TPMS Microarchitectures for Vertical Bone Augmentation and Osteoconduction: An In Vivo Study

Abstract

Triply periodic minimal surface microarchitectures (TPMS) were developed by mathematicians and evolved in all kingdoms of living organisms. Renowned for their lightweight yet robust attributes, TPMS structures find application in diverse fields, such as the construction of satellites, aircrafts, and electric vehicles. Moreover, these microarchitectures, despite their intricate geometric patterns, demonstrate potential for application as bone substitutes, despite the inherent gothic style of natural bone microarchitecture. Here, we produced three TPMS microarchitectures, D-diamond, G-gyroid, and P-primitive, by 3D printing from hydroxyapatite. We explored their mechanical characterization and, further, implanted them to study their bone augmentation and osteoconduction potential. In terms of strength, the D-diamond and G-gyroid performed significantly better than the P-primitive. In a calvarial defect model and a calvarial bone augmentation model, where osteoconduction is determined as the extent of bony bridging of the defect and bone augmentation as the maximal vertical bone ingrowth, the G-gyroid performed significantly better than the P-primitive. No significant difference in performance was observed between the G-gyroid and D-diamond. Since, in real life, the treatment of bone deficiencies in patients comprises elements of defect bridging and bone augmentation, ceramic scaffolds with D-diamond and G-gyroid microarchitectures appear as the best choice for a TPMS-based scaffold in bone tissue engineering.

Keywords: 3D printing; TPMS; additive manufacturing; bone substitute; ceramics; microarchitecture; osteoconduction; vertical bone augmentation.

Figure 1

Micro- and macroarchitecture of the scaffolds used in the osteoconduction/bone defect model. In the upper panel, the overall macroarchitectures of halved and full scaffolds are illustrated. In the lower panel, the constructs are displayed. A scale bar is provided.

Figure 2

Micro- and macroarchitecture of the scaffolds used in the bone augmentation model. In the upper panel, the constructs are displayed. A scale bar is provided. In the lower two panels, the overall macroarchitecture of full and halved scaffolds is illustrated.

Figure 3

Calvarial model for bone augmentation: (a) the four sites for cylinder placement were prepared with a trephine, and bone ingrowth was primed by a bur to generate three small defects in the outer cortical plate of the calvarial bone; (b,c) the four cylinders were screwed into the circular slits; (d) scaffolds were inserted (D, P, and G in black indicates the position of D-diamond, G-gyroid, and P-primitive); and (e) the cylinders were closed by press-fitting titanium lids. The diameter of the circular slits (a,b) is 6.00 mm.

Figure 4

Mechanical testing of TPMS microarchitectures. Cubic scaffolds 7.80 × 7.80 × 7.80 mm³ of the three TPMS microarchitectures were tested (upper panel). The graph in the lower panel visualizes the results of the testing for the maximal force in a box plot 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. Individual points outside the range are also displayed. The displayed p-values (P) show the significance level between the D-diamond and P-primitive scaffolds (upper) and between the G-gyroid and P-primitive scaffolds (lower). The compression strength of D-diamond and G-gyroid are significantly higher than for P-primitive.

Figure 5

Vertical bone augmentation according to the TPMS microarchitecture of the scaffold. Central histologic sections of cylinders harvested from the crania of rabbits after 4 weeks (upper panel). The highest point of bone ingrowth is depicted by the black line. Titanium appears black, the scaffolds appear greyish, and bone appears greyish purple. Scale bars of 1.00 mm and type of TPMS microarchitecture are provided. Bone augmentation with D-diamond and G-gyroid was significantly higher than for the P-primitive-based microarchitecture (lower panel, left). The bony regenerated area was also higher for D-diamond and G-gyroid compared to P-primitive, however, not to a level of significance. Values are displayed as box plots ranging from the 25th (lower quartile) to the 75th (upper quartile) percentile, with the median displayed as the solid black line and the whiskers extending to the minimum and maximum values. The displayed p-values (P) show the significance level between the G-gyroid and P-primitive scaffolds.

Figure 6

One-sided bone ingrowth according to the TPMS microarchitecture. Central histologic sections of one half of the 6.00 mm defects harvested from the crania of rabbits after 4 weeks are displayed for all three TPMS microarchitectures (upper panel). A scale bar for 1.00 mm is provided. Maximal bone ingrowth is depicted by the black line. The scaffolds appear dark greyish, and bone blue. One-sided bone ingrowth into the 6.00 mm defect was significantly higher in D-diamond and G-gyroid compared to P-primitive (lower panel, left side). Similar results were observed in terms of the percentage area of bony regeneration, although they did not reach a level of significance (lower panel, right side). Values are displayed as box plots ranging from the 25th (lower quartile) to the 75th (upper quartile) percentile, with the median displayed as the solid black line and the whiskers extending to the minimum and maximum values. The displayed p-values (P) report the significance level between the G-gyroid and P-primitive scaffolds.

Figure 7

The direction of bone growth in bone augmentation (upper panel) and osteoconduction (lower panel). Ground middle sections derived after 4 weeks of implantation are shown for all three TPMS microarchitectures tested. The direction of bone growth is indicated by blue arrows. Type of TPMS microarchitecture and scale bar of 1.00 mm is provided.