Can loaded interface characteristics influence strain distributions in muscle adjacent to bony prominences?

Pressure distributions at the interface between skin and supporting tissues are used in design ofsupporting surfaces like beds, wheel chairs, prostheses and in sales brochures to support commercialproducts. The reasoning behind this is, that equal pressure distributions in the absence of high pressuregradients is assumed to minimise the risk of developing pressure sores. Notwithstanding the difficultyin performing reproducible and accurate pressure measurements, the question arises if the interfacepressure distribution is representative of the internal mechanical state of the soft tissues involved.The paper describes a study of the mechanical condition of a supported buttock contact, dependingon cushion properties, relative properties of tissue layers and friction. Numerical, mechanicalsimulations of a buttock on a supporting cushion are described. The ischial tuberosity is modelled as arigid body, whereas the overlying muscle, fat and skin layers are modelled as a non-linear Ogdenmaterial. Material parameters and thickness of the fat layer are varied. Coulomb friction betweenbuttock and cushion is modelled with different values of the friction coefficient. Moreover, thethickness and properties of the cushion are varied.High shear strains are found in the muscle near the bony prominence and the fat layer near thesymmetry line. The performed parameter variations lead to large differences in shear strain in the fatlayer but relatively small variations in the skeletal muscle. Even with a soft cushion, leading to a highreduction of the interface pressure the deformation of the skeletal muscle near the bone is high enoughto form a risk, which is a clear argument that interface pressures alone are not sufficient to evaluatesupporting surfaces.