Pediatric ACL Injury Causes

Several biological and structural factors make the pediatric and adolescent knee especially vulnerable:

• Growing Ligament: The ACL itself continues to grow in width and length until the terminal stages of skeletal maturity. Younger, smaller ligaments have different biomechanical properties and failure thresholds than adult ligaments.
• Bony Avulsion Pattern: In skeletally immature patients, the attachment of the ligament to bone is a weak point. Rather than a midsubstance tear, the ligament may be completely separated from the bone with a fragment of bone still attached — known as a bony avulsion or tibial spine fracture. This injury pattern is more common in younger, pre-pubertal children.
• Growth Plate Vulnerability: The physis, or growth plate, creates a zone of relative structural vulnerability at the knee. Under certain loading conditions, physeal fractures can occur simultaneously with or instead of ACL tears.
• Neuromuscular Immaturity: A developing neuromuscular system results in different landing and pivoting kinetics compared to adults, including greater dynamic valgus — the inward collapse of the knee during movement — particularly in pre-pubertal and early pubertal athletes.
• Early Specialization & Overuse: Children are increasingly specializing in a single sport before puberty, training year-round, and competing at higher intensities. All-year-round pitch availability, improvements in footwear technology, and a higher average pace of play contribute to cumulative stress on the immature knee.

Female athletes between ages 15 and 20 account for the largest number of ACL injuries. Among high school and college athletes, females sustain ACL tears at two to six times the rate of males in comparable sports. A systematic review and meta-analysis found female athletes are approximately 1.5 times as likely as male athletes to suffer an ACL injury across all adolescent sports, with nearly a 10% cumulative risk across a high school career for multisport female athletes.

Several anatomical and physiological factors account for this disparity:

• Joint Laxity: Increased ligamentous laxity, including hyperlaxity, which is more prevalent in females and may be influenced by hormonal factors including estrogen and relaxin
• Q-Angle: A higher Q-angle (the angle between the quadriceps force vector and the patellar tendon), which increases valgus stress at the knee
• Intercondylar Notch Geometry: Narrower intercondylar notch dimensions in some female patients, which may reduce space for the ACL and increase impingement risk
• Neuromuscular Patterns: Neuromuscular patterns with greater quadriceps dominance relative to hamstring activation compared to males, resulting in greater anterior tibial shear forces during landing
• Hormonal Factors: Hormonal fluctuations across the menstrual cycle have been shown to alter ACL mechanical properties, with increased laxity at certain phases potentially elevating injury risk

Certain structural features are associated with elevated ACL injury risk in the pediatric population and can be identified on preoperative MRI:

• Increased Lateral Tibial Slope: An increased lateral tibial slope — the posterior downward angulation of the tibial plateau — is a well-documented MRI-based risk factor for pediatric ACL tears, identified in an MRI case-control study of 152 pediatric patients
• Narrow Intercondylar Notch: A narrow intercondylar notch reduces the available space for the ACL and may predispose to impingement and injury
• Patella Alta: Patella alta (high-riding patella) has also been associated with increased ACL injury risk in pediatric patients