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Pressure-related deep tissue injury (DTI) is a severe form of pressure ulcer that initiates in compressed muscle tissues under bony prominences, and progresses superficially towards the skin. Patients with impaired motosensory capacities are at high risk of developing DTI. There is a critical medical need for developing risk assessment tools for DTI. A new anatomical index, the Compression Intensity Index: CII=(BW/Rt)^{1/2}, which depends on the body weight (BW), radius of curvature of the ischial tuberosities (R) and thickness of the underlying gluteus muscles (t), is suggested for approximating the loading intensity in muscle tissue during sitting in permanent wheelchair users, as part of a clinically-oriented risk assessment for DTI. Preliminary CII data were calculated for 6 healthy and 4 paraplegic subjects following MRI scans, and data were compared between the groups and with respect to a gold standard, being a previously developed subject-specific MRI-finite-element (MRI-FE) method of calculating muscle tissue stresses (Linder-Ganz et al., J. Biomech. 2007). Marked differences between the R and t parameters of the two groups caused the CII values of the paraplegics to be ∼ 1.6-fold higher than for the healthy (p<0.001), thereby indicating on the sensitivity of this parameter to the pathoanatomical changes that occur in the buttocks with paraplegia. Data of CII correlated reasonably with the gold standard calculations of MRI-FE muscle stresses (correlation coefficient 0.65). Since CII measurements do not require highly-specialized biomechanical numerical analyses such as MRI-FE, CII has the potential to serve as a practical, quick, and cost-effective approximation of the loading intensity in muscles of wheelchair-bound or bedridden patients. Hence, CII measurements can be integrated into DTI-risk-assessment tools, the need of which is now being discussed intensively in the American and European Pressure Ulcer Advisory Panel meetings. [ABSTRACT FROM AUTHOR]

The most common complication associated with immobilization is pressure sores caused by sustained localized tissue strain and stress. Computational simulations have provided insight into tissue stress-strain distribution, subject to loading conditions. In the simulation process, adequate soft tissue material parameters are indispensable. An in vivo procedure to characterise material parameters of human gluteal skin/fat and muscle tissue has been developed. It employs a magnetic resonance imaging (MRI) device together with an MRI compatible loading device. Using the derived data as constraints in an iterative optimization process the inverse finite element (FE) method was applied. FE-models were built and the material constants describing skin/fat and muscle tissue were parameterized and optimized. Separate parameter sets for human gluteal skin/fat and muscle were established. The long-term shear modulus for human gluteal skin/fat was G_{∞, S/F}= 1182 Pa and for muscle G_{∞, M} = 1025 Pa. The Ogden form for slightly compressible materials was chosen to define passive human gluteal soft tissue material behaviour. To verify the approach, the human skin/fat-muscle tissue compound was simulated using the derived material parameter sets and the simulation result was compared to empirical values. A correlation factor of R^{2} = 0.997 was achieved. [ABSTRACT FROM AUTHOR]