Mandibular stiffness in humans: numerical predictions

The chin is a feature unique to humans. This study evaluates the effect of mandibular symphyseal design on biomechanical masticatory effectiveness as determined by structural stiffness and stress developed under flexural and torsional loading. A simple model of three symphyseal shapes (chin, flat symphysis and lingual buttress), was built to represent human, Neanderthal and higher primate symphyses and these were subjected to wishboning and torsional forces. Additionally, an anatomically detailed reconstruction was made of the CT scan of an actual human mandible, which was then also morphed into a chinless model. The results of a 3-D finite element analysis show firstly, that none of the three different symphyseal shapes is biomechanically more advantageous than the others for the given loading condition. Secondly, we show in a CT-derived model, that the presence of a chin does not confer significantly improved stiffness to torsional or flexural loading. These results indicate that the acquisition of a chin in modern humans is not related to the functional demands placed upon the mandible during mastication, but suggest that it may have developed in response to other biomechanical demands.     

Gender differences in active musculoskeletal stiffness. Part II. Quantification of leg stiffness during functional hopping tasks.

Leg stiffness was compared between age-matched males and females during hopping at preferred and controlled frequencies. Stiffness was defined as the linear regression slope between the vertical center of mass (COM) displacement and ground-reaction forces recorded from a force plate during the stance phase of the hopping task. Results demonstrate that subjects modulated the vertical displacement of the COM during ground contact in relation to the square of hopping frequency. This supports the accuracy of the spring-mass oscillator as a representative model of hopping. It also maintained peak vertical ground-reaction load at approximately three times body weight. Leg stiffness values in males (33.9+/-8.7 kN/m) were significantly (p<0.01) greater than in females (26.3+/-6.5 kN/m) at each of three hopping frequencies, 3.0, 2.5 Hz, and a preferred hopping rate. In the spring-mass oscillator model leg stiffness and body mass are related to the frequency of motion. Thus male subjects necessarily recruited greater leg stiffness to drive their heavier body mass at the same frequency as the lighter female subjects during the controlled frequency trials. However, in the preferred hopping condition the stiffness was not constrained by the task because frequency was self-selected. Nonetheless, both male and female subjects hopped at statistically similar preferred frequencies (2.34+/-0.22 Hz), therefore, the females continued to demonstrate less leg stiffness. Recognizing the active muscle stiffness contributes to biomechanical stability as well as leg stiffness, these results may provide insight into the gender bias in risk of musculoskeletal knee injury.     

Effect of displacement rate on the tensile mechanics of pediatric cervical functional spinal units

This study examined the effect of loading (displacement) rate on the tensile mechanics of cervical spine functional spinal units. A total of 40 isolated functional spinal units (two vertebrae and the adjoining soft tissues) from juvenile male baboons (10+/-0.6-human equivalent years old) were subjected to tensile loading spanning four orders of magnitude from 0.5 to 5000 mm/s. The stiffness, ultimate failure load, and corresponding displacement at failure were measured for each specimen and normalized by spinal geometry to examine the material properties as well as the structural properties. The tensile stiffness, failure load, normalized stiffness, and normalized failure load significantly increased (ANOVA, p<0.001) with increasing displacement rate. From the slowest to fastest loading rate, a two-fold increase in stiffness and four-fold increase in failure load were observed. The tensile failure strains (1.07+/-0.31 mm/mm strain) were not significantly correlated with loading rate (ANOVA, p=0.146). Both the functional (non-destructive stiffness and normalized stiffness) and failure mechanics of isolated functional spinal units exhibited a power-law relationship with displacement rate. Modeling efforts utilizing these rate-dependent characteristics will enhance our understanding of the tensile viscoelastic response of the spine and enable improved dynamic injury prevention schemes.     

Development of biomechanical response corridors of the thorax to blunt ballistic impacts

Human responses are critical to understanding injury biomechanics in blunt ballistic impacts, which are defined as 20-200 g projectiles impacting at 20-250 m/s. 13 human cadavers were exposed to three distinct ballistic impacts of the chest to determine force-time, deflection-time and force-deflection responses. Comparisons were made between biomechanical responses for ballistic impacts and those previously reported for lower speed, higher mass impacts. Impact condition B (140 g at 40 m/s) gave the largest peak force 10,602+/-2226 N and deflection 54.7+/-14.6 mm. Impact condition A (140 g at 20 m/s) involved lower impact energy and produced lower peak force 3383+/-761 N and deflection 25.9+/-3.1 mm, as did impact condition C (40 g at 60 m/s), which gave 3158+/-309 N and 20.1+/-7.8 mm. The results indicate each impact condition gives distinctive responses, which differ from those previously reported in the automotive literature for lower speed impacts. This information provides the foundation for future biomechanical research in the area of blunt ballistic impacts, specifically the development of test surrogates and evaluation of protective equipment.     

Ultimate strength of the lumbar spine in flexion--an in vitro study

The ultimate strength in flexion of 16 lumbar functional spinal units (FSU) was determined. The specimens were exposed to a combined static load of bending and shearing in the sagittal plane until overt rupture occurred (simulated flexion-distraction injuries). The biomechanical response of the FSU was measured with a force and moment platform. Mechanical displacement gauges were used to measure vertical displacements (flexion angulation) of the specimens. Photographs were taken after each loading step for determination of horizontal displacements and the centre of rotation. The lumbar FSU could resist a combination of bending moment and shear force of 156 Nm and 620 N respectively, before complete disruption occurred. The tension force acting on the posterior structures was 2.8 kN. The flexion angulation just before failure was 20 degrees and the anterior horizontal displacement between the upper and lower vertebrae was 9 mm. The centre of rotation was located in the posterior part of the lower vertebral body. The bone mineral content in the vertebrae appeared to be a good predictor of ultimate strength of the lumbar FSU. Knowledge of the biomechanical response of the lumbar spine under different static traumatic loads is a first step to better understand the injury mechanisms of the spine in traffic accidents.     

A telemetry-based device to determine the force-displacement behaviour of materials in high impact loading situations

Meaningful testing of stab resistant body armour requires the use of realistic body tissue simulants. A device for the determination of the force-displacement behaviour of materials in high impact loading situations has been developed for the testing of such simulants. Force measurement is achieved with the use of electrical foil strain gauges applied to a cylindrical load cell. A piezo-resistive accelerometer (+/- 500 g) is used to calculate the displacement of the device through double integration of its signal, with the impact velocity used as a boundary condition. The signals from the strain gauge circuit and the accelerometer are sampled at 2500 Hz. The data are transmitted to a receiver via telemetry using a 418 MHz FM transmitter and from the receiver to a laptop PC via the serial port. Calibration of the device is described and sample results showing forces up to 2500 N and displacements up to 0.04 m are presented.     

Dietary correlates of temporomandibular joint morphology in the great apes

Behavioral observations of great apes have consistently identified differences in feeding behavior among species, and these differences have been linked to variation in masticatory form. As the point at which the mandible and cranium articulate, the temporomandibular joint (TMJ) is an important component of the masticatory apparatus. Forces are transmitted between the mandible and cranium via the TMJ, and this joint helps govern mandibular range of motion. This study examined the extent to which TMJ form covaries with feeding behavior in the great apes by testing a series of biomechanical hypotheses relating to specific components of joint shape using linear measurements extracted from three‐dimensional coordinate data. Results of these analyses found that taxa differ significantly in TMJ shape, particularly in the mandibular fossa. Chimpanzees have relatively more anteroposteriorly elongated joint surfaces, whereas gorillas tend to have relatively anteroposteriorly compressed joints. Orangutans were most commonly intermediate in form between Pan and Gorilla, perhaps reflecting a trade‐off between jaw gape and load resistance capabilities. Importantly, much of the observed variation among taxa reflects differences in morphologies that facilitate gape over force production. These data therefore continue to emphasize the unclear relationship between mandibular loading and bony morphology, but highlight the need for further data regarding food material properties, jaw gape, and ingestive/food processing behaviors. Am J Phys Anthropol, 2013. © 2012 Wiley Periodicals, Inc.     

Modeling a fibula transplant in mandibular reconstructions: evaluation of the effects of a minimal number of osteotomies on the contour of the jaw.

The fibula osteocutaneous free flap has become the preferred method for most cases of mandibular reconstruction after oncologic surgical ablation. To recreate the parabolic form of the mandible, the fibula has to be divided up into segments using a closed wedge osteotomy technique. The number of osteotomies is preferably kept to a minimum so that segmental periosteal circulation is not compromised and also to keep operating time to a minimum. The limited number of osteotomies creates an angular contour. The aim of this study was to establish the degree to which overcorrection or undercorrection would occur when a subtotal reconstruction from ramus to ramus was simulated using five bony segments and four osteotomies. The study was carried out using 30 preserved jaws; the contour lines of the jaws were transferred onto tracing paper using a cardboard template. The contour of the mandible was divided into five sections (ramus, body, symphysis, body, and ramus). Because of the cutting off of the curvature in the original jaw outline, the lateral side of the body will become narrower and the chin broader. This also results in an underprojection (displacement) of the chin. To follow the original contour of the jaw as accurately as possible, all these anomalies must be minimized. The amount of under- and overprojection is calculated for a displacement of 1.0, 1.5, 2.5, 5.0, 7.5, and 10 mm of the chin. The most accurate reconstruction of the mandibular contour is achieved with a displacement of 1.5 or 2.5 mm. To preserve sufficient periosteal circulation, the minimum width of bone segments must be 15 mm or more. This concerns especially the symphysis section. On the basis of a fibula thickness of 14 mm, the internal bone width of the symphysis section is calculated. With a displacement of 1.5 mm, the average internal width of the bone segment is 14.8 mm, with a range of 9.9 to 23.0 mm (95 percent confidence interval, 12.8 to 16.7 mm). Therefore, a displacement of 2.5 mm with an internal bone width of 16.4 mm is preferred (range, 11.9 to 24.8 mm; 95 percent confidence interval, 15.5 to 18.2 mm). The loss of lateral projection is minimal (5.8 mm) and the resulting chin width is acceptable (average, 35.0 mm). In conclusion, we propose that in a subtotal procedure, an acceptable jaw reconstruction can be achieved with a limited number of osteotomies. The bone length of the symphysis section remains within safe limits. If the defect is of limited dimensions, then the resulting jaw contour is even more accurate.     

Power-time, force-time, and velocity-time curve analysis during the jump squat: impact of load

The purpose of this investigation was to examine the impact of load on the power-, force- and velocity-time curves during the jump squat. The analysis of these curves for the entire movement at a sampling frequency of 200-500 Hz averaged across 18 untrained male subjects is the most novel aspect of this study. Jump squat performance was assessed in a randomized fashion across five different external loads: 0, 20, 40, 60, and 80 kg (equivalent to 0 +/- 0, 18 +/- 4, 37 +/- 8, 55 +/- 12, 74 +/- 15% of 1RM, respectively). The 0-kg loading condition (i.e., body mass only) was the load that maximized peak power output, displaying a significantly (p 相似文献   

Use of the rat forelimb compression model to create discrete levels of bone damage in vivo

Skeletal responses to damage are significant for understanding the etiology of stress fractures and possibly osteoporotic fractures. We refined the rat forelimb-loading model to produce a range of sub-fracture damage levels during in vivo cyclic loading. A total of 98 right forelimbs of anesthetized, male, 5-month old Fischer rats were loaded cyclically (2 Hz) in axial compression. Rats were killed immediately after loading. In the first experiment, forelimbs were loaded to fracture, which occurred after an increase in peak displacement of 2.0+/-0.2 mm, independent of peak force or cycle number. In the next experiment, we loaded forelimbs at a constant peak force until the peak displacement increased by 0.6-1.8 mm (30-90% of fracture displacement). Mechanical properties of the loaded (right) and contralateral control (left) ulnae were determined ex vivo using three-point bending, and cracks were analyzed using micro-computed tomography. Results demonstrated a dose-response between increased forelimb displacement and increased ulnar damage, with four discrete damage levels. "Low" damage was produced by cyclic loading to 30% of fracture displacement, with no visible cracks and a 10% strength loss. "Mild" damage was produced by loading to 45% of fracture displacement, with variable linear cracks and 20% strength loss. "Moderate" damage was produced by loading to 60-75% of fracture displacement, with consistent linear cracks and 40% strength loss. "High" damage was produced by loading to 85-90% of fracture displacement, with branching cracks and 60% strength loss. This loading model will be useful for examining biological responses to a range of sub-fracture damage levels in future experiments.     

Excessive compression of the human tibio-femoral joint causes ACL rupture

The knee is one of the most frequently injured joints in the human body. A recent study suggests that axial compressive loads on the knee may play a role in injury to the anterior cruciate ligament (ACL) for the flexed knee, because of an approximate 10 degrees posterior tilt in the tibial plateau (J. Orthop. Res. 16 (1998) 122-127). The hypothesis of the current study was that excessive axial compressive loads in the human tibio-femoral (TF) joint would cause relative displacement and rotation of the tibia with respect to the femur, and result in isolated injury to the ACL when the knee is flexed to 60 degrees , 90 degrees or 120 degrees . Sixteen isolated knees from eleven fresh cadaver donors (74.3+/-10.5 yr) were exposed to repetitive TF compressive loads increasing in intensity until catastrophic injury. ACL rupture was documented in 14/16 cases. The maximum TF joint compressive force for ACL failure was 5.1+/-2.1 kN for all flexion angles combined. For the 90 degrees flexed knee, the injury occurred with a relative anterior displacement of 5.4+/-3.8mm, a lateral displacement of 4.1+/-1.4mm, and a 7.8+/-7.0 degrees internal rotation of the tibia with respect to the femur.     

Strain-rate sensitivity of the rabbit MCL diminishes at traumatic loading rates

This study was performed to determine whether the viscoelastic behavior of ligaments persists at high rates of loading, such as those associated with sports-related trauma or motor vehicle accidents. Medial collateral ligaments (MCLs) from 22 skeletally mature New Zealand White rabbits were tensile tested quasi-statically and via two impact conditions at displacement rates of 0.17 mm/s (n=22), 640+/-160 mm/s (n=10) and 2500+/-270 mm/s (n=12) (corresponding to strain rates of approximately 1.0%/s, 3660%/s and 14,000%/s, respectively). Despite dramatic increases in displacement rate, only a modest strain-rate effect was observed when the specimens tested quasi-statically were compared to those tested via impact (24% and 37% increases in stiffness and failure load, respectively). There were no differences in the structural (e.g. 145+/-30 and 136+/-29 N/mm stiffness values, respectively) or failure properties (e.g. 434+/-91 and 443+/-154 N failure load values, respectively) of the two impact-tested groups. Our findings suggest that the rabbit MCL is not viscoelastic at loading rates approximating those associated with high-energy trauma.     

Differential displacement of the human soleus and medial gastrocnemius aponeuroses during isometric plantar flexor contractions in vivo.

The human triceps surae muscle-tendon complex is a unique structure with three separate muscle compartments that merge via their aponeuroses into the Achilles tendon. The mechanical function and properties of these structures during muscular contraction are not well understood. The purpose of the study was to investigate the extent to which differential displacement occurs between the aponeuroses of the medial gastrocnemius (MG) and soleus (Sol) muscles during plantar flexion. Eight subjects (mean +/- SD; age 30 +/- 7 yr, body mass 76.8 +/- 5.5 kg, height 1.83 +/- 0.06 m) performed maximal isometric ramp contractions with the plantar flexor muscles. The experiment was performed in two positions: position 1, in which the knee joint was maximally extended, and position 2, in which the knee joint was maximally flexed (125 degrees ). Plantarflexion moment was assessed with a strain gauge load cell, and the corresponding displacement of the MG and Sol aponeuroses was measured by ultrasonography. Differential shear displacement of the aponeurosis was quantified by subtracting displacement of Sol from that of MG. Maximal plantar flexion moment was 36% greater in position 1 than in position 2 (132 +/- 20 vs. 97 +/- 11 N.m). In position 1, the displacement of the MG aponeurosis at maximal force exceeded that of the Sol (12.6 +/- 1.7 vs. 8.9 +/- 1.5 mm), whereas in position 2 displacement of the Sol was greater than displacement of the MG (9.6 +/- 1.0 vs. 7.9 +/- 1.2 mm). The amount and "direction" of shear between the aponeuroses differed significantly between the two positions across the entire range of contraction, indicating that the Achilles tendon may be exposed to intratendinous shear and stress gradients during human locomotion.     

Compressive strains and displacement in a partially dentate lower jaw rehabilitated with two different treatment modalities

doi: 10.1111/j.1741‐2358.2011.00572.x Compressive strains and displacement in a partially dentate lower jaw rehabilitated with two different treatment modalities Background: Understanding of the biomechanical consequences of the stresses generated to the supporting bone during occlusal loading is significant for improving the design and clinical planning process in partial edentulism therapy. Objective: The aim of this study was to analyse the distribution of strain and displacement on the partially dentate lower jaw rehabilitated with an partial denture (RPD) and to compare it to the strain and displacement distribution on a partially dentate lower jaw rehabilitated with a cantilever fixed partial denture (FPD). Material and methods: The experimental models were a partially dentate mandible with full‐arch PFM crowns and RPD and a partially dentate mandible rehabilitated with a full‐arch cantilever FPD. Strains and displacement were measured using the Digital Image Correlation Method. Results: Displacement values of the first experimental model ranged from 0.31 to 0.54 mm with strains from 1.35 to 2.34%. Analysis of the second experimental model results showed displacement values from 0 to 0.34 mm, while strains were in the range of 0–1.40%. Conclusion: Higher displacements and strains of bone tissue were observed below the RPD, especially in the region of the distal abutment and distal portion of the free‐end saddle. Strains within bone and the bone‐denture contact area were mostly influenced by the teeth and denture vertical displacement.     

Runners adjust leg stiffness for their first step on a new running surface.

Human runners adjust the stiffness of their stance leg to accommodate surface stiffness during steady state running. This adjustment allows runners to maintain similar center of mass movement (e.g., ground contact time and stride frequency) regardless of surface stiffness. When runners encounter abrupt transitions in the running surface, they must either make a rapid adjustment or allow the change in the surface stiffness to disrupt their running mechanics. Our goal was to determine how quickly runners adjust leg stiffness when they encounter an abrupt but expected change in surface stiffness that they have encountered previously. Six human subjects ran at 3 m s(-1) on a rubber track with two types of rubber surfaces: a compliant "soft" surface (ksurf = 21.3 kN m(-1) and a non-compliant "hard" surface (ksurf = 533 kN m(-1). We found that runners completely adjusted leg stiffness for their first step on the new surface after the transition. For example, runners decreased leg stiffness by 29% between the last step on the soft surface and the first step on the hard surface (from 10.7 kN m(-1) to 7.6 kN m(-1), respectively). As a result, the vertical displacement of the center of mass during stance ( approximately 7 cm) did not change at the transition despite a reduction in surface compression from 6 cm to less than 0.25 cm. By rapidly adjusting leg stiffness, each runner made a smooth transition between surfaces so that the path of the center of mass was unaffected by the change in surface stiffness.     

Effect of loading rate on the compressive mechanics of the immature baboon cervical spine

Thirty-four cervical spine segments were harvested from 12 juvenile male baboons and compressed to failure at displacement rates of 5, 50, 500, or 5000 mm/s. Compressive stiffness, failure load, and failure displacement were measured for comparison across loading rate groups. Stiffness showed a significant concomitant increase with loading rate, increasing by 62% between rates of 5 and 5000 mm/s. Failure load also demonstrated an increasing relationship with loading rate, while displacement at failure showed no rate dependence. These data may help in the development of improved pediatric automotive safety standards and more biofidelic physical and computational models.     

Denervated muscle fibers explain the deficit in specific force following reinnervation of the rat extensor digitorum longus muscle

The authors tested the hypothesis that, after denervation and reinnervation of skeletal muscle, observed deficits in specific force can be completely attributed to the presence of denervated muscle fibers. The peroneal nerve innervating the extensor digitorum longus muscle in rats was sectioned and the distal stump was coapted to the proximal stump, allowing either a large number of motor axons (nonreduced, n = 12) or a drastically reduced number of axons access to the distal nerve stump (drastically reduced, n = 18). A control group of rats underwent exposure of the peroneal nerve, without transection, followed by wound closure (control, n = 9). Four months after the operation, the maximum tetanic isometric force (Fo) of the extensor digitorum longus muscle was measured in situ and the specific force (sFo) was calculated. Cross-sections of the muscles were labeled for neural cell adhesion molecule (NCAM) protein to distinguish between innervated and denervated muscle fibers. Compared with extensor digitorum longus muscles from rats in the control (295 +/- 11 kN/m2) and nonreduced (276 +/- 12 kN/m2) groups, sFo of the extensor digitorum longus muscles from animals in the drastically reduced group was decreased (227 +/- 15 kN/m2, p < 0.05). The percentage of denervated muscle fibers in the extensor digitorum longus muscles from animals in the drastically reduced group (18 +/- 3 percent) was significantly higher than in the control (3 +/- 1 percent) group, but not compared with the nonreduced (9 +/- 2 percent) group. After exclusion of the denervated fibers, sFo did not differ between extensor digitorum longus muscles from animals in the drastically reduced (270 +/- 20 kN/m2), nonreduced (301 +/- 13 kN/m2), or control (303 +/- 10 kN/m2) groups. The authors conclude that, under circumstances of denervation and rapid reinnervation, the decrease in sFo of muscle can be attributed to the presence of denervated muscle fibers.     

Stiffness map of the grasping contact areas of the human hand

The elasticity and damping of the soft tissues of the hand contribute to dexterity while grasping and also help to stabilise the objects in manipulation tasks. Although some previous works have studied the force-displacement response of the fingertips, the responses in all other regions of the hand that usually participate in grasping have not been analysed to date. In this work we performed experimental measurements in 20 subjects to obtain a stiffness map of the different grasping contact areas of the human hand. A force-displacement apparatus was used to simultaneously measure force and displacement at 39 different points on the hand at six levels of force ranging from 1 N to 6 N. A non-linear force-displacement response was found for all points, with stiffness increasing with the amount of force applied. Mean stiffness for the different points and force levels was within the range from 0.2 N/mm to 7.7 N/mm. However, the stiffness range and variation with level of force were found to be different from point to point. A total of 13 regions with similar stiffness behaviours were identified. The stiffness in the fingertips increased linearly with the amount of force applied, while in the palm it remained more constant for the range of forces considered. It is hypothesised that the differences in the stiffness behaviour from one region to another allow these regions to play different roles during grasping.     

Analysis of the compressive strain below the removable and fixed prosthesis in the posterior mandible using a digital image correlation method

It was the purpose of this study to determine and analyse strains in the bone of posterior mandible below the removable and fixed partial dentures using Digital Image Correlation Method. Dried mandible with shortened dental arch (Kennedy Class 1) was used in the experiment. The mandible model was prepared for accepting two types of restorations for bilaterally missing molars conventional therapy, and removable and fixed partial dentures were manufactured following standard prosthetic protocol. The models with prosthetic restorations placed in situ were subjected to loading of 50-300 N, and photographed using two digital cameras as part of the digital image correlation method equipment. Afterwards, the obtained data for strains within restored mandible during loading ciclus were analysed in the software Aramis and graphically presented. Percentage size of the mandible strains within the line section are from 0.14 to 0.19% for the removable partial denture experimental model and 0-0.14% for the fixed partial denture experimental model. The study has demonstrated that Digital Image Correlation method can be used to measure strain on the mandible surface and replacements during loading and that from biomechanical perspective both therapeutic modalities can be equally useful for the restoration of the mandible with bilaterally distal edentulous spaces.