Knee extensor injuries

[Mary Mallozziberg]

Knee extensor injuries occur when people try to straighten (extend) their knee and an obstacle prevents them from doing so. Such injuries can tear the tendon that attaches the main muscle in the thigh (quadriceps) to the kneecap (patella), tear the tendon that attaches the kneecap to the shinbone (tibia), or break the kneecap or top of the shinbone.
(See also Overview of Sprains and Other Soft-Tissue Injuries.)
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

Older age
Osteoarthritis
Use of certain drugs (such as corticosteroids or antibiotics called fluoroquinolones)
Diabetes mellitus
Obesity
Hyperparathyroidism
Polyneuropathy (the malfunction of many nerves)
Anabolic steroid abuse

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 the older people.