Mechanical, Histological, and Scanning Electron Microscopy Study of the Effect of Mixed-Acid and Heat Treatment on Additive-Manufactured Titanium Plates on Bonding to the Bone Surface

Abstract

The additive manufacturing (AM) technique has attracted attention as one of the fully customizable medical material technologies. In addition, the development of new surface treatments has been investigated to improve the osteogenic ability of the AM titanium (Ti) plate. The purpose of this study was to evaluate the osteogenic activity of the AM Ti with mixed-acid and heat (MAH) treatment. Fully customized AM Ti plates were created with a curvature suitable for rat calvarial bone, and they were examined in a group implanted with the MAH-treated Ti in comparison with the untreated (UN) group. The AM Ti plates were fixed to the surface of rat calvarial bone, followed by extraction of the calvarial bone 1, 4, 8, and 12 weeks after implantation. The bonding between the bone and Ti was evaluated mechanically. In addition, AM Ti plates removed from the bone were examined histologically by electron microscopy and Villanueva-Goldner stain. The mechanical evaluation showed significantly stronger bone-bonding in the MAH group than in the UN group. In addition, active bone formation was seen histologically in the MAH group. Therefore, these findings indicate that MAH resulted in rapid and strong bonding between cortical bone and Ti.

Keywords: additive manufacturing; bone bonding; custom-made plate; mixed-acid and heat treatment; selective laser melting; titanium.

Figure 1

Shape of the Ti plates. (A): Designed plate. The dimensions of the AM Ti Plate samples are 10 mm × 10 mm × 1 mm. (B): Real image of an AM Ti Plate. The scale bar indicates 5 mm.

Figure 2

Implantation technique. The AM Ti plate is fixed to the calvarial bone.

Figure 3

Evaluation of bone bonding. The bonding between the AM Ti plate and the cortical bone is evaluated by measuring the fracture load (N). The AM Ti plate is pulled out from the calvarial bone. (A): The lateral view. (B): The frontal view.

Figure 4

Evaluation of bone bonding. The mixed-acid and heat (MAH)-treated group shows significant increases in bone bonding at 4 and 12 weeks after implantation: 16.87 ± 9.90 N and 7.07 ± 2.91 N for the MAH group and the untreatedgroup, respectively, at 4 weeks, and 49.31 ± 16.58 N and 17.32 ± 8.86 N for the MAH group and the UN group, respectively, at 12 weeks. * <0.05, ** <0.01.

Figure 5

Electron micrograph by energy dispersive X-ray mapping. White circles indicate the area of bone tissue. (A): Backscattered electron image, (B): Titanium, (C): The area of calcium, (D): The area of phosphorus.

Figure 6

Scanning electron microscopic observation. Electron micrograph of Ca distribution by energy dispersive X-ray mapping of selective laser melting Ti metal. Arrows indicate screw holes. (A): The MAH group at 1 week. (B): The UN group at 1 week. (C): The MAH group at 4 weeks. (D): The UN group at 4 weeks. (E): The MAH group at 8 weeks. (F): The UN group at 8 weeks. (G): The MAH group at 12 weeks. (H): The UN group at 12 weeks.

Figure 7

Histological observation. Active bone formation between the calvarial bone and the Ti surface is seen in the MAH group. The scale bar indicates 500 µm. Ti: Titanium plate, Bo: Calvarial bone.

Figure 8

Enlarged histological observation. The MAH group at 12 weeks, showing active bone formation from the Ti surface toward the deep part. The scale bar indicates 100 µm. Ti: Titanium plate, Bo: Calvarial bone.