Predisposing factors for an Anterior Cruciate Ligament tear

Predisposing factors for an Anterior Cruciate Ligament tear


Antonis Papoutsidakis MD, MSc, MFSEM(UK), MECOSEP

Orthopaedic Surgeon


In sports, especially in football, one of the most common knee injuries is the ACL tear, which usually (70%) occurs through non contact mechanisms. These non contact mechanisms include; sudden deceleration, landing and pivoting maneuvers which are repeatedly performed. Studies have shown that the incidence of ACL tear in female athletes is two to eight times higher than in men athletes in soccer, basketball and volleyball. The research effort to determine the risk factors for sustaining non-contact ACL injuries is increasing as concerns grow about the larger number of incidents, the greater treatment costs and the serious consequences of non-contact ACL injuries.It has been estimated that 175.000 primary ACL reconstructions are performed annually in USA with a cost of US$2 billion. 

Without a good understanding of the injury mechanisms, the risk factors for sustaining non-contact ACL injuries identified from epidemiological studies could be misinterpreted and lead to the selection of non-optimal injury prevention programs.

Mechanically, ACL injury occurs when an excessive tension force is applied on the ACL. A non-contact ACL injury occurs when a person generates great forces or moments at the knee that apply excessive loading on the ACL. Therefore, an understanding of the mechanisms of ACL loading during active human movements is crucial for understanding the mechanisms and risk factors for non-contact ACL injuries.

               There are number of studies that show that the anterior shear force at the proximal end of the tibia is a major contributor to ACL loading, while there are valgus, varus and internal rotation moments at the knee. According to these ACL loading mechanisms, a small knee flexion angle, a strong quadriceps muscle contraction or a great posterior ground reaction force can increase ACL loading.

                The literature also shows that individuals at high risk of sustaining non-contact ACL injuries have a smaller knee flexion angle during athletic tasks than individuals at low risk. Recent biomechanical studies demonstrated that female recreational athletes exhibited small knee flexion angles in running, jumping and cutting tasks. The results of other studies showed that the peak ACL strain occurred at the impact peak vertical ground reaction force shortly after initial contact between the foot and the ground.

Other studies have shown that the response of the ACL strain to knee valgus moment loading was minimal when the MCL was intact but significantly increased after the MCL rupture started due to knee valgus moment loading. Also, some studies clearly demonstrate that there is unlikely to be a complete ACL rupture due to knee valgus moment loading without a complete MCL rupture (grade 3 injury), while clinical observation shows that most ACL non-contact ACL injuries are not accompanied by significant MCL injuries.

The risk factors for non-contact ACL injuries fall into four distinct categories:  environmental, anatomic, hormonal, and biomechanical.

               Environmental risk factors

                The role of knee braces

                During the late 1970s prophylactic knee braces were introduced to protect knee collateral ligaments. Early reports indicated a decreased number of knee injuries in collegiate and high school athletes. Later on it has been proved that there is no benefit of using this prophylactic braces, but many athletes still insist in using them. It seems that it works more psychological than increasing the propioception ability of the knee.  

               Influence of shoe-surface interaction

               Recently, a high level of friction between shoes and the playing surface has been identified as a major risk factor for a non-contact ACL injury in the sport of the team handball. In football, there are studies that show that more non-contact ACL injuries occur when the surface is dry. Higher levels of friction between the shoe and the surface are generally associated with better performance but a higher injury risk. That’s why we see an increase of ACL tears in areas with artificial grass in football grounds. This happens because the players are not taught to change the football shoes from the natural type of grass to the artificial one.

               Anatomic risk factors

               There are studies that try to indentify anatomical factors that increase the risk for ACL tear but, in summary, the association of anatomic variables with an increased risk for ACL tear is intriguing, but to date no anatomic variable has been directly correlated to an increased risk for unilateral noncontact ACL tear.

               Hormonal risk factors

               In 1996, oestrogen and progesterone receptors have been found in human ACL cells, suggesting that the female sex hormones may play a role in ACL structure. Although the results of studies so far are compelling regarding the interaction between female hormone concentrations and compositional changes to the ACL, consensus is lacking regarding the relationship of menstrual cycle phase to the incidence of ACL injuries. In the light of this lack of agreement, more rigorous studies must be performed before treatment or prevention recommendations can be made.

               Biomechanical risk factors

                Role of propioception and neuromuscular control in joint stability

               Propioception plays an integrant role in maintaining functional role stability. Appropriate adaptations to preparatory activations of muscle, mediated by propioceptive signals, may provide the most efficient means of inducing prophylactic mechanisms that could shield the ACL from extreme in vivo forces and reduce the incidence of ACL injury. 

               High-Trunk contributions to ACL injury

               There are enough studies that try to correlate the high-trunk contributions to ACL injury. So far nothing has been proved. Further studies are needed to clarify the exact role of these in ACL injury. However, strengthening programs that emphasize hip control-gluteal and hamstring activation in a closed-chain fashion-have been shown to be beneficial in injury prevention programs.

               Influence of other kinetic, gravitational and muscle forces

               Examination of videotapes of non-contact ACL injury frequently reveal that just prior to an injury athletes slightly bumped or perform an awkward movement, from which they quickly recover by initiating a new movement pattern. With insufficient time to obtain information, the central nervous system tries to recover and frequently the activity becomes more quantriceps dominant as the player tries to regain balance. Unfortunately this occurs at a time when the ACL is more susceptible to the shear forces of the quantriceps.

                From video analysis, it can be hypothesized that a neuromuscular training program to aid in the prevention of non-contact ACL injuries sustained in pivoting sports should include the following elements: a kinaesthetic program to keep the centre of gravity forward and the athlete on his or her toes (including strength and endurance training of the rectus abdominis, iliopsoas and gastrocnemius-soleus muscles) and a program to encourage better lower-extremity rotational and angular control (including strength and endurance training of hip abductors and external rotator muscles).

               Future research directions

               There is a need to continue to define specific, neuromuscular, propioceptive and motor control factors associated with injury. However, until specific predictive and protective factors are identified, training and prevention programs should continue to be implemented, assessed and improved. There is a pressing need to improve public and participant awareness of the risk of ACL injury and the possibilities for prevention.         


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