Hip fractures in a city in Northern Norway over 15 years: time trends, seasonal variation and mortality

The main finding from this study with 15 years of population-based data is that the age-adjusted hip fracture incidence rates of women above 50 years are significantly lower in Harstad, Northern Norway, than in Oslo. The incidence rates in Harstad are comparable to the rates reported from two other cities, a city in the central [17] and south easternparts of Norway [16], in women, but higher than the rates in the more rural area in mid-Norway [15] (Table 2). Our results confirm that there is a great variation in hip fracture rates between different regions in Norway [7], as there is for distal forearm fractures [21]. Furthermore, the age-adjusted rates from Harstad are lower than both winter and summer rates in Oslo [8] and also lower than earlier reported rates from Copenhagen and Gothenburg, but they are higher than reported rates from the US [5].

International variations in hip fracture risk have displayed a north–south gradient [6] which has been linked to the importance of sunlight exposure [22]. A study using national data from France showed substantial heterogeneity of hip fracture risk within the country, with higher hip fracture risk in the Southern France [23]. Other studies reporting regional differences in hip fracture rates within countries explain the differences by an urban–rural gradient [24]. In a study from Australia, the age-adjusted incidence of hip fracture was 32% lower in rural compared to urban residents aged 60 years and above, 26% lower in women [25]. In comparison, the age-adjusted rates in women aged 65 years and above were 21% lower in Harstad than in the more urbanized capitol Oslo [8]. Unfortunately, with the registry data available, we do not have explanation for the indicated urban–rural difference, but another Norwegian study reported higher bone mineral density levels in rural versus urban dwellers at the hip [26], one factor which may explain differences in fracture risk. In a study by Ringsberg et al. [27], urban subjects had significantly poorer balance compared with their rural counterparts, a difference which increased with increasing age, affected gait performance and risk of falls. With an extensive prevention program running in Harstad between 1988 and 1993 [18, 19] and part of this program still integrated in the community health service, this may also explain the differences in fracture rates between Harstad and Oslo.

It could furthermore be expected that the extensive prevention program might have resulted in lower fracture rates especially in the first years after 1994. However, comparison of the two periods, 1994–1996 and 2006–2008, indicated no significant change in the age-adjusted incidence rates in any of the sexes during the time of the study. Interestingly, this stability of age-adjusted incidence rates is in accordance with data from Oslo [8] and reports from several other countries including Finland, Denmark, Norway, Switzerland, Canada, US and Australia [10, 12‐15, 28]. There are studies reporting increasing numbers of hip fracture rates in women and men in Germany and Austria [29, 30], in men in Switzerland [28], in the oldest age groups in Swedish [31] and Swiss [32] women. Conflicting results are also reported within countries where, for example, a recent paper from the Australian Capital Territory reported significant declining hip fracture rates after 2001 in women [13], while other data from Australia indicate no change in incidence [33]. The Australian report suggests that the declining hip fracture rates may be explained by increased use of anti-osteoporotic treatments [13]. In the Danish study reporting declining hip fracture rates from 1997 to 2006, it is noted that the decrease is too large to be explained by the extent of anti-osteoporotic medication and therefore must be explained by other factors [11].

As reported from several other studies, both within Norway [17] and from other countries like UK [34] and the US [35], there was a significant seasonal variation in the occurrence of hip fractures in our study. In a study comparing and observing seasonal variation of hip fractures in Scotland, Hong Kong and New Zealand [36] as well as in Taiwan [37], it was claimed evidence against a major influence of conditions underfoot causing extra falls and increased risk of fracture during winter [36]. In our study, we had information about place of injury in 90% of all cases; 64% occurred indoors with no significant seasonal variation. For the fractures happening outdoors, there was a significant seasonal variation, which can be connected to falls on ice or slippery surfaces. Unfortunately, the data from the Harstad Injury Registry do not provide enough information for exact studies of the mechanisms leading to falls and fracture indoors.

The mean age at hip fracture in persons above 50 years in Harstad, were not different from the mean age at hip fracture in Oslo, which was 82.1 years in women and 76.6 years in men [8]. A lower mean age at fracture in men, compared to women, are also reported by others [26]. With 73% of the hip fractures occurring in women, the gender distribution of hip fractures in Harstad did not differ in comparison with Oslo (78%) or other comparable studies [12, 14]. Increased mortality risk up to 10 years has been reported for hip fractures [38], although mortality is highest in the first year [3, 38]. A sex difference in mortality after hip fracture has also been indicated, with higher rates in men compared with women [2, 3, 38, 39]. In our study, mortality was higher in men than in women 3 months after fracture and persisted at 6 and 12 months after adjustment for age of hip fracture. This is in accordance with other Norwegian data showing higher mortality in men throughout the first year after hip fracture [40], and with a recent meta-analyses showing that, although the sex difference in mortality persists, the difference is greatest in the first 3 months after hip fracture, with reported relative all-cause mortality hazard of 5.75 (95% CI, 4.94–6.67) in women and 7.95 (CI, 6.13–10.30) in men [41].

One of the strengths of this study is the possibility to study the incidence of hip fractures in a well-defined municipality over a long time period and the accessibility of a well-established injury registry, which also provides the opportunity for quality assessment of the hip fracture registration. Furthermore, the injury registry provided valuable information on date and place of fracture and through the medical records we got access to mortality data. There are, however, several limitations in our dataset. Given the location of Harstad, north of the Arctic Circle, it would have been interesting to study the impact of vitamin D status on fracture risk and on the seasonal variation [42], but neither biological data nor lifestyle information was available. Because of the lack of data we cannot explore if time trends and urban–rural differences can be explained by other important factors like smoking [43] and body mass index [44].

In conclusion, the present study supports previous reports concerning significant regional differences in hip fracture incidence within Norway, which cannot be explained by a north–south gradient. A majority of hip fractures happen indoors, suggesting the need of developing effective prevention strategies towards falls and fractures at home in the elderly. Although fewer hip fractures happen outdoors, they are mostly due to falls on slippery surfaces indicating that securing outdoor areas during winter must be included in prevention of hip fractures in the elderly.