Biomimetic Design Of Fatigue-Testing Fixture For Artificial Cervical Disc Prostheses Ⅱ

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-plate05 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.

Figure 2.The deformation of pure Ti DCI within human C5–C6 cervical segments under the flexion condition

Click Here To Get To Know Cistanche Anti-fatigue Treatment

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 fifixture under the flflexion condition are shown in Figure 5. The maximum equivalent stress of pure Ti DCI within the fatigue-testing fifixture 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 was similar to those of the DCIs within the C5–C6 cervical spinal segments, as plotted in Figure 6.

Figure 3. Influence of the U-plate's elastic modulus and size on the DCI's maximum deformation.(a) U-plate thickness and width were fixed at 1 mm and 30 mm; (b) elastic modulus and width at 70,000 MPa and 30 mm; (c) elastic modulus and thickness at 70,000 MPa and 1 mm.

Figure 5. The contours of the equivalent stress of pure Ti DCI under the flexion condition: (a) within C5–C6 cervical spinal segments; (b) within the fatigue-testing fixture. 

3.3. Fatigue Simulation and Fatigue Experiment 

Figure 7 shows the contours of the fatigue life of pure Ti DCI within the C5–C6 cervical spinal segments and within the optimized fatigue-testing fifixture under the flexion condition. The minimums of the simulated fatigue life of pure Ti DCI within the C5–C6 cervical spinal segments and within the fatigue-testing fifixture were 22.397 million cycles (N = 107.3502 = 22,397,000) and 21.478 million cycles (N = 107.332 = 21,478,000), respectively. The fatigue results of the DCIs of pure Ti and Ti6Al4V within the C5–C6 cervical spinal segments and within the fatigue-testing fifixture were simulated under various loading conditions, as summarized in Table 2. The simulated fatigue life of titanium alloy exceeds 80 million cycles during flexion. In the process of extension and lateral bending, the simulated fatigue life of both pure titanium and titanium alloy exceeds 80 million times. 

The fatigue-testing results of the DCIs of pure Ti and Ti6Al4V within the fatigue-testing fifixture were obtained under various loading conditions by using an Instron-8874 fatiguetesting machine, as plotted in Table 2. It was shown that the experimental fatigue life of pure Ti DCI within the fatigue-testing fifixture under the flexion condition was 35.645 million cycles, whereas the fatigue lives of pure Ti DCIs under other experimental conditions, as well as Ti6Al4V DCIs under all experimental conditions, were more than 80 million cycles.

Figure 6. The maximum equivalent stress of DCI within C5–C6 cervical segments and within the fatigue-testing fifixture in (a) flflexion, (b) extension, and (c) lateral bending movements. 

Figure 7.The contours of the fatigue life of pure Ti DCI under the flexion condition: (a) within C5–C6 cervical spinal segments; (b) within the fatigue-testing fixture

4. Discussion

Human physiological motion is complicated, comprehensive, and cooperative, which is diffificult to represent accurately. However, its main functions are highlighted by mimick ing the main biological structures and control principles of human cervical vertebrae. 

4.1. The Rationality of the Static Load Settings 

Notably, 200 N is also the routine maximum fatigue compressive force applied in the dynamic tests of ACDs according to ASTM F2346 [21,29,30]. In order to bridge the loading parameters in biomechanical tests with cervical spines from cadaver donors and in static and dynamic tests following ASTM F2346, an extra equivalent moment can be obtained by finely adjusting the eccentric distance between the centers of the cuboid block 01 and ACD during the finite element simulations of the static and fatigue experiments. For example, in the flexion movement, a 1.8 Nm flexion moment and 73.6 N preload with a 6 mm eccentric distance between the center of the implant position of ACD and the center of C5–C6 cervical spinal segments were applied onto the top surface of C5; therefore, the comprehensive loading moment was 73.6 N × 6 mm + 1.8 Nm = 2.242 Nm. According to ASTM F2346 test methods, a similar comprehensive load in the fatigue-testing fixture can be achieved (i.e., 200 N × 11.2 mm = 2.240 Nm) only by adjusting the eccentric distance between the force loading position (cuboid block 01) and the center of ACD to 11.2 mm [29,33]. Likewise, identical comprehensive loads can be obtained for either extension or lateral bending movements. 

Meanwhile, the extra equivalent moment was also stable in the process of motion due to the tiny deformation of ACDs in the fatigue test. In consideration of experimental conditions and the various testing requirements of ACDs, the above loading methodology is not merely reasonable but also easily achieved.

4.2. Optimization of the Biomimetic Fatigue-Testing Fixture 

When the elastic modulus of U-plate 05 reached 70,000 MPa, the deformation of the DCI was 0.57 mm, which is the same value as that of the DCI with human C5–C6 cervical spinal segments, as shown in Figure 3a. Among a variety of candidate materials, 6061 Al alloy was the most suitable. Simultaneously, the optimized thickness and width of the U-plate 05 were confirmed as 1 mm and 30 mm, respectively, according to Figure 3b,c. The geometric sizes of the cuboid block 01 and the cylindrical blocks 02–04 had no obvious influence on the DCI’s maximum deformation, as shown in Figure 4. Therefore, the sizes of the blocks were determined according to those of the cervical vertebrae, while the distance between the center of the cylindrical blocks 02–04 and the rear end of the U-plate 05 should be inclined to that between the intervertebral disc and ligaments.

The optimized parameters of the biomimetic fatigue-testing fifixture are as follows: 6061 Al alloy is suitable for the U-plate 05; the thickness and width of the U-plate 05 are 1 mm and 30 mm; the hydroxyapatite-filled epoxy block 01 is a 25 mm long, 10 mm wide and 10 mm high cuboid; the radius and height of the hydroxyapatite-filled epoxy cylindrical blocks 02–04 are 12 mm and 10 mm; and the distance between the center of the cylindrical blocks 02–04 and the rear end of the U-plate 05 is 50~55 mm. 

By employing these loading conditions and the optimized fifixture, the simulated results of the maximum equivalent stress of the DCI within the C5–C6 cervical segments and within the fatigue-testing fifixture present a series of consistencies in flexion, extension, and lateral bending movements, as shown in Figure 6. 

4.3. The Safety of the Biomimetic Fatigue-Testing Fixture 

The safety of the fifixture is its foundation and premise during long periods of cyclic loading. The simulated fatigue lives of the U-plate and the blocks were more than 80 million cycles, as shown in Figure 7. Furthermore, the failure of the biomimetic fatigue-testing fixture did not occur in the 80-million-cycle fatigue experiments. Therefore, it can be concluded that the fifixture is highly safe. 

4.4. The Equivalence between the Biomimetic Fatigue-Testing Fixture and the Natural Cervical Sections

As shown in Figure 5, the curved section of the DCI is prone to forming crack sources due to large stresses; the cracks could propagate gradually during long periods of cyclic loading and finally cause the fatigue fracture of the DCI when the cyclic times accumulate beyond its fatigue life. 

Under the circumstance that the equivalent stress and deformation of the DCI within C5–C6 cervical segments are almost the same as those within the biomimetic fatigue-testing fifixture, the results of both fatigue simulations coincide well with the experimental results. The calculated fatigue lives of DCI within the C5–C6 cervical spinal segments and within the fatigue-testing fifixture were 22.397 million cycles and 21.478 million cycles, respectively, which agree well with the experimental fatigue life of 35.645 million cycles, as shown in Table 2. It is noticeable that the simulated fatigue lives and possible sites for the fatigue failure of pure Ti DCI were almost the same whether it was fixed within C5–C6 cervical spinal segments or within the fatigue-testing fifixture, as shown in Figure 7.

In brief, the prostheses within the fatigue-testing fifixture under the loads according to ASTM F2346 can achieve a functionally equivalent result to that under the biomechanical loads within normal cervical vertebrae. 

4.5. Limitations of the Biomimetic Fatigue-Testing Fixture

The present study of biomimetic fatigue-testing fixtures has two limitations. Firstly, actual cervical movement includes not only flexion, extension, lateral bending, and axial torsion but also the combinations of single movement patterns within the scope of physiology. The biomechanical axial torsion and preload cannot be equivalent to the moment, which relates to the compressive force perpendicular to the surface of cuboid block 01, because they are not in a common plane. Therefore, the biomimetic fifixture cannot meet the requirements for the torsion condition. Additionally, the biomimetic fatigue-testing fifixture is only suitable for single load patterns, such as flflexion, extension, and lateral bending. Unfortunately, single load patterns may be considered to be clinically unrealistic.

Secondly, muscle forces in spinal motions should not be neglected. Muscles in the loading spine generate spinal reaction forces, which can occupy the main portion of the total axial compression and shear forces on the spine, further affecting the life of ACD prostheses [34]. Simultaneously, posterior muscles can assist with balance in flflexion postures; accordingly, anterior muscles act in the same role in extension postures. They can reduce the reaction forces of the lower joints and keep the spine steady [35]. These aspects are signifificant for ACD prostheses, especially when dynamic or impact loading is applied [36,37]. Unfortunately, synergism among the muscles is difficult to investigate because specimens in vitro cannot mimic the role of muscles well [34].

5. Conclusions

In summary, a novel specimen fixture has been designed for testing the fatigue behavior of ACD prostheses with aspects of both structural and functional bionics. The equivalence between the designed biomimetic fixture and the natural cervical sections has been verified by numerical simulations and mechanical experiments. This biomimetic fatigue-testing fifixture represented the biomechanical characteristics of normal human cervical vertebrae with considerable accuracy. The novel specimen fixture provides a convenient and accurate way to research and evaluate the fatigue behavior of ACD prostheses. 

References 

1. Hilibrand, A.S.; Robbins, M. Adjacent segment degeneration and adjacent segment disease: The consequences of spinal fusion? Spine J. 2004, 4, S190–S194. [CrossRef] [PubMed] 

2. Matsumoto, M.; Okada, E.; Ichihara, D.; Watanabe, K.; Chiba, K.; Toyama, Y. Adjacent segment disease and degeneration after anterior cervical decompression and fusion. Neurosurg. Q. 2010, 20, 15–22. [CrossRef] 

3. Fiani, B.; Nanney, J.M.; Villait, A.; Sekhon, M.; Doan, T. Investigational research: Timeline, trials, and future directions of spinal disc arthroplasty. Cureus 2021, 13, 16739. [CrossRef] [PubMed] 

4. Joaquim, A.F.; Makhni, M.C.; Riew, K.D. Evidence-based use of arthroplasty in cervical degenerative disc disease. Int. Orthop. 2019, 43, 767–775. [CrossRef] [PubMed] 

5. Lee, H.; Lee, M.; Han, G.; Kim, H.; Song, J.; Na, Y.; Yoon, C.; Oh, S.; Jang, T.; Jung, H. Customizable design of a multiple-biomolecule delivery platform for enhanced osteogenic responses via ‘tailored assembly system’. Bio-Des. Manuf. 2022, 5, 451–464. [CrossRef]

6. Lee, H.; Lee, M.; Cheon, K.; Kang, I.; Park, C.; Jang, T.; Han, G.; Kim, H.; Song, J.; Jung, H. Functionally assembled metal platform as lego-like module system for enhanced mechanical tunability and biomolecules delivery. Mater. Design. 2021, 207, 109840. [CrossRef]

7. Gloria, A.; Causa, F.; De Santis, R.; Netti, P.A.; Ambrosio, L. Dynamic-mechanical properties of a novel composite intervertebral disc prosthesis. Mater. Med. 2007, 18, 2159–2165. [CrossRef]

8. Rosa, G.L.; Clienti, C.; Corallo, D. Design of a new intervertebral disc prosthesis. Mater. Today Proc. 2019, 7, 529–536. [CrossRef] 

9. Barker, J.B.; Cronin, D.S.; Chandrashekar, N. High rotation rate behavior of cervical spine segments in flflexion and extension. J. Biomech. Eng. 2014, 136, 121004. [CrossRef] 

10. Nuckley, D.J.; Linders, D.R.; Ching, R.P. Developmental biomechanics of the human cervical spine. J. Biomech. 2013, 46, 1147–1154. [CrossRef] 

11. Lou, J.; Hao, L.; Li, Y.; Orthopedics, D.O.; Hospital, W.C.; University, S. Biomechanical evaluation of one new artificial cervical disc prosthesis. Orthop. Biomech. Mater. Clin. Study 2016, 13, 10–13.

12. Patwardhan, A.G.; Havey, R.M. Prosthesis design influences segmental contribution to total cervical motion after cervical disc arthroplasty. Eur. Spine J. 2020, 29, 2713–2721. [CrossRef]

13. Toen, C.V.; Melnyk, A.D.; Street, J.; Oxland, T.R.; Cripton, P.A. The Effects of Lateral Eccentricity on Failure Loads and Injuries of the Cervical Spine in Head-First Impacts. In Proceedings of the Ohio State University’s 10th Annual Injury Biomechanics Symposium, Columbus, OH, USA, 18–20 May 2014; p. 18. 

14. Rezaei, A.; Giambini, H.; Carlson, K.D.; Xu, H.; Lu, L. Mechanical testing setups affect spine segment fracture outcomes. J. Mech. Behav. Biomed. 2019, 100, 103399. [CrossRef] 

15. Rahm, M.; Brooks, D.; Harris, J.; Hart, R.; Hughes, J.; Ferrick, B.; Bucklen, B. Stabilizing effect of the rib cage on adjacent segment motion following thoracolumbar posterior fixation of the human thoracic cadaveric spine: A biomechanical study. Clin. Biomech. 2019, 70, 217–222. [CrossRef]

Ask for more:

Email:wallence.suen@wecistanche.com 

Whatsapp/Tel:+86 15292862950