Background: There is limited knowledge of morphologic changes of the anterior cruciate ligament (ACL) during childhood. The purpose of the study is to describe normal ACL growth in pediatric patients. Methods: We performed retrospective review of patients under 19 years of age with at least 2 serial magnetic resonance imaging (MRI) examinations of the same knee without ACL tear. The parameters measured included length of the ACL, width of the ACL in sagittal and coronal planes, physeal status, ACL-tibial inclination angle, Notch Width Index, ACL tibial attachment width and midpoint, and anterior to posterior length and height of the tibial epiphysis. Pearson correlation coefficients were calculated to determine the strength of correlation of each parameter relative to age. Growth curves were calculated for individual parameters, producing a predictive model for growth of the ACL over time. Results: One hundred forty-seven patients (365 MRIs) were included. The average age at initial MRI was 12.2 years (range: 1.2 to 18.4 y). The Pearson correlation coefficient for each parameter was statistically significant relative to age. Younger patients had more oblique, anteriorly attached ACLs. Three distinct phases of growth were observed: patients age 1.5 to 5.75 years average 2.25 mm of growth per year; patients aged 6 to 11.5 average 1.46 mm of growth per year; growth slows at age 11.75, stopping by 18.5 years. The growth model for ACL sagittal width increased 0.45 mm/y from age 1.5 to 14.5 after which it plateaus, whereas coronal width increased 0.22 mm/y from age 1.5 to 18.75. Conclusions: In the pediatric patient, the ACL grows in length and width in a predictable manner until age 18. This model aids clinicians in predicting normal ACL parameters for anatomic reconstruction procedures in the skeletally immature patient. Level of Evidence: Level IV.

Background The incidence of anterior cruciate ligament (ACL) injury is steadily increasing in skeletally immature patients [1-7]. As a result, surgical approaches to reconstruction of the ACL without compromising the physis have been developed [6, 8, 9]. It is essential to understand how the native ACL changes with patient growth and age, in order to better understand how grafts might serve patients as they continue to grow. The purpose of this study is to characterize the morphological change over time of different ACL parameters in the pediatric and adolescent population based on serial magnetic resonance imaging (MRI). Methods After receiving Institutional Review Board approval, the patient cohort was identified retrospectively from a database of all patients who had a routine magnetic resonance imaging (MRI) of the knee between July 2005 and February 2017. Patients aged 0 to 18 years at the time of their initial MRI, who had undergone at least two serial MRI examinations of the same knee at least 4 months apart for reasons other than ACL tear were included. Patients were excluded if they had a fracture of the distal femur or proximal tibia, congenital condition that affects the knee, deformity of the femur or tibia, or if they have a pathologic process (e.g. tumor) that disrupts the anatomy of the distal femur or proximal tibia. The parameters measured included physeal status, length of the ACL, diameter of the ACL in the sagittal and coronal plane, ACL-tibial inclination angle, notch width index, the midpoint of the ACL tibial attachment, anterior to posterior dimension of the tibia, and tibial epiphyseal height. A Pearson correlation coefficient was calculated to determine the strength of correlation of each parameter relative to age. Growth curves were calculated for individual parameters, which produced a predictive model for the rate of growth of the ACL at different ages. Results 162 knees in 147 patients (365 MRI studies) were identified to have had serial MRI of their knee. 50.3% of patients were female. Ages at time of MRI ranged from 1.3 to 21.7 years (average 13.5 years, SD 3.60). The Pearson correlation coefficient for each parameter showed statistically significant relationship relative to age. The ACL grows in length and diameter with age. Younger patients had more oblique, anteriorly attached ACLs compared to older patients. The growth model for ACL length shows three distinct phases of growth: patients age 1.5 to 5.75 years average 2.25 mm of growth per year; patients aged 6 to 11.5 average 1.46 mm of growth per year; growth begins to plateau at age 11.75, stopping by age 18.5 years. The growth model for ACL sagittal diameter shows an average of 0.45 mm of growth per year between 1.5 and 14.5 years old, after which growth slows until it stops at age 18.75. The ACL coronal diameter model shows an average of 0.22 mm of growth per year between 1.5 and 18.75 years old, with growth completion by age 18.75. ANCOVA was used to assess the difference in growth rates between men and women. Growth rates for ACL length, ACL diameter in the coronal plane, transcondylar width, posterior border, center, and width of ACL-tibial attachment, and anterior-to-posterior dimension of the tibia are significantly different in men and women. Namely, for all significant differences, men had faster growth rates than women. Conclusions/Significance In the skeletally immature patient, the ACL grows in length and diameter in a predictable fashion until age 18. This model aids clinicians in predicting normal ACL parameters for reconstruction procedures in the skeletally immature patient. 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