Three-dimensional printable metamaterial intramedullary nails with tunable strain for the treatment of long bone fractures

Abstract

Novel metamaterial unit cells with tunable strain were incorporated into titanium intramedullary (IM) nails to overcome problems of dense metallic implants. Three-dimensional (3D) models of cylindrical, pillar, and reference metamaterials were tuned and analyzed for structural authentication. Further, one solid IM nail (S0) and four different cylindrical metamaterial cell IM nails with gap sizes of 0.15 mm, 0.30 mm, 0.45 mm, and 0.60 mm were used to achieve 5% (S1), 10% (S2), 15% (S3), and 20% (S4) strain, respectively, in a 3-mm fracture, to investigate the healing outcomes. The bending stiffness (callus quality) in the S2 metamaterial IM nail increased by 22% between the 7th and 16th iterations, whereas for the S0, S1, S3, and S4 IM nails, the increases were 5.6%, 8.1%, 12.4%, and 9.0%, respectively. The S2 IM nail exhibited the best healing and bending stiffness; however, the S0 IM nail exhibited the worst results. This unique design was customized for different fracture gap sizes according to patient-specific needs.