Bone Absorption

We have had to discard the conception that the adult skeleton is composed of a stable inert tissue, reacting only to local insult. Jung dates our present knowledge of the metabolic behavior of bone in infection and trauma from the publication of Leriche and Policard’s work. However that may be, no clinical test is better established to-day than radiography of the skeleton in cases of suspected deficiencies of diet and altered endocrine function. Jung analyzes all the known causes of osteoporosis and endeavors to trace the underlying factors that may be common to some of them. First he is at pains to distinguish osteoclasis, the active eating away of bone, from osteolysis, when it passively dissolves.

The difference histologically, he says, is in the presence or absence of the large phagocytic osteoclasts. In osteoclasis these cells remove the whole of the bony substance, calcium salts, connective tissue, and collagen. The framework has gone, and if the bone is to be generated it must be rebuilt. Osteolysis is less drastic – almost physiological – and occurs without apparent cellular activity. (This observation, on which the whole conception of osteolysis rests, certainly cannot yet be considered proven.)

According to Leriche, it is possible in the lass rapid absorptions to distinguish two stages, the disappearance first of the phosphates of calcium, and then of the “pre-osseous” connective tissue. The collagen fibres remain, and removal of the agent that has excited the osteolytic process is most often followed by reconstitution of the bone without any elaborate action of the cells. The radiologist often reports this partial disappearance of bone as decalcification; but it is something more than that. Osteomyelitis in Leriche’s view provides examples of both processes. Simple inflammation, through induction of hyperemia, causes osteolysis. If a sequestrum forms, then phagocytes appear and it is removed by osteoclasis. The process of hyperemic osteolysis occurs also apart from direct bone infection, as in a whitlow, and unnecessary operation are at times performed for removal of friable bone from the phalanges of fingers through the misreading of a radiogram. A sequestrum, being dead bone, should be recognized by the denser shadow it throws. Nothing preserves the skeleton better than death says Jung- a generalization that is only apparently paradoxical. Teeth without pulp show no caries.

Once we accept bone as the calcium reserve of the body we expect a reaction to metabolic disorder. Absorption of bone will be the natural corollary of wastage of calcium, and will also be looked for in the presence of large calcium deposits elsewhere. Jung relates instances of intractable diarrhoea, of prolonged biliary drainage, and of massive reserves represented by loss of bony substance can presumably be repaired by administration of calcium.

Other causes of “osteolysis” are not so easily righted. Jung recognizes three main factors: the vasomotor, the endocrine, and the chemical. Post-traumatic and infective osteoporosis are examples of the vasomotor type, the direct cause being local hyperemia.