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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)range0.3610.4766076 6279 159176mean 0.419 63 71 174Basing 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 PROCESSNo Action Relationship 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 POSTURElying raising Sitting up declining lyingTime (s) 0 013 1337 3742 42In 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 POSTURENo posture part gravityPosition of centroids (x, y, z) (cm)No posture part gravityPosition of centroids (x, y, z) (cm)1 chest 14432972 3.48, 524.48, 9.632 lumbar 13470319 4.96, 297.91, 8.753 hip 8614896 3.75,102.93,3.814lietotal 72552996 2.95,199.96,3.275 chest 15463898 84.28,358.04,339.006 lumbar 11401344 81.00,225.87,154.957 hip 9861277 84.64,100.46,47.638Lie on one sidetotal 69481027 83.03,111.70,175.989 chest 15178240 5.77,531.24,37.0410 lumbar 14073480 5.17,283.78,25.1311 hip 9037140 9.89,104.39,12.4412Sit uptotal 73641723 3.04,204.46,25.05Here 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 uncomfortable feeling. Regions covered by thin soft tissues with bone supporting are prone to cause ulcer9, 10. Mattress should be designed to fit this requirement. Mobility limited patients should be helped to release the shear force by other people after being rotated by ECB.For designers, it is necessary to design bed boards according to human surface because mismatching of bed board and human curve will probably lead to unexpected posture and force distribution. Bed board should leave enough room for hip to avoid extra pressure when rotating. The surface of bed board below Ischial region, pressure focused region on the back should be able to extend or move to avoid shear force and friction.SECTION 4.ConclusionsResults of the two methods to study on the force distribution are almost consistent. Only when both back and legs were lifted together, measured pressure was obviously bigger than simulation result. That means the simulation method can be utilized to evaluate and predict the force distribution. In almost all situations, pressure over 32mmHg is unavoidable, sitting up or turning aside may cause obviously larger pressure (even more than 20%), Shear force can not be ignored too. All postures provided by tested ECBs lead to pressure concentration on hip, that means some other functions such as turning aside should be taken into consideration to eliminate the regions pressed but hard to relieve.For all these reasons, available ECBs could be applied to offer assistance but it can only be applied in a recommended way and regular help from other people is still necessary. Several suggestions have been given basing on the study.ACKNOWLEDGEMENTSWe thank Shao-Jie Xin, Ting Wu, Ting-Jun Wang for their technical assistance. The research is supported by the multi-robots system project (national 863 project) and Shanghai key project.基于虚拟人床系统的电子护理床生物力学分析人与电动护理床的关系是复杂而特殊的。本研究旨在降低使用ECB 时的失误发生率并提高舒适性。为了分析长期跟踪护理床过程中对人体施加的力分布，评价护理床新功能的可行性，我们邀请了一组老年人在三种类型的电动护理床上完成一系列实验。采用压力测图系统和虚拟人床系统对各种体位的受力进行量化，评价体位和体重的影响，探讨床运动与受力分布的关系。结果可用于改善所有体位的 ECB 病理特征，并可用于评价或预测电动护理床新增功能的可行性。第 1 部分 简介60 岁以上的人每年增长 1.9%，80 岁以上的人每年增长 3.8%。到 2030 年，全球 65 岁以上的老人比例将达到 20%。活动受限患者，如老年人和脊髓损伤患者。年龄越大，越容易受到伤害。大约 70%的溃疡发生在 70 岁以上的人身上 1。老年人的表皮变得越来越薄，对剪切力的抵抗力也越来越弱。导致溃疡的内在原因有很多，例如，心力衰竭等一些严重疾病的活动受限和合并症可明显增加溃疡的发生概率，这被认为是比老化本身更重要的原因2。溃疡的外在原因主要由褥垫系统引起。当软组织受到的接触压力超过 32mmHg 且不能经常缓解时，毛细血管小动脉肢的真皮血管会闭合，引起溃疡3。除气动床外，普通床引起的压力通常在 45mmHg-75mmHg 范围内。美国卫生保健政策和研究机构建议，压力性溃疡应根据安排的时间表重新定位至少 2 小时4。理论上，电动护理床可以帮助缓解压力并提供简单的帮助。护理任务可以减轻，而护士最危险的任务是提升或运送病人，欧洲央行在这类任务中几乎无法提供任何帮助5，因此本研究不考虑对护士的影响。如果用电动床取代传统的床，人们将被移动并长期保持许多姿势。几乎所有关于压力分布、床垫、溃疡6 和舒适性的相关结论都是基于传统床7。如果不能准确了解ECB 的力分布和动力学特征，简单设计的床会对处于易受伤害位置的患者施加不可预测的力，从而造成严重伤害。基于压力测绘系统的结果，建立了虚拟人床系统，对其进行了仿真。因为护理床是传统床和机械辅助系统的结合。丰富的相关研究可作为研究的基础。例如，卧床不起引起的慢性病，软组织的耐受性，护理任务的工作量。当用 ECB 替代传统的床位时，研究结果不能直接利用，尤其是当患者已经或已经丧失运动能力或失去意识时。设计不良的 ECB 可能会造成以下影响：（1）造成强烈和不可靠的接触压力。未缓解的压力是溃疡的主要原因。有许多方法可以处理不可靠的接触压力，定期复位，电 32mmHg 时间过长，软组织会被缺血破坏。 （2）如果人体跟随 EMCB 移动，摩擦几乎是不可避免的。可剥除角质层，为压疮等疾病提供条件。 （3）根据主观评价，将剪切效应放大。只有当力平行于表面施加时，才会发生这种情况。剪切可能使软组织变形并引起缺血。 （四）尿失禁、出汗引起的水分，软化角质层也可引起其他疾病。第 2 部分 研究方法为了探讨人床系统的生物力学特性，
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