Why should we reconsider how fracture risk is determined?

Historically, Bone Mineral Density (BMD) has been the default value for determining fracture risk. However, it is now recognized that, although low BMD values are associated with an increased risk of fracturing, BMD is not the absolute determinant of fracture risk. There are individuals with low BMD values that never fracture and conversely, a large percentage of individuals who sustain fragility fractures have BMD values that are above the osteoporotic range. There have been recent improvements in the understanding of bone structure and new capabilities are now available for quantitative measurements using novel technologies. Therefore, it is now universally accepted that multiple properties of bone need to be assessed to be able to correctly predict fracture risk. “Bone quality” is a term that is now ascribed to important structural properties of bone. 

The “strength “of a material is a standard engineering term that describes the capacity of the material to withstand applied forces and not fail. Therefore, it is appropriate to use the term strength to describe the ability of a bone not to fracture due to an applied force. The strength of a material is a function of multiple physical properties of the material which include its microstructural properties.  BMD and bone quality are two of the microstructural properties of bone. Infrared spectral analysis of bone biopsy specimens, determined that factors such as bone crystal size and collagen maturity, have a predictive value for fracture risk. It is now recognized that bone quality is a composite description of multiple microstructural properties of the bone which include the bone-mineral composition, the microarchitecture and the presence and the amount of micro fracturing present in addition to the amount and the integrity of the collagen components and the correct collagen-crosslinking. These properties are all integral components of the bone mineral crystal. BMD and bone quality are now considered to be the critical properties that determine bone strength. Consensus is growing that the term “bone strength” should replace “bone quality” when assessing bone structure. 

In summary, the ability of a bone to withstand applied forces and not fracture should now be referred to as the bone strength. Bone strength is believed to be a composite property of the bone which consists of the density (BMD) and the bone quality. Recommendations are for both properties to be assessed to determine bone health and fracture risk.