This work explores the viability of 3D printed intervertebral lumbar cages based on biocompatible polycarbonate (PC-ISO® material). Several design concepts are proposed for the generation of patient-specific intervertebral lumbar cages. The 3D printed material achieved compressive yield strength of 55 MPa under a specific combination of manufacturing parameters. The literature recommends a reference load of 4,000 N for design of intervertebral lumbar cages. Under compression testing conditions, the proposed design concepts withstand between 7,500 and 10,000 N of load before showing yielding. Although some stress concentration regions were found during analysis, the overall viability of the proposed design concepts was validated.
The combination of biotechnology and 3D printing has led to the rise of 3D bioprinting, which is a processing technique that promises to solve critical issues while finding printable biomaterials, increasing the capacity of precise positioning and including cell sources, in order to be successfully applied in diagnosis, personalized medicine, and regenerative medicine [
3D printing of implants, prosthesis, and other medical devices can be considered an important stage of the full development of 3D bioprinting applications. Particularly, design of prosthesis and implants is nowadays embracing the use of 3D printing technologies as FDM (Fused Deposition Modeling), in order to solve the need for customization and the need for providing a fast response in surgical interventions [
A specific case of the need of customized implants is column surgery. This is performed in order to ease pathologies associated with back pain that are sometimes caused by deterioration of surrounding fibrous ring of intervertebral discs, resulting in spinal disc herniation. The column surgery that deals with this illness is usually known as spinal fusion surgery, where the vertebrae gradually fuse into a single body with the introduction of an intervertebral cage implant. Spinal fusion is done most commonly in the lumbar region of the spine, but it is also used to treat cervical and thoracic regions. The main function of an intervertebral cage implant is to fill the intervertebral space in order to facilitate the process of osseointegration and to provide mechanical support through an optimal load distribution and an interbody fusion balance fixation.
Figure
Representation of the lumbar cage implants (vertebrae images courtesy of Centro de Tecnologia da Informação Renato Archer).
There are a limited number of research studies that explore the design and rapid manufacturing of patient-specific implants for column surgery. de Beer and van der Merwe developed a study of the rapid manufacturing of metallic implants. They used computed tomography (CT), direct laser metal sintering (DMLS), and mechanical testing in order to propose a process chain for customization of intervertebral disc implants [
There is a clear need to evaluate the viability of the use of 3D printing for the customization of column surgery implants. Thus, the main objective of this research is to validate a series of design concepts of a specific case of column implants (called “intervertebral lumbar cages”) with the aid of FDM, computational simulations, and experimental testing. Furthermore, this work is intended to lay the basis for the implementation of these medical devices in orthopedic surgeries.
This section covers the set-up of a proposed process chain for lumbar cage material selection, design, prototyping through FDM, mechanical testing, and Finite Element Analysis.
The proposed polymer for this research is PC-ISO (polycarbonate-ISO), an industrial thermoplastic that can be sterilized by several methods like ethylene oxide and gamma radiation, according to the work of Perez et al. In this work sterility testing was performed with successful results for material deposited with FDM process [
In the case of the material biocompatibility, there are several studies ordered by the material supplier that confirm that the material is not toxic, does not present allergenic potential, and does not have irritant effects. Among the studies mentioned above there is, for example, the ISO Acute Systemic Injection Test, which was designed for screening PC-ISO extracts for potential toxic effects as a result of a single-dose systemic injection in mice. The study confirmed that the animals did not present signs of toxicity in comparison with the control. Regarding mechanical properties of the chosen material, it has a specified ultimate tensile strength of 57 MPa and a modulus of elasticity of 2 GPa, properties that made this polymer competitive with other engineered materials for implants [
The implant design for FDM took into account several design guidelines for the appropriate deposition of the polymeric material, in order to improve its mechanical performance during tensile or compressive testing and for increasing the dimensional and geometrical accuracy. Some of these design guidelines were formulated by Ahn et al., based on the results of extensive experimentation [ A negative air gap, meaning that two FDM layers partially occupy the same space, will increase both strength and stiffness. The build orientation could improve the part accuracy and strength.
Prior to the exploration of patient-specific lumbar cage process chain, cylindrical specimens with two different configurations of build directions were subject to compression strength testing (Figure
System specifications of FDM machine.
Configuration | |
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Build envelope ( | 406 × 355 × 406 mm |
| |
Material delivery | Two build material canisters 1508 cc |
Two support material canisters 1508 cc | |
Autochangeover between canisters | |
| |
Material options | PC-ISO |
| |
Layer thickness | 0.330 mm |
0.254 mm | |
0.178 mm | |
| |
Support structure | Breakaway Support System (BASS™) |
Fused Deposition Modeling process specifications.
Nozzle diameter | 0.30 mm | 0.40 mm |
---|---|---|
Slice height | 0.17 mm | 0.25 mm |
Contour width | 0.35 mm | 0.50 mm |
Part raster width | 0.35 mm | 0.50 mm |
Visible surface raster | 0.30 mm | 0.43 mm |
Internal raster | 0.40 mm | 0.61 mm |
Build direction and part interior style of cylindrical test specimens: (a) transverse-vertical and (b) horizontal-axial configurations.
The design criteria for lumbar cage geometries were established by the analysis of patents and commercial products, in which the predominant factor found was the load distribution for providing minimum damage to vertebrae [
It is important to point out that the mechanical response to fatigue testing must be considered in further research since there is evidence in the literature that repetitive loading can cause vulnerability to column mechanical damage [
The evolution of all design attempts is shown in Figure
Evolution of lumbar cage design concepts.
In this work, four different lumbar cage design concepts are proposed, as an optimization alternative in the search for appropriate 3D printed implants. These lumbar cage design concepts were tested in a similar set-up to that used for cylindrical specimens. Solid models of the proposed lumbar cages were drawn using a generic CAD tool. The dimensions ranges of the prototypes were about 13.4 mm in width and 28.7 mm in length, having a height of 13 millimeters. Also, all the designs included geometrical features that facilitate osseointegration. It is important to point out that the design concepts include antiskid systems required for appropriate implant fixation, but these geometrical features could generate stress concentration. Figure
Proposed lumbar cage new designs.
Compressive strength tests for each cylindrical specimen and each lumbar cage design concept were conducted using a Shimadzu Universal Testing Machine equipped with a 25 kN load cell. The tests performed on cylindrical specimens were based on the “ASTM.D695.2010, Standard Test Method for Compressive Properties of Rigid Plastics” [
Test system equipped with a 25 kN load cell.
The Finite Element Analysis (FEA) was performed with the aid of COMSOL Multiphysics 4.3 software, to automatically generate tetrahedral elements and for performing the computational simulation. It is a common practice to use automatic tetrahedral mesh generators to discretize complex 3D structural components. This type of mesh generators can handle complex geometries with a minimum of human intervention (as compared to, e.g., the manual generation of a mesh of hexahedral elements). The solver used to perform the 3D calculations was embedded in the software. Mesh size was predefined in a range of 0.286 and 2.290 mm with a maximum element growth rate of 1.45. Mechanical properties of the proposed lumbar cages were assumed to be homogeneous and linear elastic. The mechanical properties of Table
Mechanical properties of PC-ISO cylindrical test specimens.
Mechanical properties | Unit | 0.30 mm | 0.40 mm |
---|---|---|---|
Compressive yield strength | MPa | 40 | 55 |
Young’s modulus | GPa | 1.2220 | 1.2580 |
Poisson’s ratio | 1 | 0.3928 | 0.4287 |
Density | Kg/m3 | 1,060 | 1,110 |
In order to establish a good approximation of the load that must be used for simulating average column loads, an additional literature review was made. Wilke et al. estimated an interdiscal pressure of 1.8 MPa, equivalent of 3,240 N in a disc area of 1800 mm2, in the L4-L5 disc while an average person was holding a 20 kg object 600 mm away from the chest [
The results of this screening experimentation for horizontal-axial build configuration were discarded since they presented lower strength behavior (7,800 N) in comparison to those with transverse-vertical build configuration for similar conditions (8,300 N). Figure
Comparison of the compression test of PC-ISO cylindrical test specimens, printed with 0.30 mm and 0.40 nozzle configuration.
Table
Simulations results are explained in terms of the average von Mises stresses exhibited in different cross sections of the four design concepts. Figure
Volumetric analysis: von Mises stress distribution with 4,000 N as reference load.
Max 76.227 MPa
Max 95.750 MPa
Max 99.668 MPa
Max 133.680 MPa
Cross section analysis for middle plane: von Mises stress distribution with 4,000 N as reference load.
Max 21.083 MPa
Max 22.484 MPa
Max 20.916 MPa
Max 58.115 MPa
Figure
Cross section analysis for top plane: von Mises stress distribution with 4,000 N as reference load.
Max 62.635 MPa
Max 78.757 MPa
Max 71.447 MPa
Max 65.423 MPa
Figure
Compression testing for design concept (a), with different FDM nozzle configurations (0.30 versus 0.40 mm).
Figure
Compression testing for all design concepts with 0.40 mm nozzle configuration.
Based on the cylindrical samples, the reference compressive yield strength to consider is 55 MPa. The middle plane simulation analysis shows that, on average, stress level is well below the compressive yield strength of the FDM-printed PC-ISO material. Only in the case of concept (d), the maximum stress level exceeds the limit by small percentage (6%).
From the compression testing experimental results of the cylindrical specimens and the design concepts (Figures
Local maximum stress levels are above the material strength in some regions. For the top plane analysis, all regions with maximum stress are above the material strength (between 14 and 43% higher stress compared to the material strength). The volumetric analysis shows regions that exceed the material strength by a large percentage (up to 143% in the case of design concept (d)).
In contrast to the FEM analysis results, using 4,000 N as the reference load, the actual compression test shows that the various design concepts are robust. For the case of design concepts (a) and (b), the yield behavior starts at approximately 10,000 N. A load of approximately 7,500 N is supported by design concepts (c) and (d) before yielding is observed.
The FDM process cannot produce sharp corners due to geometry of the filament used for generation of each layer. Therefore, this additive manufacturing process is beneficial to avoid stress concentration regions in the printed intervertebral lumbar cages. In addition, the FEM analysis shows that the stress concentration areas are quite localized. Therefore, it can be concluded that the high loads supported by the different design concepts are due to a combination of (a) localized stress concentration regions, (b) additive manufacturing process that intrinsically reduces stress concentration geometries, and (c) ductile nature of the PC-ISO material.
In order to further validate the proposed design concepts, additional testing under dynamic conditions is needed in order to assess the fatigue response of the material. In terms of the design concepts, additional refinements are required in order to reduce the amount of stress concentration due to sharp changes in geometry.
This work has shown the viability of FDM-printed intervertebral lumbar cages based on biocompatible polycarbonate (PC-ISO material). Several design concepts are proposed for the generation of patient-specific intervertebral lumbar cages. Finite Element Analysis and compression testing show the viability of the proposed design concepts. The part interior style has a more significant influence than the build direction of the material deposition. Furthermore, PC-ISO material showed a high repeatability in the manufacture process for transverse-vertical build direction and solid part interior style parameters, achieving compressive yield strength of 55 MPa. The literature recommends a reference load of 4,000 N for design of intervertebral lumbar cages. Under compression testing conditions, the FDM-printed intervertebral lumbar cages withstand between 7,500 and 10,000 N of load before showing yielding.
Further research must be carried out with in vitro and in vivo testing in order to guarantee the full viability of the intervertebral lumbar cage implants that have been proposed in this work. Specifically, the validation should be carried out through the analysis of biocompatibility and osseointegration with the surrounding tissue, towards the full integration of 3D printed implants in the medical practice. The benefits of using 3D printing for each specific patient should potentially increase the ergonomics, simplify the procedure, and bring overall better personalized results once validated.
The authors declare that there are no competing interests regarding the publication of this paper.
The authors would like to acknowledge the support of Tecnológico de Monterrey through its Research Group in Advanced Manufacturing. Support was also provided from the MADiT “National Lab of Additive Manufacturing, 3D Digitizing and Computerized Tomography” at Universidad Nacional Autónoma de México and from Centro de Tecnologia da Informação Renato Archer. Finally, the authors of this work would like to express their gratitude to Jan Lammel Lindemann, for his valuable comments.