Risk factors for hip fracture in very old people: a population-based study

This analysis employed data from the Umeå 85+/Gerontological Regional Database (GERDA) study, a population-based cohort study conducted by Umeå University, Sweden, in 2000–2002, 2005–2007, and 2010–2012. From a randomized starting point, every other 85-year-old and every 90- and ≥95-year-old inhabitant of one urban and five rural municipalities in the county of Västerbotten was selected from National Tax Agency registers. Written information was first sent to all eligible participants by mail, and oral informed consent to participate was obtained during telephone calls placed shortly thereafter. When appropriate due to cognitive impairment, relatives or otherwise authorized representatives provided informed consent. The Regional Ethics Review Board of Umeå approved the study (§ 99–326, § 05-063 M, § 09-178 M, § 13-432-32 M).

Trained assessors with prior medical knowledge (physicians, medical students, nurses, and physiotherapists) performed the interviews and assessments in the participants’ homes to enable individuals with, for example, severe dementia and multimorbidity to participate. Care personnel and/or relatives were also interviewed when the participants lived in residential care facilities or when required due to cognitive impairment. Medical records were reviewed to confirm diagnoses and medications. For participants who took part in more than one data collection period, data from the earliest occasion including home visitation and review of medical records were used in the present analyses.

From the 1368 persons invited to participate between 2000 and 2012 (see Fig. 1), the present study included the 953 individuals consenting to home visitation and review of medical records (participation rate 69.7 %; Fig. 1). Age did not differ between the 415 individuals who declined to participate in this study and the 953 individuals who consented (P = .812), but a higher proportion of women than men declined to participate (32.3 % vs. 26.4 %, P = 0.028).

Data on occurrence and type of femur fracture between 1 January 1980 and 31 October 2013 were collected through review of medical records and discharge registers from the three local hospitals (Umeå, Skellefteå, and Lycksele), maintained by the County Council of Västerbotten. Hip fractures were categorized as cervical (femoral neck) or trochanteric (inter- and sub-trochanteric regions), based on International Classification of Diseases (ICD) codes. Hip fractures occurring before baseline were classified according to ICD-8 (820.00/01, 820.10/11, 820.90/91), ICD-9 (820A–D), and ICD-10 (S72.00–S72.21) codes. Hip fracture incidence was followed for each participant and hip fracture type, categorized using ICD-10 codes. Follow-up started at the date of study inclusion and ended at the first occurrence of one of the following events: hip fracture, death, the last day of the maximum 5-year follow-up period, or—for individuals included in 2010–2012—the last date of data collection (31 October 2013). Dates of death were collected from death certificates, electronic medical records, and population registers.

Variables assessed at baseline were chosen based on associations with hip fracture, falls, or osteoporosis in previous studies of older people (Table 1). Body weight and height were measured, and body mass index (BMI, kg/m²) was calculated. Vision was rated as impaired when a participant was unable to read a sentence printed in 4-mm-high capital letters, with or without glasses. Hearing was rated as impaired when a participant was unable to hear a conversation at normal speaking volume from a 1-m distance, with or without a hearing aid. Cognitive function was assessed using the Mini-Mental State Examination (MMSE) [21]. MMSE scores range from 0 to 30, with scores ≤23 considered to indicate impaired cognition. The Barthel ADL Index (scores, 0–20) was used to measure dependence in personal activities of daily living (P-ADL) [22], with a score of 20 indicating total independence. The Barthel ADL Index “mobility on level surface” item was singled out to describe participants’ ability to move indoors. The ADL staircase, a development of the Katz ADL Index [23], was used to assess dependence in instrumental ADL and P-ADL. ADL staircase scores were dichotomized at 0 (independence in all ten activities) to compare independent with dependent individuals. To measure functional capacity, participants’ ability to rise once from a chair and then sit down independently, without using the hands, was tested. Nutritional status was evaluated using the Mini-Nutritional Assessment (MNA) Scale [24]. MNA Scale scores range from 0 to 30, with a score <17 indicating malnutrition, scores of 17–23.5 indicating risk of malnourishment, and scores ≥24 indicating good nutritional status. Socio-demographic information and data on falls in the previous year were collected during interviews, as were medical history and current use of medication, which were verified later by reviewing medical records. A fall was defined as an event in which the individual involuntarily ended up on the floor/ground. A specialist in geriatric medicine either confirmed pre-existing diagnoses of osteoporosis or set new clinical diagnoses of osteoporosis, mainly based on low-energy fractures and/or dual-energy X-ray absorption (DXA) assessment. The same specialist also confirmed diagnoses of dementia, depression, and delirium according to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision criteria [25] by review of medical records, current medical treatment, and baseline assessments, including the 15-item Geriatric Depression Scale [26] and the Organic Brain Syndrome scale [27].

Differences between women and men, as well as those between individuals who agreed and declined to participate, were tested using Pearson’s chi-squared test and Student’s t test. Univariate Cox proportional hazards regression models were used to analyze associations between baseline characteristics and time to hip fracture during follow-up. The Barthel ADL Index “mobility on level surface” item was dichotomized, as the increased risk of incident hip fracture was similar in participants who were able to walk independently or with help and those who were independently mobile in wheelchairs or immobile (data not shown). Non-linear associations between incident hip fracture and continuous or ordinal baseline variables were analyzed according to established cutoff scores or quartiles. As a result, BMI was dichotomized at 22.0, which indicates underweight in people aged >70 years [28]. The proportionality of hazards was tested using Schoenfeld residuals.

Baseline variables associated with risk of incident hip fracture at P < 0.15 in univariate Cox analyses were included in a multivariate Cox regression model, with the exception of the MNA variable. MNA was excluded due to singularity since it includes data on BMI, type of residence and indoor mobility, which are included as separate variables in the multivariate model. The correlations between all variables in the multivariate model were tested using Pearson and Spearman coefficients, and no strong correlations (r > 0.6) were found. Step-wise backward deletion was performed manually, with the least-significant variable eliminated until only significant variables remained. These variables, adjusted for sex, formed the final model and were re-tested using Schoenfeld residuals.

Individuals with unilateral and bilateral hip prostheses were included in the baseline hip prosthesis variable. Sensitivity analyses excluded individuals with bilateral hip prostheses, which greatly reduce the probability of future hip fracture.