Age-Related Increases in Graft Tendon Size and Stiffness During Skeletal Growth Enhance Anterior Cruciate Ligament Graft Function and Joint Stability in an Early Adolescent Porcine Model

Anterior cruciate ligament (ACL) reconstruction in pediatric patients has a higher graft failure rate compared to adults. Restoring joint stability and reducing graft failure is essential. However, how graft biomechanical properties change with age and affect reconstruction outcomes remains unclear. This study investigated the biomechanical development of porcine flexor tendons across skeletal growth and evaluated how graft size and stiffness influence knee biomechanics in a pediatric porcine model. Flexor tendons (n = 57) were harvested from pigs at 0.5, 1.5, 5, and 9 months of age to measure cross-sectional area (CSA), stiffness, and failure load. ACLs in nine early adolescent porcine knees were reconstructed using both 1.5- and 5-month-old (1.5 mo and 5 mo) grafts and tested under anterior-posterior, compressive, and varus-valgus (VV) loading at 40 deg of flexion using a robotic testing system. ACL and graft forces were calculated, and in situ properties were derived from force-displacement curves. Tendon CSA, stiffness, and failure load increased with age, and stiffness associated with CSA. The CSA of 5 mo tendons was 57% greater than that of 1.5 mo tendons, but stiffness increased only 20%. ACL reconstruction with 5 mo grafts resulted in 29% less anterior-posterior tibial translation and 44% higher graft force compared to 1.5 mo grafts. In situ stiffness of 5 mo grafts was 51% higher than 1.5 mo grafts. These findings highlight the differences between tendon size and biomechanical development, which together contribute to the improvements in joint function following ACL reconstruction.