Bone Stress Injuries - Clinical Tree

Introduction and Epidemiology

Bone stress injuries (BSIs) result when bone is unable to withstand repetitive, mechanical loading due to factors that disrupt bone load, bone strength, or bone remodeling. BSIs are a common concern in runners. Studies suggest that BSIs account for 0.7%–20% of all sports medicine clinic injuries.

Competitive cross-country and track-and-field athletes have the highest incidence of BSIs compared with other athletes. The 1-year prospective incidence of BSI in competitive cross-country and track-and-field athletes ranges from 4.9% to 21.1%.

BSIs exist along a pathology continuum progressing from radiographic findings on magnetic resonance imaging (MRI) of periosteal edema with varying degrees of bone marrow edema, to more advanced injuries showing evidence of a cortical fracture line. Early identification and proper management of BSIs may help prevent injuries from progressing along this pathology continuum.

BSI may represent a broader, systemic deficit in metabolic, hormonal, or nutritional status. Low energy availability results from insufficient caloric intake and/or excessive energy expenditure and can impact bone health. The terms Relative Energy Deficiency in Sport (RED-S), the Female Athlete Triad (Triad), and the parallel Triad in males all have energy availability as a key element. Proper screening for low-energy availability and other systemic deficits can help prevent BSI recurrence. The metabolic, hormonal, and nutritional aspects of BSI are covered more fully in Chapter 15 , “Bone Health of the Runner: Metabolic Work-up and Impact on Fracture Risk,” in Clinical Care of the Runner .

This chapter reviews evidence-based medicine and rehabilitation principles for BSIs. Within this framework, the pathophysiology, risk factors, clinical evaluation, and management of BSIs are detailed, followed by a comprehensive discussion of BSI evaluation based on specific anatomic location.

Pathophysiology of Bone Stress Injuries

Trabecular, or cancellous, bone serves as the internal tissue of skeletal bone and is less dense and more elastic than cortical bone, which typically forms the outer casing of long bones. Cortical bone represents approximately 80% of bone mass and consists of the outer periosteum and inner endosteum.

Gonadal hormone irregularities can predispose athletes to cortical and trabecular BSI and/or osteopenia or osteoporosis. Athletes with nutritional deficiencies, hormonal irregularities, and lower bone mineral density (BMD) have an increased incidence of cortical and trabecular BSI compared with athletes without these attributes.

According to Wolff's law, external mechanical forces cause adaptive changes in the internal architecture of the trabeculae, followed by secondary adaptive changes in the external architecture of the cortical bone at the osteoids. In cortical bone, the initial response to an increase in mechanical force is osteoclastic activity, leading to resorption of bone. Osteoblastic activity fills resorption cavities with lamellar bone, but formation of bone is slower than resorption of bone. In trabecular bone, external mechanical force may result in microdamage of the trabeculae, which is repaired by microcallus. If insufficient time is given to adapt to an external mechanical force, an imbalance can occur between bone remodeling and bone damage in both cortical and trabecular bones.

The resulting pathophysiology is currently debated, but accumulating microdamage may coalesce to progress down the BSI pathology spectrum. Stress reactions are associated with periosteal and/or bone marrow edema, and stress fractures have a discernible fracture line.

Bone exhibits anisotropic mechanical behavior or varying strength as a function of direction of load applied. Tensile loading causes elongation and narrowing of a structure, whereas compressive loading causes shortening and widening of a structure. The mechanical properties of bone enable bone to be stronger in compression than in tension. Due to intrinsic stability of the fracture pattern and direct osteogenesis, compression stress fractures are more likely to heal with conservative measures compared with tension stress fractures.

Risk Factors for Bone Stress Injury

Identification and modification of risk factors can help prevent BSI, improve healing times when a BSI does occur, and improve overall bone health of runners. Warden et al. group BSI risk factors into two categories: (1) factors modifying load applied to bone and (2) factors influencing the ability of bone to resist load and prevent accumulation of damage. Table 14.1 highlights risk factors.

Table 14.1

Risk Factors for BSI.

Adapted from Warden A, Davis I, Fredericson M. Management and prevention of bone stress injuries in long-distance runners. J Orthop Sports Phys Ther. 2014;44(10):749–765. https://doi.org//10.2519/jospt.2014.5334.

Factors That Modify Bone Load	Factors That Enable Bone to Resist Load
Biomechanical factors (foot type, leg length discrepancy)	Athletic history (participation in ball sports)
Training patterns (volume, intensity, race frequency)	Past medical history (prior fracture, medications, genetics)
Footwear (orthotics, shoe age)	RED-S and bone health (energy availability, bone mineral density); calcium and vitamin D status

BSI , bone stress injury; RED-S , Relative Energy Deficiency in Sport.

Factors that modify the load applied to bone influence the magnitude, rate, frequency, duration, and direction of bone stress. Factors that influence the ability of bone to resist load influence bone mass, bone size, and histopathologic properties of bone.

Factors That Modify Bone Load

Biomechanical factors

Biomechanical risk factors include both static alignment from anatomic features and abnormal dynamic biomechanical loading patterns. Leg length discrepancy, smaller calf girth, and planus or pes cavus feet are anatomic features that can contribute to BSI risk from altered magnitude or direction of bone loading. Greater average vertical loading, higher peak acceleration, and greater peak free mass are dynamic biomechanical loading patterns that can contribute to BSI risk. Furthermore, higher peak adduction, knee internal rotation, knee abduction, tibial internal rotation, and rear foot eversion during gait may increase BSI risk.

Training patterns

Increases in velocity, duration, or frequency of run training can increase bone stress. An increase in racing distance or frequency can contribute added stress. Studies in adolescent runners indicate that running volumes greater than 32 km (20 miles) per week increase risk of BSI. The influence of running surface and terrain on overall injury and BSI risk is complex. Studies show that runners alter their leg stiffness when running on surfaces of different compliances, which adjusts ground reaction force loading rates. Based on clinical evidence, recent changes in running surface, terrain, or hills should still be assessed for potential BSI risk.

Footwear

The influence of footwear and orthotics on overall injury and BSI risk is also complex, subject to frequent debate, and beyond the scope of this chapter. An assessment for shoe advanced age or recent footwear change is still clinically important in BSI evaluation. For a more comprehensive discussion on runners' footwear and the impact on injury, please review Chapter 9 , “The Interaction of Foot Strike and Footwear in Runners,” and Chapter 10 , “Considerations in Selection of a Running Shoe,” in Clinical Care of the Runner .

Factors That Enable Bone to Resist Load and Prevent Accumulation of Damage

Athletic history and medical history

A history of long-distance running involves lower-impact loads and has not been definitively shown to improve bone health. However, adolescent participation in high-impact and multidirectional loading sports may improve bone density and bone geometry. Participation in ball sports such as basketball and soccer during adolescence was protective against stress fractures in adult male and female distance runners.

In both male and female runners, prior fracture is a risk factor for development of BSI. Medications such as steroids, anticonvulsants, antidepressants, and antacids may impair bone health. Genetic factors impact BMD. Family history of BSI or osteoporosis/osteopenia can indicate potential risk.

Energy availability and bone health

The Triad is defined by the interrelationship of energy availability, menstrual function, and BMD. Under the Triad definition, a female runner could have one or more components of the Triad. In female athletes, a greater number of Triad factors are associated with increased risk of BSI.

As a parallel to the Triad, male athletes may experience low energy availability, hypogonadotropic hypogonadism, and low BMD. Kraus et al. identified BSI risk factors in male athletes and found that risk factors were similar to female athletes with the Triad. Multiple risk factors resulted in increased cumulative risk for BSI in male runners.

RED-S is an alternate term that also incorporates low energy availability as a key element. RED-S can occur in both male and female runners and results from insufficient caloric intake and/or excessive energy expenditure. RED-S can impair bone health and can also progress to the same Triad physiological endpoints including impaired metabolism, menstrual function, immunity, protein synthesis, cardiovascular, and psychological health.

While different terms may be used in scientific literature and clinical practice, it is important to recognize that all terms address the same underlying physiological processes and endpoints. For a more comprehensive discussion of bone health, please review Chapter 15 , “Bone Health of the Runner: Metabolic Work-Up and Impact on Fracture Risk,” in Clinical Care of the Runner .

Calcium and vitamin D status

Calcium is part of the mineralized matrix that gives strength to bone. Vitamin D promotes absorption of calcium in the gut and kidneys to enable bone growth and remodeling. In a study evaluating female runners aged 18–26 years, females who consumed less than 800 mg of calcium a day had nearly six times the stress fracture rate of females who consumed more than 1500 mg of calcium.

In male and female NCAA Division I distance runners, serum vitamin D levels were inversely related to time lost to BSI injury. A 10-unit increase in vitamin D was associated with a 17% decrease in time lost due to BSI.

Clinical Evaluation

History and Exam

A complete history and physical exam is important in athletes presenting with potential BSI. It is important to assess running history to look for changes in training patterns, footwear, alterations in biomechanics, and race schedule. Screening for Triad risk factors including low energy availability, endocrine abnormalities, and low BMD is also imperative. In female athletes, altered reproductive hormone function can be assessed by history of menstrual cycle dysfunction and age of menarche. In males, a history of loss of libido or decrease in morning erections may offer insight into suboptimal reproductive hormone status.

Evaluate medical history with a focus on history of fractures, disordered eating, personal or family history of low BMD, hormonal or oral contraceptive use, and medications that influence bone health.

Understand the quality, onset, and time course of presenting pain. In most circumstances, BSIs start as a mild, diffuse ache that either does not resolve or worsens as running continues. As injury progresses, pain may present earlier during training or occur during walking. In more advanced stages, pain may occur at night or while resting. In other circumstances, particularly with sacral and more axial BSIs, intense pain may occur all at once.

On physical examination, inspect for local swelling, erythema, and bruising. Bony tenderness is the hallmark clinical finding in BSIs. Evaluate for bony tenderness to palpation and pain with direct and indirect percussion. Direct palpation can be difficult at deeper sites including the femur, lumbar spine, and sacrum. In these circumstances, special tests can help elicit and differentiate pain.

The single-leg hop test is considered a bone-loading test that can be attempted to elicit BSI pain in appropriate clinical context. This test imparts stress on the bone and should only be attempted if runners present with minimal weight-bearing pain. The following more specific tests can be considered based on differential diagnosis and location of pain.

Sacral/pelvic bone stress injury

Sacral and pelvic BSIs may present with pain elicited by sacroiliac joint provocative maneuvers including the thigh thrust, pelvic distraction, pelvic compression, and flexion, abduction, and external rotation of the hip (FABER maneuver). Since the specificity of these exam maneuvers is quite low, a high index of clinical suspicion should lead to further investigation of a sacral or pelvic BSI, no matter the outcome of these exam maneuvers.

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