Partially Demineralized Macroporous (PDM) Allografts for Cranial Tissue Engineering

poster abstractDecompressive Craniectomy is a cranial surgery where a large part of the cranial bone is removed in order to mitigate swelling in the brain tissue. Consequently, a scaffold biomaterial is required to substitute the lost bone. Ideal cranioplasty biomaterials should have the following features: fit the cranial defect and achieve complete closure, radiolucency, resistance to infections, no dilation with heat, resistance to biomechanical wear, pliability, and inexpensive. Partially Demineralized Macroporous (PDM) allografts exhibit such properties to correct these cranial defects. The main objectives of this study include: (1) examining the effects of demineralization and macroporosity formations on the mechanical and biological properties of allograft bone disks; (2) conducting finite element analysis (FEA) to stimulate the mechanical properties of the PDM allografts; and (3) evaluating the in vitro response of the PDM allografts utilizing pre-osteoblast cell lines. Tibias were harvested from Ossabaw mini-pigs and cylindrical cortical bone sections of 2 mm in thickness and 8 mm in diameter were obtained. Macropores of 600 micrometers in diameter were created to generate porosity levels of 0-40% in the bone disks. The bone disks were then demineralized in 14-wt% EDTA for 6 to 48 hours at 37℃. The relative stiffness was determined for each class using a material testing machine with a loading rate of 1 mm/min using a piston-on-ring set up. To analyze the deformation characteristics, FEA software LS-DYNA was employed. In order to understand the in vitro response, biocompatibility of PDM scaffolds were evaluated by culturing MC3T3-E1 cell lines where XTT and ALP assays were conducted. PDM allografts display the suitable stiffness required for cranial defects. The PDM allograft scaffolds aid in osteogenic proliferation and differentiation of pre-osteoblast cell lines in vitro. However, there will be further in vivo testing regarding the validity of PDM allografts in rat cranial defects. Mentor: Tien-Min Gabriel Chu, Department of Restorative DentistryDecompressive Craniectomy is a cranial surgery where a large part of the cranial bone is removed in order to mitigate swelling in the brain tissue. Consequently, a scaffold biomaterial is required to substitute the lost bone. Ideal cranioplasty biomaterials should have the following features: fit the cranial defect and achieve complete closure, radiolucency, resistance to infections, no dilation with heat, resistance to biomechanical wear, pliability, and inexpensive. Partially Demineralized Macroporous (PDM) allografts exhibit such properties to correct these cranial defects. The main objectives of this study include: (1) examining the effects of demineralization and macroporosity formations on the mechanical and biological properties of allograft bone disks; (2) conducting finite element analysis (FEA) to stimulate the mechanical properties of the PDM allografts; and (3) evaluating the in vitro response of the PDM allografts utilizing pre-osteoblast cell lines. Tibias were harvested from Ossabaw mini-pigs and cylindrical cortical bone sections of 2 mm in thickness and 8 mm in diameter were obtained. Macropores of 600 micrometers in diameter were created to generate porosity levels of 0-40% in the bone disks. The bone disks were then demineralized in 14-wt% EDTA for 6 to 48 hours at 37℃. The relative stiffness was determined for each class using a material testing machine with a loading rate of 1 mm/min using a piston-on-ring set up. To analyze the deformation characteristics, FEA software LS-DYNA was employed. In order to understand the in vitro response, biocompatibility of PDM scaffolds were evaluated by culturing MC3T3-E1 cell lines where XTT and ALP assays were conducted. PDM allografts display the suitable stiffness required for cranial defects. The PDM allograft scaffolds aid in osteogenic proliferation and differentiation of pre-osteoblast cell lines in vitro. However, there will be further in vivo testing regarding the validity of PDM allografts in rat cranial defects

Structural and Mechanical Improvements to Bone Are Strain Dependent with Axial Compression of the Tibia in Female C57BL/6 Mice

Strain-induced adaption of bone has been well-studied in an axial loading model of the mouse tibia. However, most outcomes of these studies are restricted to changes in bone architecture and do not explore the mechanical implications of those changes. Herein, we studied both the mechanical and morphological adaptions of bone to three strain levels using a targeted tibial loading mouse model. We hypothesized that loading would increase bone architecture and improve cortical mechanical properties in a dose-dependent fashion. The right tibiae of female C57BL/6 mice (8 week old) were compressively loaded for 2 weeks to a maximum compressive force of 8.8N, 10.6N, or 12.4N (generating periosteal strains on the anteromedial region of the mid-diaphysis of 1700 με, 2050 με, or 2400 με as determined by a strain calibration), while the left limb served as an non-loaded control. Following loading, ex vivo analyses of bone architecture and cortical mechanical integrity were assessed by micro-computed tomography and 4-point bending. Results indicated that loading improved bone architecture in a dose-dependent manner and improved mechanical outcomes at 2050 με. Loading to 2050 με resulted in a strong and compelling formation response in both cortical and cancellous regions. In addition, both structural and tissue level strength and energy dissipation were positively impacted in the diaphysis. Loading to the highest strain level also resulted in rapid and robust formation of bone in both cortical and cancellous regions. However, these improvements came at the cost of a woven bone response in half of the animals. Loading to the lowest strain level had little effect on bone architecture and failed to impact structural- or tissue-level mechanical properties. Potential systemic effects were identified for trabecular bone volume fraction, and in the pre-yield region of the force-displacement and stress-strain curves. Future studies will focus on a moderate load level which was largely beneficial in terms of cortical/cancellous structure and cortical mechanical function

Load/Strain Calibration Curve.

<p>This figure demonstrates the linear relationship between applied load and resulting tensile strain at the periosteal mid-diaphysis. For all data points, error bars represent the standard deviation from n = 5 mice.</p

Timeline for tibial loading study and waveform profile.

<p>A) The right tibia of each mouse was loaded using the waveform to a set force (8.8N, 10.6N and 12.4N) to elicit a desired periosteal mid-diaphyseal tensile strain level (1700 με, 2050 με and 2400 με) over a 14 day period. B) The loading profile consisted of four haversine waveforms followed by 3 seconds of rest repeated 55 times for a total of 220 cycles of loading per day.</p

Schematic Representations of Standard Site Geometric Profiles.

<p>A) 1700 με group. B) 2050 με group. C) 2400 με group. There was a dose-dependent increase in cortical parameters with robust periosteal and endocortical formation, especially at the higher strain levels. D) 2400 με group with the addition of those animals that experiences a woven bone response due to loading.</p

Schematic representations of mechanical testing curves.

<p>A) Representative structural-level force/displacement curves. B) Estimated tissue-level mechanical curves. At 1700 με, there was no mechanical effect noted. Those limbs loaded to 2050 με experienced significant increases in structural- and tissue-level strength and energy dissipation. The 2400 με group also experienced gains. However, when animals that experienced a woven bone response were removed from the analysis, the gains were more modest (especially at the tissue-level) and most failed to reach significance versus the contralateral control limb given the loss in power. For all data points, error bars represent the standard error of the mean (SEM).</p