DENTAL HARD TISSUE DESTRUCTION 95 



Firstly, the resorption of alveolar bone during growth and eiaip- 

 tion of the teeth, which usually occurs at the bone surfaces sub- 

 jected to physiological pressure, is synchronized with processes of 

 bone apposition at opposite zones of tension. Thus teeth are kept 

 in functional balance during long periods of growth, eruption, and 

 alveolar bone remodeling. Indeed, it is in part this very biological 

 fact, rather than mechanical appliances per se, which makes possible 

 orthodontic tooth mo\'ement in definitive directions (Sognnaes, 

 1955, 1960fl, 1960/7). 



Secondlv (and this is the other biological blessing to which ortho- 

 dontics owes its very existence), the dental root cementum — albeit 

 within close neighborhood of alveolar bone, and notwithstanding 

 that its constituents are almost identical with those of bone — is re- 

 markably well protected from resorption. This is presumably due to 

 minor but yet adequate differentials in the immediate vascular and 

 cellular environments. 



Thirdly, in the process of normal shedding of the teeth, the 

 alveolar bone sockets (as well as all the specialized dental tissues, 

 cementum, dentin, and even enamel) undergo extensive localized 

 resorption from one jaw region to another, one tooth after another 

 (in man, 20 deciduous teeth) over a long period of time (in man, 

 from 6 to 13 vears of age) seemingly unrelated to any general 

 svstemic control, in contrast to the simultaneous, sex hormone-con- 

 trolled shedding of antlers (see chapter 13 by Goss). 



Fourthly, in the presence of metabolic disease which otherwise 

 tends to affect the human skeleton, the alveolar jaw bone may in 

 some conditions be among the verv first to show signs of destruction 

 (lamina dura resorption in hvperparathvroidism and "loose teeth" 

 in scurvv), or in other conditions may be relatively unaffected in 

 the presence of severe generalized bone disease, as in the case of 

 osteoporosis ( see chapter 15 by Urist et al. ) . 



Fifthlv, although it is the primary responsibility of the dentist — 

 assisted bv routine roentgenological examinations — to recognize 

 raref\'ing jaw lesions of dental origin ( abscesses, cysts, and odonto- 

 genic tumors ) , there are numerous nondental conditions which may 

 cause extensive resorption of the jaw bones, including nondental 

 local and metastatic tumors, as well as primary and secondary ef- 



