An ACL injury is the tearing of the anterior cruciate ligament inside the knee joint. This type of injury occurs most frequently in sports that involve sudden stops and changes in direction such as soccer, basketball and tennis.
Ligaments are strong bands of tissue that connect the bones. The ACL connects the thighbone (femur) to the shinbone (tibia) and stabilizes the knee joint. ACL injuries cause immediate knee swelling, instability and severe pain especially when bearing weight.
Treatment for a severe ACL injury may include surgery to replace the torn ligament and rehabilitation exercises to strengthen the muscles and ligaments and regain stability.


The signs and symptoms of an ACL injury include:

  • A loud pop/snap sound
  • Severe pain and inability to continue with activity
  • Knee swelling that increases for hours after injury
  • Instability when bearing weight


Most common ACL injuries occur during sports and fitness activities. Tearing of the ligament can be caused by suddenly stopping to change direction during running or rotating a firmly placed foot thus twisting or hyperextending the knee.

Other causes of ACL injuries include downhill skiing, badly landing from a jump, football tackles or motor vehicle accidents.

Risk factors

Women have a greater risk of an ACL injury than man in the same sports. Women’s quadriceps (front thigh muscles) are stronger than the muscles at the back of the thigh (hamstrings) thus causing a strength imbalance.
When landing from a jump, women may be more likely to land in an awkward position that increases stress on the ACL.


Experiencing an ACL injury increases the risk of later development of knee osteoarthritis. Knee osteoarthritis causes the joint cartilage to deteriorate and its surface to roughen. Arthritis can occur even if reconstructive surgery of the ligament is performed.


The following suggestions can help reduce the risk of an ACL injury:

  • Improve the physical fitness: Training programs are effective in strengthening and stability exercises such as aerobic conditioning, plyometric exercises, balance exercises, jump training and risk awareness training.
  • Strengthen the hamstrings: It is important for women in particular to strengthen the hamstring and quadriceps muscles.
  • Exercise proper techniques: Technique training and strengthening the hip muscles can help reduce the risk of an ACL injury while playing sports. For instance, if the sport involves jumping it is imperative to learn to land properly.
  • Proper sports gear: The sports gear should be chosen and checked carefully. For example, in downhill skiing, the ski bindings should be adjusted correctly so that the skis will release during a fall.