R.H.M. Goossens
Delft University of Technology
The Netherlands
March 2006
In medical literature the effect of a mechanical load on the diffusion of oxygen and
metabolites to the cells has been studied extensively, especially in relation to pressure sores (or decubitus). A similar relationship to discomfort is also to be expected. Most authors agree that the most important cause of decubitus is prolonged tissue ischemia that is caused by the mechanical load on the tissue which causes the capillaries (the smallest blood vessels) to be closed. Therefore, the measurement of the mechanical load, especially pressure, is often used to compare different cushions that support the body. However, another important part of the mechanical load, namely shear, is often not taken into account. A drawback of the measurement of the mechanical load on the tissue is that the blood flow in the skin is evaluated in an indirect way. This can be overcome by direct measurement of blood perfusion in the tissue by means of laser Doppler.
Shear force is an important factor in the mechanical load on the tissue during human / material interaction. In order to gain insight in the shear force a special sensor was developed that can measure shear on the contact surface between the tissue and a material (Goossens, 1997). In a study performed by Goossens (2001) with this shear sensor, it was concluded that a seat cushion with LiquiCell® technology produces significantly lower shear stress on the skin, compared to either a foam or a gel cushion.
The aim of the present study is to compare the blood perfusion in the tissue during real life sitting conditions at the ischial tuberosities when using a LiquiCell® cushion, a foam cushion and a gel cushion.Blood perfusion was measured on 15 healthy subjects while sitting on three different cushions; a LiquiCell® cushion, a gel cushion, and a foam cushion. Measurement of blood perfusion was done using a Vasamedics Laserdoppler Softflo sensor. The sensor was attached to the bare skin of the right ischial tuberosity of the subject. The subject was then seated in office chair (brand: Hag Credo model: 2260) wearing a pair of jogging trousers. The office chair was ergonomically adjusted to match the dimensions of the subject. The angle of the seat was fixed and tilted 3 degrees backward.
After being seated for a period of 60 minutes, the blood perfusion was significantly better in subjects who had been sitting on the LiquiCell® cushion, 1.8 times better compared to the foam cushion (P=0.02) and 2.3 times better compared to the gel cushion (P=0.005). These results show that the reduction of shear force on the tissue attributable to the LiquiCell® cushion resulted in a better load situation inside the tissue, which resulted in better blood perfusion. More...