1 Division of Epidemiology, Department of Health Research and Policy, Stanford University School of Medicine, Stanford, CA.
2 Stanford Center for Research in Disease Prevention, Department of Medicine, Stanford University School of Medicine, Stanford, CA.
3 Division of Research, Kaiser Permanente Medical Care Program, Oakland, CA.
Received for publication February 13, 2003; accepted for publication July 25, 2003.
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ABSTRACT |
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accidental falls; aging; biomechanics; foot; forearm; fractures; pelvis; shoes
Abbreviations: Abbreviations: CI, confidence interval; OR, odds ratio.
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INTRODUCTION |
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The aim of this study was to assess both extrinsic circumstances of the fall and personal characteristics of fallers that affect the risk of fractures at each of five sites: foot, distal forearm, proximal humerus, pelvis, and shaft of the tibia/fibula. We hypothesized that the factors that differentiate injurious falls from noninjurious falls would, to some extent, differ by fracture site according to the point of impact of the fall. Other more general factors, such as those that impair protective responses, overall fitness, or bone mineral density, may influence the risk of fracture at most of these sites.
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MATERIALS AND METHODS |
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Cases
Cases for the entire study were identified on a weekly basis through inpatient (4 percent) and outpatient (96 percent) records. Information on diagnosis and fracture site was abstracted from computerized radiology reports and medical records by a trained record abstractor. To meet the case definition, the fracture must have been confirmed by radiography, bone scan, or magnetic resonance imaging. Classification of fractures followed the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (9).
To accrue relatively large numbers of racial/ethnic minorities, all persons whose computerized hospital inpatient files and medical records listed a minority or unknown race/ethnicity were included for all fracture sites. Because pelvis and shaft of the tibia/fibula fractures were uncommon, all Whites with these fractures were included. Until May 1, 2000, 50 percent of Whites with proximal humerus and distal forearm fractures and 25 percent of Whites with foot fractures were randomly sampled. Starting on May 1, 2000, all persons with distal forearm fractures and 50 percent of the Whites with foot or proximal humerus fractures were included to increase numbers. In the parent study, the participation rate for cases was 73 percent; participants and nonparticipants did not differ by age but differed somewhat by gender (75 vs. 70 percent female) and race/ethnicity (40 vs. 31 percent White; 42 vs. 50 percent unknown) as indicated by inpatient medical files.
Controls
Controls for the entire study were accrued over the same time period as cases. Every 3 months, 25 percent of Kaiser Permanente members were sampled and were stratified into 5-year age and gender groups, which were then randomly ordered, and the first 34 females and seven males in each 5-year age group were retained. Race/ethnicity, if available, was ascertained for these subjects. All minorities and persons of unknown race/ethnicity, and a random sample of 39 percent of White females and 78 percent of White males, were selected in order to obtain approximately the same number of controls in each age group within each gender group. In the parent study, the participation rate for controls was 66 percent; participants and nonparticipants did not differ by gender, age, or race/ethnicity as indicated by inpatient medical files.
Exclusions
Pathologic fractures caused by specific diseases, such as metastatic cancer or Pagets disease, were excluded from the study, as were persons who spoke languages other than Spanish or English. Additional exclusions are presented in table 1. Cases with a history of fracture since the age of 45 years at the same site as that for which they were selected were excluded from the analysis, and controls with a prior fracture since the age of 45 years at any of the five fracture sites were excluded from the common control group. Excluding cases with any of the five fractures since the age of 45 years did not substantially change the findings, so data on the larger case groups are presented here.
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Most of the questions on the circumstances of the falls were categorical. Dummy variables were created to describe the activity at the time of the fall, the direction of the fall ("other" direction included falling in multiple directions or tumbling/rolling), and the type of shoes and shoe soles, with the largest groups serving as the reference. Vigorous exercise referred to running, jogging, or performing other vigorous recreational activities.
Age referred to the age at fracture for cases and the age at interview for controls. Self-reported adult height and weight were combined into body mass index (weight (kg)/height (m)2). Self-reported alcohol intake was estimated by using the average number of drinks per week of beer, wine, and hard liquor in the past year. Average leisure-time physical activity in the past year was weighted by the type of activity by using metabolic equivalents (10), quantified in number of hours per month spent performing various activities and then placed into quartiles based on the distribution of the control group.
Lower limb dysfunction was defined as confinement to a bed/chair, requirement of ambulatory aids, or help walking up/down stairs or one-half mile (0.8 km) in the month prior to the fracture or interview (5). Upper limb/back dysfunction was based on four questions in which the subject rated his or her ability to pull/push large objects, lift 10 pounds (4.5 kg), extend the arms above shoulder level, and handle small objects. Subjects who answered "a lot of difficulty" to any of the questions or were confined to a bed or chair were categorized as most impaired, while those who answered "no difficulty" to all questions were categorized as least impaired (5). The middle level included subjects who answered "some difficulty" or "a little difficulty" with at least one activity (5). The total number of falls in the past year was used as a summary variable.
Statistical analyses
To examine the association between one variable and a fracture, controlling for the effects of other variables, we used unconditional logistic regression with SAS version 8 software (SAS Institute, Inc., Cary, North Carolina) to compute adjusted odds ratios. Because of the nature of the sampling of subjects, the odds ratios were adjusted for gender, 5-year age group, and race/ethnicity as indicated by inpatient medical files (White vs. non-White or unknown). In addition, the odds ratios were adjusted for type of interview, age in years to account for any residual confounding, and self-reported race/ethnicity categorized into four groups. Persons were able to place themselves in more than one race/ethnicity category and were classified into one category according to their answers to questions on acculturation.
Variables were entered into the multivariable models if they were associated with fractures after adjustment for age, gender, race/ethnicity, and type of interview at an alpha level of 0.15, and they were retained if they were associated with fractures at an alpha level of 0.10 or if a priori hypotheses supported their inclusion. Because a longer time could have elapsed between when the fall occurred for controls versus cases, analyses were also conducted by using only those controls who fell in the 3 months prior to interview. The results were compared with those in which data on all controls who fell were used. The odds ratios did not differ by more than 10 percent; therefore, the results using all controls are presented here.
Effect modification was assessed for the variables in the multivariate models for the foot and distal forearm (for which numbers were larger than for the other sites) by visual inspection and cross-product terms in logistic regression. The potential effect modifiers assessed were age (>65 years, 65 years), gender, race/ethnicity, and lower limb dysfunction. There was little evidence of effect modification, and this issue will not be discussed further in this paper.
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RESULTS |
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Extrinsic characteristics
Hitting the foot (odds ratio (OR) = 17.9, 95 percent confidence interval (CI): 13.0, 24.4), hand (OR = 5.5, 95 percent CI: 4.2, 7.2), shoulder (OR = 13.5, 95 percent CI: 9.7, 18.6), pelvis (OR = 20.1, 95 percent CI: 11.5, 34.9), and shin (OR = 6.6, 95 percent CI: 4.1, 10.6) increased the odds of fracture at the foot, distal forearm, proximal humerus, pelvis, and shaft of the tibia/fibula, respectively. Other extrinsic circumstances of falls potentially associated with fractures in those who fell are presented in table 2. Falls from more than a standing height and on a hard surface increased the risk of fracture at most sites, while the activity at the time of the fall varied by site of fracture. Table 3 shows that narrow-width shoe heels and medium-/high-heeled shoes were associated with an increased risk of fracture at all sites.
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Distal forearm fracture
Falling while performing a vigorous activity or on a hard surface and wearing shoes at the time of the fall that were worn less than three times per week were associated with an increased risk of distal forearm fracture (table 6). Carrying something in ones arms, current use of hormone therapy by women, and numbness/weakness in the feet or legs were associated with decreased risks of distal forearm fracture.
Proximal humerus fracture
Turning around, performing a vigorous activity, and falling on a hard surface were associated with increased risks of proximal humerus fracture, while descending stairs was associated with a decreased risk (table 7). (A history of Parkinsons disease was not included in the multivariate model because the small number of people with this disease made the odds ratio unstable.) Cigarette smoking in the past, current use of hormone therapy by women, and limping were associated with decreased risks of proximal humerus fracture.
Pelvis fracture
Turning around, reaching up, sitting/lying, or performing another outside activity at the time of the fall showed trends toward increasing the risk of pelvis fracture, while getting into or out of a bed, sofa, chair, or toilet was associated with decreased risks (table 8). A higher body mass index and taking any medication within 4 hours of the fall were associated with decreased risks of pelvis fracture. Being taller than the median height, being a current smoker, and ever using anticonvulsants for a year or longer was associated with increased risks of pelvis fracture.
Shaft of the tibia/fibula fracture
High-heeled shoes were associated with an increased risk of shaft of the tibia/fibula fracture (table 9). A higher body mass index and numbness/weakness in the legs/feet were associated with decreased risks of this type of fracture.
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DISCUSSION |
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For all sites except the pelvis, increasing leisure-time physical activity in the past year was associated with a reduced risk of fracture among fallers, possibly because physical activity can increase physical functioning, maintain mobility, increase muscle strength and balance, improve bone mineral density (11), and improve reaction time. However, certain types of physical activity can also increase the risk of falls and fractures. In our study, for instance, most falls occurred while walking, descending stairs, engaging in vigorous exercise, or participating in another outside activity. Previous studies have found moderate physical activity or a physically active lifestyle to be associated with a 2060 percent decrease in the risk of hip fracture, but the findings for other fracture sites have been less clear (11).
Persons who broke their fall reduced their risk of all fractures except distal forearm fractures, where putting the hand out increased the risk by nearly fourfold. Whereas breaking the fall with the hand places the force of the fall on the distal forearm, grabbing or hitting something with the body before landing may reduce the force of the fall and the risk of fractures in general. A previous study also found that fallers who initiated protective responses were less likely to sustain a hip fracture and that landing on the hand increased the risk of distal forearm fracture (3). In the present study, persons who were carrying something in their arms at the time of the fall were at a decreased risk of distal forearm fracture, probably because they were less likely to put out their arm to break the fall.
Falls from more than a standing height increased the risk of fractures at all sites, a finding supported by previous studies of fall-related injuries (12, 13). Taller persons also may generate more energy during a fall and have been found to be at a greater risk of hip (3, 5, 1416) and distal forearm (3) fractures. In our study, taller persons were found to have a twofold increased risk of pelvis fracture.
Higher body mass index was associated with a reduced risk of pelvis and shaft of the tibia/fibula fractures, possibly because these areas have more soft tissue to absorb the energy of the fall than the other fracture sites considered in this study. Although previous studies have found fewer hip fractures in fallers with higher body mass (5, 6, 13), probably in part because of a higher bone mineral density and greater fat padding associated with higher body mass index (17), higher body mass index has not been found consistently to reduce the risk of distal forearm fracture (1821). Similarly, the risks of distal forearm and proximal humerus fractures were increased if the fall occurred on a hard surface, consistent with previous studies of fall-related serious injuries (3, 13, 22). The risk of fractures may be reduced as a result of measures that reduce the impact of the fall, such as installing softer flooring (i.e., wall-to-wall carpeting).
Prior studies have found increases in bone mineral density and reductions in the risk of hip, vertebral, nonvertebral, and distal forearm fractures with recent use of hormone therapy in studies not limited to fallers (17). Our results support these findings. Most previous studies also suggest that tobacco use is associated with reduced bone mineral density and an increased risk of fractures (23). In our study, current smoking was positively related to the risk of pelvis fracture and showed opposite trends at the foot and proximal humerus. We can offer no explanation for the latter findings.
The effectiveness of protective responses depends on the speed of execution and the strength of the muscles initiating the protective movement (24). Preexisting neurologic conditions, upper- and lower-limb dysfunction, and use of ambulatory aids increased the likelihood of hip fracture in studies that did (5) and did not (13, 15, 25) limit the analyses to fallers. In our study, only Parkinsons disease was associated with an increased risk of proximal humerus fracture, while upper- or lower-limb dysfunction and symptoms such as numbness/weakness in the legs/feet, limping, or dizziness were consistently associated with a lower risk of fracture at all sites. Persons with these symptoms were less likely to be performing vigorous exercise at the time of the fall (data not shown), which is consistent with a previous study (26) that found a higher percentage of falls resulting in serious injury among vigorous (22 percent) compared with frail (6 percent) elderly. Our findings suggest that persons with these symptoms may move at a slower pace and be less likely to perform some of the activities that would place them at increased risk if they fell.
In addition, the sedating effects of alcohol and certain medications may impair protective responses in the event of a fall. In this study, drinking any alcohol within 4 hours of the fall increased the risk of foot fractures by over threefold. To our knowledge, past studies have not considered alcohol intake in the 4 hours preceding a fall, but use of seizure medications and benzodiazepines has been associated with foot (27) and hip (14) fractures in studies not limited to fallers. In our study, only pelvis fracture was associated with use of anticonvulsant medications. On the other hand, use of any medication within 4 hours of the fall was associated with a reduced risk of pelvis fracture, a finding difficult to interpret because of the wide variety of medications used and because medication is usually a marker of the presence of disease.
To our knowledge, this study is the first to consider risk factors for fractures at these sites for fallers in a group of cases and controls with equal access to health care. On the other hand, we cannot rule out the possibility of recall bias; cases might have reported information more fully because they thought more about why their fracture occurred, the fall probably had a significant effect on their lives, and the fall occurred during their recent fracture, whereas controls provided information on a fall that could have occurred up to 12 months before they were interviewed. However, the mean time from fall/fracture to interview was similar for cases and controls, and, when the analysis was limited to controls who fell 3 months prior to the interview, the results were similar to the findings for all controls. Second, 3 years into the study, most interviews began to be conducted over the telephone rather than in person, which could have introduced some error because some sections of the questionnaire included cards illustrating certain medications and medical conditions (cards were not used in telephone interviews). In the analyses, we found no evidence of effect modification by type of interview. Third, the small number of cases with fractures at the less common sites of the pelvis and shaft of the tibia/fibula and, to some extent, the proximal humerus limited our ability to detect associations.
In conclusion, we identified risk factors associated with fractures at five sites for people who fell. Wearing proper shoes with low, wide heels that cover and stay on the foot in the event of a fall could reduce the risk of fractures. In this study, breaking the fall by putting a hand out was associated with a lower risk of all fractures except the distal forearm, and breaking the fall by grabbing or hitting something with the body before landing was associated with a lower risk of distal forearm fractures. In addition, an increasing amount of leisure-time physical activity in the past year was associated with a lower risk of fractures at all five sites. Physical activity may help maintain mobility, increase muscle strength and balance, improve physical functioning, and increase bone mineral density (11) so that in the event of a fall, fractures are less likely. On the other hand, the positive benefits of physical activity must be balanced with the higher risks of falling and fracturing during certain activities. Our findings also suggest that factors that minimize the impact of the fall, such as softer flooring, can reduce the risk of fractures.
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ACKNOWLEDGMENTS |
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The authors thank Beverly Peters and Luisa Hamilton for project management, Michael Sorel for computing and database management, and Carolyn Salazar for medical record abstraction.
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NOTES |
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REFERENCES |
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