The distribution of 99mTc-EHDP in the tissues of the dog and its application in the assessment of fracture healing.

Technetium-labelled ethane hydroxydiphosphonate (99mTc-EHDP) is a commonly used bone-scanning agent. After injection it leaves the circulation to enter bone and to be cleared by the kidney. The transcapillary exchange of 99mTc-EHDP in bone was examined and found to be low. The capillary movement was compared with that of sucrose, a freely diffusible substance, and it was found that the permeability ratio of 99mTc-EHDP to 14C-sucrose was similar to the diffusion coefficient ratio, suggesting that 99mTc-EHDP passes through the capillaries by the process of passive diffusion. The renal clearance of 99mTc-EHDP was 24 ml/min and was unaffected by the action of parathyroid hormone. After a fracture the bone blood flow increases, although the transcapillary extraction of 99mTc-EHDP does not change. This is because there is an increase, from recruitment and dilatation of capillaries, in the surface area available for exchange. Therefore the increased isotopic activity seen on a bone scan after a fracture is primarily related to an increase in bone blood supply from capillary enhancement within the cortex.