基于虛擬人床系統(tǒng)的電子護(hù)理床生物力學(xué)分析
人與電動(dòng)護(hù)理床的關(guān)系是復(fù)雜而特殊的。本研究旨在降低使用ECB時(shí)的失誤發(fā)生率并提高舒適性。為了分析長(zhǎng)期跟蹤護(hù)理床過(guò)程中對(duì)人體施加的力分布，評(píng)價(jià)護(hù)理床新功能的可行性，我們邀請(qǐng)了一組老年人在三種類(lèi)型的電動(dòng)護(hù)理床上完成一系列實(shí)驗(yàn)。采用壓力測(cè)圖系統(tǒng)和虛擬人床系統(tǒng)對(duì)各種體位的受力進(jìn)行量化，評(píng)價(jià)體位和體重的影響，探討床運(yùn)動(dòng)與受力分布的關(guān)系。結(jié)果可用于改善所有體位的ECB病理特征，并可用于評(píng)價(jià)或預(yù)測(cè)電動(dòng)護(hù)理床新增功能的可行性。
第1部分 簡(jiǎn)介
60歲以上的人每年增長(zhǎng)1.9%，80歲以上的人每年增長(zhǎng)3.8%。到2030年，全球65歲以上的老人比例將達(dá)到20%。活動(dòng)受限患者，如老年人和脊髓損傷患者。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(lái)越薄，對(duì)剪切力的抵抗力也越來(lái)越弱。導(dǎo)致潰瘍的內(nèi)在原因有很多，例如，心力衰竭等一些嚴(yán)重疾病的活動(dòng)受限和合并癥可明顯增加潰瘍的發(fā)生概率，這被認(rèn)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(dāng)軟組織受到的接觸壓力超過(guò)32mmHg且不能經(jīng)常緩解時(shí)，毛細(xì)血管小動(dòng)脈肢的真皮血管會(huì)閉合，引起潰瘍[3]。除氣動(dòng)床外，普通床引起的壓力通常在45mmHg-75mmHg范圍內(nèi)。
美國(guó)衛(wèi)生保健政策和研究機(jī)構(gòu)建議，壓力性潰瘍應(yīng)根據(jù)安排的時(shí)間表重新定位至少2小時(shí)[4]。理論上，電動(dòng)護(hù)理床可以幫助緩解壓力并提供簡(jiǎn)單的幫助。護(hù)理任務(wù)可以減輕，而護(hù)士最危險(xiǎn)的任務(wù)是提升或運(yùn)送病人，歐洲央行在這類(lèi)任務(wù)中幾乎無(wú)法提供任何幫助[5]，因此本研究不考慮對(duì)護(hù)士的影響。如果用電動(dòng)床取代傳統(tǒng)的床，人們將被移動(dòng)并長(zhǎng)期保持許多姿勢(shì)。幾乎所有關(guān)于壓力分布、床墊、潰瘍[6]和舒適性的相關(guān)結(jié)論都是基于傳統(tǒng)床[7]。如果不能準(zhǔn)確了解ECB的力分布和動(dòng)力學(xué)特征，簡(jiǎn)單設(shè)計(jì)的床會(huì)對(duì)處于易受傷害位置的患者施加不可預(yù)測(cè)的力，從而造成嚴(yán)重傷害?；趬毫y(cè)繪系統(tǒng)的結(jié)果，建立了虛擬人床系統(tǒng)，對(duì)其進(jìn)行了仿真。
因?yàn)樽o(hù)理床是傳統(tǒng)床和機(jī)械輔助系統(tǒng)的結(jié)合。豐富的相關(guān)研究可作為研究的基礎(chǔ)。例如，臥床不起引起的慢性病，軟組織的耐受性，護(hù)理任務(wù)的工作量。當(dāng)用ECB替代傳統(tǒng)的床位時(shí)，研究結(jié)果不能直接利用，尤其是當(dāng)患者已經(jīng)或已經(jīng)喪失運(yùn)動(dòng)能力或失去意識(shí)時(shí)。設(shè)計(jì)不良的ECB可能會(huì)造成以下影響：（1）造成強(qiáng)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(fù)位，電32mmHg時(shí)間過(guò)長(zhǎng)，軟組織會(huì)被缺血破壞。（2）如果人體跟隨EMCB移動(dòng)，摩擦幾乎是不可避免的。可剝除角質(zhì)層，為壓瘡等疾病提供條件。（3）根據(jù)主觀評(píng)價(jià)，將剪切效應(yīng)放大。只有當(dāng)力平行于表面施加時(shí)，才會(huì)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(zhì)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(xué)特性，9名老年人被要求使用三種ECBS床?！皉”值是重量/高度（kg/cm）的結(jié)果，床由測(cè)試人員操作。老年人信息見(jiàn)表1。
TABLE I. SUMMARY OF SUBJECTS
R
年齡
體重/kg
身高/cm
范圍
0.361–0.476
60–76
62–79
159–176
平均
0.419
63
71
174
通過(guò)對(duì)居家護(hù)理的調(diào)查，根據(jù)盡可能多地替代人力的原則，選擇和測(cè)量了居家護(hù)理的主要姿勢(shì)。只有經(jīng)常使用和保存很長(zhǎng)時(shí)間的姿勢(shì)才被選為主要姿勢(shì)。
根據(jù)表1所示的r值，將受試者分為三組。他們被要求悠閑地躺在床上，在接受簡(jiǎn)單訓(xùn)練后跟隨ECB移動(dòng)。作為壓力測(cè)量系統(tǒng)的一部分，床墊上放置了一塊特制的毯子，里面的壓力傳感器不足2000個(gè)。用于記錄施加在床墊上的壓力。系統(tǒng)記錄壓力分布和平均壓力。為了與人床刺激系統(tǒng)的結(jié)果進(jìn)行比較，要求受試者以兩種方式放置手臂，一種是自然放置，另一種是胸部折疊。
TABLE II.
No
動(dòng)作
與床的聯(lián)系
1
從躺著到坐起來(lái)（雙腿伸直）
受剪切力和摩擦的長(zhǎng)周期壓制
2
從躺著到坐起來(lái)（兩腿并舉）
受剪切力和摩擦的長(zhǎng)周期壓制
從一側(cè)轉(zhuǎn)向另一側(cè)可能導(dǎo)致姿勢(shì)以不可預(yù)測(cè)的方式發(fā)生變化，因此沒(méi)有模擬轉(zhuǎn)向過(guò)程。像俯臥這樣笨拙的姿勢(shì)也沒(méi)有被模仿。極端姿勢(shì)中的力分布特征主要是決定人和壞人的形狀，也在一定程度上決定了運(yùn)動(dòng)過(guò)程。三種床的功能和結(jié)構(gòu)不同，A、B型床可以幫助患者在不考慮人體曲線的情況下坐起來(lái)。類(lèi)型C提供了一個(gè)功能，將背部和腿部一起抬起，姿勢(shì)如圖1所示。坐骨區(qū)和大腿區(qū)的曲線與特殊設(shè)計(jì)相匹配。C型提供了一個(gè)功能，可以將背部和腿部一起從臥位提升和旋轉(zhuǎn)到坐位，然后再回到臥位。圖2顯示了整個(gè)過(guò)程中的壓力值。
圖1背部和腿部一起抬起
圖2.壓力與時(shí)間的關(guān)系
表3. 每種姿勢(shì)的持續(xù)時(shí)間
躺
起身
坐起
下降
躺
Time (s)
0
0–13
13–37
37–42
42
在上升過(guò)程中，平均壓力大大增加。在極限位置，壓力達(dá)到峰值，比初始值高出26%，隨床到臥位，壓力恢復(fù)到初始水平。
圖3壓力與時(shí)間的關(guān)系
從測(cè)量結(jié)果可以看出，超過(guò)30%的人體表面面積被超過(guò)32mmHg的壓力擠壓。躺位時(shí)壓力分布最均勻，而身體再回到躺位時(shí)壓力分布則不同，對(duì)于脂肪受試者（r≥0.471），剪切力的影響增大。在腿抬高或不抬高的坐姿中，受試者在跟隨A、B型移動(dòng)時(shí)承受較大的峰值壓力、剪切力和摩擦力，因?yàn)楹?jiǎn)單直床板的角部沒(méi)有為臀部提供任何空間，而臀部是整個(gè)身體的主要支撐位置和旋轉(zhuǎn)中心，床板移動(dòng)時(shí)，整個(gè)身體移動(dòng)。因此，髖關(guān)節(jié)的摩擦力和剪切力是不可避免的。更糟糕的是，如果床旋轉(zhuǎn)不止一次，受試者可能會(huì)處于尷尬的位置，腰部在拐角處。
建立了虛擬人體模型，模擬人體的重力分布，通過(guò)采集各姿態(tài)關(guān)鍵點(diǎn)的三維位置建立了虛擬人體。將每個(gè)體位的虛擬人體模型分為25個(gè)部分，每個(gè)部分的密度設(shè)為1g/com3。由于卡車(chē)是與床的主要接口，五個(gè)部分都由不同的骨骼支撐，因此根據(jù)人體的自由度對(duì)其進(jìn)行了劃分，而軀干則由五個(gè)部分組成。每個(gè)部分的模型都是封閉的。整個(gè)模型先制作，然后分成若干部分。有時(shí)一些部分可以看作是一個(gè)較大的部分，例如，當(dāng)模擬坐姿時(shí)，頸部和頭部可以作為一個(gè)單獨(dú)的部分計(jì)算。用這種方法建立的模型不僅計(jì)算簡(jiǎn)單，而且符合人體的真實(shí)形態(tài)。
根據(jù)壓力測(cè)量結(jié)果，確定了虛擬人體與床墊的主要接觸面積。虛擬人可以發(fā)現(xiàn)并預(yù)測(cè)人體各部分如何與床層相互作用，可以計(jì)算出諸如剪切力之類(lèi)的未知力。該結(jié)果可用于預(yù)測(cè)和量化所有體位的潰瘍風(fēng)險(xiǎn)，或評(píng)估ECB在設(shè)計(jì)過(guò)程中新功能的可行性。
為了記錄人床系統(tǒng)中各部分的位置，建立了一個(gè)坐標(biāo)系，在模型的對(duì)稱平面上選擇其原點(diǎn)，使其處于臥位，與床垂直，位于胸骨最下邊緣和背部之間的長(zhǎng)度中間。如圖4所示。這一起源比較容易測(cè)量，相對(duì)穩(wěn)定，它提供了一種根據(jù)人體特征而不是通常首先考慮的床來(lái)設(shè)計(jì)人-床系統(tǒng)的方法。這種原產(chǎn)地選擇方法有助于產(chǎn)品的設(shè)計(jì)完全符合人類(lèi)的要求。
利用質(zhì)心位置和零件的重力來(lái)模擬每個(gè)零件所受的力。主要姿勢(shì)主要部位數(shù)據(jù)見(jiàn)表四。
表4.每個(gè)姿勢(shì)的時(shí)間
No
姿勢(shì)
部分
重力
質(zhì)心位置坐標(biāo)00
1
平躺
胸部
14432972
3.48, 524.48, 9.63
2
腰椎
13470319
?4.96, 297.91, 8.75
3
臀部
8614896
?3.75,102.93,3.81
4
全部
72552996
?2.95,199.96,3.27
5
側(cè)躺
胸部
15463898
?84.28,358.04,339.00
6
腰椎
11401344
?81.00,225.87,154.95
7
臀部
9861277
?84.64,100.46,47.63
8
總計(jì)
69481027
?83.03,111.70,175.98
9
坐姿
胸部
15178240
5.77,531.24,37.04
10
腰椎
14073480
5.17,283.78,25.13
11
臀部
9037140
9.89,104.39,12.44
12
總計(jì)
73641723
3.04,204.46,25.05
下面是一個(gè)示例，演示如何使用模型。在圖5中的姿勢(shì)中，計(jì)算從頭部到腰部，最后到臀部施加在每個(gè)部分的力。頭部的摩擦忽略不計(jì)。腿提供的支撐剪力不能直接測(cè)量，也可以通過(guò)計(jì)算施加在臀部的力來(lái)知道。
圖5。零件受力分析
該系統(tǒng)具有以下優(yōu)點(diǎn)：
它易于調(diào)整和改進(jìn)，即在設(shè)計(jì)過(guò)程中取得進(jìn)展，而無(wú)需建立真實(shí)的模型，該系統(tǒng)可用于探索新姿態(tài)的可行性。
該模型提供了一種更清晰的描述力效應(yīng)的方法。
一些不能直接測(cè)量的力可以這樣計(jì)算，例如剪切力和摩擦力。在計(jì)算剪切力時(shí)，必須考慮運(yùn)動(dòng)過(guò)程來(lái)決定剪切力和摩擦力的方向。
計(jì)算結(jié)果與壓力繪圖系統(tǒng)的結(jié)果進(jìn)行了比較。兩種方法的結(jié)果表明，背部和大腿是人體和床墊的主要界面，小腿在某些姿勢(shì)中也很重要。肘部、手部和腳部很少引起長(zhǎng)時(shí)間的高壓，這些部位很少發(fā)生潰瘍，因此這些部位被忽略了。主要姿勢(shì)的壓力分布如下圖所示。每一點(diǎn)表示垂直于被測(cè)對(duì)象的一條線上的最大壓力。
圖6。簡(jiǎn)單坐著時(shí)的壓力分布
圖7。背部和腿部一起抬起時(shí)的壓力分布
當(dāng)背部和腿部一起抬起時(shí)，平均壓力明顯增加，臀部成為最受壓迫的部位，受試者很難調(diào)整壓迫區(qū)域。在壓力較大的情況下，平均壓力始終大于虛擬模擬的壓力結(jié)果，而壓力區(qū)域保持不變，甚至增大。這意味著當(dāng)身體被迫進(jìn)入一個(gè)尷尬的位置時(shí)會(huì)有某種阻力。
圖8。壓力分布，床墊覆蓋的床
圖9。壓力分布，取下床墊
結(jié)果表明，床墊可以更均勻地分配壓力，降低峰值壓力，而軟床墊也會(huì)產(chǎn)生額外的剪切力。唯一可靠的結(jié)論是胖人應(yīng)該比瘦人使用更薄的床墊。
第三部分 討論
通過(guò)對(duì)壓力測(cè)量和虛擬人床系統(tǒng)的比較，發(fā)現(xiàn)了以下現(xiàn)象。
受壓區(qū)的分布不僅取決于重力，還取決于剪切力。在整個(gè)操作過(guò)程中，剪切力和摩擦力的方向是相反的，因此，受力區(qū)域經(jīng)常發(fā)生變化，從而使受力區(qū)域放大并向剪切力的方向移動(dòng)。在壓力測(cè)圖系統(tǒng)提供的圖片中可以清楚地看到結(jié)果。這一現(xiàn)象對(duì)脂肪物體更為明顯。
從虛擬人床系統(tǒng)的受力分析可以看出，當(dāng)模型跟隨床板上升時(shí)，剪切力增大，以平衡軀干的重力。當(dāng)床旋轉(zhuǎn)時(shí)，人體臀部表面延伸超過(guò)50%，而床A和床B的床面不能延伸到與之匹配，這就是造成摩擦力和剪切力的原因之一。另一個(gè)原因是坐著時(shí)剪切力隨重力增大，床板垂直旋轉(zhuǎn)45°時(shí)剪切力達(dá)到峰值。C床使用時(shí)，由于其臀部以下的床板可隨皮膚伸展，軀干重力平衡，床板上的曲線提供足夠的空間容納臀部，因此沒(méi)有出現(xiàn)明顯的受剪面積。
同時(shí)由于A、B床不能與人體曲線相匹配，每次坐姿過(guò)程后受力分布都會(huì)發(fā)生變化，所以本應(yīng)施加在髖部的壓力都會(huì)變回背部，所以脊柱的壓縮力會(huì)增大，軀干被迫適應(yīng)一個(gè)意想不到的曲線，這對(duì)臥床病人來(lái)說(shuō)是非常不舒服的。
實(shí)驗(yàn)證明，單點(diǎn)平均壓力隨受試者體重的變化不明顯。然而，很明顯，肥胖的受試者與床墊的互動(dòng)區(qū)域要大得多。過(guò)厚的床墊和軟組織為軟組織變形和剪切力提供了條件。這是從人床模擬和壓力測(cè)量中知道的?；兪侨毖脑颉：竦能浗M織也不利于水分調(diào)節(jié)。對(duì)于“r”值小于0.361的瘦受試者，坐起后壓力分布圖幾乎保持不變，但全身有一些小區(qū)域支撐，過(guò)度集中的壓力是潰瘍和不適的主要原因。壓力比較如圖10和圖11所示。
圖10。壓力分布，r≥0.471
圖11。壓力分布，r≤0.361
為避免潰瘍，保證用戶的舒適性，并改進(jìn)ECB的設(shè)計(jì)，提出以下建議：
躺著時(shí)平均接觸壓力最小，單純坐著可能使平均壓力增加20%以上。與床板一起移動(dòng)也會(huì)產(chǎn)生摩擦力和剪切力。因此，建議患者不要長(zhǎng)時(shí)間保持坐姿，尤其是背部和腿部同時(shí)抬起的坐姿。每次改變姿勢(shì)時(shí)，患者應(yīng)獲得幫助，使背部和臀部至少稍微抬起一點(diǎn)，以釋放剪切力，這是令人失望的，因?yàn)檫@將是一項(xiàng)艱巨的工作和風(fēng)險(xiǎn)的保育[8]。
坐在床上時(shí)，衣服是減少摩擦所必需的。無(wú)論是躺著還是坐著都不能完全緩解坐骨區(qū)的接觸壓力，因此對(duì)于臥床的病人來(lái)說(shuō)，每隔一段時(shí)間將身體轉(zhuǎn)向一邊也是必要的。更胖的人更容易受到剪切力的威脅。瘦的人更容易承受壓力峰值和不舒服的感覺(jué)。有骨支撐的薄軟組織覆蓋的區(qū)域容易引起潰瘍[9]，[10]。床墊的設(shè)計(jì)應(yīng)符合此要求?；顒?dòng)受限的患者在ECB旋轉(zhuǎn)后，應(yīng)幫助其他人釋放剪切力。
對(duì)于設(shè)計(jì)人員來(lái)說(shuō)，有必要根據(jù)人體表面來(lái)設(shè)計(jì)床板，因?yàn)榇舶迮c人體曲線的不匹配可能導(dǎo)致意外的姿勢(shì)和力分布。床板應(yīng)留有足夠的空間，以便臀部旋轉(zhuǎn)時(shí)避免額外的壓力。床板表面以下的坐骨區(qū)、背部的壓力集中區(qū)應(yīng)能伸縮或移動(dòng)，以避免剪切力和摩擦力
第四部分 結(jié)論
兩種方法的研究結(jié)果基本一致。只有當(dāng)背部和腿部同時(shí)抬起時(shí)，測(cè)量的壓力才明顯大于模擬結(jié)果。這意味著可以利用模擬方法來(lái)評(píng)估和預(yù)測(cè)力的分布。在幾乎所有的情況下，超過(guò)32mmHg的壓力是不可避免的，坐起來(lái)或轉(zhuǎn)過(guò)去可能會(huì)造成明顯較大的壓力（甚至超過(guò)20%），剪切力也不能忽略。經(jīng)測(cè)試的ECB提供的所有體位都會(huì)導(dǎo)致髖關(guān)節(jié)壓力集中，這意味著應(yīng)考慮一些其他功能，如轉(zhuǎn)向一邊，以消除壓迫但難以緩解的部位。
出于所有這些原因，可用的ECB可用于提供援助，但只能以推薦的方式應(yīng)用，還需要其他人的定期幫助。在此基礎(chǔ)上提出了幾點(diǎn)建議。
附：原外文文獻(xiàn)
A biomechanical analysis of electric nursing care beds basing on a virtual man-bed system
2
Author(s)
Qi Wang ; Xuehua Tang
Abstract:
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
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Contents
SECTION 1.
Introduction
Individuals 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.
Method
To 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 SUBJECTS
R
Age (year)
weight (kilogram)
height (cm)
range
0.361–0.476
60–76
62–79
159–176
mean
0.419
63
71
174
Basing 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 PROCESS
No
Action
Relationship with mattress
1
From lying to sitting up (legs straight still)
Long period pressed with shear force and friction
2
From lying to sitting up (legs raised together)
long period pressed with shear force and friction
Turning 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 together
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Figure 2. Relationship between pressure and time
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TABLE III. DURATION OF EACH POSTURE
lying
raising
Sitting up
declining
lying
Time (s)
0
0–13
13–37
37–42
42
In 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 time
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It 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 origin
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Position 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 POSTURE
No
posture
part
gravity
Position of centroids (x, y, z) (cm)
1
lie
chest
14432972
3.48, 524.48, 9.63
2
lumbar
13470319
?4.96, 297.91, 8.75
3
hip
8614896
?3.75,102.93,3.81
4
total
72552996
?2.95,199.96,3.27
5
Lie on one side
chest
15463898
?84.28,358.04,339.00
6
lumbar
11401344
?81.00,225.87,154.95
7
hip
9861277
?84.64,100.46,47.63
8
total
69481027
?83.03,111.70,175.98
9
Sit up
chest
15178240
5.77,531.24,37.04
10
lumbar
14073480
5.17,283.78,25.13
11
hip
9037140
9.89,104.39,12.44
12
total
73641723
3.04,204.46,25.05
Here 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 parts
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The 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 sitting
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Figure7. Pressure distribution when back and legs are lifted together
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When 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 mattress
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Figure9. pressure distribution, with mattress taken away
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It 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.
Discussion
By 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. Wh