Anterior Cruciate Ligament Injury Prevention and Rehabilitation

Abstract

At all levels of American football, knee injuries are common, with injuries to the anterior cruciate ligament (ACL) making up a significant proportion. Historically, ACL injuries were career-altering for professional players, but innovative techniques in surgery and rehabilitation have returned many to the field. While there is a consensus on surgical techniques for ACL reconstruction, significant discrepancies remain on injury prevention and rehabilitation programs. This review article describes the burden of ACL injury on players in the National Football League, best practices in injury prevention and rehabilitation, and evidence-based recommendations for preparing injured athletes to return to play.

Introduction

Knee injuries are common in American football, with anterior cruciate ligament (ACL) injuries being some of the most devastating. It has been reported that ACL ruptures occur at a rate of 68.6 per 100,000 person-years in the United States [53], with numerous studies documenting significantly higher rates in the National Football League (NFL) [12,44]. In addition, it has been shown that incidence among players is dependent on their position; running backs, wide receivers, and linebackers are at the highest risk in both college and professional football [12,23].

While risk factors for ACL injury in the general population include neuromuscular coordination, hamstring to quadriceps strength ratio, and landing mechanics, football players have unique risks, including high speed contact with other players, frequent cutting and pivoting maneuvers, and artificial playing surfaces with altered mechanics compared with natural grass [20,22].

Consensus for the treatment of acute ACL ruptures has been reached among college and NFL team physicians. Most surgeons reconstruct the ACL once swelling subsides and range of motion has returned; the most common approaches are bone-patellar tendon-bone (BTB) autografts, single bundle reconstruction techniques, and an accessory anteromedial portal for femoral tunnel drilling [12,26].

ACL injuries pose a significant problem for professional athletes. To enable return to play at the highest level possible, a holistic approach to understanding the risk factors, prevention, treatment, and rehabilitation of ACL injuries is necessary. This review article discusses ACL injury prevention along with postoperative rehabilitation and return to play.

ACL Injury Prevention

Nonmodifiable risk factors for ACL injury include decreased intercondylar notch width, hormonal differences in ligamentous laxity, limb length discrepancies, increased lateral tibial slope, and anatomic variants that lead to restricted range of motion in adjacent joints, such as lack of internal rotation through the hip due to femoroacetabular impingement [7,46,47]. Modifiable risk factors include strength and neuromuscular issues such as proprioception and neuromuscular control when landing from jumping, side-stepping, and pivoting. These factors rely on strength variables such as quadriceps to hamstring strength ratios, but the focus of preventing these injuries is usually modification of excessive valgus and internal rotatory forces upon the tibia, which increase strain on the ACL [7,9]. Overall, the focus of injury prevention programs is to identify modifiable risk factors and target correction of these factors to limit their influence upon movement in games and practice.

The success of the numerous ACL injury prevention programs available has historically been anecdotal, since there were few high-quality studies [57]. However, as interest has increased, several studies have confirmed the success of some interventions. Gagnier et al found a 50% reduction in ACL injury in adolescents and adults who completed neuromuscular and educational intervention programs [28]. Similarly, in a study largely involving soccer players, Sadoghi et al demonstrated a risk reduction of 52% in female athletes and up to 85% in male athletes following ACL injury prevention programs [52].

ACL injury prevention programs vary significantly in recommended exercises, dosage, and duration, and studies have yet to identify a “best” training program [52]. A clinical practice guideline and systematic review found that programs employing more than 1 combination of neuromuscular and proprioceptive training strategies (such as strengthening, stretching, and plyometric exercises) were more successful than those that focused on 1 technique [2]. In addition, the researchers identified success in programs that involved multiple training sessions per week, training sessions that lasted longer than 20 minutes, and training volumes longer than 30 minutes per week. Similarly, Sadoghi et al found successful programs included at least 10 minutes of training 3 times per week, with a focus on neuromuscular training. They found no evidence supporting newer protocols over older ones [52].

FIFA-11 and Harmoknee are 2 injury prevention programs that have been used successfully. Both include running exercises and specific dynamic movements aimed to mitigate modifiable injury risk factors such as poor strength, power, balance, speed, proprioception, and joint range of motion [4]. The FIFA-11 program, initially designed for amateur soccer players, demonstrates a significant reduction in non-contact lower extremity injuries in this population—however, no studies have attempted to demonstrate its efficacy in football [4,41,51]. This program consists of 3 stages of 15 exercises performed at least twice a week during warm-up for 10 to 12 weeks. The first stage involves running exercises. The second consists of 6 exercises designed to target core stabilization, eccentric thigh strength, and dynamic stabilization. The third and final stage consists of advanced running exercises [4,51].

The Harmoknee program has also been shown to demonstrate a reduction in lower extremity injuries, although, like FIFA-11, the majority of supporting data has been derived from soccer players [4]. This 5-part program includes a warm-up, muscle activation, balance, strength, and core stability [35]. It is performed as a substitute for a normal warm-up routine 3 times a week for at least 4 weeks [4]. While these programs have demonstrated reduction in lower extremity injuries, it is worth noting that for any program to be successful, strict adherence to proper mechanical alignment must be maintained while performing exercises. As such, it is recommended that these programs are conducted by trained staff [4].

Many ACL injuries occur in a non-contact situation, when an athlete’s knee is placed in an “at risk” position, usually caused by poor positional control [33]. Consequently, many ACL injury prevention programs focus on increasing an athlete’s positional control through core stability. Jeong et al found that a 10-week core strengthening program alone resulted in improved motor control and joint kinematics thought to be associated with risk of ACL injuries [33]. They noted an increase in the trunk flexion angle, vastus medialis to lateralis activation ratio, hamstring to quad activation ratio, and reduced knee valgus and hip abduction angles in the intervention group. The program included exercises such as the leg raise, crunch, “Superman,” and plank hip twist and was performed 3 days a week on alternating days [33].

Another risk factors of sustaining ACL injuries in game and practice settings is the effect of playing surface, which may be natural grass or artificial turf. There are 3 generations of artificial turf: the first generation is short blades of grass and minimal padding; the second generation is longer fibers and sand filling; and the third generation is longer fibers with a rubber filling. There is concern that modifying the playing surface leads to changes in shoe-surface and landing mechanics with increased friction and rotational stiffness and that this predisposes players to twisting injuries, such as meniscal and ACL injuries [24,60,65].

Studies on the effect of playing surface on the incidence of ACL injury have come to significantly different conclusions. Numerous studies have reported that artificial turf increases risk of lower extremity injury, especially ACL injuries [24,37,38]. There is particular concern that the third-generation surfaces contribute to a significantly higher rate of injury than other surfaces. Specifically, Dragoo et al demonstrated a significantly higher rate of non-contact mechanisms of injury on third-generation turf compared with natural grass [24]. This raises concern that the altered shoe-surface mechanics may lead to increased sticking to the surface and increase risk of rotational injuries. However, other studies have refuted these claims and have shown no difference in the ACL injury rates between natural grass and artificial turf [12,44,54]. While the effect of playing surface on ACL injury may still be up for debate, an argument can be made that turf alters the interaction with the shoe surface, potentially predisposing athletes to lower extremity injuries.

Also up for debate is the role of prophylactic bracing, which is used to decrease coronal plane loading and tibial rotation during jumping and pivoting motions and during direct contact to the knee. While it could have significant benefit, numerous sources including the American Academy of Orthopaedic Surgeons (AAOS) clinical practice guidelines have demonstrated that prophylactic bracing has limited efficacy in preventing ACL injury [10,11,50,56]. Bracing may also present a disadvantage to players due to increased energy expenditure, prevention of fluidity of lower extremity movement/increased intramuscular pressure, and the constant need to adjust the brace [10,11]. In fact, studies in favor of bracing advocate for its use only for specific positions such as offensive and defensive linemen whose ACL injury is often secondary to direct contact. A few studies have demonstrated bracing’s efficacy in preventing direct contact injuries without significantly impairing function [15,59]. Therefore, prophylactic bracing is best used on a case-by-case basis, with the understanding that there is limited supporting evidence, and even some documented disadvantages.

ACL Rehabilitation and Return to Sport

The concept of return to sport after an ACL injury continues to evolve. In the past, this was considered a career-altering injury. While there has certainly been an increase in the success of athletes’ return to play after ACL reconstruction, literature looking at NFL athletes has found successful return to play rates of 63% to 73% [25,34,49]. Further exploration of these data reveals significant variability depending upon numerous factors. Certain skill positions tend to have significantly lower rates of return than others. Wide receivers, tight ends, and running backs have some of the lowest rates of return to play, while quarterbacks, kickers, and defensive players have much higher rates of return [18,25,27,39]. Not only were return to play rates lower in certain position players, but the performance of the players was adversely affected as well [18,39]. The extent of injury is also a considerable factor.

Discuss expectations for success and return to previous performance with athletes who have suffered a multi-ligamentous or associated meniscal injury with their ACL injury. While it may be possible to achieve a full return to play, this was often not the case for NFL players who had an isolated ACL reconstruction [6,14,34]. Much of the discrepancy between return to play rates for elite NFL athletes and the general population can be explained by the demands that this level of play necessitates. Even slight alterations in muscular balances, ligamentous stability, and range of motion can lead to changes in performance on the field. This is manifest in the differences between return to play in early draft picks and those drafted in later rounds. Those drafted in earlier rounds (most studies found prior to fourth round) had significantly higher return to play rates and higher performance upon return [25,34,67]. There may be numerous reasons for this. While it may be that their physical ability correlates with a better ability to resume sports activity, these athletes also likely represent a larger investment for the team. Unlike lower round draft pick players who may be cut from the team if injured, higher round draft pick players may be considered worth holding onto for the length of time required for surgery and rehabilitation.

The rehabilitation process is a crucial element of recovery, although it is extremely variable. Most surgeons agree that patients should be allowed to return to play once the knee has full range of motion, no pain, no swelling, and equal isokinetic testing of quadriceps strength bilaterally [12,26,56]. However, ideas differ on further requirements. For example, surgeons typically hold patients back for 6 to 12 months, but this can change depending upon clearance and return to play criteria.

Most rehabilitation programs have similar postoperative protocols in the early weeks following ACL reconstruction. The graft must be protected in the first few weeks to months. Some surgeons prefer to use a functional ACL brace while other surgeons only restrict activity. The goal for this early stage of rehab is to regain full range of motion, resolve effusions, and begin working on quadriceps, core, and hamstring strengthening via closed chain exercises. [1,27]. It is important to note that the rehabilitation process should proceed upon patient progression through milestones documented by the physical therapist. It should not rely upon set timelines. While attitudes have been changing about this approach, a significant number of therapists still prefer to follow rigid timelines for return to sport guidelines, even though most surgeons now prefer a functional progression milestone approach [30,63].

Rehabilitation might progress through a range of activities including movement from static and dynamic strength building to sports-specific training such as plyometrics, agility drills, return to running, sports-related non-contact movement. The process will culminate with sports-specific contact training for football. The injured limb tends to favor positions of increased load on the ACL postoperatively, which includes decreased hip rotation, increased tibial internal rotation, and increased knee valgus [29]. The goal is to train neuromuscular asymmetries out of the 2 limbs in order to prevent functional alterations that could occur during training or games. In 2020, Buckthorpe et al described an ACL reconstruction rehabilitation process that includes pre-reconstruction rehab, early-stage rehab, middle-stage rehab, late-stage rehab, and return to sport phase. These phases incorporate a mix of strengthening, straight-line jogging, and neuromuscular activation in the early stages. As rehab progresses, there is a slow incorporation of more dynamic strengthening exercises and agility training. Finally, return to sport is aimed at sport-specific drills and secondary injury prevention [16].

A few key muscle groups are targeted during these strength-building phases. The core musculature, including the rectus abdominis and oblique muscles, is crucial in stabilizing the lower extremity coronal plane axis; this limits hip adduction and knee valgus, which decreases stress placed upon the ACL. In addition, it increases the hamstring to quadriceps and vastus medialis to vastus lateralis activation ratios [33,36]. Strengthening the hamstring strengthening is crucial. It provides a posterior force upon the tibia and functions as another check in to reduce strain on the ACL. Importantly, continued deficits in hamstring strength can lead to ipsilateral ACL graft re-tear or contralateral ACL rupture [13,16]. Hamstring-strengthening activities should include knee-dominant exercises such as Nordic hamstring curls and single leg roll outs. They should also include hip extension-dominant exercises such as split squats and single leg loaded deadlifts [16]. These should occur as a progression from closed chain linear exercises to the more complex exercises such as split squats, which involve numerous muscle groups. Finally, developing quadriceps strength is also fundamental to the strengthening period. Quadriceps strength is essential to sagittal plane motion of the knee. Failure to properly strengthen the quadriceps musculature can lead to asymmetrical knee axis firing, which can lead to worse performance, increased rate of re-tears, and worse patient-reported outcomes [31,43]. However, many athletes return to sport with persistent quadriceps weakness [45]. This weakness results from a few main mechanisms following ACL reconstruction. First, knee effusions trigger arthrogenic muscular inhibition, which effectively shuts down the quadriceps of the ipsilateral leg, which induces atrophy [29,42]. Second, the central nervous system (CNS) inhibits the voluntary activation of the rectus femoris bilaterally, which also complicates the use of the non-injured leg as a control for normal quadriceps strength [29,61]. Third, there is some component of disuse leading to atrophy, although the debate continues about its effect on quadriceps function in the rehabilitation process. Typical exercises aimed at quadriceps function begin with closed chain exercises. Eccentric exercises with a neuromuscular focus have been demonstrated to have the best benefit in quadriceps strength and function [29]. Early exercises focused on quadriceps strengthening include leg press and partial depth squats. Other modalities that target quadriceps without requiring as much strain on the knee include blood flow restriction training (BFRT) and neuromuscular electrical stimulation [5,32].

More dynamic movements can be incorporated following the early neuromuscular activation and strengthening period. As the athlete progresses through these stages of recovery, the therapist and surgeon must assess the knee to ensure that there is still no effusion or pain. There should also be full range of motion and near symmetrical strength [5]. While strengthening can occur throughout the course of ACL rehabilitation, the goal at this stage is functional recovery. Again, referencing Buckthorpe et al, this would occur during the late stage of rehabilitation [16]. During this stage, training can incorporate dynamic activities, such as agility drills, change of direction movements, and other plyometric-based activities. In general, these exercises should start simply and become more complex. Verstegen et al in 2012 recommended football-specific movements as football is one of the most dynamic and fluid sports [64]. A general progression of movements would begin with linear explosive movements, followed by a combination phase involving cutting and side-stepping as well as explosiveness such as t test and pro agility shuttles. During the final phase, these movements could be incorporated into drills including rapid and random changes of direction, pivoting, and cutting to simulate practice and game movement conditions [64].

While assessment takes place throughout the rehabilitation process, a formal evaluation to assess for clinical signs of ACL recovery is normally conducted before the player begins the return to sport process. Physicians look for the presence of knee effusion, knee pain, and ACL ligamentous stability. Patient outcome measures may include the Knee Outcome Survey–Activities of Daily Living or Anterior Cruciate Ligament–Return to Sport after Injury. NFL players may also benefit from a functional movement screen [64]. In addition, assessing for a leg symmetry index of typically >90% on a single hop, triple hop, cross-over hop, or some variation is typically conducted. Assessing leg symmetry for quadriceps and hamstring isokinetic strength tends to be more varied. However, it normally involves at least 80% leg symmetry index for the different muscle groups and is measured anywhere from 60°/s to 180°/s [1,17,26,31]. Recently, leg symmetry index has been called into question due to the fact that it relies on the contralateral extremity, which can be problematic. Obtaining non-injured baseline measures on these tests could help with postoperative assessment; however, this is not frequently done [17]. Football players may also benefit from a final, on-field assessment that puts the athlete through bilateral position-specific movements to identify any functional asymmetries [64]. Once these return to sport criteria are satisfied, the athlete may begin progressive incorporation into non-contact drills and practices, before returning to full contact activities.

The goal of rehabilitation is to seamlessly incorporate the athlete back into play. This is accomplished by resolving strength asymmetries, working on neuromuscular activation strategies, and progressively adding more dynamic and functional exercises into the process. Typically, the main focus is getting the player back on the field. However, there is a growing trend for secondary prevention of ipsilateral re-rupture of the ACL graft or contralateral limb ACL rupture. Elite athletes are highly prone to these injuries; one study found that they occurred in 17.8% of professional European soccer players [21]. Similar percentages have been quoted in other studies as well [31,34]. The highest rates of these re-tears or contralateral tears were found in patients with non-contact injuries. This creates a unique opportunity as these athletes would likely benefit from a neuromuscular retraining similar to primary injury prevention programs. The ACL-SPORTS trial was conducted on male and female athletes during the return to sport period. The athletes enrolled after completed late-stage rehabilitation. The program included balance, plyometrics and landing mechanics, strengthening, and perturbation training [66]. Since outcomes for the perturbation and non-perturbation training male groups were equal, the groups were combined to add power to the study. The group was followed for 2 years. Two-year outcomes showed a 100% return to sport rate with a 95% rate of return to prior performance level and only a 2.5% ACL re-injury rate [3]. Secondary prevention of ACL injuries is similar to the primary prevention programs that have previously been discussed and may begin to play an important role in the rehabilitation process.

ACL rehabilitation is often considered either conservative or accelerated. Conservative rehabilitation simply means that the process will take around 12 months to complete. Accelerated rehabilitation allows the patient to bear weight, work on range of motion, and come out of the functional brace much earlier; some surgeons even forgo the brace altogether. The goal is to shorten the entire process, so the patient begins strengthening and returns to functional activities at shorter intervals. The desired outcome is return to sport at 6 months. Early studies have demonstrated the efficacy of accelerated rehabilitation compared with the conservative approach and found advantages including the prevention of arthrofibrosis, quicker return to sport, increased muscular strength, better range of motion, and overall lower costs [19,62,58]. When compared with the traditional model, these benefits may plateau at the 5- to 6-month mark. However, more recent studies have called the advantage of the accelerated programs into question. Even after completing an accelerated rehabilitation program, athletes may still have residual muscular asymmetries and deficits in joint proprioception and neuromuscular coordination [8,40,48,55]. These are significant risk factors for new injury or ACL re-injury once the athlete returns to intense practice and game conditions. Overall, this will continue to be an area of interest for ACL rehabilitation strategies. Nevertheless, it does make clear that rehabilitation needs to be highly individualized. While it is not likely that early weight-bearing and range of motion significantly alters outcomes, in order to ensure that the patient is ready to return to play, significant therapist interaction and oversight is crucial throughout the rehabilitation process.

Conclusion

ACL injuries pose a significant burden to football players at all levels. While the importance of injury prevention and rehabilitation has been acknowledged, the best strategies continue to be a source of debate. Current literature suggests that the two most important variables are a variety of exercises and the frequency of sessions. While the FIFA-11 and Harmoknee programs have demonstrated success in soccer, additional studies are necessary to establish their efficacy in football. In addition, studies of factors that may predispose athletes to ACL injury—such as playing surface and prophylactic knee bracing—continue to produce conflicting evidence. New, high-level research is necessary.

ACL rehabilitation programs and prevention programs can greatly vary. One point of consensus, however, is that before returning to play athletes should have full range of motion, no pain, no swelling, and equal isokinetic testing of quadriceps strength bilaterally. Quadriceps and hamstring strengthening is extremely important to a successful recovery and secondary prevention of re-injury. Evidence suggests that accelerated programs result in increased strength, range of motion, and quicker return to sport. However, they achieve this at the cost of reduced coordination and residual muscle asymmetry—outcomes more likely to result from conservative programs.