MRI Findings for Bone Stress Injury
On MRI, bone stress injuries demonstrate a fracture line (appearing as linear T1 and T2 hypointense signal) surrounded by bone marrow edema (T1 hypointense and T2 hyperintense signal), best visualized on fluid-sensitive sequences like STIR (Short Tau Inversion Recovery) and T1-weighted images. 1
Key MRI Characteristics
Primary Findings
- Fracture line: Linear areas of low signal intensity on both T1 and T2-weighted images, continuous with the cortex 2
- Bone marrow edema:
- T1-weighted: Hypointense (dark) signal in surrounding marrow
- T2-weighted/STIR: Hyperintense (bright) signal in surrounding marrow 1
Additional Findings
- Periosteal edema: Juxtacortical and/or subperiosteal areas of high signal intensity on T2-weighted images 2
- Stress reaction (without fracture): Bone marrow edema without a visible fracture line 1
- Muscle/tendon injuries: Can be identified concurrently using fluid-sensitive sequences 1
Optimal MRI Protocol
Use MRI without IV contrast - fluid-sensitive sequences are the favored initial screening sequences 1. There is no evidence supporting the use of contrast, as it provides no additional diagnostic information for stress injuries 1.
Clinical Significance
Why MRI is Preferred
MRI is the procedure of choice for early diagnosis of both fatigue and insufficiency stress fractures, outperforming all other imaging modalities 1. It demonstrates stress abnormalities as early as bone scintigraphy but with considerably more specificity 1.
Prognostic Value
MRI provides both diagnostic and prognostic information about bone stress injuries 1. The Fredericson grading system uses MRI findings to predict severity and return-to-sport timing 3:
- Grade 1: Periosteal edema only
- Grade 2: Bone marrow edema on T2-weighted images
- Grade 3: Bone marrow edema on both T1 and T2-weighted images
- Grade 4a: Multiple focal intracortical signal abnormalities
- Grade 4b: Linear intracortical signal abnormalities (most severe, longest recovery)
Common Pitfall
Do not wait for radiographs to become positive - radiographs remain insensitive for 2-3 weeks and may never show abnormalities in early stress reactions 4. MRI should be obtained when clinical suspicion is high and radiographs are negative or indeterminate 1.