Silk Based Devices To Modulate Fracture Healing

INTRODUCTION: Rigid fixation using plates and screws derived from metal alloys are the accepted mode of repair for fractures. However, these devices may have drawbacks. Recently, use of resorbable plates and screws formed from synthetic polyesters has improved bone remodeling but are potentially associated with inflammatory reaction, incomplete bone remodeling, and strained dynamics between device strength retention and wound healing. We have developed a new family of resorbable screws and plates prepared from silk proteins for fracture repair.1–5 METHODS: Silk-based blanks were prepared and then treated to induce crystalline, beta sheet structures, for stability. The blanks were then machined into standard machine screws, plates, or cylindrical pins. The screws, plates, and pins were evaluated in vitro in dry and in hydrated environments to mimic implantation conditions. Mechanical tests were performed using double lap shear (pins) and pull-out strength (screws). The screws were assessed in vivo in rat femurs and were shown to be self-tapping, remained fixed in the bone for a four week trial, and exhibited no negative host response. Histological analysis was done. RESULTS: Histological analysis showed initial resorption of the screws and initial remodeling of the bone after 4 weeks in vivo. The properties of these new silk-based devices compared favorably with those of current poly-lactic-co-glycolic acid resorbable fixation systems. Our results demonstrate that silk biomaterial-based devices exhibit potential for resorbable fixation systems due to their mechanical strength, ease of implantation, lack of a negative host response, and ability to effectively aid in bone healing. CONCLUSION: Silk protein can be used for resorbable plates and screws in an animal model. Their mechanical properties compared favorably with that of current fixation systems, while offering a number of advantages. We anticipate our first iteration of these novel materials as a potential starting point for new advanced resorbable medical devices that utilize the versatility of silk (Figures 1 and 2).Figure 1: Initial Silk Screws and Plates. (A) 1:0 25% silk screw (1-72 machine screw designation: major diameter = 0.0730 in; pitch diameter = 0.0640 in; minor diameter = 0.0560 in). (B) SEM image of 1-72 silk screw. (C) Combination of silk screws and plates on model skull orbital bone to show fixation potential. (D) SEM image of milled surface of silk plate.Figure 2: Histology of subcutaneous implants after one week in vivo: moderate neutrophil infiltration denoted by “N”. The silk implant is the uniform shade of translucent purple and is denoted by “S.” (A) 400X magnification. (B) 4X magnification.