To the Editor:
Quantitative histomorphometry of bone is a well-recognized technique with selected applications in clinical patient evaluation and in clinical research and drug development that allows assessment of the safety of drugs and their mechanism of action. Recently, two publications have examined issues in modern histomorphometry and have updated the ASBMR Histomorphometry Nomenclature Committee's recommendations on nomenclature, symbols, and units.1, 2 Here we draw attention to an issue originally described by Parfitt and colleagues3 which has implications for the conclusions that might be drawn from dynamic assessment of bone formation.
In most histomorphometry applications, the bone is labeled with a tetracycline that binds to the mineralizing surface of bone. This has become a standard method that allows calculation of the rate of bone formation in all four bone compartments (cancellous, endocortical, intracortical, and periosteal bone). In most clinical situations, two separate labels of the same form of tetracycline (usually tetracycline HCl, oxytetracycline, or demethylchlortetracycline) are administered, each separated from the other by a period of 12 or to 14 days, and the biopsy is performed about 1 week after completion of the second label. For some applications, a different label may be used for the second of the two labels; the difference in the color of the two labels under UV light allows unambiguous assignment of single labels to the first or second labeling period.
Several years ago we described a quadruple-labeling technique4 that allows short-term longitudinal data in a single biopsy with the first two tetracycline labels given in the usual format, but separated by several weeks from the second set of labels. In order to distinguish the first and second sets of labels under UV light, we used the same tetracycline for the first set of two labels and a different one for the second set. Using that technique, with demethylchlortetracycline as the first pair of labels and tetracycline HCl for the second set, we demonstrated the early effects of teriparatide on bone formation at the cellular level. With that sequence, we found that in cancellous bone of subjects on no osteoporosis medication, the second set of labels was almost always shorter than the first set, and consequently the proportion of the surface covered by label, the mineralized surface (MS/BS), calculated as double + 50% of the single-labeled surface, was less with the second set of labels (2.94 ± 1.13% demethylchlortetracycline versus 2.14 ± 0.83% tetracycline HCl), a difference that did not achieve statistical significance (n = 9). A similar trend was seen at the endocortical junction. Given the crescent-shaped anatomy of many cancellous bone packets, we might have expected that, if anything, the second set of labels should be longer, even in untreated control subjects. The effects of teriparatide were clearly evident, with a marked increase in MS/BS (that exceeded by several-fold any expected differences in label length due to the different forms of tetracycline).
We now report in a separate experiment, in which tetracycline HCl was given as the initial set of labels and demethylchlortetracycline as the final set, in cancellous bone of untreated subjects, that the second labels are now longer than the first set (2.82 ± 0.76 tetracycline HCl versus 3.76 ± 0.65 demethylchlortetracycline; p < 0.05, n = 14). This strongly supports the original data from Parfitt and colleagues3 which suggested that the demethylchlortetracycline label was always longer than the oxytetracycline label, irrespective of the order in which the labels were administered. We believe that our data confirms the recommendation3 that if two different tetracyclines are being administered the differences in label length need to be taken into consideration. We suggest that for consistency across studies demethylchlortetracycline always be used for the initial set of labels followed by tetracycline HCl for the second set, and that a correction factor be applied using control data from the same laboratory, to correct for the differences in label length that are likely due to just differential binding to mineralizing bone.