CORR Insights®: Biomechanical Function and Size of the Anteromedial and Posterolateral Bundles of the ACL Change Differently with Skeletal Growth in the Pig Model

Where Are We Now?

The incidence of ACL injuries is rising among athletes who are skeletally immature [1, 5, 17]. Following ACL injury, pediatric patients are at increased risk of knee degeneration, instability, further meniscal/chondral damage, and the inability to resume sporting activity [5, 6, 12, 13]. While the increasing incidence of ACL injury in pediatric patients has been established, it is difficult to study pediatric patients for a number of reasons, including the relatively small number of available participants, ethical concerns regarding participation of children in trials, differences in the allowable level of risk to which children may be exposed in the context of clinical trials, and challenges associated with obtaining parental consent for the child to participate in a research study [9, 10]. Furthermore, because of the extreme rarity of pediatric cadaveric specimens, there is a paucity of data available describing the intrinsic biomechanics of the ACL during development [14, 16].

In the current study, Cone and colleagues [2] present an in vitro biomechanical study that aimed to define the anatomic and functional characteristics of the anteromedial and posterolateral ACL bundles throughout growth. The authors found that joint laxity decreased as the patients aged, and that as early adolescence approached, the cross-sectional area and functional contribution of the anteromedial bundle increased, while in the posterolateral bundle the cross-sectional area plateaued, and the functional contribution decreased [2]. The biomechanical foundation provided in this study suggests future research should be directed toward age-specific approaches to ACL reconstruction. In addition, these data will be useful in future studies, particularly those aimed at developing methods for studying pediatric joint biomechanics.

Where Do We Need To Go?

Our ultimate goal should be to develop a complete understanding of the natural history, biomechanics, and vulnerabilities of the ACL throughout the human life cycle. We should also aim to improve our techniques for both prevention of ACL injury and ACL reconstruction to avoid further knee degeneration and the need for revision operations, respectively. Both are commonly encountered after ACL reconstruction in the pediatric population [5, 6]. In addition, we have a social responsibility to support injury awareness and prevention through policy development, sports injury prevention programs, and improvements in youth sports safety [1].

While there have been volumes of research on the intrinsic biomechanical properties of the adult ACL [8, 11, 18], few studies have attempted to define similar characteristics in skeletally immature patients [3, 19]. And although the authors of the current study determined that the properties of the anterolateral and posteromedial bundles of the ACL change during development, their assessment was limited by the use of an in vitro model system. We should aim for direct, serial assessment(s) of the in vivo biomechanical properties of the human ACL during development. Furthermore, as ACL reconstruction becomes more common in the pediatric population, it is crucial that we develop a thorough understanding of how grafts integrate biomechanically into the growing knee joint [18].

How Do We Get There?

The functional behavior of any tissue depends on its structural characteristics—size, orientation, material properties—and the forces applied to that tissue. In order to understand the natural history and biomechanics of the ACL throughout the human life cycle, future studies should aim to incorporate prospective serial in vivo measurements using advanced imaging techniques or other non-invasive methods. While these studies should document the structural characteristics of the human ACL throughout growth and development, execution of such a study presents technical, administrative, and financial challenges. For example, serial imaging of pediatric patients should avoid ionizing radiation. However, MRI is limited by long imaging times and the potential need for sedation in young patients. Furthermore, from the financial and administrative perspectives, it is difficult and expensive to maintain enrollment in pediatric clinical trials over the course of the several years required to conduct such a trial.

Additional in vitro trials or creative case-cohort studies will be required to identify the unique vulnerabilities of the ACL during growth and development. In vitro studies that apply calibrated loads to the immature ACL are technically feasible, but present their own challenges with respect to selection of appropriate model systems, like the one used in the current study [2], and generalization of those results. Case-cohort studies may be used to provide insight into the mechanisms of ACL injury in a skeletally immature population, however, these studies will be limited in their ability to draw conclusions regarding the structural properties of the ACL. In reality, a combination of in vivo trials with in vitro and case-cohort studies will likely be required to fully characterize the structural and functional properties of the human ACL throughout the human life cycle.

Once we can identify growth stages and quantify major structural and functional changes that occur in the ACL during growth, data on the age-specific functions of the ACL (and the individual ACL bundles) will be useful in developing age-specific procedures for the prevention and treatment of childhood ACL injuries. For example, the stability of the knee joint during activity depends on a combination of dynamic forces applied by the knee flexors and extensors, their associated tendons, and static/passive stability provided by the intrinsic knee ligaments. The balance of these static and dynamic forces during loading determines an individual’s ability to withstand ACL injury. Using MRI and clinical biomechanical techniques, Davidson and colleagues [4] performed a cross-sectional study of skeletally immature women athletes divided into three distinct maturation groups. They determined that the ACL may be particularly vulnerable to injury during middle maturation when the cross-sectional area of the ACL decreases, and the balance of loading is shifted to favor knee extensors over flexors [4]. This study and others designed to investigate differences in knee loading, lower limb neuromuscular control strategies [7, 15], and the effects of targeted interventions (such as improving hamstring strength/activation) during maturation represent initial steps toward development of age-specific procedures for the prevention of childhood ACL injuries.

For treatment of ACL injuries once they occur, Cone and colleagues [2] suggested that future research may support matching age-specific function via graft selection and placement in children undergoing ACL reconstruction. While it would be ideal to reach the point of data-driven age-specific graft selection and placement criteria for children undergoing ACL reconstruction, substantial work remains necessary before such steps could or should be taken. Again, the initial steps toward achieving this goal require in-depth characterization of the structural properties of the ACL throughout growth and development, as described above. Investigations into the outcomes and specific limitations of current graft selection and placement in the skeletally immature population will also advance us toward the ultimate goal of achieving the perfect graft in the perfect location/orientation for each individual based on age and functional characteristics.

Knowledge of the natural history, biomechanics, and vulnerabilities of the human ACL will form a foundation upon which to base new preventative strategies and clinical interventions for ACL injury. Ideally, these data should be used to support policy development, sports injury prevention programs, and improvements in youth sports safety [1] in an attempt to slow the trend of the increasing incidence of ACL injuries in the pediatric population.