Last modified: 2014-06-09
Abstract
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.