Chalmers Conferences, 9th European Conference on Mathematical and Theoretical Biology

Computational modeling to assess guidelines, risk factors and cushions addressing sitting-acquired pressure ulcers
Amit Gefen

Last modified: 2014-06-09


Pressure ulcers (PUs) develop in soft tissues that are subjected to

sustained large deformations, which typically occur under bony prominences

during weight-bearing postures, particularly in the seated buttocks of

wheelchair users. These injuries onset mainly in persons with impaired

mobility and sensation, such as those with a spinal cord injury (SCI).

Guidelines for PU prevention usually include a recommendation to place soft

and thick-enough support surfaces under the weight-bearing tissues, in order

to better distribute contact pressures and shear as well as deep tissue strains

and stresses. The present talk will describe utilization of magnetic resonance

imaging (MRI) coupled with finite element (FE) modeling as tools for

evaluating performances of cushions in distributing tissue loads in the seated

buttocks. Specifically, we demonstrate how FE simulations can assist in

studying the effects of sitting-down after a push-up maneuver that patients

with a SCI are trained to perform, on internal tissue loads in the buttocks. We

also consider examples of sitting on an air-cell-based cushion versus foam

cushions, as means to explore the influence of the design of the cushion on

tissue loading conditions. For quantifying the distributions of mechanical

strains and stresses in the seated buttocks, we based our FE modeling on a

4-mm-thick MRI slice acquired from a 21 years-old male, one year post a SCI.

Segmentation and meshing of the buttocks tissues and cushion were

performed using Simpleware®. Simulations were all processed using the

FEBio 1.5.1 software package. Loading conditions were selected to simulate

vertical descent of the ischial tuberosities under the load of the trunk.

Mechanical properties of tissues were adopted from the literature. In the

sitting-down simulations, while the fat tissues of the buttocks were loaded at a

nearly constant rate, skin loads increased nonlinearly - with a greater

load/time slope at the early phase of skin-support contact. In the context of

tissue health and prevention of PUs, this theoretically indicates that the more

sensitive period with respect to skin integrity and health is at initial skinsupport

contact. In simulations incorporating skin scarring, e.g. due to a

previous PU which healed, we further found that the edges of a pre-existing

scar are more susceptible to injury, and the greater risk for that is when a

hypertrophic scar in skin was present in the simulations. The practical

implication of this finding is that patients should reposition themselves

gradually and gently, as oppose to "falling" back into the wheelchair after

finishing a push-up maneuver. We further found that the immersion of the

buttocks in the air-cell-based cushion and the envelopment of the body by this

cushion type are considerably better than what can be achieved with flat foam

cushions. The resulting strains and stresses in the soft tissues of the buttocks

are therefore substantially lower when sitting on the air-cell-based cushion,

with respect to sitting on standard flat foams. Overall, the studies indicated the

power and potential of using FE for design and assessment of cushion

technologies and products aimed at protecting patients against PUs.