Introduction Charateristics of orthopaedic implants depend structural properties (this topic) material properties Structural characteristic differs from strength characteristic not only depends on the material, but also the structural configuation of the object (cylinder, rectangle) the stuctural properties can also be demonstrated in a stress vs. strain curve Bending Rigidity (stiffness) Definitions defined as the slope of the curve in the elastic range on a structure stress-strain curve stress shielding of proximal bone in THA is related to implant stem stiffness Solid Cylinder proportional to the radius to 4th power for a solid cylinder cylinder A has great rigidity than cyliner B on illustration above (and thus has greater radius) Hollow Cylinder proportional to the radius to the 3rd power for a hollow cylinder Rectangular Object proportional to the thickness to the 3rd power Area Moment of Inertia (I) Area moment of inertia is a property of shape that is used to predict deflection, bending and stress in beams, and is proportional to Length to the power of four is a function of: structure width, thickness, and polar moment of inertia (J) measure of the flexural stiffness of a beam (i.e. bending rigidity) Polar moment of inertia represents an object resistance to torsion Moment of inertia is a measure of an object's resistance to change in rotation direction Deflection Deflection is the degree to which a part of a structural element is displaced under a load (because it deforms) Proportional to: (applied force/elastic modulus)(area moment of inertia)