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

Abstract

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.

Figure 1

Experimental protocol overview. (A) Paired deep digital flexor tendons were collected from pigs at 0.5, 1.5, 5, and 9 months of age. 3D scanning and tensile testing were performed to assess the size and tensile properties of the flexor tendons. (B) A 6-degree-of-freedom robotic testing system was utilized to perform biomechanical testing. The 4.5-month-old porcine knee was tested in the intact and ACL-transected states, followed by ACL reconstruction (ACLR) using 1.5-month-old and 5-month-old tendon grafts. Joint stability and ACL/graft forces were calculated and compared across different joint states. Created in BioRender. Fisher, M. (2025) https://BioRender.com/f7bu8fe

Figure 2

Size and tensile properties of flexor tendon increase during skeletal growth. (A) Cross-sectional area (CSA) and (B) stiffness of the 5-month-old tendon and trimmed 9-month-old tendon were greater. (C) 5-month-old tendon showed significantly higher load at failure compared to other age groups. Data points presented with mean and 95% CI. Statistical significance (P < .05) between states indicated (*).

Figure 3

Flexor tendon stiffness correlated with cross-sectional area (CSA) throughout skeletal growth but not within individual age groups. (A) Linear regression for flexor tendon stiffness vs. CSA, independent of age. (B) Linear regressions for flexor tendon stiffness vs. CSA within 0.5-month-old, 1.5-month-old, and 5-month-old groups. Statistical results are shown in the graph.

Figure 4

ACL reconstruction (ACLR) with 5-month-old (5mo) graft showed better joint stability in early adolescent porcine joints compared to using 1.5-month-old (1.5mo) graft. Joint stability was measured under different loading conditions. (A) Anterior-posterior tibial translation (APTT) under anterior load. (B) Delta APTT relative to intact joint. (C) Anterior tibial translation (ATT) under compressive load. (D) ATT relative to intact joint. (E) Varus-valgus rotation (VV) under VV torque. (F) Delta VV rotation relative to intact joint. Data points presented with mean and 95% CI. Statistical significance (P < .05) between states indicated (*).

Figure 5

The 5-month-old graft carried greater anterior force compared to the 1.5-month-old graft. (A) Under anterior loading, the 5-month-old graft carried significantly more anterior force than the 1.5-month-old graft, though neither matched the native ACL. (B) Under compression, the 1.5-month-old graft carried less anterior force compared to the native ACL. Under varus (C) and valgus (D) rotational loading, anterior force carried by the 1.5- and 5-month-old grafts was similar. ⨂ represents the ACL anterior force direction pointing into boards. Data points presented with mean and 95% CI. Statistical significance (P < .05) between states indicated (*).

Figure 6

5-month-old graft showed greater in situ stiffness compared to the 1.5-month-old raft. (A) Anterior-posterior (A-P) force-displacement curve was recorded to calculate submaximal properties. In situ joint slack (B) and stiffness (C) following ACL reconstruction (ACLR) with either graft were similar but did not return to intact joint levels. (D) The 5-month-old graft exhibited significantly greater in situ stiffness than the 1.5-month-old graft. Data points presented with mean and 95% CI. Statistical significance (P < .05) between states indicated (*).