The patella, commonly known as the kneecap, is a unique bone positioned at the front of the knee joint. It is the largest sesamoid bone in the human body, embedded within the quadriceps tendon. Its placement serves a significant purpose, acting as a protective shield for the knee joint and enhancing the leverage of the quadriceps muscle. This mechanical advantage allows for efficient extension and straightening of the leg, with the patella acting as a fulcrum.
Observable Physical Changes
A broken kneecap, or patellar fracture, often presents with visual signs around the knee. One of the most immediate and noticeable changes is rapid swelling. This swelling occurs as blood and fluid accumulate within the joint space due to the injury, a condition known as hemarthrosis. The knee may appear puffy and larger than its uninjured counterpart, with the extent of swelling varying depending on the severity of the fracture.
Accompanying the swelling, bruising develops around the front of the knee. This discoloration, which can range from red to purple or black, is a result of blood leaking from damaged blood vessels into the surrounding soft tissues. The bruising may not appear immediately but can become more pronounced over hours or days, spreading across the knee area. The affected area will also be tender to the touch.
In severe cases, a visible deformity may be present. The patella might appear displaced from its normal position, or there could be an indentation or gap where the bone has fractured. For instance, a transverse fracture might cause a palpable gap in the kneecap. In rare instances, bone fragments might protrude through the skin. The specific appearance of the deformity depends on the type of fracture, such as a stable or comminuted fracture.
Immediate Sensations and Functional Limitations
Beyond the visible changes, a broken kneecap is characterized by intense pain localized over the kneecap. This pain is severe and can make any attempt at movement or weight-bearing difficult. The discomfort often worsens with attempts to bend or straighten the knee.
A functional limitation accompanying a patellar fracture is the inability to straighten or extend the leg. This occurs because the fracture disrupts the “extensor mechanism” of the knee, which involves the quadriceps muscle, patella, and patellar tendon working together to extend the leg. When this mechanism is compromised, the leg cannot be held out straight, and attempts to do so will be unsuccessful and painful.
Bearing weight on the affected leg becomes difficult or impossible. The instability and pain caused by the fracture prevent the knee from supporting the body’s weight, making standing or walking challenging without assistance. Weight-bearing may be restricted even with a knee immobilizer or brace.
At the moment of injury, some individuals might hear or feel a popping or grinding sensation or sound. This can indicate the sudden break or displacement of bone fragments within the knee joint. While not always present, this sensation can indicate a knee injury.