Comparison of mechanical properties of third generation silk protein for use in osteosynthesis fixation systems

Osteosynthesis systems are widely used in the bony skeleton, such as in post-oncologic or traumatic reconstruction and orthognathic surgery. Silk protein-based biomaterials are novel alternatives to metal fixation systems, with studies showing degradability, good biocompatibility, nonsensitivity to temperature, and minimized stress-shielding. Mechanical properties of three generations of silk-based hardware were studied and compared to conventional metal and resorbable systems.

Silk plates and screws were prepared using three techniques: (1) hexafluoro-2-propanol-derived (HFIP)-based approach, (2) aqueous-derived approach and (3) thermally molded silks formed by direct fusing amorphous silk nanomaterials ASN (ASN, diameters from 30 nm to 1 μm), at high pressure. Three-point bending, tensile, compression, and double lap shear tests were performed.

Mechanical properties of thermal silk plates varied depending on the hydration condition. Dry thermally processed silk plates had a higher flexural modulus (2.4 -7.8 GPa) than both dry HFIP-derived (1.7 - 4.4 GPa) and aqueous-derived silk plated (2.7 GPa), suggesting superiority in flexural load bearing without permanent deformation. Hydrated thermal silk plates had excellent tensile toughness (0.9-10.5 MJ ·m-3) compared to current resorbables (0.1-5 MJ ·m-3). Silk pins performed similarly to current resorbables in terms of maximum shear strength. Silk bulk materials exhibited mechanical tolerance above trabecular bone and approached that of cortical bone. The closely matched elastic moduli reduce stress shielding.

Thermally processed silk is a promising biomaterial with favorable properties compared to current metal systems, resorbables, and earlier iterations of silk fabrication techniques. Hydration status allows further refinement of mechanical properties of silk osteofixation systems.