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2. THESE PRINCIPLES ARE :-
PRESSURE EQUILIBRIUM
THE LEVER
ARM
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3. The pressure is equal to the total force per
unit area.
 [ P= F/A ] where
 F = force, A= area of application
It means that the larger the area of a pad
or plastic shell of an Orthosis is, the less
force will be placed on the skin.
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4. It says the sum of the forces and the
bending forces created must be equal to
zero.
This means that three-point pressure
system or loading system occurs when
three forces are applied to a segment in
such a way that a single primary force is
applied b/w two additional counter forces
with the sum of all three forces equalizing
zero.
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5. The farther the point of force from the joint
is, the greater will be the moment arm and
the smaller the magnitude of force is
required to produce a given torque at the
joint.
THIS IS WHY MOST ORTHOSIS ARE
DESIGNED WITH LONG METAL BARS
OR PLASTIC SHELL THAT ARE THE
LENGTH OF ADJACENT SEGMENT.
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6.  So while designing or evaluating the orthotic device, we
should check that-
1. There is adequate padding covering the greatest area
possible for comfort.
2. The total force acting on the involved segment is equal
to zero or there is equal pressure throughout the
Orthosis and no areas of skin irritation.
3. The length of the Orthosis is suitable to provide an
adequate force to create the desired effect and to avoid
increased transmission of shear forces against the
anatomic tissues.
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