Studies of Iliac Crest Bone from Controls and Patients with Bone Disease 75 



to be significantly lower than normal (Fig. 2). Consequently the P/HP ratio was 

 found to be significantly higher than normal. 



The nine cases of hyperadrenocorticism for which a P/HP ratio is given, showed 

 histologically a decreased amount of trabecular bone (UVo, range 6— 18"/o), as com- 

 pared with the controls (200/0, range 18— 240/o, van der Heul et ai, 1964). 



4. Discussion 



The degree of variability of the results of the chemical analysis is illustrated by 

 the data in Table 1, in which bone from the left and right iliac crest is compared. 



Table 1. C hemic j! analysis of hitman iliac crest hone 



The low phosphorus content which was found in most cases of hyperpara- 

 thyroidism is in agreement with the abnormally low mineral content found by micro- 

 radiography (Smeenk, 1961). To eliminate the expression of one variable (the mineral 

 or collagen content) in another dependent one (the weight of the bone) (Arnold, 

 1960), we have used the phosphorus-hydroxyproline ratio. 



Changes in the P/HP ratio, however, leave the question open whether they are 

 due to a change in the phosphorus or the hydroxyproline content. In hyperpara- 

 thyroidism the relative decrease of the P/HP ratio is less conspicuous than that of 

 the phosphorus content per mg bone. This suggests that there is an increased water 

 content of bone in addition to the subnormal degree of mineralization which exists 

 in hyperparathyroidism. Concerning the high P/HP ratio (and the decreased hydroxy- 

 proline content per mg bone) in bone in Cushing's syndrome it is not possible to 

 decide from our data alone whether the alteration is caused by an abnormally high 

 bone salt content or an abnormally low collagen content per unit of bone volume. 



This is due to the fact that bone salt constitutes such an important part of the 

 weight of bone that an abnormally high bone salt content per unit of volume is not 

 so easily reflected in a high bone salt content per unit of weight, whereas it may 

 entail an abnormally low hydroxyproline content per unit of weight. However, in 

 osteoporosis in elderly people Mrs. Birkenhager-Frenkel (in press) has recently 

 found a low hydroxyproline content of iliac crest bone, both per unit of volume and 

 of weight, with a high P/HP ratio. The normal values she found for phosphorus and 

 hydroxyproline per unit of bone weight agree reasonably well with ours. These 

 findings in osteoporosis of hyperadrenocorticism and in osteoporosis of elderly people 

 suggest the existence of "collagen-deficient" bone in developing or established osteo- 

 porosis. In using hydroxyproline as representative of collagen we are conscious of the 

 fact that it is not known whether in pathologic conditions the collagen molecule still 

 contains the normal amount of hydroxyproline (13.6*'/o of weight). 



