A stainless steel femur rod does not set off airport metal detectors because it is made of non-magnetic stainless steel.
Stainless steel is made in two forms: austenitic and ferritic. Austenitic stainless steel is nonmagnetic. It is the common form used in cooking pots and for surgical hardware.
Ferritic stainless steel is magnetic. It is used in some knives and guns and for various industrial purposes.
Non-stainless steel (carbon steel) is magnetic. It is used in most knives and guns.
Metal detectors do not detect metals as such; they detect the distortion of a magnetic field produced by materials of high permeability.
Magnetic materials have high permeabilites which greatly distort the field and so are ready to detect.
Nonmagnetic materials have low permeabilites and as a result have little effect on the field and are difficult, if not impossible, to detect.
The small bunch of keys detected at the airport is probably on a magnetic steel ring because most keys are made of nonmagnetic metals, such as brass or aluminum.
One can avoid setting off airport metal detectors by limiting the number of bank credit cards (they have magnetic stripes) and by touching all the metal things with a strong magnet and not carrying those it attracts.
