Wednesday, May 23, 2012

Injury Series: Tibial stress fractures and stress reactions: The role of bone structure, impact, and calf strength

Tibial stress fractures are related to medial tibial stress syndrome, which you can read about here

This installment of the Injury Series deals with tibial stress fractures, one of the most serious of the common running injuries.  A stress fracture, or “hairline fracture,” is a small crack in a bone that develops due to repeated stress on the bone, usually from weight-bearing activity.  Typically, a stress fracture develops gradually, as high stresses progressively overcome the bone’s ability to heal itself.  Blaming stress fractures just on high stresses is misleading, though.  In a healthy athlete, exposure to high loading and impact strengthens bones, as they remodel according to the stresses they experience in daily life.  So, all else being equal, a weight lifter or construction worker would have much stronger bones than someone confined to bed rest. 

A stress fracture is a quite serious injury, and virtually all cases will require 6-8 weeks or more away from running, often with some of that time spent in a “boot” or walking cast.  Recovery is predicated on the body’s ability to heal the crack in the bone, and unfortunately it is a slow process which cannot be hurried along.  How soon you’ll be able to return to running depends on the severity of your fracture, which can only be determined by advanced medical imaging.  As such, this article is more focused on preventing tibial stress fractures in the first place and understanding why they occur, since there will be no quick and easy exercise protocol to recover from a stress fracture in a few weeks.  As it turns out, the dominant factors in the development of tibial stress fracture seem to be related to “intrinsic” factors like bone structure, bone composition, and biomechanics, versus “extrinsic” factors like mileage, running surface, and so on.

Anatomy

The tibia is your “shinbone,” the long, thick bone that forms the front of your lower leg.  To date, most researchers believe that tibial stress fractures are caused by repeated bending of the tibia.  Much like a thin steel rod under a compressive load, the tibia bends slightly on impact with the ground.  Repeated loading of the tibia stimulates the body to strengthen the bone, but the bone matrix must be resorbed first, which puts the bone in a vulnerable position, since if the stress from running outpaces the body’s ability to rebuild the bone, the end result will be a stress fracture.


According to Popp et al. and Bennell et al., damage to the tibia occurs due to bending in the sagittal plane.


Tibial stress fractures may occur anywhere along the tibia, but they are particularly common between one-third and two-thirds of the way down from the knee, as most peoples’ tibia is at its narrowest at this point.  It was this observation that led to the conclusion that tibial bending is to blame for stress fractures.  Any type of column, whether it’s a bone in your body or a steel girder in a skyscraper, is most susceptible to bending where its cross-sectional area is thinnest.  If we examined a cross-section of the tibia, we’d see a thick outer “shell” surrounding the bone marrow in the middle.  The thick shell is the cortical part of the bone, which provides most of the mechanical strength.  It’s possible for two bones of the same external dimensions to have different cortical thicknesses, and hence different resistance to stress.

CT Scan cross section of a healthy tibia. 
Symptoms and diagnosis

A tibial stress fracture usually manifests gradually.  A runner will initially feel a dull pain on his or her shin that hurts at the beginning and end of runs, calming somewhat during the middle.  Over the course of a few days or weeks, the pain will localize to a small area less than two inches in size.  Often, it will hurt to press directly on this area of the shin, and hopping on one foot will hurt as well.  If the runner continues to train, he or she will begin to experience pain with walking, and sometimes an aching sensation even at rest.  While runners who reach this stage are generally in too much pain to run, continuing to put stress on the bone at this stage can result in a true bone fracture, which can put a stop to your running for a very long time.  Sharp, localized shin pain associated with running that does not go away after a few days’ rest always warrants a trip to the doctor’s office. 

While most orthopedic doctors will conduct an X-ray if they suspect a tibial stress fracture, it is not a particularly useful or accurate diagnostic tool when it comes to tibial injuries.  Instead, the best methods of diagnosis are via MRI or bone scintigraphy (also known as a “bone scan”).  MRI exams are probably familiar to most readers, but bone scans are less well-known.  In a bone scan, you are given an intravenous injection with a small amount of radioactive material—usually technetium-99.  99Tc is not radioactive enough to pose a health risk, but does emit enough radiation to be detected by sensitive instruments.  When a damaged bone is attempting to heal, it is absorbing significantly more nutrients from the blood than the surrounding tissue, and over the course of a few hours, the 99Tc is absorbed as well.   A bone scan shows any abnormal areas of uptake in the skeleton, and can fairly accurately diagnose a stress fracture.1 

Bone scans (right) can reliably diagnose stress fractures, but X-rays (left) cannot. From Iwamoto et al.
One problem with bone scans, and to a lesser extent, MRIs, is that healthy bone remodeling shows characteristics similar to that of a healing stress fracture. While findings like reduced cortical density and localized reduction in bone density correlate very well with tibial stress injuries, up to 50% of healthy runners have “abnormal” findings in their tibias upon examination with bone scintigraphy, MRI, or CT imaging. 2, 3  Even over a year later, these healthy runners did not develop shin pain.  Ironically, while this is not good news in the diagnostics department, it actually helps further our understanding of why stress fractures develop.  Regardless, MRI is the preferred method of diagnosing tibial stress fractures; bone scans are also sufficiently accurate but offer less insight into the severity of the fracture.  An experienced orthopedist or radiologist can use a grading system to estimate how soon you can return to running.
Tibial stress fracture on an MRI.  From Kijowski et al.

Tibial stress fractures need to be distinguished from medial tibial stress syndrome (MTSS), or “shin splints,” which is another common running injury (and topic for another day).  For a long time, researchers suspected that MTSS was caused by muscles pulling on the tibia, irritating the periosteum, a “skin” of tissue that shrouds the tibia, and that tibial stress fractures were a different injury entirely.  However, anatomic studies have found that muscle insertion points do not correlate with the areas where pain is most commonly felt in runners with shin splints, and microscopic examination of samples of the periosteum do not show signs of inflammation in runners with shin splints.  Right now, the prevailing opinion in the research community is that medial tibial stress syndrome, tibial “stress reactions,” and true tibial stress fractures exist on a spectrum of severity.  That is, all cases of shin pain in runners are related to stress on the tibia from abnormal bending.  In an extensive 2009 review, Moen et al. present four pieces of evidence for this claim:1

Recently bony overload of the medial tibia has been shown to be important as the underlying problem. There are four important findings that support the theory that bony overload forms the primary pathophysiological basis for MTSS: (i) on triple-phase bone scans the last phase is abnormal, showing that the bone and periosteum are involved (ii) on high-resolution CT scan the tibial cortex is found to be osteopenic, as can be seen in patients as well as in asymptomatic athletes as a sign of bone remodelling (iii) on MRI images, bone marrow oedema as well as a signal along the periosteum can be seen (iv) in patients with MTSS,  bone mineral density is reduced compared with controls, and when symptoms improve, the bone density returns to normal values.

This conclusion is enlightening, as it tells us that the causative factors for shin splints are likely to be the same as the ones for tibial stress fractures.  However, it brings up some new questions.  Why do some runners have shin splints for weeks or even months, without ever developing a stress fracture? Taking a look at who gets tibial stress fractures might provide some answers.

CT tibial cross sections of A) a healthy runner B) a healthy runner showing normal signs of bone resorption/growth (dots) C) a runner with medial tibial stress syndrome and marked areas of low bone density and resorption (arrows) D) a runner with MTSS and a larger area of osteopenia/low bone density and bone resorption associated with overloading of the tibia (arrows).  None of these runners had yet developed a stress fracture. Adapted from Gaeta et al. and Gaeta et al.

Epidemiology—who gets it and why

The tibia is the most-often injured bone in the runner’s body.  One study reviewing ten years’ worth of athletes at the University of Minnesota found that a total of 3.2% of all track and cross country runners suffered a stress fracture during their college career—tibial stress fractures being the most common.4  An Australian study found that 21% of elite track and field athletes (of all events) suffer a stress fracture in a given year, again with most occurring in the tibia.5  Women are two to three times more likely to suffer a tibial stress fracture.  While extrinsic factors like mileage and running surface may play a role in the development of stress fractures in individual runners, multiple studies indicate that on the whole, runners who develop tibial stress fractures aren’t running more mileage or on different surfaces than runners who stay healthy.6, 5  This indicates that we probably need to look inward for the source of the injury, examining intrinsic risk factors.  As we’ve seen before on this blog, an intrinsic risk factor is some inherent property about your body—height, weight, and muscular strength are all examples of intrinsic factors.  And, obviously, some of these can be changed and some cannot. 

Evidence from prospective studies

There are very few well-designed prospective studies that examine tibial stress fractures.  This is likely because of the relative rarity of the injury, making it difficult to gather a sample size large enough to find significant differences in runners who go on to  develop a tibial stress fracture and those who do not.  Most prospective studies have looked at tibial stress fractures in military recruits, but we need to be careful when applying these results to runners: military recruits usually undergo a huge jump in training at the start of boot camp, and much of their activity is marching in boots, not running in trainers. 

Among the possible risk factors identified in military studies are low initial physical fitness (being out of shape), menstrual irregularity in women,7 lighter weight, and a generally slighter (leaner) build, as measured by various “girths”: the circumference of the thigh or calf, for example.8 Bone characteristics also play a significant role.  A pair of studies by Beck et al. which compared bone geometry and composition of a large group of male and female Marine Corps recruits concluded:9

In both genders, fracture cases were less physically fit, and had smaller thigh muscles compared with controls.  [...]  Female cases had thinner cortices and lower areal bone mineral density (BMD), whereas male cases had externally narrower bones but similar cortical thicknesses and areal BMDs compared with controls. In both genders, differences in fitness, muscle, and bone parameters suggest poor skeletal adaptation in fracture cases due to inadequate physical conditioning prior to training.

So, while bone composition and geometry seems to play a role in both male and female military recruits, the specifics differ on some important points. 

Prospective studies among runners are rare, although there is one impeccable study published in 1996 by Kim Bennell et al. at the University of Melbourne in Australia.5  Bennell et al. examined 111 national-caliber Australian track and field athletes, then followed them for a year, documenting who suffered a stress fracture and who stayed healthy.  Perhaps because these athletes were at such a high level of competition, a remarkable 21% suffered a stress fracture during the study.  Among females, a lower bone density, a history of menstrual irregularity, smaller calf muscles, a leg length discrepancy, and a low-fat diet increased the risk of a stress fracture.  In fact, by combining the age of menarche (the age at which a female first got her period) and the circumference of the calf, the researchers were able to predict with 80% accuracy who would suffer a stress fracture.  All else being equal, for every 1cm decrease in calf circumference, the risk of stress fracture in women increases fourfold.  Unfortunately, none of these factors proved helpful in predicting stress fractures in men, although this perhaps should not be surprising: remember, the pair of studies by Beck et al. blamed bone density for causing stress fractures in females, but bone geometry in males. Bennell et al. did not examine bone dimensions.

Evidence from retrospective studies

Having exhausted the relatively limited pool of prospective studies on tibial stress fractures, we now turn to retrospective studies.  If you’ve read any of the previous articles in the injury series, you’ll know that a retrospective study examines runners who have already become injured.  While confusing correlation and causation is already an issue in any retrospective study, it’s particularly thorny in the case of tibial stress fractures.  Take, for example, calf girth.  If a runner gets a bad tibial stress fracture, she might spend 3-4 weeks in a walking cast.  This would certainly reduce the size of her calf muscles! Regardless, there have still been some interesting findings from retrospective studies.

Muscular strength

In previous Injury Series articles, we’ve seen how inadequate hip muscle strength can increase forces through the knee and IT band.  Surprisingly, there is only one study that has investigated any marker of muscular strength as it relates to tibial injuries.  Madeley, Munteanu, and Bonanno10 published a 2007 study of athletes (both male and female!) with medial tibial stress syndrome (or “shin splints”), which, as we saw earlier, exists on the continuum of tibial bone injuries, but is not nearly as serious as a stress fracture.  The three researchers used a simple single-legged calf raise test to compare whether the athletes, mostly runners and soccer players, had weaker calves than healthy controls.  Indeed, they did.  The healthy athletes were able to manage 33 single-leg calf raises in a row, while the injured ones managed only 23.  This is perhaps the most helpful finding of all the studies in this article, as it is the closest to providing a practical solution for preventing tibial injuries.  Perhaps poor calf muscle endurance forces the tibia to absorb more of the shock from impact? Of course, first we have to see whether impact is even related to the incidence of tibial stress fractures.

The impact loading rate (the slope from 20%-80%) may be related to tibial stress fracture. From 11

Research by Irene Davis at the University of Delaware has examined how biomechanical factors affect stress on the tibia.  In a pair of studies published in 2006 and 2007, Davis’ lab conducted a biomechanical evaluation of female runners who had suffered and recovered from a tibial stress fracture.  The researchers compared their impact loading rate, knee stiffness, and tibial shock with that of healthy runners who had never suffered a tibial stress fracture.  The findings indicated that runners with a history of tibial stress fracture have significantly higher impact loading rates—the rate at which the force of your footstrike ramps up as your foot hits the ground—as well as a stiffer knee and a higher tibial shock, as measured by an accelerometer taped to the shin.11, 12  Along with calf strength, impact loading and knee stiffness are both factors that we can actively change with training interventions, so Davis’ research provides another promising route for injury prevention.


Bone geometry

On the bone geometry front, recall that Bennell’s 1996 prospective study failed to discover any factors predictive of stress fracture in men.  Three years later, Bennell’s lab followed up with a retrospective study, this time of 23 male athletes with a history of stress fracture.  Just as we suspected from Beck et al., men with a history of tibial stress fractures have a smaller tibial cross-sectional area, relative to body size, than men who have never suffered a tibial stress fracture, but no difference in tibial bone density.13

One very thorough retrospective study by Popp et al. published just a few years ago examined female runners with a history of stress fractures (again, most in the tibia), analyzing both their bone geometry and composition across the entire tibia using high-tech CT scans.  Popp et al. found that this group of women tended to have a smaller tibial cross-sectional area, lower calculated bone strength, and a lower muscular cross-sectional area compared to healthy runners.14 Muscular cross-sectional area can be thought of as a more high-tech version of calf girth.  Interestingly, when they adjusted the measures of bone strength and cross sectional area to be proportional to the overall size of the lower leg, the differences disappeared.  The reason, say Popp et al., is that bone responds to the demands put on it—and if you have small, weak muscles in your lower leg, you won’t be able to put much force on your tibia during exercise.  As such, you won’t develop very thick bones. This jives very well with Bennell’s findings in her prospective study, since women with small calves would necessarily have more slender bones.  Regrettably, Popp et al. did not include men in their study, but given the results of Bennell et al., I suspect the same relationship applies.  Keep in mind that only 12 of the 20 women in Popp et al.’s study had tibial stress fractures.  The rest had metatarsal or femoral stress fractures, so we have to interpret these results with caution.

Bone composition and density

As far as bone composition, we’ve already seen a lot of evidence that bone density and remodeling play a key role in the development or avoidance of a tibial stress fracture.  In response to stress on the tibia (or any bone), the body remodels and strengthens it to resist stress injuries.  During the remodeling period, there is a window of time, reported to be around a month or so long15 where the bone is actually weaker due to the resorption of bone material that precedes bone growth.  While most people continue to train straight through this window with no ill effects, if tibial stress continues to outpace bone growth over a long period of time, a tibial stress injury is inevitable.  Women are particularly vulnerable to less-than-adequate bone remodeling for two reasons.  First, they naturally have thinner and less-dense bones than men.  Second, and more importantly, their bone health is strongly affected by disturbances in the menstrual cycle. 

A 1988 study by Gray Barrow and Subrata Saha16 at Louisiana State University highlights the alarming degree to which irregular menstrual cycles can affect bone health.  Using surveys, Barrow and Saha evaluated 240 female collegiate long-distance runners, asking questions about their eating habits, menstrual cycles, and history of stress fracture.  The results, summarized in the two tables below, are eye-opening.


Eating disorders, menstrual health, and stress fractures
in female collegiate distance runners

Menstrual cycles per year
Prevalence of stress fracture
0-5 (very irregular)
49%
6-9 (irregular)
39%
10-13 (regular)
29%


Menstrual cycles per year
Respondents admitting
to an eating disorder
0 (amenorrhea)
47%
1-5 (very irregular)
20%
6-9 (irregular)
10%
10-13 (regular)
7%


For women, it’s clear that missing more than one or two menstrual cycles a year substantially increases your risk of a stress fracture.  Furthermore, just as bone health is linked to menstrual health, so too is menstrual health linked to an adequate diet.  The proportion of female runners who admit to having an eating disorder skyrockets as the number of menstrual cycles per year drops, and this is no surprise—it’s well-known that menstrual disturbances in female runners are almost always caused by inadequate calorie and nutrient intake.  It should be no surprise that the runners with very irregular periods weighed significantly less than the healthy athletes.  Also recall that Bennell et al.’s study identify a low-fat diet as a risk factor for stress fractures in females.  Keep in mind that this study was a survey study, and relied on the subjects to self-report whether they exhibited behavior characteristic of an eating disorder.  I suspect the true incidence of disordered eating approached 100% in the very irregular and amenorrhea groups. 

Barrow and Saha’s work is bolstered by an earlier study by Myburgh et al., which found that a group of 25 runners with stress fractures (19 of which were women) had a higher incidence of menstrual disturbance, lower spinal and femoral bone density, and, interestingly, lower dietary intake of calcium.17


Summarizing the gender differences

There is an apparent gender discrepancy when it comes to the factors associated with the development of tibial stress fractures, as we’ve seen in the raft of studies listed above.  Tibial stress fractures in women are dominated both by differences in bone composition and bone geometry.  Bone density in women is regulated by their menstrual cycle, which in turn depends on dietary intake—disordered eating and insufficient caloric intake in women sets up a chain reaction: the absence of the menstrual cycle lowers the levels of estrogen in the blood, which in turn severely hampers bone strength.  As such, the body is not able to recover from the stresses of training, and the damage to the tibia outpaces the body’s ability to repair it.  Bone thickness in women is also proportional to calf girth.  Stronger calf muscles increase the cortical area of the bone, making it stronger and more resilient.  Women who develop stress fractures have smaller calves and often, thinner bones than those who do not.  Additionally, women with a history of stress fracture have higher impact loading rates when running, as well as higher knee stiffness.

Men who get tibial stress fractures tend to have bones that are narrower and smaller in cross-sectional area than men who do not, but bone density is not typically an issue.  Calf size may be implicated in the development of tibial stress fractures in men, but only studies of military recruits have observed this relationship.  It may be that calf size plays a smaller role than in women and larger studies are needed to detect the effect, or that calf strength (since size and strength are not synonymous!) plays a more important role. 

In any case, most studies have found that runners of either gender who develop tibial stress fractures do not run more mileage than those who do not.  However, they seem to have higher impact loading rates, higher leg stiffness, and, in women, disturbances in their menstrual cycle.  While lowering your own personal training load should, in theory, lower your risk of a tibial stress fracture, the scientific evidence shows that a large number of runners get stress fractures even at very modest mileage (20-25 miles per week).  When searching for strategies to prevent tibial stress fracture, we should turn our attention to intrinsic factors, attempting to correct or ameliorate the effects of inadequate bone strength, weak lower leg muscles, high impact loading rates, high leg stiffness, and menstrual/dietary disturbances.

Prevention and Treatment: looking for solutions

There are very few studies which have actually showed a reduction in the incidence of tibial stress fractures.  So, while the focus of this article is on preventing tibial stress fracture, much of what follows is inferred from the prospective and retrospective studies above.  We don’t know for sure, for example, whether reducing impact loading rates will lower your risk of a tibial stress fracture, but it’s a good place to start. 

The only study I’m aware of which has showed a true reduction in the incidence of tibial stress fractures comes from Lappe et al., who examined stress fractures in female Navy recruits.  Providing vitamin supplements with 200% of the RDV of calcium and vitamin D reduced the incidence of stress fracture from 8.6% to 6.8%, a reduction of 21%.18  Although this study examines military recruits, not runners, there’s little drawback to adding calcium and vitamin D to your diet, so it’s a good starting point.

The benefits of calcium supplementation bring us to bone geometry and composition.  Is it realistic to attempt to change these? It may not be as far-fetched as it seems. Popp et al. showed that tibial size is generally proportional to lower-leg muscle mass, and several of the studies we’ve reviewed above have connected small calf girth (which implies small lower-leg muscle mass) with the development of tibial stress fractures.  Bone will adapt to the size of the muscle that surrounds it, as Popp et al. describe:14

When bone measurements were adjusted for MCSA [muscle cross-sectional area], differences in bone parameters were no longer significant between groups, suggesting that participants in both groups had bones that were adequately adapted to their MCSA regardless of fitness, training, or nutritional factors. Although the cross-sectional nature of the study precludes the ability to make causal inferences, these findings are consistent with the mechanostat and related theories, which suggest that bone adapts its strength to the highest peak voluntary loads. Because muscles [not impact!] produce the highest loads on bone, bone strength should be adapted to muscle force.

Popp et al. go on to describe how sufficiently strong calf muscles could oppose the bending forces on the tibia during running, as illustrated below:

Calf strength may be able to oppose tibial bending moments (exaggerated), preventing tibial injury.

This theory is bolstered by Madeley, Munteanu, and Bonanno’s work10 which showed reduced calf muscle endurance in runners with shin splints.  It seems that the calf muscles play a significant role in the absorption of impact forces during running—whether they reduce loading on the tibia by actively absorbing impact or by opposing bending forces on the tibia is not clear, but fortunately we don’t need to know this to recommend doing calf muscle strengthening.

Female runners have additional issues to worry about when it comes to bone strength.  We’ve seen how disturbances in the menstrual cycle, caused by inadequate caloric intake, can disturb bone growth, reduce bone density, and increase the risk of stress fracture.  Often, the lack of calories is a result of disordered eating or a misguided desire to maintain an unhealthy “racing weight.”  Menstruation, eating disorders, and weight are touchy subjects for many coaches, but reluctance to discuss them doesn’t help the very real issue of stress fractures in female runners.  The facts remain: amenorrhea, or missing your period, is a dangerous condition which damages bone health by creating hormonal imbalances, and the root cause is usually insufficient caloric and nutrient intake in your diet.  I recommend all coaches, trainers, and doctors investigate the menstrual and dietary health of any female runner who develops a stress fracture.  Missing your period more than twice a year raises your risk of stress fracture by ten percent and missing it for several months at a time nearly doubles your risk.  And disordered eating can lead to issues far beyond stress fractures.  Whether you are a coach, athlete, or parent, don’t mess around with these sorts of things.  Talk to a doctor or nutritionist who is experienced in dealing with female athletes and who understands how the “female athlete triad” can affect hormonal and bone health.

Another approach to reducing the risk of tibial stress fracture is by attempting to lower the stress on the tibia.  Irene Davis’ work suggests that we can also reduce the risk of tibial stress fracture by reducing impact loading rates and decreasing knee stiffness.  Although we might propose running in more cushioned shoes or on a softer surface to decrease loading rates, it’s not clear that either of these strategies will actually change loading rates.  During running, the body does its best to keep your center of gravity at the same height above ground.  This is accomplished by adjusting the stiffness of your leg to keep the overall stiffness of the ground, shoe, and leg constant.  So, if you run in cushioned trainers on a soft surface, your body will increase leg stiffness, keeping the overall impact forces the same, as Benno Nigg argues in a very influential 2001 paper.19  Ironically, softer surfaces may increase tibial shock, as Davis’ work has also shown higher leg stiffness to be correlated with higher impact loading.  So, paradoxically, the evidence supports running in thinner shoes on a hard surface to decrease tibial shock.  This, in theory, should decrease knee stiffness, lowering the stress on the tibia. 

But why do some runners find softer surfaces more forgiving when it comes to their shins? I suspect theanswer lies in the fact that soft surfaces are also uneven, and as such, distribute stress differently than a very smooth, flat surface.  While nearly every step on a road is identical to the last, small variations in the surface on grass fields, gravel roads, and dirt trails may vary the stress on your leg. 

Increasing your stride frequency is a more sure way to reduce impact loading.  While knee stiffness does increase at higher stride frequencies, it’s well-established that the overall tibial shock does not.20  In fact, joint loading seems to decrease when stride frequency is increased by 5-10%.21 Another way to reduce loading is simply by slowing down! Taking it easy on your easy runs will definitely lower tibial shock, as long as you’re careful not to adopt a lazy, loping, low stride frequency.

Finally, whether or not custom shoe inserts (“orthotics”) can reduce tibial loading is unclear.  No studies have identified them as a proven method of preventing or treating tibial injuries, but many podiatrists have great success treating patients with custom orthotics.  The rationale for their use is that they are able to shift stresses on the tibia to alter the bending of the bone, though there’s disagreement about which way the tibia is bending.  Kevin Kirby, a prominent podiatrist in California, argues that the main problem is lateral bending (as evidenced by the fact that pain occurs on the medial side of the tibia), which can be addressed by custom orthotics.22  Popp et al.14 and Bennell et al.5 argue that the main problem is forward bending, which could possibly be reduced by increasing calf strength and would likely not be impacted by orthotics.  Additionally, according to Popp et al., the most common location for tibial stress fractures is on the posterior (back) face of the tibia.  Until more evidence is available, you can consider orthotics as an option (seeing as many clinicians report success using them), but I can’t endorse them given the lack of scientific evidence. 

Conclusion

Tibial stress fractures are a serious injury.  They are the most severe case on the tibial stress injury spectrum, which ranges from mild cases of “shin splints” to true stress fractures.  Tibial injury seems to be connected to excessive bending of the tibia during running.  When the damage to the tibia outpaces the body’s ability to repair it, tibial injury occurs.  If the damage to the tibia is severe enough and continues for long enough, a tibial stress fracture will develop.  Runners with long-standing cases of “shin splints” that have diffuse pain along the shin but not a sharp, localized, aching pain on the bone should take care not to increase their training load until they have recovered, lest they further damage the bone.  Tibial stress fractures require advanced medical imaging to properly diagnose, so if you have a sharp, localized pain in your shin when running that does not disappear after a few days’ rest, see a sports medicine doctor.  There is no known treatment, other than rest, to speed recovery.  Additionally, there is little hard evidence indicating methods to prevent stress fractures in distance runners.  However, after reviewing a host of prospective and retrospective studies, we can make some recommendations based on the scientific findings to date.

First, any runner with a history of stress fracture should consider supplementing his or her diet with 200% of the RDV of calcium and vitamin D.  This has been shown to reduce the risk of stress fractures in female Navy recruits by 21%, and one study found runners with stress fractures had inadequate calcium intake. 

Second, all runners should not make drastic changes in training volume over a short period of time.  In military recruits, the majority of stress fractures occur in the first month of basic training, which coincides with the 30-day “latency period” of bone remodeling.  According to Belinda Beck at Stanford University, the tibia is weakened during the first month or so after an increase in tibial loading due to the bone resportion which precedes bone.15  While runners traditionally increase their training at a constant rate (say, increasing mileage 10% every week), this model suggests it may be a better idea to either increase training in larger “chunks” every month or so (say, weekly mileage of 40-40-60-60-60-60-80-80....) or to take “down weeks” every three to four weeks (ex. mileage of 55-62-70-55-65...).  Additionally, runners who are new to the sport will be at an increased risk of tibial injury if they have little experience in high impact sports like soccer or basketball.  High school coaches in particular should keep this in mind, encouraging new runners to gradually ramp up their training for at least a month before the cross-country or track season begins. 

Third, all runners should incorporate calf strength and calf endurance training into their weekly routines.   As of yet there is no consensus on what an acceptable protocol for calf strengthening would consist of, but a reasonable plan to build strength might be 3 sets of 15 calf raises, with weight, 5-7 days a week.  Building endurance can be accomplished by doing single-leg calf raises to failure 3-5 times a week.  These should not be done without the approval of a doctor or physical therapist if you currently have a tibial stress fracture. 

Fourth, you should work to decrease the impact loading on your tibia by increasing your stride frequency by 5-10%, slowing down the pace on your runs, and perhaps consider changing to a shoe with a thinner, firmer midsole if you’re willing to be a ‘guinea pig.’  Avoid ultra-minimalist shoes like the Vibram Fivefingers, as early reports indicate that these may increase the risk of metatarsal stress fractures.23  You can determine your stride frequency by counting the number of times your right leg hits the ground in 30 seconds of running, then multiplying by four.  Efficient runners tend to have stride frequencies at or above 180 steps per minute (45 counts in 30 seconds), even at slow paces.  Custom orthotics may prove to be useful in decreasing tibial shock, but the evidence is not yet in.

Finally, female runners should examine their diet and review their menstrual health.  Missing your period more than once or twice a year sharply increases your risk of a stress fracture, and indicates that you are not getting enough calories and fats in your diet.  If you exhibit signs of disordered eating, including fatigue, weight loss/a low BMI, a preoccupation with food/weight, and amenorrhea, seek help from your coach and family doctor.  Missing your period severely impacts your hormonal and bone health and puts your overall well-being at risk, not to mention your athletic performance.

If you’ve been diagnosed with a tibial stress fracture, you are probably looking at 6-8 weeks’ time away from running.  During this break, take the opportunity to review your training history and develop a plan to prevent future injury.  Work with your doctor and possibly a physical therapist to address any issues unique to you, develop a recovery plan, and ensure that you don’t return to running too soon.  

On the research front, I recommend more investigation into the role of calf strength, size, and endurance to develop a protocol to prevent stress fractures.  Additionally, more work needs to be done to pin down the causes of stress fracture in males, as evidence indicates their injury etiology differs from that of females.  Lastly, more work needs to be done on the role of custom foot orthotics and various footwear conditions in preventing tibial injuries.  Can a medially-posted orthotic reduce lateral tibial bending and therefore tibial strain? Does running in thinner shoes on a firm surface decrease tibial shock and tibial bending? What type of calf strengthening routine is ideal? Do the other muscles of the lower leg play a role in stabilizing the tibia? These are all big questions that still need to be answered. 


References

1.         Moen, M. H.; Tol, J. L.; Weir, A.; Steunebrick, M.; De Winter, T. C., Medial tibial stress syndrome: a critical review. Sports Medicine 2009, 39 (7), 523-546.
2.         Bergman, A. G.; Fredericson, M.; Ho, C.; Matheson, G. O., Asymptomatic Tibial Stress Reactions: MRI Detection and Clinical Follow-up in Distance Runners. American Journal of Roentgenology 2004, 183 (3), 635-638.
3.         Fredericson, M.; Bergman, A. G.; Hoffman, K. L.; Dillingham, M. S., Tibial stress reaction in runners: Correlation of Clinical Symptoms and Scintigraphy with a New Magnetic Resonance Imaging Grading System. American Journal of Sports Medicine 1995, 23 (4), 472-481.
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12.       Milner, C. E.; Hamill, J.; Davis, I., Are knee mechanics during early stance related to tibial stress fracture in runners? Clinical Biomechanics 2007, 22 (6), 697-703.
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13 comments:

  1. This is great. One of my friends just got diagnosed with a tibia stress fracture yesterday, 300 miles into her thru-hike bid on the PCT. Funny that you should publish this on the same day, and the two of you show up on the same page of my facebook newsfeed talking about the same injury. I've been soaking up mostly anecdotal information about tibia stress fractures since last July, when the doctors and I were pretty sure I had one as I was thru-hiking the Appalachian Trail (it turned out I had something else, a bone bruise in my ankle that was causing the trademark tibia stress-fracture pain and increased uptake in my shin. The P.A. said it was something he hadn't seen before). But this is obviously much more comprehensive and current and reliable than what I heard from one doctor and a bunch of other hikers and a few cursory Google searches. Thanks for providing a good read.

    - Chris Burke

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  2. I've started to add single-leg calf raises to my weekly regimen. I think exercising on a decline squat board is also useful for calf-strengthening? Care to comment on this?

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  3. Hi Emil, sorry about the delay!

    The decline squat might strengthen the calf isometrically, as it's used to stabilize your lower leg/ankle, but if I was a betting man, I'd bet that single-leg raises would be more effective. Stability in general (balance, etc) might also be a good predictor of injury risk, but there haven't been any studies on this that I'm aware of.

    ---John

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  4. This comment has been removed by a blog administrator.

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  5. Hi,

    Thanks so much for writing this article, I've found it really helpful. You write very clearly and thoroughly and it's great to be able to read an overview of the literature. I'm the end of 6 weeks of complete rest from running with a tibial stress fracture. The injury was sustained last october (2011) but I stupidly continued training through the pain, as my doctor simply sent me for an xray (which showed nothing) and so I thought it was just something muscular. Well, I now know that it's to do with muscle weakness amongst other things, but that it was a true stress fracture (MRI in August confirmed this). I came across your blog from a google search looking for training plans for coming back from injury, and I'm very glad to have read your article.

    Keep up the good work, and good luck with your own training!

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  6. A few comments:

    1) I just saw a documentary about Kenyan runners involving a study by Danish exercise physiologists. They correlated skinny calves with improved running performance! Any data on stress fractures in East African runners?
    2) Diagram of tibia bending and calf contraction looks like a contracting calf muscle would make the tibia bend more, not less.
    3) Any studies of stress fractures and running turns on the track? High speed curve running puts tremendous stress on the lower leg. I know both Seb Coe and Michael Johnson had lower leg stress fractures in their careers although it might have been fibula, not tibia.

    Thanks for a great article and awesome website! Peter

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    Replies
    1. Hi Peter,

      I have actually heard that one factor that affects the running economy of the Kenyans is their skinny calves—but unfortunately I have not seen any information on injury rates in Kenyans. Keep in mind that the Kenyans WE see on the international track and road racing circuits are the ones who made it through the extremely tough training, so they are all probably naturally injury-resistant. A Kenyan who is prone to shin injuries might not make it out of the country to race at all.

      As for the diagram, yes it's a bit simplistic. But the basic idea from the research papers is that calf strength can prevent bending in the frontal plane, though the mechanism is probably more akin to a thicker pillar resisting bending more than a thinner one.

      A study done in the '90s looked at about 100 Australian track athletes for a year and found a 21% incidence of stress fracture, which is higher than studies on more casual participants. Additionally, around 70% of the stress fractures were in the dominant leg. However, to what extent this is a result of the stress of the turns is not clear—it could just be that the track athletes trained hard all the time, and that was what caused the higher incidence of stress fracture. To really get a good idea, you'd need to get a force plate or something and put it on the curve of a track to measure the forces on the left and right foot. That's something I'd like to see!

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  7. Your article will surely help a lot of people! My friend has the same injury as yours and he is currently under recovery. He is now seeing a chiropractor to help him with tibial stress fracture as well to avoid future injuries from his sport, running. Since you are a runner, it is best to seek a chiropractic care to refrain from this same injury to occur.

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  8. Your article has been very informative. My 13 year old son who runs cross country and the mile in track has been suffering from shin splints. We just discovered from an MRI that he has a stress reaction of the tibia which is a precursor to a fracture. He is built with a very small build and only weighs 100 pounds. What can he benefit from to prevent him from getting a stress fracture in the future?

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  9. You might consider looking into having him do some calf strength exercises—after he's recovered, obviously. Being of a slight build probably means he's lacking muscular strength in his legs, which as I described in the article can lead to smaller and weaker bones. Aside from that, increasing stride frequency and several of the other options listed at the end of the article may also be helpful. Best of luck!

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  10. So glad to find your article...I suffered from stress fx's twice, and had to take months off from running since my ortho guy was conservative. Now, a couple of years later, I'm running again, (not much mileage wise...1-3 times per week, 3-6 miles per week), and having the same pain that I had before. The pain is located on the right side of my left tibia. I've always had muscular legs and calves, so I'm not thin, and I don't do long distance running, but sports specific high impact training and running lead to inflammation and this specific pain. Any suggestions? One sports doc told me that I have to ice my leg after working out for 15 minutes every time for the rest of my life, and was unable to tell me why this was happening other than that my stride was too long. I shortened my stride - still an issue. I would love to find a solution that doesn't include "for the rest of my life". I'm definitely going to make sure I'm taking Ca+ and D, and I'm thinking that I'll do the calf exercises as well. Currently I can do 40 single leg calf raises before it cramps up, and the pain I have on palpation is not evident during the calf raises. Any help would be completely appreciated. Thanks!

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  11. I realize this is an older post, but I found it very informative and from experience can see a direct correlation of my own injury to your research. I'm a 40 y/o female who has had an active eating disorder for many years. It is under control, however my running has recently suffered with a tibial stress reaction in the lower third - left leg. I am at 8 weeks now since the original injury date and since a solid run. I've kept up my cardio with lap swimming and cycling. The MRI was 5 weeks ago and my ortho just released me this week to re-introduce running with a walk/run. I did 3 miles and now, about 36 hours later I am experiencing discomfort and point tenderness again. It is mild but still concerning. I've had very irregular menstrual cycle during this time and up until now have not found the solid research supporting the fact I am at a higher risk due to the ED and the change in menses. Thank you for the great information.

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  12. Thank you for this post!! Its the first article I've come across that has given me hope and some sort of explanation about my stress fracture. My original stress fracture happened April 2011 and since then its been this constant "cat and mouse" game - I get back to running, shin stays good then flares up with no obvious rhyme or reason. Don't get me wrong - I've managed to do some good running in between but its always been a case of managing my shin on this fine line and never managing to get consistent running in. I will now diligently add in calf raises and a few other exercises to help that and see how I go. I have HUGE calves but they are very weak so hopefully strengthening them will help. Thanks again for your post.

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