3D patient-specific numerical modeling of the soft palate considering adhesion from the tongue

Collapse of the soft palate in the upper airway contributes to obstructive sleeping apnea (OSA). In this study, we investigate the influence of the adhesion from the tongue on the soft palate global response. This is achieved using a cohesive zone finite element approach. A traction-separation law is determined to describe the adhesion effect from the surface tension of the lining liquid between the soft palate and the tongue. According to pull-off experimental tests of human lining liquid from the oral surface of the soft palate, the corresponding cohesive properties, including the critical normal traction stress and the failure separation displacement, are obtained. The 3D patient-specific soft palate geometry is accounted for, based on one specific patient’s computed tomography (CT) images. The calculation results show that influence of the adhesion from the tongue surface on the global response of the soft palate depends on the length ratio between the cohesive length and the soft palate length. When the length of the cohesive zone is smaller than half of the soft palate length, the adhesion’s influence is negligible. When the adhesion length is larger than 70 percent of soft palate length, the adhesion force contributes to preventing the soft palate from collapsing towards to the pharynx wall, i.e. the closing pressure is more negative than in the no adhesion case. These results may provide useful information to the clinical treatment of OSA patients.     

Snoring source identification and snoring noise prediction

This paper investigates the snoring mechanism of humans by applying the concept of structural intensity to a three-dimensional (3D) finite element model of a human head, which includes: the upper part of the head, neck, soft palate, hard palate, tongue, nasal cavity and the surrounding walls of the pharynx. Results show that for 20, 40 and 60Hz pressure loads, tissue vibration is mainly in the areas of the soft palate, the tongue and the nasal cavity. For predicting the snoring noise level, a 3D boundary element cavity model of the upper airway in the nasal cavity is generated. The snoring noise level is predicted for a prescribed airflow loading, and its range agrees with published measurements. These models may be further developed to study the various snoring mechanisms for different groups of patients.     

Movement of the human upper airway during inspiration with and without inspiratory resistive loading

The electromyographic (EMG) activity of human upper airway muscles, particularly the genioglossus, has been widely measured, but the relationship between EMG activity and physical movement of the airway muscles remains unclear. We aimed to measure the motion of the soft tissues surrounding the airway during normal and loaded inspiration on the basis of the hypothesis that this motion would be affected by the addition of resistance to breathing during inspiration. Tagged MR imaging of seven healthy subjects was performed in a 3-T scanner. Tagged 8.6-mm-spaced grids were used, and complementary spatial modulation of magnetization images were acquired beginning ～200 ms before inspiratory airflow. Deformation of tag line intersections was measured. The genioglossus moved anteriorly during normal and loaded inspiration, with less movement during loaded inspiration. The motion of tissues at the anterior border of the upper airway was nonuniform, with larger motions inferiorly. At the level of the soft palate, the lateral dimension of the airway decreased significantly during loaded inspiration (-0.15 ± 0.09 and -0.48 ± 0.09 mm during unloaded and loaded inspiration, respectively, P < 0.05). When resistance to inspiratory flow was added, genioglossus motion and lateral dimensions of the airway at the level of the soft palate decreased. Our results suggest that genioglossus motion begins early to dilate the airway prior to airflow and that inspiratory loading reduces the anterior motion of the genioglossus and increases the collapse of the lateral airway walls at the level of the soft palate.     

OSAHS患者上气道和软腭的流固耦合有限元模型的建立

目的:重建OSAHS患者上气道和软腭的流固耦合有限元模型,研究OSAHS患者上气道及软腭气流动力学特征,为进一步探讨OSAHS的的发病机制奠定基础。方法:对一名中度OSAHS患者的上气道及周围组织进行MRI扫描,将以DICOM格式存储的扫描数据导入Mimics15.0软件中进行预处理,得到上气道和软腭的模型;再利用逆向工程软件Geomagic Studio 2013建立了2 mm气道壁;然后在3-D重建软件NX中,生成气道壁和气道以及软腭之间的组合模型;最后将该组合模型导入ANSYS Workbench13.0软件中,通过网格划分、定义材料属性、设定模型的边界条件操作建立了上气道和软腭的流固耦合有限元模型。结果:利用Mimics、Ansys等软件建立了完整的上气道和软腭的流固耦合有限元模型。共得到气道:2806835单元和529281个节点;气道壁:2304348单元和3487609个节点;软腭:131855单元和204784个节点。结论:本研究建立的上气道及软腭的流固耦合有限元模型符合人体的生物力学特点,为下一步的数值模拟实验提供了一个更真实、可靠的模型。     

Dynamic interaction between the tongue and soft palate during obstructive apnea in anesthetized patients with sleep-disordered breathing.

Little is known about the mechanisms of persistence of obstructive apnea. Structurally, the dorsum of the tongue locates anterior to the soft palate. On the basis of the observation of posterior displacement of the tongue during obstructive apnea, we hypothesized that the dorsum of the tongue pushes the anterior wall of the soft palate posteriorly during inspiratory efforts, maintaining closure at the retropalatal airway. To test this hypothesis, we measured the pressure between dorsum of the tongue and anterior wall of the soft palate (PT&P) during experimentally induced obstructive apneas in anesthetized patients with sleep-disordered breathing. P(T&P) changes during the obstruction significantly depended on collapsibility of the retroglossal airway. Progressive increase in the P(T&P) during obstructive apnea was observed only in patients with highly collapsible retroglossal airways. Significant increase in the P(T&P) during inspiratory effort in accordance with positive deflection pattern of P(T&P) tracing was evident in the patients with highly collapsible retroglossal airways. The results indicate significant dynamic interaction between the tongue and soft palate during both obstructive apnea and each inspiratory effort, possibly maintaining closure at the retropalatal airway.     

Passive movement of human soft palate during respiration: A simulation of 3D fluid/structure interaction

This study reconstructed a three dimensional fluid/structure interaction (FSI) model to investigate the compliance of human soft palate during calm respiration. Magnetic resonance imaging scans of a healthy male subject were obtained for model reconstruction of the upper airway and the soft palate. The fluid domain consists of nasal cavity, nasopharynx and oropharynx. The airflow in upper airway was assumed as laminar and incompressible. The soft palate was assumed as linear elastic. The interface between airway and soft palate was the FSI interface. Sinusoidal variation of velocity magnitude was applied at the oropharynx corresponding to ventilation rate of 7.5L/min. Simulations of fluid model in upper airway, FSI models with palatal Young's modulus of 7539Pa and 3000Pa were carried out for two cycles of respiration. The results showed that the integrated shear forces over the FSI interface were much smaller than integrated pressure forces in all the three directions (axial, coronal and sagittal). The total integrated force in sagittal direction was much smaller than that of coronal and axial directions. The soft palate was almost static during inspiration but moved towards the posterior pharyngeal wall during expiration. In conclusion, the displacement of human soft palate during respiration was mainly driven by air pressure around the surface of the soft palate with minimal contribution of shear stress of the upper airway flow. Despite inspirational negative pressure, expiratory posterior movement of soft palate could be another factor for the induction of airway collapse.     

Characterization of a hyper-viscoelastic phantom mimicking biological soft tissue using an abdominal pneumatic driver with magnetic resonance elastography (MRE)

The purpose of this study was to create a polymer phantom mimicking the mechanical properties of soft tissues using experimental tests and rheological models. Multifrequency Magnetic Resonance Elastography (MMRE) tests were performed on the present phantom with a pneumatic driver to characterize the viscoelastic (μ, η) properties using Voigt, Maxwell, Zener and Springpot models. To optimize the MMRE protocol, the driver behavior was analyzed with a vibrometer. Moreover, the hyperelastic properties of the phantom were determined using compressive tests and Mooney-Rivlin model. The range of frequency to be used with the round driver was found between 60 Hz and 100 Hz as it exhibits one type of vibration mode for the membrane. MRE analysis revealed an increase in the shear modulus with frequency reflecting the viscoelastic properties of the phantom showing similar characteristic of soft tissues. Rheological results demonstrated that Springpot model better revealed the viscoelastic properties (μ=3.45 kPa, η=6.17 Pas) of the phantom and the Mooney-Rivlin coefficients were C(10)=1.09.10(-2) MPa and C(01)=-8.96.10(-3) MPa corresponding to μ=3.95 kPa. These studies suggest that the phantom, mimicking soft tissue, could be used for preliminary MRE tests to identify the optimal parameters necessary for in vivo investigations. Further developments of the phantom may allow clinicians to more accurately mimic healthy and pathological soft tissues using MRE.     

Assessment of in vivo and post-mortem mechanical behavior of brain tissue using magnetic resonance elastography

The knowledge of in vivo brain tissue mechanical properties is essential in several biomedical engineering fields, such as injury biomechanics and neurosurgery simulation. Almost all existing available data have been obtained in vitro by invasive experimental protocols. However, the difference between in vivo and post-mortem mechanical properties remains poorly known, essentially due to the lack of a common method that could measure them both in vivo and ex vivo. In this study, we report the use of magnetic resonance elastography (MRE) for the non-invasive assessment of in vivo brain tissue viscoelastic properties and for the investigation of their evolution after the death. Experiments were performed on seven adult male rats. Shear storage and loss moduli were measured in vivo, just after death and at post-mortem time of approximately 24h. A significant increase in shear storage modulus G(') of approximately 100% was found to occur just after death (p=0.002), whereas no significant difference was found between in vivoG(') and G(') at 24h post-mortem time. No significant difference was found between shear loss modulus G(')in vivo and just after death, whereas a decrease of about 50% was found to occur after 24h (p=0.02). These results illustrate the ability of MRE to investigate some of the critical soft tissue biomechanics-related issues, as it can be used as a non-invasive tool for measuring soft tissue viscoelastic properties.     

Magnetic resonance elastography in nonlinear viscoelastic materials under load

Characterisation of soft tissue mechanical properties is a topic of increasing interest in translational and clinical research. Magnetic resonance elastography (MRE) has been used in this context to assess the mechanical properties of tissues in vivo noninvasively. Typically, these analyses rely on linear viscoelastic wave equations to assess material properties from measured wave dynamics. However, deformations that occur in some tissues (e.g. liver during respiration, heart during the cardiac cycle, or external compression during a breast exam) can yield loading bias, complicating the interpretation of tissue stiffness from MRE measurements. In this paper, it is shown how combined knowledge of a material’s rheology and loading state can be used to eliminate loading bias and enable interpretation of intrinsic (unloaded) stiffness properties. Equations are derived utilising perturbation theory and Cauchy’s equations of motion to demonstrate the impact of loading state on periodic steady-state wave behaviour in nonlinear viscoelastic materials. These equations demonstrate how loading bias yields apparent material stiffening, softening and anisotropy. MRE sensitivity to deformation is demonstrated in an experimental phantom, showing a loading bias of up to twofold. From an unbiased stiffness of \(4910.4 \pm 635.8\) Pa in unloaded state, the biased stiffness increases to 9767.5 \(\pm \,\)1949.9 Pa under a load of \(\approx \) 34% uniaxial compression. Integrating knowledge of phantom loading and rheology into a novel MRE reconstruction, it is shown that it is possible to characterise intrinsic material characteristics, eliminating the loading bias from MRE data. The framework introduced and demonstrated in phantoms illustrates a pathway that can be translated and applied to MRE in complex deforming tissues. This would contribute to a better assessment of material properties in soft tissues employing elastography.

     

弥猴单侧软腭肌肉对针式电极刺激的反应

目的建立针电极口内刺激猴软腭肌肉诱发腭咽闭合运动的模式,取得软腭肌肉运动的有效刺激数值,为软腭肌肉功能重建奠定基础。方法通过解剖成年猕猴软腭的五组肌肉,确定其体表位置;利用实验动物用腭部肌肉电极定位刺激器及针式电极对软腭肌肉进行有效刺激;结合鼻咽纤维镜、头颅侧位X片及软腭造影技术观察、记录肌肉收缩及腭咽闭合动作。结果在猕猴口内定位目标肌肉进行针电极刺激可诱发肌肉收缩。刺激电压为3 V、刺激频率为20 Hz时均能诱发单侧软腭肌肉的有效收缩;单侧腭帆提肌在刺激电压为5 V、20 Hz时可发生腭咽闭合动作。咽腭肌、舌腭肌在刺激电压5 V、刺激频率100 Hz时发生软腭下降动作。腭帆张肌仅发生收缩,而未发生腭咽闭合。应用鼻咽纤维镜和X线成像技术配合能记录腭咽闭合动作。结论弥猴可作为研究软腭肌肉运动模式的实验动物。应用电极刺激软腭肌肉,可初步建立腭咽闭合的动作模式。     

Influence of tongue muscle contraction and dynamic airway pressure on velopharyngeal volume in the rat.

The mammalian pharynx is a collapsible tube that narrows during inspiration as transmural pressure becomes negative. The velopharynx (VP), which lies posterior to the soft palate, is considered to be one of the most collapsible pharyngeal regions. I tested the hypothesis that negative transmural pressure would narrow the VP, and that electrical stimulation of extrinsic tongue muscles would reverse this effect. Pressure (-6, -3, 3, and 6 cmH2O) was applied to the isolated pharyngeal airway of anesthetized rats that were positioned in a 4.7-T MRI scanner. The volume of eight axial slices encompassing the length of the VP was computed at each level of pressure, with and without bilateral hypoglossal nerve stimulation (0.1-ms pulse, one-third maximum force, 80 Hz). Negative pressure narrowed the VP, and either whole hypoglossal nerve stimulation (coactivation of protrudor and retractor muscles) or medial nerve branch stimulation (independent activation of tongue protrudor muscles) reversed this effect, with the greatest impact in the caudal one-third of the VP. The dilating effects of medial branch stimulation were slightly larger than whole nerve stimulation. Positive pressure dilated the VP, but tongue muscle contraction did not cause further dilation under these conditions. I conclude that the narrowest and most collapsible segment of the rat pharynx is in the caudal VP, posterior to the tip of the soft palate. Either coactivation of protrudor and retractor muscles or independent contraction of protrudor muscles caused dilation of this region, but the latter was slightly more effective.     

The Distribution of TRPV1 and TRPV2 in the Rat Pharynx

Immunohistochemistry for two nociceptive transducers, the transient receptor potential cation channel subfamily V members 1 (TRPV1) and 2 (TRPV2), was performed on the pharynx and its adjacent regions. TRPV1-immunoreactivity (IR) was detected in nerve fibers beneath and within the epithelium and/or taste bud-like structure. In the pharynx, these nerve fibers were abundant in the naso-oral part and at the border region of naso-oral and laryngeal parts. They were also numerous on the laryngeal side of the epiglottis and in the soft palate. TRPV2-IR was expressed by dendritic cells in the pharynx and epiglottis, as well as in the root of the tongue and soft palate. These cells were located in the epithelium and lamina propria. TRPV2-immunoreactive (IR) dendritic cells were numerous in the naso-oral part of the pharynx, epiglottis, and tongue. Abundance of TRPV2-IR dendritic processes usually obscured the presence of TRPV2-IR nerve fibers in these portions. However, some TRPV2-IR nerve fibers could be observed in the epithelium of the soft palate. Retrograde tracing method also revealed that sensory neurons which innervate the pharynx or soft palate were abundant in the jugular–petrosal ganglion complex and relatively rare in the nodose ganglion. In the jugular–petrosal ganglion complex, TRPV1- and TRPV2-IR were expressed by one-third of pharyngeal and soft palate neurons. TRPV2-IR was also detected in 11.5 % pharyngeal and 30.9 % soft palate neurons in the complex. Coexpression of TRPV1 and CGRP was frequent among pharyngeal and soft palate neurons. The present study suggests that TRPV1- and TRPV2-IR jugular–petrosal neurons may be associated with the regulation of the swallowing reflex.     

Sleep-related obstructive disordered breathing in cleft palate patients after palatoplasty

Sleep-disordered breathing is frequently associated with children presenting congenital midface defects. Because of structural and functional anomalies in the upper airway, children with cleft palate, especially after surgery, may carry a higher risk of developing sleep-disordered breathing. However, the presence of such sleep-disordered breathing in older cleft palate children has not been emphasized. The aim of this comparative overnight cardiorespiratory sleep study was to evaluate cleft palate patients according to sleep-disordered breathing. A group of 43 cleft palate children (17 girls and 26 boys; mean age, 12.1 +/- 3.8 years) was compared with a control group of 20 randomly selected, noncleft children matched for age, sex, and body mass index. None of the patients suffered from manifest sleep-disordered breathing. Cleft palate patients had a statistically significantly higher respiratory disturbance index and snoring index, but no increased apnea index. The data suggest that cleft palate patients having undergone primary closure of the palate demonstrate microsymptoms of nocturnal upper airway obstruction.     

An upper airway resonator model of high-frequency inspiratory sounds in children with sleep-disordered breathing.

The goal of this study was to determine how high-frequency inspiratory sounds (HFIS) are generated by sleeping children with obstructive sleep-disordered breathing (OSDB). We hypothesized that HFIS are generated when a high-velocity jet of air, generated by a narrowed upper airway, induces the upper airway to act as a resonating chamber. We tested two predictions of this hypothesis: 1) the upper airway is narrowed in children who make HFIS and 2) the length of the upper airway, calculated from HFIS harmonic intervals, is similar to that calculated from magnetic resonance imaging (MRI) scans. The study was conducted in the setting of a sleep laboratory. Participants included 29 children between 6 and 12 yr of age with adenotonsillar hypertrophy suspected of having OSDB. Minimum cross-sectional airway area and airway long dimensions (lips to larynx or soft palate) were measured in awake children with MRIs. Later that night, sound was recorded with a microphone suspended above their bed while the children underwent polysomnography. Sounds were later analyzed with fast Fourier transforms. We found that sleeping children who generated HFIS had significantly narrower upper airways compared with children who did not make HFIS [minimum airway area 20.5 +/- 4.4 vs. 70.9 +/- 22.5 mm(2) (mean +/- SE), respectively; P = 0.02]. There was a significant inverse correlation between the log(10) of the narrowest airway area and the number of HFIS recorded per hour (r(2) = 0.55, P < 0.00001). The harmonics characteristics of HFIS predicted that they were generated by sound resonating in chamber whose length was 12.0 +/- 0.9 cm, which is similar to the MRI measured distance from the lips to the larynx of 12.8 +/- 0.4 cm. In conclusion, these data suggest that children generate HFIS when 1) they have a narrowed upper airway and 2) their upper airway acts as a resonating chamber.     

Simulation of turbulent airflow using a CT based upper airway model of a racehorse

Computational model for airflow through the upper airway of a horse was developed. Previous flow models for human airway do not hold true for horses due to significant differences in anatomy and the high Reynolds number of flow in the equine airway. Moreover, models that simulate the entire respiratory cycle and emphasize on pressures inside the airway in relation to various anatomical diseases are lacking. The geometry of the airway was created by reconstructing images obtained from computed tomography scans of a thoroughbred racehorse. Different geometries for inhalation and exhalation were used for the model based on the difference in the nasopharynx size during the two phases of respiration. The Reynolds averaged Navier-Stokes equations were solved for the isothermal flow with the standard k-epsilon model for turbulence. Transient pressure boundary conditions for the entire breathing cycle were obtained from past experimental studies on live horses. The flow equations were solved in a commercial finite volume solver. The flow rates, computed based on the applied pressure conditions, were compared to experimentally measured flow rates for model validation. Detailed analysis of velocity, pressure, and turbulence characteristics of the flow was done. Velocity magnitudes at various slices during inhalation were found to be higher than corresponding velocity magnitudes during exhalation. The front and middle parts of the nasopharynx were found to have minimum intraluminal pressure in the airway during inhalation. During exhalation, the pressures in the soft palate were higher compared to those in the larynx, epiglottis, and nasopharynx. Turbulent kinetic energy was found to be maximum at the entry to the airway and gradually decreased as the flow moved inside the airway. However, turbulent kinetic energy increased in regions of the airway with abrupt change in area. Based on the analysis of pressure distribution at different sections of the airway, it was concluded that the front part of the nasopharynx requires maximum muscular activity to support it during inhalation. During exhalation, the soft palate is susceptible to displacements due to presence of high pressures. These can serve as critical information for diagnosis and treatment planning of diseases known to affect the soft palate and nasopharynx in horses, and can potentially be useful for human beings.     

The influence of physiological aging and atrophy on brain viscoelastic properties in humans

Physiological aging of the brain is accompanied by ubiquitous degeneration of neurons and oligodendrocytes. An alteration of the cellular matrix of an organ impacts its macroscopic viscoelastic properties which can be detected by magnetic resonance elastography (MRE) – to date the only method for measuring brain mechanical parameters without intervention. However, the wave patterns detected by MRE are affected by atrophic changes in brain geometry occurring in an individual''s life span. Moreover, regional variability in MRE-detected age effects is expected corresponding to the regional variation in atrophy. Therefore, the sensitivity of brain MRE to brain volume and aging was investigated in 66 healthy volunteers aged 18–72. A linear decline in whole-brain elasticity was observed (−0.75%/year, R-square = 0.59, p<0.001); the rate is three times that determined by volume measurements (−0.23%/year, R-square = 0.4, p<0.001). The highest decline in elasticity (−0.92%/year, R-square = 0.43, p<0.001) was observed in a region of interest placed in the frontal lobe with minimal age-related shrinkage (−0.1%, R-square = 0.06, p = 0.043). Our results suggest that cerebral MRE can measure geometry-independent viscoelastic parameters related to intrinsic tissue structure and altered by age.     

Effect of upper airway negative pressure on proprioceptive afferents from the tongue.

We examined whether receptors in the tongue muscle respond to negative upper airway pressure (NUAP). In six cats, one hypoglossal nerve was cut and its distal end was prepared for single-fiber recording. Twelve afferent fibers were selected for study on the basis of their sensitivity to passive stretch (PS) of the tongue. Fiber discharge frequency was measured during PS of the tongue and after the rapid onset of constant NUAP. During PS of 1-3 cm, firing frequency increased from 17 +/- 7 to 40 +/- 11 (SE) Hz (P < 0.01). In addition, 8 of the 12 fibers responded to NUAP (-10 to -30 cmH2O), with firing frequency increasing from 23 +/- 9 to 41 +/- 9 Hz (P < 0.001). In two fibers tested, the increase in firing frequency in response to NUAP was not altered by topical anesthesia (10% lignocaine) applied liberally to the entire upper airway mucosa. Our results demonstrate that afferent discharges from the hypoglossal nerve are elicited by 1) stretching of the tongue and 2) NUAP before and after upper airway anesthesia. We speculate that activation of proprioceptive mechanoreceptors in the cat's tongue provides an additional pathway for the reflex activation of upper airway dilator muscles in response to NUAP, independent of superficially located mucosal mechanoreceptors.     

Comparison of surface electromyographic (sEMG) activity of submental muscles between the head lift and tongue press exercises as a therapeutic exercise for pharyngeal dysphagia

Objective: The present study compared surface electromyographic (sEMG) activity obtained from the submental muscle group for a tongue press and a head lift exercise as potential therapeutic exercises for dysphagic elderly. Materials and methods: Fifty‐three healthy volunteers with a mean age of 35.3 participated in this study. Subjects were required to perform an isometric task, pressing their tongue against the hard palate, and an isotonic task requiring sustained lingual force against the hard palate. Pressure sensors were used to measure the amount of lingual pressure against the hard palate. Submental sEMG data from these tasks were compared with those obtained from the isometric and isotonic aspects of a head lift exercise. Results: No sEMG differences were identified between the isometric tongue press task and head lift exercise. Isotonic tongue press exercises resulted in significantly higher maximum and mean sEMG values compared with the isotonic head lift exercise (p < 0.05). The submental sEMG activity from the tongue press exercise was equal (isometric) to, or greater (isotonic) than comparable muscle activation obtained during the head lift exercise. Conclusions: The tongue press exercise may be less strenuous than the head lift exercise while achieving the same therapeutic effect.     

The surface structure of the completely and incompletely orthokeratinized oral epithelium in the rat: a light, scanning and transmission electron microscope study.

Mucosa from the hard and soft palates, molar gingiva, cheek and dorsal surface of the tongue of the rat was examined in the light microscope, following Mallory's triple connective tissue stain, and in the scanning and transmission electron microscopes. The epithelium covering the hard palate, gingiva, the smooth band of mucosa at the junction of the hard and soft palates, intermediate zones of the soft palate, fungiform papilla-like structures in the central zone of the soft palate, the fungiform papillae, and the more superficial part and posterior surfaces of the filiform papillae of the tongue all exhibited complete orthokeratinization. The oral surfaces of the epithelial cells in all these areas had a honeycomb pattern of interconnecting ridges surrounding depressions. Imprints of the overlying cells that had been desquamated were apparent, and the lateral boundaries between the cells were formed by two raised ridges separated by a gap. The epithelium covering the cheek, central zone of the soft palate apart from the fungiform papilla-like structures, lateral zones of the soft palate, gingival crevice, and the mucosa between the fungiform and filiform papillae of the tongue all exhibited incomplete orthokeratinization. The oral surfaces of the epithelial cells in all these areas were relatively smooth and did not exhibit a honeycomb pattern of interconnecting ridges. Imprints of the overlying cells that had been desquamated and the lateral boundaries between the cells were only very occasionally found. In the transmission electron microscope the outlines of the cells were compatible with the surface patterns seen in the scanning electron microscope. The possible relationships between the degree of orthokeratinization and ultrastructure of the various epithelia are discussed.     

A mechanical model representation of the in vivo creep behaviour of muscular bulk tissue

In vivo mechanical properties of bulk tissue have not yet been explored sufficiently. One of the major problems researchers face is the lack of agreement between the constitutive models and the standardised methodologies for experimental studies. The object of this study was to obtain bulk modulus of the upper arm under relaxed and controlled contraction that was 25% of the maximum voluntary contraction. A new testing machine was designed to generate constant load on the upper arm and measure the deformation over time. This device is effectively a cuff that applies controllable pressure on a 47-mm wide band of the upper arm. Six different loads (10, 20, 30, 40, 50 and 60 kgf) were applied over a time of up to a maximum of 120 s. The deflection-time curves obtained show strongly non-linear responses of the bulk tissue. The non-linearity manifested by these deflection-time curves is in terms of both time- and load-dependency. A specific mechanical model was developed to represent the creep behaviour of the bulk tissue. The creep behaviour of the upper arm can be simulated by using four Voigt viscoelastic models in series. The three obvious soft tissues of the upper arm, namely skin, fat and muscle, were modelled in series. The effects of blood vessels and connective tissue were also modelled in series with the previous ones. A mechanical model would provide a more controlled method of studying the mechanical properties of the bulk tissue. The purpose of the current research, therefore, was to develop a mechanical model, which would predict the non-linear, viscoelastic behaviour of the human muscular bulk tissue.