An Overview of the Diagnosis and Treatment of the Torn Anterior Cruciate Ligament in the Skeletally Immature Athlete

Introduction

Intrasubstance tears of the anterior cruciate ligament (ACL) were once considered a rare injury in skeletally immature athletes; however, they are now observed with increasing frequency. A dramatic rise in youth competitive athletic activity, early sport specialization, year-round training and competition, and lack of free play, along with increased awareness of ACL injuries in children, have contributed to a commensurate increase in the frequency of ACL tears in the skeletally immature. In fact, a recent epidemiological study in New York state revealed that the rate of ACL reconstruction in children under age 20 had increased nearly threefold over a 20-year period from 1990 to 2009, and it indicated that adolescents and teenagers represent the largest per capita demographic of ACL reconstructions.

In light of the increasing frequency and awareness of ACL injuries in children, treatment strategies have evolved, catering to the unique anatomy of the skeletally immature patient. Similarly, there are unique diagnostic modalities that should be considered in evaluating a child with an ACL tear. Current literature now supports the trend toward early operative treatment to restore knee stability and prevent progressive meniscal and/or chondral damage, but the optimal surgical technique remains controversial.

This chapter will review anatomy unique to the skeletally immature knee, as well as evaluation, diagnostic imaging, treatment, outcomes, and rehabilitation of children with ACL injuries. Treatment strategies will be outlined and discussed; however, detailed surgical techniques and their complications will be covered elsewhere in this text.

Anatomy

The embryology and anatomy of the knee is covered elsewhere in this text; however, there are unique features of the skeletally immature knee that are of paramount importance to understand when treating children with ACL injuries. These include the physis, overall limb length and alignment, and the development of the intercondylar notch.

The tibial and femoral physes are the greatest contributors to overall lower limb longitudinal growth. The distal femoral physis contributes 70% of the femoral length and 37% of the overall limb length over the course of skeletal development at an average rate of 10 mm/year. The distance of the femoral physis from the femoral origin of the ACL remains unchanged from gestational age through skeletal maturity and averages approximately 3 mm. The proximal tibial physis contributes approximately 55% of the tibial length and 25% of the overall limb length over the course of skeletal development at a rate of 6 mm/year. Although skeletal maturity occurs around age 14 in girls and age 16 in boys, negligible (<1 cm in each limb segment) growth remains around the knee after age 12 in girls and 14 in boys. Until these ages, careful consideration must be given to physeal-respecting ACL reconstructions. While the rate of growth disturbance after ACL reconstruction in children is not precisely known, a survey of 108 surgeons from the Herodicus Society and the ACL study group revealed 15 cases of postoperative deformity due to physeal injury. They included distal femoral valgus deformity, tibial recurvatum, genu valgum, and significant leg-length discrepancy.

In addition to limb length, alignment of the lower extremity is of paramount importance. Lower limb alignment follows a predictable pattern, with mechanical axis changes happening during normal development up to age 7. After age 7, lower limb alignment changes very little, and children will unlikely outgrow any residual varus or valgus deformity. This is an important consideration during the initial patient evaluation and will be explained in the section that follows.

Finally, femoral intercondylar notch development is important to understand in children with ACL injuries. When compared with age- and gender-matched controls, children who sustain ACL ruptures have significantly smaller intercondylar notch volumes. During development, the intercondylar notch width increases steadily through skeletal growth until age 11, at which time there are no significant increases in anterior notch width. This understanding may help the surgeon when considering appropriate graft sizing in developing children.

Risk Factors

Several risk factors for ACL injury have been identified and are typically categorized as intrinsic or extrinsic. Furthermore, some intrinsic risk factors are modifiable while others are not. Examples of nonmodifiable anatomic risk factors include sex, increased pelvic tilt, decreased femoral notch width and volume, and decreased ACL volume. More recent evidence has linked increased posterior tibial slope (particularly in the lateral compartment) with increased risk of ACL injury. Furthermore, increased quadriceps angle, increased femoral anteversion, and greater generalized ligamentous laxity have also been associated with an increased ACL injury risk. While a greater total number of ACL injuries occur in males, females have a 2.1–3.4 times increased risk of ACL injury for sex-comparable sports, with some studies showing an overall risk of ACL injury per athletic exposure upwards of 8–9 times greater than in males. Several studies have attempted to find an association between receptors for sex hormones located within the ACL, including estrogen, testosterone, and relaxin, and ligament injury. These sex hormones may alter the mechanical properties of the ACL, but the precise mechanism has not been fully elucidated.

Modifiable risk factors that predispose children to ACL injuries include strength, flexibility, and knee biomechanics. Increased dynamic valgus in adolescent females have been strongly linked to ACL injury. Decreased dynamic muscular control leads to prolonged reliance on ligamentous stability creating more valgus motion, and quadriceps dominance during cutting activities is also associated with ACL injury. These risk factors are modifiable through neuromuscular training protocols that have been shown to be cost effective when routinely used in pediatric and adolescent athletes, and may decrease incidence of ACL rupture by 62% in meta-analysis.

Extrinsic risk factors are all modifiable because they are risk factors to which children are exposed. These include participation in high-risk cutting and pivoting sports, footwear (increased cleat density transferring greater forces across the knee), field and weather conditions, and playing surfaces.