Documentation:FIB book/Non-Contact Anterior Cruciate Ligament (ACL) Injuries in indoor sports

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Knee Structure
Knee Structure

The Anterior Cruciate Ligament (ACL) is one of knee’s four crucial ligaments, which maintains the stability of the knee joint, prevents tibial translations, and limits the rotation [1]. It accounts for up to 86% of the overall resistance against anterior displacement [2]. ACL injuries range from relatively mild, involving the stretching of the ligament's fibers, to more severe injuries including partial or complete ruptures [3]. 10 to 27% of ACL tears are partial and if not taken care of, they can progress to complete rupture [4]. A non-contact ACL injury is one of the most common injuries in sports such as basketball and volleyball, typically resulting from leg cutting, pivoting, or drop landing [5][6]. This review gathers information about non-contact ACL injury in sports, most notably, in basketball and volleyball, which are played on the hard surface of gymnasiums as well as possible prevention and treatment strategies.



It is estimated that up to 91% of ACL injuries occur during sport, particularly in non-contact instances [7]. Around 200,000 to 250,000 ACL injuries occur annually in the United States alone [8][9][10], resulting in surgical costs exceeding $13,000 USD per procedure [10]. A significant recover and rehabilitation period of at least a 2-year is recommended before resuming pre-injury levels of physical activity [11]. There may be long-term consequences, such as alteration of knee kinematics, associated meniscus, and cartilage damage, and ultimately knee osteoarthritis [12]. It is estimated that one-third of players do not regain their former level of sports participation, and half of them do not resume participation in competitive sports [13][14]. Moreover, for those who return, the overall incidence rate of a second ACL injury within two years after ACL reconstruction, is 15 times greater than healthy athletes [15]. ACL injuries in sport are categorized into contact and non-contact groups, and most of them fall to the latter category for sports such as volleyball and basketball, where up to 70-90% of ACL injuries non-contact instances [5][16][17]. In an epidemiological study of the Italian professional women's volleyball league, it was observed that 97% of ACL ruptures were non-contact [18].

In volleyball, most injuries occurred during games rather than training sessions [6]. There have been ACL injury studies in different sports, especially in soccer or American football, which are played on natural turf . These differences suggest that the context of the sport is an important factor in determining injury, and these must be carefully considered when analyzing and predicting ACL risk.

The probability of ACL tears in female players is 2 to 8 times higher than in male basketball players [5][14][19][20][21]. These injuries are significantly more common within athletes between 21 and 25 years old [18][22].

Knee joint`s degree of freedom
Knee joint`s degree of freedom

Biomechanical Mechanisms

The knee joint can move in six degrees of freedom, which include three translational and three rotational. Rotating about the three principal axes shown in Figure 2. results in internal-external rotation, flexion-extension, and varus-valgus rotation [23]. Non-contact ACL injuries arise from the substantial force or torque produced by the player on their own knee joint surpassing its maximum load-bearing capacity [16]. Landing with excessive varus/valgus, internal/external tibial rotations or with low flexion angles increases the risk of ACL injury. Anterior tibial shear force, mainly from the sagittal and coronal plane, high knee laxity, and excessive force on quadriceps may also lead to this kind of injury [5][16][24]. However, there is not a single mechanism responsible for ACL injury in general, but a combination of several factors which vary from case to case [24].

Causes and risk Factors

Several factors have been found to be associated with increased risk of ACL injuries include body kinematics and kinematics, physiology and anatomy, and external factors.

Body Kinematics and kinetics

75% of ACL injuries in volleyball practices happen during the jump-landing phase [25]. Players who generate force quickly during their jumps tend to experience more force when they land. Additionally, the time it takes to reach the peak power during the jump is also related to the force at landing. Specifically, players who take longer to reach their peak power during a jump tend to experience more force when they land [26]. After some movements such as blocking and spiking in volleyball, players may land on one leg or two, with different knee flexions and tibial positions relative to the femur [6][16][27]. Volleyball players often land on one foot after a spike shot because of the shift in their center of gravity and loss of balance. This may increase non-contact ACL injuries because of higher peak vertical ground reaction force, loading rate, knee valgus angle, and knee flexion moment than both-legs landings [16]. Podraza and White, (2010) found that the knee flexion angle plays a crucial role in the risk of non-contact ACL injuries and that an extended knee landing position with knee flexions between 0° and 25°, poses the highest risk for ACL injuries [28]. This is because the ACL is most in tension when the knee is extended [28][29][30], leading to increased foot–ground impact forces [22][30][31][32][33]. Landing with a more bent knee decreases the ground reaction force as the antigravity muscles are positioned in a way that absorbs the impact’s kinetic energy more [28]. According to a study by DeMorat et al. (2004), excessive quadriceps force at or near knee’s full extension may also contribute to ACL injuries. Using radiographic and mathematical analyses, their cadaveric experiment estimated the maximum isometric quadriceps contraction in a normal person as 3000 N on average, and 6000-7000 N in young, trained male athletes. Applying a force of 4500 N to the quadriceps was sufficient to induce damage to the ACL in all tested knees. This is consistent with a study by Wall et al. (2012), which concluded that the combination of compressive forces and quadriceps load had a synergistic effect [9].  Upon adding a load to the quadriceps, the compressive force required to cause an ACL injury was much lower than when compressive forces were applied alone [34]. Furthermore, abnormal anterior tibial translation, varus and valgus, and internal and external rotation of tibia are other common kinetic factors of ACL injury [5][7][27][30][35][36].

Physiological and Anatomy  

Sex differences

Most previous studies agree that women are 2-8 times more susceptible to ACL injuries and this may be because of differences in their anatomical, physiological, and biomechanical features  [5][14][17][20][37]. Fluctuating hormones in females, such as estrogen and progesterone may be responsible for the higher rate of ACL injuries in women due to an increase in the knee laxity [38][39][40].

Due to female and male biomechanical differences, the angle between hip and knee is larger in women than in men, which increases the risk of ACL injury risks [41]. However, some studies emphasize movement techniques regardless of players’ sex. In a study implemented on 19 male and 19 female basketball players, it was found that female athletes tend to land with less knee flexion and encounter higher ground reaction forces at the maximum knee flexion compared to their male counterparts during cutting movements. Individual athletes, regardless of gender, demonstrating similar traits, may face an increased risk of ACL injury  [42].  


Fatigue appears to contribute to ACL injuries by negatively impacting proprioception and altering biomechanics during the demanding phases of volleyball games, particularly during landing [6]. When players are tired, they are more likely to land with a more extended knee which increases the ACL risk injury [43]. As such it is crucial for athletes to rest and avoid over-exertion to minimize injury risk.

External Factors  

Landing surface and shoes

Upon landing from a jump, both footwear and the surface naturally dampen a segment of the impact energy [44]. The material deformity and the ability of the landing surface to absorb the impact and prolong the interval time, is an important risk factor. Additionally, surfaces that exhibit a higher friction coefficient with shoes preventing significant sliding and feet’s free rotation, are more likely to contribute to ACL injuries [45][46]. On the other hand, landing on natural and artificial turf allow for greater sliding, which minimizes the joint moments and lower limb injuries [45].


There are measures which may be taken to reduce the risk of ACL injuries.  It is suggested that athletes participate in prevention programs, 20 to 30 minutes, several times per week [47][48], starting at an early age [49]. Verbal and visual feedback can also be very helpful. If the coach verbally emphasizes during the play that players need to bend their knees as much as they can when landing or if players review their performance in the videos and try to correct their movements and focus on improving their landing mechanics in order to reduce knee valgus collapse and excessive knee rotations [48]. The neuromuscular training is focused on ensuring controlled landing, optimizing absorption of ground reaction force, as well as analysis and progressive feedback, from basic to advanced [42]. The proper use of gluteal muscles, hamstrings, gastrocnemius, and soleus is crucial to control the lower limb and therefore, prevent ACL injuries [6].  Moreover, when knee braces are used, reduction in knee valgus and internal rotation during tasks such as pivoting and single-leg landing may be seen [50] as well as knee extension on landing, and the anterior tibial translation by up to 40% [51]


In case of partial and complete tears, conservative treatments such as physiotherapy and special exercises are often leveraged. In most cases, it is accompanied by activity modifications, especially avoiding pivoting [52]. ACL Reconstruction surgery is one of the most common treatment techniques in which an autograft (the transplantation of tissue from one part of an individual's body to another part of the same individual's body) or allograft (The transplantation of tissue from one individual to another individual of the same species) is used to reconstruct the ACL [53]. Many studies have identified the disadvantage of reconstruction surgery by mentioning its serious side effects including meniscal damage, premature osteoarthritis as well as the fact that not all athletes can return to sport after the operation [54]. It is critical for players to be informed about all treatment options during counseling, so that they can decide according to their aims and expectations [55]. Others like Smith et al., in 2014 however, found that functional braces have no effect in improving the injury, but reduces the probability of a second injury [56].

Limitation and future work

One of the most important limitations of this review is the fact that due to the complexity of the knee joint, and due to individualized physical and physiological features, ACL injuries can be attributed to various factors. Moreover, there were considerably fewer studies about ACL injury in women, although they are up to 8 times more susceptible than male athletes. Another limitation was the limited sample size in most studies, which usually affects the accuracy of results. Studies with soft tissue models had the problem of complexity and high cost. There was also a lack of enough studies about prevention techniques and about the link between knee laxity and its impact on ACL injury mechanism. There are also conflicted ideas about the role of internal and external tibial rotation on ACL tear.

In the future, further research on sex differences, computer modeling techniques, prevention methods, the effect of knee laxity as well as internal and external tibial rotation impact are needed. There is also a need to study rehabilitation techniques to avoid surgery or reinjury. In addition, more research is needed on ACL joint loading during normal activities.


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