Bone Stress Injuries

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Original Editor - Puja Gaikwad

Top Contributors - Puja Gaikwad, Kim Jackson, Vidya Acharya, Rucha Gadgil and Lucinda hampton  

Introduction

Bone Stress Injuries (BSI) are overuse injuries associated with repeated loading of bone by strenuous weight-bearing activities (such as running, jogging, marching) and inadequate recovery periods. BSI’s represent the failure of skeleton bone to withstand repetitive loading, leading to structural fatigue, localized bone pain, and tenderness around the area. Bone Stress Injuries (BSI) are commonly seen in avid runners, track and field athletes, endurance athletes, military recruits, gymnasts, dancers, but also among otherwise healthy people who have recently started the new or intensive physical activity accounting for 10% of all sports-related injuries. BSI differs in severity, with initial findings of periosteal edema and marrow edema. In more severe conditions, stress fractures with distinct fracture lines are present. Stress fractures account for > 10% of total sports-related injuries and it could be as high as 30% in running.

Pathophysiology

When the bone is subjected to mechanical forces the forces cause adaptive changes in the trabecular (i.e internal architecture of bone), followed by secondary adaptive changes in the bone cortex (i.e external architecture of bone). In trabecular bone, the initial response to mechanical forces is the microdamage of the trabecular which is repaired by a micro-callus. In cortical bone, the initial response to an increase in mechanical forces is osteoclastic activity (bone break down) which leads to resorption of bone. Osteoblastic cellular activity fills the resorption cavities with a lamellar bone. However, bone formation is slower than bone resorption.

In normal healthy bone, new activities or prolonged amounts loading stimulates turn-over of bone cells. First, the body breaks down selected portions of the bone, temporarily weakening it, and then within 1-2 weeks, new bone tissue is laid down. Once established, the new bone is stronger, denser, and better accustomed to the new activity or increase load. However, if we continue high loading or impact activities during this turn-over period if we don’t give our bones adequate time to recover. Over time, the overloaded bone will accumulate micro-fractures and eventually break down, developing into what we call a bone stress injury.

Bone Stress Continuum

There is a continuum of bone stress injury varying from "normal to bone strain to stress reaction to the ultimate progression of a stress fracture. These injuries sit on a continuum whereby they start with a stress reaction (often called a ‘hot spot) which can progress towards a stress fracture and finally towards a complete bone fracture. Essentially, as the BSI progresses along the continuum, the longer it takes to recover. Therefore, an understanding and early recognition of these injuries are critical to any athlete and their sports-specific goals.

On the continuum, bone is accruing microdamage following mechanical forces from loading, for example from running. Microdamage accrues proportionate to the number of loading cycles, rate of loading, and strain magnitude of the skeleton bone. When the bone is given adequate time between loading cycles to recover the rate of bone resorption is met by the rate of bone repair and remodeling. However, when insufficient time is given for the bone to adapt to external mechanical forces, an imbalance may occur between bone remodeling and microdamage to the bone resulting in structural deformation of bone and fatigue This accumulative microdamage can continue to progress and result in further pathology. It can progress from the bone being in a state of stress reaction through to stress fracture and at times even frank cortical fracture.

Stages of Bone Stress Injuries

There are three progressive stages of bone stress injuries:

1. Bone strain: there are signs of bone stress on medical imaging but an athlete is not yet experiencing symptoms.
2. Stress reaction: bone stress injury is severe enough to cause localized pain on the bone, which aggravates with sporting activities. Athletes will often also experience tenderness to the touch.
3. Stress fracture: the final stage of the bone stress continuum when a fracture or break in the bone develops, which can be seen on medical imaging. Typically pain is very pronounced in a clear localized area, and often bearing weight on the limb is enough to provoke pain.

Common Locations

Common sites for bone stress injuries and bone stress fractures are primarily seen in the lower limbs. Common locations of Bone Stress Injuries are as follows:

• The front or inside of the tibia (shin bone)
• Fibula
• Metatarsal or carpal bones of the foot
• Medial malleolus of the ankle
• Neck or shaft of the femur
• Calcaneum

Risk Factors

Bone stress injury risk factors could be grouped into two categories:

• Biomechanical factors- factors that modify and affect the load applied to a bone/skeleton
• Biological factors- factors that modify how bone resists strain and the resultant accumulation of damage. (see table 1 below)

Biomechanical Factors	Biological Factors
Training errors: sudden increases in training loads (not allowing the bone sufficient time to adapt)	Athletic/physical activity history: previous history of BSI
Faulty biomechanics: Poor running form, and techniques	Medical history: Family history of BSI or osteoporosis/osteopenia, history of medications like the use of oral steroids, hormone therapies, cancer medications, antidepressants, antacids, anticonvulsants, etc.
Footwear: for example, wearing the wrong shoes	Low Energy availability/RED-S
Rigid training surface	Bone health: Lower bone mineral density
Poor muscle strength and endurance	Insufficient calcium and Vit D
Misalignment or anatomic problems like lean body mass, leg length discrepancy, small calf circumference, pes cavus, or pes planus, etc.	Poor diet and nutrition
Dynamic loading patterns: for example, greater vertical loading, peak hip adduction, and rearfoot eversion, etc. during gait.	Stress

Table 1: Risk factors as proposed by Warden

Classification

Injury classification systems may help to grade severity, choose treatment options, and determine prognosis. In athletes who show imaging changes compatible with the symptoms of BSIs, the anatomical location and imaging findings of the lesion are important factors. Based on the understanding of the biological healing process, knowledge of blood supply to the bone in different areas of the body, and the direction of loading on the bone, these locations have been classified into low, medium, and high risk of fracture (see table 2 below). Low-risk BSIs are susceptible to compressive forces and normally heal well with activity modification, in which case normal weight-bearing is likewise possible. High-risk BSIs, on the other hand, are vulnerable to tensile forces and occur in areas with decreased blood flow. They are susceptible to delayed union or nonunion and are more likely to advance to complete fractures in a later stage.

Low	Medium	High
Posteromedial tibia	Pelvis (sacrum and pubic rami)	Femoral neck
Fibula/lateral malleolus	Femoral shaft	Patella
Calcaneus	Proximal tibia	Anterior tibial diaphysis
Diaphysis of second to fourth metatarsals	Cuboid	Medial malleolus
	Cuneiform	Talus (lateral process)
		Great toe sesamoid
		Navicular

Table 2: Anatomical Location and Risk of BSI

Evaluation and Diagnosis

History taking and physical examination

Bone stress injuries are an overuse injury that is typified by a gradual onset of pain with activity. A thorough review of the athlete's training history is essential, looking for a spike, or rapid change in training load. In female athletes, evaluation of female athlete triads is vital, and dietary restriction behaviors, daily intake of calcium and vitamin D, and menstrual irregularities should be reviewed. The previous fractures and family history of low bone mineral density (BMD) should also be investigated. Hormonal use, like oral contraceptives, and the use of drugs like steroids and antacids, which can affect bone health, should be reported. In the early stages of the continuum, pain can be a mild diffuse ache during running that settles on rest. As the stress reaction progresses, pain may be significant during loading and in the late stages, pain can be present even at rest or with low load tasks like walking or at night.

On physical examination, Pain is usually localized and will be tender to palpate. Because direct palpation of deep areas is difficult at deeper sites, a single-leg hop test or fulcrum test of the femur shaft can be performed to provoke pain. In more severe cases, changes in skin color and local edema can be visible.

Assessment of bone health

For athletes with diagnosed BSIs, clinicians must measure BMD and perform an endocrine function assessment. According to the American College of Sports Medicine, Z-scores less than −1 are considered to be low bone mass in female athletes who take part in weight-bearing sports. For male athletes, no criteria have yet been established. Z-scores less than −2 in both genders are considered as low bone density by the International Society for Clinical Densitometry.

Imaging

Often, imaging such as an X-ray or MRI is needed to confirm the diagnosis. A stress reaction will show swelling (bone edema) on MRI. No true fracture line will be seen either on the MRI or plain x-ray. Recently, the use of diagnostic ultrasound is increasing and may be used in the field, enabling early detection of BSIs and continued use in the follow-up.

MRI Grade	Nattiv et al.	Fredericson et al.
1	Mild marrow or periosteal edema on T2; T1 normal	Mild to moderate periosteal edema on T2; normal marrow on T2 and T1
2	Moderate marrow or periosteal edema plus positive T2	Moderate to severe periosteal edema on T2; marrow edema on T2 but not T1
3	Severe marrow or periosteal edema on T2 and T1	Moderate to severe periosteal edema on T2; marrow edema on T2 and T1
4	Severe marrow or periosteal edema on T2 and T1 plus fracture line on T2 or T1	Moderate to severe periosteal edema on T2; marrow edema on T2 and T1; Fracture line present.

Table 3: MRI grading scale