US has been shown to be virtually useless in lots of studies. but, i do believe it is safe to use over titanium (opinions vary on this topic - see article below). if you want a joint to improve its movement then mobilize it (unless the hardware physically restricts it from moving).

From the literature (I cut a bunch out but it is worth finding and reading):
"OBJECTIVES: To determine the amount of agreement among general rehabilitation sources for both superficial heating and therapeutic ultrasound contraindications... DATA SYNTHESIS: Eighteen superficial heat and 20 ultrasound sources identified anywhere from 5 to 22 and 9 to 36 contraindications/precautions, respectively. Agreement among sources was generally high but ranged from 11% to 95%, with lower agreement noted for pregnancy, metal implants, edema, skin integrity, and cognitive/communicative concerns. Seventy-two percent of superficial heat sources and 25% of ultrasound sources failed to reference at least 1 contraindication claim."
Batavia M. Contraindications for superficial heat and therapeutic ultrasound: do sources agree? Arch Phys Med Rehabil. 2004 Jun;85(6):1006-12.