Answer to Dr. Frost's letter to the editor.

Abstract

We are disturbed by Dr. Frost's letter for several reasons. First, we are disturbed by his comments because he has contributed so much in the area of morphometric assessment of skeletal renewal. Also, however, his statements of "serious error which invalidate the method" and his "purpose of preventing others from making glaring errors" cause concern. Perhaps much of the problem is due to his misunderstanding of the method. The quotation on page 194 (Harris, 1972) is U B+ T + De+ P~ U B-+- T + Dc+ De-I-D ~ [all determinations in this equation are at T 8 (time of sacrifice)]. Dr. Frost raises the question that the value of U B (unlabeled bone) at sacrifice (Ts) is probably less than the value for unlabeled bone at T o (beginning of the experimental period). He is concerned similarly about T (Terramycin labeled bone). We agree and point this out on page 193. However, this effect of intracortical resorption is taken into account by making the assumption that the bone had no porosity at T o (which creates a bias agalnst showing an increase in skeletal mass) and then adding the full value of P (porsity at sacrifice) and Do (Deelomycin labeled bone in the cortex at sacrifice) to that side of the equation. This accounts fully for the intraeortical bone resorption during the experimental period. In fact it overestimates this figure. He also is concerned that the basic expression lacks a term for cortical porosity at time zero. The article addresses itself to this in detail. Since no method is available for obtaining this figure, the assumption is made that this figure is zero. Clearly this creates a bias against showing an increase in mass during the experimental period since Po is obviously not zero. Thus the true magnitude of s ny increase in mass shown by the method exceeds the derived value by the unknown magnitude of this bias. A positive result remains valid. A negative result of small magnitude is ambiguous. Both total cross-sectional area and marrow cavity area can be measured at T~ but cannot at T o.