In Belgium alone there are 15,000 hip-fractures every year. Given the increase in life expectancy, this figure is likely to explode if preventive measures are not taken. In fact, according to predictions, it should triple by the year 2050.

Between osteoclasts and osteoblasts

In the pathology of osteoporosis, the positive point is that it can be readily diagnosed. A bone mineral density (BMD) test – the main method is the dual-energy x-ray absorptiometry (DXA) test ‑ which makes it possible to measure the quantity of calcium in the bone and therefore the mineral density of the bone. “The exam is painless and above all involves lower radiation levels than a chest x-ray”, explains Jean-Yves Reginster.

Diagnosis of osteoporosis is made when the bone mineral density is 2.5 standard deviations or more below the mean peak bone mass (average of young healthy adults). In such a case, a normal x-ray of the vertebral column must be carried out as a complement to this, in order to detect or confirm possible vertebral compression fractures. Moreover, the biological markers of bone remodelling must be measured. “Bone density gives us a ‘photograph’ of the current situation, while the biological markers reflect the level of remodelling of the skeleton and therefore its predictable development in the long term,“ comments the head of the Bone and Cartilage Metabolism Research Unit. In fact, the skeleton constantly renews itself with 70 days of formation following 20 days of resorption.

Here we are at the heart of the intimate mechanics which when “unbalanced”, lays the foundation for osteoporosis. In the adult, bone remodelling is the result of a balance between, on the one hand, the depositing of a bone matrix, whose synthesis and mineralisation control is carried out by the osteoblasts, and on the other hand, the deterioration of the mineralised matrix by the osteoclasts. This permanent process, which results in the renewal of around 10% of the adult skeleton in one year, is therefore efficiently orchestrated over space and time. In order for growth to take place in children and teenagers, the osteoblast is predominant. Then there is a period of balance which extends over a period of several years when the osteoclast “takes control”, and bone resorption takes over from the formation of hydroxyapatite crystals, those natural derivatives of calcium apatite which are constituents of the mineral part of the bone matrix.

To study the level of skeletal remodelling, we refer to markers of the respective activity of osteoblasts - generally bone enzymes of alkaline phosphatase – and osteoclasts – most often, C-telopeptide of type 1 collagen. By comparing the level of these markers, it is possible to establish whether the bone is deteriorating or whether its structure is stable or not.