3D finite element simulation of the opening movement of the mandible in healthy and pathologic situations

One of the essential causes of disk disorders is the pathologic change in the ligamentous attachments of the disk-condyle complex. In this paper, the response of the soft components of a human temporomandibular joint during mouth opening in healthy and two pathologic situations was studied. A three-dimensional finite element model of this joint comprising the bone components, the articular disk, and the temporomandibular ligaments was developed from a set of medical images. A fiber reinforced porohyperelastic model was used to simulate the behavior of the articular disk, taking into account the orientation of the fibers in each zone of this cartilage component. The condylar movements during jaw opening were introduced as the loading condition in the analysis. In the healthy joint, it was obtained that the highest stresses were located at the lateral part of the intermediate zone of the disk. In this case, the collateral ligaments were subject to high loads, since they are responsible of the attachment of the disk to the condyle during the movement of the mandible. Additionally, two pathologic situations were simulated: damage of the retrodiscal tissue and disruption of the lateral discal ligament. In both cases, the highest stresses moved to the posterior part of the disk since it was displaced in the anterior-medial direction. In conclusion, in the healthy joint, the highest stresses were located in the lateral zone of the disk where perforations are found most often in the clinical experience. On the other hand, the results obtained in the damaged joints suggested that the disruption of the disk attachments may cause an anterior displacement of the disk and instability of the joint.     

Effect of hyperactivity of the lateral pterygoid muscle on the temporomandibular joint disk

In this study, the effect of hyperactivity of the lateral pterygoid muscle (LPM) on the temporomandibular joint (TMJ) disk during prolonged clenching was examined with a mathematical model. Finite element models of the TMJ were constructed based on magnetic resonance images from two subjects with or without internal derangement of the TMJ. For each model, muscle forces were used as a loading condition for stress analysis for 10 min clenching. Furthermore, an intermittent increase of the LPM force with intervals of 1 min was applied. In the asymptomatic model, large stresses were found in the central and lateral part of the disk at the onset of clenching. In the retrodiscal tissue, stress relaxation occurred during the first 2 min of clenching. When the force of the LPM increased temporarily, the disk moved anteriorly and returned to its original position afterward. In the symptomatic model, large stresses were observed in both the posterior region of the disk and the retrodiscal tissue throughout clenching. Upon temporary increase of the LPM force, the disk was elongated anteriorly, which appeared to be irreversible. These results indicate that hyperactivity of the LPM may be involved in the progression of disk displacement.     

Biomechanical influence of disk properties on the load transfer of healthy and degenerated disks using a poroelastic finite element model

Spine degeneration is a pathology that will affect 80% of the population. Since the intervertebral disks play an important role in transmitting loads through the spine, the aim of this study was to evaluate the biomechanical impact of disk properties on the load carried by healthy (Thompson grade I) and degenerated (Thompson grades III and IV) disks. A three-dimensional parametric poroelastic finite element model of the L4/L5 motion segment was developed. Grade I, grade II, and grade IV disks were modeled by altering the biomechanical properties of both the annulus and nucleus. Models were validated using published creep experiments, in which a constant compressive axial stress of 0.35 MPa was applied for 4 h. Pore pressure (PP) and effective stress (S(E)) were analyzed as a function of time following loading application (1 min, 5 min, 45 min, 125 min, and 245 min) and discal region along the midsagittal profile for each disk grade. A design of experiments was further implemented to analyze the influence of six disk parameters (disk height (H), fiber proportion (%F), drained Young's modulus (E(a),E(n)), and initial permeability (k(a),k(n)) of both the annulus and nucleus) on load-sharing for disk grades I and IV. Simulations of grade I, grade III, and grade IV disks agreed well with the available published experimental data. Disk height (H) had a significant influence (p<0.05) on the PP and S(E) during the entire loading history for both healthy and degenerated disk models. Young's modulus of the annulus (E(a)) significantly affected not only S(E) in the annular region for both disk grades in the initial creep response but also S(E) in the nucleus zone for degenerated disks with further creep response. The nucleus and annulus permeabilities had a significant influence on the PP distribution for both disk grades, but this effect occurred at earlier stages of loading for degenerated than for healthy disk models. This is the first study that investigates the biomechanical influence of both geometrical and material disk properties on the load transfer of healthy and degenerated disks. Disk height is a significant parameter for both healthy and degenerated disks during the entire loading. Changes in the annulus stiffness, as well as in the annulus and nucleus permeability, control load-sharing in different ways for healthy and degenerated disks.     

Nonlinear analysis of intervertebral disk under dynamic load

This study pertains to the response of intervertebral joint under dynamic axial load. The numerical model represents two vertebral bodies with an interposed disk and uses three-dimensional elements. A transversely isotropic material law is adopted for cortical bone and an isotropic law for cancellous bone. Annulus collagen fibers are modelled using truss elements with no compressive resistance. The disk material is assumed hyperelastic, using a mixed finite element approach, allowing a representation of the disk involving the incompressibility characteristics for the material. The analysis considers finite displacement and strain fields under dynamic load. Intensity, trend and distribution of loads on the vertebral body are deduced from the literature. The problem is investigated with reference to different compressibility levels of disk material related to disk degenerationn phenomena.     

The influence of different playing surfaces on the biomechanics of a tennis running forehand foot plant

Research suggests that heightened impacts, altered joint movement patterns, and changes in friction coefficient from the use of artificial surfaces in sport increase the prevalence of overuse injuries. The purposes of this study were to (a) develop procedures to assess a tennis-specific movement, (b) characterize the ground reaction force (GRF) impact phases of the movement, and (c) assess human response during impact with changes in common playing surfaces. In relation to the third purpose it was hypothesized that surfaces with greatest mechanical cushioning would yield lower impact forces (PkFz) and rates of loading. Six shod volunteers performed 8 running forehand trials on each surface condition: baseline, carpet, acrylic, and artificial turf. Force plate (960 Hz) and kinematic data (120 Hz) were collected simultaneously for each trial. Running forehand foot plants are typically characterized by 3 peaks in vertical GRF prior to a foot-off peak. Group mean PkFz was significantly lower and peak braking force was significantly higher on the baseline surface compared with the other three test surfaces (p<0.05). No significant changes in initial kinematics were found to explain unexpected PkFz results. The baseline surface yielded a significantly higher coefficient of friction compared with the other three test surfaces (p<0.05). While the hypothesis is rejected, biomechanical analysis has revealed changes in surface type with regard to GRF variables.     

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

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

Homing behaviour of stream-dwelling brook charr following experimental displacement

Precise homing of 24 brook charr Salvelinus fontinalis (157–215 mm L_F), displaced from their site of capture in a natural stream, was rare both in fish displaced upstream and those displaced downstream. Most fish settled in preferred habitats (pool or glide) away from their home site and showed restricted movement up to the pre-spawning period. Partial return responses, however, were stronger in fish displaced downstream than in fish displaced upstream. Most fish displaced downstream showed directional upstream movement soon after displacement, whereas fish displaced upstream did not show directional upstream movement, suggesting that it is primarily olfactory cues from upstream locations, and not positive rheotaxis, which mediate the return response. Homing success was unrelated to distance of displacement, body size, or population density in the home site, but daily mobility increased with body size for fish displaced downstream. There was no evidence of switching between specific areas between night and day, nor of differences between daytime and night-time mobility, suggesting that diel movement cycles did not bias the evaluation of homing success. Site attachment in stream-dwelling brook charr may be characterized as persistent (undisturbed fish remain stationary over long periods), but readily disrupted by involuntary displacement.     

Kinematic, kinetic and EMG patterns during downward squatting.

The aim of this study was to investigate the kinematic, kinetic, and electromyographic pattern before, during and after downward squatting when the trunk movement is restricted in the sagittal plane. Eight healthy subjects performed downward squatting at two different positions, semisquatting (40 degrees knee flexion) and half squatting (70 degrees knee flexion). Electromyographic responses of the vastus medialis oblique, vastus medialis longus, rectus femoris, vastus lateralis, biceps femoris, semitendineous, gastrocnemius lateralis, and tibialis anterior were recorded. The kinematics of the major joints were reconstructed using an optoelectronic system. The center of pressure (COP) was obtained using data collected from one force plate, and the ankle and knee joint torques were calculated using inverse dynamics. In the upright position there were small changes in the COP and in the knee and ankle joint torques. The tibialis anterior provoked the disruption of this upright position initiating the squat. During the acceleration phase of the squat the COP moved posteriorly, the knee joint torque remained in flexion and there was no measurable muscle activation. As the body went into the deceleration phase, the knee joint torque increased towards extension with major muscle activities being observed in the four heads of the quadriceps. Understanding these kinematic, kinetic and EMG strategies before, during and after the squat is expected to be beneficial to practitioners for utilizing squatting as a task for improving motor function.     

Shielding disk position in intra-operative electron radiotherapy (IOERT): A Monte Carlo study

PurposeIn IOERT breast treatments, a shielding disk is frequently used to protect the underlying healthy structures. The disk is usually composed of two materials, a low-Z material intended to be oriented towards the beam and a high-Z material. As tissues are repositioned around the shield before treatment, the disk is no longer visible and its correct alignment with respect to the beam is guaranteed. This paper studies the dosimetric characteristics of four possible clinical positioning scenarios of the shielding disk. A new alignment method for the shielding disk in the beam is introduced. Finally, it suggests a new design for the shielding disk.MethodsAs the first step, the IOERT machine “Mobetron 1000” was modeled by using Monte Carlo simulation, tuning the MC model until an excellent match with the measured PDDs and profiles was achieved. Four possible shielding disk positioning scenarios were considered, determining the dosimetric impact. Furthermore, in our center, to prevent beam misalignment, we have developed a shielding disk equipped with guiding rods. Having ascertained a correct alignment between the disk and the beam, we can propose a new internal design of the shielding disk that can improve the dose distribution with a better coverage of the treated area.ResultsAll MC simulations were performed with a 12 MeV beam, the maximum energy of Mobetron 1000 and a 5.5 cm diameter flat tip applicator, this applicator being the most clinically used. The simulations were compared with measurements performed in a water phantom and showed good results within 2.2% of root mean square difference (RMSD). The misplacement positions of the shielding disk have dosimetric impacts in the treatment volume and a small translation could have a significant influence on healthy tissues. The D-scenario is the worst which could happens when the shielding disk is flipped upside down, giving up to 144% dose instead of 90% at the surface of the Pb/Al shielding disk. A new shielding design used, together with our alignment tool, is able to give a more homogeneous dose in the target area.ConclusionsThe accuracy of shielding disk position can still be problematic in IOERT dosimetry. Any method that can ascertain the good alignment between the shielding disk and the beam is beneficial for the dose distribution and is a prerequisite for an optimized shield internal design that could improve the coverage of the treated area and the protection of healthy tissues.     

Downstream displacement of post‐emergent brown trout: effects of development stage and water velocity

Brown trout Salmo trutta were introduced at hatching into distinct sections of two parallel artificial channels, one with a constant low velocity (control) and one with velocity changes (experimental), at such times as to produce 12, 3 and 0 day old fish (age after emergences) when the velocity was changed in the experimental channel. This experimental design was repeated in 2002 and 2003 at comparable dates. Young brown trout were sensitive to an increased water velocity for 5 to 6 days after emergence. Water velocity modified the displacement patterns qualitatively but not quantitatively. Eighty per cent of fish moved downstream at all water velocities. Velocity changes, however, advanced the time by which 80% of the fish had displaced downstream.     

Tibio-femoral movement in the living knee. A study of weight bearing and non-weight bearing knee kinematics using 'interventional' MRI

The aim of this study was to image tibio-femoral movement during flexion in the living knee. Ten loaded male Caucasian knees were initially studied using MRI, and the relative tibio-femoral motions, through the full flexion arc in neutral tibial rotation, were measured. On knee flexion from hyperextension to 120 degrees , the lateral femoral condyle moved posteriorly 22 mm. From 120 degrees to full squatting there was another 10 mm of posterior translation, with the lateral femoral condyle appearing almost to sublux posteriorly. The medial femoral condyle demonstrated minimal posterior translation until 120 degrees . Thereafter, it moved 9 mm posteriorly to lie on the superior surface of the medial meniscal posterior horn. Thus, during flexion of the knee to 120 degrees , the femur rotated externally through an angle of 20 degrees . However, on flexion beyond 120 degrees , both femoral condyles moved posteriorly to a similar degree. The second part of this study investigated the effect of gender, side, load and longitudinal rotation. The pattern of relative tibio-femoral movement during knee flexion appears to be independent of gender and side. Femoral external rotation (or tibial internal rotation) occurs with knee flexion under loaded and unloaded conditions, but the magnitude of rotation is greater and occurs earlier on weight bearing. With flexion plus tibial internal rotation, the pattern of movement follows that in neutral. With flexion in tibial external rotation, the lateral femoral condyle adopts a more anterior position relative to the tibia and, particularly in the non-weight bearing knee, much of the femoral external rotation that occurs with flexion is reversed.     

Tactic responses of the parasitic mite, Psoroptes ovis, to light and temperature

The astigmatid mite, Psoroptes ovis (Hering) (Acari: Psoroptidae), is an obligate, non-burrowing ectoparasite of vertebrates, of particular economic importance in domestic sheep flocks where it causes clinical psoroptic mange. To help understand the behaviour which facilitates transmission via the environment, the responses of P. ovis derived from rabbits (syn. Psoroptes cuniculi) to temperature and light were examined in the laboratory. On a vertical surface of uniform temperature, the presence and direction of illumination had a significant effect on the distance and direction moved by the mites. In darkness or with illumination from both above and below, the mites moved relatively little, but this movement was upwards. In contrast, with illumination from above only, mites moved downwards. When the direction of the illumination was reversed so that it came from below only, the mites moved upwards. On a vertical surface with a temperature gradient, in darkness or with illumination from both above and below, the mites moved up or down towards the area of highest temperature, depending on whether this was above or below, respectively. However, the movement of the mites in response to the temperature gradient was strongly displaced up or down by the presence of unidirectional illumination from above or below, respectively. The results indicate that the movement of these mites is strongly directed towards areas of high temperature but away from higher light intensity. These behaviours might be expected to maintain the position of the mites on a host animal and help them locate the skin surface of a new host when displaced into the environment.     

Fluctuation in the microtubule sliding movement driven by kinesin in vitro.

We studied the fluctuation in the translational sliding movement of microtubules driven by kinesin in a motility assay in vitro. By calculating the mean-square displacement deviation from the average as a function of time, we obtained motional diffusion coefficients for microtubules and analyzed the dependence of the coefficients on microtubule length. Our analyses suggest that 1) the motional diffusion coefficient consists of the sum of two terms, one that is proportional to the inverse of the microtubule length (as the longitudinal diffusion coefficient of a filament in Brownian movement is) and another that is independent of the length, and 2) the length-dependent term decreases with increasing kinesin concentration. This latter term almost vanishes within the length range we studied at high kinesin concentrations. From the length-dependence relationship, we evaluated the friction coefficient for sliding microtubules. This value is much larger than the solvent friction and thus consistent with protein friction. The length independence of the motional diffusion coefficient observed at sufficiently high kinesin concentrations indicates the presence of correlation in the sliding movement fluctuation. This places significant constraint on the possible mechanisms of the sliding movement generation by kinesin motors in vitro.     

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

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

Regeneration of the Cornea in Adult Newts: Overall Process and Behavior of Epithelial Cells

Regeneration of the cornea in adult newts was studied by means of light- and electron-microscopic techniques. We focused our analysis particularly on the behavior of epithelial cells during the initial process of wound healing after we had excised a central disk about 0.5 mm in diameter through the entire thickness of the cornea. Fine fibrous material, assumed to be fibrin, appeared within 30 min to form an acellular layer of mucous consistency which sealed the wound opening completely. The cut edge of corneal epithelium moved centripetally on this layer by coordinate movement of individual epithelial cells. Almost all cells of the remained epithelium were completely rearranged within 5 h after excision. Some desmosomes among the epithelial cells persisted during the process of cellular rearrangement. Thus, the wound opening was covered completely within 24 h by the epithelium alone without cell proliferation. Cytochalasin B or D completely inhibited movement of the corneal epithelium on the stroma in conditions in vitro, suggesting active participation of intracellular contractile microfilaments in such movement of the epithelium. Active growth of cells in the epithelium started on day 3 and the epithelium recovered its normal thickness by day 10 after excision.
After the recovery of the epithelium, keratocytes moved out from the wounded edge of the remained corneal stroma. These keratocytes actively proliferated in the wound area under the newly formed epithelium and participated in the stromal reconstitution, which proceeded gradually for more than 5 weeks.     

Social status and growth rates as determinants of site attachment in juvenile Atlantic salmon

This paper describes an experimental study of the effects of food supply, growth rates and social interactions on homing by juvenile Atlantic salmon Salmo salar in response to displacement. Groups of five fish were housed in a section of an artificial stream and given either rations allowing maximum growth (the rich condition) or 0·1 of this amount (the poor condition); daily specific growth rates were significantly higher in the rich condition. After a 6-day settlement period, the fish were captured, displaced downstream and their movements recorded over the next 3 h. Prior to displacement, the fish showed a high degree of site fidelity and high levels of aggression. Dominant fish and those with stronger site attachment grew faster prior to displacement, these effects being independent. Following displacement, 24% of all fish returned to their previously favoured site and stayed there, 23% returned home initially, but subsequently moved on, 5% settled in a new site and 49% failed to move. The distribution of responses was identical for the rich and poor conditions, but fish that homed were dominant and had grown faster during the pre-displacement period.     

Structural models for human spinal motion segments based on a poroelastic view of the intervertebral disk

Analytical and finite element models (FEMs) were used to quantify poroelastic material properties for a human intervertebral disk. An axisymmetric FEM based on a poroelastic view of disk constituents was developed for a representative human spinal motion segment (SMS). Creep and steady-state response predicted by FEMs agreed with experimental observations, i.e., long-time creep occurs with flow in the SMS, whereas for rapid steady-state loading an "undrained," nearly incompressible response is evident. A relatively low value was determined for discal permeability. Transient and long-term creep FE analyses included the study of deformation, pore fluid flow, stress, and pore fluid pressure. Relative fluid motion associated with transient creep is related to nuclear nutrition and the overall mechanical response in the normal disk. Degeneration of the disk may be associated with an increase in permeability.     

A technique to determine friction at the fingertips

This article proposes a technique to calculate the coefficient of friction for the fingertip- object interface. Twelve subjects (6 males and 6 females) participated in two experiments. During the first experiment (the imposed displacement method), a 3-D force sensor was moved horizontally while the subjects applied a specified normal force (4 N, 8 N, 12 N) on the surface of a sensor covered with different materials (sandpaper, cotton, rayon, polyester, and silk).The normal force and the tangential force (i.e., the force due to the sensor motion) were recorded. The coefficient of friction (mu(d)) was calculated as the ratio between the tangential force and the normal force. In the second experiment (the beginning slip method), a small instrumented object was gripped between the index finger and the thumb, held stationary in the air, and then allowed to drop. The weight (200 g, 500 g, and 1,000 g) and the surface (sandpaper, cotton, rayon, polyester, and silk) in contact with the digits varied across trials. The same sensor as in the first experiment was used to record the normal force (in a horizontal direction) and the tangential force (in the vertical direction). The slip force (i.e., the minimal normal force or grip force necessary to prevent slipping) was estimated as the force at the moment when the object just began to slip. The coefficient of friction was calculated as the ratio between the tangential force and the slip force. The results show that (1) the imposed displacement method is reliable; (2) except sandpaper, for all other materials the coefficient of friction did not depend on the normal force; (3) the skin-sandpaper coefficient of friction was the highest mu(d) =0.96+/-0.09 (for 4-N normal force) and the skin-rayon rayon coefficient of friction was the smallest mu(d) =0.36+/-0.10; (4) no significant difference between the coefficients of friction determined with the imposed displacement method and the beginning slip method was observed. We view the imposed displacement technique as having an advantage as compared with the beginning slip method, which is more cumbersome (e.g., dropped object should be protected from impacts) and prone to subjective errors owing to the uncertainty in determining the instance of the slip initiation (i.e., impeding sliding).     

Differential repositioning of the second transmembrane helices from E. coli Tar and EnvZ upon moving the flanking aromatic residues

Aromatic tuning, i.e. repositioning aromatic residues found at the cytoplasmic end of transmembrane (TM) domains within bacterial receptors, has been previously shown to modulate signal output from the aspartate chemoreceptor (Tar) and the major osmosensor EnvZ of Escherichia coli. In the case of Tar, changes in signal output consistent with the vertical position of the native Trp-Tyr aromatic tandem within TM2 were observed. In contrast, within EnvZ, where a Trp-Leu-Phe aromatic triplet was repositioned, the surface that the triplet resided upon was the major determinant governing signal output. However, these studies failed to determine whether moving the aromatic residues was sufficient to physically reposition the TM helix within a membrane. Recent coarse-grained molecular dynamics (CG-MD) simulations predicted displacement of Tar TM2 upon moving the aromatic residues at the cytoplasmic end of the helix. Here, we demonstrate that repositioning the Trp-Tyr tandem within Tar TM2 displaces the C-terminal boundary of the helix relative to the membrane. In a similar analysis of EnvZ, an abrupt initial displacement of TM2 was observed but no subsequent movement was seen, suggesting that the vertical position of TM2 is not governed by the location of the Trp-Leu-Phe triplet. Our results also provide another set of experimental data, i.e. the resistance of EnvZ TM2 to being displaced upon aromatic tuning, which could be useful for subsequent refinement of the initial CG-MD simulations. Finally, we discuss the limitations of these methodologies, how moving flanking aromatic residues might impact steady-state signal output and the potential to employ aromatic tuning in other bacterial membrane-spanning receptors.