The Epidemiology of Fragility Hip Fracture in Southern Norway: A study on incidence, mortality and predictors of mortality
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Background and objectives Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of the bone tissue which impairs bone strength causing increased risk of fracture. Fragility hip fracture is a direct consequence of osteoporosis. Osteoporosis and risk of falling explain more than 90% of all hip fractures. The mortality and morbidity of the fragility hip fracture is high and the disease is also a challenge for the health care system. Geographic differences in hip fracture incidence have been reported with Oslo in Norway reporting the highest incidence worldwide. No data however, on the hip fracture epidemiology, exist for Southern Norway. Several predictors of hip fracture risk and subsequent mortality have been recognized, but controversies continue to exist. Clinical epidemiological studies are important in order to understand the risk factors for hip fracture and hip fracture mortality and also to understand the impact that the disease brings to the single individual and society. Thus, exploring the epidemiology is important when aiming to reduce the burden of a disease. The aim of this thesis is to study the epidemiology of the fragility hip fracture in Southern Norway with regard to incidence, mortality and predictors of mortality in fragility hip fracture patients. Methods The study objects have been addressed in three miscellaneous papers. In paper I, the incidence of fragility hip fracture in the two most Southern counties in Norway in a two-year period (2004 and 2005) has been studied. I also compared adjusted incidence rates from my study with other national and international hip fracture studies. In paper II, the mortality of the hip fracture population has been examined in comparing with randomly selected individuals from the background population matched for age, gender and residency. In paper III, I searched for predictors of short- and long-term mortality in hip fracture patients. For papers I and II all the fragility hip fracture patients were identified and included retrospectively. Non-residents, patients with high energy or other type of hip fracture were excluded. For paper III, a prospectively recruited cohort of hip fracture patients assessed at the osteoporosis centre in Kristiansand hospital was studied. Diverse tools of statistical analyses were used as appropriate in each paper: z-test, Student’s t-test, chi-squared tests, survival analyses, and regression analyses. Results In paper I, a total of 951 (271 males, 680 females) individuals aged ≥ 50 years with hip fracture were identified. The age-adjusted incidence rate was 34.6 for males and 75.8 for females per 10 000 person-years. Age specific incidence rates were significantly higher in females than in males, but only for age groups between 70 and 90 years. No differences in incidence rates were observed between rural and urban areas. The incidence rates were significantly higher during winter months compared to the other seasons. The age-adjusted incidence rates for females from Southern Norway were among the lowest reported from Norway, Denmark and Sweden. However, compared to other non-Nordic countries, they continued to be one of the highest in the world. In paper II, a total of 942 (267 males and 675 females) patients with a hip fracture were identified. In the hip fracture patients overall mortality rate after one year was 21.3% (males 30.7% and females 19.1%, p<0.005) and after 5 years 59.0% (males 70.0% and females 54.6%, p<0.005). The corresponding figures for matched controls was 5.6% (males 5.9 %, females 5.4%, p=0.6) and 24.9% (males 25.9%, females 24.5%, p=0.4), respectively. The Odds ratio (OR) for hip fracture mortality was highest during the first 3 months after fracture for both males (10.5) and females (6.5). Thereafter, the OR appeared to stabilize, but persisted at higher levels than the background population for the whole 5-year period. In the performed multivariate analysis, the mortality risk was significantly associated with male gender, older age (>80 years) and the fracture event at 1 and 5 years after the fragility hip fracture. No significant differences in mortality rates were observed between rural and urban areas or between hospitals, either at 1 or 5 years after the event of hip fracture. In paper III, a total of 432 hip fracture patients (129 males and 303 females) were prospectively identified. Among them 296 (85 males and 211 females) patients (mean age 80.7 years) were assessed at the Osteoporosis Centre. Plethora of variables were individually associated with increased short- and long-term mortality in both males and females. Nevertheless, in multivariate analysis, no associations were observed with mortality for 3 months after the fragility hip fracture. After 1 year, variables independently associated with short-term mortality were in females older age (OR 6.95) and in males older age (OR 5.74) and pulmonary disease (OR 3.20). Variables independently associated with 5 years mortality in males were osteoporosis (OR 3.91) and older age (OR 6.95), and dementia (OR 4.16) and older age (OR 2.80) in females. Conclusion The results presented in this thesis shows that the incidence of the fragility hip fracture in Southern Norway is lower than in Oslo and comparable to other Norwegian studies, but continues to be one of the highest worldwide. The mortality after a fragility hip fracture is high, not only in the short-term, but also five years after the event. Males suffer higher mortality rates than females. After 1 year, variables independently associated with short-term mortality were older age in females, older age in males and pulmonary disease. Variables independently associated with 5 years mortality in males were osteoporosis and older age, and in females dementia and older age.