Usually titanium, but yeah sometimes medical stainless steel. Both are non-ferromagnetic, especially titanium. These balls look like they could well be normal steel or any other metal. Also, implants - unlike these metal balls - are usually screwed firmly into your bones. So yeah, implants should be mostly fine on MRI. Loose balls of dubious metal? Wouldn’t advise it. Keep in mind MRIs are literally powerful enough that metals in tattoo ink can even be an issue - which the article you linked mentions itself.