Titanium is an inert, nonmagnetic, metal. It should not cause and alarm to go off.
Newer model detectors can detect both ferrous and non-ferrous metals. It probably depends on the amount of shielding provided by skin and tissue. Plus, I doubt all ports or airports have the latest equipment - with ports probably being well behind the curve.
I may be wrong, but aren't joint replacements not solely pure titanium but include titanium alloys and other metals?