*3.1.2 Neuromuscular risk factors*

In Contrast to anatomical risk factors which non-modifiable without surgical intervention, neuromuscular deficit are often modifiable risk factors. Intervention could reduce the risk of ACL injuries. Better control over the core body and improved proprioceptive control over the lower extremity have said to be associated with reduced risk of ACL injury. Proprioceptive performance can be improved by exercises improving muscle strength, synergistic coordination etc. which in turn help to reduce the risk of ACL injury. The mechanism of ACL injury occurs when the athlete takes off from valgus positioned knee, during which the knee is typically in 10–30° of flexion and tries to internally rotate the externally rotated foot aiming to suddenly change the direction.

There is a greater risk of injury to ACL injury when the knee is in abduction, there are intersegmental abduction movements and increased ground reaction force with decreased stance time. A study by Hewett et al. showed that when landing from jump in double leg stance, increased knee abduction angle and intersegmental forces, greater ground reaction force and shorter stance time caused increased ACL injuries [44–48].

A small knee flexion angle coupled with strong quadriceps contraction during sports activity will cause increased posterior loading on the knee. As a result, this increases ACL injury risk.

In female athletes, while landing from heigh during a jump, they perform cutting and pivoting manoeuvres with less knee flexion and hip flexion, increased valgus at the knee, increased internal rotation of the hip coupled with increased external rotation of the tibia and increased quadriceps muscle activation caused increased ACL injury due to increased strain on the knee.

Females have poor neuromuscular control of hamstrings and weaker gluteus medius strength, weaker hip abductors which coupled with poor landing mechanism increases the risk of ACL injury.

Balance training, core strengthening, jump training, dynamic joint stability and plyometric exercises training increased core stability and improved proprioception, which reduced the risk of ACL injury.

When the knee is in valgus loading the medial collateral ligament becomes taut and lateral compression occurs. This as well as the anterior force vector caused by quadriceps contraction causes the lateral femoral condyle to shift posteriorly and the tibia shifts anteriorly and internally rotates, resulting in ACL rupture.

### *3.1.3 Sex differences*

Anterior cruciate ligament tears vary in incidence by gender being more common among women. In fact, studies have shown that the rate of ACL tears could be 9 times more common in women as compared to men [49]. Many studies have tried to study the exact reason for this sex-based discrepancy in ACL injuries. Though the exact reason still remains unclear, it appears there are various intercalated intrinsic factors that lead to this. The possible factors could be due to their unique effects of sex hormones, anatomic differences in the female ACL s and/or neuromuscular control variations among the sexes [50].

Female athletes have been found to have different movement and muscle activation patterns [51, 52]. Females while jumping, due to their increased quadriceps activation coupled with decreased hip and knee flexion increases the load on the ACL, thus injuring it [53]. A study conducted by Anderson et al. have found that the lack of stiffness and strength in the quadriceps and hamstrings in females along with anatomically smaller ACLs predispose them to injuries [52]. Furthermore, female athletes displayed greater knee laxity values as compared to their male counterparts [54].

Anthropometric studies on ACL have shown Intercondylar notch is an anatomic factor that has links to the risk of ACL injuries [55, 56]. Subjects with a narrow/ stenotic intercondylar notch have a higher incidence of ACL injury. Therefore, females by the virtue of having smaller intercondylar notch and notch width index, have an increased likelihood of sustaining an ACL injury [57].

### *3.1.4 Hormonal risk factors*

There is wide variations in hormonal milieu over the course of menstrual in females. The occurrence of ACL injuries has been found to have an association with the menstrual cycle phase. The reason for this may be because of the presence of progesterone and oestrogen receptor sites on the ACL [58–60]. However, all the studies on this have been either in vitro or in animal models and the presence of these receptors on human ACLs have not been proved. The hypothesis is that oestrogen has an effect on the synthesis and breakdown of the matrix components of ACL. The rate of occurrence of ACL injuries is more during the ovulatory phase of the menstrual cycle, which is hallmarked by high concentrations of serum oestrogen [58, 59]. What's perplexing about these injuries in women is that they have been found to occur during non-contact event usually due to deceleration or a change of direction manoeuvre rather than a direct impact injury [61]. Oestrogen decreases the rate of proliferation of fibroblast and synthesis of types I procollagen whilst progesterone promotes the same [62]. Hence this variation in the concentration of oestrogen and progesterone in the various phases of menstruation influence the materialistic properties of ACL. Thus, ACL injuries in women are more common during the pre-ovulatory phase of menstruation, when the serum oestrogen levels are high [63].

#### *3.1.5 Familial predisposition*

There is some evidence to prove that ACL injuries do have a familial predisposition. A study conducted by Flynn et al. showed that people with anterior cruciate ligament tear were twice as likely to have a relative who has an anterior cruciate ligament tear as compared to the controls [64]. They concluded from this study that there is a familial predisposition to an anterior cruciate ligament tear. Another study by Harner et al. found that the incidence of anterior cruciate ligament tear was higher in the patients who have had a family history of ACL injury [65]. However, these studies have not looked into the similarities between the patients and their family. A study by Goshima et al. looked into the mechanism, situation and the types of sports played between patients and their family members to avoid bias [66]. They found that there was a strong familial predisposition to ACL injuries. Furthermore, individuals with FH of ACL injury had an increased risk of repeat ACL injuries and thus require prevention programs.

#### *3.1.6 Genetic risk factors*

Mutation in the specific genetic sequence variants of genes that code for the extracellular matric of the ACL are found to predispose such individuals to ACL injury. These genes include COL1A1, COL5A1 and COL12A1. The COL1A1 gene codes for the primary subunit of type 1 collagen which I the primary constituent of the ACL matrix. The TT genotype of the COL1A1 gene was found to be

#### *Risk Factors of ACL Injury DOI: http://dx.doi.org/10.5772/intechopen.99952*

underrepresented in patients with ACL ruptures as compared to those with control. These studies concluded that individuals with TT genotype of COL1A1 gene are less prone to ACL tears [67]. The COL5A1 gene codes for a major subunit of type V collagen which is a minor constituent of the ACL. A recent study showed that mutation of this gene was associated with ACL ruptures in these individuals [68]. The COL12A1 gene codes for collagen XII which is involved in the fibrillogenesis of ACL. In a recent study by Posthumus et al., it was found that COL12A1 AluI RFLP was found to be associated with ACL ruptures [69].

Matrix metalloproteins that are physiological mediators of collagen cleavage and removal are located on chromosome 11q22. A study done by Posthumus et al. this found that underrepresentation of the AG and GG genotypes caused an increased incidence of ACL injuries [70]. However, these factors are usually associated with other intrinsic and extrinsic factors and their independent association is difficult to determine. It is important that genetic variants be determined in various at-risk population and their phenotypes identified.
