Why is the evidence scant?

It has previously been acknowledged that prospective studies examining stress fractures are difficult to perform given the large populations that must be followed for long time periods, and thus, retrospective studies are often pursued, even though they are unable to establish a causal relationship.18 Additionally, one could argue that the lack of available evidence may be secondary to the complex nature of stress fracture research in the running population. One of the complications in stress fracture research in runners is the interdependence of variables and how individual differences may influence response to confounding factors. Tenforde et al's15 findings, in which some apparently significant risk factors (eg, training volume, body mass, racing performance) became non-significant when multivariate analysis was conducted, serves as a prime example of the interdependent nature of risk factors for stress fractures. While many of these factors were examined in various studies that met the inclusion criteria, they were often not examined synchronously. The individual nature of stress fracture risk, and thus, the need to identify risk factors to prevent them, is also clear when one considers athletes performing nearly identical training regimens within a team. While some athletes remain injury free, others develop soft tissue injuries (eg, tendinopathy), whereas other develop stress fractures at different sites. Finally, there is emerging evidence that various single nucleotide polymorphisms may be associated with stress fractures in athletes,19 however, considerable research is necessary before more definitive conclusions can be made regarding genetic risks for stress fractures in runners.

Bone mineral density and calcium as risk factors

Mechanical loading is associated with increased bone mineral density,20 thus, runners with greater training volumes should, theoretically, have higher bone mineral density. Although ORs could not computed from mean data, bone density in runners with a history of stress fracture was lower than those without stress fractures, yet greater than that of non-athletes.10 This suggests that healthy bone mineral density requirements of active running individuals may be greater than that required by the general public.

This systematic review found conflicting results regarding whether calcium supplementation is associated with increased15 or decreased13 ,14 risk for stress fractures. Though dietary interventions focused on calcium intake are theoretically beneficial to reduce risk of stress fracture in runners, no prospective clinical trial data is available to support this recommendation. A full understanding of dietary risk factors is further complicated by differences in methodology used to assess nutrition, as well as variety and crossover in predictor variables (eg, dairy intake vs calcium intake).

Menstrual dysfunction

Although menstrual dysfunction is commonly cited as a risk factor for stress fractures in athletes, Tenforde et al's15 was the only study identified which described an association between the variables, in that age of menarche and current amenorrhoea were associated with significant increases in risk for stress fracture. Conflicting findings in this realm may relate to fallacies within the traditional ‘female athlete triad’ paradigm.21 ,22 Menstrual dysfunction may be considered simply as one group of signs which occurs secondary to the issue of ‘relative energy deficit in sports (RED-S)’.21 It is possible that female athletes who experience RED-S, and are at elevated risk of stress fracture, have suboptimal menstrual function that is not clinically apparent,21 ,22 and such subclinical issues are not recognised in risk factor studies.

Biomechanical risk factors

Biomechanical factors, including kinetic, kinematic and static alignment variables, are commonly implicated as a risk factor for stress fractures, however, no studies which met the inclusion criteria examined biomechanical factors. One commonly cited study prospectively links biomechanical variables (average and instantaneous vertical loading rate, tibial shock) to stress fractures in competitive female runners, however, it is not a peer-reviewed article, but rather a 2004 research abstract with only five individuals with stress fractures.23 Other studies which have focused on biomechanical ‘predictors’ in runners have chiefly examined biomechanical differences (ie, peak hip adduction, knee rotation, rearfoot eversion angle) in individuals with a previous history of stress fracture,24–27 or have examined biomechanical variables which, theoretically, could lead to stress fracture without prospectively tracking the incidence of injury.28–30 Also noticeable by its absence is the almost complete lack of investigation of the association of biomechanically relevant clinical examination components with stress fracture risk such as arch height, alignment of lower extremity, range of motion, and leg-length inequality.

Military population data should not be translated directly to runners

A wealth of research has been conducted on risk factors associated with stress fractures; however, much of this data comes from studies involving military personnel, with far more sampling populations and considerably larger sample sizes than those specific to runners.4 Studies involving military personnel include smokers and individuals with poor overall physical fitness, both of whom would not be expected in a population of serious runners, yet are significantly associated with stress fractures31 and overall training-related injuries.32 ,33 Military recruits often run <10 miles per week34 and often take 12 weeks to accumulate 40 miles of running,35 whereas runners often perform 40 miles of running at various intensities within 1 week, with some competitive runners exceeding 100 miles per week.36 Additionally, soldiers regularly carry heavy loads37 and use footwear that is not as effective for injury prevention as typically found in running shoes,38 further prohibiting a direct comparison with running athletes. Stress fractures in the military often occur within the first 16 weeks of training,4 but risk factors during this initial acclimation period are not likely applicable to athletes who experience their first stress fracture after years of running. Although military data are commonly extrapolated to the general running population, this is likely inappropriate due to major differences between these populations.

Clinical implications

We found little or conflicting evidence for extrinsic or modifiable risk factors such as training regimen, nutritional status, or biomechanics. Only one study reported on training volume (eg, mileage) associated with increased risk of stress fracture, however, the reported threshold for increased risk was a relatively low volume (32 km/week) in relation to the training regimens many runners undertake.36 ,39 Additionally, running volume itself is not an overall indicator of mechanical stress placed upon the musculoskeletal system. A typical training week may involve a combination of different intensities of running, which place different magnitudes of load on the skeleton. Rapid changes in training volume and intensity are often considered in the development of injuries, though this is difficult to quantify in epidemiological studies.

Thus, future research examining training factors should also consider incorporating a holistic measure, such as training impulse,40 while also seeking to describe the stability of training practices, and reporting injury rates in terms of both time (eg, injuries per week) and volume (eg, injuries per mile). In the absence of methodologically sound high-quality studies which quantify training practices, including volume, intensity and progression, clinicians have little evidence to support the widespread anecdote that higher mileages are associated with increased risk of stress fracture, or the corresponding recommendation that reducing running volume can prevent stress fractures.

Limitations

There is an overall lack of prospective studies focused on stress fractures within the running population, some of which do not include the OR associated with the identified risk factor which is recognised as the proper method for reporting risk factor ratios. Only four independent cohorts made up the eight included studies of this review highlighting the complete lack of research performed in the running population. While three of the cohorts exclusively focused on middle and long distance runners, one cohort included a small sample of track and field athletes from other events.12 Likewise, this systematic review focused on lower extremity stress fractures, but some cohorts included isolated cases of pelvic stress fractures. Studies which examined stress fracture risk in a heterogeneous group of athletes,41 or those which examined general risk factors for all types of running injuries42 were not included, as data reported in these broader studies are not sufficient to determine the risk factors specific to stress fracture in runners.

The available data may be at risk of bias given the considerable variation among the studies with respect to diagnostic criteria (radiograph, nuclear scintigraphy, or MRI), with studies using scintigraphy and MRI potentially reporting a higher incidence. With only a single cohort43 reporting distribution of stress fracture by site and mode of diagnosis, risk factors for stress fractures at specific anatomic sites (eg, fifth metatarsal vs femoral neck) could not be considered, and this may be important if the mechanism of injury varies by location.

In conclusion, our systematic review revealed little evidence for most risk factors for stress fracture within the running population. Previous stress fracture was identified as the greatest risk factor for a stress fracture, however, it is unknown whether this represents new injuries or a previous stress fracture which was not fully healed. Female runners appear to be at greater risk of stress fractures than their male counterparts, and the evidence is inconclusive as to whether this risk may be elevated in females with a history of menstrual dysfunction, low bone mineral density, low calcium intake, and training at higher volumes.

Given the current state of literature, perhaps the most important role for clinicians to reduce the impact of stress fractures are through proper education and time-efficient diagnosis. When patients present to clinicians for pre-participation physicals prior to team sport enrolment or beginning a self-guided training programme, they should be encouraged to beware of the signs of musculoskeletal injuries and to take appropriate measures at the onset of suspected injury.