t()() J. .lOWSEY 



pleted. As large resorption cavities are more frequent than small 

 ones, and as the average length of surface per cavity is higher for 

 resorption than for formation, it is probable that resorption also 

 takes place more rapidly in a new, small resorption cavitv than in 

 an older, large one. However, despite the local variations in the 

 rates of formation and resorption, an average figure should be valid, 

 and measurements of the length of surface occupied by resorption 

 and formation should also be valid and refer to actual bone turnover. 

 The measurements can be converted to mass of tissue added to or 

 removed from bone if one assumes the figure of 1 micron per day 

 for the rate of formation and resorption to be correct and knows the 

 relation between the amount of inactive surface and the area of the 

 sample (Fig. 4) (20 ju, X 10 "-/mm^ in 20- to 40-year-old individuals 

 and SO [x X lO""/!!!!""^" hi 60- to 80-year-old individuals). This in- 

 crease in the amount of surface per unit area of bone means that if 

 resorption and formation are to be considered in terms of area or 

 mass rather than surface, the actual values will show a greater in- 

 crease in old as compared with young adults. In other words, from 

 the graph (Fig. 4), resorption in young adults is 2 /x/100 fx of in- 

 active surface, or 1.5 mm^/100 mm^/year, which is balanced by 

 similar values for bone formation. In older individuals the value in 

 terms of length of surface is 4.8 per cent and in terms of mass it is 

 5.2 mni'VTOO mm'Vyear, while bone formation is 3.3 per cent in 

 terms of length of surface and 3.6 per cent per year in terms of 

 mass. Therefore, at this rate a piece of cortical bone should lose 16 

 per cent of its mass over a 10-vear period, a figure that is consistent, 

 as far as can be judged, with the values for porosity and cortical 

 thickness. 



Osteoporosis 



In comparison with the normal series, the amount of bone forma- 

 tion in osteoporotic bone seems not to be different from the levels 

 of formation found in normals of similar age ( Fig. 4 ) . However, 2 

 out of the 10 cases showed remarkably low values, and these had 

 been bedridden for 2 to 12 months before autopsy; there seems to 

 be an effect of partial immobilization on bone formation. The levels 



