外文原文- 基于虛擬人床系統(tǒng)的電子護理床生物力學分析

A biomechanical analysis of electric nursing care beds basing on a virtual man-bed system 2 Author(s)Qi Wang ; Xuehua TangAbstract: Relation between human and electric care beds (ECB) is complex and special. This study aims at reducing incidence of ulcer and improve comfort when using ECB. To quantify the force distribution exerted upon human body in process of following the care beds for a long period and evaluate the feasibility of new functions of care beds, a group of old people were asked to finish a series of experiments on three kinds of electric care beds. Pressure mapping system and a virtual man-bed system were utilized to quantify the forces in all postures and assess the influences of postures and body weight, explore the relationship between bed movement and force distribution. The results can be utilized to improve the pathological characteristics of ECB in all postures, and furthermore to evaluate or predict the feasibility of new functions added to electric nursing care beds.Published in: 2009 IEEE 10th International Conference on Computer-Aided Industrial Design & Conceptual Design Date of Conference: 26-29 Nov. 2009 Date Added to IEEE Xplore: 08 January 2010 ISBN Information: INSPEC Accession Number: 11084135 DOI: 10.1109/CAIDCD.2009.5375263 Publisher: IEEE Conference Location: Wenzhou, China AdvertisementContents SECTION 1.IntroductionIndividuals over the age of 60 increase 1.9% each year and those over 80 increase 3.8%. By 2030, 20% of world population will be over 65. Mobility limited patients such as the elderly and the spinal cord injury patients. The older, the more vulnerable. Approximately 70% of ulcers occur in individuals over the age of 701. Epidermis of old people gets thinner and less resistant to shearing forces. There are many intrinsic reasons that lead to ulcer, for instance, mobility limitation and comorbidities of some serious diseases such as heart failure can increase the probability of ulcer obviously, which has been known as a much more important reason than aging itself2. Extrinsic reasons of ulcer are mainly caused by bedding system. When contact pressure exerted upon soft tissues is over 32mmHg and can not be often relieved, dermal vessels in the arteriolar limb of capillary will be closed and ulcer is caused3. Pressure caused by common beds is often in the span of 45mmHg-75mmHg except for air powered bed.It is recommended by US Agency for health Care Policy and Research that pressure ulcers should be repositioned at least 2 hours according to an arranged schedule4. Theoretically, electric care beds can help to relieve pressure and offer simple assistance. Nursing tasks can be relieved, while the most dangerous tasks for nurses are lifting or transporting patients, ECB could hardly offer any help in such tasks5, so influence to nurses is not taken into consideration in this study. If traditional beds are replaced by electric beds, people will be moved and kept in many postures for long periods. Almost all relative conclusions about pressure distribution, mattress, ulcer6 and comfort are based on traditional beds7. Were force distribution and kinetics characteristics of ECBs can not be exactly known, simply designed beds will cause great injury by exerting unpredictable forces upon patients on vulnerable positions. A virtual man-bed system was established basing on the result of pressure mapping system to simulate the situation.As the care bed is a combination of traditional bed and mechanical assistant system. Abundant relative research can be utilized as basis for the research. For example, chronics caused by bedridden, tolerance of soft tissues, work loading of nursing tasks. When traditional bed is replaced by ECB, the research results can not be utilized directly, especially when the patient has or been motion ability disabled or lost consciousness. Poor designed ECB may cause the following influences: (1) Cause intense and unrelieved contact pressure. Unrelieved pressure is the main reason of ulcer. There are many kinds of methods to deal with unrelieved contact pressure, regular reposition, electric 32mmHg for too long a period, soft tissues will be undermined by ischemia. (2) Friction is almost unavoidable if human body follows the EMCB to move. It may strip the stratum corneum and provide condition for pressure ulcer and other diseases. (3) According to subjective evaluation, effect of shear is amplified. It happens only when the forces is exerted parallel to surface. Shear may distort soft tissues and cause ischemia. (4) Moisture caused by incontinence or perspiration also cause other diseases by softening the stratum corneum.SECTION 2.MethodTo explore the biomechanical characteristics of man-bed system, 9 old people were asked to use three kinds of ECBs beds. “R” value is the result of Weight/height(kg/cm), The beds were manipulated by testers. Information of old people are shown in table 1. TABLE I. SUMMARY OF SUBJECTSRAge (year)weight (kilogram)height (cm)range0.3610.47660766279159176mean0.4196371174Basing on the investigation about home care, main postures were chosen and measured basing on the discipline of replacing human force as much as possible. Only postures that are often used and kept for a long time were chosen as main postures.Subjects were divided into three groups according to the value of R shown in table 1. They were asked to lie on the beds leisurely and follow the ECBs to move after being simply trained. A specially designed blanket with less than 2000 pressure Sensors in is placed on the mattress as a part of pressure measure system. It is used to record the pressure exerted upon mattress. Pressure distribution and mean pressure were recorded by the system. To compare the result with man-bed stimulation system, the subjects were asked to put their arm in two ways, once placed naturally and again folded before chest. TABLE II. MANIPULATION PROCESSNoActionRelationship with mattress1From lying to sitting up (legs straight still)Long period pressed with shear force and friction2From lying to sitting up (legs raised together)long period pressed with shear force and frictionTurning from one side to the other may cause posture changed in unpredictable ways, so the turning process was not simulated. Awkward postures such as prone position were not simulated too. Force distribution characteristics in extreme postures is mainly decided shape of bad and man, also decided by motion process to some extend. Function and construction of the three types of beds were different, Type A, B can help patients to sit up without taking the curve of human body into consideration. Type C offers a function to lift back and leg together, the posture is shown in Fig1. Curves in ischial region and thigh region have been matched with special design. Type C provides a function to lift and rotate back and legs together from lying position to sitting position and then return to lying. Fig2 shows value of pressure during a whole process. Figure 1. Figure 1 back and legs were raised togetherView AllFigure 2. Relationship between pressure and timeView AllTABLE III. DURATION OF EACH POSTURElyingraisingSitting updeclininglyingTime (s)00131337374242In process of being raised, mean pressure increased greatly. The pressure got to the peak which was 26% more than the original value at limit position, and then return to original level when following the bed to lying position. Figure 3. Relationship between pressure and timeView AllIt could be seen in the measurement that more than 30% of area on the surface of human body were pressed by pressure of over 32mmHg. Pressure is most evenly distributed in lying position, while the distribution has been different when the body come back to lying again, for fat subjects(R0.471), effect of shear force has been increased. In sitting positions with legs lifted or not, subjects suffered from larger peak pressure, shear force and friction when following type A, B to move, because the corner of simple straight bed board did not offer any room for hip which is the main support position and rotation center of the whole body, when the bed board moves, the whole body moves with hip, as a result, friction and shear force are unavoidable. Even worse, if the bed rotates for more than once, the subject may be kept in awkward position with lumbar on the corner.Virtual human model was established to simulate the gravity distribution of human body, the virtual human was established by collecting 3D position of critical points in all postures. The virtual human model in each posture was divided into 25 parts with density of all parts were set to be1g/com3. The parts were divided basing on the degree of freedom of human body except for the trunk which is composed of five parts because the truck is the main interface with bed and all the five parts were supported by different bones. Models of each part is closed. The whole model is made first and then divided into parts. Sometimes a few parts can be seen as a larger part, for example, neck and head can been calculated as a single part when simulating sitting postures. Models established in this way is not only easy to calculate but also comply with real shape of human body.Basing on the results of pressure measuring, main contact area between virtual human and mattress are known. The virtual human make it possible to find out and predict how the human parts are interacting with the beds, unknown forces such as shear can be calculated. The result is used to predict and quantify the risk of ulcer in all postures, or evaluate the feasibility of new functions of ECB in design process.To record the position of all parts in the man-bed system, a coordinate was set up, its origin was chosen in the symmetric plane of the model in lying posture, vertical to bed, at the middle of the length between lowest edge of sternum and back. It is depicted in Fig 4. This origin is easier to measure and relatively stable, it offers a method to design man-bed system basing on the characteristics of human body, not from bed Which is often considered first. This origin choosing method can help to design the product totally for the requirements of human. Figure 4. Position of the originView AllPosition of centroids and gravity of parts were used to simulate the force exerted on each part. Data of main parts in main postures is shown in table IV. TABLE IV. TIME OF EACH POSTURENoposturepartgravityPosition of centroids (x, y, z) (cm)1liechest144329723.48, 524.48, 9.632lumbar134703194.96, 297.91, 8.753hip86148963.75,102.93,3.814total725529962.95,199.96,3.275Lie on one sidechest1546389884.28,358.04,339.006lumbar1140134481.00,225.87,154.957hip986127784.64,100.46,47.638total6948102783.03,111.70,175.989Sit upchest151782405.77,531.24,37.0410lumbar140734805.17,283.78,25.1311hip90371409.89,104.39,12.4412total736417233.04,204.46,25.05Here is a sample to show how to use the models. In the posture in Fig5, force exerted upon each part is calculated, from head to lumbar, finally to the hip. Friction on the head is neglected. The support shear offered by leg which can not to measured directly is also known by calculating the forces exerted on the hip. Figure 5. Force analysis of partsView AllThe system has the following valuable advantages:It is easy to adjust and improve, that means making progress in design process without making real models and the system can be utilized to explore the feasibility of new postures.The model offers a much clearer description method to show the effect of forces.Some forces which can not be directly measured can be calculated in this way, for instance, shear and friction. When calculating the shear force, motion process must be taken into consideration to decide the direction of shear force and friction.All calculation results are compared with results got from pressure mapping system. Results from both methods show that Back and thigh are main interface between body and mattress, Calf is also important in some postures. Elbow, hands and feet seldom cause long time high pressure and these parts hardly suffer from ulcer, so these parts are neglected. Pressure distribution in main postures are show in the following Figs. Each point depicts the largest pressure in a line vertical to the subjects on the measured plan. Figure6. Pressure distribution when simply sittingView AllFigure7. Pressure distribution when back and legs are lifted togetherView AllWhen the back and legs were lifted together, the mean pressure was obviously increased, and hip became the most intensely pressed part, it was hard for subjects to adjust the pressed area. It is unexpected that the mean pressure is always more than pressure result made from virtual simulation when pressed area has remain unchanged or even amplified because of larger pressure. That means there is some kind of resistance when body is forced to an awkward position. Figure8. pressure distribution, bed covered by mattressView AllFigure9. pressure distribution, with mattress taken awayView AllIt proves that Mattress can be used to distribute pressure more evenly and decrease peak pressure, while soft mattress also cause extra shear force. The only solid conclusion is fat people should use thinner mattress than thin people.SECTION 3.DiscussionBy comparing results got from pressure measurement and virtual manbed system, the following phenomena are found.Distribution of pressed areas are not only decided by gravity, but also by shear force. In the whole manipulation process, direction of shear force and friction has been inversed, so pressed areas often change, as a result, pressed areas are amplified and moved in the direction of shear force. The result can be clearly found in the picture provided by pressure mapping system. This phenomenon was more obvious for fat objects.It could be seen in force analysis to virtual man-bed system that when the model follow the bed board to sit up, the shear force increases to balance the gravity of trunk. When bed is rotating, human surface on the hip extend more than 50%, while the bed surface of beds A and B can not extend to match it, that is one reason for causing friction and shear force. Another reason is shear force increases with gravity when sitting up, the shear force reach its peak when then bed board rotates 45 to vertical. No obvious area caused by shear force appeared when using Bed C because its bed board below hip can extend with skin and gravity of trunk is balanced and the curve on bed board provide enough room to contain the hip.Also because bed A, B can not match the human curve, force distribution alter after every sitting process, so pressure that should has been exerted on hip were changed to back, so spine compression is increased and the trunk forced to fit an unexpected curve, that is very uncomfortable for bed bound patients.It was proved in the experiment that mean pressure on single points did not change with weight of subjects obviously. However, obviously fat subjects interact with mattress in a much larger area. Too thick mattress and soft tissues provide condition for soft tissue distortion and shear force. This was known both from man-bed simulation and pressure measurement. Distortion is a reason for ischemia. Thick soft tissue is also bad for moisture adjustment. For thin subjects with R” value less than 0.361, mapping of pressure distribution almost kept unchanged after sitting up, but the whole body is supported by a few small areas, too intensely focused pressure is a main reason for ulcer and uncomfortable. Pressure comparison is shown in Fig 10 and Fig 11. Figure 10. Pressure distribution, R 0.471View AllFigure 11. Pressure distribution, R 0.361View AllFollowing suggestions are given to avoid ulcer and ensure the user comfortable, also to improve design of ECB:Mean contact pressure is smallest when lying, Simply sitting up may increase mean pressure by more than 20%. Moving with bed board also cause friction and shear force. So, it is recommended that patients should not be kept in sitting postures for long especially in postures such as lifting back and legs together. Every time the posture is changed, the patient should get help to make back and hip at least a little lifted to release shear force, that is disappointing because that will be a tough work and a risk for nursesf8.Clothes is necessary to minimize friction when sitting up with bed. Neither lying nor sitting can thoroughly relieve the contact pressure exerted on Ischial region, so turning aside at intervals is also necessary for bed bound patients. Fatter people are more endangered by shear force. Thin people are easier to suffer from peak pressure and