Mechanisms of bone destruction in multiple myeloma: the importance of an unbalanced process in determining the severity of lytic bone disease.

In order to clarify the mechanisms involved in the occurrence of lytic bone lesions (BL) in multiple myeloma (MM), we have compared the presenting myeloma-induced histological bone changes of 14 previously untreated MM patients with lytic BL with those of seven MM patients lacking lytic BL at presentation despite similar myeloma cell mass. A major unbalanced bone remodeling (increased bone resorption with normal to low bone formation) was the characteristic feature of patients presenting lytic BL. Furthermore, this unbalanced process was associated with a significant reduction of bone mass. Unexpectedly, a balanced bone remodeling (increase of both bone resorption and bone formation, without bone mass reduction) rather than a true lack of an excessive bone resorption was the usual feature of patients lacking lytic BL. Our current work clearly shows that a majority (72%) of patients with MM present an important unbalanced bone remodeling at diagnosis, leading to bone mass reduction and bone destruction (unbalanced MM). Some patients (20%) retain a balanced bone remodeling with initial absence of bone destruction (balanced MM). Few (8%) patients have pure osteoblastic MM without bone destruction.