(See also Overview of Sprains and Other Soft-Tissue Injuries Overview of Sprains and Other Soft-Tissue Injuries Sprains are tears in ligaments (tissues that connect one bone to another). Other soft-tissue injuries include tears in muscles (strains) and tears (ruptures) in tendons (tissues that connect... read more 
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In healthy people, knee extensor injuries occur only when force is significant (as occurs in a jump from a height or in high-impact car crashes). However, people may have certain conditions that make these injuries more likely. These conditions include
People with one of these conditions can injure their knee when they stumble on stairs or catch their foot when walking.
Extending the knee involves several structures. The quadriceps tendon attaches the main muscle in the thigh (quadriceps) to the kneecap (patella). The patellar tendon attaches the kneecap to the shinbone (tibia).
In healthy people, the tendons are so strong that the kneecap often fractures before a tendon tears. The quadriceps tendon is injured more often than the patellar tendon, particularly in older people.
Symptoms of Knee Extensor Injuries
Diagnosis of Knee Extensor Injuries
By examining the knee, doctors may be able to tell which structure is injured. If a person has knee swelling and pain after an injury, doctors ask the person to sit and try to extend the injured leg or to lie on their back and lift their injured leg. If the person cannot extend or lift the leg, a knee extensor mechanism injury is likely.
Doctors also take x-rays of the knee. Often, x-rays can show whether a kneecap is displaced or fractured. For example, x-rays may show a knee cap that is positioned higher than its usual place over the knee joint (called a high-riding or raised patella). This finding suggests that the patellar tendon is torn. However, x-rays may appear normal.
Magnetic resonance imaging (MRI) can confirm the diagnosis.
