Abstract
The main mechanical functions of the skeleton are support, protection and movement. In order to fulfil them, the bones that comprise the skeleton must have certain characteristics in terms of elasticity and stiffness.
The long bones are stiff because they have to bear high loads and resist deformation; they are stronger under compression than under tension. The vertebrae, on the other hand, are elastic (flexible) and able to withstand dynamic stresses, deforming without breaking.
When bone is loaded excessively (≥ 6 kg/mm2 = 3000 μ strain) it undergoes permanent deformation associated with the appearance of microcracks. Microcracks are a means of releasing energy and they serve as a protection mechanism against fractures proper.
In the presence of balanced bone metabolism, microcracks are “repaired” through remodelling, in other words through ongoing processes of resorption and bone formation.
However, when bone metabolism is unbalanced and the resorption process prevails, there is an accumulation of microcracks, resulting in reduced bone strength and a greater predisposition to fractures.
The first sign of “abnormal” structural transformation in bone is probably due to reduced bone-forming capacity of osteoblasts, which become incapable of repairing microcracks.
Vertebral fracture deformities resulting from a build-up of microcracks lead to permanently altered statics (accentuation of dorsal kyphosis, reversed lumbar lordosis, loss of height with disproportion of limbs to trunk) as a response to the need, in any case, to maintain the upright posture; this has clear repercussions on gait and on quality of life.
Today, we have at our disposal new instruments able to counter the progression of microcracks and appearance of fractures. In particular, research has recently given us new drugs with bone-forming actions that could favour the repair of microcracks, preventing their accumulation from evolving into complete fractures. Strontium ranelate is one particularly interesting treatment option on account of its dual action: inhibition of resorption and stimulation of bone formation. The effect of strontium ranelate on these two parameters is not as marked as the single effects of antiresorptive agents (powerful inhibitors of bone resorption) and osteoinducers (powerful stimulators of bone formation); it acts, above all, to re-balance bone turnover, preserving the capacity of the osteoclasts to resorb damaged areas and of the osteoblasts to rebuild them. Preclinical studies have shown that strontium ranelate increases bone mass and improves the microarchitecture of trabecular and cortical bone and thus the mechanical properties of bone, reducing its biological deterioration (Bain et al. Osteop Int 2008).
In view of the mechanism of action of strontium ranelate and of recent clinical evidence that seems to be moving in the direction of kyphosis progression prevention and spinal pain reduction in osteoporotic women, it is possible to hypothesise new field of application, alongside the established use of the drug in the reduction of vertebral and hip fractures.
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