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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 70[1]. 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 itself[2]. 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 caused[3]. 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 schedule[4]. 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 tasks[5], 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, ulcer[6] and comfort are based on traditional beds[7]. 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)range0.361–0.47660–76 62–79 159–176mean 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 0–13 13–37 37–42 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(R≥0.471), 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 nursesf[8].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 ulcer[9], [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.基于虛擬人床系統(tǒng)的電子護理床生物力學分析人與電動護理床的關系是復雜而特殊的。本研究旨在降低使用ECB 時的失誤發(fā)生率并提高舒適性。為了分析長期跟蹤護理床過程中對人體施加的力分布，評價護理床新功能的可行性，我們邀請了一組老年人在三種類型的電動護理床上完成一系列實驗。采用壓力測圖系統(tǒng)和虛擬人床系統(tǒng)對各種體位的受力進行量化，評價體位和體重的影響，探討床運動與受力分布的關系。結果可用于改善所有體位的 ECB 病理特征，并可用于評價或預測電動護理床新增功能的可行性。第 1 部分 簡介60 歲以上的人每年增長 1.9%，80 歲以上的人每年增長 3.8%。到 2030 年，全球 65 歲以上的老人比例將達到 20%?；顒邮芟藁颊撸缋夏耆撕图顾钃p傷患者。年齡越大，越容易受到傷害。大約 70%的潰瘍發(fā)生在 70 歲以上的人身上 [1]。老年人的表皮變得越來越薄，對剪切力的抵抗力也越來越弱。導致潰瘍的內在原因有很多，例如，心力衰竭等一些嚴重疾病的活動受限和合并癥可明顯增加潰瘍的發(fā)生概率，這被認為是比老化本身更重要的原因[2]。潰瘍的外在原因主要由褥墊系統(tǒng)引起。當軟組織受到的接觸壓力超過 32mmHg 且不能經常緩解時，毛細血管小動脈肢的真皮血管會閉合，引起潰瘍[3]。除氣動床外，普通床引起的壓力通常在 45mmHg-75mmHg 范圍內。美國衛(wèi)生保健政策和研究機構建議，壓力性潰瘍應根據(jù)安排的時間表重新定位至少 2 小時[4]。理論上，電動護理床可以幫助緩解壓力并提供簡單的幫助。護理任務可以減輕，而護士最危險的任務是提升或運送病人，歐洲央行在這類任務中幾乎無法提供任何幫助[5]，因此本研究不考慮對護士的影響。如果用電動床取代傳統(tǒng)的床，人們將被移動并長期保持許多姿勢。幾乎所有關于壓力分布、床墊、潰瘍[6] 和舒適性的相關結論都是基于傳統(tǒng)床[7]。如果不能準確了解ECB 的力分布和動力學特征，簡單設計的床會對處于易受傷害位置的患者施加不可預測的力，從而造成嚴重傷害。基于壓力測繪系統(tǒng)的結果，建立了虛擬人床系統(tǒng)，對其進行了仿真。因為護理床是傳統(tǒng)床和機械輔助系統(tǒng)的結合。豐富的相關研究可作為研究的基礎。例如，臥床不起引起的慢性病，軟組織的耐受性，護理任務的工作量。當用 ECB 替代傳統(tǒng)的床位時，研究結果不能直接利用，尤其是當患者已經或已經喪失運動能力或失去意識時。設計不良的 ECB 可能會造成以下影響：（1）造成強烈和不可靠的接觸壓力。未緩解的壓力是潰瘍的主要原因。有許多方法可以處理不可靠的接觸壓力，定期復位，電 32mmHg 時間過長，軟組織會被缺血破壞。 （2）如果人體跟隨 EMCB 移動，摩擦幾乎是不可避免的?？蓜兂琴|層，為壓瘡等疾病提供條件。 （3）根據(jù)主觀評價，將剪切效應放大。只有當力平行于表面施加時，才會發(fā)生這種情況。剪切可能使軟組織變形并引起缺血。 （四）尿失禁、出汗引起的水分，軟化角質層也可引起其他疾病。第 2 部分 研究方法為了探討人床系統(tǒng)的生物力學特性，