Physiological Load-Response in Synthetic Ligaments through Active Filament-Mixing Fabrication

Additive manufacturing has enhanced medical trainers and their functionality, but synthetic tissues that are both anatomically accurate and capable of replicating musculoskeletal load responses remain largely unexplored. The purpose of this study is to demonstrate fabrication of synthetic tissue using active filament-mixing fabrication to enable local composition control with thermoplastic compositions tuned to match the modulus of elasticity of the respective tissues and functionally graded transitions between ligament and bone.

Synthetic femoral anterior cruciate ligament (ACL) tibial complexes were fabricated and tested at 30° flexion along the tibial and anatomical load orientations matching the test procedure performed on cadaveric femoral ACL tibial complexes by Woo et al. in 1991. Load parameters were facilitated by section grips integrated into the design of test specimens without requiring a multi-axis jig.

Anatomical accuracy was achieved by modeling tissues from magnetic resonance imaging data. Specimens loaded in the tibial orientation have an ultimate load of 287 ± 37 N and stiffness of 62.8 ± 11 N/mm. Specimens loaded in the anatomical orientation have an ultimate load of 312 ± 6.5 N and a stiffness of 43.5 ± 3.4 N/mm.

Although load response in the synthetic tissue was less than its cadaveric counterpart due to bending load from flexion, the methods of this study are versatile, repeatable, and compatible with a wide variety of printable thermoplastics and composites which can be applied to other tissues, joints, and load conditions.