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Research Papers

# Effects of Intramuscular Fat Infiltration, Scarring, and Spasticity on the Risk for Sitting-Acquired Deep Tissue Injury in Spinal Cord Injury Patients

[+] Author and Article Information
Ran Sopher

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

Jane Nixon, Claudia Gorecki

Clinical Trials Research Unit, University of Leeds, Leeds LS2 9PH, UK

Amit Gefen1

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

See www.epuap.org.

See www.npuap.org.

1

Corresponding author.

J Biomech Eng 133(2), 021011 (Jan 31, 2011) (12 pages) doi:10.1115/1.4003325 History: Received August 11, 2010; Revised December 07, 2010; Posted December 22, 2010; Published January 31, 2011; Online January 31, 2011

## Abstract

Sitting-acquired deep tissue injury (DTI) is a severe form of pressure ulcer (PU) often affecting patients with spinal cord injury (SCI) who also tend to suffer from intramuscular fat infiltration, soft tissue scarring (due to previous PU), and/or muscle spasticity in their buttocks. We previously used finite element (FE) modeling to evaluate whether abnormal bodyweight is a risk factor for sitting-acquired DTI. Here we hypothesize that fat infiltration, scarring, or spasms increase internal loads in the gluteus muscles in the vicinity of the ischial tuberosities during sitting, which consequently put SCI patients with these conditions at a higher risk for DTI. Our objective was to determine changes in gluteal strains and stresses and tissue volumes exposed to elevated strains/stresses associated with these factors. Thirty-five FE models of coronal slices through the seated buttocks, simulating these conditions at different severities, were developed. We calculated peak strains and stresses in glutei and percentage volumes of muscle tissue exposed to above-critical strains/stresses (compression $strain≥50%$, compression/von Mises $stress≥2 kPa$, and strain energy $density≥0.5 kPa$). Progressive intramuscular fat infiltration increased all the aforementioned outcome measures. Increase in size of scar patterns that were contained in both muscle and fat tissues similarly elevated the outcome measures. Spasms increased muscle stresses and volumetric exposures to stress, but tissue volumes at risk were $∼1–2%$ and increases due to spasticity were slight. We conclude that the above potential risk factors can be listed according to the following order of importance: (i) fat infiltration, (ii) scars contained in both muscle and fat tissues, and (iii) spasms. This information should be considered when prioritizing prevention means and resources for patients with SCI.

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## Figures

Figure 1

MRI-T1 image of an individual with spinal cord injury who suffers from intramuscular fat infiltration. The intramuscular fat is seen as white strips within the dark gray area representing the gluteus maximus muscle.

Figure 2

Example of finite element meshing of the reference anatomy preceding (b) and following (d) simulation of sitting on a stiff support. The MRI images were used to produce the reference model (a) and evaluate the sagging of the ischial tuberosities (c).

Figure 3

Example finite element model geometries simulating the three different levels of intramuscular fat infiltration into the gluteus maximus muscle. The corresponding values of intramuscular fat volumes are provided in Table 1. An arrow indicates one strip of intramuscular fat infiltration.

Figure 4

Example finite element model geometries simulating the different patterns and dimensions of scars simulated herein. The corresponding values of scar volumes are provided in Table 2. Scars are indicated by brighter spots.

Figure 5

Distributions of strain energy densities in soft tissues of the buttocks for the same model configurations of intramuscular fat infiltration, which are shown in Fig. 2. A region of the gluteus muscle and overlying fat under the right ischial tuberosity is magnified in each simulation.

Figure 6

Distributions of von Mises stresses in soft tissues of the buttocks for the same model configurations of intramuscular fat infiltration, which are shown in Fig. 2. A region of the gluteus muscle and overlying fat under the right ischial tuberosity is magnified in each simulation.

Figure 7

Peak values of (a) principal compression strain, (b) maximal shear strain, and (c) strain energy density in the gluteus muscle for the simulations of intramuscular fat infiltration

Figure 8

Peak values of (a) von Mises stress, and (b) maximal shear stress in the gluteus muscle for the simulations of intramuscular fat infiltration

Figure 9

Percentage of gluteus muscle volume (excluding intramuscular fat), which exceeds critical load levels of (a) principal compression strain=50%, (b) strain energy density=0.5 kPa, and (c) von Mises stress=2 kPa for the simulations of intramuscular fat infiltration

Figure 10

Distributions of von Mises stresses in soft tissues of the buttocks for the same model configurations of scarring, which are shown in Fig. 3. A region of the gluteus muscle and overlying fat containing the scar, under the right ischial tuberosity, is magnified in each simulation.

Figure 11

Peak values of (a) principal compression stress, and (b) shear stress, plotted versus the relative muscle stiffness (Gins’ property ratio), for the muscle-spasticity simulations. (c) Percentage of gluteus muscle volume, which exceeds critical load level of principal compression stress=2 kPa, plotted versus the relative muscle stiffness (Gins property ratio), for the muscle spasticity simulations.

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