Preventive Biomechanics: A Paradigm Shift with a Translational Approach to Biomechanics

Abstract

Background:

Preventive medicine techniques have alleviated billions of dollars in economic burden from the medical care system through the implementation of vaccinations and screenings prior to the onset of disease symptoms. These similar concepts are applicable to non-contact musculoskeletal injuries to reduce the economic burden of sports medicine treatment and enhance the long-term health of athletes. Knowledge of biomechanical tendencies has progressed rapidly over the past 20 years to the point where clinicians can identify, in healthy athletes, the underlying mechanisms that lead to catastrophic injuries such as anterior cruciate ligament rupture.

Purpose:

Illustrate the practical medical benefits that could be gained from preventive biomechanics as well as the need and feasibility for broad implementation of these principles.

Study Design:

Meta-analysis

Methods:

Recent literature pertinent to musculoskeletal injury screening and prevention was reviewed and compiled into a meta-analysis commentary on the current state and applicability of preventive biomechanics.

Results:

Investigators have identified neuromuscular training protocols that screen for and correct the underlying biomechanical deficits that lead to ACL injury. The literature shows that when athletes comply with these prescribed training protocols, the incidence of injury is significantly reduced within that population. Such preventive biomechanics practices employ basic training methods that would be familiar to athletic coaches and have the potential to save billions of dollars in cost in sports medicine.

Conclusion:

Widespread implementation of preventive biomechanics concepts could profoundly impact the field of sports medicine injuries with a minimum of initial investment.

INTRODUCTION

We define “Preventive Biomechanics” as the implementation of clinical measures within a standard training setting that demonstrate the capacity to diagnose relative risk and reduce the incidence rate of musculoskeletal injuries prior to onset. In opposition to complex and expensive laboratory assessments, the realization of preventive biomechanics is rooted on the cost effective integration of clinical knowledge and sports performance to protect athletes. The objective of preventive biomechanics is to employ basic training techniques in manners that are clinically proven to reduce injury incidence while enhancing performance. The overarching objective of preventive biomechanics is the widespread reduction of musculoskeletal injuries in athletics.

One particular injury that could benefit from the implementation of preventive biomechanics is anterior cruciate ligament (ACL) rupture. Over the past 17 years, the incidence rate of ACL injury among collegiate athletes in cutting and pivoting sports has declined in the United States.^1–3 In NCAA male and female soccer players the incidence rate of ACL injury per 1000 athletic exposures was respectively reported as 0.12 and 0.33 from 1994–1998,¹ then as 0.12 and 0.32 from 1989–2004,² and most recently as 0.07 and 0.25 from 2011–2014.³ Similarly, incidence among NCAA male and female basketball athletes were 0.07 and 0.29 from 1994–1998,¹ then 0.08 and 0.28 from 1989–2004,² and most recently 0.07 and 0.20 from 2011–2014.³ The period of these investigations (1999–2016) represents the advent and rise in popularity of targeted neuromuscular training designed to prevent ACL injuries through the alteration of an athlete’s movement biomechanics.^4–8 These preventive measures have gained popularity, especially among female athletes who are more susceptible to ACL injury than their male counterparts, but have yet to realize widespread incorporation across sport and medicine. Despite a lack of widespread adoption, these preventive measures still seem to have made an impact on ACL injury outcomes, as incidence among NCAA female athletes reduced by 22% - 29% between 2004 and 2014. This concept also pertains to lateral epicondylitis, where home exercise relieved pain better than a wait-and-see approach, but not as well as clinic-based exercise program;⁹ elbow injuries, where poor pitching biomechanics increased varus torque on the ulnar collateral ligament;¹⁰ shoulder injuries, where strengthening the scapula stabilizing musculature reduced injury incidence in tennis players;¹¹ as well as ankle sprains, where prophylactic interventions reduced incidence among basketball players.¹²

The concept of preventive medicine has proliferated since it was introduced by Leavell and Clark in 1965.¹³ The overarching goal of preventive medicine is to maintain patient health through the institution of and investment in measures that reduce or eliminate a healthy individual’s risk of acquiring a medical complication. The idea is that maintaining health across a whole population represents a reduced financial burden compared to treating individual cases of a medical complication. Indeed, it was estimated that in 2000, approximately 50% of American deaths were from preventable causes.¹⁴ This represents a significant, and potentially unnecessary, financial burden on the health care system as well as an emotional and personal burden patients and their families.

The benefits of preventive medicine may be difficult to quantify monetarily because it remains unknown how each patient will respond to preventive interventions and it is impossible to quantify the value of human life. However, from 2000 to 2010, preventable infant deaths in the 42 countries responsible for 90% of the worlds infant deaths were reduced from 9.6 million to 7.6 million.¹⁵ Within these countries, medical coverage of standard preventive care measures implemented in developed parts of the world is under 50%.¹⁶ It is estimated that an additional 55% of these deaths could be avoided with basic preventive interventions such as improved nutrition, sanitation, and vaccination techniques.¹⁶ Within the United States the implementation of the polio vaccine as a preventive measure reduced the burden of this disease from an average of greater than 15,000 paralytic cases per year in the 1950s to complete elimination by 1994.¹⁷ Overall, it is estimated that for every $1 spent on polio vaccinations over $5 has been saved in treatment costs. This circumstance is similarly true for vaccines of measles-mumps-rubella (reduced by >98%, saves $16.34 for every $1 spent) and diphtheria-tetanus toxoids-pertussis (reduced by >91%, saves $6.21).¹⁸ Similarly, population based medical risk screenings such as mammograms have been widely implemented as early detection of breast cancer can reduce treatment costs from $140,000 to $11,000.¹⁹ These, along with numerous other preventive medicine measures, relieve significant financial burden from the medical care system as well as improve the quality of life for patients who avoid these respective complications.

Despite the benefits documented, preventive medicine has, up to this point, experienced limited integration into the subspecialty of sports medicine. Most preventive measures prescribed within sports medicine have been directed around the general incorporation of exercise at a moderate-or-greater level of intensity into daily routine.^20,21 Commitments to general exercise exhibit positive influence on overall cardiovascular health and mortality.²² While general exercise and speed training have a slightly positive effect on the overall incidence of lower extremity musculoskeletal injuries, there is little evidence to indicate that such non-targeted training significantly impacts the incidence rate of catastrophic soft injuries such as ACL ruptures that sideline athletes for extended periods of time.^23,24 Injuries such as ACL rupture can devastate athletic careers and long-term quality of life.^25–27 These injuries occur in non-contact scenarios 70% of the time and are primarily the result of poor mechanical control that leads to the application of high loads within and athlete’s joints, consequently placing them “at risk” for injury.^28–34 Recently, significant work has been done to indicate that the incidence rate of these primarily non-contact injuries can be significantly reduced through appropriate training of an athlete’s neuromuscular control.^4,5,7,8 With an ever increasing numbers of adolescent and adult sports participants, and the corresponding injury incidence, the widespread application of preventive biomechanics measures stands to have a great impact on athlete health, just as preventive exercise and preventive medicine have had on the general population.

The objective of this commentary is to illustrate the practical medical benefits that could be gained from preventive biomechanics as well as the need and feasibility for broad implementation of these principles. Using data culled from the last 20 years of anterior cruciate ligament (ACL) injury research, this manuscript will outline a prospectus on how advanced laboratory biomechanical measures can be applied in clinical and pre-clinical settings in order the improve the overarching spectrum of sports health.

STAGES OF PREVENTION

According to the literature there are three traditional stages of preventive medicine.^13,35 Primary prevention addresses the elimination of disease causes in order to prevent the establishment of the disease condition. Secondary prevention addresses the disease itself in an attempt to disrupt the disease process before symptoms appear. Finally, tertiary prevention focuses on containment in an attempt to limit the physical and social repercussions of a symptomatic disease.

The vast majority of sports injuries are mechanical in nature, which presents an obvious difference in pathology from disease progression. However, the three pillars of preventive medicine can easily be adapted into three similar concepts that form the base principles of preventive biomechanics. The primary prevention measure of preventive biomechanics would be screening for injury risk. As with disease progression, athletic injuries are often preceded by identifiable risk factors that can be assessed in a lab, clinical, or athletic setting.^{32,33,36–45} The secondary prevention measure for preventive biomechanics would incorporate the training of athletes identified to be at high-risk for injury. Just as not every woman who has a mammogram requires a mastectomy, not every athlete who undergoes biomechanical screening will require corrective neuromuscular training. Certainly, preventive protocols can be applied to the athletic population at-large; however, it has been shown that the athletes who exhibit high-risk movement patterns are the individuals who will register the greatest change in response to corrective biomechanics training.⁴⁶ Finally, the tertiary prevention measure in preventive biomechanics would focus on evidence-based treatment and rehabilitation of injury occurrences to limit damage to the body and minimize the risk of further re-injury. As with many forms of preventive medicine, while the implementation of primary and secondary preventive biomechanics will reduce injury incidence,^4,5,7,8,23 they will not fully eliminate occurrence. At a minimum, these injured athletes need to return to activities of daily living, and often desire a full return to sport. Accordingly, the third pillar of prevention should be to treat injury occurrence in a manner that most efficaciously balances short term recovery and long term health.

INJURY RISK SCREENING

It has been recommended that football injury prevention efforts be focused at the knee and ankle as these joints account for 37.6% and 36.9% of all time-loss injuries at the high school and collegiate levels respectively.⁴⁷ Within this population, ACLs were the third most common knee injury and second most common knee surgery.⁴⁸ Incidence and variance of knee injuries in elite college football players] In the NBA, the tibiofemoral joint was the seventh most frequently injured structure of the body (4.4%), but accounted for the greatest time loss due to injury (13.6%).⁴⁹ Accordingly, for the past three decades, investigators have worked to identify the underlying mechanisms that lead to ACL injury. It is the desire of these investigators to identify treatable, high-risk biomechanical trends within athletes prior to the onset of catastrophic injuries. Indeed, a number of modifiable biomechanical factors have been related to increased risk of ACL injury.^{32,33,36–45,50} Many of these factors can be organized into four general categories of ligament dominance, quadriceps dominance, leg dominance, and trunk dominance.^{28–31,51–54} Ligament dominance is characterized by an inhibition to dissipate the impulse reaction forces generated during ground contact in the musculature. Instead, the loads are propagated through the passive restraints in the knee, resulting in larger stresses and strains that may induce rupture.^28–31,54 Specifically, during a prospective in vivo investigation, knee abduction moments and angles generated when landing from a jump were found to be significant predictors of subsequent ACL injury status.³² Quadriceps dominance is characterized by an imbalance in muscle recruitment between the knee flexors and extensors.^{28,30,31,51,52,54,55} The quadriceps muscle works as an antagonist to the ACL as it induces anterior tibial translation and ACL strain when contracted, while the hamstrings serve as a counteracting ACL agonist.^56–59 Thus, a quadriceps-to-hamstrings activation ratio below 60% is associated with increased risk of ACL injury as this was prospectively observed in females who went on to rupture and is believed to add strain to the ligament during dynamic activity.⁶⁰ Leg dominance is characterized by an imbalance of muscle recruitment and dynamic control between contralateral limbs.^{28,30,31,51,53–55} Athletes with asymmetric quadriceps recruitment have demonstrated similarly asymmetric mechanics in knee flexion excursion, trunk flexion, and knee extension moment.⁶¹ These, along with asymmetries in transverse plane hip kinetics, frontal plane knee kinematics, sagittal plane knee moments, and postural stability, are risk factors for subsequent ACL injury.⁶² Trunk dominance is characterized by poor control of the trunk during dynamic activity that moves the center of mass outside of the base of support of the body.^30,31 Approximately 60% of young female athletes demonstrate biomechanical profiles that express deficits in one or more of these four categories, which places them at elevated risk for ACL injury.³¹

These mechanical risk factors that precede ACL rupture can be used to parse out which athletes are most susceptible to injury prior to onset. In particular, knee abduction moment predicted ACL injury status with 78% sensitivity and 73% specificity (Figure 1).³² Direct laboratory measures of peak knee abduction angle, peak knee extensor moment, knee flexion range of motion, BMI Z-score, and tibia length account for 78% of the variance in knee abduction moment and predict high valgus knee loads (> 2.25 Nm) in an athletes with 85% sensitivity and 93% specificity.⁶³ Similarly, active proprioceptive response has been related to ACL injury risk and was used to predict injury status in a cohort of collegiate athletes with 91% accuracy.⁶⁴ The inherent problem associated with the identification of these risk factors is cost. Joint kinematics and kinetics are assessed via three dimensional motion analysis techniques that require facilities with access to quarter-million-dollar motion systems and trained personnel to conduct the time-intensive data analyses. Such expenses make these gold standard measures of diagnosing relative ACL injury risk cost-prohibitive to implement on nationwide scale with millions of adolescent athletes at the middle and high school levels.

Figure 1:

Data stratification of 205 young female athletes who were prospectively followed for ACL injury. The nine subjects who went on to ACL rupture demonstrated greater than average knee abduction angles and moments when landing from drop. This is indicative of the potential for biomechanical behaviors to serve as screening tools to identify those athletes who are most susceptible to particular injuries. Image reproduced with permission from Hewett, et al. 2005. Am J Sports Med, 33(4):492–501.

The cost-inhibition related to widespread implementation of advanced scientific metrics is not lost on sports medicine investigators. Unfortunately, two-dimensional (2D) video has been shown to correlate poorly with 3D laboratory measures and, therefore, cannot be interchanged as a reliable surrogate in a clinical setting.⁶⁵ Likewise, subjective 2D video analysis of 3D motion and subsequent classification of high and low ACL risk without the use of specific variable measures exhibited “inadequate” levels of sensitivity.⁶⁶ Using such and undefined protocol, raters failed to detect up to 33% of high-risk individuals. Accordingly, some investigators have worked to develop more clinically-applicable tools that extrapolate similar injury risk predictions without the expense of a full, detailed biomechanical analysis.^67–71 The clinic-based algorithm, a prediction tool using clinical surrogates for the laboratory-based 3D variables, predicted high KAM status with 84% sensitivity and 67% specificity using knee valgus motion, knee flexion range of motion, body mass, tibia length, and quadriceps to hamstrings strength ratio (Figure 2).⁶⁷ A similar prediction model has assessed high KAM status with 73% sensitivity and 70% specificity.⁶⁸ As illustrated previously, KAM is a leading predictor for risk of ACL injury.³² Each of these simplified screening mechanisms were executed with standard clinical tools including a physician’s scale, measuring tape, camcorder, free Image J software (National Institutes of Health, Bethesda, MD, USA), and an isokinetic dynamometer.⁷² With these clinical correlates and reduced equipment demands, it is now possible to implement effective biomechanical injury screening within academic and athletic team settings.

Figure 2:

Multiple clinical biomechanical risk factors can be combined into a single algorithm and offer strong predictive screening models that maintain high sensitivity and specificity without the prohibitive expense of obtaining precise laboratory measures. Image reproduced with permission from Myer, et al. 2010. Am J Sports Med, 38(10):2025–2033.

While clinically-based screening tools may accurately predict the presence of underlying mechanisms that lead to ACL injury and contribute to injury risk, there remains room for improvement in the prospective prediction of an actual ACL injury event. The landing error scoring system (LESS), a 2D tool that is reliable in clinical assessment of biomechanical deficiencies during a DVJ task,^71,73 was unable to predict non-contact ACL tears across a population of matched high school and collegiate athletes.⁷⁴ However, poor LESS scores did correspond with increased odds ratio of ACL rupture, which indicated potential for the tool to serve as a screen for relative injury risk. Such a concept was best illustrated within a cohort of 829 elite soccer athletes.⁷⁵ The LESS system predicted the need for ACL injury-prevention training with 86% sensitivity and 64% specificity within this group, but the 1-season risk of injury was 1.37% in athletes who scored worse than the established LESS threshold and 0.13% in athletes who scored better. A screening tool diagnosis of high injury risk does not guarantee that future ACL incidence will occur.

Muscular strength and activation have provided a second series of ACL injury risk prediction mechanisms for a clinical environment. A model based on EMG data from the vastus medialis, vastus lateralis, rectus femoris, biceps femoris, and semi-tendinosus predicted that the probability of sustaining ACL injury was 50% for athletes that expressed greater than one standard deviation in difference between vastus lateralis and semitendinosus pre-activity (mean 47% ± 14%) during a sidestep cut task.⁷⁶ Such a concept was later supported by isometric hip strength data that showed athletes who went on to ACL injury had significantly lower hip external rotation and abduction strength than their healthy counterparts.⁷⁷ The sensitivity and specificity of these hip strength prediction variables were 87–93% and 59–65%, respectively. While most clinical sports medicine environments will have access to isokinetic / isometric dynamometers to evaluate isolated muscle group strength, such equipment may be cost prohibitive at the middle and high school level. Fortunately, it is possible to assess strength deficits with standard weightlifting equipment, though the reliability of these measures have not been correlated to dynamometers.

The wealth of prediction tools presented provides medical practitioners, coaches, trainers, and teachers with a variety of tools to actively screen their patients, athletes, and students in a non-laboratory athletic environment. Widespread application of these tool are plausible as literature has shown a lack of differences in assessment reliability between medical practitioners and other groups in field-based DVJ screening tools.⁷⁸ The underlying purpose of all these screens is to ascertain easily identifiable biomechanical markers that occur within athletic tasks that can be used to identify athletes who are at high risk of catastrophic injury. In-turn, these high risk athletes can be treated and rehabilitated before an adverse event, or “pre-habilitated”, in order to optimize the probability of injury prevention.

TRAINING

The knee joint performs optimally when it functions as a hinge joint with minimal frontal or transverse plane rotation. Musculoskeletal modeling has demonstrated that no amount of sagittal plane flexion torque can lead to ACL failure;⁷⁹ however, both frontal and transverse plane moments serve as precursors to ACL failure as they invoke ligament loading.^80–83 Athletes who invoke high levels of neuromuscular control during the performance of athletic tasks tend to exhibit smaller magnitudes of excursion and instability outside the sagittal plane and, consequently, reduced loading on the ACL. Conversely, athletes with poor neuromuscular activation lack this control and demonstrate the clinically screenable biomechanical tendencies described above.^{28,32,84–86} Once an athlete has been identified with high-injury-risk potential, it is possible to correct his or her negative biomechanical tendencies through targeted intervention training. ^4,5,7,8 In as little as 6–10 weeks, this pre-habilitation can reduce neuromuscular deficits to the point where it influences injury incidence outcomes.⁵ Specifically, pre-habilitation training in high-risk athletes has reduced ACL injury incidence compared to untrained controls by upwards of 50% across multitude of investigations, with individual study results as high as 88%.^4–8 Within a population of female soccer players, controlled randomized implementation of injury-prevention pre-habilitation programs reduced overall ACL incidence rate from 0.67% in controls to 0.28% in trained athletes.⁸⁷ However, it should be noted that compliance has a direct impact on the effectiveness of injury prevention programs, as high-compliance athletes demonstrated an 88% reduction in ACL injury rate when compared to low-compliance athletes.⁸⁷ Implementation of pre-habilitation measures are not just isolated to ACL injuries. Many of the same risk factors for ACL failure are shared by patellofemoral pain syndrome (PFPS) patients, which is the most common disorder of the knee (Figure 3).^88,89,90 Correspondingly, pre-habilitation has demonstrated similar effectiveness in PFPS reduction.⁹¹

Figure 3:

Multiple injury types can share similar mechanism and, therefore, similar biomechanical predictors. This data stratification depicts that knee abduction moment was both a predictor for ACL injury and PFPS. The ACL injury prediction was predicated by a greater abduction moment, so it is also possible that the magnitude of a biomechanical factor can be related to the magnitude of injury that it predicts. Image reproduced with permission from Myer, et al. 2015. Am J Sports Med, 49(2):118–122.

Pre-habilitation protocols target modifiable risk factors associated with ACL injury risk.⁹² Specifically these training programs look to correct neuromuscular deficits through targeted interventions. Neuromuscular training varies between protocols, but programs incorporate a combination of plyometric and agility training, balance and stability training, resistance training, speed training, as well as strengthening of the musculature around the core, hips, pelvis, knees, and ankles.^5,92 However, when parsed out, none of these specific components were individually associated with improved ACL injury outcome measures.⁴ For example strength training alone will improve strength, but has not been shown to alter lower extremity movement patterns.⁹³ This data indicates that the injury reduction observed from pre-habilitation culminates from the correction of a multitude of biomechanical factors that contribute to ACL loading. Apart from biomechanical benefits such as reduced hip internal rotation, knee valgus moment, and knee extensor moments as well as increased joint flexion, hip abduction, and hamstrings muscle activation, these neuromuscular training protocols can also lead to enhanced performances that are directly tangible on the field of play.⁴ Direct side-effects from pre-habilitation training have included increased strength, jumping performance, speed, and reaction time.^5,24,94,95 Though biomechanical improvements have a direct influence on injury prevention, these concepts may appear abstract to an athletic coach or trainer. Accordingly, the enhanced performance attributes ascribed to targeted neuromuscular training should intrigue athletic coaches and trainers in addition to the efficacious health benefits for their athletes.

Muscle activation, specifically hip muscle activation, works to maintain the body’s center of mass in a planar base. Poor neuromuscular control leads to the trunk and center of mass deviating from this planar base; and thus, extends the moment lever arm acting on the knee and other lower extremity joints. This leads to greater frontal plane excursion of the knee⁵⁷ which, in turn, produces large magnitudes of torque at the joint. Pre-habilitation training incorporates a significant amount of plyometric jump and perturbation tasks that will strengthen an athlete, but also correct the neuromuscular activation pathways that lead to out-of-plane motion at the knee.⁵⁷ The end objective is enhanced joint control as forces are actively dissipated across the knee via musculature and less likely to be passively absorbed by the ligaments.

A wide variety of exercises can be employed to address each of the modifiable ACL injury risk factors that arise from poor neuromuscular control.⁵⁷ Muscle activation deficits have been linked to poor control of the lower extremities^96,97 and can be modified through single-leg hoping and landing exercises that improve hip strength and necessitate trunk and lower extremity control to minimize frontal plane moments.^57,98 These single leg anterior and lateral plyometric jumping tasks can similarly be implemented to gain greater control of postural instability that is associated with ACL injury risk.^57,62,99 Excessive trunk motion can increase ACL injury risk;^100,101 thus, lateral, prone, and kneeling trunk stability exercises may lead to increased proprioception and decreased frontal plane knee moments.⁵⁷ Hamstrings serve as an agonist to ACL strain during dynamic activities;⁵⁶ therefore, hamstring strength training through weightbearing activities such as single leg dead lift can improve quadriceps-to-hamstrings strength rations and contribute to reduced injury risk.^57,102 ACL rupture is related to limited and asymmetrical knee flexion;¹⁰³ thus, lunges, lunge jumps, and tuck jump tasks that encourage symmetrical, deep knee flexion while maintaining frontal plane control of the knee can contribute to injury prevention.^57,104 Each the neuromuscular modifying tasks presented are described in detail by Di Stasi, et al. What makes these presented exercises particularly applicable is that each one is already a common athletic training task. As such, coaches and trainers at all levels should be familiar with these activities and easily able to execute them in conjunction in both individual and team settings with minimal equipment or educational investment.

TREATMENT

Knee injuries account for between 10–40% of adolescent sports injuries dependent on definition and criteria.^105,106 Additionally, knee injuries are the most common cause of surgery in high school sports, accounting for 60% of sports injury operations.¹⁰⁷ ACL injuries specifically account for approximately 20.5–25.4% of these sports related knee injuries and result in the greatest time lost from sport participation in young female athletes.^43,106,108 With such high prevalence, it is unlikely that any amount of pre-habilitation will entirely eradicate non-contact ACL injuries. However, prevention of just one ACL rupture saves over $17,000 in medical costs for reconstructive surgery and rehabilitation.¹⁰⁹ Further, if the annual estimated incidence of 250,000 ACL ruptures and 125,000 ACL reconstructions in the United States can be reduced by greater than 50% via widespread application of pre-habilitation techniques, the overall saving to the American medical system would be more than $1.1 billion annually in immediate care costs alone.^109–112 The cost footprint on the medical system is then further reduced as the long-term pain treatments and total joint replacements necessitated by early onset arthritis, which is expected in 50–90% of patients within 20 years of injury,^25–27 may be avoided. Consequently, implementation of pre-habilitation in young, high-injury-risk athletes has the potential to alleviate significant financial burden from the national medical system.

As indicated, even widespread implementation of pre-habilitation training will not eradicate ACL injuries. Those athletes who suffer an initial ACL tear represent the most likely cohort to suffer additional failures as the incidence of secondary ACL failure is as high as 30%.^57,113 This secondary failure rate was highest amongst young athletes (<25 years old) who returned to sport following initial reconstruction.¹¹⁴ Correspondingly, injured athletes represent the cohort that stands to benefit the most from preventive biomechanics as they are 5 to 40 times more likely to suffer future injury than healthy counterparts,^113–115 and secondary ACL injuries can be devastating to athletic careers. Following primary ACL injury, 63% of athletes successfully return to sport at their previous level of competition,^116,117 while only 23% return after a secondary tear.¹¹⁸ Further, following rehabilitation from ACLR, under 50% of young athletes report being able to perform at their pre-injury level.¹¹⁹ In healthy controls, screening measures serve to separate the high-injury-risk athletes from low-injury-risk counterparts to determine need for pre-habilitation. Injury occurrence itself is the gold-standard screening tool as this cohort of athletes exhibits, by far, the highest incidence rate of future injury.^113–115

As in healthy athletes, there are both modifiable and non-modifiable risk factors that contribute to secondary ACL injury. The modifiable factors much resemble those indicated in primary failure.⁵⁷ In particular, it was demonstrated that neuromuscular impairments in both the reconstructed and contralateral limb predict secondary ACL injury with 92% sensitivity and 88% specificity.⁶² Once again, neuromuscular deficits lead to dynamic knee valgus that precipitates increased loading on the ACL or ACLR graft. The effectiveness of neuromuscular training on the reduction of secondary ACL injuries has not been directly investigated.⁵⁷ However, as previously described, these practices are well documented in the correction of primary injury biomechanical deficits, ^4–8 so it is likely their implementation during ACLR rehabilitation would similarly impact the prevention of secondary tears through a reduction in incidence. A specific protocol for late-phase ACLR rehabilitation was previously proposed, using objective patient-specific performance criteria as qualifiers for enrollment.⁵⁷ Patients who demonstrated full and pain free ROM relative to the contralateral limb, minimal joint effusion, >70% strength symmetry, and ability to hop in place without pain or apprehension were to being a targeted neuromuscular training with exercises designed to “(1) activate muscles hypothesized to be deficient at the time of sport, (2) utilize surfaces and movements that elicit muscle co-activation capable of modifying mechanics theorized to be related to injury risk, and (3) elicit movement that may replicate conditions experienced during sport.”⁵⁷ Specifically, the program incorporates the same single leg anterior and lateral hop, lunge, lunge jump, tuck jump, lateral jump, lateral trunk, trunk stability, posterior chain, and single-leg dead lift exercises utilized in pre-habilitaton, to improve ACLR rehabilitation outcomes. These activities are applied bilaterally to target typically-observed post-ACLR deficits and side-to-side asymmetries in the gluteals, trunk, quadriceps, and hamstrings musculature (Figure 4).⁵⁷ Though the proposed protocol has yet to be validated as efficacious in the prevention of secondary, each incorporated exercise has been shown to alleviate factors related to primary injury.^57,98,99,120 As both primary and secondary injuries share biomechanical risk factors it is expected that dynamic exercises designed around the reduction of ligament dominance, quadriceps dominance, trunk dominance, and leg dominance will also have efficacious impact on secondary prevention.

Figure 4:

Progressive, bilateral training involving plyometric, perturbation, and stability that is used to incrementally adapt an athlete to positive neuromuscular patterns that are likely to reduce risk of biomechanically induced injury. The progressive model starts the athlete at a basic task level and incrementally increases difficulty as the subject’s mechanics adapt. Image reproduced with permission from Di Stasi, et al. 2013. J Ortho Sports Phys Ther, 43(11):777-A11.

CONCLUSION

ACL injury and rehabilitation are the number one cause for time loss due to injury in NCAA athletics. These injuries exact an exorbitant cost on both patients and the medical community at large. This cost is not limited to just monetary expense as ACL ruptures frequently lead to decreased quality of life for the patient. The efficacious impact of pre-habilitation techniques on the reduction of ACL injury incidence is well documented in the literature. This reduction of incidence is possible through the combination of screening, training, and treatment measures into an overarching preventive biomechanics program. Nationwide implementation of preventive biomechanics techniques at the level of young, adolescent, and collegiate could be executed at a minimum of cost with the potential for a high-reward return on investment. Coaches, athletic trainers, and physical therapists already possess the knowledge and much of the equipment necessary to appropriately execute effective preventive biomechanics. As coaches and trainers begin to observe that preventive biomechanics methods not only preserve their athletes’ health, but also improve their performance measures, a natural progression of general acceptance within the sporting community should follow. Further, preventive biomechanics are not limited to just the ACL rupture condition as these same principles have been applied to reduce PFPS. Accordingly, it is expected that preventive biomechanics could be uniquely adapted to sport-specific needs in order to more effectively reduce incidence of additional traumatic and overuse injuries such as ankle sprains, epicondylitis, ulnar collateral ligament tears, or lower back pain. Widespread implementation of preventive biomechanics in today’s athletic community is feasible and could significantly improve health outcomes as well as reduce medical expenditures.