MICRORADIOGRAPHY OF BONE RESORPTION 457 



Quantitative Measurements 



Normal. In order that abnormality may be appreciated, the 

 normal state of affairs must first be described. It is always a little 

 difficult to choose criteria for what shall be considered normal and 

 what abnormal. Since one of the main purposes of this study was 

 to compare normal with osteoporotic bone, certain individuals were 

 rejected from the normal group because a gross slab x-ray through 

 a 1-cm-thick longitudinal slice of the 5th lumbar vertebra showed 

 definite porosis; only those showing unremarkable density in this 

 x-ray were retained as normals. This normal group shows the char- 

 acteristic pattern of bone turnover that has already been demon- 

 strated (Jowsey, 1960), with the exception that, as a result of ex- 

 cluding those individuals that showed loss of bone mass in the spine, 

 there was less increase in the amount of resorption in older individ- 

 uals. From Fig. 4 it seems that from the age of 49 onward the 

 level of resorption is above that of formation, but that the difference 

 does not continue to increase after the age of 70. The result of this 

 imbalance is visible in the values for porosity; that is, the number 

 of osteons less than two-thirds closed is increased, while the thick- 

 ness of the cortex decreases. These graphs do not paint the whole 

 picture: from the microradiographs themselves (Fig. 5) it can be 

 seen that there are generally a greater number of incompletely 

 mineralized osteons in the older individuals and that the increase 

 in number of osteons less than two-thirds closed appears to be the 

 result of a preponderance of resorption cavities which have become 

 only partly filled in with new bone. 



Resorption and formation rates. The actual addition or sub- 

 traction of tissue mass to or from the bone can be calculated by mul- 

 tiplying the values for surface activities by the rate of formation or 

 resorption which is seen in the thickness of new tissue in a cross 

 section. The results reported here assume that the rates of bone 

 formation and resorption are similar and that they do not vary a 

 great deal without leaving an indication in the microradiograph. 

 Measurements have been made of the rate of new bone formation in 

 cross sections of cortical bone using tetracycline markers (Frost, 



