Exertional Leg Pain in Runners

Introduction

MTSS is one of the most common leg injuries in athletes and military personnel with an incidence between 4% and 35%. In runners, the incidence of MTSS has been reported to be between 13% and 20%. Other terms, such as shin splints, shin soreness, shin splints syndrome, and tibial stress syndrome, have been used to describe the same condition. MTSS refers to pain on the posteromedial tibial border during exercise which is reproduced with palpation of the tibia over a length of at least 5 cm. Yates and White defined MTSS as “pain along the posteromedial border of tibia that occurs during exercise, excluding pain from ischemic origin or signs of stress fracture.”

Pathophysiology

The exact cause of MTSS is not fully understood. The prevailing theories include the traction theory and a model based on pathologic adaptation of the tibial cortex to repetitive bending resulting in a painful stress reaction of bone.

In the traction theory, posterior compartment leg muscles are thought to invoke a traction reaction along the tibia resulting in periostitis. This theory relies on functional anatomy and the location of muscle attachments to the posteromedial tibia in the region where MTSS pain is experienced by runners. Cadaveric studies have been performed to examine the relationship between the location of pain in MTSS and the muscle attachments of the posterior compartment muscles. Beck et al. dissected 50 cadaver legs and found that muscle attachments to the distal third of the tibia are not abundant, calling into question the rationale for the traction theory. In the distal third of the tibia, very few muscle fibers of the soleus and flexor digitorum longus (FDL) were present and no tibialis posterior (TP) fibers were present. A recent study by Bouche and Johnson provided supportive evidence for the traction theory. In three cadavers, tension was applied to the tibial periosteum through the soleus, TP, and FDL muscles. Tension in the respective muscles and the tibial fascia (i.e., periosteum) increased in a linear fashion.

Histological support of periostitis as a result of traction to the periosteum from muscle contraction is limited. Small studies have shown a mild inflammatory response upon biopsy. The largest histological studies to date have failed to demonstrate inflammatory cells in the periosteum of patients with MTSS. Although inflammatory cells have been demonstrated in the crural fascia and wide lymphatics of the periosteum in MTSS subjects.

Pathologic adaptation of the tibial cortex as a result of repeated tibial bending and bowing has been implicated as a cause of MTSS. Repeated bending of the tibia causes an adaptive reaction of bone primarily at the site where bending forces are greatest. The most bending occurs at the junction of the middle and distal third of tibia where the tibial diaphysis is narrowest. According to Wolff's law, repeated force applied to bone stimulates a cellular response to repair microdamage with a goal of adaptation to strengthen the bone to resist future loading. Bones can respond to stress up to a certain threshold, but strain above the threshold may lead to microdamage that escapes repair and accumulates over time.

A study by Franklyn et al. used tibial radiographs and computed tomography (CT) to compare the bone characteristics of sedentary individuals, healthy aerobic control subjects, and those with MTSS and tibial bone stress injury (TBSI). Male patients with MTSS and TBSI had smaller cortical areas than the aerobic controls. Both male and female MTSS subjects had lower cross-sectional cortical area as compared with the aerobic control group. The aerobic controls were also better adapted to axial loading and torsion and had superior bending rigidity than subjects with MTSS and TBSI. This study suggests that runners with MTSS are less adapted than healthy controls to handle tibial loads. In comparison with healthy controls, low tibial bone density (TBD) is found in patients with MTSS. Magnusson et al. found low TBD in athletes with MTSS compared with healthy athletes. Using dual-energy X-ray absorptiometry, TBD in the middle to distal tibia of MTSS patients was 23% ± 8% (mean ± SD) less than healthy controls. TBD normalized when the athletes recovered from MTSS after a mean of 5.7 years (range 4–8 years).

Studies have also found a connection between increased tibial bending and weaker muscles. Those with lower muscle strength may be less able to oppose tibial bending which negatively impacts bone adaptation increasing strain on the tibial cortex. Similarly, a recent study by Milgrom et al., showed greater tibial strain when muscles were fatigued. Still others have suggested the etiology of MTSS is a combination of traction and a failed bone adaptation response. Despite histologic and environmental similarities, no study supports progression of MTSS to a stress fracture.

History and physical examination

Runners with MTSS present with ELP located along the middle to distal third of the posteromedial tibia. Initially, symptoms are absent at rest and begin with activity. Pain may subside as the activity continues. As MTSS worsens, pain may not resolve during exercise and can also be felt after stopping activity. Continued pain at rest after exercise is also seen in those with TBSI and thus must be considered. Intrinsic risk factors associated with MTSS should be assessed ( Table 19.2 ).

Physical examination of the runner with MTSS is focused on anatomic localization of the symptoms, evaluation of risk factors, and ruling out competing diagnoses with similar presentations. In patients with MTSS, diffuse pain with palpation along the distal two-thirds of posteromedial tibia should be present. Pain with palpation in this region is thought to be more sensitive than pain on hopping or pain on percussion. The navicular drop test can be used to assess for foot pronation.

The presentation of MTSS may be confused with that of TBSI and CECS. Differentiating between these conditions can usually be done with a thorough history and physical examination. When uncertainty exists, diagnostic imaging and intracompartmental pressure (ICP) testing can confirm the specific diagnosis.

Diagnostic studies

In patients with uncomplicated MTSS, imaging is not indicated. If there is concern for TBSI, imaging is necessary to rule out a stress fracture. Plain radiography has limited value in the diagnosis of MTSS but may be helpful in ruling out other causes of leg pain. Bone scintigraphy has historically been used in the diagnosis of MTSS but has a high false-positive rate and its routine use is limited. The role of CT is unclear and not commonly used. MRI has been increasingly used over bone scintigraphy and CT in the diagnosis of ELP. MRI can accurately differentiate between MTSS and TBSI. There are two prospective studies examining the sensitivity and specificity of MRI in MTSS. MRI had a 79%–88% sensitivity and 33%–100% specificity. Fredericson et al. developed a grading system for MTSS and TBSI based on clinical findings and MRI ( Table 19.3 ). MTSS is a clinical diagnosis, and imaging should be ordered only when the diagnosis is unclear.

Treatment

The initial treatment of MTSS begins with rest (i.e., stop aggravating activities). The duration of relative rest is based on the Fredericson MRI criteria ( Table 19.3 ). If an MRI has not been completed, the runner should rest until pain-free with daily activities and nontender to palpation over the affected region of the tibia. Although the duration of rest will vary with the severity of the condition, 4–6 weeks is often required.

Decreasing weekly running distance, frequency, and intensity by 50% has been shown to improve symptoms sometimes without complete activity cessation. Most often, complete rest from the aggravating activity will be required. NSAIDs and acetaminophen are often used for analgesia although there is no data supporting their efficacy for MTSS pain. Common modalities for pain control are employed but have not been shown to be efficacious over rest alone. After exercise, cryotherapy is commonly used in the acute phase. Biomechanical deficits and modifiable risk factors should be addressed ( Table 19.2 ). If the navicular drop test is positive, overpronation should be treated (e.g., foot strengthening, shoe modifications, orthotics, taping, or gait retraining). Extracorporeal shock-wave therapy, combined with a graded running program, has been shown to shorten the duration of symptoms compared with a graded running program alone.

In the military population, three RCT's are available regarding the treatment of MTSS. In these studies, rest was equal to any intervention. Surgery can be considered if conservative treatment fails.

Surgical treatment of MTSS is rarely indicated. There are no RCTs regarding surgical outcomes, and the studies available are of poor methodological quality. Surgery often involves fasciotomy along the posteromedial border of the tibia. Another study added removal of a strip of periosteum from the posteromedial border to decrease traction. For relief of pain, these studies report good to excellent results in 69%–92% and return to preinjury activity levels in 31%–93% of patients.

Return-to-running guidelines

During the period of relative rest, focus is placed on cross-training with low-impact exercises that do not cause pain (e.g., deep water running, antigravity treadmill (AGT) running, swimming, elliptical machine, and stationary cycling). The athlete should progress from restricted activities to unrestricted running in a stepwise, gradual fashion. Return-to-running (RTR) is considered when the athlete is pain-free with daily activities, low-impact exercise, and nontender to palpation along the affected area.

Running should begin on a treadmill to provide more shock absorption and less strain on the lower limb. Treadmill running provides absolute control over the pace, duration, and incline/decline of the run. If available, AGT running can be considered prior to use of a traditional treadmill. The AlterG Anti-Gravity Treadmill provides body weight support up to 80% in 1% increments allowing for a graded exercise program which can be precisely controlled. However, no study has evaluated the use of AGT running in MTSS and a recent study found a significant difference between the reported and measured body weight supported by the AGT. Running duration should be increased before intensity with slow increases of no more than 10% per week. The athlete is slowly transitioned to running outside, initially on a track or uniform surface with moderate firmness to provide more shock absorption. Clinicians should emphasize proper running technique and gait retraining. Gradually, the runner may increase training intensity, volume, and add high intensity intervals specific to the requirements of their running event.

Prevention

Prevention programs do not appear to influence the rate of MTSS in military personnel. They have not been studied in competitive runners. In military populations, shock-absorbing insoles have been shown to reduce the incidence of MTSS. Runners should change shoes every 250–500 miles as most shoes lose about 40% of their shock-absorbing capability at that mileage.