Professor Rutgers, The State University of New Jersey, United States
Introduction:: Significant bone loss can occur due to trauma or disease resulting in large bone defects. Typically, autografts and allografts are used as bone grafts, but these are not widely available and carry risk of disease transmission. Synthetic bone graft substitutes have a number of disadvantages. Tissue-engineered bone grafts are viable alternatives. In order for these to be successful, we must first address two important issues - lack of cell infiltration throughout their structure and inadequate vascularization. We have developed 3D-printed polylactic acid (PLA) scaffolds with distinct cortical and trabecular geometries. These are highly porous structures promoting capillary inflow of blood and bone marrow. They are also mineralized and prevascularized to promote regeneration of both bone and its vasculature. These scaffolds were evaluated in a load-bearing, critical-sized radial defect rabbit model and show promise.
Materials and Methods:: Scaffolds were designed using Solidworks and 3D-printed using the Ultimaker 2+/S3 3D printers. They were tested under compression, and their mechanical properties were compared to the previous iterations of scaffolds developed in our lab. Glycerol-water mixtures were used to mimic the viscosities of blood and bone marrow and study scaffolds’ ability to promote capillary inflow. In order to improve their ability to wick up these fluids, the polymer surface was pretreated with oxygen plasma prior to mineralization using concentrated simulated body fluid. Scaffolds were prevascularized by allowing vascular endothelial cells to grow within the cortical structure. They were decellularized after two weeks leaving behind a pro-angiogenic matrix. Scaffolds with and without autologous bone marrow were implanted in a critical-sized radial defect rabbit model with allografts serving as controls. Radiographic and computed tomography imaging was used to track bone regeneration over the course of twenty weeks. At the end of the study, histology will be performed to analyze regenerated bone and vasculature.
Results, Conclusions, and Discussions:: Our new scaffold design displayed superior mechanical properties when compared to the previous design. Our previous design displayed Ultimate Compressive Stress and Compressive Modulus which fall within the limits for trabecular bone (0.2-10 MPa and 7-200 MPa) but fall short of those required for native whole bone (200 MPa and 1 GPa respectively). With our new design, we have been able to achieve a mean Ultimate Compressive Stress of approximately 40 MPa and a mean Compressive Modulus of approximately 600 MPa (Figure 1). Slow capillary rise of fluids mimicking viscosities of blood and bone marrow was observed in the 3D-printed scaffolds. Presence of mineral resulted in them quickly wicking up fluid followed by a more gradual rise. In our in vivo evaluation, X-ray images showed progressive bone regeneration from two to ten weeks, originating first from the ulnar ridge just below where the radius was removed and from the two ends where the radius contacted the implant (Figures 2A and 2B). The CT images taken twelve weeks post-operatively showed significant bone regeneration (Figure 2C).
We have developed load-bearing scaffolds that promote infiltration of bone marrow into and throughout the scaffold, reducing the need to add stem cells prior to implantation in vivo. Using endothelial cells to prevascularize our scaffolds, we have created decellularized lumens within our cortical geometry to promote growth of vasculature upon implantation. Our ongoing in vivo evaluation has shown that our scaffolds can maintain their structural integrity while promoting bone remodeling and show promise as bone graft alternatives.
