Biomechanical Function and Size of the Anteromedial and Posterolateral Bundles of the ACL Change Differently with Skeletal Growth in the Pig Model

Abstract

Background ACL injuries are becoming increasingly common in children and adolescents, but little is known regarding age-specific ACL function in these patients. To improve our understanding of changes in musculoskeletal tissues during growth and given the limited availability of pediatric human cadaveric specimens, tissue structure and function can be assessed in large animal models, such as the pig. Questions/purposes Using cadaveric porcine specimens ranging throughout skeletal growth, we aimed to assess age-dependent changes in (1) joint kinematics under applied AP loads and varus-valgus moments, (2) biomechanical function of the ACL under the same loads, (3) the relative biomechanical function of the anteromedial and posterolateral bundles of the ACL; and (4) size and orientation of the anteromedial and posterolateral bundles. Methods Stifle joints (analogous to the human knee) were collected from female Yorkshire crossbreed pigs at five ages ranging from early youth to late adolescence (1.5, 3, 4.5, 6, and 18 months; n = 6 pigs per age group, 30 total), and MRIs were performed. A robotic testing system was used to determine joint kinematics (AP tibial translation and varus-valgus rotation) and in situ forces in the ACL and its bundles in response to applied anterior tibial loads and varus-valgus moments. To see if morphological changes to the ACL compared with biomechanical changes, ACL and bundle cross-sectional area, length, and orientation were calculated from MR images. Results Joint kinematics decreased with increasing age. Normalized AP tibial translation decreased by 44% from 1.5 months (0.34 ± 0.08) to 18 months (0.19 ± 0.02) at 60° of flexion (p < 0.001) and varus-valgus rotation decreased from 25° ± 2° at 1.5 months to 6° ± 2° at 18 months (p < 0.001). The ACL provided the majority of the resistance to anterior tibial loading at all age groups (75% to 111% of the applied anterior force; p = 0.630 between ages). Anteromedial and posterolateral bundle function in response to anterior loading and varus torque were similar in pigs of young ages. During adolescence (4.5 to 18 months), the in situ force carried by the anteromedial bundle increased relative to that carried by the posterolateral bundle, shifting from 59% ± 22% at 4.5 months to 92% ± 12% at 18 months (data for 60° of flexion, p < 0.001 between 4.5 and 18 months). The cross-sectional area of the anteromedial bundle increased by 30 mm2 throughout growth from 1.5 months (5 ± 2 mm2) through 18 months (35 ± 8 mm2; p < 0.001 between 1.5 and 18 months), while the cross-sectional area of the posterolateral bundle increased by 12 mm2 from 1.5 months (7 ± 2 mm2) to 4.5 months (19 ± 5 mm2; p = 0.004 between 1.5 and 4.5 months), with no further growth (17 ± 7 mm2 at 18 months; p = 0.999 between 4.5 and 18 months). However, changes in length and orientation were similar between the bundles. Conclusion We showed that the stifle joint (knee equivalent) in the pig has greater translational and rotational laxity in early youth (1.5 to 3 months) compared with adolescence (4.5 to 18 months), that the ACL functions as a primary stabilizer throughout growth, and that the relative biomechanical function and size of the anteromedial and posterolateral bundles change differently with growth. Clinical Relevance Given the large effects observed here, the age- and bundle-specific function, size, and orientation of the ACL may need to be considered regarding surgical timing, graft selection, and graft placement. In addition, the findings of this study will be used to motivate pre-clinical studies on the impact of partial and complete ACL injuries during skeletal growth.

Anterior cruciate ligament remnant‐preserving and re‐tensioning reconstruction: a biomechanical comparison study of three different re‐tensioning methods in a porcine model

“…Nonetheless, the key anatomic features and functional characteristics of the porcine model are similar to those of human knees [34]. Moreover, porcine models are commonly used in biomechanical studies of the meniscus [27,35]. Third, we measured the mechanical strength with tensile loading at 30°flexion [7]; however, the actual human knee moves dynamically through a wide range of angles, rotation, and pivoting, and the axial tensile load used in this study might not reflect the behavior of the knee during real-life functional activities.…”

Sex-Specific Function and Morphology of the Anterior Cruciate Ligament During Skeletal Growth in a Porcine Model

“…28; 29 The porcine model is widely used for musculoskeletal applications 30 and is a good model to study function of the mature ACL. 31; 32 The female porcine ACL increases in length, CSA, and angular orientation throughout skeletal growth 22; 23 , similar to humans. 5–8; 33 These studies also revealed age-specific differences in growth and function of the AM and PL bundles.…”

“…Data from female specimens has been previously reported. [22][23][24] was not certified by peer review) is the author/funder. All rights reserved.…”

“…Our group and others have used the porcine model to study the skeletally immature ACL [22][23][24][25][26][27] and sex-specific ACL behavior. 28; 29 The porcine model is widely used for musculoskeletal applications 30 and is a good model to study function of the mature ACL.…”

“…5–8; 33 These studies also revealed age-specific differences in growth and function of the AM and PL bundles. 23 However, these findings leave open the question of whether functional maturation of the ACL and its bundles is sex-specific.…”