In Reply.—We fully agree with Dr Harkavy's observation that low BMC is not equivalent to rickets. The common confusion of these two appears to arise from misunderstanding about what quantity is represented in BMC. Although Dr Harkavy points out one aspect of this misunderstanding, his letter suffers from several others.
The letter states that BMC is not bone density. There are, of course, nearly as many types of density as there are denominators. Of particular relevance to the measurement of BMC are linear density, area density, and volume density. Linear density relates most closely to bone quantity and is expressed in mass per unit length. Bone mineral content is an example of a linear density. Area density is the quantity that determines roentgenographic absorption and is expressed in mass per unit area. Volume density relates most closely to bone quality (ie, rickets) and is expressed in mass per unit