Recent hip fracture trends in Sweden and Denmark with age-period-cohort effects

Abstract

This study used nationwide hip fracture data from Denmark and Sweden during 1987-2010 to examine effects of (birth) cohort and period. We found that time trends, cohort, and period effects were different in the two countries. Results also indicated that hip fracture rates may increase in the not so far future.

Introduction: The reasons for the downturn in hip fracture rates remain largely unclear but circumstances earlier in life seem important.

Methods: We ascertained hip fractures in the populations ≥50 years in Denmark and Sweden in national discharge registers. Country- and sex-specific age-period-cohort (APC) effects during 1987-2010 were evaluated by log-likelihood estimates in Poisson regression models presented as incidence rate ratios (IRR).

Results: There were 399,596 hip fractures in SE and 248,773 in DK. Age-standardized hip fracture rate was stable in SE men but decreased in SE women and in DK. Combined period + cohort effects were generally stronger in SE than DK and in women than men. IRR per period ranged from 1.05 to 1.30 in SE and 0.95 to 1.21 in DK. IRR per birth cohort ranged from 1.07 to 3.13 in SE and 0.77 to 1.67 in DK. Relative period effects decreased with successive period in SE and described a convex curve in DK. Relative cohort effects increased with successive birth cohort in both countries but with lower risks for DK women and men and SE women born around the 1930s (age 75-86 years today and responsible for most hip fractures) partly explaining the recent downturn. Men and women born thereafter however seem to have a higher hip fracture risk, and we expect a reversal of the present decline in rates, with increasing hip fracture rates in both Denmark and Sweden during the upcoming decade.

Conclusions: Time trends, cohort, and period effects were different in SE and DK. This may reflect differences in general health as evident in known differences in life expectancy, healthcare organization, and prevention such as use of anti-osteoporosis drugs. Analyses indicate that hip fracture rates may increase in the not so far future.

Keywords: Age-period-cohort; Hip fracture; Men; Trends; Women.

Fig. 1

Annual age-standardized hip fracture rate (per 10,000) and number of hip fractures in Danish and Swedish men and women (Denmark year 1980 to 2010 and Sweden 1987 to 2011). By direct standardization with the mean total population of both countries during the observation years 1987–2010 as reference

Fig. 2

Estimation of departure from linearity for birth cohort effects and period effects for age-period-cohort (APC) models in Swedish and Danish men and women. Note that, because there is a linear relationship among year of birth, year of hip fracture, and age at hip fracture (i.e., if any two are known, then the third can be calculated), the individual birth cohort effects from the APC model do not necessarily have an interpretation in terms of relative risk (in contrast to the combined period-cohort effects derived from the AC or AP models in Table 3)