Modeling the development of in vitro and in vivo pressure-induced muscle damage

Pressure ulcers are localized areas of soft tissue breakdown resulting from sustained mechanical loading. The aim of the present thesis was to increase our understanding of the pathways leading to deep pressure ulcers that start in skeletal muscle tissue, also called pressure-induced deep tissue injury. Thorough knowledge of this aetiology is necessary for early detection, objective risk assessment and adequate prevention. Ischaemia has always been thought to play an important role in causing pressure ulcers, but the tolerance of skeletal muscle cells to ischaemia makes it unlikely that this is the only cause of damage. Previous experimental studies indicated that cellular deformation could directly lead to muscle cell damage (Bouten et al., 2001; Breuls et al., 2003a; Stekelenburg et al., 2007; Gawlitta et al., 2007a). This thesis focuses on theoretical modeling to further elucidate the contributions of pressure-induced ischaemia and deformation to skeletal muscle damage as observed in in vitro and in vivo experimental studies. A microstructural finite element model was developed to study the interactions between cells with respect to deformation-induced hypoxic damage. In the model, external compression decreased capillary cross-sections, thereby decreasing the oxygen supply to the cells, eventually resulting in cell death. Upon cell death, metabolism ceased and mechanical stiffness was reduced. The latter effect led to a change in the micro-mechanical environment, affecting the extent of capillary occlusion. Together, these effects delayed or even prevented the ensuing damage development. To integrate deformation-induced damage with ischaemic damage, a cellular damage law for deformation-induced damage had to be established. The hypothesis was that such a law could be based on intracellular calcium accumulation which was thought to result from a deformation-induced disruption of the cell membrane integrity. This was tested in single-cell compression experiments in which the intracellular calcium concentration was monitored. The heterogeneity in the responses emphasized the significance of the cell level in damage processes, but there was no consistent increase in the intracellular calcium concentration as was hypothesized. The combined effects of deformation- and ischaemia-induced damage were analyzed using a theoretical description of in vitro experiments from Gawlitta et al. (2007b). In those experiments, tissue-engineered muscle constructs were subjected to ischaemia and/or mechanical compression. Concentrations of metabolites and a cell death marker (LDH) were measured in the medium surrounding the construct. Compression did not lead to an increase in the LDH concentration, which contradicted previous findings (Gawlitta et al., 2007a; Breuls et al., 2003a; Bouten et al., 2001). The theoretical model showed that this lack of effect of compression could be explained by the compression-induced decrease in diffusivity. Compression did lead to considerable cell death but diffusion of LDH to the medium was limited. To study the local relation between muscle damage and deformation, in vivo animal experiments from Stekelenburg et al. (2007) were used. With an MR-compatible loading device, both the short-term spatial damage distribution and the internal strain distribution could be measured, using T2-weighted MRI and MR tagging respectively. Since these two techniques could not be combined in one protocol, a dedicated finite element model was developed to calculate the internal strain distribution for each animal specifically. The model was validated with MR tagging measurements. Analysis of damage in conjunction with the numerical strain calculations proved the existence of a strain threshold for damage initiation. When maximum shear strains in the muscle exceeded this threshold, damage was observed. A local comparison between measured damage and calculated maximum shear strains revealed a monotonic increase in damage with increasing strain. Moreover, this relationship was very similar for the individual animals, suggesting that the sensitivity for strain-induced damage is a tissue property. In conclusion, this thesis shows that the development of theoretical models can be a valuable addition to both in vitro and in vivo experiments. Taking into account diffusion properties is important for the analysis of indirect measurements. It proved that deformation caused damage, although it could not be measured in the experiments of Gawlitta et al. (2007b). Moreover, for the first time a strain threshold for the initiation of damage in skeletal muscle tissue was identified, using a combined experimental-numerical approach. When this threshold was exceeded, there was a monotonic increase in damage with increasing strain, and the differences among the animals were remarkable small. Thus, the results strongly propose that deformation is an important contributor to damage initiation of pressure-induced deep tissue injury