Biomechanical evaluation of an osteoporotic anatomical 3D printed posterior lumbar interbody fusion cage with internal lattice design based on weighted topology optimization

June 6, 2023

doi: 10.18063/ijb.697.

eCollection 2023.

Affiliations

¹ Department of Biomedical Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan.
² Innovation and Translation Center of Medical Device, Department of Biomedical Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan.
³ National Applied Research Laboratories, Taiwan Instrument Research Institute, Hsinchu, Taiwan.
⁴ Department of Orthopedics, Tri-Service General Hospital Songshan Branch, National Defense Medical Center, Taipei, Taiwan.
⁵ Department of Surgery, Tri-Service General Hospital Songshan Branch, National Defense Medical Center, Taipei, Taiwan.

Item in Clipboard

Shao-Fu Huang et al.

Int J Bioprint.

2023.

doi: 10.18063/ijb.697.

eCollection 2023.

Affiliations

¹ Department of Biomedical Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan.
² Innovation and Translation Center of Medical Device, Department of Biomedical Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan.
³ National Applied Research Laboratories, Taiwan Instrument Research Institute, Hsinchu, Taiwan.
⁴ Department of Orthopedics, Tri-Service General Hospital Songshan Branch, National Defense Medical Center, Taipei, Taiwan.
⁵ Department of Surgery, Tri-Service General Hospital Songshan Branch, National Defense Medical Center, Taipei, Taiwan.

Item in Clipboard

Abstract

In this study, we designed and manufactured a posterior lumbar interbody fusion cage for osteoporosis patients using 3D-printing. The cage structure conforms to the anatomical endplate’s curved surface for stress transmission and internal lattice design for bone growth. Finite element (FE) analysis and weight topology optimization under different lumbar spine activity ratios were integrated to design the curved surface (CS-type) cage using the endplate surface morphology statistical results from the osteoporosis patients. The CS-type and plate (P-type) cage biomechanical behaviors under different daily activities were compared by performing non-linear FE analysis. A gyroid lattice with 0.25 spiral wall thickness was then designed in the internal cavity of the CS-type cage. The CS-cage was manufactured using metal 3D printing to conduct in vitro biomechanical tests. The FE analysis result showed that the maximum stress values at the inferior L3 and superior L4 endplates under all daily activities for the P-type cage implantation model were all higher than those for the CS-type cage. Fracture might occur in the P-type cage because the maximum stresses found in the endplates exceeded its ultimate strength (about 10 MPa) under flexion, torsion and bending loads. The yield load and stiffness of our designed CS-type cage fall into the optional acceptance criteria for the ISO 23089 standard under all load conditions. This study approved a posterior lumbar interbody fusion cage designed to have osteoporosis anatomical curved surface with internal lattice that can achieve appropriate structural strength, better stress transmission between the endplate and cage, and biomechanically tested strength that meets the standard requirements for marketed cages.

Keywords:

3D printing; Biomechanics; Cage; Finite element; Topology optimization.

Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

**Figure 1**

The superior/inferior endplate morphologies were measured from the second (L2) to the fifth (L5) osteoporotic lumbar vertebrae, and the positions of the curved surface endplate variation were measured at 25%, 50%, and 75% of the endplate length in the coronal and the sagittal planes.

**Figure 2**

Finite element model generation processes included computed tomography image processing, computer-aided design model generation, mesh generation, model validation, and endplate morphology modification.

**Figure 3**

The weight topology optimization (WTO) analysis included the lumbar spine subjected to 21.5% for flexion/extension, 33% for bending, and 24% for axial rotation in the individual topology poetization (middle part). Top right part shows the reserved element after of WTO analysis. Bottom right part shows that the shape of a single posterior cage can be projected from the contours of half of the transverse cross-section plane and the sagittal plane.

**Figure 4**

Two CS-type and P-type forms were designed for implant between L3 and L4 spine body. Top right part shows the dimension of cage, that is, 25 mm in length, 16 mm in width, and 16.2 mm/12 mm in anterior/posterior height. Bottom right part shows the solid and mesh models of CS-type and P-type cages.

**Figure 5**

(A) 3D-printed CS-type cage. (B-D) The clamping device of *in vitro* tests under compression, compression-shear and torsion, respectively.

**Figure 6**

The maximum stress values of CS-type and P-type cages at the L3 inferior (A) and L4 superior (B) endplates under flexion, extension, lateral bending, and torsion.

**Figure 7**

The von Mises stress distributions of L3 inferior and L4 superior endplates for CS-type and P-type cages under all load conditions.

**Figure 8**

The CS-type cage fracture pattern after *in vitro* test: (A) ISO view and (B) back view.

**Figure 9**

Status of contact areas between cage and endplate for CS-type and P-type cages under all load condition simulations. Red ovals/circles indicate the positions of fractures or damages. Bottom left: two red oval regions show the possibility superior/inferior contact areas of the CS-type cage; bottom right: four red circle regions show the possibility of superior/inferior contact points of the P-type cage.

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