Design and study of additively manufactured Three periodic minimal surface (TPMS) structured porous titanium interbody cage

Abstract

Objective: TPMS porous structures have adjustable stiffness, a smooth surface, and highly connected pores, which help avoid stress concentration within the dot-matrix structure and promote cell adhesion and proliferation. A cervical interbody cage based on this type of porous structure was designed and fabricated, and its mechanical properties and biocompatibility were evaluated.

Methods: TPMS porous structures have adjustable stiffness, a smooth surface, and highly connected pores, which help avoid stress concentration within the dot-matrix structure and promote cell adhesion and proliferation. A cervical interbody cage based on this type of porous structure was designed and fabricated, and its mechanical properties and biocompatibility were evaluated.

Results: The volume fraction of the 3D-printed TC4-based Tubular-G structure was linearly related to compressive strength. Adjusting the volume fraction resulted in a Tubular-G structure with a modulus and yield strength similar to human bone, without stress concentration within the structure. The designed and fabricated TC4-based Tubular-G porous cervical interbody cage demonstrated excellent anti-sagging properties and biocompatibility.

Conclusions: The volume fraction of the 3D-printed TC4-based Tubular-G structure was linearly related to compressive strength. Adjusting the volume fraction resulted in a Tubular-G structure with a modulus and yield strength similar to human bone, without stress concentration within the structure. The designed and fabricated TC4-based Tubular-G porous cervical interbody cage demonstrated excellent anti-sagging properties and biocompatibility.

Keywords: 3D printing; Cervical interbody cage; Three periodic minimal surfaces; Tubular-G.

Graphical abstract

Fig. 1

Offset parameter versus volumetric fraction.

Fig. 2

Measurement of the pore size.

Fig. 3

20 %, 30 %, 50 %, and 70 % volume fractions after 4∗4∗4 array Tubular-G.

Fig. 4

3D printed Tubular-G structures in four volume rates, from left to right, 20 % volume fraction, 30 % volume fraction, 50 % volume fraction, and 70 % volume fraction.

Fig. 5

The body-centered cubic structure.

Fig. 6

Four cage models: (a): Type A cage; (b): Type B cage; (c): Type C cage; (d): Type D cage.

Fig. 7

Mechanical testing of cervical interbody cage; (a): Static compression of cervical interbody cage; (b): Sinking experiment of cervical interbody cage.

Fig. 8

Compressed image of Tubular-G structure at 50 % volume fraction.

Fig. 9

Image of equivalent modulus of elasticity and yield stress with respect to relative density; (a): Ashby's formula for relative density versus Young's modulus; (b): Ashby's formula for relative density versus yield strength.

Fig. 10

Stress contour plots of the five models under peak load. (a) Tubular-G structure with 20 % volume fraction; (b) Tubular-G structure with 30 % volume fraction; (c) Tubular-G structure with 50 % volume fraction; (d) Tubular-G structure with 70 % volume fraction; (e) Body-centered cubic structure; (f) Connecting points of the Body-centered cubic structure.

Fig. 11

Titanium interbody cage made by 3D printing; (a): Cervical interbody cage with 20 % volume fraction Tubular-G structure, from left to right, Types A, B, C, D of the design in 2.5, respectively; (b): Cervical interbody cage with 30 % volume fraction Tubular-G structure, from left to right, Types A, B, C, D of the design in 2.5, respectively.

Fig. 12

Stiffness and yield load of the cervical interbody cage; (a): Stiffness; (b): Yield Load.

Fig. 13

Compression curves of the cervical interbody cage; (a): 20-A; (b):30-A.

Fig. 14

Load-displacement curves of the two types of cervical interbody cages and the polyurethane block.

Fig. 15

Cell attachment to the cage; (a) 20-A cage; (b) 30-A cage.