Key Engineering Materials
ISSN: 1662-9795, Vols. 270-273, pp 2061-2066
doi:10.4028/www.scientific.net/KEM.270-273.2061
© 2004 Trans Tech Publications, Switzerland
Citation &
Copyright (to be inserted by the publisher)
Online: 2004-08-15
A Nondestructive Diagnostic Modeling for Muscular Dysfunction of
Human Pharynx using Finite Element Method
Sung Min Kim1, Sung Jae Kim1, Ha Suk Bae2, Byeong Chul Choi3,
Joung Hwan Mun4
1
2
Dept. of Biomedical Eng., College of Medicine, Konkuk University, Chungbuk, Korea.
Dept. of Rehabilitation Medicine, College of Medicine, Yonsei University, Seoul, Korea
3
4
Dept. of Biomedical Eng., Choonhae College, Ulsan, Korea
Dept. of Bio-Mechatronics, Sungkyunkwan University, Suwon, Korea
Keywords: Nondestructive Diagnosis, Biomechanical Modeling
Abstract. Pharynx is a organ which transports food into the esophagus through peristaltic motion
(repeat of contraction and expansion movement) and functions as an air passage. In this study, a
structural change of human pharynx caused by muscular dysfunction is analyzed by a biomechanical
model using CT and FEM(finite element method). For the biomechanical model, a loading condition
is assumed so that equal internal pressure loaded sequentially to the inside of pharyngeal tissue. In
order to analyze the pharyngeal muscular dysfunction by the biomechanical model, the pharyngeal
dysfunctions are classified into 3 cases in the clinical complication by neuromuscular symptoms such
as pharyngeal dysfunction after stroke. Those cases frequently show a change of material property and
we investigated muscular tissue stiffness, deformation of cross sectional area of the pharynx by
increasing the stiffness to 25%, 50%, 75% on the basis of stress-strain relationship of the pharyngeal
tissue in each case. With three-dimensional reconstruction of pharyngeal structure using FEM and the
optimization process by using inverse dynamic approach, the biomechanical model of the human
pharynx is implemented and the muscular dysfunction is simulated.
Introduction
Dysfunction of Pharynx may cause various kinds of complication called dysphagia and the weakening
of pharyngeal function accompanied especially by aging and other diseases. However, this case is not
to be found at the early stage of various complications. Therefore, the synthetic analysis of pharyngeal
dysfunction is essential to this case. In general, previous investigations has put the key point in a
visible phenomenal description of a pharyngeal swallowing and one of the research [1] has tried to
measure actual degree of the muscle activity and pressure of the pharyngeal cavity through
electromyography of main pharyngeal muscle. This type of study shows various important aspects.
Considering physiological variables, however, it cannot provide reliable methodology.
It has been reported that 45% of acute aproplexy (By Gorden et al. [4]) and 51% of serious brain
damaged patients have dysphagia(by Elliott[3]). In diagnosis of dysphagia, a case history or a physical
examination of the patient is a basic methodology, which is insufficient for clinical judgment. A video
fluoroscopy test is one of the standard methods used in most cases. Ultrasonic endoscope, ECG,
manometry[5] test has also been used , but many problems remain unsolved yet. The results of those
methods may be integrated into the objective diagnostic index. Fixed quantity measurement is
compared with the video fluoroscopy. The result is that it has an advantage which estimation of the
pharyngeal cavity pass time can be measured[6][7].
Even though a biomechanical pharyngeal model by finite element analysis is used as an
approximate method, the evaluation of dysphagia through a biomechanical model of pharyngeal
dysfunction can be effective for noninvasive diagnosis of dysphagia. A biomechanical esophageal
model based on fluid mechanics describing esophageal motion by bolus transport is proposed by
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Brasseur et al [8]. The investigations on the changes of cross-sectional area caused by bolus volume
were reported by Kahrilas et al[12, 13, 14].
In this study, we present a new approach for the diagnosis of the pharyngeal muscular dysfunction
through a biomechanical model by utilizing FEA(Finite Element Analysis) in a structural analysis in
which we simulate a functional disorder of pharynx.
Methods
Material properties. To express a viscoelastic material behavior of pharyngeal tissue, we test several
human pharyngeal tissues in order to obtain several important material properties such as stress
relaxation test data and elastic response. The characteristic of the material of pharyngeal tissue used
for biomechanical model is assumed to be isometric. Poisson’s ratio is given as v=0.45. Several
material variables of viscoelastic behavior are obtained by stress relaxation test and uniaxial tensile
test.
Finite element analysis of pharyngeal structure. A pharyngeal 3-D model for FEA(finite
element analysis) using 8 level pharyngeal 2-D CT image data is implemented by taking 7mm interval
CT from the base of tongue to cricopharyngeus of a normal subject’s pharynx with no clinical
history(Fig. 1-(a)). The subject swallowed 10ml radio-opaque bolus and we obtained the total CT
image to reach from the oral cavity to the cricopharyngeus. The structure of the pharynx was
implemented with time in the image information and deformation of cross-sectional area variation on
the course of the section of the 5 step were observed in an each level to a 0.1 seconds interval.
(a)
(b)
(c)
Fig. 1. (a) Schematic image of pharynx and adjacent structures. (b) Schematic posterior view of the
musculature of hypopharynx and adjacent esophagus (c) Finite element model of pharynx.
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Fig. 2. Finite element model of pharyngeal muscular dysfunction
The 3-D 8-node elements are used in each level (Fig. 1-(c)). As for loading condition, it is assumed
that equal pressure is loaded sequentially to the inside of pharyngeal structure. The origin of the
coordinate is located at the center of superior pharynx; medial-lateral and anterior-posterior directions
are defined as x-axis and y-axis. The thickness of the pharyngeal tissue is assumed to be uniform in
the model. The structure of the model is showed at Fig.2-(a). Pressure values are increased with each
6mmHg in the pressure range from 3mmHg to 36mmHg, The nodes on the uppermost level (base of
tongue) are fixed and served as fixed boundary conditions. We assume that the central portion of the
posterior pharyngeal geometry is constrained by the cervical spine, and this is also considered as a
fixed boundary condition.
In this study, pharyngeal muscular dysfunctions are classified into 3 cases as follows.(Fig
2-b),(c),(d))
Case1 : dysfunction of half in the medial-lateral direction in the model
Case2 : dysfunction of 2, 3, 4 level in the model(mainly cricopharyngeal muscle)
Case3 : dysfunction of 7, 8 level in the model(connection part of esophagus and pharynx)
Bolus driving force in pharyngeal cavity can be expressed as pressure gradient. However, since the
measurement of the pressure gradient is practically difficult, the value of the pressure in pharyngeal
cavity is estimated by calculating the deformation of pharyngeal structure due to the change of
pressure (3mmHg~36mmHg, in this study) by FEM, and comparing the value with the one obtained
from CT image data. Taking into account the clinical complication by neuromuscular symptom such
as pharyngeal dysfunction by stroke, may result in muscular tissue stiffness caused by the change of
material property. Changes of cross section area of the pharynx are observed by increasing the
stiffness by 25%, 50%, 75% on the basis of stress-strain relationship.
Results
Finite Element Analysis of Pharynx Subjected to Assumed Forcess. All input and output variables
are presented with respect to the coordinate system fixed in the most superior level. The output
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variables of the model are the resultant displacements of the internal lumen of the pharynx, and the
pressure necessary to counterbalance the external load on pharyngeal wall.
Each solution is obtained from an optimization procedure. The resultant counterbalance pressure
magnitude is then equivalent to the intraluminal “swallowing” pressure.
In general finite element analysis, geometry, the material properties, and loading conditions on the
structure are known, and the nodal displacements (the deformations) are directly solved. However, in
the present study, pharyngeal geometry of the “deformed” structure, which comes from cine CT
image, is known, and the loading condition (in this case, pressure) remains as unknown variable
(inverse to dynamic approach). A finite element method is used to determine geometry of pharynx
during swallowing, corresponding to an assumed force history. The nodes on the uppermost level
(base of tongue) are fixed and served as fixed boundary conditions. The central portion of the
posterior pharyngeal geometry is assumed to be constrained by the cervical spine, which is also
considered as a fixed boundary condition.
The analysis is completed with a series of different assumed pressure patterns acting on the inner
surface of the pharynx with spatial, as well as temporal, variation.
A computer-predicted geometry of the pharynx is compared with the geometry of the pharynx as
depicted by the cine CT scan images at corresponding instances. The difference between the observed
geometry and the computer-predicted geometry is defined as a percent error and is to be minimized
via multiple modeling iterations. When the percent error approximates zero, a good kinematic match
occurs, and hence, the corresponding pressures are expected to be reliable estimates of the
intraluminal pressure gradients of pharyngeal swallowing.
Similarity between the result of the simulation of the change of cross section area due to the
arbitrary pressure and that of measuring the pressure gradient by means of CT is obtained. The
pharyngeal muscular dysfunction simulation by the model, i.e., pharyngeal tissue stiffness increases
by 25%, 50%, and 75% each, is performed. From the result, the tendency of change of cross section
area due to the stiffness of pharyngeal muscular tissue shows the maximum decrement when the
pressure is at 30mmHg on the first level in half model. The cross-sectional area decrement ratio
shows a trend to increase by 30 mmHg in all cases whereas it decreases after 30mmHg. The FEA
results of pharyngeal muscular dysfunction show that, in the Case 1 model, when pressure increases
by 9mmHg, the cross-sections of all levels show the decreasing ratio of 1%-8% when the stiffness
increases by 25 %. When it is compared with the normal one, the upper part of the model on the basis
of UES(Upper Esophageal Sphincter) shows 8-10 % decreasing ratio at 30 mmHg ,
1) When the stiffness increases by 50%, the model shows the decreasing ratio of 15-21% ,
2) When the stiffness increases by 75%, the model shows the decreasing ratio of 20-30% .
And lower part (level 5,6,7,8) in Case 1 shows the decreasing ratio of 3-5 % at 36 mmHg when
the stiffness increases by 25%,
1) When the stiffness increases by 50 %, the model shows the decreasing ratio of 5-12% .
2) When the stiffness increases by 75% , the model shows the decreasing ratio of 7-15% .
In the Case 2 model (dysfunction of 2, 3, 4 level in the model), when pressure increases until
9mmHg, the cross-sections of all levels show the decreasing ratio of 1%- 7% when the stiffness
increases by 25 %. When it is compared with the normal one, the upper part on the basis of UES
(Upper Esophageal Sphincter) shows the decreasing ratio of 7-9 % at 30 mmHg,
In the Case 3 model (dysfunction of 7, 8 level in the model), the level 7,8 (lower part ) shows the
decreasing ratio of 2-5% at 36 mmHg when the stiffness increases by 25 %, Table .1 represents the
results of all 3 case models simulation results.
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Table 1. Estimated cress-sectional area decreasing ratio of pharynegeal muscular dysfunction
Stiffness Increase Ratio(%)
Range of
decrease ratio
(each level)
Case1
Case2
Case3
125%
150%
175%
L1234
L5678
8-10%
3-5%
15-21%
5-12%
20-30%
7-15%
at 30mmHg
at 36mmHg
L123
L45
L678
L123456
L78
7-9%
5-6%
Under 5%
Under 7%
2-5%
15-18%
9-11%
Under 5%
Under 7%
4-9%
20-27%
12-15%
under 5%
Under 7%
6-12%
at 30mmHg
at 36mmHg
at 36mmHg
Discussion
In this study, mathematical model of viscoelasticity of pharynx is implemented based on viscoelastic
theory proposed by Y. C. Fung and the parameters of the model are obtained from experimental test.
By way of three-dimensional reconstruction of pharyngeal structure by using optimization process
based on inverse dynamic approach method and directly applying the results to the model, we may
estimate consecutive pressure gradient of the pressure created internally when the pharynx functions.
The change of the various pressure gradients in the pharyngeal structure was estimated by quasi-static
method and the simulation results shows that the pressure gradient of the pharynx is changed in all
levels from proximal to distal part. Also, maximal change of the pharyngeal deformation is observed
at the anterior portions rather than at the posterior ones and localized stress concentration is assumed
to be at the posterior region. From the model simulations, the trajectory of the pressure gradient in the
pharynx by given deformation of each level’s pharyngeal cross section area is compared with previous
investigations and shows similar patterns with those.
Conclusions
In this study we estimates pressure gradient of the pharynx in various pharyngeal muscular
dysfunction cases caused by stiffness of tissue resulted in the tendency of cross sectional deformation
change, applying the results as the clinical index to diagnosing the degree of abnormality of
pharyngeal tissue. To make this method effective for the early diagnosis of pharyngeal dysfunction,
the model should include various biomechanical parameters. Also, it is necessary to characterize the
active mechanism of pharyngeal tissue by EMG measurement,
The results can be applied as a useful diagnosis model which is able to discover pharyngeal
disorder at an early stage and the mechanism of pharyngeal function. Such a biomechanical model can
simulate the structural or pathological variation by the pharyngeal paralysis, cancer, tumor resection
and pharyngo-esophageal stenosis.
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