A hairline kneecap fracture is a simple crack in the bone (the patella is still in one piece). Hairline fractures are also known as stress fractures and are rare in the kneecap. They can occur in athletes, such as marathon runners, and may be due to overuse. A stress fracture of the patella may be difficult to see on an X-ray. The main symptom is a pain in the front of the knee that gets worse over time.