Pathways of transmission of ultrasound energy through the distal metaphysis of the second phalanx of pigs: an in vitro study.

Ultrasound (US) has been studied by several researchers to assess its possible use in screening for osteoporosis; among other sites the phalanxes have been proposed as a possible site for investigation with US. In the present experimental work we studied the morphostructural characteristics of the second phalanx of the pig; then, in vitro, we investigated the behaviour of an ultrasound signal at 1.25 MHz crossing the distal metaphysis of the second phalanx. In particular, we studied the effects of milling or drilling on US velocity, and on the energy and shape of the signal generated by the US at the receiving probe. We demonstrated that the US velocity decreases by an average of 496 m/s (-21%) when axial perforations are made in the central marrow. A decrease is also noted in the number of peaks, and the normalized energy of the US signal received falls on average by 11.3 mV microseconds (-84%). The characteristics of the signal at the receiving probe can be broadly reconstituted if, after extensive drilling, the bone cavity is filled with polymerized styrene resin. In contrast, if phalanx milling is performed to remove the outermost bone tissue, the normalized energy increases by 15.5 mV microseconds (+84%) and the velocity of US increases by 163 m/s (+7%). It was also noted that the complexity of the signal received (i.e. number of peaks) and the signal normalized energy depend on the integrity of the bone structures traversed. The results reported here provide useful indications for interpreting the findings of clinical investigations with US, most specifically those performed on the phalanx of the hand.