A 52-year-old man with a 3-month history of right knee pain and swelling undergoes a technetium-99m bone scan. The scan shows focal increased uptake in the medial femoral condyle with normal surrounding bone. Plain radiographs are unremarkable. Which of the following is the most likely diagnosis?

Explanation

## Interpretation of Bone Scan Findings The clinical presentation of a 52-year-old man with localized knee pain and a focal area of increased uptake on technetium-99m bone scan in the medial femoral condyle, with normal radiographs, is classic for a **stress fracture**. ### Why Stress Fracture is the Best Answer **Key Point:** Bone scans are highly sensitive for detecting stress fractures, often showing increased uptake 24–72 hours after injury, while plain radiographs may remain normal for 2–3 weeks. Stress fractures result from repetitive microtrauma causing incomplete fractures. The increased osteoblastic activity at the fracture site leads to increased technetium-99m uptake. The focal nature of the uptake (medial femoral condyle) and normal surrounding bone are consistent with a localized stress fracture rather than a systemic process. ### Bone Scan Phases in Stress Fracture | Phase | Timing | Appearance | |-------|--------|------------| | Blood flow (angiographic) | Immediate | Increased flow to area | | Blood pool (soft tissue) | 5 min | Soft tissue uptake | | Delayed (osseous) | 3–4 hours | Focal uptake at fracture site | The delayed phase showing focal uptake is pathognomonic for stress fracture. **Clinical Pearl:** Stress fractures commonly occur in the femoral neck, tibia, and metatarsals in athletes and military recruits. The medial femoral condyle is a recognized site, particularly in runners. ### Why Other Diagnoses Are Less Likely **Osteomyelitis** would typically show: - More diffuse uptake extending beyond the focal area - Clinical signs of infection (fever, elevated inflammatory markers) - Soft tissue swelling on imaging - The 3-month chronic course is atypical for acute osteomyelitis **Metastatic bone disease** would present with: - Multiple focal areas of uptake (polyostotic pattern) - History of primary malignancy (not mentioned) - Age-appropriate risk factors - Progressive constitutional symptoms - A single focal lesion in a young-to-middle-aged patient without cancer history is unlikely **Osteoarthritis** causes: - Diffuse, bilateral uptake in the joint space (not focal) - Degenerative changes on plain radiographs (which are normal here) - Gradual onset over years, not acute 3-month presentation - Increased uptake is diffuse and symmetrical, not focal and unilateral ## Diagnostic Algorithm ```mermaid flowchart TD A[Focal increased uptake on bone scan]:::outcome --> B{Radiographs normal?}:::decision B -->|Yes| C{Acute or chronic onset?}:::decision B -->|No| D[Fracture visible on X-ray]:::outcome C -->|Acute, localized pain| E[Stress fracture]:::action C -->|Chronic, systemic signs| F[Infection or malignancy]:::action E --> G[MRI or CT for confirmation]:::action F --> H[Blood cultures, tumor markers]:::action ``` **High-Yield:** The combination of a normal radiograph with a focal bone scan abnormality is the hallmark of early stress fracture detection. This is why bone scans remain valuable in sports medicine and occupational health screening. [cite:Harrison 21e Ch 404]