Biomimetic Design Of Fatigue-Testing Fixture For Artificial Cervical Disc Prostheses Part 1
Sep 12, 2023
Abstract: To investigate the biomechanical performances of artificial cervical disc (ACD) prostheses, many studies have been conducted, either with cervical sections of cadavers under physiological loads or with block-like testing fixtures obeying the ASTM F2346 standard. Unfortunately, both methods are almost impossible to utilize for accurate results of lifetime anti-fatigue experiments for at least 10 million cycles due to the difficulties in cadaver preservation and great deviations of natural cervical bodies, respectively. Based on normal human cervical structural features, a novel specimen fixture was designed for testing the fatigue behavior of ACD prostheses under flexion, extension, and lateral bending conditions, with aspects of both structural and functional bionics. The equivalence between the biomimetic fatigue-testing fixture and the natural cervical sections was investigated by numerical simulations and mechanical experiments under various conditions. This study shows that this biomimetic fatigue-testing fixture could represent the biomechanical characteristics of the normal human cervical vertebrae conveniently and with acceptable accuracy.
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Keywords: artificial cervical disc (ACD); specimen fixture; fatigue behavior; biomimetic
1. Introduction
Disc arthroplasty is a new surgical treatment for intervertebral degeneration and instability. Compared with traditional cervical fusion surgery, its advantages are that it restores the range of motion of the cervical spine and can lower the incidence of adjacent segment degeneration in the long term [1–4]. ACD prostheses are intended to bear alternating loads within the scope of physiology and should theoretically last for several decades in the body without failures. The life of ACD prostheses and their minimum acceptable clinical life are disputed, however. Titanium and titanium alloys are widely used in orthopedic hard tissue repair and artificial cervical intervertebral disc manufacture because of their good biocompatibility and non-toxicity [5,6]. The optimum life span has generally been demonstrated to be 80 million movements, while 10 million movements are suggested to be the ideal minimum testing cycle [7,8].
Traditional implant trials often choose a series of cervical spine specimens from cadaver donors moisturized with saline solution. Cervical spine specimens are dissected free from soft tissues, musculature, and single-segmental cervical intervertebral discs, while the ligaments and integrity of post-zygapophysial joints are carefully preserved, and then ACD prostheses are implanted for testing [9–12]. These specimen tests must be completed in a short time to avoid causing side effects in the process of the biological disintegration of cadavers [13–17]. Unfortunately, it is almost impossible to carry out a lifetime anti-fatigue experiment for at least 10 million cycles with fresh-frozen cadavers due to the time and cost limitations of cadaver preservation.
In dealing with such problems, the tests conducted by ASTM F2346 allow for the analysis of individual disc replacement devices and the comparison of the mechanical performance of multiple artificial intervertebral disc designs in a standard model [18–23]. Specialized test fixtures have been developed by leading machine manufacturers to conduct both static and cyclic testing of ACDs following the ASTM F2346 standard in recent decades. Due to the obvious difference between these test fixtures and the natural cervical spine, the obtained data about the static and dynamic behaviors of ACD prostheses are less accurate. Therefore, the results from these tests may not directly predict in vivo performance; however, they can be used to compare the mechanical performance of different ACD prostheses [19,20].
In dealing with the ultra-high cost and inevitable data deviation of the above-mentioned testing methods, the biomimetic methodology could be a promising solution for testing ACD prostheses with better accuracy and efficiency.
The objective of this study was to design a biomimetic fatigue-testing fixture using synthetic materials similar to human cervical vertebrae for cost-effective, accurate static and dynamic tests of ACDs, to evaluate the equivalence of the stresses and deformations (i.e., range of motion) of ACDs within the designed fatigue-testing fixture, and within C5–C6 cervical spinal segments. Fatigue simulations of ACDs within C5–C6 cervical spinal segments and within the fatigue-testing fixture and fatigue experiments of ACDs within the fatigue-testing fixture were also carried out. By comparing results, the feasibility of the designed biomimetic fatigue-testing fixture can be discussed thoroughly.
2. Materials and Methods
2.1. Biomimetic Design of Fatigue-Testing Fixture
2.1.1. Structural Bionics of Fatigue-Testing Fixture
The design of the biomimetic fatigue-testing fixture is shown in Figure 1. Based on normal human cervical structural features, four epoxy blocks filled with 70 wt% hydroxyapatite powder, the same content as natural bone, were used to simulate human cervical vertebrae, as the elastic modulus of epoxy blocks is close to that of human cervical vertebrae. A metallic flexible U-plate of the fatigue-testing fixture limited the movement of blocks to simulate the function of normal human cervical ligaments and facet joints.
In terms of the size measurement and deformation properties of the C5–C6 cervical spinal segments, as shown in Figure 1, the fixture is cost-effective and reasonable, composed of a cuboid block (01), three cylindrical blocks (02–04) and a U-plate (05). Among blocks 01–04, the cylindrical blocks 02–04 are concentric, whereas the position of cuboid block 01, as the site for compressive force to be applied, can be varied to form different loading conditions. According to preliminary estimations, the thickness and width of the U-plate (05) were 1~2 mm and 30~45 mm. The length, width, and height of the cuboid block (01) were 25~35 mm, 8~15 mm, and 8~15 mm. The radius and height of the cylindrical blocks (02–04) were 10~15 mm and 8~15 mm. The distance between the center of the cylindrical blocks (02–04) and the rear end of the U-plate (05) was 45~65 mm. Using numerical simulation analysis, the fixture design was further optimized. Additionally, the lower surface of the cylindrical block (02) and the upper surface of the cylindrical block (03) were polished to meet the body's normal physiological curvature of the cervical lordosis.
The fixture was able to work with the integration prostheses and the majority of other ACD prostheses, such as Dynamic Cervical Implant (DCI), Z-braze Dynamic Fusion, and Prestige LP, as shown in Figure 1. Among various ACDs, DCI is a single-component U-shaped implant maintaining the spinal kinematics, which imposes minimum influence on the adjacent soft tissues compared with other types of prostheses [24]. The U-shaped open structure of the DCI is more favorable for the direct observation of static and dynamic behaviors during the numerical simulations of implants. Therefore, DCI (14 mm long, 16 mm wide, and 0.8 mm thick) was selected as the ACD specimen for the study of the effectiveness of the designed biomimetic fatigue-testing fixture in substitution of the natural cervical spine in static and dynamic experiments.
2.1.2. Functional Bionics of Fatigue-Testing Fixture
The geometric models of the fatigue-testing fixture with DCI and the cervical bodies with DCI were established, which were input into the finite element analysis software ANSYS Workbench 16.0 (Ansys, Canonsburg, PA, USA) and assigned with corresponding material properties, as shown in Table 1 [25,26].
In the numerical calculation of the equivalent stress and deformation of DCI within the C5–C6 cervical spinal segments during the static test, the maximum routine loading parameters in biomechanical tests with cervical spines from cadaver donors were followed; namely, a 73.6 N preload was applied to the top surface of C5, with an extra 1.8 Nm flexion moment for flexion movement and a 1.8 Nm extension moment for extension or a 1.0 Nm lateral bending moment for bending, respectively, while the bottom surface of C6 was fixed in six degrees of freedom in the finite element model [26–28].
As for the simulations of the static test with the biomimetic fatigue-testing fixture, a 200 N load, the routine loading force in static and dynamic tests following ASTM F2346, was applied on the upper surface of the cuboid block 01, while the lower surface of the cylinder 04 was fixed in six degrees of freedom [19,29,30]. Furthermore, by finely adjusting the distance between the centers of DCI and the cuboid block 01, an extra equivalent moment can be obtained by multiplying the force on the top surface of the cuboid block 01 and the eccentric distance. Finally, the equivalent stress and deformation of DCI within the fatigue-testing fixture were calculated by using finite element simulation software ANSYS Workbench 16.0 (Ansys, Canonsburg, PA, USA). The equivalent stress and deformation of DCI within the fatigue-testing fixture can be made similar to those within human C5–C6 cervical spinal segments through further optimization of the parameters of the biomimetic fatigue-testing fixture.
2.2. The Process of Fatigue Simulation and Fatigue Test
The previous analysis results of DCI within C5–C6 cervical spinal segments and within the optimized fatigue-testing fixture were input into the FE-SAFE6.5 software (Dassault Systèmes Simulia Corp., Providence, RI, USA). According to the estimation methods of Seegers’ material data in FE-SAFE6.5 software, SN curves were generated by inputting the tensile strength and elastic modulus, which were modified afterward [31,32]. A frequency of 10 Hz was applied with a triangular-wave load channel file. The Goodman method was used for mean stress correction, and the fatigue full-life analysis was carried out according to SN curves. After the fatigue calculation, the analysis results were re-input into ANSYS Workbench 16.0 (Ansys, Canonsburg, PA, USA) for post-processing.
In the present fatigue experiments, the DCI was fixed between the cylindrical hydroxyapatite-filled epoxy blocks 02 and 03 by epoxy AB glue in the biomimetic fatigue-testing fixture of the optimized parameters. It is used to simulate the bony fusion between the upper and lower surfaces of the DCI prosthesis and the corresponding vertebral body after the implant. The loading parameters of the cervical segment biomechanical tests were consistent with static and dynamic tests based on ASTMF2346 by adjusting the distance between the geometric center of the block and the rotating center of the artificial cervical disc. Then, the fixture, together with the DCI, was clamped between the vise and actuator of an Instron-8874 fatigue testing machine (Instron Corporation, Canton, MA, USA), as shown in Figure 1. During fatigue experiments, the fixture was loaded with the corresponding loadings at the calculated eccentric position to provide different moments. Finally, fatigue tests were carried out until fatigue failure occurred; if not, the tests were continued until 80 million cycles were reached on the biomimetic fatigue-testing fixture.
3. Results
3.1. Optimization of the Biomimetic Fatigue-Testing Fixture
The maximum deformation of pure Ti DCI within human C5–C6 cervical segments under the flexion condition was numerically calculated as 0.57 mm, as shown in Figure 2. On the foundation that the stress of the DCI within the fatigue-testing fixture is similar to that within C5–C6 cervical spinal segments, the deformations of pure Ti DCI within the designed fatigue-testing fixture of different parameters were numerically calculated and shown in Figures 3 and 4. With the other factors being equal, the maximum deformation decreased markedly with an increase in the elastic modulus of the material and the thickness and width of the U-plate 05, respectively, as shown in Figure 3, whereas the length, width, and height of the cuboid block 01, as well as the radius and height of the cylindrical blocks 02–04, lacked an obvious influence on the DCI’s maximum deformation, as shown in Figure 4a–e. Additionally, the DCI’s maximum deformation increased slowly as the distance between the center of the cylindrical blocks 02–04 and the rear end of the U-plate 05 increased, as shown in Figure 4f. Likewise, the influencing tendencies of the abovementioned factors on the deformation of DCI under the flexion condition coincide with those of DCI under either extension or lateral bending conditions. Finally, the optimizations of the various designing factors were conducted by keeping the coincidence of the stresses and deformations of the DCI within the designed biomimetic fatigue-testing fixture and within human C5–C6 cervical segments.
3.2. Simulation of DCI within the Optimized Fixture under Static Mode
The contours of the equivalent stress of pure Ti DCI within the C5–C6 cervical spinal segments and within the optimized fatigue-testing fixture under the flexion condition are shown in Figure 5. The maximum equivalent stress of pure Ti DCI within the fatigue-testing fixture was 396.5 MPa, which agreed well with 394.6 MPa, the result of DCI within the C5–C6 cervical segments. More importantly, both maximum equivalent stresses appeared in the same location of the DCI. Furthermore, the contours of the equivalent stress of the DCIs of pure Ti and Ti6Al4V were simulated under various loading conditions within the fatigue-testing fixture, which were similar to those of the DCIs within the C5–C6 cervical spinal segments, as plotted in Figure 6.
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