Anterior Cruciate Ligament Injury Prevention: Concepts, Strategies, and Outcomes

Allston J. Stubbs

Mininder S. Kocher

Introduction and History

A recent 10-year survey of knee injuries in 6,434 patients revealed that tears of the anterior cruciate ligament (ACL) were responsible for 20% of joint complaints.¹ It is estimated that each year between 75,000 and 250,000 individuals in the United States will suffer a new injury to the ACL. The burden of disease is particularly high in the female athlete. It is estimated that 1.4 million women and girls have torn their ACL during the last decade, and the incidence of noncontact ACL injury in female collegiate athletes is as high as 1 in 10.²

The epidemic of ACL injuries in both noncontact and contact athletics has resulted in a two-pronged strategy of prevention and treatment. As surgical treatment for ACL injuries carries with it morbidity, long rehabilitation and recovery, and large costs, attention has been focused on prevention of the sentinel ligament injury. During the last 35 years, orthopaedic research has contributed to a better understanding of cruciate ligament biomechanics, physiology, and failure patterns. ACL injury prevention models have been developed to address differences in cruciate injury among female athletes³ ⁴ ⁵ ⁶ and contact athletes.⁷ ⁸ ⁹

A complete tear of the ACL has been historically regarded as a significant injury. This opinion was based on the morbidity of surgical treatment, the length of time for recovery and rehabilitation, and the cost of medical treatment. The evolution of arthroscopic knee surgery, with reliable ligament reconstruction techniques, has limited the associated surgical morbidity of ACL treatment, but issues of recovery time and cost remain. Typically, ACL reconstruction requires a minimum of 6 months of functional therapy to ensure adequate muscle strength and knee joint proprioception. Additionally,
the direct medical costs of a torn ACL approach $17,000 within the first year; a cumulative yearly cost of $1.5 billion in the United States.⁴ This figure does not account for long-term cost of posttraumatic knee osteoarthritis or emotional cost of a major injury to active individuals.

A successful prevention strategy must address goals of efficacy, compliance, reproducibility, and cost. Several approaches to prevention have been taken, including identifying high-risk athletes, encouraging better biomechanics, and providing structural support to a healthy knee. This chapter presents current information on concepts, strategies, and outcomes related to ACL injury prevention.

Biomechanics and the Female Athlete: The Noncontact Anterior Cruciate Ligament Injury

The first concept of ACL injury prevention begins on the tissue level and expands to include the interaction among biologic tissues and systems. A simple hypothesis would state that achieving optimal biomechanics of a tissue system would lead to a lower chance of injury to that system. The corollary argument being that less than optimal biomechanics of a tissue system would lead to a higher chance of injury to that system. Expanding this hypothesis to the competitive athlete, one would propose that well-conditioned athletes have a lower rate of injury and that poorly conditioned athletes have a higher rate of injury. More specifically, an athlete with worse biomechanics would have a greater risk of ACL injury. A dynamic, multifactorial model of sports injury etiology proposes that internal risk factors combine to make an athlete predisposed to injury. Exposure to external risk factors lead to a susceptible athlete who is then exposed to an inciting event, leading to injury (Fig. 5.1).¹⁰

What often distinguishes this injury from that of the female basketball player, however, is the presence of an external force beyond that of gravity; hence, “a contact sport.” It is thus the challenge to further define the proposed hypothesis in the context of contact versus noncontact activities. The female athlete has been used as the model for the stated hypothesis in the setting of noncontact ACL injury and, thus, is the basis for this discussion.

According to Hewett et al.,¹¹ gender differences between male and female ACL injuries are thought to result from differences in anatomy, hormones, and neuromuscular patterns. Another study has defined these differences as nonmodifiable and modifiable.¹² Nonmodifiable differences, such as femur length,¹³ femoral notch width,¹⁴ patient height,¹² and menstrual cycle hormone levels¹⁵ ¹⁶ ¹⁷ contribute in different capacities to ACL injury risk. Modifiable differences, such as neuromuscular patterns, appear to result from a lack of synchronization between growth and maturity of the lower extremity and appropriate neuronal control of the lower extremity in high-risk sporting movements.¹⁸ ¹⁹ ²⁰ ²¹

ACL injury of the female athlete has typically two defining characteristics: noncontact and deceleration. Based on these characteristics, investigators have sought to understand the implications of modifiable biomechanics on female ACL injury risk patterns and the subsequent effects of
neuromuscular training designed to protect at-risk athletes. The female ACL injury risk pattern is best defined as a pathokinetic chain.²² This chain begins as an increased adductor moment at the hip leading to lower extremity valgus and, ultimately, increased ACL strain. Hewett et al.¹² prescreened 205 female athletes for neuromuscular control using measurements of lower extremity joint angles and moments during a jump landing task. They noted that the nine athletes with subsequent ACL tears had an 8 degree greater abduction angle (knee valgus), 2.5 times greater abduction moment, and 20% higher ground reaction force as noted on the prescreening examinations. They concluded that landing task knee motion and loading are predictors of ACL injury risk in female athletes.

Figure 5.1. A dynamic, multifactorial model of sports injury etiology. (Adapted from

Bahr R, Krosshaug T. Understanding the injury mechanisms: a key component to prevent injuries in sport. Br J Sports Med 2005;39:324–329.

)

Other investigators have defined the at-risk position for ACL injury to be either knee varus or valgus with flexion of 10 to 30 degrees (Fig. 5.2 and Fig. 5.3).²³,²⁴ Markolf et al.²⁵ were more specific in their study of ACL forces and specified that a varus force at the knee in combination with an internal rotation moment at the knee placed the ACL at greatest risk for tearing.

Encouraged by earlier investigations, Sell et al.²⁶ examined gender differences in planned and reactive stop-jump tasks of different directions. Comparing 18 males with 17 females of high school age, biomechanical and neuromuscular patterns were observed in the right knee. Of the three jumping directions, lateral jumping tasks to the medial aspect of the right knee demonstrated the greatest ACL risk profile by increasing ground-reactive forces, increasing proximal tibial shear forces, increasing valgus and flexion moments, and lower flexion angles. These findings were potentiated in female athletes and by switching from a planned to a reactive task. The authors concluded that future neuromuscular and proprioceptive ACL injury prevention programs should incorporate reactive task training as well as lateral jumping strategies.

Figure 5.2. Valgus lower extremity alignment puts the anterior cruciate ligament at risk of injury. This alignment consists of hip adduction, femoral internal rotation, knee abduction, tibial external rotation, and ankle eversion. Ext, external. (Reprinted with permission from

Hewett TE, Shultz SJ, Griffin LY, eds. Understanding and Preventing Noncontact ACL Injuries. Champaign, IL: Human Kinetics; 2007.

)

Figure 5.3. Clinical example of the valgus lower extremity at-risk alignment. (Reprinted with permission from

Hewett TE, Shultz SJ, Griffin LY, eds. Understanding and Preventing Noncontact ACL Injuries. Champaign, IL: Human Kinetics; 2007.

)

Anterior Cruciate Ligament Injury-Prevention Studies

Several ACL injury prevention studies have been performed, with many showing a reduction in the risk of noncontact ACL injury. A summary of ACL injury prevention studies is shown in Table 5.1.

Most injury-prevention schemes have focused on neuromuscular control and proprioception. Caraffa et al.³ studied 600 soccer players in Sweden. Half (300 players) were assigned proprioceptive training and the other half no training. The players were followed for three soccer seasons. The group treated with proprioceptive training demonstrated a statistically significant decrease in the rate of ACL injury compared with the untreated group.

Hewett et al.⁴ examined the effect of corrected jump and landing technique training on 11 female subjects (Fig. 5.4 and Fig. 5.5). They noted significantly reduced abduction moments at the knee. A similar study showed that phase-oriented and technique neuromuscular training in female athletes reduced the rate of ACL injuries to that seen in a male cohort.¹¹ A similar risk reduction was seen in elite female

Norwegian handball players by Myklebust et al.⁵ In the elite players, they noted that a five-phase program of neuromuscular control and planting/landing skills significantly reduced the risk of an ACL tear over the course of two seasons. This effect was not seen in a broader skill set of female players. This led the study authors to conclude that compliance and time for training likely play a role in the success of such prevention programs. Furthermore, in a randomized trial of handball clubs in Norway, Olsen et al.²⁷ found that a structured program of warm-up exercises to improve running, cutting, and landing technique as well as neuromuscular control, balance, and strength resulted in a reduction of knee and ankle injuries. However, a follow-up video-based intervention program studied by Arnason et al.²⁸ did not show a reduction in ACL injury.

Table 5.1 Summary of Anterior Cruciate Ligament Injury-Prevention Studies

Study	Sport	Duration	Randomized	Equipment	Strength	Flexibility	Agility	Plyometric	Proprioception	Strengths	Weaknesses	Outcome
Caraffa et al.,³ 1996	Soccer semi-professional and amateur; N = 600 males on 40 teams (20 intervention, 20 control)	3-season intervention (preseason)	Prospective, nonrandom	Rectangular, oblique, circular, and BAPS boards (20 min from level I to V) over 3 to 6 days a week with self-determined advancement to next level = 30 preseason days	PNF exercises	No	No	No	Balance board activities: multi-level I-V on four boards	Mechano-receptor/proprio training	Additional equipment (BAPS); not cost-effective in a large scale cohort	87% decrease in NC ACL injury: 1.15/team/season in control group compared to 0.15/team/season in intervention group (p <0.001)
Ettlinger et al.,^a 1995	Alpine skiing; N = 4,000 ski personnel in 20 ski areas	1-year intervention (1993-94) with two previous years of historic controls (1991-93)	Prospective, nonrandom	Educational video clips of skiers sustaining ACL injuries and those that avoided injury in very similar falls; injury prevention education utilized (mechanism of injury, avoidance of high-risk behavior, fall technique)	No	No	No	No	No	Fall analysis and accident and injury analysis; cost-effective intervention and highly feasible with large skiing populations	Nonrandomized; not all potential participants trained; historic controls; exact diagnosis of serious knee sprains not always available; exact exposure to risk cannot be precisely determined	Severe knee sprains reduced by 62% among trained skiers (patrollers and instructors) compared to unperturbed group, who had no improvement during study period
Gilchrist et al.^b 2004 (abstract only)	Soccer U-18 to U-22; N = 561 females from 61 Div I NCAA universities	1-year intervention	Yes	Educational video, cones, soccer ball	Glut med, abd, ext, HS, core training	Yes	Decel-eration, sport specific	Hip and knee position, landing technique, multiplanar	Strength on field perturbation on grass	Instructional video; Web site, compliance monitored (random site visits)	Randomized, 1-year intervention, begun at day 1 of season	Overall 72% reduction in ACL injury; 100% reduction in practice contact and NC ACLs; 100% reduction in contact and NC ACLs in last 6 weeks of season
Griffis et al.,^c 1989	BB females	8-year intervention	Prospective, nonrandom	No	No	No	Yes	Landing technique (knee and hip flexion)	Rounded cut, deceleration patterns (3-step shuffle)	Changing cutting, deceleration, and landing techniques (encouraging knee and hip flexion)	Nonrandomized; not published (abstract only)	89% decrease in NC ACL injury in female basketball athletes
Mandelbaum et al.,⁶ 2005	Soccer U-14 to U-18; N = 1041 (year 1) and 844 (year 2) females	2-year intervention	Prospective, nonrandom	Educational video, 2 in. cones, soccer ball	HS, core training	Yes	Soccer specific with deceleration techniques	Hip and knee, landing technique, multiplanar	On-field program: strength, plyo, agilities on grass	Instructional video; Web site, compliance monitored (random site visits)	Nonrandomized; inherent selection (motivational) bias	Injury rates: year 1-88% reduction in NC ACL injury; year 2%-74% reduction in NC ACL injury
Myklebust et al.,⁵ 2003	European team handball; N = 900 females Div I-III	3-year intervention, five-phase program	Prospective, nonrandom	Educational videotape, poster wobble board, balance foam mats	No	Yes	Planting, cutting, NM balance control activities	Landing technique (knee and hip flexion)	Balance activity on foam mats and boards	Compliance monitored by PT; instructional video poster	Nonrandomized; insufficient power	In elite division, risk of injury reduced among those who completed the program (OR: 0.06 [0.01-0.54]) compared with control; overall 53.8% and 61.5% reduction of ACL injury
Pfeiffer et al.,^d 2004	Soccer, VB, BB HS females; N = 577 intervention, N = 862 control	2-year prospective intervention over a 9-week treatment, 15 min, two times a week	Prospective, nonrandom	No	Yes	No	No	Landing technique	No	Compliance monitored; sig. reduction in GRF and RFD in intervention	No decrease in injury in intervention group; performed posttraining; fatigue phenomenon; only 9 weeks in duration	6 NC ACL injuries: 3 intervention and 3 control = no effect
Soderman et al.,^e 2000	Soccer females; N = 121 (control N = 100); only 62 intervention and 78 control completed study	1-season intervention (April-Oct), 10-15 min	Prospective, randomized	Balance board, 10-15 min training program in addition to regular training	No	No	No	No	Balance	Randomized clinical trail; sig. more injuries in control vs. intervention	37% dropout rate; not all subjects received same amount of training; unknown if training other than balance board was the same; numbers of ACL injuries very small	The training did not reduce the risk of primary traumatic injuries to the lower extremities; four of five ACL injuries occurred in the intervention group
Hewett et al.,¹¹ 1999	BB, VB, soccer; N = 1263: male (N = 434), female (N = 366 trained and 463 untrained)	6-week pre-season intervention, 1-year monitoring, 60-90 min/day for 3 days a week	Prospective, nonrandom	Plyometric jump box, and balance	Yes	Yes	No	Yes	Yes	Videotape, decrease peak landing forces, decrease valgus/varus perturbation, increase vertical leap, increase hamstring strength and decrease time to hamstring contraction	Nonrandomized; low VB enrollment; motivational bias; 1-on- 1 program in sport facility; not feasible to implement across large cohort	14 ACLs reported: female injury rates 0.43 untrained vs. 0.12 trained vs. male control 0.9 over 6-week program; untrained group 3.6-4.8 higher injury rates of ACL injury
Heidt et al.,^f 2000	Soccer; N = 300 females	7-week pre-season intervention, 1-year monitoring, 3 days a week (one plyo and two treadmill)	Prospective, nonrandom	Treadmill, sports cord, plyometric box jump	Yes	Yes	Yes	Yes	Yes	Increase strength, lower overall injury rates	Not stat. sig.; 7 weeks insufficient time for NM re-education to occur at mechano-receptor level	61.2% injuries in knee/ankle; 2.4% injury rate in intervention vs. 3.1% in control
Olsen et al.,²⁷ 2005	European team handball (123 teams); N = 1837 players: 1586 female, 251 male	15-20 min training, 8 months (one handball season), 15 consecutive sessions and once a week thereafter	Randomized, controlled cluster trial	Wobble board (Norpro), balance foam mats (Airex)	Squats and power (bounding)	Yes	Planting, cutting, NM control	Knee over toe, proper landing technique	Balance activity on single/ double leg, mats and boards	Randomized; compliance monitored; reduction of injury; structured warm-up	Uncertain what parameter of program effective; male and female; cannot extrapolate to other sports	129 acute knee and ankle injuries overall; 81 in control (0.9 overall, 0.3 trained, 5.3 matched) vs. 48 in intervention (0.5 overall, 0.2 trained, 2.5 matched); 80% reduction of ACL injuries
Wedderkopp et al.,^g 2003	European team handball; 236 females (17-18 years), 20 teams	10-month intervention (one season)	Randomized, controlled cluster trial	Balance board (proprio-ceptive) in four levels	Yes	No	No	Yes	Balance training with ankle discs	Randomized clinical trial	Specific injury types not given; description of ankle disc training not given; “warm-up” exercises also provided to trained group but not specified; compliance with all exercises not mentioned	Ankle injuries sig. greater in control group (2.4 vs. 0.2); unspecified knee injuries not sig. less in trained group (6.9 vs. 0.6); 5 knee sprains and 1 knee “luxation” in control group vs. 1 knee sprain in trained group
BAPS, biomechanical ankle platform system; PNF, proprioceptive neuromuscular facilitation; propio, proprioceptive; NC, noncontact; ACL, anterior cruciate ligament; U, under; NCAA, National Collegiate Athletic Association; Glut med, gluteus medius; abd, abduction; ext, external; HS, high school; BB, basketball; plyo, plyometric; NM, neuromuscular; PT, patient; OR, odds ratio; VB, volleyball; GRF, ground-reaction force; RFD, rate of force development; sig., significantly; stat., statistically.
Adapted from: Griffin LY, Albohm MJ, Arendt EA, et al. Understanding and Preventing Noncontact Anterior Cruciate Ligament Injuries: A Review of the Hunt Valley II Meeting, January 2005. Am J Sports Med 2006;34:1512–1532.
^a 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;23:531–537. ^b Gilchrist JR, Mandelbaum BR, Melancon H. A randomized controlled trial to prevent anterior cruciate ligament injuries in female collegiate soccer players (abstract 6-7). Paper presented at: American Orthopaedic Society for Sports Medicine Specialty Day; March 13, 2004; San Francisco, CA. ^c Griffis ND, Nequist SW, Yearout K, et al. Injury prevention of the anterior cruciate ligament. Abstracted in the American Orthopaedic Society for Sports Medicine: Meeting Abstracts, Symposia, and Instructional Courses, 15th Annual Meeting; June 19–22, 1989; Traverse City, MI. ^d Pfeiffer RP, Shea KG, Grandstrand S, et al. Effects of a knee ligament injury prevention (KLIP) program on the incidence of noncontact ACL injury: a two-year prospective study of exercise intervention in high school female athletes. Podium presentation at American Orthopaedic Society for Sports Medicine Specialty Day; March 13, 2004; San Francisco, CA. ^e Soderman K, Werner S, Pietila T, et al. Balance board training: prevention of traumatic injuries of the lower extremities in female soccer players? A prospective randomized intervention study. Knee Surg Sports Traumatol Arthrosc 2000;8:356–363. ^f Heidt RS, Sweeterman LM, Carlonas RL, et al. Avoidance of soccer injuries with preseason conditioning. Am J Sports Med, 2000;28:659–662. ^g Wedderkopp N, Kaltoft M, Holm R, et al. Comparison of two intervention programmes in young female players in European handball: with and without ankle discs. Scand J Med Sci Sports 2003;13:371–375.

Figure 5.4. Plyometric neuromuscular training. (Reprinted with permission from

Hewett TE, Shultz SJ, Griffin LY, eds. Understanding and Preventing Noncontact ACL Injuries. Champaign, IL: Human Kinetics; 2007.

)

Figure 5.5. Balance board training with biofeedback. (Reprinted with permission from

Hewett TE, Shultz SJ, Griffin LY, eds. Understanding and Preventing Noncontact ACL Injuries. Champaign, IL: Human Kinetics; 2007.

)

More recently, Paterno et al.²⁹ examined the role of neuromuscular training on 41 female high school students. The intervention lasted 6 weeks and measured single-limb postural stability, anteroposterior stability, and medial-lateral stability. With focused neuromuscular training, the authors noted a significant increase in overall single-limb stability and anteroposterior stability. There was no change in medial-lateral stability. The potential for this type of neuromuscular training to reduce ACL injury in a susceptible female population will need to be determined across a larger study group.

Bracing the Intact Native Anterior Cruciate Ligament

One of the most tangible historical and current interventions for protection against ACL injury has been knee bracing. Knee braces have been developed for prophylactic, functional, and rehabilitative purposes. Support for prophylactic knee bracing (PKB) has come from athletic trainers, coaches, physicians, and, most importantly, an athlete’s family members. The effectiveness of knee bracing in protecting native ligament integrity has been studied both biomechanically and clinically.

The Biomechanical Experience

In assessing PKBs, the use of cadaveric and surrogate knee models has been viewed with skepticism. The braced knee models, while controlled, do not account for weightbearing, physiologic loads, or muscle tone. In light of these limitations, the laboratory investigations of PKBs off the playing field have provided some insight into their effectiveness in protecting against knee ligamentous injury. Further, newer laboratory methods that examine additional risk factors for noncontact injuries have led to research that accounts for the deficiencies of previous studies, including weightbearing and muscle tone.

In 1987, Paulos et al.³⁰ published a study of 18 cadaveric knees tested with one of two lateral stabilizing braces: Anderson Knee Stabler (Vision Quest Industries, Inc., Irvine, CA [Out of Production]) or McDavid Knee Guard (McDavid Knee-Guard, Inc. Woodridge, IL). In the presence or absence of one of the lateral braces, the application of valgus force was analyzed with respect to joint line opening and resulting ligament tensions/failure. No correlation was
seen between the use of a brace and reduced joint line opening or ligament failure with application of valgus stress. The authors correlated these results to a lack of protection of the medial collateral ligament (MCL).

Also in 1987, Wojtys et al.³¹ focused specifically on the influence of the Lennox Hill Brace, a functional brace, in protecting the knee from excessive anterior translation or external rotation moments. The authors used four cadaveric specimens tested at 30 degrees of knee flexion to compare nonbraced and braced ligament-intact and ligament-deficient knees. Although the sample size was too small for statistical significance, the brace was noted to decrease anterior translation in the ACL intact knee and the isolated ACL-deficient knee. No effect was noted in the braced MCL-/ACL-deficient or MCL-/lateral collateral ligament-deficient knee. With regard to rotation, the brace effectively limited external rotation in all specimens regardless of ligament condition.

The variability of cadaveric knees in biomechanics testing led some research teams to test brace wear using a surrogate knee model. These models were typically made from composite materials that could be manufactured to mimic the bulk, size, and mechanics of human tissue. Additionally, some research teams employed hybrid cadaveric-surrogate knee models to capture brace effects in a reproducible and anatomic way.

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