Causes / typical injury mechanism: Distal femur fractures have 2 typical injury mechanisms that vary by age. Young patients sustain these injuries secondary to acute trauma with high-energy mechanisms. These fractures can also result from low-energy mechanisms, which tends to happen in elderly osteoporotic patients.

Classic history and presentation: Look for a history of trauma with a high-energy mechanism (eg, motor vehicle or motorcycle crash, struck pedestrian, fall from a height) or low-energy mechanism (eg, mechanical ground-level falls, rotational / twisting injuries).

Prevalence: Distal femur fractures account for < 1% of all fractures and 4%-6% of all femur fractures. Of these, 19%-54% are open fractures, most commonly Gustillo type III. High-energy mechanisms (approximately 58% of cases) are more common than low-energy mechanisms. Annual incidence in the United States is approximately 31 per million.

• Age – Bimodal age distribution. Young patients (aged 10-30 years) and older patients (aged 60-80 years).
• Sex / gender – Young patients are typically male, and older patients are typically women.

Risk factors: Osteopenia, osteoporosis, and proximal or distal femur implants that act as stress risers (ie, total knee replacement, intramedullary nail, plate and screw combo).

Pathophysiology: Excessive stress placed on the distal femur causes fracture. Displacement of the fracture is usually greater in high-energy mechanisms compared to low-energy mechanisms.

Grade / classification system: Müller AO / Orthopedic Trauma Association (OTA) classification for distal femur fractures:

• A – Extraarticular
• B – Partial articular
• C – Complete articular