Genetic Risk Factors for Anterior Cruciate Ligament Ruptures

anterior cruciate ligament, or ACL, is one of four ligaments that connect the bones of the knee joint providing roughly 90% of the knee’s stability (Health Information Publications, 2011). The ACL is one of the most important of the four ligaments and is often injured during athletic activities although it causes of ACL injury can be from non-athletic activities (Health Information Publications, 2011).

In order to better understand the ACL functions, injury causes, prevention of further or future injury, and the techniques used in surgically repairing a torn ACL, a brief discussion of the anatomy of the knee is both helpful and useful. The knee is a hinge joint and is the meeting place for the femur, tibia, and patella (Netter, 2011). Cartilage provides a smooth layer, which provides somewhat of a cushion for these bones such that the bones may smoothly glide over one another (Netter, 2011). Four ligaments, including the ACL firmly hold the knee together while still allowing range of motion (Netter, 2011). The ACL is located within the knee joint along with the posterior cruciate ligament, or PCL (Netter, 2011). The ACL and PCL form a cross (cruciate is Latin for cross hence the name cruciate) in the center of the knee and play a role in controlling forward (anterior) and backward (posterior) knee motions as well as rotation (Health Information Publications, 2011). The other two ligaments that hold the knee in place are the medial collateral ligament (MCL), which is located on the outside of the inner knee, and the lateral collateral ligament (LCL), which is located on the outside of the outer knee. One other ligament that is very important is the patellar (knee cap) ligament, which connects the patella to the upper portion of the tibia. In ACL reconstruction, physicians will often use a portion of the patellar ligament (Health Information Publications, 2011).

Menisci are crescent shaped structures located either side of the knee and made up of a type of cartilage. Each meniscus acts as a shock absorber to help bear the weight loading between the weight bearing ends of bones in the knee. When the ACL is torn, it is very common for the menisci to tear as well (Bahr & Krosshaug, 2005).

Ligaments help stabilize the knee joint, menisci act as shock absorbers, and muscles aid in movement control, therefore, muscles play an integral role in keeping the ligaments healthy, and aiding in controlling weight loading of the joint (Ageberg, Thombe, Neeter, Gravare Silbernagel, & Roos, 2008). The most important muscles involved are the quadriceps, which are involved in knee extension, and the hamstrings, which are involved in knee flexion (hamstrings).

The quadriceps comprise of a groups of four muscles called the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis. The hamstrings comprise of a group of three muscles called the biceps femoris, which has two muscle heads known as the long and short head, the semitendinosus, and semimembranosus. The long and short head of the biceps femoris wrap together as the outer hamstring and merge as a tendon with insertion below the knee joint. The semitendinosus and a tendon from the semimembranosus wrap together along with the garacilis (groin) muscle to form the inner hamstring. Tendons, like ligaments, are made up of very tough tissue. Tendons connect muscle to bone, while ligaments connect bone over bone. Hamstring tendons are also used in ACL repair and reconstruction (Health Information Publications, 2011).


Regular physical activity is important for a number of reasons in order to maintain a healthy lifestyle and for long-term health (Bahr & Krosshaug, 2005). Along with physical activity comes the risk of injury, in particular soft, musculoskeletal injuries (Bahr & Krosshaug, 2005). Knee and ankle injuries tend to be the most common injury and amongst those injuries, ACL ruptures and tears are a serious concern, perhaps even the most severe (Bahr & Krosshaug, 2005). Specifically regarding the knee, over 11.2 million people visit their healthcare provider because of injury (Healthcare Information Publications, 2011). Of those 11.2 million knee injuries roughly 100,000 are specific to ACL injuries occur every year, even though in the general population, ACL injuries tend to be fairly low (Grindstaff, Hammill, Tuzson, & Hertel, 2006). Of those 100,000 injury cases, around 50,000 are surgical ACL reconstructions (Health Inofrmation Publications, 2011). Even still, the injury consequences in terms of surgery, rehabilitation, and time lost from activity are just as serious as in an elite, or recreational athlete (Grindstaff et al., 2006). The surgical costs alone are around $17,000 per case not including the subsequent medical costs such as physical therapy, and future complications that often arise such as osteoarthritis (Grindstaff et al., 2006). Determining risk factors for injury can greatly aid in prevention, and scientific research studies are an effective method of determining the frequency, risks, and cause of ACL injury.

The precise etiology and mechanisms of ACL injury aren’t entirely known, however, there are generalizations as to what causes and ACL to tear (Bahr & Krosshaug, 2005).

The ACL usually tears when specific knee movements place intense strain on the ACL (Health Information Publications, 2011). Examples of these movements include hyperextension of the knee especially if the knee is extended 10 degrees further than its normally extended position as this forces the lower leg abnormally forward in relation to the upper leg, or femur (Health Information Publications, 2011). Pivoting motions are also common causes of ACL tears, such as excessive inward rotation of the lower leg (Health Information Publications, 2011). Either of these two types of ACL injury can be through contact, or non-contact force although most ACL injuries occur during non-contact athletic sports (Yeager, 2010). One of the specific movements that can lead to inward rotation or hyperextension is planting and cutting whereby the foot is firmly planted on the ground followed by the leg of that foot and body simultaneously turning (pivoting) as in making a fast cut and then changing directions (Yeager, 2010). Football is an example of this. Another specific injury causing movement is a straight-knee landing, which can result when a person strikes their foot against the ground with a straight knee, which can occur in sports such as gymnastics the foot strikes the ground with the knee straight (Yeager, 2010). Movements such as a one-step-stop landing with a hyperextended knee can cause injury and result in a sudden stop with a hyperextended leg with additional force placed upon hyperextension (Yeager, 2010). An example of this is a baseball player sliding into base such that the leg abruptly stops while in an over-straightened position (Yeager, 2010). Lastly, ACL injuries can result from a rapid deceleration movement followed by planting the foot, then pivoting the knee, an example being the movements a soccer player performs during a game (Yeager, 2010). Non-athletic causes of ACL injury usually result from wear and tear on the knee causing small tears that progress further, car and motorcycle accidents, contact injuries to the knee (Health Information publications, 2011). Injury severity depends on the direction and force of injury, knee position at time of injury as well as other factors that may predispose an individual to injury, sport, etc.

Other factors that may cause, or even predispose, a person to ACL injury are, as already mentioned, a frequent topic of scientific research studies. Research has, and is, often focused on intrinsic factors such as anatomical (e.g., pelvic tilt, ACL geometry, pronation of the foot, etc.), hormonal, and neuromuscular reasons. One of the main anatomical reasons for increased susceptibility to ACL rupture is the quadriceps (Q) angle as it can alter the alignment of the lower leg in relation to the upper leg altering the kinematics of the knee (Posthumus, 2009). Specifically, the Q. angle refers to the anatomical, geometric relationship between the center of the patella to the hip, and the center of the patella to the tibia (Posthumus, 2009). This is believed to influence the pelvic angle, hip rotation, tibial rotation, patella and foot position, and there are inherent gender differences between males and females with regards to the Q. angle (Posthumus, 2009). Studies have shown that ACL injury is much higher in female athletes compared to male athletes (Grindstaff et al., 2006). Possible contributing factors are the Q. angle, which affects, as mentioned previously, the anatomical structure therefore placing females at greater risk (Posthumus, 2009). Hormonal changes due to menstruation, muscle activation ratios, joint laxity, and styles / techniques of athletic play, also may play a role in placing females at greater risk for ACL injury (Grinstaff et al., 2006).

There are some research studies that have investigated a genetic component to ACL injuries though the data is limited (Posthumus, 2009). Studies have been attempting to identify specific hereditary markers that predispose a person to ACL ruptures, as well as what role they may play in the development of these injuries, with some success (Posthumus, 2009). Research has recently identified a specific binding site polymorphism that appears to correlated with acute ligament injuries such as cruciate ligament ruptures and shoulder dislocation (Posthumus, 2009). Researchers believe that incorporating evidence-based prevention methods can decrease the incidence of ACL ruptures, but an understanding of the etiology and mechanisms of sports injury are a necessary to do this (Posthumus, 2009).

The highest prevalence of extrinsic ACL injuries tends to occur in organized sports especially adolescents participating in pivoting type sports such as football, basketball, and team handball (Bahr & Krosshaug, 2005). In addition to any intrinsic factors and adolescent may have, or predisposing factors, environmental factors surrounding organized sports can also play a role in injury. For example, weather conditions (slippery surface), type of surface sport is played on (grass vs. pavement), proper footwear, and protective bracing (Posthumus, 2009).

A growing concern regarding these injuries and the populations that incur them is that these injuries increase the risk of osteoarthritis (Bahr & Krosshaug, 2005). In fact, after ten years, around half of all people that have suffered from an ACL injury show signs of osteoarthritis, and studies show that almost all of these ACL injury sufferers will have osteoarthritis after 15-20 years irrespective of treatment choice (Bahr & Krosshaug, 2005). In preventing osteoarthritis, having good muscle function is key, but it isn’t understood very well if reconstructive surgery to repair muscle function is helpful (Ageberg et al., 2008).


There are many intervention programs that are being used in an attempt to reduce the risk of ACL injuries most of which are geared toward addressing neuromuscular function (Grindstaff et al. 2006). For example, plyometrics, strength training, balance training, core training, and stretching are all techniques that are being used to better train the body for sport on muscular strength and neuromuscular level in order to enhance technique with great success (Grindstaff et al., 2006). These training techniques also improve agility and increase awareness of foot and body placement, and positioning, specific to the sport being played (Bahr & Krosshaug, 2005). However, limited knowledge of injury causation inhibits knowing exactly which training component is the most important in preventing knee and ankle injuries (Bahr & Krosshaug, 2005).


In conclusion, understanding more about possible intrinsic and extrinsic factors that can impact an athlete is crucial in order to develop more effective preventative methods for ACL injuries (Bahr & Krosshaug, 2005). Prevention isn’t only about avoiding the high costs of surgery and associated medical costs, it is also about preventing time lost in recovery and away from activities that a person loves, as well as being able to maintain a healthy lifestyle.

In addition to understanding potential risk factors, it is also important to understand the biomechanics of the musculoskeletal system of the athlete who is performing the sport, and the biomechanical demands of the sport (Bahr & Krosshaug, 2005). The factors involved in the athletic event that correlates with the injury needs to be assessed in detail with regards to playing situation, the athlete, the actions of the opponents in the play (Bahr & Krosshaug, 2005). Along with this, an analysis of the body and joint mechanics that preceded the event and were involved in the event at the time of injury occurrence also needs to be evaluated (Bahr & Krosshaug, 2005). In summary, a combination of further scientific studies, good reporting and analysis of injury events and their occurrence will be invaluable to improving injury prevention, and knowing which prevention techniques are best given a set of risk factors, whether they be predisposing risk factors, or extrinsic risk factors.


Ageberg, E., Thombe, R., Neeter, C., Gravare Silbernagel, K., Roos, EM. Patients With Anterior Cruciate Ligament Injury Treated With Training and Surgical Reconstruction or Training Only: A Two to Five-Year Followup. Arthritis Care & Research. 2008; 59(12):1773-79.

Bahr, R., Krosshaug, T. Understanding injury mechanisms: a key component of preventing injuries in sport. Br J. Sports Med. 2005; 39:324 — 329.

Grindstaff, TL., Hammill, RR., Tuzson, AE., Hertel, J. Neuromuscular Control Training Programs and Noncontact Anterior Cruciate Ligament Injury Rates in Female Athletes: A Numbers-Needed-to-Treat Analysis. Journal of Athletic Training. 2006; 41(4):450 — 456.

Health Information Publications (2011). What is the anterior cruciate ligament? eHealthMD retrieved from

Netter, FH. Atlas of Human Anatomy. 5th Ed. 2011. Philadelphia: Saunders Elsevier Publishing.

Posthumus, M. (2009). Genetic Risk Factors for Anterior Cruciate Ligament Ruptures. Retrieved from

Yeager, Caroline R. (2010). Anterior Cruciate Ligament (ACL): Causes of Injury, Adverse Effects and Treatment Options (Muscular System-Anatomy, Functions and Injuries). New York: Nova Science Publishers.

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