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medial arch will be able to bear a significant proportion of the force applied
to the plantar surface, unloading the heel and potentially the forefoot, too.
This benefit is lost as soon as the heel comes off the ground, because
the orthosis under the arch of the foot will no longer be in contact with
the ground.
Figures 3.6
and
3.7
show that a rigid orthosis with only
1.8 mm of rigid material under the heel can still reduce heel pressures by
up to 50 per cent at specific sites under the heel, simply by redistribution
of load, and no shock absorption. Do not assume pressure reductions
require soft and compliant materials.
It has been widely assumed that making a foot orthosis to the shape
of the foot (using a cast) would provide the best means of maximizing
contact area and thus reducing foot pressures under the forefoot. One
difficulty with this is the fact that when pressures are greatest under the
forefoot, the heel and midfoot are off the ground, so any arch support
and heel cup that are created from the cast of the foot are largely
ineffective in terms of increasing contact area at that point in the gait
cycle. A further difficulty is that foot shape may not relate to the contact
area between the plantar aspect of the foot and the shoe. Recent orthotic
innovations have focused on the integration of foot shape information with
pressure pattern (contact area pattern) data, and manufacturing the ante-
rior edge of the arch and metatarsal head area according to the contact
pattern rather than the foot shape. This has produced consistently greater
reductions in foot pressures compared with orthoses based only on the
shape of the foot (
Owings et al 2008
).
Perhaps the most effective pressure-reducing orthoses combine all
these properties: a rigid arch support to redistribute load as much as
possible, use of compliant materials under the forefoot to increase local
contact area at the metatarsal heads, and reduce loading rates owing to
their compliance. The positioning of these forefoot features must take
account of the pattern of contact between foot and shoe.
It is thought that shear forces (sliding forces) are as damaging as verti-
cal pressures under the foot and can lead to serious complications such
as foot ulcers. Under the forefoot, shear stress is greatest during propul-
sion when the foot is pushing backwards against the ground. If the plantar
skin is adhered to the sock and shoe then the shear forces may become
concentrated within the soft tissues of the forefoot and cause tissue
damage. To reduce shear forces under the forefoot, the forefoot must be
allowed to slide backwards inside the shoe at this period of gait, so that
the shear forces that are applied to the skin surface are reduced. However,
a well fitted shoe will prevent this due to a strong heel counter, good arch
support and appropriate lacing and throat. As with the pressure changes
already mentioned, local changes in shear (that is, changes occurring
