A 28-year-old woman with a 2-year history of recurrent bone infections affecting multiple sites is evaluated for chronic osteomyelitis. Imaging shows involvement of the femur, tibia, and humerus. What is the most common site of osteomyelitis in children and young adults?

Explanation

## Most Common Site of Osteomyelitis **Key Point:** The **distal femur (metaphyseal region)** is the single most common site of acute haematogenous osteomyelitis in children and young adults, followed closely by the proximal tibia. Together, these two metaphyseal regions account for > 50% of all cases. **High-Yield:** Osteomyelitis has a predilection for the metaphyseal regions of long bones, particularly around the knee joint (distal femur and proximal tibia), because of the unique vascular anatomy in these areas. The distal femur is consistently cited as the #1 site in standard references (Robbins Pathology, Apley's Orthopaedics, Tachdjian's Paediatric Orthopaedics). ### Anatomical Basis: Why the Metaphysis? **Vascular anatomy of the metaphysis:** 1. **Capillary loops:** The metaphyseal vessels form slow-flow capillary loops that turn sharply, creating areas of relative stasis. 2. **Reduced blood flow:** Blood velocity decreases in these loops, allowing bacteria to settle and adhere. 3. **Lack of phagocytic filtration:** The metaphyseal capillaries do not filter out bacteria efficiently. 4. **Proximity to growth plate:** The metaphysis is adjacent to the avascular growth plate, creating a "dead zone" where bacteria can proliferate unchecked. ### Sites of Osteomyelitis: Frequency Distribution | Site | Frequency | Age Group | Notes | |------|-----------|-----------|-------| | **Distal femur (metaphysis)** | **25–30%** | **Children & young adults** | **Most common single site** | | Proximal tibia (metaphysis) | 20–25% | Children & young adults | Second most common | | Distal tibia | 10–15% | All ages | Less common | | Ilium | 5–10% | Children | Often missed; atypical presentation | | Humerus | 5–10% | All ages | Less common | | Fibula | < 5% | Rare | Rarely affected | **Clinical Pearl:** The "around the knee" region (distal femur + proximal tibia) accounts for approximately 50% of all acute haematogenous osteomyelitis in children. The distal femur is the single most frequently affected bone, making it the standard textbook answer for "most common site." **Mnemonic:** **"Distal Femur First"** — Distal femur > Proximal tibia > Distal tibia > Humerus > Ilium. ### Why Distal Femur > Proximal Tibia? - The distal femur has the largest metaphyseal surface area of any long bone. - Blood flow is slowest in the distal femoral metaphysis due to the widest capillary loops. - It is the fastest-growing metaphysis in the body, with the highest metabolic demand and greatest bacterial seeding risk. **Warning:** Do not confuse metaphyseal osteomyelitis with epiphyseal involvement. Acute haematogenous osteomyelitis typically starts in the metaphysis and may spread to the epiphysis only if the growth plate is breached (rare in children; more common in infants < 1 year and in adults where the growth plate is closed). *Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.; Apley & Solomon's System of Orthopaedics and Trauma, 10th ed.*