A 65-year-old man with a history of osteoporosis presents with groin pain and a positive Trendelenburg sign after a minor fall. Plain radiographs of the pelvis show no obvious fracture line, but there is subtle sclerosis in the femoral neck. Which investigation is most appropriate to detect a stress fracture of the femoral neck?

## Investigation of Choice for Stress Fracture of Femoral Neck ### Clinical Scenario Stress fractures of the femoral neck are subtle injuries that may not be visible on initial plain radiographs. Early detection is critical to prevent progression to complete fracture and avascular necrosis. ### Why MRI is Superior for Stress Fractures **Key Point:** MRI is the most sensitive imaging modality for detecting stress fractures of the femoral neck. It shows bone marrow edema before any radiographic changes appear. **High-Yield:** In stress fractures, MRI can detect the injury within **24–48 hours** of symptom onset, whereas plain radiographs may remain normal for **2–3 weeks** until callus formation occurs. This early detection allows timely intervention. ### Imaging Findings on MRI - **T1-weighted:** Low signal intensity (dark) at fracture site - **T2-weighted / STIR:** High signal intensity (bright) due to bone marrow edema - **Sensitivity:** 100% for stress fractures ### Comparison of Modalities for Stress Fracture Detection | Investigation | Sensitivity | Timeline to Positivity | Clinical Use | |---|---|---|---| | Plain radiographs | 10–30% initially | 2–3 weeks (callus formation) | Baseline; may be normal early | | MRI | 100% | 24–48 hours (marrow edema) | Gold standard for early detection | | Bone scan | 80–90% | 3–5 days (hyperemia + osteoblastic activity) | Alternative if MRI unavailable | | CT scan | 85–95% | 1–2 weeks | Preoperative planning; not first-line | ### Clinical Pearl **In osteoporotic patients with groin pain and negative plain radiographs, MRI should be obtained urgently.** Early diagnosis of femoral neck stress fractures allows conservative management (rest, protected weight-bearing) before progression to complete fracture, which requires surgical intervention. ### Why Delayed Radiographs Are Inadequate Waiting 2 weeks for radiographic changes risks progression of the stress fracture to a complete fracture, especially in elderly patients with compromised bone quality. MRI provides definitive diagnosis within days.