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TECHNICAL BRIEFS

# Viscoelastic Properties of Ovine Adipose Tissue Covering the Gluteus Muscles

[+] Author and Article Information
Amit Gefen1

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

Einat Haberman

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

1

Corresponding author.

J Biomech Eng 129(6), 924-930 (Apr 19, 2007) (7 pages) doi:10.1115/1.2800830 History: Received September 07, 2006; Revised April 19, 2007

## Abstract

Pressure-related deep tissue injury (DTI) is a life-risking form of pressure ulcers threatening immobilized and neurologically impaired patients. In DTI, necrosis of muscle and enveloping adipose tissues occurs under intact skin, owing to prolonged compression by bony prominences. Modeling the process of DTI in the buttocks requires knowledge on viscoelastic mechanical properties of the white adipose tissue covering the gluteus muscles. However, this information is missing in the literature. Our major objectives in this study were therefore to (i) measure short-term $(HS)$ and long-term $(HL)$ aggregate moduli of adipose tissue covering the glutei of sheep, (ii) determine the effects of preconditioning on $HS$ and $HL$, and (iii) determine the time course of stress relaxation in terms of the transient aggregate modulus $H(t)$ in nonpreconditioned (NPC) and preconditioned (PC) tissues. We tested 20 fresh tissue specimens (from 20 mature animals) in vitro: 10 specimens in confined compression for obtaining the complete $H(t)$ response to a ramp-and-hold protocol (ramp rate of $300mm∕s$), and 10 other specimens in swift indentations for obtaining comparable short-term elastic moduli at higher ramp rates $(2000mm∕s)$. We found that $HS$ in confined compression were $28.9±14.9kPa$ and $18.1±6.9kPa$ for the NPC and PC specimens, respectively. The $HL$ property, $10.3±4.2kPa$, was not affected by preconditioning. The transient aggregate modulus $H(t)$ always reached the plateau phase (less than 10% difference between $H(t)$ and $HL$) within $2min$, which is substantially shorter than the times for DTI onset reported in previous animal studies. The short-term elastic moduli at high indentation rates were $22.6±10kPa$ and $15.8±9.4kPa$ for the NPC and PC test conditions, respectively. Given a Poisson’s ratio of 0.495, comparison of short-term elastic moduli between the high and slow rate tests indicated a strong deformation-rate dependency. The most relevant property for modeling adipose tissue as related to DTI is found to be $HL$, which is conveniently unaffected by preconditioning. The mechanical characteristics of white adipose tissue provided herein are useful for analytical as well as numerical models of DTI, which are essential for understanding this serious malady.

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## Figures

Figure 1

Experimental setups: (a) confined compression test by means of a uniaxial material testing machine (Model 5544, Instron Co., High Wycombe, UK); (b) swift indentation test. A pneumatic piston, “armed” with pressurized air (4bars), drives a flat-surface cylindrical indentor (diameter of 12mm) onto the surface of the tissue specimen, at a speed of 2000mm∕s. A stopper plate stops the indentor’s movement at a depth of 4mm. The indentation chamber is covered to minimize dehydration of the tissue’s upper surface.

Figure 2

Examples of transient aggregate modulus H(t) data (from specimen 1, Table 1) in the NPC mode (black marks) and PC mode (gray marks). The first 45s of stress relaxation are magnified (on the top right frame) to show the biexponential curve fits (dashed lines, Eq. 2) at the region of dominant changes in H(t).

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