Bone Mineral Density vs Bone Fragility

Bone mineral density (BMD) refers to the amount of hard material contained within a certain volume of bone. The hard material is primarily made-up of hydroxyapatite, a mineral compound composed of carbon, phosphorus, oxygen and hydrogen and collagen, a structural protein found widely in biological tissues. The organisation of hydroxyapatite and collagen can be in a very compact structure which is termed cortical bone or a much more honeycomb like structure which is called cancellous bone. The density of the two types of bone is quite different with the cortical bone being much denser than the cancellous bone. All of the bones in the skeleton have an outer rim of cortical bone and a variable amount of cancellous bone within. Understanding the density of bone provides little information about the likelihood of that bone fracturing because bone density does not describe how well made the bone is. This is particularly the case for the WHO diagnostic categories “normal” and “osteopenia”.  There is a somewhat better correlation between bone mineral density and the risk of fractures in the elderly who have osteoporosis, but this may not hold true for younger, active women with low bone mineral density.

 

To fully characterise the behaviour of bone mechanically, it's structure needs to be understood and this is only made possible by techniques that examine the degree of bone toughness in the spine and the hip. Toughness is the inverse of fragility - the lower the fragility the tougher the bone.

 

REMS measures the fragility of the hip and the four bones in the spine independently of bone mineral density. The ultrasound signals from the probe are analysed alongside those that are used to generate bone mineral density and two scores or provided with each REMS scan result within a few minutes. The BMD results can be compared to those that are generated by DEXA machines, but the Fragility Score (FS) is unique to REMS, which is the only technology available in the world that can provide real-time measurement of bone toughness at the hip and spine in a clinic setting.

 

Understanding the interaction between bone density and fragility is key to assessing bone health in a holistic fashion. A woman with a low body mass index, who is otherwise completely well and who has no other risk factors for impaired bone health, is likely to have low bone density. However, if she is very active and spends a considerable time putting a variety of different loads through her skeleton, she is likely to have tough bone. As a result, her risk of sustaining a fracture will below. The converse is also true and someone with a high BMI is likely to have a high bone density, but if they are not active their fragility might also be high and they may be more prone to fracturing. This is the situation that can be encountered in people who have type 2 diabetes. With advancing age and if people have conditions that predispose to both osteoporosis and fragility, decreased bone mineral density and increased fragility are to be expected and these women may sustain fractures as a result of very minor injuries. However, they do not make up the majority of women who
have fragility fractures. These are women who have either osteopenia or normal bone on bone densitometry. For many years it was speculated that there was a problem with bone fragility but it could not be demonstrated routinely. Now, with the REMS fragility score module being available in the clinic, this conundrum has been solved and it is frequently the case that women with bone density not in the osteoporosis category have high fragility hence their predisposition to low energy fractures.