The ACL or anterior cruciate ligament is one of the four main ligaments in the knee. Ligaments are made of connective tissue and join bones together and give support to a joint. Other ligaments in the knee are the PCL (posterior cruciate ligament), MCL (medial collateral ligament), and LCL (lateral collateral ligament).
The shape of the bones that form the knee joint give the knee a high degree of mobility, which allows the joint to bend quite far. The ACL is found near the front of the knee, just beneath the patella (knee cap) and helps to limit how much the lower leg bone (tibia) can move away the thigh bone (femur). Its angled position also helps prevent the tibia from twisting too much when an athlete’s foot is on the ground. .
Explore how ligaments in the knee help provide stability
Researchers have studied ACL injuries extensively over the past two decades and have identified many factors that can make athletes susceptible to ACL injuries. Most of these are directly related to the athletes themselves, which we call internal factors. Others are associated with other outside or external factors.
Internal factors that put young athletes at risk of ACL injury include the following:
- Neuromuscular Factors
Every athlete has differing levels of strength, balance, coordination, flexibility, and agility. Differences in these neuromuscular factors allow each athlete to perform athletic activities and movements in their own unique way. Imbalances or deficits can cause an athlete to move in a way that places excessive force and strain on the ACL, greatly increasing the risk of an injury.
Movements such as jumping and landing (plyometrics), quickly changing direction (cutting), and quickly changing speed (acceleration or deceleration) with improper technique have been studied extensively over the past decade in motion analysis laboratories and have been identified as the cause of many non-contact ACL injuries. Researchers have also found that limited strength in the hip and core muscles significantly affect how an athlete moves and can greatly increase the risk of an ACL injury.
Learn more about the Leon Root Motion Analysis Laboratory at HSS
Female athletes have a higher risk of injuring their ACL during certain sports, with injury rates more than two times greater for soccer players and more than three times greater for basketball players when compared with their male counterparts. Males and females move differently changing speed, changing direction, and jumping and landing, putting females at greater risk of injury. Females' ACLs are also generally smaller and less stiff than males. This lack of stiffness is called laxity, and is often seen in sports that require incredible flexibility, such as gymnastics and cheerleading.
- Previous Injury
Athletes who've had a previous ACL injury or surgery are more likely to experience another ACL injury in either the same knee or the opposite knee. A recent study found that athletes with an ACL injury in the previous year were over 11 times more likely to suffer another ACL injury compared to those who were uninjured. Athletes who had experienced an ACL injury longer than one year prior were still more likely to have another ACL injury than those who had not been previously injured.
- Body Composition
Having a high body mass index (BMI) has been shown to increase the risk of ACL injury in female athletes. High BMI in females is sometimes linked to a higher proportion of fat mass and less lean (muscle) mass. Athletes with lower lean mass often have greater laxity in their ligaments and deceleration and changing directions may be more strenuous and less efficient as a result.
- Fatigue and Dehydration
Being tired or dehydrated can increase ACL injury risk by impairing an athlete's performance. Fatigued athletes often experience changes in strength, body posture and neuromuscular control over the course of a practice or game. Being tired can change the way an athlete jumps or lands and can result in an injury.
Dehydration and heat stress can impair an athlete's ability to perform repeated sprinting and jumping movements. Dehydration also impairs neuromuscular control and balance and raises the risk of joint injuries. Researchers have shown that dehydrated athletes had more unwanted hip, knee, and ankle movements and swayed more when standing on one leg during exercise testing compared to athletes that were well hydrated.
External factors that put young athletes at risk of ACL injury include the following:
- Shoe-Surface Interface
The way an athlete's shoes connect with playing surface can greatly affect knee mechanics and ACL injury risk. The type of playing surface and weather conditions can influence how much friction there is between the shoe and the playing surface. Too much or too little friction between the ground and an athlete’s shoes can alter their ability to maintain a stable contact point. Wet surfaces decrease the amount of friction and can lead to an unexpected fall or loss of balance.
ACL injuries have also been linked to athletic shoes with long cleats on the outside borders of the shoe and shorter cleats in the middle. This design creates higher levels of turning force (torsion) than shoes with shorter or fewer cleats.
The composition of the playing surface also matters: athletes who play on synthetic indoor surfaces are more than twice as likely to experience an ACL injury as those that play on wooden surfaces.
Researchers have also found that the rate of ACL injury among college football players on artificial outdoor turf was 1.4 times greater than the injury rate on natural grass surfaces.
ACL Injuries can be divided into two main categories: Contact and non-contact injuries.
Contact ACL Injuries
A contact injury occurs when an athlete collides with another player or object. This happens much less often than non-contact injuries. Athletes who play contact sports such as football, lacrosse, and rugby are at the highest risk of suffering a contact injury of their ACL than athletes in other sports, though ACL contact injuries can occur during any sport. The ACL may be injured when an athlete’s knee hyperextends or if they collide in a way that causes the knee to bend or twist unnaturally. Contact injuries may also result in damage to other ligaments or structures in the knee, such as meniscus, tendons, or bones.
Non-Contact ACL Injuries
Non-contact injuries occur when the forces produced by athletes themselves are higher than the ACL can bear. Jumping and landing, quickly changing direction, or changing speed when running are common activities for most athletes. These types of activities can result in a non-contact ACL injury if the stress applied to the ligament during these motions is too great. The ligament is most vulnerable when the lower leg turns inward (tibial internal rotation) and the knee is forced inward toward the middle of the body (knee valgus).
Excessive forward or lateral (sideways) movement of the upper body and trunk can cause the lower leg to rotate internally and cause tibial internal rotation and knee valgus. Poor posture, insufficient core stability, and weak hip and pelvic muscles can contribute to excessive tibial internal rotation and knee valgus.
- 78% to 98% of professional football and basketball athletes return to pre-injury athletic performance levels within one year after surgery.
- Approximately 33% of amateur athletes return to their pre-injury levels within one year of surgery and 66% return to their pre-injury levels between two to seven years later.
- Approximately one of every four young athletes who sustain an ACL injury and return to high-risk sports will experience another ACL injury.
- ACL injuries can significantly increase the rate of cartilage loss and development of osteoarthritis.
- A recent HSS study found that all patients with ACL tears sustained cartilage damage that increased over time.
- Seven to eleven years after injury, the risk of cartilage loss to the lower portion of the femur was 50 times greater than that seen at the time of injury.
- The risk of cartilage loss in the kneecap was 30 times higher than baseline levels.
- Patients treated without ACL reconstruction were six times more likely to have cartilage degeneration in the tibia and five times more likely to have degeneration in the kneecap than those patients who underwent surgical reconstruction.
- Patients with ACL reconstruction had a slower rate of progression of osteoarthritis than those who did not have ACL surgery, though both have faster rates of progression than athletes without ACL injury.