Although it may seem trivial to use national diagnosis databases to identify the number of patients, analyze time trends, and make projections, here we use hip fracture to illustrate the complexity and difficulty of estimating the reliable number of injury events. Readmissions after the index hip fracture are common, and register identification of a real second fracture is crucial for data accuracy. Therefore, we aimed to estimate the absolute and age-standardized number of hip fractures in Sweden during the past two decades to produce time trends and future projections. We also illustrate how the naïve use of register data leads to invalid estimates.
We used nationwide Swedish register data from 1998 to 2019 and a validated algorithm to calculate the annual absolute and age-standardized number of incident hip fractures over time. According to our algorithm, the total hip fracture burden was 335,399 incident events over the 22-year period, with a change from 16,180 in 1998 to 13,929 in 2019, a 14% decrease. One decade after the index hip fracture event, 80% of the patients had died and 11% had a new hip fracture. After considering the steady growth of the older population, the decline in the age-standardized number of hip fractures was 30%. With a continued similar reduction in hip fracture incidence, we can predict that 14,800 hip fractures will occur in 2034 and 12,000 in 2050 despite a doubling of the oldest old (≥80 years). Without an algorithm, a naïve estimate of the total number of hip fractures over the study period was 539,947, with a second 10-year hip fracture risk of 35%.
Accordingly, the naïve use of register data can dramatically inflate estimates of the number of fractures. We note an ongoing decline in absolute and age-standardized actual number of hip fractures in Sweden, with consequences for future projections.