This retrospective analysis was conducted to analyse the incidence of stress fractures in an elderly cohort. While osteoporosis is certainly one of the leading risk factors for stress fractures, especially in elderly patients, only 24.7 % of the patients within our study cohort actually had a t-score assessed by DXA within the osteoporotic range. Overall, the mean t-score for both the lumbar spine and the femur was within the osteopenic range. These findings may either indicate that stress fracture risk in the elderly already increases in the osteopenic t-score range or that impaired bone microarchitecture leading to stress fracture occurrence cannot be fully assessed according to the BMD of the proximal femur and lumbar spine as measured by DXA.
To gain further insight, we analysed additional risk factors that could contribute to the incidence of stress fractures. Particular attention was given to calcium homeostasis and vitamin D status, both of which are known to be of paramount importance for bone and skeletal health [28
]. For instance, vitamin D insufficiency leads to a lower serum calcium concentration, which in turn results in secondary hyperparathyroidism [33
]. Moreover, the demand for calcium further increases in physically active people, as bone formation is stimulated and microfractures require repair [34
]. As previously shown, vitamin D serum levels of at least 30 ng/ml or 75 nmol/l are considered to be necessary for skeletal health [28
]. However, the majority of elderly patients in our study group as well as the younger controls presented with insufficient vitamin D levels. These observations are consistent with other studies, which found that vitamin D insufficiency significantly increases the risk for stress fractures in young and active patients. In a study on Israeli soldiers, Givon et al. found that vitamin D levels were lower in patients with high grade stress fractures compared to controls [36
]. Ruohola et al. prospectively followed 800 randomly chosen male Finnish recruits and reported vitamin D serum levels below the median of 75.8 nmol/l to be a significant risk factor for the incidence of stress fractures [33
]. Moreover, Lappe and colleagues analysed 5201 female Navy recruits in a randomised, double-blind, placebo-controlled study and were able to show a 20 % reduction in stress fracture incidence in a cohort with daily supplementation of 2,000 mg calcium and 800 IE vitamin D [34
]. A case–control study by Burgi et al. analysed 600 female Navy recruits (mean age 19.5 years) with stress fractures of the tibia or fibula and found a two-fold higher risk of stress fractures of tibia and fibula in women with a vitamin D level of less than 20 ng/ml compared to women with concentrations of over 40 ng/ml [3
]. They concluded that a target for stress fracture prevention would be a vitamin D serum level of 40 ng/ml or greater and underlined the importance of a balanced calcium and vitamin D metabolism in the prevention of stress fractures. Overall, this study on elderly patients from the general population supports the aforementioned findings on young and active people regarding a possible relationship between vitamin D insufficiency and an increased risk of stress fracture.
In addition, the widespread use of PPI among elderly patients has recently been linked to increased fracture risk and disrupted calcium absorption [37
]. Yang and co-workers found an association between long-term PPI therapy and an increased risk of hip fracture, which led us to investigate this issue within our study cohort. Although PPI therapy was reported in 22.5 % of affected patients within the study group, there was no significant difference in use of PPIs in comparison with younger controls. Previous studies further indicated an increased stress fracture risk for patients suffering from anorexia nervosa or other eating disorders such as lactose intolerance that can result in an insufficient calcium supply [1
]. However, in our study group only one elderly woman reported suffering from anorexia nervosa and only four women were lactose intolerant. In addition, anorexia nervosa was observed significantly more often in younger controls, indicating that lactose intolerance as well as anorexia nervosa are not typical risk factors for the occurrence of stress fractures in elderly patients.
We also considered stress fracture locations. Studies on young military recruits and athletes found that the tibia was the most common stress fracture location [24
]. In contrast, in our elderly cohort from the general population the metatarsals were the most frequent stress fracture location. One reason for this observation might be that the physical stress leading to the incidence of stress fractures in elderly patients is different and often lower than that in professional athletes or military personnel. This was reflected by the fact that elderly patients often reported that long hikes or recently started endurance training had led to the stress fracture. On the other hand, 75.5 % of patients in the study cohort were not engaged in regular physical activity more than once a week. Thus, their bone microarchitecture could not adequately adapt to the sudden increased stress level that occurred during long mountain hikes or endurance sports. This is in accordance with Shaffer and colleagues who analysed 152 recruits with stress fractures and identified low aerobic fitness as the only modifiable risk factor for stress fractures within their study [24
]. The authors further pointed out that training intensity should be increased gradually before exposure to larger loads to prevent stress fractures. Although all these studies were performed on young female military recruits, the principle may also apply to elderly patients. Thus elderly patients should be encouraged to increase their physical activity and improve their aerobic fitness. However, the intensity of physical exertion should not be increased too quickly.
Law and colleagues reported an increased risk for hip fracture in postmenopausal smokers over the age of 50 years [39
]. In addition, Lappe et al. analysed 3,758 female recruits and found that after eight weeks of basic training stress fractures were more likely to be reported by recruits with a current or past history of smoking or regular consumption of alcohol [5
]. We also analysed lifestyle risk factors and found nicotine abuse in 46.7 % of male patients within the study group and thus significantly more often compared to female patients. Overall, the incidence of smoking was more than twice that of daily alcohol consumption (26.3 % vs. 12.5 %) among study group patients. Thus our findings indicate that modifiable risk factors such as nicotine or alcohol abuse may also increase the risk of stress fracture incidence in the elderly as reported for young military recruits.
There are some limitations to this study. Firstly, this was a retrospective analysis with no control group of elderly patients without stress fractures was available. However, we included a control group of younger patients with stress fractures to look for possible risk factors, especially those related to increasing age. Second, the sample size in this study was relatively small compared to the large and prospective studies on stress fractures conducted on military recruits and athletes. However, comparably large and prospective studies on stress fracture incidence in the elderly general population are difficult to perform.