Introduction:
Stress fractures are overuse injuries caused by repetitive submaximal stress on the bone, exceeding its ability to remodel. Commonly seen in athletes, military recruits, and individuals with altered biomechanics or osteoporosis.
Two major types:
- Fatigue fractures: Normal bone under abnormal stress
- Insufficiency fractures: Abnormal bone under normal stress
Common Sites:
- Tibia (especially posteromedial cortex)
- Metatarsals (2nd and 3rd most common)
- Femoral neck
- Pelvis (sacrum, pubic rami)
- Tarsal bones (navicular, calcaneus)
- Ribs
- Pars interarticularis (spondylolysis)
Clinical Clue:
- Gradual onset localized pain, worsens with activity, improves with rest
- May have local tenderness and swelling
Imaging Modalities:
1. Plain Radiograph:
- Often normal in early stages (first 1–2 weeks)
- Later findings:
- Cortical lucency
- Periosteal reaction
- Sclerosis or callus formation
- Fracture line
- Sensitivity: ~15–35% early; improves with time
2. MRI: (Modality of Choice)
- Highly sensitive and specific (detects early bone stress changes)
- Detects bone marrow edema even before cortical disruption
- Staging system (Fredericson for tibia):
| Grade | MRI Appearance |
|---|---|
| 1 | Periosteal edema on STIR |
| 2 | Periosteal + marrow edema |
| 3 | Extensive marrow edema |
| 4 | Visible fracture line + edema |
- Best sequences:
- STIR / T2 fat-sat: Marrow and periosteal edema
- T1: Fracture line (low signal), helps assess healing
3. Bone Scan (Tc-99m MDP):
- Detects increased osteoblastic activity
- Shows focal increased tracer uptake at fracture site
- Useful when MRI not available
- Less specific – may show uptake in tumors or infection
4. CT Scan:
- Good for cortical detail
- Useful in:
- Navicular stress fracture
- Femoral neck fracture
- Spondylolysis
- Detects fracture line not seen on MRI occasionally
- Used for surgical planning or in complex locations
5. Ultrasound:
- Not primary tool, but may show:
- Periosteal thickening
- Hypoechoic fracture line
- Soft tissue edema
Examples of Classic Stress Fractures:
| Location | Imaging Clue |
|---|---|
| Tibia (posteromedial) | STIR hyperintensity along cortex |
| Metatarsals | 2nd metatarsal “March fracture” |
| Femoral neck | Compression side (inferior) – more stable |
| Sacrum | H-shaped vertical + horizontal lines on MRI |
| Pars interarticularis | “Scotty dog” collar defect on oblique X-ray |
Conclusion:
MRI is the most sensitive modality for early detection of stress fractures, especially when radiographs are normal. Radiography and CT help evaluate chronic or cortical involvement. A high index of suspicion and appropriate imaging ensures early diagnosis and prevents complications like complete fracture or non-union.
