MICRORADIOGRAPHY OF BONE RESORPTION 449 



in in vivo procedures or in chemical analyses of whole biopsy sam- 

 ples, there must always be uncertainty in these methods as to the 

 proportion of the retained isotope measured that reflects bone for- 

 mation. 



The most successful attempts to measure bone formation have 

 consisted of sampling small areas of growth in sections of bone from 

 specific sites in the skeleton (Jowsey, 1960; Frost and Villanueva, 

 1961 ) . Microradiographs of undecalcified sections are perhaps more 

 informative than the more conventional decalcified sections, since 

 they demonstrate the distribution of mineral in the tissue besides 

 showing a most characteristic appearance in areas of formation and 

 resorption. It is possible with tetracyclines (Vanderhoft et al., 1962; 

 Harris et ah, 1962) or a bone-seeking radioactive isotope (Jowsey 

 et al, 1953; Plonlot, 1960) to see with the aid of a microscope where 

 growth has taken place, as the new tissue will be marked by a band 

 of the fluorescent material or the radioisotope, or will lie between 

 two bands if the label is given twice at different times. By compar- 

 ing such areas of growth with a microradiograph of the same section, 

 the characteristic appearance of new bone formation has been estab- 

 hshed as an area of low density with new lamellae lying parallel to 

 a smooth surface (Fig. 1); most characteristic is the continuous 

 decrease in density right up to the edge of the bone. A bone surface 

 that has recently been growing but has stopped will show a sclerotic 

 ring adjacent to the haversian canal (Fig. 3). Measurements of 

 bone formation may be made using microradiographs of unlabeled 

 material. 



It becomes apparent on looking at such measurements in bone 

 from normal elderly people and in specimens of some bone disorders 

 that, generally, there is no great variation in bone formation. It 

 would seem that it is increases or decreases in resorption that are 

 important in producing the gross changes seen in abnormal bone, 

 so that attempts to evaluate bone formation only would not measure 

 the cause of the upset in the balance of bone turnover. Resorption 

 must be measured. 



Bone resorption is unfortunately more difficult to characterize 

 than bone formation, since it consists of the removal of tissue, and 

 it is possible onlv to deduce from circumstantial evidence what a 



