Incidence and seasonal variation in hip fracture incidence among elderly women in Norway. The HUNT Study

Abstract

There is a substantial variation in hip fracture incidence between populations. The Scandinavian countries have the highest incidence of hip fractures worldwide, and latitude and seasonal variation have been discussed as possible reasons for the high fracture incidences. The purpose of this study was to investigate time dependent and seasonal variation of hip fractures in a population based cohort of women aged 65+ residing in a rural county in Norway and followed for 9.3 years.

Information at baseline was collected as part of The Nord-Trøndelag Health Study (HUNT) during 1995–97, and 8362 women with no previous hip fracture and with a mean age of 74.3 years were included in the study. All hip fractures occurring after inclusion in the health study were registered (mean follow-up: 9.3 years) by medical journals and x-ray reports. A total of 5661 of the women had their forearm bone mineral density (BMD) measured by single energy x-ray bone densitometers (SXA) as part of HUNT.

In total, 782 women sustained a first hip fracture during follow-up, and the overall hip fracture incidence rate per 1000 person-years was 13.1 (95 % CI: 12.2-14.1). The hip fracture incidence increased exponentially by age from 2.1 (95% CI: 1.2-3.8) in the age group 65–69 years to 49.7 (95% CI: 41.2–59.8) among the women aged 90+, respectively. In age-stratified analyses no changes in the incidence of hip fractures were observed during the nine years of follow up. The occurrence of fractures varied by season of the year, characterized by higher fracture rates during the winter months. In conclusion, the hip fracture rates in this population of elderly women are highest in the winter months. There was, however, no indication of an increasing hip fracture incidence in this rural area. Compared to similar studies from more urban areas in Norway, the hip fracture rates in this population seem somewhat lower.

Introduction

The variation in hip fracture incidence between populations is substantial [1]. The highest incidence of such fractures is found in the Scandinavian countries [2], [3], somewhat higher than in North America or Oceania [1]. An eleven-fold range in apparent hip fracture incidence amongst women has been demonstrated between various countries in Europe [4]. In addition, a considerable regional variability in fracture incidence is found within countries, with higher rates in urban than in rural areas [2], [5], [6], [7], [8]. This difference also accounts for bone mineral density (BMD) that has been found higher in rural compared with urban areas of Norway, which might explain some of the difference in fracture incidence [9], [10]. The hip fracture incidence in Oslo in 1996–1997 is the highest ever reported [2].

Epidemiologic data of hip fractures are necessary for planning of health care and for targeting fracture prevention [11]. Due to aging of the population, the annual number of hip fractures is expected to rise [1], [12]. Earlier studies showed an increase in age-adjusted hip fracture incidence in several Western countries [13]. More recently, several studies have reported a levelling off [2], [14], [15], and some have even reported a decline in hip fracture incidence [16], [17].

The variation in fracture incidence between countries suggests a large heterogeneity in fracture risk. Latitude and seasonal variation have been discussed as potential reasons for the high fracture incidence in the Scandinavian countries [2], [15], [18], [19]. In the northern latitudes cold winters with ice and snow could be responsible for more falls, and frequent falling is a known risk factor for hip fractures [20], [21]. There also are fewer hours of daylight which could make it more difficult for persons with compromised vision to move about. In addition, less exposure to ultraviolet radiation leads to diminished formation of vitamin D. Seasonal variation in hip fracture rates has been described in USA, Canada, Scotland, Sweden, Hong Kong and New Zealand [18], [19], [22], [23], [24]. Even though there is evidence for a seasonal effect on the hip fracture incidence, this has not been a universal finding, and in some studies showing an overall effect, it could not be demonstrated in all sub-groups [2], [25], [26], [27]

The aim of this study was to investigate time-dependent and seasonal variation in the incidence of hip fractures in a population based cohort of women aged 65 + residing in a rural county in Norway.