Viscoelastic properties of ovine adipose tissue covering the gluteus muscles

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 (H(S)) and long-term (H(L)) aggregate moduli of adipose tissue covering the glutei of sheep, (ii) determine the effects of preconditioning on H(S) and H(L), 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 300 mms), and 10 other specimens in swift indentations for obtaining comparable short-term elastic moduli at higher ramp rates (2000 mms). We found that H(S) in confined compression were 28.9+/-14.9 kPa and 18.1+/-6.9 kPa for the NPC and PC specimens, respectively. The H(L) property, 10.3+/-4.2 kPa, was not affected by preconditioning. The transient aggregate modulus H(t) always reached the plateau phase (less than 10% difference between H(t) and H(L)) within 2 min, 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+/-10 kPa and 15.8+/-9.4 kPa 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 H(L), 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.