Mechanical Properties of Soft Tissues in the Human Chest, Abdomen and Upper extremities

Transcription

1 Mechanical Properties of Soft Tissues in the Human Chest, Abdomen and Upper extremities Dr. A. Chawla 1, Member Dr. S. Mukherjee, Non-member B. Karthikeyan Non-Member ABSTRACT In this paper we review the mechanical properties of soft tissues available in literature. Human body regions are split into different parts to pursue this study. This review paper focuses on the soft tissues in the chest, abdomen and upper extremities. Material properties, which are directly extracted from the experimental methods, and the constitutive properties that have been used in finite element models are looked at. Isolated tissue tests, subsegmental tests and full-scale tests used for validating the respective finite element models are investigated. Static and dynamic properties are sorted according to the tissue type. Variations in the data from different sources has been studied and summarized. Scatter in the static properties and less frequently available dynamic properties indicate the need for further testing and alternate material models. Keywords: Material properties, Human soft tissues, Chest, Abdomen, Upper extremity INTRODUCTION Human body finite element (FE) models, if based on a realistic geometry and bio-fidelic material properties, can be useful in designing safer vehicles in order to reduce incidences of injuries and fatalities in road crashes 1,2. To identify the reliability and variations within the material properties reported in literature, a review has been conducted on the soft tissues in the whole human body. Human body regions are divided into three parts a) lower extremities b) head, neck and spine and c) chest, abdomen and upper extremity. The present study reviews the soft tissues in chest, abdomen and upper extremity. Reported data within these regions are collated. Constitutive properties of soft tissues used in the finite element models and the validating experimental procedures are reviewed. Mechanical properties are categorized and analyzed in the following sections according to major tissue type in the respective body regions. CHEST, ABDOMEN AND PELVIS Torso is an anatomical term for the upper part of the human body without the head and limbs. The torso includes the chest, back, and abdomen. Torso and pelvic injuries constitute 22% among all injuries with an Abbreviated Injury scale (AIS) of 2 to 6 in pedestrian car accidents 3 and are also a significant amongst injuries to occupants in side impacts 4. The chest is the region of the body between the neck and the abdomen. Thorax is the cage which lies in the upper part of the torso, extending from the base of the neck to the lower ribs. Thorax is mainly protected and supported by the ribcage, spine, and shoulder girdle. Most of the studies often use the words thorax and chest interchangeably as the biomechanical studies conducted between these regions are closely interrelated. The portion of the body which lies between the thorax and the pelvis is called as abdomen. It contains a cavity (abdominal 1 Corresponding Author Dr. A Chawla, Associate Professor, Department of Mechanical Engineering, IIT Delhi, Hauz Khas, New Delhi achawla@mech.iitd.ernet.in

2 cavity) separated by the diaphragm from the thoracic cavity above and by the plane of the pelvic inlet from the pelvic cavity below where the most of internal organs are placed. There are very few independent studies reported for the torso region. Most of the studies were conducted jointly for the chest, abdomen and pelvis region or in a combination of any two. Chest and Abdomen Soft Tissues Chest and abdomen regions consist of main internal organs of the body including the like heart, bronchi, spleen; liver, lung, stomach, pancreas, trachea, kidney, aorta, pectoral muscles, trapezius muscles, diaphragm, vena cava, oesophagus and intestinal digestive systems. Readers are encouraged to refer to a text on anatomy 5, for a detailed anatomical description. Chest And Abdomen Injury Load Cases Under blunt impacts, the thorax injuries can be attributed to three mechanisms: the compression of the thorax, the viscous loading within the thorax cavity, and the inertial loading to the internal organs 6,7. Experiments on Chest Abdomen and Pelvis Regions and Related Injury Assessment Functions Injury criterion or injury assessment functions developed to establish the degree of human tolerance to chest, abdomen and thorax impacts are discussed in this section. Several experimental studies on the thorax region were performed to evaluate the occupant injuries in lateral and frontal impacts. Injury assessment functions were reported for predicting both regional and local injuries. The injury criterions are classified as acceleration criterion, force criterion, compression criterion and viscous criterion 8. The acceleration measured at one point in the spine is said to be a reasonable indicator of whole body response and the severity of crash. An improved acceleration based criterion, the Thoracic Trauma Index (TTI) has been evaluated using the lateral acceleration measured at rib and thoracic vertebrae along with information of weight and age of the subject 9. Some studies on thorax have quantified thoracic injury tolerance and reported the mechanical response for blunt, mid sternal and anterior posterior impact 10,11. These studies indicate the maximum chest compression as an injury indicator. This criterion is considered a better injury indicator of chest injury severity than single point acceleration and force criterion. The compression criterion is considered suitable mainly for frontal impacts. As the compression criterion becomes inadequate when the velocity of deformation exceeds 3 m/s, a viscous injury criterion based on the severity and the time of occurrence of soft tissue injury was predicted by Viano and Lau 8,12. The viscous criterion is rate dependant and measures the risk of soft tissue injury during impact. This viscous injury assessment function is defined as a product of velocity of deformation and amount of compression of the body. This viscous response gives a better correlation with chest and abdominal injuries in lateral impacts than the compression criterion 13. It also indicates that chest and abdominal injury may occur by a viscous mechanism during rapid compression, and both viscous and compression responses should be used to assess injury risk in side impacts. Viano 14 conducted blunt lateral impacts on fourteen unembalmed cadavers at chest and abdomen with initial velocities 4.5, 6.7 and 9.4 m/s using 15-kg and 25-kg flat circular face impactors. Viscous criterion tolerance for the chest and abdomen are reported as 1.47m/s and 1.98 m/s respectively. Maximum compression criterion is considered as 38% for chest and 44% for abdomen. Cavanaugh and Zhu 15,16 performed seventeen-cadaver side impact tests to find the injury response of thorax. The tests were conducted using horizontally accelerated sled and Heidelberg type seat fixture with three surface conditions, flat rigid wall, Unpadded sidewall with 6 pelvic offset and Flat padded side wall. They propose average spine acceleration (ASA), and compare it with the viscous criterion. The ASA is calculated by taking the slope of spine velocity obtained by integrating the acceleration measured at thoracic vertebrae. Cesari et al, have shown that when the arm is placed along the thorax it does not alter the impact forces on the chest and hence the injuries 17. The overall stiffness of the lower abdomen was found to be both velocity and impactor mass dependent 18. A comprehensive review on the development of a combined thoracic injury assessment criterion has been done by Hassan and Nusholtz 19.

3 The injury criterions discussed predict injury risk from external mechanical load. Soft tissue injuries are often critical and the injury mechanisms in case of blunt impacts are not well understood 6. Viscous tolerance criterion which predicts the risk of soft tissue injury does not predict individual tissue or organ response. Hence studies to understand injuries at the tissue level are needed. Computational models with a detailed geometry and biofidelic material properties can provide a better insight on the individual injuries. With substantial improvement in geometry input using MRI and CT Scans, a review revealing the gaps in the tissue properties would enhance the research towards developing biofidelic finite element models. Hence a review of individual tissue properties obtained either from experiments or inversely mapped using finite element modeling is presented. Evolution of Finite Element Models on Chest and Abdomen Huang 20 developed a three-dimensional finite element model of human body to simulate side impact experiments 15, 16 and pendulum impact configurations 14. This model includes ribs, visceral contents, muscles, skin, ligament and coastal cartilage as soft tissues. Ribs and visceral contents were represented by elastic solid material with discrete dampers. Muscles and ligaments between ribs were modeled as a single layer of membrane elements. Skin and part of outer muscles were modeled as solid elements with stiffening, which allowed the skin to contact vehicular surfaces without bottoming out. A foam model was used to model the skin. Average Young s modulus of muscle for initial 20% elongation was represented. They recommended an improvement in damping properties, high Poisson s ratio and a non-linear material model in order to simulate the internal viscera contents. Lizee 21,22 developed a 3-D finite element model of thorax including shoulder and abdomen. The model was designed to predict human responses and kinematics under most crash conditions (frontal, oblique and lateral, sitting and standing posture). The model represented the 50 th percentile adult male in a seated position. Twentytwo validation test configurations were selected and 50 validation corridors were conducted for creating an experimental database. Impact tests were performed in frontal, oblique and lateral directions for sitting and standing posture. Guided impactors were used for frontal and lateral impacts at initial velocities of m/s and a pendulum type impactor having mass of 23.4 kg was used for oblique standing positions. Belt compression tests were conducted by laying the subjects on a rigid flat table. Soft tissues filling the ribcage, shoulder, abdomen and the pelvis cavity were modeled by a uniform mesh of brick elements. Internal organs like heart, spleen and lungs are not described in this study. Other internal organs, lungs and heart; spleen and stomach; lower abdomen and intestine were modeled with individual stiffness. Intercoastal ligaments and coastal cartilage were represented as a part of the ribcage. Muscles were represented as a non-linear material. The parameter values chosen for the muscle and internal organs did not have a good predicting capability at high compression levels indicating the need for better material characterization. Deng 23 developed FE human thorax model to study the injury mechanisms under impact. The model was validated against frontal impact 24,25 and lateral impact 26 against reported cadaver tests. Intercostal muscles were modeled as membrane elements to provide force in tension and buckle in compression. Fabric material model was used for modeling these muscles and the material properties were based on Myres 27. Internal organs such as heart and lung muscles were represented by 8-node brick element and the material properties were based on a strain energy function 28,29. A pendulum with diameter cm and mass 23.4 kg was used to impact the sternum with initial velocities of 4.5 and 6.7m/s. Lateral impacts were conducted with the center of impactor striking the 6 th rib. Good correlation between the model response and the force and deflection curves of cadavers was reported between the model and the cadaver specimens. Furusu 30 developed a thorax model for side impact using a similar impactor with initial impact velocities 3.3 m/s and 5.7 m/s. Coastal cartilage, diaphragm and intercostal muscles were considered in this study. Internal organs were modeled as homogenous viscous with dynamic properties. The entire internal cavity was considered as one linear viscoelastic material. Time history plot of deflection and load deflection for simulated response and cadaver test had good agreement with experimental corridors.

4 Tissue Level Finite Element Models and Constitutive Models of Soft Tissue Organs in the abdominal region are divided into hollow and solid organs. Hollow organs in the abdominal region are the stomach and intestines; while the solid organs are the liver, spleen, pancreas, and kidney. Solid organs are more frequently injured than hollow organs. These internal organs exhibit size-related mechanical properties but extrapolating the results from one tissue to other is not recommended because of varying degrees of nonlinear elastic and viscoelastic responses exhibited 31. Hip capsule when subject to tensile loading exhibits non linear material response and also displays significant regional differences within the material properties 32. Similarly bovine kidney exhibited nonlinear characteristics and strain softening under shear loading 33. The limit of linear viscoelasticity in the kidney was of the order of 0.2%. Nonlinear viscoelastic constitutive models for liver and kidney capable of predicting strain rate dependency from 0.2 to 22.5 /s for nominal compressive strains up to 35% have been reported 34. Existence of significant changes in the stress distributions due to anisotropic properties of leaflets were demonstrated by Li 35 in the porcine aortic valve. Similarly constitutive properties for arterial wall 36,37,38,39, heart epicardial tissue 40, myocardium 41, aorta 42, vena cava, carotid artery and ventricle 43 are available in the literature but their applicability to automobile related accidents is uncertain as these studies are primarily developed for conditions pertaining to surgical simulations. Experimental studies using isolated organs or tissues and studies on dynamic properties of these internal organs are scarce 2,44. UPPER EXTREMITY Upper extremity injuries constitute 8.2% in pedestrian car collisions for AIS 2-6 injury range 3. In addition, an injury due to airbag deployment is another area of concern 45,46. Evolution of Experimental Methods, Finite Element Models and Injury Assessment Functions Pintar 47 conducted three-point bending test on 30 human cadaver forearms to determine its tolerance under a dynamic bending mode (initial velocities 3.3 m/s and 7.6 m/s). Their investigation offers quantitative information regarding tolerance of the human forearm and is useful in designing injury mitigating-devices. Results indicated significantly greater biomechanical parameters for males as compared to females. The bending tolerance of the human forearm, however, was found to correlate with bone mineral density, bone area, and forearm mass. This study indicates occupant with lower bone mineral density and lower forearm cross-section / mass is at a higher risk. Although there are appreciable differences between quasi-static loading and dynamic testing, the present study does not find significant differences in the mechanical parameters between the two dynamic rates chosen. This may be due to either a plateauing effect or the two loading rates chosen were not separate enough to statistically separate the parameters. Iwamoto 48 developed a finite element model of the human shoulder in order to understand the relationship between shoulder and multiple rib fractures. Cartilages were modeled as linear isotropic elastic 8-node solid elements. Major ligaments around the shoulder were modeled using anisotropic, non-linear, elastic, 4-node membrane elements with non-linear material properties. The effect of the strain rate dependency was investigated. The model contains major shoulder muscles that largely contribute to the motion. These muscles and their associated tendons were modeled using anisotropic non-linear elastic 4-node membrane elements. Muscles which were expected to sustain high forces such as deltoid, biceps, triceps, supraspinatus, infraspinatus, subscapularis, and teresminor were modeled using nonlinear isotropic elastic 8-node solid elements. To investigate responses of the shoulder model, pendulum impact test data published by Bendjellal 49 was used. In this study, the shoulder model was used for lateral impacts with an impact velocity of 4.5 m/s. The first peak of the FE force time history was higher than the experimental value when the Young s modulus of 2.6 MPa was assigned. They recommend a Young s Modulus of 0.5 MPa for deltoid, triceps and biceps muscles. Additionally, ligaments were modeled with higher stiffness values than the reference experimental data to prevent them from rupture. Subsequently impact force response fell within the corridor. They recommend a strain rate dependent non-linear viscoelastic material model for the muscles.

5 To understand the transmission of force through fingertips to the musculoskeletal system, Wu 50 proposed a lumped-parameter non-linear viscoelastic model of the fingertip tissue to model fingertip force-displacement characteristics during a range of rapid, dynamic tapping tasks. Eight human subjects tapped with their index finger on the surface of a rigid load cell while an optical system tracked fingertip position using an infrared LED (light emitting diode) attached to the fingernail. Four different tapping conditions were tested: normal and high-speed taps with a relaxed hand, and normal and high-speed taps with the other fingers contracted. Non-linear viscoelastic model consisting of an instantaneous stiffness function and a viscous relaxation function reproduced fingertip tissue force response due to pulp compression under these four different loading conditions. The model could successfully reconstruct very rapid (less than 5 ms) force transients, and forces occurring over time periods greater than 100 ms, within a 10% accuracy. Model parameters varied by less than 20% over the four conditions, despite almost 3-fold differences in average forces and 38% differences in fingertip velocities. Fingertip pulp model parameters, including the energy absorbed, did not substantially vary across the four conditions tested. Mukherjee 51 conducted 3-point bending test on goat cadavers. Six specimens of upper rear leg were tested using 2.4 kg cylindrical impactor with 3m/s velocity. Both flesh and bone were considered in this study during the impact. The finite element model developed validated for a range of impacts. The soft tissue of model was considered as viscoelastic material with dynamic properties. Duma et al 52 have used small female cadaver upper extremities to develop the wrist tolerance as a conservative estimate of the most vulnerable section of the driving population. A Wrist injury criterion based on the axial force was proposed. This criterion did not take into account the effect of combined loading and also did not focus on soft tissue injuries. Since upper extremity injuries are neither life threatening like head injuries nor as frequent as lower extremity injuries, very few finite element studies of the upper extremity incorporate soft tissues. To identify the gaps in the existing literature the available material properties of soft tissues in the chest abdomen and upper extremity regions are categorized according to the tissue and indicated in the tables listed in appendix-a (Refer Table A1-Table A3). Figure 1, Figure 2 and Figure 3 graphically show the reported elastic modulus, Poisson s ratio and the density of the different tissues Logarithamic Elastic Modulus (kpa) Blood vessels (23) Cartilage (21) Coastal cartilage (20) Costal Cartilages (30) Intercostal cartilage (21) Diaphragm (23) Intercoastal muscles (23) Muscle (20) Muscle (21) Muscles-Scapula (21) Skin (21) Visceral skin (21) Sternal cartilage (21) Visceral contents (20) Tissue (Reference) Figure 1 Elastic Modulus of the soft tissues in chest, abdomen and upper extremity region

6 0.5 Poisson's Ratio Blood vessels (23) Cartilage (21) Coastal cartilage (20) Costal Cartilages (30) Intercostal cartilage (21) Diaphragm (23) Intercoastal muscles (23) Muscle (20) Muscle (21) Muscles-Scapula (21) Skin (21) Visceral skin (21) Sternal cartilage (21) Visceral contents (20) Tissue (Reference) Figure 2 Poisson s Ratio of the soft tissues in chest, abdomen and upper extremity region Density (kg/m3) Cartilage (21) Coastal cartilage (20) Costal Cartilages (30) Intercostal cartilage (21) Intercostal muscles (23) Muscle (20) Muscle (21) Muscles-Scapula (21) Skin (21) Visceral skin (21) Sternal cartilage (21) Visceral contents (20) Viscus (30) Tissue (Reference) Figure 3 Density of soft tissues in chest, abdomen and upper extremity region

7 Figure 1 shows the range of elastic modulus reported for soft tissues in the chest, abdomen and upper extremity regions. Poisson s ratio of soft tissue ranges from 0.3 to 0.47 for the above mentioned regions. Except the reported density of coastal cartilage by Huang 20, the remaining values for soft tissues spread between 1000 and 1500 kg/m 3. A more detailed comparison between the individual tissue types could not be carried out because of lack of sufficient data. Dynamic properties are rarely reported in terms of short term and long term shear modulus for constituting the behavior in viscoelastic model. From the available data presented we can conclude that: a. Very few studies have been conducted for the upper extremity regions. b. Most of the impact studies conducted on the chest and thorax do not incorporate the internal organs or soft tissues. c. Nonlinear viscoelasticity, anisotropy and rate dependency are not well characterized and more tissue level experiments are needed for their characterization. d. Better material models capable of predicting the soft tissue behavior under impact have to be developed. e. There are no studies that include muscle behavior and its active tones for the upper extremity, shoulder and back regions. CONCLUSIONS A body of knowledge about mechanical properties of soft tissues in chest, abdomen and upper extremity regions, assembled in recent years, is collated. Reported experimental methods, injury assessment functions and finite element models are investigated and the observations for improvements have been listed. Scatter and uncertainty among the reported material properties of human soft tissue tissues are observed in both static and dynamic properties. It is felt that isolated specimen tests aimed at developing the material models needed in finite element analysis should be prioritized. This will help understand the complex behavior of these tissues and subsequently aid in injury prediction using finite elements. REFERENCES 1. I Watanabe, K Furusu, C Kato, K Miki, J Hasegawa. Development of practical and simplified human whole body FEM model. JSAE Review, vol. 22, pp M Iwamoto, Y Kisanuki, I Watanabe, K Furusu, K Miki, J Hasegawa. Development of a finite element model of the total human model for Safety THUMS and application to injury reconstruction. Proceedings of the 2002 International IRCOBI Conference on the Biomechanics of Impact, Munich, Germany, 2002, pp Y Mizuno. Summary of IHRA Pedestrian safety WG activities (2005) - proposed test methods to evaluate pedestrian protection afforded by passenger cars. Proceedings of the 19 th International Technical Conference on the Enhanced Safety of Vehicles J M Cavanaugh, T Walliko, J Chung, A I King. Abdominal injury and response in side impact. Proceedings of the 40th Stapp Car Conference, SAE Paper number , H Gray. Anatomy of the Human Body. Philadelphia: Lea & Febiger, 1918; Bartleby.com, J S H M Wismans, E G Janssen, M Beusenberg, W P Koppens, H A Lupker. Injury Biomechanics - Course Notes. Faculty of Mechanical Engineering - Division of computational and experimental mechanics - Eindhoven University of Technology, J Yang. A report to European safety network on Review of injury biomechanics in car-pedestrian collisions, Crash Safety division, Machine and vehicle systems, Chalmers university of technology, I V Lau, D C Viano. The viscous criterion - bases and applications of an injury severity index for soft tissues, Proceedings of the 30th Stapp car conference, SAE Paper Number , 1986, pp

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11 APPENDIX A- MECHANICAL PROPERTIES OF SOFT TISSUE RELATED TO CHEST ABDOMEN AND UPPER EXTREMITY Table A1. Elastic Modulus of soft tissues References (Other sources cited therein) Elastic modulus (MPa) Soft tissue 23 (Abe 1996) Blood vessels 5 21 Cartilage Coastal cartilage (Plank 1994, Costal Cartilages 24.5 Lizee 1998) 21 Intercostal cartilage (Plank 1994, Diaphragm 10 Lizee 1998) 23 (Myers 1995) Internal and external Intercoastal muscles (Yamada 1970) Muscle Muscle Scapula Muscles Skin Visceral skin 5 21 Sternal cartilage Visceral contents 8.42E-03 Table A2. Poisson s ratio of soft tissues References (Other sources cited therein) Poisson's ratio Soft tissue 23 (Abe 1996) Blood vessels Cartilage (Viano 1986) Coastal cartilage (Plank 1994, Lizee 1998) Costal Cartilages Intercostal 0.4 cartilage 23 (Plank 1994, Lizee 1998) Diaphragm (Myers 1995) Internal and 0.4 external Intercoastal muscles 20 (Yamada 1970) Muscle Muscle Muscles -Scapula Skin Visceral skin Sternal cartilage Visceral contents 0.47 Table A3. Density of soft tissues References (Other sources cited therein) Density (kg/mm 3 ) Soft tissue 21 Cartilage (Plank 1989) Coastal cartilage (Plank 1994, Lizee 1998) Costal Cartilages (Thorax) 21 Intercostal cartilage 23 (Myers 1995) Intercostal muscles (Human Thorax) (Yamada 1970) Muscle Muscle Muscles (Scapula) Skin Visceral skin Sternal cartilage Visceral contents (Plank 1994) Viscus (Thorax) 1000

12 April 30, 2006 The Director (Technical) The Institution of Engineers (India) 8 Gokhale Road Kolkata Dear Sir, Kindly find enclosed four copies of a manuscript of our paper, titled, Mechanical Properties of Soft Tissues in the Human chest, abdomen and upper extremity, co-authored by myself, Dr. S Mukherjee and B Karthikeyan, along with a CD containing the soft version for consideration for publication in the Institution of Engineers, Journal of Mechanical Engineering. I request you to kindly consider for the same. The author form and the paper submission forms are also enclosed. Being a review paper the word count (not including figures, tables and appendices) has reached I hope you will be able to consider the same as in a review paper of this kind, it was very difficult to stick to these limits and we believe that this paper will have a very strong archival value and in the interest of quality of Indian Journals, it should appear in Indian Journals. Yours truly, (A Chawla)