Biomechanical changes on the typical sites of pressure ulcers in the process of turning over from supine position: theoretical analysis, simulation and experiment

6 Background: Pressure ulcer is a typical disease, which is common in long-term 7 bedridden patients and difficult to cure. It is necessary to study the biomechanics of 8 the typical sites of pressure ulcers in turning over from supine position, which is an 9 important reference for clinical medical nursing and and guides an assisted 10 exoskeleton robot design. 11 Methods: The typical sites of pressure ulcers mainly focus on the scapula and the 12 hip-sacrum of the trunk in turning over from the supine position. Based on the 13 requirements of rehabilitation technical aids and the anatomy theory, the simple 14 model of the scapula and the hip-sacrum were established for a force analysis in the 15 process of turning over from the supine position, and the theoretical contact pressure 16 between the human body and the bed surface was obtained. Then, three-dimensional 17 models of the scapula and hip- sacrum were reconstructed and the maximum stress 18 under different boundary conditions was obtained by finite element analysis. Finally, 19 the pressure distribution sensor was used to carry out the human experiment of 20 turning over from the supine position, and the pressure cloud diagram and the 21 maximum contact pressure curve of the shoulder blade and the hip were obtained under different angles of turning over. 23 Results : The results from theoretical analysis, simulation and experiment were 24 almost the same change trends, and the curves and the stress diagrams showed the 25 contact pressure change of the typical sites of pressure ulcers in turning over. The 26 angle threshold of the optimal comprehensive pressure can improve the use 27 efficiency of the equipment to assist human turning over and reduce the incidence of 28 pressure ulcers in the use of assisted bed in long-term bedridden patients. 29 Conclusions: In response to the less research on the mechanism of pressure ulcer, 30 biomechanical changes have been revealed, which helps to explain the causes of 31 pressure ulcer disease and provide basis for improving clinical nursing, and the 32 relevant results provided a reference that contributes to the man-machine coupling 33 design of the assisted rollover robot. and recurrence of the related results helpful a for

under different angles of turning over. 23 Results: The results from theoretical analysis, simulation and experiment were 24 almost the same change trends, and the curves and the stress diagrams showed the 25 contact pressure change of the typical sites of pressure ulcers in turning over. The 26 angle threshold of the optimal comprehensive pressure can improve the use 27 efficiency of the equipment to assist human turning over and reduce the incidence of 28 pressure ulcers in the use of assisted bed in long-term bedridden patients. 29 Conclusions: In response to the less research on the mechanism of pressure ulcer, 30 biomechanical changes have been revealed, which helps to explain the causes of 31 pressure ulcer disease and provide basis for improving clinical nursing, and the 32 relevant results provided a reference that contributes to the man-machine coupling 33 design of the assisted rollover robot. The pressure ulcer, also known as decubitus, is localized injury to the skin and/or 38 subcutaneous tissue caused by pressure or pressure in combination with shearing 39 force [1]. This phenomenon has a physiological effect restricting blood flow, with 40 the restriction in blood flow, perfusion of oxygen is limited and removal of 41 metabolic waste is inhibited which leads to cells death in the affected area resulting 42 in ischemia followed by pressure ulcers formation [2]. Pressure ulcers usually occur 43 in areas of bony prominence, for instance, the elbow, heel and back of skull, and 44 Multiple mechanical parameters including friction, shear stress, pressure and 45 microclimate are believed to be included in the potential triggering factors [3][4][5][6]. It is 46 more cost-effective to prevent the occurrence of ulcers than provide treatment especially for the stage four ulcers [7]. The risk of death after suffering the pressure 48 ulcers will increase 4-fold in the critical patients [8]. Nevertheless, the incidence of 49 pressure ulcers can decrease by 50-60% through effective prevention [9]. So it is 50 significant to study the biomechanics of the typical sites of pressure ulcers in turning 51 over from supine position, and it helps to achieve better care. 52 The prevention of pressure ulcers is mainly about relieving backpressure, decrease 53 the exposure time and improving skin health [10]. The common method is to change 54 the patient's body position every two hours by turning over, and as technology 55 advances, rehabilitation robots may also be used that needs to have good investigations into the biomechanics of human structure leading to pressure ulcers. 71 According to the structure of the body, the scapula, sacrum and calcaneus form a plane and support the entire body weight in the supine position [20], and the tilting 73 can affect the strain distribution, taking away the highest peak strains of the sacrum. 74 In different parts of the body, the probability of pressure ulcers is different, and in the 75 same area, there are biomechanical changes in the process of turning over. It is 76 necessary to reveal the biomechanics mechanism leading to pressure ulcers in order to 77 better guide pressure ulcers care, such as the optimal care angle and the specific care     Figure 1 The typical sites of pressure ulcers in turning over from supine position.

Figure 2
Theoretical force analysis of the scapula in the process of turning over from supine position.   The measurement of the contact pressure distribution.

Figure 6
The theoretical contact pressure of the scapula and hip-sacrum and the bed in different angle.

Figure 7
The nite element analysis outcome of the soft tissue of scapula and hip-sacrum.

Figure 8
The pressure distribution changes of scapula and hip in turning over from supine position.