Sir.—We read with interest the article by Vyhmeister et al,1 who measured bone mineral content (BMC) at the radial and humeral sites using the Norland 278A (Norland Corp, Fort Atkinson, Wis) photon absorptiometer.
The statement in the abstract that "We tested... photon absorptiometric bone density measurements... The humerus was a more reliable site of measurement" is misleading. The sentence refers only to the machine the authors used, and not to photon absorptiometry. The study shows only that the Norland 278A is not a very sensitive instrument, and not that the humerus is a more reliable site. Using a larger bone (the humerus), the authors obtained a coefficient of variation (7% for instrument-reading error) larger than that obtained by Greer et al2 (3.9%) on the smaller radius site using the Lunar instrument (Lunar Radiation Corp, Madison, Wis).
A potential danger of using an insensitive instrument to measure BMC
MIMOUNI F, TSANG RC. Bone Mineral Content in Infants: Which Machine or Which Bone?. Am J Dis Child. 1988;142(9):919. doi:10.1001/archpedi.1988.02150090017011