Compression-induced damage and internal tissue strains are related.

Prolonged mechanical loading of soft tissues adjacent to bony prominences can lead to degeneration of muscle tissue, resulting in a condition termed pressure-related deep tissue injury. This type of deep pressure ulcers can develop into a severe wound, associated with problematic healing and a variable prognosis. Limited knowledge of the underlying damage pathways impedes effective preventive strategies and early detection. Traditionally, pressure-induced ischaemia has been thought to be the main aetiological factor for initiating damage. Recent research, however, proposes tissue deformation per se as another candidate for initiating pressure-induced deep tissue injury. In this study, different strain parameters were evaluated on their suitability as a generic predictive indicator for deep tissue injury. With a combined animal-experimental numerical approach, we show that there is a reproducible monotonic increase in damage with increasing maximum shear strain once a strain threshold has been exceeded. This relationship between maximum shear strain and damage seems to reflect an intrinsic muscle property, as it applied across a considerable number of the experiments. This finding confirms that tissue deformation per se is important in the aetiology of deep tissue injury. Using dedicated finite element modeling, a considerable reduction in the inherent biological variation was obtained, leading to the proposal that muscle deformation can prove a generic predictive indicator of damage.

Microstructural analysis of deformation-induced hypoxic damage in skeletal muscle.

Deep pressure ulcers are caused by sustained mechanical loading and involve skeletal muscle tissue injury. The exact underlying mechanisms are unclear, and the prevalence is high. Our hypothesis is that the aetiology is dominated by cellular deformation (Bouten et al. in Ann Biomed Eng 29:153-163, 2001; Breuls et al. in Ann Biomed Eng 31:1357-1364, 2003; Stekelenburg et al. in J App Physiol 100(6):1946-1954, 2006) and deformation-induced ischaemia. The experimental observation that mechanical compression induced a pattern of interspersed healthy and dead cells in skeletal muscle (Stekelenburg et al. in J App Physiol 100(6):1946-1954, 2006) strongly suggests to take into account the muscle microstructure in studying damage development. The present paper describes a computational model for deformation-induced hypoxic damage in skeletal muscle tissue. Dead cells stop consuming oxygen and are assumed to decrease in stiffness due to loss of structure. The questions addressed are if these two consequences of cell death influence the development of cell injury in the remaining cells. The results show that weakening of dead cells indeed affects the damage accumulation in other cells. Further, the fact that cells stop consuming oxygen after they have died, delays cell death of other cells.

Validation of a numerical model of skeletal muscle compression with MR tagging: a contribution to pressure ulcer research.

Sustained tissue compression can lead to pressure ulcers, which can either start superficially or within deeper tissue layers. The latter type includes deep tissue injury, starting in skeletal muscle underneath an intact skin. Since the underlying damage mechanisms are poorly understood, prevention and early detection are difficult. Recent in vitro studies and in vivo animal studies have suggested that tissue deformation per se can lead to damage. In order to conclusively couple damage to deformation, experiments are required in which internal tissue deformation and damage are both known. Magnetic resonance (MR) tagging and T2-weighted MR imaging can be used to measure tissue deformation and damage, respectively, but they cannot be combined in a protocol for measuring damage after prolonged loading. Therefore, a dedicated finite element model was developed to calculate strains in damage experiments. In the present study, this model, which describes the compression of rat skeletal muscles, was validated with MR tagging. Displacements from both the tagging experiments and the model were interpolated on a grid and subsequently processed to obtain maximum shear strains. A correlation analysis revealed a linear correlation between experimental and numerical strains. It was further found that the accuracy of the numerical prediction decreased for increasing strains, but the positive predictive value remained reasonable. It was concluded that the model was suitable for calculating strains in skeletal muscle tissues in which damage is measured due to compression.