Casting over Metal Method Used in Manufacturing Hybrid Cobalt-Chromium Dental Prosthetic Frameworks Assembles

Abstract

Cobalt-chromium (Co-Cr) alloys are the most widely used materials for removable and fixed dental prosthetic frameworks. The fitting accuracy between these components in dental prosthetic frameworks assembles (DPFAs) is largely influenced by the manufacturing method. This study presents a novel manufacturing method that combined two common techniques for obtaining one single framework: casting of Co-Cr inserts on top of parts previously manufactured by selective laser melting (SLM) of Co-Cr powder (CoM). Horizontal (n = 4) and vertical (n = 3) surfaces were microscopically analyzed (n = 770 count sum). The results revealed a high precision of the process and high fitting accuracy between the hybrid frameworks. The average distance measured between the frameworks in joined position was 41.08 ± 7.56 µm. In conclusion, the manufacturing of Co-Cr alloys DPFA using the CoM method reduced the deformation of hybrid frameworks and improved the joining accuracy between them.

Keywords: accuracy; casing over metal method; cobalt–chromium alloy; dental alloys; dental prosthetic framework assemble; selective laser melting.

Figure 1

Diagram showing the technological process of manufacturing the dental crown hybrid framework: (1) master cast (blue); (2) Co–Cr coping produced by SLM (grey); (3) wax pattern (orange) milled by CNC machine fitted over metal coping; (4) spruing wax (orange); (5) wax pattern over the metal coping detached from the master cast; (6) coating in refractory material (pink); (7) lost wax technique and the resulting mold (white); (8) casting over metal coping (grey) molten alloy (red); (9) cooling of the alloy in the mold; (10) dental crown hybrid framework (red-grey); and (11) mechanical preparation of the parallel surface using milling tools (purple).

Figure 2

Diagram showing the technological process of manufacturing the removable partial denture hybrid framework: (12) dental crown hybrid framework (red–grey) on the master cast (blue); (13) Co–Cr framework produced by SLM (grey); (14) wax pattern (orange) produced by manual additive method (orange) joining Co–Cr framework and dental crown hybrid framework external surface; (15) spruing wax (orange); (16) wax pattern over metal coping detached from the master cast; (17) coating in refractory material (pink); (18) lost wax technique and the resulting mold (white); (19) Casting over metal framework (grey) and the molten alloy (green); (20) cooling of the alloy in the mold; (21) removable partial denture hybrid framework (green–grey); (22) removable partial denture hybrid framework fitted on the DC hybrid framework and the resulting hybrid dental prosthetic frameworks assembles.

Figure 3

(a) Steel master cast (C1); (b) simulation of shape transfer from the prosthetic field (steel master cast) using a silicon impression; (c) dental laboratory stone cast.

Figure 4

(a) Scanning of the dental laboratory stone cast using structural blue light; (b) computer-aided design (CAD) software screen view of a digitalized dental cast (abutment); (c) CAD software screen view of the 3D coping framework design.

Figure 5

(a) CAD Software screen view of the 3D copy framework design; (b) external surface of Co–Cr copy framework; (c) the thickness of the copy wall measured with a mechanical micrometer; (d) the scale of the instrument indicated a thickness of 35 µm.

Figure 6

(a) Digital design of the complementary dental crown framework with geometrical surface; wax pattern milled by the five-axes CNC machine; (b) external view; (c) internal view; (d) para-axial view (pattern on the dental laboratory stone cast abutment).

Figure 7

(a) Preparation of the wax pattern for molding; (b) insertion of the metallic framework and the wax pattern during the process of molding.

Figure 8

(a,b) Milling the lower step; (c) milling the pin-lock; (d) dental crown framework.

Figure 9

(a) Dental crown framework (STL format); (b) work in progress design; (c) virtual design of removable partial denture framework; (d) removable partial denture framework design in relation with dental crown framework; (e) removable partial denture framework (external surface); (f) removable partial denture framework (mucosal surface).

Figure 10

Geometric map of sections through frameworks assemble (removable partial denture hybrid framework—green, dental crown hybrid framework—red) in contact with master cast (dental abutment and mucosal area—blue); I1, A1, A2, A3, and A4 are the areas resulting from horizontal sections through the assemble; V2A and V2B are the areas resulting from vertical sections at the marginal–cervical area of dental crown hybrid framework at the level of contact with the dental abutment.

Figure 11

Sections through the framework assemble: (a) A1; (b) A2; (c) A3; (d) A4.

Figure 12

The areas V3A and V3B resulted from the vertical sections at the marginal–cervical zone of the dental crown hybrid framework at the level of contact with the dental abutment; V1MO, V1O, and V1DO are the areas that result from vertical section true removable partial denture framework at the level of contact with mucosal area.

Figure 13

Sections through the framework assemble: (a) V1MO, V1O; (b) V3A, V3B; (c) V1O, V1DO.

Figure 14

Micrographic segment (200×) after measuring the length of the gap between the joined frameworks of the dental prosthetic frameworks assembles (A3—area 3, segment g7).

Figure 15

Micrographic segments (1000×) after measuring the length of the gap between the joined frameworks of the dental prosthetic frameworks assembles (I1—area 1, segment g2).

Figure 16

Micrographic segment (200×) after measuring the length of the gaps between dental abutment and dental crown framework at cervical area (V2B—area, segment g2).

Figure 17

Micrographic segment (200×) after measuring the length of the gap between removable partial denture framework and mucosal area at the V1 area (V1—area, segment d6).