Prosthetic Legs and Arms

 Prosthetic limbs must be able to sustain large magnitudes of stress, therefore the materials selected for their construction is extremely important.  Most are now made of an nonliving solid because they are subject to deformation, and thus, must demonstrate elastic qualities. Flex-Foot, inc.

Physical Concepts for Prosthetic Limb Design.

*Young's Modulus:
Y, measures the tension or compression of the object.  This modulus is used in deciphering
the material(s) to use for the prosthetic device.

Y=  tensile stress      = F/A
tensile strain          =  L/Lo

 Common Materials Characteristics Aluminum Sturdy, durable, non bulky, withstands drastic temperature changes, and can support heavy weights. Fiberglass Easily manipulated, natural looking, is compressible, can withstand moderate to heavy weights. Wood Not frequently used, inexpensive, bulky, natural looking, can withstand heavy weights, but must be replaced often.

* Shear Modulus:
Measures the elasticity of Shape.  This modulus is used in designing the
appropriately shaped limb for a  particular person.

S= shear stress.F/A
shear strain.       X/h

These moduli can determine and predict the necessary size and shape of a prosthetic limb based on the weight and activity level of the amputee.  This can be done by finding a particular material that can withhold the person's weight(mg) and will respond with compressions and elasticity to the movement of the person.

Problems encountered in the field.
*when deciding what material to use for a person of particular weight, or activity level, the person's height, weight, and size(dimensions), must be known.
* Height = 66inches=1.68m
*length of desired limb= 19inches=.48m
* desired cross sectional area=( 7E-3m)
(the area of the limb they wish to match)
*Maximum weight to carry = 350lbs ,1550N
(weight of the person, and weight they can hold)
*Using Aluminum first, the distance compressed will be measured.

Using Young's modulus:  Y=FLo/A(delta)L
delta L= FLo/YA
=(1550N)(.48m)
(7E10) (7E-3m)

=  1.5E-6m
*We thus find that aluminum is a suitable source for the limb*