The cellular basis of bone turnover and bone loss: a rebuttal of the osteocytic resorption--bone flow theory.

There is now sufficient evidence to conclude that the osteocytic resorption--bone flow theory of bone turnove is untenable. According to this theory bone is resorbed not from the surface by osteoclasts but from within by osteocytes, towards which bone flows through tissue space away from bone forming surfaces. The need to invoke resorption by osteocytes stems from the belief that too few osteoclasts are present to account for normal bone resoption, a belief which reflects unawareness of the enormous capacity of the osteoclast and the rapidity of its advance. The belief that osteocytes resorb substantial amounts of bone rests on invalid conclusions from indirect techniques, various artifacts of specimen processing and unawareness of the microscopic characteristics of woven bone. Osteocytes enlarge their lacunae by resorbing bone only as a prelude to resorption from the surface, the osteocyte and osteoclast working together as a resorbing unit. The belief that bone can flow is incompatible both with the physical properties of bone and with a substantial body of evidence relating to Haversian remodelling; the experimental data purporting to demonstrate such flow can all be explained by conventional concepts of bone turnover.