strontium (Sr) /stron″sh(ē)əm/ [Strontian, Scotland] , a metallic element. Its atomic number is 38; its atomic mass is 87.62. Chemically similar to calcium, it is found in bone tissue. Isotopes of strontium are used in radioisotope scanning procedures of bone. Strontium 85 (85Sr) and strontium 87 (87Sr) mimic calcium metabolism and are used in studies of bone physiological characteristics and disorders. These radionuclides can be counted with any standard detector or imaged at a very early stage in bone disease, whereas radiographic films of bone without the use of a radioactive tracer can show decreased density only after approximately 50% of bone is decalcified. Most 85Sr or 87Sr is deposited in bone within 1 hour after injection. Increased deposition of these radionuclides is strongly linked to osteoblastic activity and new bone formation. In addition to four naturally occurring isotopes (88Sr, 87Sr, 86Sr, and 84Sr), 12 artificial strontium isotopes are produced by nuclear reactions. Strontium 90, the longest-lived, is the most dangerous constituent of fallout from atomic bomb tests. It can replace some of the calcium in food, become concentrated in teeth and bones, and continue to emit electrons that can cause death in the host. Strontium 90 becomes concentrated in cow’s milk.
