Condylar motions and forces were within expected ranges. The study indicates that key parameters of the pig's chewing cycle can be simulated by combining general biomechanical principles, individual-specific data and a dynamic modelling approach frequently used in mechanical engineering.

Forces resisting jaw displacement in relaxed humans: a predominantly viscous phenomenon.

Forces opening the relaxed human jaw are resisted by intrinsic restraints, including passive tensions in the jaw-closing muscles. These muscle tensions have been modelled as viscoelastic elements, and static measurements suggest their elastic portions contribute approximately a total of 5 N resistance at wide gape. As the viscous damping properties of muscles which affect the jaw's dynamic behaviour are unknown, we measured the jaw opening force required to reach maximum gape during fast and slow opening in six relaxed subjects. These data were then incorporated in a dynamic mathematical jaw model to determine the damping properties of the masticatory system. During the 3 and 8 s opening trials, forces increased with gape (6.7 +/- 3.3 and 3.9 +/- 2.3 N, respectively, at 50% gape) and reached their maxima at wide gape (19.9 +/- 4.5 and 13.2 +/- 4.4 N, respectively). The muscle damping constant needed by the model to emulate these results was 150 Nsm(-1), approximately 25% lower than the calculated critical damping constant. This study suggests low forces are required to open the jaw in relaxed humans, and that jaw viscosity, not elasticity, provides the major resistance to motion.

Length changes in the human masseter muscle after jaw movement.

The human masseter is a multilayered, complex muscle contributing to jaw motion. Because variations in stretch may cause muscle fibers to function over different portions of their length-tension curves, the aim of this study was to determine how parts of the masseter lengthen or shorten during voluntary jaw movements made by living subjects. Magnetic resonance (MR) imaging and optically-based jaw-tracking were used to measure muscle-insertion positions for four parts of the muscle with six degrees of freedom (DOF), before and after maximum-opening, jaw protrusion and laterotrusion in four adult males. Muscle part lengths and intramuscular tendon lengths were calculated, and these data, with fiber-tendon ratios published previously, were used to estimate putative changes in fiber-length. During maximum jaw-opening, the largest increases in muscle length (34-83%) occurred in the medial part of the deep masseter, whereas the smallest changes occurred in the posterior-most, superficial masseter (2-19%). Smaller changes were found during movement to the ipsilateral side, than during protrusion and movement to the contralateral side. On maximum opening, putative fibers in the deep masseter lengthened up to 83%, whereas those of the superficial masseter stretched up to 72%. The masseter muscle does not stretch uniformly for major jaw movement. Jaw motion to the ipsilateral side causes little length change in any part, and the effect of tendon-stretch on estimated fiber lengths is not substantial. The stretch that occurs infers there are task-related changes in the active and passive tensions produced by different muscle regions.

Mass properties of the pig mandible.

Specification of mass properties is an essential step in the modeling of jaw dynamics, but obtaining them can be difficult. Here, we used three-dimensional computed tomography (CT) to estimate jaw mass, mean bone density, anatomical locations of the mass and geometric centers, and moments of inertia in the pig jaw. High-resolution CT scans were performed at one-mm slice intervals on specimens submerged in water. The mean estimated jaw mass was 12% greater than the mean wet weight, and 33% more than the mean dry weight. Putative bone marrow accounted for an extra 13% of mass. There was a positive correlation between estimated mean bone density and age. The mass center was consistently in the midline, near the last molar. The mean distance between the mass center and geometric center was small, especially when bone marrow was taken into account (0.58 +/- 0.21 mm), suggesting that mass distribution in the pig jaw is almost symmetrical with respect to its geometric center. The largest moment of inertia occurred around each mandible's supero-inferior axis, and the smallest around its antero-posterior axis. Bone marrow contributed an extra 9% to the moments of inertia in all three axes. Linear relationships were found between the actual mass and a mass descriptor (product of the bounding volume and mean bone density), and between the moments of inertia and moments of inertia descriptors (products of the mass descriptor and two orthogonal dimensions forming the bounding box). The study suggests that imaging modalities revealing three-dimensional jaw shape may be adequate for estimating the bone mass properties in pigs.

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

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

The role of passive muscle tensions in a three-dimensional dynamic model of the human jaw.

The role of passive muscle tensions in human jaw function are largely unknown. It seems reasonable to assume that passive muscle-tension properties are optimized for the multiple physiological tasks the jaw performs in vivo. However, the inaccessibility of the jaw muscles is a major obstacle to measuring their passive tensions, and understanding their effects. Computer modelling offers an alternative method for doing this. Here, a three-dimensional, dynamic model was used to predict active and passive jaw-muscle tensions during simulated postural rest, jaw opening and chewing. The model included a rigid mandible, two temporomandibular joints, multiple dental bite points, and an artificial food bolus located between the right first molars. It was driven by 18 Hill-type actuators representing nine pairs of jaw muscles. All anatomical forms, positions and properties used in the model were based on previously published, average values. Two states were stimulated, one in which all optimal lengths for the length-tension curves in the closing muscles were defined as their fibre-component lengths when the incisor teeth were 2 mm apart (S2), and another in which the optimal lengths were set for a 12.0 mm interincisal separation (S12). At rest, the jaw attained 3.6 mm interincisal separation in S2, and 14.8 mm in S12. Activation of the inferior lateral pterygoid (ILP) and digastric (DG) muscles in various combinations always induced passive jaw-closer tensions, and compressive condylar loads. Maximum midline gape (from maximum bilateral co-activation of DG and ILP) was 16.2 mm in S2, and 32.0 mm in S12. When both model states were driven with muscle patterns typical for human mastication, recognizable unilateral and vertical "chopping" chewing cycles were produced. Both states revealed condylar loading in the opening and closing phases of mastication. During unilateral chewing, compressive force on the working-side condyle exceeded that on the balancing side. In contrast, during the "chopping" cycle, loading on the balancing side was greater than that on the working side. In S2, chewing was limited in both vertical and lateral directions. These results suggest that the assumptions used in S12 more closely approximated human behaviour than those in S2. Despite its limitations, modelling appears to provide a useful conceptual framework for developing hypotheses regarding the role of muscle tensions during human jaw function.

Modeling of jaw biomechanics in the reconstructed mandibulectomy patient.

STATEMENT OF PROBLEM: Biomechanics of occlusal force and indirect calculation of temporomandibular joint loading in patients after surgery for head and neck cancer is poorly understood. PURPOSE: This study compared occlusal force values of 6 mandibulectomy subjects with reconstructed mandibles to 6 noncancer subjects with intact mandibles and reports occlusal force predictions from a developed computer model simulation of both a mandibulectomy subject with a reconstructed mandible and noncancer subject with an intact mandible. MATERIAL AND METHODS: Maximum occlusal force was recorded at the first molar and incisal edge in 6 mandibulectomy subjects who had bony reconstruction of the mandible and 6 noncancer subjects with an intact mandible. Clinical data were then qualitatively compared with occlusal force values generated from an existing computer model simulating an average adult, and a developed model simulating an average mandibulectomy subject who had bony reconstruction of the mandible. The biomechanical parameters modeled also included an estimation of joint force magnitude and direction when biting with maximal force on the first molar. RESULTS: Clinical data revealed no significant differences in occlusal force between the 6 mandibulectomy subjects with bony reconstruction of the mandible and 6 noncancer subjects with an intact mandible; however, average molar and incisal occlusal force values were 22% and 32% less in mandibulectomy subjects with bony reconstruction. Computer simulations of a reconstructed mandibulectomy subject predicted that reconstructed subjects would have 45% less molar occlusal force, 50% less incisal occlusal force, and a higher joint/tooth force ratio compared with a simulated noncancer patient with an intact mandible. CONCLUSIONS: There were no significant differences in first molar or incisal occlusal force between reconstructed mandibulectomy subjects and noncancer subjects with intact mandibles. Trends calculated from computer simulations were consistent with clinical findings.

The association among occlusal contacts, clenching effort, and bite force distribution in man.

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

Functional movements of putative jaw muscle insertions.

BACKGROUND: The craniomandibular muscles control jaw position and forces at the teeth and temporomandibular joints, but little is known regarding their biomechanical behaviour during dynamic function. The objective of this study was to determine how jaw muscle insertions alter position during different jaw movements in living subjects. METHODS: Computer 3D reconstruction of MR images and jaw-tracking were combined to permit the examination of movement with six degrees of freedom. Maximum mandibular opening, protrusive and laterotrusive positions were recorded in four subjects, and the translation and rotation of the putative insertions of masseter, temporal, medial, and lateral pterygoid muscles were measured. RESULTS: The sizes and shapes of regional attachments varied markedly among subjects, and their displacement patterns were different in specific muscles. For instance, when the jaw closed to the dental intercuspal position from maximum gape, the region near the superior insertion site of the masseter moved backward and upward, whereas the region near the inferior insertion site displaced mainly forward. In three subjects, the jaw's rotational center during this act was approximately 26-34 mm below the mandibular condyles. CONCLUSIONS: Since the movements of each muscle part differ according to variations in the size and shape of insertion areas, individual musculoskeletal form, and patterns of jaw motion during function, the prediction of motion-related muscle mechanics in any one subject is unlikely to be possible without direct measurement of the motion of visualized muscle parts. The present study shows that this information can be obtained.

Motor unit territory in relation to tendons in the human masseter muscle.

In the human masseter, motor-unit (MU) territories are reportedly focal and it is possible that they are confined to the muscle's internal aponeuroses. However, previous electromyographic (EMG) investigations of MU sizes have not correlated them with the muscle's internal architecture. In this study, 162 single-MU territories were assessed by scanning EMG recordings throughout the masseter muscles of 4 subjects. The needle-electrode scans were stereotactically located with magnetic resonance imaging and an optical system capable of tracking the needle movement in three dimensions. Mediolateral territorial dimensions were then displayed graphically within muscle reconstructions. The mean territorial width was 3.7 +/- 2.3 mm and varied between 0.4 and 13.1 mm. The widths were comparable in size to those of previous reports, and were related to the subject's muscle size. Most MU territories were confined, and lay between tendons, although 10% of the units clearly extended across at least one tendon. The dispersion of most territories within discrete tendon-bounded compartments in the masseter provides an anatomical basis for selective activation of the muscle. However, it is also possible that this arrangement provides a flexible means for ensuring tendon stiffness and mechanical adaptation of the multipennate masseter during growth and development, whether or not the muscle is activated selectively.

Internal organization in the human jaw muscles.

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

Mandibular forces during simulated tooth clenching.

Differential, functional loading of the mandibular condyles has been suggested by several human morphologic studies and by animal strain experiments. To describe articular loading and the simultaneous forces on the dental arch, static bites on a three-dimensional finite element model of the human mandible were simulated. Five clenching tasks were modeled: in the intercuspal position; during left lateral group effort; during left lateral group effort with balancing contact; during incisal clenching; and during right molar clenching. The model's predictions confirmed that the human mandibular condyles are load-bearing, with greater force magnitudes being transmitted bilaterally during intercuspal and incisal clenching, as well as through the balancing-side articulation during unilateral biting. Differential condylar loading depended on the clenching task. Whereas higher forces were found on the lateral and lateroposterior regions of the condyles during intercuspal clenching, the model predicted higher loads on the medial condylar regions during incisal clenching. The inclusion of a balancing-side occlusal contact seemed to decrease the forces on the balancing-side condyle. Whereas the predicted occlusal reaction forces confirmed the lever action of the mandible, the simulated force gradients along the tooth row suggest a complex bending behavior of the jaw.

Stereotactic location of EMG needle electrode scans relative to tendons in the human masseter muscle.

A method is presented for locating the positions of moving electromyographic needle electrodes relative to internal tendinous boundaries within the human masseter muscle. It combines scanning electromyography, needle electrode tracking, magnetic resonance imaging, and three-dimensional stereotactic reconstruction. The technique is useful for investigating motor unit territory within the masseter and other complex craniofacial muscles.

Deformation of the human mandible during simulated tooth clenching.

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