Documentation:FIB book/ACL Injuries in Soccer

Knee joint anatomy

The anterior cruciate ligament (ACL) is a ligament in the knee that provides joint stability by preventing anterior translation of the tibia as well as internal rotation of the tibia. It works with the medial collateral ligament (MCL) and lateral collateral ligament (LCL). ACL tear injuries are common and often require surgical reconstruction and extensive rehabilitation. The knee is the second most commonly injured body site in sports and is the leading source of sports-related surgeries. More than 50% of all knee injuries involve ACL tears. Soccer is a leading cause accounting for 35.2% of ACL injuries in high school students of which 81.5% required surgical repair. The prevalence of ACL tears in soccer is potentially due to frequent and sudden changes in speed and direction, such as cutting or pivoting and landing on one leg. These movements may lead to hyperextension, varus and valgus, or internal rotation of the knee, which are the most common ACL injury mechanisms.

Background

File:AcL CAUSES.jpg

Torn ACL

Epidemiology

Epidemiological studies estimate that in the US, there are 200,000 ACL injuries per year^[4]. One study analyzed National Collegiate Athletic Association (NCAA) soccer players between 1989 and 2004 and found an ACL injury incidence rate of 14 per 100,000 athletic exposures (A unit of susceptibility to injury, defined as one athlete participating in one game, in which he/she is exposed to the possibility of athletic injury) per year.^[4].

Coaches from 100 nationally delegated US secondary schools announced athlete-exposure and injury data between the academic years 2007/08 - 2011/12. Athletic trainers reported 617 ACL injuries occurring in 9 ,452, 180 athletic exposures (AEs), for an overall rate of 6.5 ACL injuries per 100 000 AEs^[2].

Long-term effects

Long-term effects of ACL lesions contribute to the development of knee osteoarthritis, leading to pain and functional impairment.   Osteoarthritis is a disorder where cartilage degeneration occurs, leading to bone-on-bone rubbing. The severity of degeneration depends on the individual’s age, sex, genetics, muscle strength, obesity, re-injury and activity^[5]. ACL injuries cause a change in normal physiological loading conditions, cartilage load adaptation, and variations in cartilage structure, which can often lead to cartilage death and osteoarthritis, even after treatment^[6].

When the ACL tears, multiple ligament knee injuries may occur as a result of the trauma. If the ACL, PCL (posterior cruciate ligament), and MCL (medial collateral ligament) rupture, this may result in a  knee dislocation, which can be limb-threatening.^[8].

ACL tears can be accompanied by other detrimental effects such as bone bruising, which is a common injury for soccer players due to running and can lead to post-traumatic arthrosis even after the knee has been reconstructed. Trauma to the knee caused by an ACL injury can cause further damage to the femur, tibia, and patella. A bruised bone causes swelling, stiffness, and painful movement to the knee, limiting its range of motion

Risk Factors

Age, sex, and other factors may put an individual at higher risk to injure their ACL. Studies show that female athletes suffer from ACL injuries at 3 times the rate as their male counterparts. The specific reason for this is unknown, but potential causes include differences in muscle control, conditioning, and strength. Individuals who have previously torn their ACL are 15% more likely to experience reinjury than someone with no previous injury. Age also influences the risk of an ACL tear. This injury is most common in demographics between the ages of 15 and 45, primarily due to their more active lifestyle and higher participation in sports.

Injury Mechanisms

Example of a contact injury

Contact Injuries

A study that sampled high school players, contact-based ACL injuries were found to be the most common ACL injury mechanism. Player-player contact was found to be the most common injury mechanism followed by non-contact, then player-surface contact. They encompass approximately 40% of all ACL injuries in both genders in high school soccer in the USA. While ACL tears due to contact under dynamic loading are common in soccer, there are also other contributing factors to be considered, such as the individual's notch morphology, age, and strain rate. Cadavers were used to reproduce ACL injuries by varying the input strain rate to the hyperextended ligament (high strain rate to simulate pedestrian-motor vehicle accidents; low strain rate to simulate sporting contact), but there were no significant differences found in the ACL tear rate.. However, it is noted that this study was limited by the lack of cadaveric muscle activation.

Please refer to the following link as a more clear example.

Non-Contact Injuries

Example of potential non-contact injury scenario during running

Non-contact ACL tearing is caused by forces generated by the subject themselves as opposed to external sources. They are different from contact injuries that involve forces induced from another player. They encompass 21.6% of boy’s soccer injuries and 7.5% of girl’s soccer injuries. Commonly found mechanisms of injury are repetitive acceleration/deceleration, landing, and pivoting, which cause dynamic valgus, knee abduction, ankle eversion, and femoral adduction. These movements are often found in soccer, as athletes are required to quickly change speed and direction to out-maneuver their opponents.

The exact loading mechanism that causes a non-contact ACL injury is uncertain. Studies in cadaveric knees suggest that a combination of anterior shear force at the proximal end of the tibia, knee valgus/varus, internal rotation moments, and high sagittal plane forces impact ACL loading. An examination of female athletes reveal that previously-injured athletes have greater valgus flexibility and can sustain higher moments in the knee compared to healthy athletes, suggesting that valgus movement is a predicate of non-contact ACL injuries.

However, other research opposes the stance that varus/valgus movement is the primary indicator for injury. The risk of non-contact ACL injury was found to be highly correlated with a medial collateral ligament (MCL) injury, but knee varus was not a significant contributor in cases where the MCL was intact and healthy.  Clinical data still shows that most non-contact ACL injuries are not accompanied by MCL injuries, so the exact mechanisms that cause non-contact ACL injuries are still undetermined.

Injury Prevention

Despite the prevalence of ACL injuries, prevention programs have been successful at reducing injury rates by 50% across various sports. This success has only been attributed to exercise programs, however, not engineered preventative devices. The effectiveness of using a knee brace to reduce ACL elongation in lateral impact scenarios was tested with instrumented cadaveric knees. While studies did not find a significant reduction of the ligament elongation, braces were able to absorb and reduce much of the impact force. There are also drawbacks with using preventative braces. Despite the positive effects exhibited in lateral and medial impacts, knee braces are found to behave neutrally in posterior impacts and harmfully in anterior impacts with respect to strain change of the ligament and joint accelerations. Additionally, no benefits of the knee brace were found for the application of internal and external moments on the knee.

Another study tested the biomechanical effect of ACL kinesio taping on the knee with athletes performing drop vertical jumps and found a significant difference in the knee abduction angle from 100ms before initial contact (IC), at IC, and for 100ms after IC. This would appear to be effective in minimizing injury potential in the valgus injury mechanism.

Limitations & Future Work

While ACL injuries are common in soccer, currently there are few studies that are able to completely capture the whole picture of the injury due to technology available today. There are limitations to what data can be collected during biomechanics studies because experimenters are unable to instrument players in a non-invasive way for direct measurements. It is also challenging to measure the biomechanics of ACL injuries during sports because athletes are unable or unwilling to wear accelerometers during play, either due to discomfort or because the equipment may impede natural movements. To overcome this, researchers use ways to closely simulate ACL injuries, but these methods come with limitations.

Cadaver studies are beneficial as they accurately reflect human anthropometry, but their mechanics are not realistic in many scenarios. This is due to a lack of muscle activation, possible degeneration of tissue, as well as the lack of functioning bodily systems which would otherwise provide internal pressures or bleeding. Nerve damage and pain also cannot be studied due to the lack of a functional nervous system. Finally, the number of samples in each study is often small since cadavers are difficult to obtain and manage in large quantities.

Additionally, because the ACL is a soft tissue, its properties dramatically change depending on tissue treatment methods and may not provide data that reflects in-vivo properties. Despite these drawbacks, future cadaver studies are needed to better understand the general structural mechanisms involved in ACL tears.

A limitation of current ACL research is the lack of an exact, agreed upon mechanism or metric to measure thresholds for ACL injuries. Without a general metric, it is difficult to compile and compare data or develop preventative devices. Researchers have determined contributing factors such as anterior shear force, knee varus/valgus, internal rotation moments, and high sagittal plane forces, but future work can be done to determine the relationship between injury and these criteria.

Additional work can be done in the field of engineered prevention devices for ACL injuries. While there is awareness that ACL injuries are prevalent in soccer, many procedures for prevention are exercise or rehabilitation based. For example, the Vancouver-based company Embrace Orthopaedics is attempting to develop injury prevention devices that provides support without impeding movement. In the process of developing such a device, extensive research and testing would be required to determine its effectiveness in preventing ACL injury, perhaps in conjunction with research on an injury threshold.