Stiffening of muscle tissue under bone compression is a key factor in formation of pressure sores

Pressure sores (PS) are the result of intensive and prolonged mechanical loading of a vascularized soft tissue that inhibits or obstructs the nutrient supply and waste clearance necessary for maintaining the tissue viable. In this study, we characterized changes in mechanical properties of rat gracilis muscles exposed to prolonged intensive compression in vivo, and applied the modified properties to an anatomically accurate model of the pelvis of a recumbent, paralyzed patient. Using uniaxial tension testing, we found that tangent moduli of muscles exposed to interfacial pressure of 11.5 KPa for 2 to 6 hours were statistically indistinguishable from controls, but muscles exposed to 35 and 70 KPa were stiffer than controls by 60% in average, at strains of 2.5 and 5% (p<0.05). Histological evaluation (PTAH staining) showed that stiffening accompanies extensive necrotic damage, evident by loss of cross-striation. Incorporating the effect of stiffening of injured muscles into the model of a human patient, we were able to simulate, using the finite element method, a mechanism of deterioration in which PS injury is gradually spreading. Injury is diffused by exposure of normal muscular tissue to elevated stresses inducted by damaged muscular tissue under the bone-muscle contact surface.