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Research Papers

# Stress Analyses Coupled With Damage Laws to Determine Biomechanical Risk Factors for Deep Tissue Injury During Sitting

[+] Author and Article Information
Eran Linder-Ganz

Faculty of Engineering, Department of Biomedical Engineering, Tel Aviv University, Tel Aviv 69978, Israel

Amit Gefen1

Faculty of Engineering, Department of Biomedical Engineering, Tel Aviv University, Tel Aviv 69978, Israelgefen@eng.tau.ac.il

http://www.npuap.org

1

Corresponding author.

J Biomech Eng 131(1), 011003 (Nov 18, 2008) (13 pages) doi:10.1115/1.3005195 History: Received January 04, 2008; Revised June 14, 2008; Published November 18, 2008

## Abstract

Deep tissue injury (DTI) is a potentially life-threatening form of pressure ulcer that onsets in muscle tissue overlying bony prominences and progresses unnoticeably to more superficial tissues. To minimize DTI, the efficacy of wheelchair cushions should be evaluated not only based on their performance in redistributing interface pressures but also according to their effects on stress concentrations in deep tissues, particularly muscles. However, a standard bioengineering approach for such analyses is missing in literature. The goals of this study were to develop an algorithm to couple finite element (FE) modeling of the buttocks with an injury threshold for skeletal muscle and with a damage-stiffening law for injured muscle tissue, from previous animal experiments, to predict DTI onset and progression for different patient anatomies and wheelchair cushions. The algorithm was also employed for identifying intrinsic (anatomical) biomechanical risk factors for DTI onset. A set of three-dimensional FE models of seated human buttocks was developed, representing different severities of pathoanatomical changes observed in chronically sitting patients: muscle atrophy and “flattening” of the ischial tuberosity (IT). These models were then tested with cushions of different stiffnesses representing products available on the market and semirigid supports. Outcome measures were the percentage of damaged muscle tissue volumes after $90min$ and $110min$ of simulated continuous immobilized sitting as well as muscle injury rates post-$60min$, -$90min$, and -$110min$ of continuous sitting. Damaged muscle volumes grew exponentially with the level of muscle atrophy. For example, simulation of a subject with 70% muscle atrophy sitting on a soft cushion showed damage to 33% of the muscle volume after $90min$ of immobilized sitting, whereas a comparable simulation with a nonatrophied muscle yielded only 0.4% damaged tissue volume. The rates of DTI progression also increased substantially with increasing severities of muscle atrophy, e.g., 70% atrophy resulted in 8.9, 2.7, and 1.6 times greater injury rates compared with the “reference” muscle thickness cases, after $60min$, $90min$, and $110min$ of sitting, respectively. Across all simulation cases, muscle injury rate was higher when a “flatter” IT was simulated. Stiffer cushions increased both the extent and rate of DTI at times shorter than $90min$ of continuous sitting, but after $110min$, volumes and rates of tissue damage converged to approximately similar values across the different cushion materials. The present methodology is a practical tool for evaluating the performances of cushions in reducing the risk for DTI in a manner that goes far beyond the commonly accepted measurements of sitting pressures.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

Flowchart of the algorithm used in this study for simulating the onset and progression of deep tissue injury. The algorithm incorporates finite element modeling as well as injury threshold and damage-stiffening law for muscle tissue obtained in previous experimental studies (47,49). IT, ischial tuberosity.

Figure 2

The FE models: (a) Model geometries showing the different muscle atrophy levels (“reference,” 30%, 50%, and 70%) and IT radii of curvature (7mm, 10.75 (reference) mm, and 13mm) that were simulated herein. The MRI image used to build the reference model geometry is shown on the top left frame. (b) Example of a FE mesh and description of the boundary conditions (vertical sagging of the IT and contact conditions at the cushion).

Figure 3

Mechanical properties measured for the cushion products in uniaxial compression tests. The tangent elastic modulus of the cushion at 80% strain, Ec, is plotted against the cushion’s density.

Figure 4

Effects of the level of muscle atrophy on maximal principal compression stress, principal tension stress, shear stress, and von Mises stress in muscle tissue for a soft cushion (tangent elastic modulus Ec=40kPa), a harder cushion (Ec=100kPa), and a semirigid support (Ec=1GPa).

Figure 5

Principal compression stress distributions in a 70% atrophied gluteus muscle along 110min of simulated immobilized continuous sitting on a (a) 100kPa cushion and (b) semirigid (1GPa) cushion. For clarity, only the muscle tissue component of the model is shown. The solid line depicts the predicted injured muscle volume at each time step. In this case, the size of the injured muscle region increased substantially between 75min and 90min and again between 90min and 110min as the stress-time tolerance levels of the tissue (Eq. 2) were exceeded in large parts of the muscle at these times. IT, ischial tuberosity and Ec, tangent elastic modulus of the cushion.

Figure 6

Effects of the level of muscle atrophy (“reference” and 50% atrophy) on maximal stress measures (a) and on the injured muscle area (b) for the different cushion stiffnesses (in terms of tangent elastic modulus of the cushion Ec). Smooth and dashed lines represent the injured muscle volumes post-90min and -110min of immobilized continuous sitting, respectively. IT, ischial tuberosity and Ec, tangent elastic modulus of the cushion.

Figure 7

Effects of the radius of curvature of the IT on maximal stress measures (a) and on the injured muscle area (b) for “reference” and 50% atorphied muscles while sitting on a 80kPa cushion. Smooth and dashed lines represent the injured muscle volumes post-90min and -110min of immobilized continuous sitting, respectively. Ec, tangent elastic modulus of the cushion.

Figure 8

Injury rates (percent injured muscle volume per minute) calculated after 60min, 90min, and 110min of simulated sitting for (a) “reference,” (b) 30% atrophied, (c) 50% atrophied, and (d) 70% atrophied muscles. IT, ischial tuberosity and Ec, tangent elastic modulus of the cushion.

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