The label escape error: Comparison of measured and theoretical fraction of total bone-trabecular surface covered by single label in normals and patients with osteoporosis

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Thirty-four normal caucasian women and 69 patients with osteoporosis were given two bone-labeling agents. Transilial biopsies were obtained and embedded undecalcified. Fractional surfaces covered by double label and single label were determined, along with total surface and double label width by fluorescent microscopy. Mean wall thickness was measured on toluidine blue-stained sections. The mathematical model for predicting the amount of single-labeled surface was compared to the actual amount of surface covered by single label. We found an excess of single labels compared to the model in both groups, more so in the normals, and suggest the explanation is that bone-formation pauses at formation sites in both groups but tends not to resume in the patients. This results in a shortened functional life span of the osteoblast and bone loss. Further, the data suggest that for accurate expression of bone formation rates in trabecular bone using a volume referent, one should use the whole bone tissue including bone and marrow and should express the bone-forming surface as the double-labeled surface plus one-half the measured single-labeled surface.