van Haeff, & Saris, 2007; Hartigan, Axe, & Snyder-Mackler, 2010; Keays, Bullock-Saxton, Keays, & Newcombe, 2001). The risk of developing knee osteoarthritis is relatively similar whether a person chooses nonoperative management or ACL reconstruction (Lohmander et al., 2004; Myklebust, Holm, Maehlum, Engebretsen, & Bahr, 2003; von Porat, Roos, & Roos, 2004; Wellsandt et al., 2018), although one systematic review suggests that osteoarthritis may be less prevalent in those who choose nonoperative management (Luc et al., 2014). A review by Oiestad, Engebretsen, Storheim, & Risberg (2009) reported that the prevalence of knee osteoarthritis after isolated ACL injury is approximately 13%; concomitant medial collateral ligament (MCL) or meniscal injuries are linked to much higher
risk (21% and 48%, respectively). Several of the same authors recently conducted an updated systematic review on the same topic and found wide-ranging results: the prevalence of radiographic tibiofemoral osteoarthritis in the included studies ranged from 0% to 100% and ranged from 1% to 80% in the high-quality studies (Lie et al., 2019). Risk factors associated with clinical symptoms of posttraumatic osteoarthritis after ACL reconstruction include subsequent surgery, meniscus pathology (especially meniscectomy (Lie et al., 2019), and chondral injury (Jones & Spindler, 2017). Therapists should counsel patients on the risk of developing knee osteoarthritis whether they have surgery or choose nonoperative management of an ACL injury.
RISK FACTORS
Identifying the risk factors that contribute to an initial or second ACL injury allows clinicians to develop risk profiles, screen individuals to identify those at the greatest risk for injury, and develop targeted interventional strategies to potentially reduce the risk of injury (Cameron, 2010). Risk factors may be categorized as nonmodifiable or modifiable. Although nonmodifiable risk factors such as female sex, narrow femoral notch width, or increased joint laxity cannot be altered, their identification permits clinicians to counsel athletes on the inherent risk of injury, enabling them to make informed decisions regarding sports participation. The incidence rate of a second ACL injury to either knee is highest in patients who are under 20 years of age at primary surgery (at a rate of approximately 29%) with odds ratio (OR) of 6.3 for ipsilateral graft rupture and OR of 3.1 for contralateral ACL injury (Webster, Feller, Leigh, & Richmond, 2014). Returning to high-risk sports involving cutting and pivoting increases the odds of ipsilateral graft rupture by almost fourfold and of contralateral ACL rupture by nearly fivefold (Webster et al., 2014). Athletes younger than 25 years who return to sports have a second ACL injury rate of approximately 23% (Wiggins et al., 2016). Female athletes have a substantially greater rate of injury compared to their male counterparts (Arendt & Dick, 1995), with an injury rate ratio of 3.4 for girls relative to boys in sex- comparable sports (Joseph et al., 2013). Other risk factors for females are joint laxity, knee recurvatum, increased posterior tibial slope, and hormonal changes (Griffin et al., 2006). In a comprehensive systematic review of risk factors in male athletes, dry weather, artificial turf compared to natural grass, and a greater posterior slope of the lateral tibial plateau may increase the risk of noncontact ACL injuries (Alentorn-Geli et al., 2009). The identification of modifiable factors, such as neuromuscular control, biomechanics, muscle strength, and movement patterns, allows physical therapists and physical therapist assistants to tailor rehabilitation programs to lower the risk of an ACL injury. Neuromuscular imbalances in the lower extremity have been associated with ACL injury mechanisms during landing from a jump. Four underlying neuromuscular factors have been identified: ligament dominance, quadriceps dominance, leg dominance, and trunk dominance (Boden, Torg, Knowles, & Hewett, 2009; Myer, Ford, Khoury, Succop, & Hewett, 2011). Ligament dominance results from the insufficient absorption of the ground reaction forces by the surrounding musculature, causing the knee joint and ligamentous structures to absorb the high force levels. The bony architecture and static stabilizers of the knee must mitigate these high ground reaction forces over a short time. This profile demonstrates higher knee valgus ROM and high peak knee valgus moments (Pappas, Shiyko, Ford, Myer, & Hewett, 2016). Quadriceps dominance results from repeated use of the quadriceps muscles to stabilize the knee joint, thereby reducing the amount of knee flexion during activities such as landing from a jump. This action causes an anterior shear force to the tibia and ACL. It is categorized as a combination of high quadriceps and leg dominance deficits (Pappas et al., 2016).
Leg dominance results from an individual’s continual favoring of one leg over the other, often due to underlying limb-to-limb asymmetries. These limb-to-limb asymmetries can increase the risk of future ACL injury (Hewett et al., 2005). Finally, trunk dominance is the lack of precision trunk control, categorized by a combination of trunk and leg dominance deficits (Pappas et al., 2016). Neuromuscular control and instability in the trunk may create increased lateral positioning of the trunk during cutting or landing. Increased lateral positioning can result in increased loads at the knee. Lack of precision trunk control contributes to the increased risk of ACL injuries (Zazulak, Hewett, Reeves, Goldberg, & Cholewicki, 2007). Female athletes with lower knee valgus ROM, lower peak knee valgus moments, lower trunk side flexion ROM, and lower limb-to-limb differences in hip ROM fit into a low-risk profile (Pappas et al., 2016). Multiple training programs that aim at reducing ACL injury risk by improving dynamic neuromuscular control through conditioning have been proposed (Hewett, Ford, Hoogenboom, & Myer, 2010; Steffen et al., 2013). For example, Hewett and colleagues (2010) have developed a training program to address each of the four underlying neuromuscular factors in an attempt to reduce the risk of initial ACL injury. In addition, recent findings suggest a hip focused prevention program may reduce ACL injury risk in female basketball players (Omi et al., 2018). Clinicians should implement exercise-based knee injury prevention programs for all athletes involved in cutting and pivoting sports to reduce the number of knee injuries. These programs should involve multiple components, particularly stretching/flexibility exercises, lower extremity and core strengthening, power exercises, balance and dynamic stabilization training, plyometric training, technique training, and/or sports-specific drills (Arundale, Cummer, Capin, Zarzycki, & Snyder-Mackler, 2017). Programs should have a longer duration (greater than 20 minutes), be done multiple times per week (two to three times per week in the preseason and one to three times per week in the competitive season), and be used with high compliance. Programs should not be used after training sessions or games (Arundale et al., 2017). Implementing exercise-based knee injury prevention programs carries little risk to the athlete, with the primary side effect being muscular soreness when the program is initiated. Implementation of exercise-based knee injury prevention programs can reduce the risk of knee injuries in male soccer players and in female athletes, particularly youth and adolescent athletes (Arundale et al., 2017). Other modifiable neuromuscular factors are predictive of a second injury once athletes have returned to their sporting activities after ACL reconstruction. Paterno and colleagues (2010) found that 23.2% of young athletes sustain a second ACL injury after ACL reconstruction within 12 months of testing around the time of return to sport; at 24 months after return to sport, 29.5% of young athletes sustain a second injury to either the ipsilateral or contralateral knee (Paterno et al., 2014). These researchers were able to identify factors that place athletes at risk for a retear of the ACL graft or the contralateral ACL. Neuromuscular control deficits of the hip external rotators,
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