In a previous blog, I discussed the implication of communicating mortality risk to patients with a fracture. In this article, we put forward the new concept of “skeletal age” to quantify the impact of fracture on mortality.
It is easy to determine the chronological age of an individual. The number of years that pass from the date of birth (elapsed time) is chronological age. However, for practical health purposes, the age of our body is more informative than our chronological age. In engineering, the idea of ‘effective age’ is used to assess the age of a structure based on its current condition rather than its chronological age.
In medical application, for the sake of illustration, we can start with the idea of ‘heart age’. We know that being overweight is a risk factor for cardiovascular mortality, and the heart age of an individual is determined by the presence of excess weight. For example, a 65-year old man may have a ‘heart age’ of 70 (according to this UK calculator), because the man has a body mass index of 26 kg/m2 which places him in the ‘overweight’ category. Knowing his heart age helps the man take preventive measures to mitigate his risk of cardiovascular mortality.
In the same way, the idea of ‘skeletal age‘ is quite relevant in osteoporosis. Skeletal age is defined as the age of our skeleton as a consequence of fracture or exposure to risk factors for fracture. Therefore, if an individual’s skeletal age is greater than their chronological age, then the individual has a higher risk of fracture.
It has been known for some time that fragility fracture is a risk factor for mortality. Women who have sustained a hip fracture have a 2.4-fold increase in mortality risk. We can translate this fracture-mortality relationship into skeletal age.
From 35 years of age, the annual mortality risk increases by an average of ~9.7% and 10.4% per year for men and women, respectively. The 10% annual increase in mortality risk is remarkably constant across populations around the world. More than 15 years ago, Brenner et al (1993) showed the number of years lost or gained in effective age for a risk factor is a simple function of the annual risk of mortality in the general population, and the hazard ratio of mortality associated with the risk factor ielog (hazard ratio) / log (increase in annual risk of mortality).