Design of a Transforaminal Lumbar Interbody Fusion (TLIF) Spine Cage

Lumbar Interbody Fusion is a technique used to treat various spinal disorders, which has many types, such as the Transforaminal Lumbar Interbody Fusion (TLIF) Technique. With TLIF being one of the most well-known techniques, which many spinal surgeons are trained and skilled at, there are various types of TLIF Spine Cages available on the market. In this paper, we designed a TLIF Cage and compared the simulation's analysis with the prototype's experimental testing. The design was developed using the reverse engineering method, and findings on the jaws profile and other design considerations through literature review. The design was then analyzed through a simulated compression test using Ansys Software. The simulation showed that the designed TLIF spine cage in this paper can withstand the force usually given to an implanted lumbar spinal cage.

3D Design; Interbody Fusion; Spine Cage; TLIF implant

        There are several spine cage implant options available that differ in their geometry depending on the approach for insertion. First is Posterior Lumbar Interbody Fusion (PLIF) which many spinal surgeons are well-trained to use. It provides better nerve root visualization than other cages. But it also requires high neural retraction. Then there is Anterior Lumbar Interbody Fusion (ALIF) as the most efficacious and predominant treatment for discogenic low back pain which can maximize implant size and surface area. But ALIF insertion surgery can cause some complications, such as retrograde ejaculation, and visceral and vascular injury. For sagittal and coronal deformity correction, lateral lumbar interbody fusion (LLIF) and oblique lumbar interbody fusion (OLIF) are suitable. Both cages can be performed with rapid postoperative mobilization and aggressive deformity correction. These cages, unfortunately, can cause lumbar plexus, psoas, bowel, and vascular injuries. The last cage type is Transforaminal Lumbar Interbody Fusion (TLIF).  This technique is the best for stabilization and treatment of degenerative lumbar disease following failed conservative treatment (Rahyussalim et al., 2017). Despite the disadvantages, such as paraspinal iatrogenic injury from prolonged muscle retraction, it can still perform direct and unilateral access to the intervertebral foramen and preserve ligamentous structures. This access can reduce direct dissection, the prior chance of damaging back muscles and the thecal sac, minimize bleeding, and improve postoperative recovery (Hammad et al., 2019; Mobbs et al., 2015). The neural foramen is also opened on one side only, so damage to the nerves is less compared to the other technique (Mobbs et al., 2015). With all those advantages, TLIF has become one of the most commonly used techniques for Disc Degeneration Disease treatment and is worth developing.

2.1. Determining the Size and 3D model of the TLIF Spine Cage

where:

        UVW        : upper vertebral width

        UVD         : upper vertebral depth

        DH           : disc height

        PH            : pedicle height

        PDW        : pedicle width

The UVW, UVD, DH, PH and PDW will be acquired from obtained data from previous research. 

2.2. Prototyping of the TLIF spine scage

    In order to execute the 3D design of the spine cage, an additive manufacturing technique commonly known as stereolithography (SLA) was conducted. A Photon Mono X (Anycubic, Shenzhen, China) with LCD-based SLA technology that has faster printing speeds and a larger volume was utilized. The previously determined solid model file in.stl format was now ready to be transferred to the SLA machine. The Photon Mono-X machine uses a bio-photopolymer resin mixed with bio-poly lactic acid from eSUN (Shenzhen, China). The PLA-resin has a low viscosity and has mechanical properties as detailed in Table 1.

Table 1 Mechanical Properties of eResin-PLA at 25?C

2.3. Numerical simulation of TLIF spine cage

      A finite element analysis (FEA) was conducted using ANSYS 2022 workbench software (2022 R2 version, Canonsburg, Pennsylvania, USA). In the FEA simulation, the spine cage was assumed to be isotropic (Ahmad et al., 2020). The material that is being simulated for the spine cage is polyetheretherketone (PEEK). The data shows that this material has a compressive strength of 120-300 MPa (depending on the molecular weight) and elongation of a break at around 1.6-43%.

    The environmental simulation needed the mechanical properties of the PLA-resin materials. Therefore, a compressive test was conducted to acquire the mechanical parameters of the PLA-resin material. A 3D printed block with the size of 20mm x 10mm x 10mm was prepared as the testing material. The compression was conducted using the universal testing machine MCT-2150 from A&D Company (Tokyo, Japan). The compression rate was set at 10 mm/min.

3.1. Size Determination and 3D Model of the Spine Cage Implant

Figure 1 The image processing result of Indonesian Lumbar dimension in sagittal plane (a) and axial plane (b)

Table 1 Lumbar Dimension Indonesian Lumbar dimension in sagittal plane (a) and axial plane (b)

        Spine cage sizes were derived from the dimensions of the above spine section. Note that the spine cage was not designed to cover all the spine areas. The spine cage was designed to be as small as possible compared to that spine section, but the cage must withstand the load from the human body. Based on our study, we converted the dimension of TLIF spine cages to adapt a minimum insertion size at around 8 mm. The formula to calculate the length (L), Width (W), Height (H), and Lordosis Angle (LA) of the TLIF spine cages are given in Equations 1, 2, 3, and 4 based on L4 sizes. The L was calculated to adjust the length area of the cages that can cover the spine area. The W was arranged from the PDW size as the insertion side of the cage. The LA was formulated from the measure of the lordotic angle between two lumbar bodies.

        Besides the dimension and angle of the cage, it is known that several factors might influence the biomechanical stability of a lumbar interbody spine cage construct, such as geometry, contact area, and integrated fixation (Triwardono et al., 2021). Therefore, in this paper, we designed a banana-shaped spine cage with a slanted side to facilitate TLIF placement. Based on the lumbar morphometry, formulas, and those biomechanical stability factors, geometry, and sizes of the spine cage design are shown in Table 2.

Table 2 Design Fixture of Spine Cage

        By following the lumbar morphometry and biomechanical stability factors in Table 2, the TLIF spine cage design is shown in Figure 2. The spine cage consists of a vertical middle hole and two horizontal holes to insert bone graft materials (Figure 2c). The banana-shaped facilitated the placement of the implant through the posterior side. The designed implant has slightly different measurements compare to those available on the market. The length of the designed implant was 27.7 mm and had an angle of 7.27 degrees.

Figure 2 The 3D design of spine cage considering the adjustment of size and features of bone graft holes and pyramid jaws: a) top view; b) side view and perspective view

The fixation for this implant is designed to use a pedicle screw fixation, as an integrated screw fixation is usually not enough to give the biomechanical stability needed for a spine cage implant. For the contact area, the profile of the jaws was made to increase friction and limit the micromotion of the spine cage. Increasing the surface roughness of the spine cage is suitable for fixating the implant to the bone (Triwidodo et al., 2021). Therefore, we designed the surface area equipped with a pyramid jaws profile to gain a better osteogenic process.  with bone graft than a simple one-type jaws profile design. The pyramid jaws are depicted in Figure 2c.

3.2. Prototyping of the TLIF Spine Cage

The prototyping was conducted using additive manufacturing technology that involves stereolitography of liquid resin. The machine realized the structure as a predetermined design in the 3D model file. The fabrication result was depicted in Figure 3, with an accuracy of around less than 0.5 mm according to the technical specification. Moreover, the realized geometry indicated that the deviation was less than 1 mm compared to the design dimension. It can be concluded that this additive manufacturing technique can be used as a reference model or a prototype of the implant before its transfer to industrial scale. (Syuhada et al., 2018).

Figure 3 TLIF design printing with PLA Resin: a) top view and b) side view

3.3. Numerical Simulation of TLIF Spine Cage

The numerical simulation predicts that our design would withstand applied loading on the spine cage. The simulation calculates the peak of von Mises stress (PVMS) value as the failure criteria of the selected material of the spine cage (Izmin et al., 2020). Moreover, this study also ensures the safety design of the spine cage with our geometrical arrangement. The stress visualizations of the spine cage simulation are shown in Figure 4. Here, a force of 500 N was applied to the spine cage in an axial direction, following the highest possible loading of the human body (Alief et al., 2019).

Figure 4 Finite Element Analysis results for spine cage implant: a) Total Deformation - Compression and b) Von Misses – Compression

The simulation results were summarized further in Table 5 to include the maximum point of the spine case during the loading scenario. As shown in Table 5, the von misses results are far below the tensile strength of the simulated material, i.e. PEEK. It suggested that the implant geometry and material successfully support the spinal movement (figure 4a). Also, in torsion and shear simulations, the result does not indicate the failure of the spine cage implant (table 5). A relatively small deformation value from the results also indicate that the implant will be able to function properly (figure 4b). Since compression, tensile, shear, and torsion tests only indicate a simple vertical movement of the spine.

Table 3 Differentiation of Lumbar Interbody Fusion Implant

3.4. Validation of the model

        A comparison of numerical simulation and the experimental setup is needed to validate the numerical study in our previous section. We simulate a compressive test of the realized resin-PLA spine cage in the finite element environment. This phenomenon was followed by compression using the universal testing machine.  Figure 5 shows the Finite Element Analysis of the sample block in terms of its total deformation and calculated von Misses Stress. 

Figure 5 Finite Element Analysis results for PLA sample block: a) Von Misses and b) total deformation in compression testing mode

The compressive test on a 3D printed acquire the young’s modulus value at around 55.6 MPa. This value was confirmed by our previous study (Supriadi et al., 2021, Saseendran et al., 2017). A 500 N vertical compression and tensile force were given from the top surface to calculate Von Mises and deformation of the implant (He et al., 2021). Shear stress with 200 N and 25 Nm torsion was also evaluated in this implant simulation (Krijnen et al., 2006; Pitzen et al., 2000). Table 4 shows the mechanical parameters in the Ansys simulation software.

Table 4 Parameters for Finite Element Analysis

Density (g/mm3)	Young’s Modulus (MPa)	Poisson’s Ratio	Reference
1.13	55.6	0.35	(Saseendran et al., 2017)

Figure 6 Compressive test result for PLA sample block compare with the numerical calculation of elastic modulus

     Figure 6 presents the experimental result of block test (with four repetitions of samples). It showed its maximum strength at around 30 MPa. The numerical simulation gives the dotted line in Figure 6. The line was projected from the force-displacement relation from the environmental simulation (Figure 5). It showed a deviation of 9% compared to the experimental result. Consequently, it can be suggested that the simulation result of spine cage has a 9% gap mostly in the elastic region (red line area in Figure 6).

        A TLIF spine cage based on Indonesian morphometry with a 28 x 9 x 11 mm dimension was designed with a pyramid-shaped jaw profile, multiple holes as a space for bone graft, and a thread hole for insertion of the implant into the disc space.  We performed a finite element analysis simulation with the Ansys software, performing compression and tensile tests to stimulate the stress impacted on the implant. Shear and torsion test was also simulated in this research. This simulation was done to test the strength of the design using PLA Resin material. The results showed that the design is strong enough to withstand the force given. Our numerical study also showed that a deviation around 9% between experimental loading and numerical calculation might occurred. However, it is believed that this gap between experimental and realization might give important information when the candidate material, PEEK, will be used in the future application. 

    The authors would like to acknowledge the Matching Fund Grant No.279/PKS/WRIII-DISTP/UI/2022 from Kementerian Pendidikan, Kebudayaan, Riset, dan Teknologi.

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