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Bisphosphonate Use and Atypical Fractures of the Femoral Shaft
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Jorg Schilcher, M.D., Karl Michaelsson, M.D., Ph.D., and Per Aspenberg, M.D., Ph.D.
From the Department of Experimental and Clinical Medicine, Faculty of Health Science, Linkoping University, Linkoping (J.S., P.A.); and the Department of Surgical Sciences, Section of Orthopedics, and Uppsala Clinical Research Center, Uppsala University, Uppsala (K.M.) - both in Sweden.
N Engl J Med 2011; 364:1728-1737May 5, 2011
ABSTRACT
Background: Studies show conflicting results regarding the possible excess risk of atypical fractures of the femoral shaft associated with bisphosphonate use.
Methods: In Sweden, 12,777 women 55 years of age or older sustained a fracture of the femur in 2008. We reviewed radiographs of 1234 of the 1271 women who had a subtrochanteric or shaft fracture and identified 59 patients with atypical fractures. Data on medications and coexisting conditions were obtained from national registries. The relative and absolute risk of atypical fractures associated with bisphosphonate use was estimated by means of a nationwide cohort analysis. The 59 case patients were also compared with 263 control patients who had ordinary subtrochanteric or shaft fractures.
Results: The age-adjusted relative risk of atypical fracture was 47.3 (95% confidence interval [CI], 25.6 to 87.3) in the cohort analysis. The increase in absolute risk was 5 cases per 10,000 patient-years (95% CI, 4 to 7). A total of 78% of the case patients and 10% of the controls had received bisphosphonates, corresponding to a multivariable-adjusted odds ratio of 33.3 (95% CI, 14.3 to 77.8). The risk was independent of coexisting conditions and of concurrent use of other drugs with known effects on bone. The duration of use influenced the risk (odds ratio per 100 daily doses, 1.3; 95% CI, 1.1 to 1.6). After drug withdrawal, the risk diminished by 70% per year since the last use (odds ratio, 0.28; 95% CI, 0.21 to 0.38).
Conclusions: These population-based nationwide analyses may be reassuring for patients who receive bisphosphonates. Although there was a high prevalence of current bisphosphonate use among patients with atypical fractures, the absolute risk was small. (Funded by the Swedish Research Council.)
Background
Bisphosphonates reduce the overall risk of fracture among patients with osteoporosis, with a long-lasting beneficial effect.1 However, since bisphosphonates reduce bone remodeling, they might "freeze" the skeleton, allowing accumulation of microcracks over time, leading to fatigue fractures (also called stress fractures).2 Fatigue fractures are well known in mechanical engineering, and they occur with age and overload in many materials. They result from the slow propagation of cracks, resulting in a peculiar appearance. In bone, their appearance is characterized by a straight fracture line running perpendicularly to tractional forces. Such fractures are not uncommon in athletes, and if they are not displaced, they heal slowly with minor external bone formation, often appearing as cortical thickening.
Stress fractures in compact bone occur at sites with high tensional stress, such as the lateral cortex of the proximal femoral shaft; this site corresponds to the locations of reported bisphosphonate-associated atypical fractures. 2-11 In published case reports that classified femoral fractures as stress fractures or nonstress fractures according to their radiographic appearance, among patients admitted to tertiary centers, such stress fractures (commonly called atypical femoral fractures, the term used here) were more common in patients who received bisphosphonates than in those who did not.3,7 A large proportion of patients who were receiving bisphosphonate treatment and who had atypical fractures did, however, also receive other medications, especially systemic glucocorticoids and proton-pump inhibitors. This finding has prompted the suggestion that these two types of drugs, in addition to being markers of coexisting conditions, are important contributors to the risk of the development of atypical femoral fractures.9,10
In contrast, an association between atypical fractures and bisphosphonate use was not shown in nationwide cohort analyses based on registry identification of "atypical" femoral fractures or in randomized trials in which the classification of fractures was based on radiologic reports.12-14 It might be that either the case series were biased or the registry studies and the randomized studies did not identify a sufficient number of atypical fractures accurately. We reviewed radiographs of all femoral subtrochanteric and shaft fractures that occurred in 2008 in the entire population of Sweden. Individual linkage of these fracture cases to national registries allowed both a nationwide cohort analysis and a population-based case-control study to elucidate the relation between atypical femoral fractures and bisphosphonate use.
Methods
Study Population
In 2008, according to the National Swedish Patient Register, 12,777 women 55 years of age or older sustained a fracture of the femur (Figure 1Figure 1Identification of Stress (Atypical) Fractures in the Study Population.). Of these women, 1271 had a femoral subtrochanteric or shaft fracture (International Classification of Diseases, 10th Revision [ICD-10] diagnosis code S722 or S723 with external-cause code W [i.e., excluding any type of transportation accident]). Digitized radiographs from 1234 patients were obtained from the involved hospitals. The remaining 37 case patients were excluded because of an insufficient quality of the radiographs (30 patients) or because the radiographs were not available (7 patients). The study was approved by the regional ethics committee, and patients were enrolled without consent being obtained.
The 1234 radiographs were reviewed and classified according to the pattern of fractures, as described below. After the fractures were classified, data on drug use and inpatient and outpatient care were obtained from registries of the Swedish National Board of Health and Welfare. The Swedish Prescribed Drug Register contains data on all prescriptions dispensed to the entire Swedish population (approximately 9 million inhabitants) from July 2005 onward. The Swedish National Patient Register contains discharge diagnoses for hospitalizations in Sweden, with complete coverage since 1987.15 This registry also contains data on outpatient visits since 2001. Complete linkage between the registries is possible through the use of the personal identification number provided to all Swedish citizens.
Classification of Fractures
Patients who were identified in the National Swedish Patient Register as having subtrochanteric or shaft fractures were categorized into four major groups. The first group included 47 patients with typical stress fractures that were transverse on the lateral side without intermediate fragments, together with thickening of the lateral cortex at the fracture site. The fracture did not involve the trochanteric or the condylar area. The femur did not have implants, and the fracture was not pathologic. The second group included 12 patients with suspected stress fractures. These fractures were the same as those described in the first group, but without clear thickening of the lateral cortex or with a separate intermediate fracture fragment. The third group included 263 control patients with fractures that did not appear to be stress fractures but were relevant for comparison with stress fractures. None of the shaft fractures were transverse on the lateral side, and there was no trochanteric or condylar engagement, except for minor trochanteric fracture extensions. There were no implants or pathologic fractures. The fourth group included 912 patients with fractures that were not relevant for comparison with stress fractures. Of these fractures, 625 were associated with major involvement of a trochanter or distal condyle, 238 were associated with implants, and 16 were pathologic. This last group also included 24 femoral-neck fractures and 9 fractures that were not femoral.
During the revision of this article, a consensus document was published in which stress fractures without cortical thickening were defined as "atypical."16 We therefore analyzed the first two groups together (Figure 1).
After completion of our analysis, we reviewed all the routine radiologic reports. These reports mentioned a transverse fracture in 23 of the 59 case patients and a fatigue (also called stress or insufficiency) fracture or an atypical fracture pattern in only 1 case patient. The remainder mentioned only the location of fractures.
Before the retrieval of registry information, 72 of the 1234 fractures (including 22 atypical fractures) were randomly selected for reclassification by one of the authors who was not informed about the previous categorization or the number of atypical fractures in the sample. There was complete agreement between the two classifications.
Results
Cohort Study
Of all 12,777 women with a fracture of the femur, we identified 59 with atypical fractures (Figure 1). The incidence proportion of atypical fracture among users and nonusers of bisphosphonates in the entire Swedish population is shown in Table 1Table 1Risk of Atypical Femoral Fracture Associated with Bisphosphonate Use during the 3 Years (2005-2008) Preceding the Fracture.. The age-adjusted relative risk of atypical fracture with any use of bisphosphonates was 47.3 (95% confidence interval [CI], 25.6 to 87.3). The difference in the risk of atypical fracture between users and nonusers of bisphosphonates was 5 cases (95% CI, 4 to 7) per 10,000 patient-years, corresponding to an average number needed to harm (i.e., the number of bisphosphonate users needed for one case of fracture to occur) of 2000 per year of use. The risk was higher with a longer duration of use. Most bisphosphonate users with atypical fractures had a history of recent use.
Bisphosphonate users also had a higher relative risk of registry-identified fractures of the hip (relative risk, 1.32; 95% CI, 1.25 to 1.40), the subtrochanteric region alone (relative risk, 1.80; 95% CI, 1.50 to 2.17), and the femoral shaft (relative risk, 3.83; 95% CI, 3.08 to 4.78) as compared with persons who did not receive bisphosphonates (Table 2Table 2Risk of Fracture Associated with the Use of Bisphosphonates.). After exclusion of irrelevant fractures and atypical fractures in the subtrochanteric and shaft regions by our review of radiographs, the relative risk of other types of fractures (i.e., in the controls in our case-control study) was 1.27 (95% CI, 0.85 to 1.90) - an estimate similar to that for hip fractures (Table 2).
Case-Control Study
The characteristics of the study participants are listed in Table 3Table 3Characteristics of the Case Patients and Controls.. Case patients with atypical fractures were younger than control patients. Case patients were also less frequent users of antidepressants and were less likely to have sustained a previous osteoporotic or hip fracture, even after adjustment for the age difference. A greater proportion of the bisphosphonate users (both cases and controls) had received systemic glucocorticoids and had a diagnosis of osteoporosis or inflammatory joint disease.
Of the case patients, 78% had received bisphosphonates (81% of the women with stress fractures and 67% of the women with suspected stress fractures), as compared with 10% of the controls (Table 4Table 4Odds Ratios for Atypical Femoral Fractures Associated with Bisphosphonate Use.). This finding corresponds to a multivariable-adjusted odds ratio of 33.3 (95% CI, 14.3 to 77.8), with a similar estimate for women who had received alendronate and risedronate. No atypical fractures were associated with etidronate, but few women received this bisphosphonate. At the time of the latest prescription, bisphosphonates were predominantly taken once a week (in 83% of the women) or daily. None of the patients received injections.
The risk of an atypical fracture was higher with an increasing duration of bisphosphonate use, with an odds ratio of 1.3 (95% CI, 1.1 to 1.6) per 100 prescribed daily doses. This risk was approximately 10 times as high as a normal level of risk within the first 2 years of use and 50 times as high thereafter (Table 4). The data did not suggest that the risk continued to increase with a longer duration of treatment. In Sweden, patients renew their prescriptions every 3 months. Therefore, in an additional analysis, we excluded women who received prescriptions for bisphosphonates during the first 3 months after the initiation of the Swedish Prescribed Drug Register in order to specifically evaluate the association in women who had drug exposure for 2.0 to 2.9 years. Of these women, 14 of 17 were case patients with atypical fractures, corresponding to a multivariable-adjusted odds ratio of 61.7 (95% CI, 14.0 to 272.6).
Most atypical fractures associated with bisphosphonate use occurred within 1 year after the last prescription. There was a 70% reduction in risk for every year since the last use (multivariable-adjusted odds ratio, 0.28; 95% CI, 0.21 to 0.38). The risk reduction was similar if the bisphosphonates had been prescribed for less than 2 years (odds ratio, 0.45; 95% CI, 0.26 to 0.76) or for 2 years or more (odds ratio, 0.26; 95% CI, 0.19 to 0.35).
The association between the risk of atypical fracture and bisphosphonate use was not increased among users of glucocorticoids or proton-pump inhibitors as compared with nonusers (see the Table in the Supplementary Appendix, available with the full text of this article at NEJM.org). The increase in risk associated with bisphosphonate use was similar for women younger than 85 years and those 85 years of age or older.
Discussion
These population-based nationwide analyses should be reassuring for bisphosphonate users. Although there was a high prevalence of current bisphosphonate use among patients with atypical fractures, the magnitude of the absolute risk was small. With a correct indication, the benefits of fracture prevention with bisphosphonate use will greatly outweigh the risk of atypical femoral fracture (i.e., the numbers needed to treat14 will be lower than the numbers needed to harm).
The risk appeared to be unrelated to the use of systemic glucocorticoids and other drugs with effects on bone and was independent of coexisting conditions and age. Previous nationwide population-based studies and randomized trials have shown only modest site-specific increases in the risk of femoral fractures with bisphosphonate use.12-14 The use of registry classification alone enabled us to confirm these results.12,13 The increased risks of femur fracture, irrespective of fracture location, that were associated with bisphosphonate use are probably the consequence of osteoporosis.12 The relative risk with bisphosphonate use among our controls who had radiologically defined fractures of the femoral shaft was similar to the risk of hip fractures, indicating a similar and somewhat increased base risk of femur fracture in this fracture-prone subpopulation. Earlier studies relied on registry data or hospital records, whereas the present investigation used reclassification of fractures according to radiographs. The specific radiographic classification is important, since our analysis shows that the rare atypical femoral fracture will be overshadowed by other types of fractures in registry studies, impeding the detection of their association with bisphosphonates.
Our results appear to contrast with those of a recent reanalysis of three randomized trials14 in which 10 patients with fractures that were located at the femoral shaft or subtrochanteric areas were identified from radiologists' reports. No relation to bisphosphonates was shown.14 The specific type of fracture was not reported, and our data suggest that in most cases, routine radiologic assessment does not detect the atypical fracture pattern. If identified, the atypical fractures in these 10 women would probably have been few, and the results are therefore not incongruent with our data.
In our study, the women who sustained atypical fractures did not appear to be especially frail. They had a lower frequency of previous osteoporotic fractures than the controls. It is possible that for the women with bisphosphonate-associated atypical fractures, the drug had not been prescribed for a proper indication. However, more than one third of all patients receiving bisphosphonates in this study were receiving systemic glucocorticoids, which is a correct indication for prophylactic use of bisphosphonates.
We19 and other investigators3 have previously presented data similar to the current results. The present study adds to the previous literature with a larger number of cases, estimates of relative and absolute risk, and strictly population-based comparisons both with patients with other types of fractures and with the general population. We also conducted analyses of the type of bisphosphonate, the duration of treatment, and how recently the drug was used, and in the analyses we considered the concomitant use of other drugs and coexisting conditions. It has been proposed that glucocorticoids and proton-pump inhibitors are likely to contribute to the risk of atypical fractures,3,9 but our data suggest that this is not the case.
The risk of atypical fractures decreased more rapidly after drug withdrawal than would be expected, given the prolonged presence of the drug in the bone. This observation and the increased risk during the first year of treatment are difficult to reconcile with the etiologic hypothesis that these fractures are a consequence of increased age of the cortical bone because of reduced remodeling. Thus, the pathogenic role of bisphosphonates probably involves faster processes. Bisphosphonates accumulate on fracture surfaces immediately after dosing. Ongoing treatment thus makes these surfaces highly resistant to resorption, whereas previous treatment does not. It is therefore conceivable that ongoing bisphosphonate treatment blocks targeted remodeling of cracks, which can grow and cause a stress fracture. This explanation, which implies that the long-lasting protective effect after bisphosphonate withdrawal1 is not associated with a similarly lasting risk of atypical fracture, argues in favor of intermittent use.16
Our study has several limitations. First, the observational design precludes definite causal interpretation of the results. Nonetheless, our study design precluded any bias associated with differential recall of bisphosphonate use and covariate information. Second, drug use before 2005 was not measured. We were therefore unable to estimate the risk pattern with a longer duration of use. Third, results adjusted for coexisting conditions and the use of other drugs were based on the case-control analysis only. Fourth, the study was performed in women and in a Northern European country, limiting the generalizability to men and other ethnic groups. Finally, no data were available to evaluate whether the risk of atypical fracture was dependent on bone density.
We conclude that the absolute risk of atypical fracture associated with bisphosphonates for the individual patient with a high risk of osteoporotic fractures is small as compared with the beneficial effects of the drug.
Dr. Aspenberg reports receiving consulting fees from Eli Lilly and Amgen and grant support to his institution, Linkoping University, from Eli Lilly and Amgen, as well as holding stock in AddBIO, a company trying to commercialize a method for bisphosphonate coating of implants to be inserted in bone, and holding a patent for this method. No other potential conflict of interest relevant to this article was reported.
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