Using ultrasound elastography to monitor human soft tissue behaviour during prolonged loading: A clinical explorative...

Pressure ulcers are localized injuries to the skin or underlying tissues over a bony prominence, as a result of pressure, or pressure in combination with shear. Ultrasound imaging techniques are widely applied to detect and to investigate pressure ulcer pathogenesis and healing, but due to missing structural alterations, very early signs of tissue alterations are not detectable. Ultrasound elastography was proposed as a new method to measure very early signs of pressure ulcer development but this was not investigated in humans so far. The aim of this study was to investigate for the first time whether US elastography is able to measure potential changes in dermal and subcutaneous tissue stiffness during prolonged loading.

An exploratory study including nine healthy volunteers (mean age 70.1 (SD 4.8) years) was conducted. Study participants were requested to follow a standardized lying protocol, consisting of two loading phases of 90 and 150 min in supine position on a standard hospital mattress. Three pressure ulcer predilection sites (lateral heel over the calcaneus, sacral, and upper back area) were measured using B-mode and elastographic ultrasound system at baseline - immediately, after 90 min, and after 150 min loading.

Mean baseline shear wave velocities were highest in the heel skin (2.7 m/s) and lowest in the upper back skin (1.9 m/s) indicating that heel skin was stiffest. Also the subcutaneous soft tissue stiffness was highest for the heel (2.7 m/s) and lowest for the upper back region (1.3 m/s). After 90 and 150 min loading there was a mean stiffness increase of the skin layers and a pronounced stiffness decrease of subcutaneous heel and sacral tissues.

Continuous loading of skin and underlying subcutaneous soft tissues leads to dynamic changes of tissue stiffness which are considered to play key roles in pressure ulcer development. Superficial skin and deep tissues seem to react differently. Elastography is able to quantify the dynamic of skin and subcutaneous soft tissue stiffness changes non-invasively in vivo. Shear wave velocity might serve as a new parameter for quantifying pressure ulcer damage risk in superficial and deeper tissues prone to pressure ulcer development.