Optimization Potential of the Method - Preventive Biomechanics - page 321

Fig. 7.42 Plot of absolute
direct stress and
corresponding shear stress
from Fig. 7.41
Generally, from a biomechanical point of view, this shows that a premature
conclusion such as ''soft support material provides pressure relief (the internal
tissue stress situation)'' is not valid. Rather, very soft foam at a lower cushion
thickness (cf. foam stiffness scaled with factor 0.25, see Fig. 7.42 ) is associated
with blocking effects which require consideration.
Since progression of direct and shear stress depend on each other and tend to be
inversely proportional, cushion optimization should incorporate more than one
cushion material, when considering plain block-shaped support geometry. This
could mean, for example, employing softer material at the cushion periphery to
restrict vertical tissue displacement in combination with a stiffer core inlay in the
center. Such an approach, however, requires knowledge about tissue damaging
(or comfort) effects of the particular stress and/or strain quantity to judge the
significance of the single stress/strain components.
7.2.3 Support Shape Optimization
7.2.3.1 Procedures
In addition to evaluating cushion thickness effects on tissue, the influence of the
support shape is investigated using an optimization algorithm to seek a support
shape that reduces internal tissue stress (and/or strain). In the optimization process
the guidelines outlined in Sect. 3.4 'Parameter Identification', specifically Sect.
3.4.6 , are followed. In contrast to Sect. 5.2 where human tissue material param-
eters have been optimized using target function values to establish the sum of