Sorry, I didn't intend to imply anything with the question, Kiren. It's completely fine to do a probe-gel-skin technique. I have done this using a small head probe on a 5th MCP fracture which was k-wired without any untoward event.

But to some hands that are bony and may have very uneven surfaces and in the absence of a small probe, you would want to use underwater technique. That was all i wanted to point out. Even direct contact would be fine so long as there was enough gel. US should not heat the appliance...well in theory and in most cases...