© 2005 by the American Diabetes Association, Inc.
Elevated Hip Fracture Risk in Type 1 Diabetic PatientsA Population-Based Cohort Study in Sweden
1 Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden Address correspondence and reprint requests to Weimin Ye, MD, PhD, Assistant Professor, Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Box 281, SE 171 77, Stockholm, Sweden. E-mail: weimin.ye{at}meb.ki.se
OBJECTIVEPatients with type 1 diabetes often have low bone mineral density, but epidemiological data on fracture risk are sparse and imprecise, particularly for men. RESEARCH DESIGN AND METHODSIn the Swedish Inpatient Register, we identified a population-based cohort of 24,605 patients (12,551 men and 12,054 women) who were hospitalized for diabetes before age 31 years during 1975 through 1998. Follow-up for hip fracture was accomplished through cross-linkage in the Inpatient Register until the end of 1998. Censoring information was obtained from the registers of Death and Migration. Using the Kaplan-Meier method, we calculated the cumulative probability of getting a hip fracture. Standardized hospitalization ratios and their 95% CIs estimated relative risks with the age-, sex-, and calendar periodmatched Swedish general population as reference. RESULTSIn total, 70 and 51 first hip fractures were ascertained in men and women, respectively, corresponding to a cumulative probability (both sexes) of 65.8/1,000 until age 65 years. Markedly elevated risks were observed in both men and women (standardized hospitalization ratios = 7.6 [95% CI 5.99.6] and 9.8 [7.312.9], respectively), increasing with follow-up time. Ophthalmic, nephropathic, neurological, and cardiovascular complications were indicators of particularly high risks. CONCLUSIONSBoth male and female type 1 diabetic patients are at increased risk for hip fracture. Although optimal preventive measures still need to be defined, the co-occurrence with other diabetes complications suggests that tighter metabolic control might reduce the risk.
Abbreviations: SHR, standardized hospitalization ratio
Both male and female patients with type 1 diabetes are reported to have low bone mineral density (14), one important risk factor for hip fracture (5). Osteopenia may be obvious already in the post-teenage years (6). However, epidemiological studies on the risk of hip fracture among type 1 diabetic patients have usually been of limited sample size, and their results are conflicting. An increased hip fracture risk among postmenopausal women with type 1 diabetes was reported in some (7,8), but not all (9,10), previous investigations. The sparse data that exist regarding men with type 1 diabetes have been unable to confirm any significant excess risks (8,10). The incidence of type 1 diabetes appears to be increasing worldwide (11). With tight metabolic control, drug treatment of hyperlipidemia, and lifestyle modifications, life expectancy among patients with type 1 diabetes is gradually increasing. Accordingly, the prevalence of type 1 diabetes is growing, and many more patients will survive long enough to develop hip fracture. Hip fracture is associated with considerable morbidity and long-term mortality (12), posing a major and growing burden on health care. To quantify the cumulative and relative risk of hip fracture in both male and female type 1 diabetic patients, we performed a nationwide population-based retrospective cohort study in Sweden.
This study was approved by the regional ethics committee at the Karolinska Institutet.
Registers
Type 1 diabetes cohort We initially identified 25,221 records with a discharge diagnosis of type 1 diabetes. These records were further linked to the Register of Total Population, the Register of Domestic and Foreign Migration, and the Death Register. These linkages resulted in the exclusion of 171 records with national registration numbers that could not be found in any of the registers, i.e., without a link to any currently or previously existing person. Further excluded were 376 patients with date of emigration before the index hospitalization, 43 patients who died on the index hospitalization, and 26 patients who already had hip fractures before or on the index hospitalization. Hence, our final type 1 diabetes cohort included 24,605 patients.
Follow-up
Statistical analysis
Additional analyses were stratified according to follow-up duration, and a
To isolate the independent effects of explanatory variables, we estimated relative effects on the standardized hospitalization ratios using a multivariate Poisson regression method with the expected number as the offset, assuming multiplicative effects between outcome and explanatory variables. The Pearsons
Both mean and median age at entry was 20.7 years. The cohort members were followed for an average of 9.9 years, yielding 242,428 accumulated person-years at risk. Of this person-time, 30,377 person-years occurred during ages 40 through 49, and 5,221 above the age of 50. The highest attained age during follow-up in the cohort was 64.5 years. During follow-up, a total of 121 incident cases of hip fracture were recorded (51 among women, 70 among men). Anatomic site of the fracture was the femoral neck in 64 (53%) and the pertrochanteric area in 57 (47%) patients. The mean age at diagnosis of first hip fractures was 43.1 years for women and 41.3 years for men (Table 1).
Figure 1 shows Kaplan-Meier estimates of the cumulative risk of hip fracture among patients with type 1 diabetes stratified by sex. The incidence of hip fractures before age 30 was almost negligible but increased quickly thereafter. The cumulative probability of getting a hip fracture was similar in both sexes before age 40, but thereafter, it increased more rapidly among men. At the age of 65, the cumulative probability for both sexes combined was 6.6%.
Among men, the SHR estimating relative risk for hip fracture relative to the age-, sex-, and calendar periodmatched general population was 7.6 (95% CI 5.99.6). The excess risks were evident across different durations of follow-up but without a clear trend. An analysis stratified by attained age showed much higher relative risk for hip fracture among men who had reached age 40 years or older than among those who were younger. Analogously, men born before 1950 had a much higher excess risk than those born in 1950 or later. Men ever hospitalized with microvascular complications, such as ophthalmic or nephropathic complications, neuropathy, or cardiovascular complications, had a substantially higher relative risk for hip fracture as a whole or divided into subtypes than men without documented diabetes complications (Table 2).
An increased relative risk for hip fractures was also observed among women with type 1 diabetes (SHR 9.8 [95% CI 7.312.9]). The excess increased with follow-up duration (P = 0.02 for trend). Similar to men with type 1 diabetes, women had much higher relative risk for hip fracture when they were >40 years old at follow-up or born before 1950. Remarkably greater relative risk elevations were also observed among women with type 1 diabetes ever hospitalized with microvascular, neurological, or cardiovascular complications than among women with no records of such complications (Table 3).
Multivariate analysis There was no obvious difference of SHRs for hip fracture between men and women, either in the univariate or multivariate analysis. After controlling for effects of the other explanatory variables, SHRs over 10 years of follow-up were close to that with follow-up <5 years. Type 1 diabetic patients had an over fourfold relative risk when their attained ages at follow-up were 40 years, whereas no obvious difference of SHRs was noted between patients born in 1950 or later and those born before 1950. Patients with diabetes complications had generally higher relative risks, particularly among those with nephropathic or neuropathic complications (2.0 [95%CI 1.03.8] and 2.4 [1.34.6], respectively) (Table 4). The Pearsons 2 statistic for the multivariate model was 500.5 with 330 degrees of freedom, which indicated modest overdispersion of the model. A scale parameter, the square root of the Pearsons 2 divided by the degrees of freedom, was thus used to correct standard errors.
We observed unequivocally increased hip fracture risks among both men and women who had been hospitalized for type 1 diabetes. Presence of diabetic microvascular (ophthalmic or nephropathic), neurological, or cardiovascular complications indicated excess risks that ranged from 17- to 42-fold. The putative mechanism nearest at hand is an impaired bone quality due to the lower bone mineral density observed among type 1 diabetic patients (13). Moreover, a link between microvascular complications of type 1 diabetes and long-term bone loss has been reported (15), notably between neuropathy and decreased bone mineral density of the femoral neck (16). Another mechanism for the increased risk could be diabetes-related nonskeletal risk factors, such as a propensity for falls (17), conceivably mediated through impaired proprioception, balance, and gait due to neuropathy, visual impairment from diabetic retinopathy and cataracts (18), or frequent nocturia (19).
To date, there has been remarkably little data about the influence of diabetes on hip fracture risk among men. With 12,551 type 1 diabetic men, our cohort provided a unique chance to explore this association. The observed number of first hip fractures in our cohort exceeds that in all previous studies including men. Other strengths of our study include the cohort design, the population-based sample, the use of medical information rather than reliance on self-reports of exposure and outcome, and the virtually complete follow-up in view of the essentially obligatory need for hospital care after hip fracture and the absence of hospitals not covered by the Inpatient Register. Despite the many advantages of our study, we should point out its limitations. First, information on potential confounding factors such as weight change, smoking, and body mass/stature was unavailable for adjustment. It is, however, unlikely that any confounding from these risk factors could produce relative risk elevations of the magnitude observed in our study. Second, our study relied solely on age ( There is no simple answer to the question of whether our restriction to hospitalized patients may have affected the observed relationship. Because of the often dramatic onset and the need for careful evaluation, we believe that most pediatric patients with type 1 diabetes during the studied period were hospitalized at least once. Thus, no important selection bias is expected. On the other hand, our cohort included a relatively large proportion of patients who were around 20 years of age or older at index hospitalization. Many of these patients may have had an earlier diabetes onset with an initial hospitalization that occurred before inpatient registration was started in their respective counties of residence. These patients might have been rehospitalized because of an atypically severe course of the disease, and the possible concentration of such patients in the cohort may have somewhat inflated the observed risks. Although causality cannot be proven in this observational study, the strength of the association, the support provided from some, albeit not all, previous studies, the dose-response relationship reflected by the increasing relative risk with follow-up time and severity of the disease, and the biological plausibility based on bone density measurements in diabetic patients make us inclined to conclude that the observed association is likely to be causal. We were unable to study the risk of other fractures simply because few other fractures lead to obligatory hospital admissions. If there is a real choice between in- and outpatient treatment (the latter is not recorded in the Swedish Inpatient Register), the concomitant presence of diabetes might shift the balance toward hospital admission, leading to ascertainment bias. Prospective studies of the risk of all kinds of fractures are, however, clearly warranted, and we hypothesize that the susceptibility for fracture is not only confined to the hip. Given that as many as 1 in 15 patients with type 1 diabetes may sustain a hip fracture before the age of 65, development of methods for primary prevention should be put high on the agenda. Although candidate treatments should ideally be evaluated in randomized intervention trials, our data provide some hints regarding the importance of tight metabolic control; the covariation with other diabetes complications and the considerably higher relative risk among patients born before 1950, who may have had a considerable part of their disease trajectory before the importance of meticulous metabolic control was clearly demonstrated, suggest that such control may also be a cornerstone in the prevention of diabetic hip fractures.
This study was partly funded by grants from K.I. Fonder (to W.Y. and O.N.), the Family S. Persson Foundation, and the Swedish Diabetes Foundation (to K.B.).
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. Received for publication June 2, 2005. Accepted for publication September 11, 2005.
Find additional patient-related information at:
This article has been cited by other articles:
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||