Poster N7 - Regeneration of Critical Size Bone Defects Using 3D Printed Biomimetic Polymer Scaffolds

Introduction: Critical size defects do not heal without surgical intervention and there are currently no reliable treatments. Previous studies in our lab have demonstrated that 3D printed polybutylene terephthalate (PBT) scaffolds printed in a biomimetic pattern increase bone regeneration compared to 3D printed scaffolds with simple geometric structures. Furthermore, when biomimetic scaffolds are coated with tricalcium phosphate particles and combined with autologous stem cells, they can regenerate a 4.2cm critical size defect in sheep in 6-months. The purpose of this study was to compare the mechanical properties and volume of regenerated bone when the implanted scaffolds are supported with a non-dynamized intramedullary nail, a dynamized intramedullary nail, and following removal of the stabilizing hardware.

Materials and

Methods: PBT scaffolds were implanted in a critical size defect during an approved IACUC animal surgery. PBT cylindrical scaffolds were 3D printed measuring 4.2cm in height and 2.2cm in diameter. The cylindrical scaffold consisted of a biomimetic interior pattern and a solid outer shell for stability. Two weeks before surgery, adipose derived stem cells were extracted from the tail fat pad of the animal and expanded in culture. 1 day before surgery, stem cells were seeded onto the scaffold and maintained at 37C and 5% CO2 until the start of surgery. Sheep underwent the creation of a 4.2cm mid-diaphyseal defect and stabilization with an intramedullary nail and interlocking screws. A scaffold was placed in the defect site for 16 sheep and 3 control sheep did not receive a scaffold. At 6 months, 10 sheep underwent dynamization of the stabilizing hardware by the removal of the proximal interlocking screw. At 9 months, 4 sheep who had undergone dynamization underwent the total removal of the stabilizing hardware. Control (n=3), non-dynamized (n=6), and dynamized (n=6) were euthanized at 9 months post implantation surgery while the sheep who underwent total hardware removal (n=4) were kept up to 2 years. Femurs were extracted, radiographed, and tested under mechanical testing and microCT analysis.

Results, Conclusions, and Discussions: Following 9 months, unions occurred in 1/3 control sheep, 1/6 non-dynamized sheep, 5/6 dynamized sheep, and 4/4 sheep following total hardware removal. Mechanical testing in axial compression, shown in Figure 1, demonstrates a significant increase in axial stiffness in the total hardware removal (1932+/-346 N/mm) compared to the control (458+/-513 N/mm) and non-dynamized (558+/-500 N/mm) groups (p < 0.05). Quantitative microCT analysis (Figure 2) demonstrated dynamized (14.69+/-4.58 cm3) and total hardware removal (12.70+/-1.346 cm3) had a higher total bone volume than control (10.04+/-3.39 cm3) (p>0.05). Qualitative microCT showed increasing bone remodeling traits in the dynamized and no hardware with higher number of unions and lower cortical bone porosity (Figure 3 I: Control, II: Non-Dynamized, III: Dynamized, IV: Hardware Removed). X-rays and microCT demonstrated that 3D printed biomimetic PBT scaffolds allow for the regeneration of critical size defects in a sheep model. Axial compressive stiffness was significantly higher after the removal of the stabilizing hardware. Although the no hardware group showed a lower total bone volume (p>0.05) than the dynamized group, qualitative analysis showed lower cortical porosity which would demonstrate that the regenerated bone underwent extensive remodeling resulting in a more organized bone tissue.

Acknowledgements (Optional): The Army Medical Research Acquisition Activity, 820 Chandler St., Ft Detrick MD 21702-5014 - awarding & administering acquisition office. Work supported by; Assist. Sec. of Defense for Health Affairs endorsed by the DoD, Broad Agency Announcement for Extramural Medical Research; Award W81XWH-18-1-0490. Opinions, interpretations, conclusions and recommendations are not necessarily endorsed by the DoD.