Medial Tibial Stress Syndrome : Who ’ s at risk ?

Background and Purpose The purpose of this review is to identify risk factors in the athletic population for the development of Medial Tibial Stress Syndrome (MTSS). MTSS is a lower extremity pathology that frequently impacts athletes and disrupts their ability to participate. Identifying risk factors and other direct causes of this condition can provide insight regarding treatment techniques and methods of prevention.


Introduction
Lower extremity injuries are a very common problem in the athletic population, therefore they are frequently addressed by physical therapists and other medical professionals. These injuries can include stress fractures, chronic compartment syndrome and Medial Tibial Stress Syndrome (MTSS) [1]. The highest incidence is seen in MTSS [1]. It has been estimated that this condition has accounted for 4 to 35% of overuse leg injuries in the athletic and military populations [2]. Research has also shown that MTSS is responsible for 13.2% to 17.3% of all running injuries and approximately 22% of injuries in aerobic dancers [1]. According to these statistics and others, this injury has become a very prominent issue that remains difficult to not only diagnose and treat, but also to identify individuals who are susceptible to its development.
The formal definition of MTSS varies throughout the existent literature. It was first qualified as "a symptom complex seen in athletes who complain of exercise induced pain along the posteromedial border of the tibia" [1]. The exact pathophysiology of MTSS remains unknown, however until recent research emerged, inflammation of the periosteum as a result of increased traction was viewed as the most viable explanation. Studies have now shown that it is more likely that MTSS is a bone stress reaction that progresses to a painful state as opposed to an inflammatory process [1]. The metabolic changes in bone that occur when an individual begins an exercise program result in an increased porosity on the posteromedial border of the tibia. In a normal individual, new bone is laid down in order to resist compressive forces and to increase the strength of the bone. However, in an individual who has suffered from longstanding MTSS, the tibia will remain more porous by approximately 15% according to one study [1].
In order to diagnose MTSS, a thorough clinical history and physical examination must be conducted. It will often present as "diffuse, palpable pain, localized to the posteromedial tibial border". The pain can be located anywhere along this border, however it is commonly found from the middle to distal thirds. It is described as a dull ache after exercise. It may last for several hours up to days [1]. MTSS is often diagnosed as shin splints, shin pain, periostitis, and exercise related lower leg pain [3]. Imaging and diagnosis MTSS with radiograph is not appropriate however, Magnetic Resonance Imaging (MRI), has increasingly been used for studying MTSS.
Currently, there is no accepted successful treatment plan that consistently alleviates the symptoms of MTSS in all populations. Many studies describe conservative options for treatment including ice application, aspirin, heel cord stretching and walking casts. Other researchers suggest more exercise based interventions such as graded running programs, strengthening, and stretching exercises specifically targeting the calf muscles. Sports compression stockings are also frequently used in athletic populations [2]. However, despite the volume of research that has been conducted on interventions for MTSS, there is a lack of agreement upon the most effective method. One reason for such lack of consensus might be the multitude factors leading to development of MTSS. With the several different risk factors known, treatment for and prevention of MTSS as a single problem is difficult and may encompass a wide range of options. This has led to a focus on attempting to determine the most relevant risk factors that contribute to the development of this condition. It is the intention of many medical professionals to identify these risk factors in individuals most known for developing MTSS, such as athletes, in order to implement prevention strategies.
Thus far, what is known about risk factors is that there are both intrinsic and extrinsic aspects that can possibly play a role in the development of MTSS. Additionally, some of the known risk factors are preventable, while others are inherent and may increase susceptibility. What are still under debate however, are the most common biomechanical risk factors or other direct causes. Understanding the most prevalent risk factors for MTSS may lead to better understanding of the best intervention and prevention approaches. Therefore, it is the purpose of this review to identify the most common risk factors of MTSS, specifically in the athletic population.

Search strategy
Literature search was conducted over a period of approximately 7 days, throughout the time span of August 1, 2014-October 1, 2014, regarding risk factors and causes of medial tibial stress syndrome. The databases that were searched include PubMed, ProQuest Nursing and Allied Health, Proquest Health and Medicine, Cinahland Google Scholar. The key phrases that were included in all database searches included: shin splints and physical therapy, shoes and shin splints, shin splints and footwear, shin splints and treatment, medial tibial stress syndrome risk factors, medial tibial stress syndrome prevention, shin splints risk factors, athletes and medial tibial stress syndrome, prevention of shin splints in athletes, medial tibial stress syndrome and risk factors, medial tibial stress syndrome and athletes, and medial tibial stress syndrome and physical therapy. Table 1 displays the initial search results for such criteria without any filters, which are discussed in the next section of this review, study selection. The reference lists of the eligible articles were reviewed for any cited articles that fit the criteria of this review. Other potential sources were then subjected to the same evaluation process.

Study selection
In order to be included in this study, articles met the following eligibility criteria: Inclusion criteria: studies that had included 1) sample consisted of an athletic population 2) participants had symptoms of medial tibial stress syndrome 3) were peer-reviewed 4) outcome measures of (but were not limited to) navicular drop, foot length, and gastrocnemius and/or soleus length 5) demographic information such as BMI, gender, age, exercise amount and intensity, type of exercise, and past medical history. Exclusion criteria: 1) articles published in languages other than English 2) published before 2000 3) studies with case report designs 4) were not peer reviewed.

Data extraction and quality assessment
Data extraction was performed in a consistent manner across all eligible articles. Critically Appraised Topics (CAT's) were written for each article selected for this review. Each article was critically appraised and the pertinent data was gathered and implemented into an organized table for ease of accessibility for both authors and readers. The data of interest that was obtained from each article included: the study design, level of evidence, sample, outcome measures, and major results of the research conducted regarding risk factors for medial tibial stress syndrome, strengths and limitations of the study, and overall validity of the research. The results of the eligible articles were then compared in order to compile all possible risk factors as well as to determine if there were consistent findings across research, indicating an increased prevalence of certain risk factors. The samples were also evaluated to identify a possible at-risk population for this musculoskeletal condition. In order to ensure clinical applicability, the authors also examined the research for possible successful treatments and any prevention methods for medial tibial stress syndrome. These items were analyzed for effectiveness and practicality in the general population.
The methodological quality of each article was analyzed using the Physiotherapy Evidence Database (PEDro scale) [4]. Each paper was individually assessed for interval validity, external validity, and statistically sufficiency. A point was awarded for each question if there was sufficient evidence that the requested information was clearly stated. A maximum possible score for methodological quality was 11. A strong score was considered >6, fair 4-6, and poor <4. The quality score of the articles used ranged from 5-9. The score for each individual paper can be found in table 1.

Results
Eleven studies were analyzed as a part of this systematic review. All studies included human subjects, particularly athletes, with a range of number of participants from 15-468, mean of 118.5. The average PEDro quality score of the articles used in this review was approximately 6, which is considered fair quality. These articles were overall only at a fair quality because most articles lacked both blinding (for subjects, therapists, or assessors) and randomization (for subject recruitment, group allocation, or order of assessments completed). The details of the selected studies can be found in table 2.

Subject description
Four studies included only runners [5][6][7][8], two included military personnel [1, 3], and five included athletes of different sports [2,[9][10][11][12]. The disease severity of participants within the studies varied, as some were deemed "healthy" prior to study initiation while other were already diagnosed with MTSS. Two studies included only subjects with MTSS [1, 9], two studies used subjects with and without MTSS for comparison reasons [12,13] and the remaining seven had subjects of overall "healthy" quality prior to study initiation.
Overall, our results displayed varying risk factors for developing MTSS. The most common risk factors include general demographics and history, navicular drop, range of motion, pain level, miles ran per week, bilateral ankle strength, exercise history, cortical thickening of tibia shown with MRI, decreased repetition completion during standing heel rise test (decreased plantarflexion strength), presentation of symptoms, foot balance and duration of symptoms which were found significant in ten of the eleven studies analyzed.

Demographics and history
Seven of the eleven studies included in this review analyzed the effect of baseline characteristics on the development of MTSS. Characteristics evaluated by the various studies included age, gender, height, Body Mass Index (BMI), limb dominance, history of injury/pain, orthotic/tape use, dietary supplementation, menstrual cycle and smoking; however only increased BMI and female gender were correlated with the incidence of MTSS in six of the seven studies. Yates et al., found a significant relationship between the development of MTSS and female gender [1] where female naval recruits were twice as likely to develop the condition as male recruits. Females (n=40) demonstrated a 52.9% incidence rate as compared to the rate of 28.2% seen in males (n=84) [1]. Higher female injury rates among military recruits have been reported in past [14,15]. Injury rates are greater when women train in conjunction with men and are expected to achieve the same fitness levels [16]. This can be explained due to the fact that women have generally smaller stature and they have to "keep up" with male recruits while marching, resulting in overstrenous gait changes, thus increasing the risk of developing MTSS. Plisky et al., found that high school cross country runners with an increased BMI were at an increased risk of suffering from MTSS. However, individuals in higher BMI have overall higher incidence of lower extremity injury, not particular MTSS [5]. Both the female gender and those with increased BMI are at high risk for MTSS development because of the increased stresses that occur to the tibia during exercise.

Biomechanical characteristics
Multiple biomechanical characteristics were used as outcome measures throughout the studies analyzed in this review. The characteristics found to be significant include navicular drop, foot pronation, limited hip internal rotation range of motion, overall plantarflexion range of motion, limited straight leg raise, cortical thickness of tibia via MRI, bilateral ankle strength, standing heel-rise test, and foot balance. Overall deficiencies in lower extremity strength, proprioception, and available motion, as well as bony structure imposing postural deficits would be positive risk factors for developing MTSS. Yates et al., found that foot pronation was significantly associated with the development of MTSS in naval recruits [1]. Those recruits who demonstrated increased pronation were nearly twice as likely to develop MTSS as individuals with a normal or supinated foot posture [1]. Likewise, Hubbard, Carpenter, and Cordova demonstrated that athletes with MTSS had an average plantarflexion range of motion of 46.0° versus those in the healthy group had significantly less motion, averaging 40.6° [9].

Training characteristics
The incidence of MTSS is found to most prevalent in the athletic population; therefore studies that examine this topic often assess various qualities of participants' exercise and training regimes [1]. This systematic review included studies that analyzed type of shoes worn during physical activity, miles ran per week, running surface type and general exercise history. Those characteristics that were found to be significant include miles ran per week and exercise history. It can be inferred that a significant change in running history (increased weekly frequency, fewer years of training, etc.) places athletes at risk for the development of stress injuries, including MTSS. Such changes in characteristics place increased stress on the tibia, putting it at a higher risk for injury. For example, it was found that those with a history of fewer years running had a higher incidence of MTSS  Foot Posture Index (FPI), ankle dorsiflexion, injury incidence, exit interview Gender and foot pronation are significant risk factors for development of MTSS; past history of MTSS increased risk for development; BMI, age and ankle dorsiflexion had no effect Strengths: 1. Large sample size 2. Investigators operationally defined sample 3. Participant's representative of population from which they were drawn 4. All participants entered the study at the same stage of the condition 5. Study time frame was long enough to capture outcomes of interest 6. All outcome criteria operationally defined 7. Investigator who took physical measurements was blinded 8. Included study specific definition of MTSS 9. Specific criteria for inclusion in either MTSS group or non-MTSS The results of this study demonstrated that those recruits with a decreased level of fitness were 3.6 times more likely to develop MTSS during the training period [3]. Although not shown to be significant in the studies analyzed in this review, shoes and running surface may have an impact in MTSS development as well; abnormal forces may be placed on the tibia from improper foot position in the shoe, unsuitable cushioning, and/or surfaces that do not allow for the absorption of forces.

Symptom presentation
Pain level and duration of symptoms were both found to be positively correlated with increased incidence of MTSS. Many of the athletes studied who had previous symptoms were also found to have a higher rate of repeated occurrence. Hubbard et al., demonstrated the trend of a significant relationship between MTSS and a previous history of the condition in college athletes. The results included that 87% of the MTSS group had a previous history as opposed to only 16% of the healthy group [9]. Another study including 26 recruits with a previous history demonstrated the same relationship. Of these 26 recruits, 11 developed symptoms during the study period: an incidence rate of 42%. Therefore, it was concluded that recruits with a previous history of the condition were at a statistically increased risk of developing MTSS than those who did not [1].

Discussion
The purpose of this systematic review was to identify risk factors in the athletic population for the development of MTSS. As can be inferred through multiple research articles, there is a lack of agreement in the current literature regarding the cause of this condition, the risk factors for its incidence, as well as the most effective way to treat it [9]. To our knowledge this is the first systematic review to determine a group of risk factors in the susceptible population. Establishing such a group would provide evidence to develop prevention techniques and improve upon current treatment strategies. Although outcome measures and general demographic characteristics found to be significant throughout the examined studies were not consistent, the wealth of information collected through this review provides practitioners with potential factors to be aware of when providing care to athletes. Being aware of the impact that gender, BMI, previous history of condition, navicular drop, plantarflexion range of motion, poor fitness, and significant change in exercise frequency can have on the athletic population will make different health care providers able to better develop appropriate plans of care.
Higher BMI and female gender were shown to be associated with an increased incidence of MTSS symptoms. With a higher BMI, increased stress is automatically placed on the lower extremities. Adding in high-impact activity too significantly increases the chance of stress-related injuries because the bones cannot handle the additional stress and will not adapt to it quickly, if at all. The relationship between female gender and the development of MTSS has been examined primarily in studies with military populations. Results have shown that the incidence of injury is increased when women train with men and are therefore expected to demonstrate equivalent fitness levels. This is due partly to the fact that women are often smaller in stature than men. Therefore, when attempting to train at the same level as men, women are susceptible to overstrenuous gait changes, such as long stride lengths. This can increase the risk for development of MTSS. Current studies are investigating the separation of men and women during naval-training programs and its effect on incidence of MTSS [1].
Altered biomechanics, such as navicular drop and increased plantarflexion range of motion may also contribute to the development of MTSS symptoms. These factors have a detrimental effect on an individual's overall lower extremity posture, leading to improper forces placed on anatomical structures during activity. The forces can impact structures that are typically loaded during physical activity, however in a manner that will place an increased stress upon them. When examining these altered mechanics in the ankle/foot complex, in particular the talocrural and tarsal bones, it is evident that increased levels of stress on the structures and cyclic loading produce symptoms [9]. Specifically, navicular drop, measured by the amount of pronation occurring at the subtalar joint, contributes to the altered biomechanics of running, and changes the normal stresses placed on the tibia [6]. This tendency of altered biomechanics to predispose structures to anatomical damage is characteristic of stress injuries, including MTSS.
The results of this review suggest that exercise history and a sudden increase in running distance may place the individual at increased risk for development of MTSS as they are at an increased risk for incurring a stress injury. It has been past suggested that with more experience in athletic activity the tibia has been able to adapt and adjust according to the forces applied a concept that is based on Wolff 's law. However, when increases in athletic participation are sudden, this law is not able to be applied because the body does not have adequate adjustment time. With a longer history of athletic participation, individuals are also in better overall physical fitness. Consequently, muscles and bones will both be able to act accordingly to normal impact forces placed on them during athletic activities [9]. Additionally, it should be taken into account that when a sudden change occurs in the type of exercise, biomechanics of the actions completed are altered and muscle imbalance may play a role in MTSS development secondary to their pull on bone [10].
Finally, higher pain levels and longer duration of symptoms were found to be associated with both the initial onset of the syndrome as well as recurrence in the future. This may be due to the fact that MTSS causes chronic inflammation of the tibia. If athletic participation is not discontinued with symptom onset, the injury site is prevented from healing adequately. Without adequate rest and rehabilitation of the underlying issues that predisposed each athlete to developing MTSS, such risk factors will continue to be present and these individuals will have a continued chance of MTSS symptoms and possible stress fractures. In fact, it has past been concluded that an inverse relationship is present between stress fractures and bone mineral density [12]. The inflammation present with MTSS not only causes pain but inhibits osteoblast cell activity, weakening the bone. If activity is not significantly decreased when such pain occurs, the bone becomes gradually weaker and microfractures occur from the increased stress that the demineralized bone cannot handle. If stress fractures develop and activity is still continued, the athlete might even develop a full fracture. On the other hand, if activity is discontinued at the onset of pain, osteoblastic activity is allowed to resume and bone density is capable of increasing [9].
This systematic review has provided preliminary evidence regarding the prevalence of certain risk factors in the incidence of MTSS. As stated these are primarily related to baseline characteristics (age, gender and BMI), biomechanical factors (range of motion, strength, navicular drop, foot balance, and tibial measures), exercise regimes and history (miles run per week) and presentation of symptoms (such as pain level and duration of symptoms). Although these factors were identified in peer reviewed literature, there was a lack of consistency and agreement amongst the research. Despite the examination of a fairly representative sample of the athletic population, a significant variety of factors were found to play a role in the development of MTSS.
Previous research on MTSS has concluded many of the same things as our review has found. Many relate the limitation of this realm to the small number of studies completed on the subject matter and the wide range of methods used. Both of these create issues with coming to sound conclusions secondary to the insufficiencies. In particular, the impact of foot structure on the possibility of developing MTSS has been reviewed, and adds only that very high and very low foot arches are at increased risk for developing stress injuries on the tibia [9]. When significant findings were found by other reviews, those risk factors were of great similarity to those outlined above. Such significant risk factors include female gender, high BMI, navicular drop, and hip external rotation in males, previous history of MTSS, use of orthotics, and less experienced runners. Based on these findings, it was also suggested that males and females be evaluated separately when evaluating the risks for MTSS as the different body compositions, bony structure, and resulting kinematics may play significant roles in potentiating factors [7]. Overall, all studies that have looked at MTSS and the associated risk factors have not been able to definitively state reasons for development or propose prevention strategies based on gathered information.
Several limitations were encountered throughout this systematic review. One such limitation is that of low PEDro scales of a majority of the studies. It would have benefited this systematic review more to include studies all of which had strong scores based on the PEDro scale. Thus, the results of this study should be interpreted with caution. A second limitation of this systematic review is the variability of subjects, as not all athletes were participants of the same sport and were of different levels of competition. Such differences may impact the interpretation of the results and how they can be applied to different populations. Additionally, several of the studies used within this review had small sample sizes, which limit the generalizability of the results, as does the overall small number of studies used in this review. Of the studies included, many limitations were common between them, including limited blinding (subjects, therapists, or assessors), lack of randomization (for subject recruitment, group allocation, or order of assessments completed), self-reporting of symptoms, subjects entering studies at different stages of MTSS development, attrition, and lack of control groups.
Despite all of the limitations to this review, several aspects were conducted appropriately and led to some basic conclusions about the subject area. The review can serve as a comprehensive summary of the current and available research that exists regarding this topic. Although the resulting list of studies included was less than optimal, it provides a decent overview of the risk factors for MTSS. This was accomplished in part due to the strengths that were evident in many of the individual studies. One of the key aspects of the research that has been conducted thus far in this area is the inclusion of a definition of MTSS for the purpose of the research. This improves the accuracy and consistency of the results that are obtained. In addition, many of the studies operationally defined their samples as well as their outcome measures. These samples were typically representative of the population that of interest. Furthermore, many of the studies included time frames that were adequate in order to capture the outcomes of interest, populations with similar demographic and baseline characteristics, and reliable statistics. Finally, the current research being conducted on this topic generally requires limited to no equipment, improving the opportunity for clinical application.
Although this review successfully revealed a multitude of factors that are associated with the development of MTSS, instead of identifying a definitive list of those that can be addressed through treatment and prevention, it highlighted gaps in the information regarding this topic. Future research should focus on narrowing the group of potential risk factors to a more precise collection that have been consistently correlated with the development of MTSS and can be used in clinical practice for prognostic purposes. It may be beneficial to continue studies addressing the athletic population initially as this is the population that is most affected by MTSS. Future studies should also attempt to use larger sample sizes, from more diverse populations of athletes. Subjects could also be analyzed within groups based on the type of impact sport, such as ballistic movement versus cyclic high-impact activity. Running mechanics could also be analyzed using advanced technological resources, such as a gait mat or other computer software to analyze specific areas of the lower extremities receiving increased force or abnormal loading. Once conclusive evidence is achieved regarding the consistent risk factors that contribute to the incidence of MTSS, they can be applied to the clinical treatment and prevention of this condition to ultimately achieve a decreased impact on the quality of life of those impacted.