Data from three earlier incidence studies on hip fracture in Oslo were used [14
]. These studies include all patients ≥20 years with hip fracture in the two-year periods 1978 to 1979 (n=2067) and 1988 to1989 (n=2697), and in the one-year period from 1st of May 1996 until 30th of April 1997 (n=1290). Hip fractures (International Classification of Diseases, ninth revision (ICD-9) code 820.X) were identified through diagnosis registers, operating theatres protocols, medical records, and x-ray records. Fractures in patients residing outside Oslo or fractures due to malignancy were not included in these studies.
Hip fractures in patients aged <50 years are infrequent, and the younger hip fracture patients differ considerably from the older hip fracture patients regarding comorbidity [17
]. For this reason, patients aged <50 years were not included in the current study. For patients with more than one fracture in any of the inclusion periods, the first fracture was included in the present study.
Registration cards from the incidence studies of 1978/79 and 1988/89 were retrieved, and the cases were identified through name and date of birth. The data from the registration cards were transferred into an electronic register, and linked to the National Population Register (Norwegian Tax Administration, Oslo) to achieve the full unique Norwegian 11-digit identification number for each patient. The data from 1996/97 were stored in an electronic register with the 11-digit identification number.
In the 1978/79-cohort, 50 patients were aged <50 years. Of the patients aged ≥50 years, 80 patients were not included due to: I) unidentifiable date of fracture (n=53); II) loss to follow up (n=1); III) missing unique identification number (n=24); IV) fracture due to metastasis (n=1); V) double registration (n=1). There were no differences regarding sex, age, and type of fracture when comparing the excluded patients with the included patients (Chi-square test and Student’s t test).
In the 1988/89-cohort, 26 patients who had sustained a hip fracture in the previous inclusion period and 42 patients aged <50 years were not included. In addition, ten patients were not included due to: I) unidentifiable date of fracture (n=2); II) loss to follow up (n=4); III) missing unique identification number (n=3); IV) resident outside Oslo (n=1).
In the 1996/97 cohort, 15 patients had sustained a hip fracture in one of the earlier inclusion periods, and 21 patients were aged <50 years. All the remaining patients were included.
Data from the 5180 cases included (Table ) were linked to The Cause of Death Register provided by Statistics Norway (Statistics Norway, Kongsvinger, Norway). The patients were followed up with respect to death from all causes until 31st of December 2007 (Additional file 1
: Flow chart).
Characteristics of the included patients of the different cohorts
The study was designed as a historic cohort study. Exposure was sustaining a hip fracture in the registration periods. Outcome was death from all causes. Covariates were sex, age, and type of fracture.
Data and definitions
The hip fractures were defined as femoral neck or intertrochanteric. Subtrochanteric fractures were not included.
Hip fractures in patients aged ≥65 years are often referred to as geriatric hip fractures, and younger patient are frequently not included in survival studies [8
]. Those aged ≥ 85 years have a high absolute mortality during the first six months of follow up, and a shorter duration of excess mortality than younger hip fracture patients [5
]. The patients were therefore divided into following age groups: I) 50–64 years; II) 65–84 years; and III) ≥ 85 years.
In the 1978/79-cohort, fracture time was only registered by month and year. Date of fracture was consequently set to the first of the month to avoid negative survival time. To assess the influence of this fictive fracture date, survival analyses with the fracture date set to both the 1st and the 15th of the month were performed. The different fictive fracture days did not influence the results presented.
Comparisons of baseline data between the three cohorts were made using the Kruskal Wallis test and the One-Way-ANOVA. Kaplan-Meier-curves were estimated for each cohort stratified by sex, age group, and fracture type.
Age- and sex-specific one year-mortality rates for Oslo from 1978–2007 were provided by Statistics Norway and were used to calculate the expected survival curves for each cohort (Figure ) as described by Therneau [19
Figure 1 Expected survival and observed survival by cohort. Expected survival (dotted curves) and observed survival (continuous curves) by cohort. a) All, b) Men, c) Women, d) Patients aged 50–64 years, e) Patients aged 65–84 years, (more ...)
Standardized mortality ratio (SMR) expresses the level of excess mortality [20
], and was calculated as the ratio of patient mortality to mortality in the background population of Oslo. The background population corresponded to the study population with respect to time period, sex, and year of birth. SMRs were calculated for the three cohorts for each sex, age-group, and fracture type, for the 0–6 months, 6–12 months, 0–1 year, 1–5 years and 5–10 years intervals after fracture. Sex- and age stratified analyses were also performed for the 0–1 year interval to allow comparison with earlier studies. Confidence intervals for SMR were computed as bootstrap BCa
intervals with 10 000 replications [21
To assess the duration of the excess mortality, time-framed Kaplan-Meier curves for 5-years intervals starting at each year of the follow up time were calculated for each sex and age-group. These curves were compared with the corresponding expected survival curves. Only patients still alive at the start of the time interval were included. One sample log rank tests were used to test for statistical significance between observed and expected curves. The beginning of the last 5 year interval, where there still was statistical significance between the expected and observed curves, was set as duration of excess mortality.
The level of statistical significance was set at p<0.05. Analyses were performed using SPSS 14.0 (SPSS Inc., Chicago, IL, USA) and R 2.10.1 (The R Foundation for Statistical Computing, Vienna, Austria) with the R packages survival and boot.
Ethics and approvals
The study is performed in compliance with the Helsinki Declaration. The Regional Committee for Research Ethics approved the study. A dispensation from professional secrecy was given by the Norwegian Directorate for Health and Social Affairs. Permission to handle sensitive information were sought and received from the Data Protection Agency.