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Patellar (Kneecap) Fracture

Kneecap Anatomy

The patella covers and protects the knee

A patellar fracture is a break in the patella, or kneecap, the small bone that sits at the front of your knee. Because the patella acts as a shield for your knee joint, it is more likely to break if you fall directly onto your knee or hit it against the dashboard in a car accident. A patellar fracture is a serious injury that can make it difficult or even impossible to straighten your knee or walk.

Some simple patellar fractures can be treated by wearing a cast or splint until the bone heals. In most patellar fractures, however, the pieces of bone move out of place when the injury occurs. For these more complicated fractures, surgery is needed to repair the kneecap.

What is the anatomy of the patella?

The patella is a small bone located in front of the knee joint — where the thighbone (femur) and shinbone (tibia) meet. It protects the knee and connects the muscles in the front of the thigh to the tibia.

The ends of the femur and the undersides of the patella are covered with a smooth substance called articular cartilage. This cartilage helps the bones glide easily along each other as you move your knee.

What causes a patellar fracture?

Patellar fractures are most often caused by:

  • Falling directly onto the knee
  • Receiving a sharp blow to the knee, such as might occur during a head-on vehicle collision if your kneecap is driven into the dashboard

The patella can also be fractured indirectly. For example, a sudden contraction of the quadriceps muscle in the knee can pull apart the patella.

What are the symptoms of patellar fracture?

The most common symptoms of a patellar fracture are pain and swelling in the front of the knee. Other symptoms may include:

  • Bruising
  • Inability to straighten the knee or keep it extended in a straight leg raise
  • Inability to walk

What are the different types of patellar fractures?

Stable Fracture

Illustration and x-ray show a vertical, stable fracture of the patella.

In this type of fracture the pieces of bone may remain in contact with each other or be separated by just a millimeter or two. In a stable fracture, the bones usually stay in place during healing.

Displaced Fracture

Illustration and x-ray show a front (left) and side (right) view of a two-part fracture across the patella (transverse fracture) with slight displacement between the broken pieces of bone.

In a displaced fracture, the broken ends of the bone are separated and do not line up correctly. The normally smooth joint surface may have also moved. This type of fracture often requires surgery to put the pieces of bone back together.

Commuted Fracture

A comminuted fracture

In this type of fracture, the bone shatters into three or more pieces. Depending on the specific pattern of the fracture, a comminuted fracture may be either stable or unstable.

Open fracture. In an open fracture, the bone breaks in such a way that bone fragments stick out through the skin or a wound penetrates down to the bone. An open fracture often involves damage to the surrounding soft tissues and may take a longer time to heal. Open fractures are particularly serious because, once the skin is broken, there is a higher risk for infection in both the wound and the bone. Immediate treatment is required to prevent infection.