Three bones meet within the knee joint: the femur, or thighbone, the tibia, or shinbone, and the patella, or kneecap. A stiff blow to any of these bones, especially when complicated by low bone density or low calcium levels or degenerative bone disease, may cause the bone to break, or fracture. An automobile accident in which the patient's knee hits the dashboard often causes the patella to break. The femur, the longest and strongest bone in the body, requires severe force to break, and such trauma is often associated with damage to blood vessels, nerves, and other tissues in the lower extremity.

The tibia, or shinbone, is susceptible to two types of fractures: acute, a blow by a low moving object or person, and stress fractures, caused by repetitive jarring such as running. In addition, the tibial plateau, the ridge at the top of the shinbone, may break. A tibial plateau fracture is often accompanied by tears of the meniscus.

Most fractures are "closed," meaning that the bone is broken without breaking the skin. Severe fractures, known as "open" or "compound," break the skin, increasing the difficulty in treating it. A fracture as seen on X-rays may take one of four types:

• Transverse, in which the bone is split across its axis, usually caused by trauma to the joint.
  
• Oblique, caused by both trauma and twisting, appears slightly diagonal.
  
• Spiral fractures are more diagonal, and are caused strictly by twisting motions.
  
• "Comminuted" fractures occur when the bone is broken into three or more pieces.

Certain other types of fracture may occur in the knee as the result of cancer or disease (pathologic), repeated wear (stress), a ligament pulling part of the bone off from its origin (avulsion) or along the epiphyseal, or growth plate (epiphyseal).