Effect of scaffold material, construct length and mechanical stimulation on the in vitro stiffness of the engineered tendon construct

Introducing mesenchymal stem cell (MSC)-seeded collagen constructs into load-protected wound sites in the rabbit patellar and Achilles tendons significantly improves their repair outcome compared to natural healing of the unfilled defect. However, these constructs would not be acceptable alternatives for repairing complete tendon ruptures because they lack the initial stiffness at the time of surgery to resist the expected peak in vivo forces thereafter. Since the stiffness of these constructs has also been shown to positively correlate with the stiffness of the subsequent repairs, improving initial stiffness by appropriate selection of in vitro culture conditions would seem crucial. In this study we examined the individual and combined effects of collagen scaffold type, construct length, and mechanical stimulation on in vitro implant stiffness. Two levels each of scaffold material (collagen gel vs. collagen sponge), construct length (short vs. long), and mechanical stimulation (stimulated vs. non-stimulated) were examined. Our results indicate that all three treatment factors influenced construct linear stiffness. Increasing the length of the construct had the greatest effect on the stiffness compared to introducing mechanical stimulation or changing the scaffold material. A significant interaction was also found between length and stimulation. Of the eight groups studied, longer, stimulated, cell-sponge constructs showed the highest in vitro linear stiffness. We now plan in vivo studies to determine if higher stiffness constructs generate higher stiffness repairs 12 weeks after surgery and if in vitro construct stiffness continues to correlate with in vivo repair parameters like linear stiffness.