MICRORADIOGRAPHY OF BONE RESORPTION 453 



Summary 



Any investigation of bone may profitably include a measurement 

 of turnover, that is, the addition and removal of tissue from the 

 skeleton. Quantitative measurements of growth and resorption in 

 normal tissue may be compared with those in bone from metabolic 

 diseases and other diseases of the skeleton to discover the imme- 

 diate cause of the abnormality. A number of methods have been 

 devised for measuring bone formation, the most direct and accurate 

 one being the quantitative estimation of growth in a specific area 

 of a bone sample bv the characteristic appearance of the surface of 

 the bone, preferablv in a microradiograph of an undecalcified sec- 

 tion. In vivo administration of radioactive isotopes of calcium and 

 other alkaline earths and of sulfur in animals have made it possible 

 to establish the characteristic appearance of a growing surface as 

 one where the lamellae are of low density and lie parallel to a 

 smooth surface. Bone resorption, which is probably the most im- 

 portant cause of imbalance in tissue turnover, can be identified by 

 comparison with histological preparations showing osteoclasts and 

 with the localization of yttrium and the rare earths, which depend 

 on the mechanism of resorption for their deposition in high con- 

 centrations. Resorbing surfaces can be identified in a microradio- 

 graph by the presence of highly calcified bone with lamellae lying 

 at angles to an uneven, crenated surface. 



The sampling method of studying bone turnover has some dis- 

 advantages, the most obvious being that it is not easy to be sure 

 exactly how relevant the results from any one sample are to the 

 turnover in the rest of the skeleton. However, the results from one 

 sample area in different individuals may perhaps be treated as com- 

 parative data without reference to the rest of the skeleton. Also 

 samples from different parts of the skeleton will produce data that 

 will indicate the magnitude of the variations to be expected through- 

 out the skeleton. The disadvantages are surpassed by the advantages 

 of being able to measure bone formation and, most important, bone 

 resorption without doubt, and also of being able to investigate local 

 lesions in the skeleton. 



