Foot Problems as Risk Factors of Fractures

Theresa H. M. Keegan1, Jennifer L. Kelsey1, Stephen Sidney2 and Charles P. Quesenberry, Jr.2

1 Division of Epidemiology, Stanford University School of Medicine, Stanford, CA.
2 Division of Research, Kaiser Permanente Medical Care Program, Oakland, CA.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
This case-control study examines whether foot problems are risk factors of fractures of five sites among people aged 45 years or older at six Kaiser Permanente Medical Centers in northern California. From October 1996 to May 2001, interviewers collected information through a standardized questionnaire. Incident cases of distal forearm (n = 1,000), foot (n = 827), proximal humerus (n = 448), shaft of the tibia/fibula (n = 168), and pelvis (n = 172) fractures and 1,913 controls from the same medical centers were included. After adjustment for potential confounders and for each additional foot problem, the odds of a foot fracture increased by 8% (adjusted odds ratio = 1.08, 95% confidence interval: 1.03, 1.13). In contrast, each additional foot problem was associated with a reduction in the odds of a forearm fracture (adjusted odds ratio = 0.93, 95% confidence interval: 0.89, 0.98). In general, foot problems were not related to fractures of other sites, although diabetes, which may result in foot problems, increased the odds of a proximal humerus fracture (adjusted odds ratio = 1.65, 95% confidence interval: 1.20, 2.26). If these findings are supported by data from other studies, preventive measures to retard the development of foot problems could reduce the incidence of foot fractures.

diabetes mellitus; foot; fractures; humeral fractures; radius fractures; tibial fractures; ulna fractures


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Fractures in older persons are a major public health problem, and as the number of elderly increases, so will the magnitude of the problem. Despite the high incidence and frequent disability resulting from fractures in older people, much remains to be learned about the epidemiology of fractures, particularly of sites other than the hip. Similarly, little is known about the epidemiology of foot problems and their relation to fractures. Foot problems needing treatment are common in persons over the age of 65 years, affecting an estimated 71 percent of the population (1Go). The prevalence and severity of foot conditions increase with age, as the aging process can result in neuropathy, ischemia, and atrophy of the plantar fat pad (2Go). In addition, chronic conditions such as diabetes, peripheral vascular disease, and arthritis often involve the feet (2Go).

The slightest deformity of the foot can lead to impaired proprioception (3Go), skeletal problems, changes in gait, and pain. Diabetes is known to cause ulceration of the feet, and there is evidence that it is a risk factor for fractures of the foot (2Go, 4Go), hip (5Go, 6Go), and proximal humerus (7Go). No prior research has focused on the relation between other foot problems and fractures, although one study found that serious foot problems (defined as moderate or severe bunions, toe deformities, ulcers, and deformed nails) increased the odds of falls by 80 percent in persons 75 years or older (8Go).

The objective of this research is to determine whether individual foot problems and the total number of foot problems are risk factors of distal forearm, foot, proximal humerus, shaft of the tibia and fibula, and pelvis fractures. Data from the first 4.5 years of a case-control study designed to evaluate a variety of possible risk factors of these fractures are used.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Cases and controls were chosen from six northern California Kaiser Permanente medical centers located in Hayward, Oakland, Richmond, San Francisco, San Jose, and south San Francisco. Informed consent procedures were approved by the Kaiser Permanente and Stanford University institutional review boards. Cases and controls were identified from October 1996 through May 2001.

Cases
Cases aged 45 years or older were identified on a weekly basis through computerized radiology records and inpatient and outpatient records. Information on the diagnosis was abstracted from 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. Fractures classified as possible or suspicious were not accepted, unless there was radiographic evidence of fracture healing within 3 weeks. Classification of fractures followed the International Statistical Classification of Diseases and Related Health Problems, Tenth Revision (9Go). Fractures of the distal forearm included those that occurred in the distal third of the radius, ulna, or both, while fractures of the proximal humerus included those that occurred at the level of or proximal to the surgical neck of the humerus. Fractures of the foot included the bones of the foot (e.g., talus, navicular, cuboid, cuneiform, and metatarsal) but not the toes or ankle. Fractures of the pelvis included all pelvic bones (e.g., sacrum, ileum, pubis, acetabulum, and ischium) except the coccyx. Fractures of the shafts of the tibia and fibula included fractures of one or both of these bones but not fractures of the distal fibula. Pathologic fractures caused by specific diseases, such as metastatic cancer or Paget's disease, were excluded. For this analysis, the first-listed fracture on the medical record was taken as the fracture's site for persons with simultaneous fractures at more than one site (n = 58).

Because it was desired to accrue relatively large numbers of racial/ethnic minorities for other purposes of this study, all persons indicated as a minority or without recorded race/ethnicity on computerized hospital inpatient files and medical records were included for all fracture sites. Because fractures of the pelvis and shaft of the tibia/fibula 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 sampled. Starting May 1, 2000, all persons with distal forearm fractures and 50 percent of the Whites with foot fractures were included in order to increase numbers. It should be noted that the final classification of race/ethnicity was based on self-reported race/ethnicity from the questionnaire. Seventy-two percent of eligible and nonexcluded cases agreed to participate.

Controls
The 1,913 controls were accrued over the same time period as the cases were. Every 3 months, 25 percent of Kaiser members from the six centers in the group aged 45 years or older were randomly sampled. These members were stratified into nine 5-year age (45–49, 50–54, 55–59, 60–64, 65–69, 70–74, 75–79, 80–84, >=85) and two 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 all persons of unknown race/ethnicity and a random sample of 39 percent of White females and 78 percent of White males were selected. Persons who were previously selected as cases were not eligible to be controls. Sixty-six percent of eligible and nonexcluded controls agreed to participate.

Questionnaires
Most information on potential risk factors was obtained from a standardized questionnaire administered by trained interviewers in English and Spanish. Back-translation was used to ensure that the English and Spanish versions of the questionnaire were equivalent. Subjects who required the use of proxy respondents (n = 154) or were interviewed in Chinese (n = 7) were excluded from these analyses. For the first 3 years of the study, the interview was conducted primarily in person at the subject's home (or institution, for institutionalized subjects) or rarely in another place of convenience. To increase participation rates and the number of subjects interviewed, the interview was conducted primarily over the telephone after November 15, 2000. Whether a study subject was interviewed in person or over the telephone was assessed as a confounder and effect modifier in the analyses. No evidence of confounding or effect modification from this source was found. For cases, questions referred to the period before the fracture occurred, while for controls, questions referred to the period before the time of the interview.

The foot-associated problems of major interest to this analysis were diabetes, flat feet, high arches, lateral deviation of the big toe, hammer toe, bunions, corns, calluses, plantar warts, ingrown toenails, painful toenails, cold feet most of the time, arthritis of the toes or feet, and any other foot problems. Diabetes was ascertained by asking subjects whether a doctor or other medical practitioner had ever told them that they had diabetes (yes, no, or do not know/refused to answer). The rest of the foot problems were ascertained by asking the subjects if they had each foot problem (yes, no, or do not know/refused to answer). For in-person interviews, pictures were shown to illustrate lateral deviation of the big toe and hammer toe. The subjects were asked to specify any other foot problems. All responses of "do not know/refused to answer" were considered as "no" in the analysis. An additional variable, total number of foot problems, was created by adding the number of foot problems for each subject. For some analyses, the number of foot problems was placed into quartiles based on the distribution of the control group (the first quartile served as the reference group).

The following questionnaire-derived variables were assessed as potential confounding variables: self-reported overall health compared with that of others of his/her age; quartiles of average weekly leisure-time physical activity (the first quartile serves as the reference group); body mass index (weight (kg)/height (m)2); history of maternal hip fracture; use at least once a week for a total of 1 year or longer of thiazides and other diuretics, calcium supplements, seizure medications, and multivitamin supplements; use of hormone replacement therapy in the last month among women; ability to recognize a friend across the room with corrective lenses; whether confined to a bed/chair for the last month; average alcohol intake in the last year; and years of smoking at least one cigarette per day. In addition, six summary variables were created by summing component variables: 1) number of common medical problems that a doctor or medical practitioner had ever told subjects that they had, 2) number of symptoms or problems experienced at least sometimes (numbness/weakness in the feet or legs, problems with balance, limping, tremors or shakes anywhere, dizziness, difficulty walking in dim light, and pain/numbness in the feet or legs while not walking), 3) number of fractures incurred over the age of 45 years, 4) number of physical aides used in the past month, 5) number of physical movements that subjects had at least some difficulty doing during the previous month, and 6) number of falls to the ground during the prior year. Most variables had missing values for less than 1 percent of cases and controls. In the construction of summary values, missing values for components were assumed to be "no" for binary response variables (except for the ability to recognize a friend across the room with corrective lenses, for which subjects were given a value of "yes"), the modal value for ordinal variables, and the mean age, gender, and racial subgroup values for continuous variables.

Statistical analysis
To compute the prevalence of foot problems among case groups and the controls while taking age and gender into account, we used the direct method of standardization, with the control group serving as the standard. Odds ratios were used as the measure of association between potential risk factors and fractures. To examine the association between one variable and fracture while controlling for the effects of other variables, we used logistic regression with SAS version 8 software (SAS Institute, Inc., Cary, North Carolina) to compute adjusted odds ratios. The adjusted odds ratios took into account 5-year age, gender, and race/ethnicity, as indicated by inpatient medical files to account for the way subjects were sampled, as well as the type of interview to account for how the questionnaire was administered. In addition, self-reported race/ethnicity and age were included in the multivariate models to control more tightly for these variables. Other potential confounding variables listed in the section above did not alter the relations between foot problems and fracture and were not included in the analyses presented here.

A case with a history of fracture since the age of 45 years at the same site as that for which he/she was selected was excluded from the analysis (n = 169), and controls with a prior fracture since the age of 45 years at any of the five fractures' sites were excluded from the common control group (n = 129). When the analysis was done with unique control groups for each fracture site, with only subjects with a prior fracture at each site excluded in the analysis involving that particular site, results were similar. Self-reported race/ethnicity was categorized into four groups on the basis of similarity of odds ratios for fractures: 1) White, Native American, and other; 2) Black; 3) Chinese, Filipino, Japanese, Pacific Islander, and other Asians; and 4) Hispanic. Those subjects (n = 157) who placed themselves in more than one category were classified into one of the above four categories according to their answers to questions on immigration and acculturation.

Effect modification by race/ethnicity, age, and gender was assessed with stratification (10Go) and by logistic regression with age as a quantitative variable. Little evidence of effect modification by race/ethnicity, age, and gender was present and will not be discussed further.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Table 1 shows the distribution of the cases and controls by age, gender, self-reported race/ethnicity, and mode of interview. Women comprise approximately 75 percent of the study population, and over 39 percent of cases and controls were minority (14.9 percent Asian, 12.7 percent Black, and 11.6 percent Hispanic). In our case groups, 90.2, 63.1, 91.5, 69.6, and 79.1 percent of distal forearm, foot, proximal humerus, shaft of the tibia/fibula, and pelvis fracture cases, respectively, attributed their fracture to a fall. In addition, 9.9 percent of foot fracture cases said that their bone just broke, while 19.0 percent of foot fracture subjects attributed their fractures to causes such as twisting and hitting the foot.


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TABLE 1. Selected characteristics of the control and case groups, California, 1996–2001

 
Among cases and controls combined, 82.1 percent had at least one foot problem, 57.2 percent had at least two, and 34.1 percent had three or more foot problems. Compared with the control group, the age- and gender-standardized prevalence of foot problems was generally higher in the foot fracture group and lower in the forearm fracture group (table 2). For the other fracture sites, differences were slight and inconsistent. The "any other foot problem" category contained a large variety of foot problems (e.g., foot or nail fungus, generalized pain and swelling, bone or heel spurs, biomechanical abnormalities such as pronation and supination, and numbness) that interviewers wrote on the questionnaire in the subject's own words. None of these other foot problems was common enough to be considered separately. Because of the relatively small number of cases of fractures of the shaft of the tibia/fibula and pelvis and lack of trends found in table 2, these are not considered further.


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TABLE 2. Age- and gender-standardized* prevalence of foot problems per 100 people in the control and case groups, California, 1996–2001

 
Multivariate-adjusted odds ratios for the relation between foot problems and fractures of the distal forearm, foot, and proximal humerus are presented in table 3. Most of the specified foot problems are associated with an increased risk of foot fracture and a lower risk of distal forearm fracture. On average, each additional foot problem is associated with an 8 percent increased odds of foot fracture and a 7 percent reduction in odds of a forearm fracture. The associations between foot problems and proximal humerus fractures are small and inconsistent. Figure 1 shows the multivariate-adjusted odds ratios for the association between quartiles of number of foot problems and fractures of the distal forearm and foot.


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TABLE 3. Adjusted* odds ratios and 95% confidence intervals for the association between foot problems and fractures by site of fracture, California, 1996–2001

 


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FIGURE 1. Adjusted odds ratios and 95% confidence intervals for the association between number of foot problems and foot and forearm fractures by quartiles of number of reported foot problems with the first quartile as the reference, California, 1996–2001. The first, second, third, and fourth quartiles comprise people with zero, one, two, and three or more foot problems, respectively. The odds ratios are adjusted for the stratification variables 5-year age, gender, and race/ethnicity, as indicated by inpatient medical files (White, non-White, and unknown), and the following: age in years, self-reported race/ethnicity, and type of interview (in person vs. over the telephone).

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The foot functions to absorb a high load, allow the body to be lifted off the ground and propelled forward, absorb and dissipate shock, balance the body and counteract the body's tendency to fall, and protect the body by acting as a barrier and a source of sensory information from the external environment (11Go). If any of these functions is compromised, that person is at an increased risk for changes in gait or balance that may lead to tripping, falling, catching the foot, or other immediate causes of fracture. We found that controls with any of the foot problems fell more in the past year than did those without foot problems (data not shown), but the association between foot problems and fractures was unchanged after controlling for the number of falls in the past year.

In this study, foot problems were related to an increased risk of foot fracture. One previous study found that a history of insulin-dependent diabetes was associated with an increased risk of foot fracture (4Go). Diabetes is associated with sensory neuropathy, which has been identified as a primary factor in the development of foot ulceration, infection, and lower extremity amputation (2Go, 12Go). The reduction or absence of feeling associated with sensory neuropathy may cause someone to catch the foot or lose fine motor coordination, leading to an increased number of falls and/or fractures. Tinetti et al. (8Go) reported that having 6–7 abnormalities of balance and gait increased the odds of a fall by 90 percent, and that foot problems increased the odds of a fall by 80 percent in subjects aged 75 years or older.

Most of the foot problems were associated with a lower risk of distal forearm fracture. Although no study previously considered foot problems as risk factors of distal forearm fractures, one possible explanation of this negative association is that these fractures often occur in osteoporotic women who have good neuromuscular function (7Go), walk briskly (13Go), and, in the event of a fall, are propelled forward onto their distal forearm (14Go). Possibly, their active lifestyles, less inhibited by foot problems, increased their risk of these fractures. Conversely, persons with foot problems may be at lower risk for distal forearm fractures because of moderately impaired walking ability (15Go) and a reduced forward momentum during a fall. In this study we had no measure of walking speed to confirm this hypothesis directly.

Foot problems were not related to proximal humerus fractures. The only foot-related problem associated with proximal humerus fractures was a history of diabetes mellitus. A previous study found insulin-dependent diabetes to be associated with an increased rate of proximal humerus fractures (7Go). No association between foot problems and fractures of the shaft of the tibia/fibula and pelvis was apparent. However, these fractures are less common, and the number of subjects with these fractures in our study was smaller than the number with fractures at the other sites.

To our knowledge, this is the first study to consider foot problems as risk factors of fractures. Cases were interviewed shortly after the fracture occurred so that more accurate and detailed information could be obtained than if more time had elapsed. One limitation of the study was the small numbers of cases in the less common fracture sites of the pelvis and shaft of the tibia/fibula and, to some extent, the proximal humerus. Another limitation was the use of the case-control study design and its potential for inaccurately reported prior events. In particular, if a person broke a bone, a foot problem may have come to the attention of the physician or subject during the course of treatment. Nevertheless, it would be difficult to see how this would explain the negative association with distal forearm fractures. The measurement of foot problems was crude inasmuch as the response was limited to yes/no. Information on severity or the time of onset was not measured, and questionnaire answers were unconfirmed by medical practitioners. In particular, in ascertaining a history of diabetes, the type of diabetes was not asked. Most of the diabetics were likely to have type II diabetes because of the much lower prevalence of type I diabetes in this age group. Future studies should strive to obtain more detailed information about the nature of foot problems.

In the ascertainment of fractures, a possible bias of using radiographic examinations is their inability to detect some mild fractures and stress fractures. However, bone scans and magnetic resonance imaging were available when radiographic examinations were not informative. In our study, the vast majority of cases received only a radiographic examination to diagnose fractures. Finally, subjects interviewed in person were shown pictures of two of the foot problems, lateral deviation of the toe and hammer toe, while subjects interviewed over the telephone were not. This could bias our results because those subjects who were interviewed in person could give more accurate answers than those interviewed over the telephone. The prevalence of these two foot problems was lower in the people interviewed over the telephone, but the type of interview was not a confounder or effect modifier.

In conclusion, foot fractures are common in people over the age of 45 years, with an incidence of approximately four per 1,000 person-years in women and two per 1,000 person-years in men (16Go). Our findings need to be evaluated in other studies, but it appears that simple preventive measures such as assessment of proper footwear, prescription of orthopedic devices, and education regarding foot hygiene reduce the frequency of foot fractures. Nevertheless, preventing foot problems does not address other intrinsic and extrinsic factors, such as activity levels, that also increase the frequency of falls and one's risk of fractures at other sites such as the distal forearm.


    ACKNOWLEDGMENTS
 
Supported by grants RO1 AR42421 and T32 AR07588 from the National Institute of Arthritis and Musculoskeletal and Skin Diseases.

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.


    NOTES
 
Correspondence to Theresa Keegan, Division of Epidemiology, Stanford University School of Medicine, Room T224, HRP Redwood Building, Stanford, CA 94305-5405 (e-mail: thmccart{at}stanford.edu).


    REFERENCES
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 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication February 19, 2001. Accepted for publication December 4, 2001.