Cushions have been used on spine boards to reduce the interface pressure acting on the skin and thus prevent the formation of pressure ulcers. Several studies have focused on determining how using different types of cushions can reduce the normal interface pressure on the buttocks while lying on the spine boards. On the other hand, and while it has been agreed upon that the shear stresses contribute to the formation of pressure ulcers, this role has not been understood or quantified. The purpose of this work is to use 3-D finite element modeling to determine the contact frictional shear stresses at the buttocks while an individual is lying on a spine board when cushions of various stiffnesses are used.
The Zygote Solid 3D Male Human Anatomy model was used to construct a 3D CAD model of a section of the human body in the pelvic region. Skin, fat, muscles and bones were identified in the model. The Zygote SolidWorks model, the HyperMesh finite element preprocessor, and the ABAQUS software were used to create the finite element model. Bones were considered as an elastic isotropic material whereas skin, fat and muscles were modeled using Hyperelastic Neo-Hookean materials. Results were obtained to find the effects of body weight on the shear stresses while a person is lying flat with his buttocks contacting the spine board.
The results indicate that frictional skin shear stresses cannot be ignored since they were found to be, and depending on the cushion material, about 15% to 35% of the maximum normal pressure. We propose, and for the first time, a relationship to estimate the maximum shear stresses at the buttocks in terms of the maximum normal pressure for different Young’s moduli of cushions. These results can also be used as a guide to select cushion material that minimize normal and shear interface stresses.