& Dawkins, 1991; Sapega, Moyer, Schneck, & Komalahiranya, 1990). The posterolateral bundle is taut in full extension and in deep flexion but slackens throughout the midrange of motion. The ACL is the primary restraint to anterior translation of the tibia relative to the femur and a major secondary restraint to internal rotation, particularly when the joint is near full extension (Duthon et al., 2006). Normal knee arthrokinematics is maintained with the ACL, along with the posterior cruciate ligament (PCL), through the four-bar linkage system (Müller, 1983). The four-bar linkage system is a model of the knee in which the ACL and the PCL are depicted as rigid bars connecting to the femur and tibia. The restraints of these ligaments help to control roll and glide in the knee joint throughout a full ROM. Damage to the ACL can disrupt this system, resulting in aberrant motion during activity.
Figure 1: Knee Anatomy
Note . Retrieved from https://www.niams.nih.gov/health-topics/ knee- problems/advanced
MECHANISM OF INJURY
● Knee flexion angle less than 30° in combination with higher hip flexion and lower ankle plantar flexion angles from initial contact to peak knee flexion during landing (Carlson, Sheehan, & Boden, 2016) or cutting/pivoting maneuver (Walden et al., 2015). ● Application of a quadriceps force when combined with knee internal rotation A valgus load combined with knee internal rotation. ● Excessive valgus knee loads applied during weight-bearing, decelerating activities. Patients with a noncontact mechanism of injury may demonstrate greater dynamic knee instability compared to those who have contact injuries (Hurd, Axe, & Snyder-Mackler, 2008a).
Damage to the ACL can result from a contact or a noncontact injury. An estimated 70% of ACL injuries result from noncontact mechanisms (Hewett, Myer, & Ford, 2006). Injuries are often reported during activities involving cutting, changing direction, or landing from a jump; landing on the foot instead of the toes and being perturbed before landing both increase the risk of ACL injury (Griffin et al., 2006). Noncontact ACL injuries are likely to happen during deceleration and acceleration motions with excessive quadriceps contraction and reduced hamstrings co- contraction at or near full knee extension (Shimokochi & Shultz, 2008). ACL loading is higher during any of these situations (Shimokochi & Shultz, 2008; Walden et al., 2015):
CLINICAL COURSE
The sequelae of ACL injury include quadriceps strength deficits, neuromuscular dysfunction, biomechanical maladaptations, and the development of knee osteoarthritis (Daniel et al., 1994; Lohmander, Östenberg, Englund, & Roos, 2004). However, because individuals do not respond uniformly to an acute ACL injury, outcomes can vary. Most of those who have had such an injury decrease their activity levels both in the short- and long-term (Ageberg, Pettersson, & Fridén, 2007; Ageberg, Thomeé, Neeter, Silbernagel, & Roos, 2008; Muaidi, Nicholson, Refshauge, Herbert, & Maher, 2007; Neeter et al., 2006; Ristanis et al., 2006; Tagesson, Oberg, Good, & Kvist, 2008). Nonoperative management of patients with ACL-deficient knees can be effective for those who are willing to avoid high- risk activities (Beynnon, Johnson, Abate, Fleming, & Nichols, 2005). However, between 23% and 42% of patients after ACL injury choose to return to high-level activities after nonoperative rehabilitation (Hurd, Axe, & Snyder-Mackler, 2008b; Kostogiannis et al., 2007). The standard of care followed by most surgeons for ACL injury in the United States for young, active individuals is early ACL reconstruction (Delay, Smolinski, Wind, & Bowman, 2001; Dye, Wojtys, Fu, Fithian, & Gillquist, 1998). Epidemiological studies have found that patients who are male, younger, Caucasian, and of higher socioeconomic status, and who possess private health insurance are more likely to have ACL reconstruction than nonoperative treatment (Collins, Katz, Donnell-Fink, Martin, & Losina, 2013). Younger athletes who wish to return to high- level sports involving pivoting activities are often advised to undergo
early ACL reconstruction because of the assumed inevitable knee instability with sports-related activities (Johnson, Maffulli, King, & Shelbourne, 2003; Marx et al., 2003; Myklebust & Bahr, 2005). However, some patients are able to postpone surgery following a period of intense rehabilitation in order to finish out the athletic season or a busy season of work without further episodes of giving way (Fitzgerald, Axe, & Snyder-Mackler, 2000b; Thoma et al., 2019). Patients who participate in structured rehabilitation with an option for later ACL reconstruction, compared to those with structured rehabilitation and early ACL reconstruction, have similar results including patient-reported outcomes, activity levels, and radiographic incidence of osteoarthritis in the surgical knee at 2 and 5 years after ACL reconstruction (Frobell, Roos, Roos, Ranstam, & Lohmander, 2010; T. O. Smith, Davies, & Hing, 2010). Furthermore, participation in extended preoperative rehabilitation not only yields superior outcomes compared to criterion-based post-operative rehabilitation alone (Failla et al., 2017; Failla et al., 2016) but also may help some individuals become candidates for successful, non-operative management long-term (Thoma et al., 2019). Unfortunately, surgical reconstruction of the ACL does not ensure a return to previous levels of activity or prevent future joint degeneration (Ardern, Webster, Taylor, & Feller, 2011a, 2011b; Gobbi, Domzalski, Pascual, & Zanazzo, 2005; Nakayama, Shirai, Narita, Mori, & Kobayashi, 2000). Many people continue to exhibit knee instability, pain, quadriceps strength deficits, or reduced ROM that may make them unable to return to or maintain a high level of competition (de Jong, van Caspel,
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