Three-Dimensional Printed Hydroxyapatite Bone Substitutes Designed by a Novel Periodic Minimal Surface Algorithm Are Highly Osteoconductive

Abstract

Autologous bone remains the gold standard bone substitute in clinical practice. Therefore, the microarchitecture of newly developed synthetic bone substitutes, which reflects the spatial distribution of materials in the scaffold, aims to recapitulate the natural bone microarchitecture. However, the natural bone microarchitecture is optimized to obtain a mechanically stable, lightweight structure adapted to the biomechanical loading situation. In the context of synthetic bone substitutes, the application of a Triply Periodic Minimum Surface (TPMS) algorithm can yield stable lightweight microarchitectures that, despite their demanding architectural complexity, can be produced by additive manufacturing. In this study, we applied the TPMS derivative Adaptive Density Minimal Surfaces (ADMS) algorithm to produce scaffolds from hydroxyapatite (HA) using a lithography-based layer-by-layer methodology and compared them with an established highly osteoconductive lattice microarchitecture. We characterized them for compression strength, osteoconductivity, and bone regeneration. The in vivo results, based on a rabbit calvaria defect model, showed that bony ingrowth into ADMS constructs as a measure of osteoconduction depended on minimal constriction as it limited the maximum apparent pore diameter in these scaffolds to 1.53 mm. Osteoconduction decreased significantly at a diameter of 1.76 mm. The most suitable ADMS microarchitecture was as osteoconductive as a highly osteoconductive orthogonal lattice microarchitecture in noncritical- and critical-size calvarial defects. However, the compression strength and microarchitectural integrity in vivo were significantly higher for scaffolds with their microarchitecture based on the ADMS algorithm when compared with high-connectivity lattice microarchitectures. Therefore, bone substitutes with high osteoconductivity can be designed with the advantages of the ADMS-based microarchitectures. As TPMS and ADMS microarchitectures are true lightweight structures optimized for high mechanical stability with a minimal amount of material, such microarchitectures appear most suitable for bone substitutes used in clinical settings to treat bone defects in weight-bearing and non-weight-bearing sites.

Keywords: 3D printing; ADMS; TPMS; adaptive density minimal surfaces; additive manufacturing; bone substitute; ceramics; hydroxyapatite; microarchitecture; osteoconduction; titanium; triply periodic minimal surface.

FIG. 1.

3D printing of ADMS structures. ADMS structure during printing in the CeraFab7500 machine (a). ADMS structure after removal from the machine and attached to the building platform (b). The same ADMS structure after sintering at 1300°C (c). The diameter of the ADMS structure was 23 mm (c). Six and fifteen millimeter scaffolds for animal testing are shown from the upper and lower sides (d). Scale is provided. Diverse ADMS structures with minimal constriction diameter as listed (e). Scale is provided. 3D, three-dimensional; ADMS, adaptive density minimal surface.

FIG. 2.

Compression strength of ADMS and lattice microarchitectures from HA. In the upper panel, all tested scaffold types are displayed in the same order as in the graph. The graph, in the lower panel visualizes the compression strength of all tested scaffold types, as listed, in a box-blot ranging from the 25th (lower quartile) to the 75th (upper quartile) percentile, with the median displayed as a solid black line and whiskers extending to the minimum and maximum values. Values outside the box blot are shown individually. HA, hydroxyapatite.

FIG. 3.

Osteoconduction and bone regeneration in ADMS and lattice microarchitectures. Histological sections from the middle of the noncritical-size defects were treated with ADMS- and lattice-derived scaffolds after 4 weeks of recovery (a). Scale bars in blue indicate 1000 μm. Original magnifications were 100-fold. Bone appears as grayish purple to purple, and HA as grayish to black. Histomorphometry results (b, c). Defect bridging (b) and new bone formation (c) decrease significantly in ADMS microarchitecture with the increase in minimal constriction from 500 to 1100 μm, as evident by the Jonckheere–Terpstra test results. The lattice microarchitecture yielded the highest values but not significantly higher than with ADMS 500 μm or ADMS 800 μm scaffolds. Values are displayed as box plots ranging from the 25th (lower quartile) to the 75th (upper quartile) percentile. The median is displayed as a solid black line and whiskers extending to the minimum and maximum values. Values outside the range of the box blot are shown as individual points. p-Values are provided in the graphs.

FIG. 4.

Osteoconduction and bone regeneration in ADMS 500 μm and lattice microarchitecture in a critical-size defect. Two histological sections from the middle of the critical-size defects treated with ADMS 500 μm and a lattice-derived scaffold after 8 weeks of recovery (a). Scale bars in blue indicate 1000 μm. Original magnifications were 100-fold. Bone appears as grayish purple to purple and HA as grayish to black. Histomorphometry results (b, c). No significant difference could be detected in defect bridging (b) and new bone formation (c) between ADMS 500 μm and the lattice microarchitectures. Values are displayed as box plots ranging from the 25th (lower quartile) to the 75th (upper quartile) percentile. The median is displayed as a solid black line and whiskers extending to the minimum and maximum values.