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contact area between two bones is likely to mean lower peak loads at the
articular surfaces, which will be associated with reduced risk of cartilage
and subchondral bone damage, the precursors of arthritic changes.
A smaller range of motion at a joint will reduce the displacement
required of a particular foot or leg tendon that helps control motion at that
joint. This in turn may influence the extent of muscle shortening and the
characteristics of the muscle contraction. Reduced foot motion will also
impact on the velocity of foot bone movements and this in turn influences
the velocity of tendon movement and muscle action required. A change
in the position of a foot joint may change the muscle/tendon length at
which muscle contraction occurs, and subsequently the efficiency of how
the muscle generates the required forces. This may change the physio-
logical effort required from the muscle and there is the risk that this effort
lies outside the muscle's physiological range. Changes in joint motion
velocity or displacement may be associated with reduced risk of injury as
forces and the rates of loading of some muscle, tendon and ligamentous
tissues are also likely to be lower.
The pressure exerted on a specific site under the foot has a strong
association with sites of skin lesions, foot pain and plantar ulceration.
Pressure is a function of both the load (force) applied to the area and the
area over which the force is applied. Pressure can thus be reduced by
increasing contact area, reducing the force applied to the area, or both.
The forces under the foot can be redistributed away from areas of high
pressure to areas of lower or no load simply by increasing the contact
area between the footwear and foot with an orthosis. The use of an arch
support on the medial side of an orthosis will enable the medial arch of
the foot to bear load where it might not otherwise do so. The use of soft
deformable materials enables an orthosis to conform to the shape of the
metatarsal heads or other bony prominences, increasing the contact area
at these specific sites.
Thus orthosis shape and materials can quickly increase contact area
and reduce pressures experienced by the plantar surface of the foot. This
type of foot orthosis is often termed a 'total contact' foot orthosis
(
Figure 3.3
) and is made to a positive plaster of Paris cast of the foot that
has not been modified in any way, which encourages distribution of load
over the entire plantar aspect of the foot. This cast is made from an
impression of the plantar aspect of the foot, often taken in foam
(
Figure 3.4
).
Material properties are also critical in determining the pressure expe-
rienced by the foot. The use of thick, deformable materials will reduce the
magnitude of loads applied to the foot and the rate at which load is
