Distribution of internal strains around bony prominences in pigs

Deep tissue injury (DTI) is a type of pressure ulcer in which tissue breakdown initiates at the bone-muscle interface under intact skin. Excessive deformation in the soft tissue, particularly around bony prominences, is believed to be one of the causes leading to the development of DTI. The main goal of this study was to measure the magnitude and distribution of strains within muscles surrounding the ischial tuberosities, induced by levels of external loading that encompass the range of loading experienced by the soft tissue in seated individuals. The experiments were conducted in adult pigs with intact spinal cords (n = 5) and pigs with partial spinal cord injury (SCI) (n = 2), one of which also had a DTI. A secondary goal was to obtain a preliminary assessment regarding the capacity of intermittent electrical stimulation (IES), an intervention for preventing the formation of DTI, to counteract the muscle compression caused by external loading. In intact animals, muscles subjected to external loads equivalent to 25% of body weight experienced maximal principal strains, minimal principal strains, and shear strains of 0.68, -0.3, and 0.4, respectively. These magnitudes increased by 91.9, 17.6, and 87.5%, respectively, when external loading increased to 50% body weight. Minimal to no further increases in strain magnitudes were seen with the 75% body weight loading level. In one animal with SCI and no DTI, strain magnitudes were on average 9.7% higher than those in the intact animals at the corresponding loading levels. The presence of a DTI in another animal with SCI reduced strain magnitudes by 28% compared to intact animals. The regions in the muscle that underwent the largest deformations were those between the ischial tuberosity and the external surface, and up to 2 cm ventral to the ischial tuberosity (furthest measured). Muscle contractions produced by IES increased the thickness of the tissue between the ischial tuberosities and skin during the period of stimulation by 10-20% for loading levels up to 75% of body weight in both intact and spinal cord injured pigs. This study provides the first measurements of strain around the ischial tuberosities in an animal model that resembles humans.