Signs and symptoms of an ACL injury usually include:

• A loud "pop" or a "popping" sensation in the knee
• Severe pain and inability to continue activity
• Rapid swelling
• Loss of range of motion
• A feeling of instability or "giving way" with weight bearing

Ligaments are strong bands of tissue that connect one bone to another. The ACL, one of two ligaments that cross in the middle of the knee, connects your thighbone (femur) to your shinbone (tibia) and helps stabilize your knee joint.

ACL injuries often happen during sports and fitness activities that can put stress on the knee:

• Suddenly slowing down and changing direction (cutting)
• Pivoting with your foot firmly planted
• Landing awkwardly from a jump
• Stopping suddenly
• Receiving a direct blow to the knee or collision, such as a football tackle

When the ligament is damaged, there is usually a partial or complete tear of the tissue. A mild injury may stretch the ligament but leave it intact.

There are a number of factors that increase your risk of an ACL injury, including:

• Being female — possibly due to differences in anatomy, muscle strength and hormonal influences
• Participating in certain sports, such as soccer, football, basketball, gymnastics and downhill skiing
• Poor conditioning
• Wearing footwear that doesn't fit properly
• Using poorly maintained sports equipment, such as ski bindings that aren't adjusted properly
• Playing on artificial turf surfaces

People who experience an ACL injury are at higher risk of developing osteoarthritis in the knee. Arthritis may occur even if you have surgery to reconstruct the ligament.