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Etiology and Prevention of Noncontact ACL Injury

Barry P. Boden, MD; Letha Y. Griffin, MD, PhD; William E. Garrett Jr, MD, PhD

THE PHYSICIAN AND SPORTSMEDICINE - VOL 28 - NO. 4 - APRIL 2021


In Brief: An understanding of the etiology and prevention of noncontact ACL injuries has lagged behind diagnosis and treatment. However, a growing research implicates hormonal, anatomic, environmental, and neuromuscular factors that may predispose athletes, particularly women, to these injuries. Specific factors may include estrogen levels, the shape of the intercondylar notch, playing style, and neuromuscular control of the quadriceps and hamstring muscles. Prevention programs that involve proprioception, plyometrics, strength training, and improved jumping, stopping, and turning techniques show promising results.

The anterior cruciate ligament (ACL) is one of the most commonly disrupted ligaments in the knee. Despite the explosion of information on the ACL over the past 25 years, little attention has been focused on the causes and prevention of injury.

Each year in the United States there are approximately 250,000 ACL injuries, or 1 in 3,000 in the general population (1). Assuming that at least one third of ACL-deficient patients require surgery, at about $17,000 per reconstruction, the estimated annual cost is about $1.5 billion (2,3). This total does not include the costs of initial evaluation and treatment of those injured, the nonsurgical care of the remaining patients, or future medical treatment for those who develop post-traumatic arthritis. Hence, the actual cost for treating ACL injuries annually is much greater than merely the surgical fees. Because of this significant financial impact and the emotional toll this injury frequently takes on young athletes, developing prevention strategies for this injury is essential.

The mechanism of noncontact ACL injuries has been elucidated (see "ACL Injury Mechanisms," below). However, numerous theories have been proposed to explain what predisposes patients to noncontact ACL injury. These theories are divided into four categories: hormonal, anatomic, environmental, and neuromuscular (table 1).


TABLE 1. Proposed Causes of ACL Injury


Hormonal
Estrogen

Anatomic
ACL size
Intercondylar notch
Lower-leg alignment
Knee joint laxity
Muscle flexibility

Environmental
Playing style
Shoe-surface interface
Uneven playing surface


Hormonal Components

Recent epidemiologic studies (4) have documented a significantly higher ACL injury rate in female athletes as compared with male counterparts, especially in basketball and soccer. Women sustain two to eight times more ACL injuries for the same sport than men.

One hypothesis is that hormones such as estrogen, which can relax soft tissue, may predispose female athletes to ACL tears. Estrogen, a hormone with receptors on the human ACL, reduces collagen synthesis and fibroblast proliferation (5). It has been postulated that any rise in estrogen, such as during the midcycle of the menstrual period, may diminish the tensile strength of the ACL. In addition, estrogen has been reported to decrease fine motor skills by acting on the central and peripheral nervous systems (6). Motor skill deficits may diminish the normal neuromuscular protective mechanisms of the knee.

Estrogen's effect was analyzed in a retrospective study (7) of 28 female athletes who sustained acute noncontact ACL tears. All the athletes reported a regular menstrual cycle. The authors found an increased incidence of ACL injury in women during the ovulatory phase (days 10 to 14) of the menstrual cycle, when a surge in estrogen production occurs. These data, however, should be interpreted with caution because the sample size was small, recall may have been inaccurate, blood and urine samples were lacking, and the authors were unable to prove a causal relationship.

The clinical effect of estrogen and other cyclic hormones on ACL laxity was assessed with objective ACL laxity measurements using the KT2021 (MEDmetric Corp, San Diego) arthrometer, a device that measures anterior and posterior tibial displacement (8). The authors evaluated more than 9,000 KT2021 results and found only a slight trend toward increased knee laxity during the middle third of the menstrual cycle. Insufficient information on the association between specific hormones and the rate of ACL injuries in female athletes makes it impossible to draw definite conclusions or make specific treatment recommendations. The effect of hormones that alter the structural properties of the ACL merits further investigation.

Anatomic Factors

Intercondylar notch. Impingement of the ACL against the intercondylar notch has been proposed as a possible anatomic cause of ACL injuries (9-11). An ACL lodged in a narrow A-shaped notch, instead of a reverse U-shaped notch, may experience greater shearing forces against the bone. Kennedy et al (10) suggested that the ACL impinges on the medial border of the lateral femoral condyle with valgus stress. It has been demonstrated in cadavers that the ACL contacts the anterior intercondylar notch when the knee is in full extension (11). Computed tomography (CT) analysis of patients with bilateral ACL injuries has revealed a narrower intercondylar notch in injured patients when compared with the intercondylar notch of controls (9).

The literature on intercondylar notch stenosis as a predictor of ACL injury has several limitations. Roentgenographic techniques for measuring intercondylar notch width vary considerably depending on knee angle, magnification, and measurement locations. Both plain films and CT scans lack ratio measurements for determining ACL impingement. A recent preliminary report correlating ACL size and intercondylar notch morphology showed no mismatch in males or females between the notch width and ACL area (12).

It is plausible that ACL impingement can occur through hyperextension. However, most ACL injuries occur with the knee partially flexed. In addition, the location of ligament rupture is usually more proximal to the potential site of impingement. If notch impingement did occur from valgus force to the knee, one would expect to see a higher rate of concomitant medial collateral ligament injuries. Further investigation is needed before attributing any relationship between impingement and ACL rupture. Cadaveric knee manipulation in ACL provocative positions and clinical trials that measure notch width and ACL size should help clarify the issue.

Other factors. In women, a wider pelvis and greater average Q-angle have been postulated to contribute to ACL disruption by placing the knee in a more valgus or unstable position. Based on differences in extensor mechanism anatomy between the sexes, Nisell (13) found that, at knee flexion angles less than 60°, quadriceps contractions of equal magnitude place an increased strain on the ACL in women. This increased anterior vector may be responsible for the higher incidence of ACL ruptures in female athletes (13,14). In contrast, the hamstring muscles, as ACL agonists, exert a posterior force on the proximal tibia that protects the ACL (15).

Preliminary data (16) show that women have more knee and muscle laxity than their male counterparts. Therefore, in female athletes with above-average hamstring flexibility, the protective ability of this muscle group may be diminished, and the forces required to stabilize the knee preferentially transferred to the ligaments. Though these anatomic parameters may not be the primary cause of ACL injury, they may predispose female athletes to ACL disruption. The ability to accurately measure hamstring flexibility and the interplay between the knee flexors and extensors require further study.

Additional anatomic factors that may contribute to ACL injury are ACL size, lower-leg malalignment, abnormal extensor mechanism anatomy, knee joint laxity, and muscle flexibility.

Environmental Contributors

Several environmental parameters have been assessed as possible contributors to ACL injury. For example, a higher rate of ACL injuries has been reported in athletes who wear cleats that are placed at the peripheral margin of the sole with a number of smaller pointed cleats positioned interiorly (17). This cleat arrangement resulted in a higher torsional resistance than the other cleat designs.

An uneven playing surface, such as bumpy grass fields, may also pose a threat to the ACL. In one study (18) of ACL injury mechanics, many patients reported landing or stepping on an uneven surface at the time of the accident. The unexpected foot position may alter knee motor recruitment patterns, placing the ACL at risk of rupture.

Gender variations in athletic posture and movement patterns may contribute to ACL injury. Although an athlete's playing style may have a genetic component, it is also dependent on environmental parameters such as coaching and training techniques. Videotape analysis of male versus female athletes revealed that women tend to play sports in a more erect position (18). This was recently confirmed in a jumping study with the aid of two-dimensional motion analysis (19). A more upright position amplifies ground reaction forces that increase the load transmitted to the knee and maximizes anterior shear forces from the quadriceps, an ACL antagonist (13,14,19).

The overall effect of the shoe-surface interface, uneven surfaces, and playing style on ACL injury is difficult to determine, but all may play a minor role in noncontact ACL injuries.

Neuromuscular Elements

Recent research in the area of ACL injury risk factors has centered on neuromuscular performance. Neuromuscular control of the knee involves a complex interplay between the neurologic system and the muscles that cross the knee joint. Perhaps in noncontact ACL injury, expected motor recruitment patterns that control the knee are altered, which leads to injury. This aberration may result in a faulty or delayed neurologic signal to the knee instead of a protective muscle response.

The balance of muscle power and recruitment pattern between the quadriceps and the hamstring muscles is crucial to functional knee stability. The quadriceps, as ACL antagonists, may contribute to ACL injury. Numerous investigators have reported that quadriceps contraction increases ACL strain between 10° and 30° of knee flexion (figure 1) (20-26). Because most noncontact ACL injuries occur with the knee close to full extension, it is possible that the quadriceps play an important role in ACL disruption (18). It is well established that open-chain quadriceps extension exercises should be avoided during the early postoperative period to prevent harmful stress to the ACL graft (21,26). According to mechanical calculations, an eccentric quadriceps muscle contraction can produce forces beyond those required for ACL tensile failure (27-29).

[Figure 1]

In contrast to the quadriceps, the hamstring muscles are ACL agonists or "stress shielders." (23,25) Therefore, any weakness, increased flexibility, or delayed motor signal to the hamstrings may increase the susceptibility to ACL injury. When Huston and Wojtys (30) evaluated neuromuscular response to anterior tibial translation in male and female athletes, they found that female athletes demonstrated more anterior tibial laxity and significantly less muscle strength and endurance. In addition, the female athletes relied more on their quadriceps muscles and took significantly longer to generate maximum hamstring muscle torque after anterior tibial force was applied. Though understanding of neuromuscular characteristics of the knee is in its infancy, any alteration in the dynamic control of the knee that favors the quadriceps over the hamstrings may predispose a patient to noncontact ACL injury.

Prevention Produces Results

The goal of understanding what causes a noncontact ACL injury is to implement preventive strategies. Though there is a plethora of literature on the ACL, we found only a few reports that address prevention (31-33).

The effect of knee braces for ACL protection is negligible. Braces may slightly influence knee proprioception, but they do not alter electromyographic (EMG) firing patterns when compared with the unbraced knee (34). Additionally, no study has prospectively demonstrated a decreased incidence of ACL injury in braced versus unbraced athletes.

The focus of current prevention programs is on neuromuscular control of the knee. Henning developed the first neuromuscular prevention program for ACL injuries in the late 120210s. His program emphasized maintaining hip and knee flexion during athletic activity to minimize the quadriceps' ability to exert potential injury-producing forces on the ACL. Drills taught athletes to land on a flexed knee and to stop with a three-step stop with the knee flexed instead of a one-step stop with the knee extended. He also encouraged an accelerated rounded turn on a flexed knee rather than changing directions by planting and cutting on a straight knee. Early trials of his program demonstrated a significant reduction in ACL injury rates.

Caraffa et al (31) in Italy prospectively followed 600 male soccer players over three seasons. Half of the athletes were placed in a proprioceptive training program consisting of 20 minutes per day for a minimum of 6 weeks of balance training with and without varying types of balance boards. The authors found a sevenfold reduction of ACL injuries in the proprioception group when compared with controls.

In another study, Hewett et al (32) analyzed the effect of a 6-week plyometric training program in female athletes. Plyometrics involves stretching muscle before contraction—for example, repeated jumping that stretches and contracts the thigh muscles. The preseason program involved 90-minute workout sessions 3 days a week. Because ACL injuries often occur after landing maneuvers, the authors examined landing mechanics before and after 6 weeks of plyometric jump training.

The ACL prevention program consisted of three phases: stretching, plyometrics, and weight training. During the first few weeks, the focus was on teaching proper jumping and landing techniques that emphasize posture, knee stability, and soft landings. Subjects were trained to land on the balls of their feet with the knees flexed and the chest over the knees (figure 2). Athletes were constantly reminded to avoid any excessive side-to-side or forward-backward motion of the knees on landing. Inward buckling of the knee was discouraged during landing. Soft, silent landings with toe-to-heel rocking of the foot were recommended to decrease ground reaction forces.

[Figure 2]

Different jumps, such as wall tucks, broad jumps, squat jumps, and cone jumps were performed on soft mats with increasing repetitions and time intervals. Verbal cues used to teach proper technique included "straight as an arrow, light as a feather, recoil like a spring, be a shock absorber, and on your toes." (1) Plyometrics and weight-training exercises were performed together three times per week.

After the 6-week training period, the authors found that peak landing forces decreased 22%, which translates to diminished forces at the knee and less stress on the ligaments (32). Medial and lateral forces at the knee decreased 50%, reducing the chance of landing in a position that predisposes the athlete to ACL disruption. In addition, the training program resulted in a 10% increase in jump height and a 44% increase in hamstring strength. The hamstring-to-quadriceps strength ratio increased from 50% to 66%, a more favorable condition for the ACL.

To determine the clinical effectiveness of their neuromuscular training program, the authors prospectively evaluated three groups of athletes (33). Group 1 consisted of female athletes who participated in the jump training program for a minimum of 4 weeks. Groups 2 and 3 were female and male athletes, respectively, who did not participate in the prevention training program. A total of 1,263 high school soccer, volleyball, and basketball athletes were monitored for injury during one playing season. The results demonstrated a 3.6 times higher incidence of knee injury in female controls compared with trained female subjects. The untrained female controls also had a 4.8 times higher incidence of injury than the untrained male controls (see "Training May Reduce Knee Injuries in Female Athletes," Best of the Literature, page 24).

The mechanism of ACL injury in skiing is different from that in jumping, running, and cutting sports such as football, soccer, basketball and volleyball. In skiing, most ACL injuries result from internal rotation of the tibia with the knee flexed greater than 90°, a position that results when a skier, falling backward, catches the inside edge of the tail of the ski (35). Though the mechanism of injury in skiing differs from pivoting sports, neuromuscular conditioning has also been demonstrated to be successful at preventing ACL injury (35). The Vermont Ski Safety program teaches skiers to recognize and respond with appropriate strategies to dangerous situations and to avoid potentially compromising positions. In early trials, this ski safety program resulted in a 69% reduction in serious knee sprains.

Useful Research Applications

The degree to which each possible etiologic factor plays in contributing to ACL injury is unknown. Nonetheless, neuromuscular control of the dynamic stabilizers of the knee likely plays an important role. Fortunately, preliminary trials on knee proprioception and proper landing techniques show successful reduction in the incidence of ACL injury. We encourage those involved in high-risk sports to incorporate into their conditioning programs training techniques from one or several of the existing neuromuscular prevention programs.

References

  1. Hewett TE, Noyes F: Cincinnati Sportsmetrics: a jump training program proven to prevent knee injury, videotape. Cincinnati, Cincinnati Sportsmedicine Research & Education Foundation, 192021
  2. Kao JT, Giangarra CE, Singer G, et al: A comparison of outpatient and inpatient anterior cruciate ligament reconstruction surgery. Arthroscopy 1995;11(2):151-156
  3. Malek MM, DeLuca JV, Kunkle KL, et al: Outpatient ACL surgery: a review of safety, practicality, and economy. Instr Course Lect 1996;45:281-286
  4. Arendt E, Dick R: Knee injury patterns among men and women in collegiate basketball and soccer: NCAA data and review of literature. Am J Sports Med 1995;23(6):694-701
  5. Liu SH, Al-Shaikh RA, Panossian V, et al: Estrogen affects the cellular metabolism of the anterior cruciate ligament: a potential explanation for female athletic injury. Am J Sports Med 1997;25(5):704-709
  6. Posthuma BW, Bass MJ, Bull SB, et al: Detecting changes in functional ability in women with premenstrual syndrome. Am J Obstet Gynecol 120217;156(2):275-278
  7. Wojtys EM, Huston LJ, Lindenfeld TN, et al: Association between the menstrual cycle and anterior cruciate ligament injuries in female athletes. Am J Sports Med 192021;26(5):614-619
  8. Belanger MJ, McGovern RD, Moore DC, et al: Menstrual cycle, exercise, and knee laxity. Presented at the annual meeting of the American Orthopaedic Society of Sports Medicine, June 19-22, 1999, Traverse City, MI
  9. Harner CD, Paulos LE, Greenwald AE, et al: Detailed analysis of patients with bilateral anterior cruciate ligament injuries. Am J Sports Med 1994;22(1):37-43
  10. Kennedy JC, Weinberg HW, Wilson AS: The anatomy and function of the anterior cruciate ligament: as determined by clinical and morphological studies. J Bone Joint Surg (Am) 1974;56(2):223-235
  11. Norwood LA Jr, Cross MJ: The intercondylar shelf and the anterior cruciate ligament. Am J Sports Med 1977;5(4):171-176
  12. Anderson AF, Dome DC: Correlation of anthropomorphic measurements, strength, ACL size and intercondylar notch morphology to gender in ACL tears. Presented at the annual meeting of the American Orthopaedic Society of Sports Medicine, June 19-22, 1999, Traverse City, MI
  13. Nisell R: Mechanics of the knee: a study of joint and muscle load with clinical applications. Acta Orthop Scand Suppl 120215;216:1-42
  14. Huberti HH, Hayes WC, Stone JL, et al: Force ratios in the quadriceps tendon and ligamentum patellae. J Orthop Res 120214;2(1):49-54
  15. More RC, Karras BT, Neiman R, et al: Hamstrings—an anterior cruciate ligament protagonist: an in vitro study. Am J Sports Med 1993;21(2):231-237
  16. Wojtys EJ, Huston LJ, Ashton-Miller JA: Active knee stiffness differs between young men and women. Presented at the annual meeting of AOSSM, July 12-15, 192021, Vancouver, British Columbia
  17. Lambson RB, Barnhill BS, Higgins RW: Football cleat design and its effect on anterior cruciate ligament injuries: a three-year prospective study. Am J Sports Med 1996;24(2):155-159
  18. Boden BP, Dean GS, Feagin JA, et al: Mechanisms of ACL injury. Orthopedics, in press
  19. Vibert B, Huston LJ, Ashton-Miller JA: Gender differences in knee angle when landing from a jump. Presented at the annual meeting of the American Orthopaedic Society of Sports Medicine, June 19-22, 1999, Traverse City, MI
  20. Torzilli PA, Deng X, Warren RF: The effect of joint-compressive load and quadriceps muscle force on knee motion in the intact and anterior cruciate ligament-sectioned knee. Am J Sports Med 1994;22(1):105-112
  21. Arms SW, Pope MH, Johnson RJ, et al: The biomechanics of anterior cruciate ligament rehabilitation and reconstruction. Am J Sports Med 120214;12(1):8-18
  22. Shoemaker SC, Adams D, Daniel DM, et al: Quadriceps/anterior cruciate graft interaction: an in vitro study of joint kinematics and anterior cruciate ligament graft tension. Clin Orthop 1993;294(Sep):379-390
  23. Draganich LF, Vahey JW: An in vitro study of anterior cruciate ligament strain induced by quadriceps and hamstrings forces. J Orthop Res 1990;8(1):57-63
  24. Hirokawa S, Solomonow M, Lu Y, et al: Anterior-posterior and rotational displacement of the tibia elicited by quadriceps contraction. Am J Sports Med 1992;20(3):299-306
  25. Renstrom P, Arms SW, Stanwyck TS, et al: Strain within the anterior cruciate ligament during hamstring and quadriceps activity. Am J Sports Med 120216;14(1):83-87
  26. Wilk KE, Escamilla RF, Fleisig GS, et al: A comparison of tibiofemoral joint forces and electromyographic activity during open and closed kinetic chain exercises. Am J Sports Med 1996;24(4):518-527
  27. Noonan TJ, Yu B, Garrett WE Jr: Patella-tendon-tibia shift angle in closed kinetic chain of the lower extremity with weight-bearing. Unpublished data
  28. Woo SL, Hollis JM, Adams DJ, et al: Tensile properties of the human femur-anterior cruciate ligament-tibia complex: the effects of specimen age and orientation. Am J Sports Med 1991;19(3):217-225
  29. Stauber WT: Eccentric action of muscles: physiology, injury, and adaptation. Exerc Sport Sci Rev 120219;17:157-185
  30. Huston LJ, Wojtys EM: Neuromuscular performance characteristics in elite female athletes. Am J Sports Med 1996;24(4):427-436
  31. Caraffa A, Cerulli G, Projetti M, et al: Prevention of anterior cruciate ligament injuries in soccer: a prospective controlled study of proprioceptive training. Knee Surg Sports Traumatol Arthros 1996;4(1):19-21
  32. Hewett TE, Stroupe AL, Nance TA, et al: Plyometric training in female athletes: decreased impact forces and increased hamstring torques. Am J Sports Med 1996;24(6):765-773
  33. Hewett TE, Lindenfeld TN, Riccobene JV, et al: The effect of neuromuscular training on the incidence of knee injury in female athletes: a prospective study. Am J Sports Med 1999;27(6):699-706
  34. Branch TP, Hunter R, Donath M: Dynamic EMG analysis of anterior cruciate deficient legs with and without bracing during cutting. Am J Sports Med 120219;17(1):35-41
  35. Ettlinger CF, Johnson RJ, Shealy JE: A method to help reduce the risk of serious knee sprains incurred in alpine skiing. Am J Sports Med 1995;23(5):531-537


ACL Injury Mechanisms

Gaining a better understanding of ACL injury mechanism provides greater insight into possible predisposing factors (1,2). Noncontact ACL injuries typically occur during deceleration and change of direction with the foot fixed (figure A) (1-3). Knee torsion that results from making a sudden directional change on a planted foot has been implicated as a cause of ACL tears (4).

[Figure A]

In one report (3), researchers interviewed 89 athletes (100 injuries) who were able to recall the mechanism of their ACL disruption. Data such as contact versus noncontact, position of the knee and lower leg, direction of knee collapse, direction of body twisting, and other events were recorded. A noncontact mechanism was described in 72% of the athletes. The most common activities at the time of the injury were basketball, football, and soccer. A popping sound was described in 70% of ACL failures. The average angle of knee flexion at injury was estimated to be 21°. The most common scenario for a contact ACL disruption was a blow to the lateral aspect of the leg or knee causing valgus collapse.

An additional 27 ACL injuries captured on videotape were reviewed (3). In the noncontact injuries, most athletes were close to an opponent at the time of injury. The opponent may have disrupted the injured athlete's coordination or motor pattern.

The number of variables an athlete must respond to in team sports may explain the higher incidence of injuries in sports such as football, soccer, and basketball. Though the exact moment of injury was impossible to determine from videotape, the position of the leg before collapse in all noncontact injuries was near foot strike with the knee in slight flexion. None were associated with a sharp, pivoting motion of the body around a planted leg or varus collapse of the knee. Valgus collapse of the knee in varying degrees was noted in most injuries.

These findings have several important implications. Sharp changes in direction, landing, and rapid deceleration are normally repeated thousands of times in athletic endeavors without injury. The difference for athletes who sustain ACL injuries may be that abnormal motor patterns cause the quadriceps to preferentially activate before the hamstrings.

REFERENCES

  1. McNair PJ, Marshall RN, Matheson JA: Important features associated with acute anterior cruciate ligament injury. NZ Med J 1990;103(901):537-539
  2. Feagin JA Jr, Lambert KL: Mechanism of injury and pathology of anterior cruciate ligament injuries. Orthop Clin North Am 120215;16(1):41-45
  3. Boden BP, Dean GS, Feagin JA, et al: Mechanisms of ACL injury. Orthopedics, in press
  4. Myklebust G, Engebretsen L, Strand T, et al: Friction tests between handball floors and different types of shoes. Scand Found Med Sci Sports 1992;Nov:53


Dr Boden is an orthopedic surgeon at The Orthopaedic Center in Rockville, Maryland. Dr Griffin is an orthopedic surgeon and team physician at Georgia State University in Atlanta. Dr Garrett is professor and chairman in the Department of Orthopaedic Surgery at the University of North Carolina at Chapel Hill. Address correspondence to Barry P. Boden, MD, The Orthopaedic Center, 9711 Medical Center Dr, Suite 201, Rockville, MD 20850.


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