Fatigue fracture

Fatigue fractures are a type of stress fracture due to abnormal stresses on normal bone. They should not be confused with an insufficiency fracture, which occurs due to normal stresses on abnormal bone. Plain radiographs typically demonstrate a linear sclerotic region but have poor sensitivity, especially in early-stage injuries. MRI is the most sensitive and specific modality. Bone scans are sensitive but not specific.

Terminology

Some authors ³ use the term stress fracture synonymously with fatigue fractures, and thus some caution with the term is suggested.

Epidemiology

The demographics are usually young active patients, females more common than males, with specific locations having different demographics in keeping with the associated activity ².

Pathology

The abnormal stresses resultFatigue fractures occur when the rate of accumulated microdamages due to sudden increase in repeated microfractures at sitesfrequency, intensity, and duration of physical activity is beyond the normal pace of structural weaknessthe bony remodeling process.

Accumulation of micro injuries/ micro fractures (plastic irreversible deformations) of the normal bone reaching the failing point in which their accumulation exceeds the repair pace/capacity of the bone, hence the gross failure and incompletetraumatic fracture ensues. In other words, injury happens faster than healing results with repeated injury. These are typical in athletes.

Location

Weight bearing lower limb fatigue fracture are by far more common. In many cases of lower limb fractures, bilateral abnormalities are present, whereas in the upper limb they are more frequently unilateral and involving the dominant arm.

• pelvis and lower limb (most common)
  • medial neck of femur
    • compressive forces
    • ballet, running, gymnastics
  • pubic rami / obturator ring: bowling, gymnastics, stooping
  • tibia
    • proximal in children
    • mid to distal in adults ²
  • calcaneum
    • bilateral in up to 27% of cases
    • jumping/landing on heels/prolonged standing
  • navicular: marching/running
  • metatarsals : marching/prolonged standing/ballet
  • sesamoid of great toe: prolonged standing
• upper limb
  • spinous processes of C6, C7, T1, T2: shovelling
  • coracoid process: trap shooting
  • ribs: chronic cough, golf, carrying heavy pack
  • humerus: throwing
  • coronoid process of ulna: propelling wheelchair, throwing javelin
  • hook of hamate: racket sports, golf

Radiographic features

Early diagnosis is best made with MRI (near 100% sensitive) or bone scan (less specific than MRI) as plain radiographs may appear normal for some time - the sensitivity for early-stage injuries is ~25% (range 15-35%) and late-stage injuries is ~50% (range 30-70%) ⁴. CT scan is not first neither second line of investigation if stress fatigue fracture is suspected. CT scan is only indicated in cases which MRI is equivocal due to its higher specificity in detecting fracture lines ⁴.

Plain radiograph

• initially normal
• gray cortex sign: corticalearliest changes, cortical lucency at the site ofdue to microfracture and osteoclastic activity
• cortical thickening: periosteal reaction progressing and endosteal calllos formation
• lucent fracture line

In predominantly trabecular bones such as metaphysis of long bones on the other hand the first sign would be subtle trabecular blurring and faint sclerosis , follwed by frank sclerotic line. If fracture extends to callus formation in diaphyseal fractures

MRI

MRI is as sensitive as bone scanning but is of higher specificity, both in isolating the exact anatomic location and in distinguishing fractures from tumours or infection.

MRI signal characteristics

• T1
  • low marrow signal
  • very hypointense linear fracture line may be present
  • hypointense periosteal/endosteal new bone formation
• T2: high marrow signal with extension into adjacent soft tissues
• T1C+: enhancement can be prominent

Nuclear medicine

There is increased activity at the site of the fracture on bone scans. 

Treatment and prognosis

Treatment depends on the location and whether the fracture is complete or incomplete.

Options include conservative management, plaster cast, internal fixation. Most importantly change in behaviour to reduce the activity which has lead to the fracture is needed. In some instances, an altered technique may be sufficient to prevent re-occurrence.