I thoroughly agree with RTP's reply

WHY do we use u/s so much? - we have so many better options.

basic u/s physics will tell you that a soft tisue/metal interface = reflection of sound energy from the sound head (instead of absorption or transmission of the enery. This means the soft tissue surrounding the metal gets an extra (reflected) dose of sound energy, heating adjacent tissues much more quickly and possibly burning them. Thats one possible mechanism for pain from U/S around highly reflective areas.

The metal is not what gets hot as far as i have been taught- it's the tissue immediately adjacent.

At any rate, there is little evidence to support the use of ultrasound for most conditions it has been studied in - so does the risk outway the benefit?

we have far better tools to use than U/S