Abstract
Skeletal metabolism and the replacement of damaged tissue with the same amount of intact bone depends on the correct balance between bone formation and bone resorption.
The existence of an imbalance between bone formation and resorption is a concept central to understanding of the pathophysiology of osteoporosis and the reduction of fracture risk.
With aging, the volume of bone that is formed during the bone remodelling process and after injury is less than the volume absorbed during the bone resorption phase; this results in bone loss and increased bone fragility. In addition to bone mineral density, many other properties of bone are determined by the balance between bone formation and bone resorption. A bone that is biomechanically more fragile is also a bone that consolidates more slowly after a fracture event. Although the fracture healing stages are the same even in the presence of osteoporosis, recent studies have shown a slowdown in the process of consolidation when osteoporosis is present. In particular, strategies to reduce fracture risk and facilitate the process of consolidation of the fracture may be a primary criterion for selection.
The ability to modulate anabolic and catabolic phenomena in the skeleton, both locally and systemically, opens up a new horizon for the reduction of fracture risk and the enhancement of bone healing, particularly when the bone is qualitatively and/or quantitatively compromised.
Clinical research has recently allowed the development of therapies, such as treatment with strontium ranelate, able to increase production of bone matrix by osteoblasts and to act positively on the distribution of the skeletal microarchitecture. Strontium ranelate is able to rebalance bone turnover in favour of the formation of more resistant and elastic bone, by stimulating osteoblasts and inhibiting the resorptive activity of osteoclasts, thereby ensuring rapid and lasting protection against the risk of fractures. In vitro studies have shown that the drug is able to promote replication of the first pre-osteoblasts and their differentiation into mature osteoblasts and osteocytes interacting with the receptor CaSR and through the increased synthesis of OPG. Thanks, again, to the participation of the CaSR receptor, but also by reducing the production of RANKL, strontium ranelate decreases the resorptive activity of osteoclasts. The anabolic action of strontium ranelate in terms of mineral apposition rate in both cortical and trabecular bone was demonstrated on bone biopsies analysed by three-dimensional micro-CT. The drug was shown to increase the number of trabeculae, the cortical thickness, and the total bone volume. The bone-forming activity of strontium ranelate was also demonstrated in comparative studies versus teriparatide and antiresorptive agents. In experimental studies the bone-forming effect of strontium ranelate leads to an increase in the bone callus volume and its maturation and, in turn, to an acceleration of the consolidation of the fracture and better implant osteointegration.
In conclusion, the mechanism of action of strontium ranelate, which inhibits bone resorption in favour of new bone formation, is able to counteract, in a physiological manner, the bone loss associated with advancing age. The net effect is an increase in bone mass, trabecular and cortical bone, which explains its anti-fracture efficacy. The drug’s ability to stimulate bone formation seems to unfold at the level of the callus allowing improved fracture healing and in the case of implants potential improvement of implant osteointegration.