As a novel “compliant” material, PEEK rods are available as substitutes for traditional Ti rods [15]. Although some studies have demonstrated that PEEK rods have excellent performance, there is still a lack of sufficient biomechanical evidence to support the use of PEEK rods in TLIF. In our study, we found that PEEK rods in TLIF played a positive role in restoring normal motion and reducing stress shielding. However, the PEEK rods also simultaneously increased the risk of rod fracture, endplates collapse, and cage failure.
The L4/5 segment was chosen for fusion and fixation because it has the highest clinical incidence (58.3%), which is determined by its anatomical structure and physiological function[38]. It is generally accepted that the aim of a lumbar surgery, including TLIF, is to provide stability after decompression, essentially sacrificing part of the ROMs for symptom improvement. In our study, the ROMs of the L4/5 segment for all surgical models were decreased by 41.1–69.5% compared to that of the intact model. Compared to the Ti rod rigid fixation models, the PEEK rod semi-rigid fixation models increased the ROM at the L4/5 segment by 0.7–20%. It was suggested that PEEK rods could not only stabilize the movement of the spine to a great extent but also allow for more ROMs than that of Ti rods. In cadaveric biomechanical testing, Gornet et al. also showed that there was no significant difference in stability between PEEK rods and Ti rods [14]. Yeager et al. concluded that the stability of PEEKs rod was similar to that of Ti rods in vitro [26]. Moreover, we found that the change in the ROMs caused by PEEK rods during lateral bending and rotation was greater than that created during flexion and extension. We inferred that the axial stiffness provided by PEEK rods was similar to that of Ti rods, but the bending stiffness of PEEK rod models was lower than that of Ti rod models. It is known that axial stiffness affects flexion and extension, whereas lateral bending stiffness affects rotation and lateral bending [26]. Therefore, axial stiffness is mainly provided by the anterior column, while the posterior internal fixation is important to bending stiffness [23, 26].
ASD is a common long-term complication of lumbar fusion. Abnormal motion (quality and quantity) and intervertebral disc pressure (IDP) of adjacent segments are closely related to ASD [39]. In this study, we found that, compared to the intact model, the ROMs and intradiscal peak stresses of the L3/4 segment of all the surgical models increased by 31.8–68.0% and 18.5–39.0%, respectively. Cunningham et al. found that spinal instrumentation increased proximal IDP by as much as 45% in in vitro biomechanical testing [40]. Jin et al. showed that although both PEEK rods and Ti rods increased the inter-segmental rotation and IDP in the upper adjacent segments, the PEEK rods generated fewer changes than those of titanium rods [41]. Similarly, Nikkhoo et al. demonstrated that there was no significant difference in the ROM of adjacent segments between the PEEK rod models and the intact models, and PEEK rods reduced the disc height loss, fluid loss, and disc stresses of adjacent segments under cyclic loading compared with those of Ti rods [30]. Additionally, during flexion, the increase in disc stresses at L3/4 was highest (29.7–39.0%), and this was similar to that reported by Hsieh et al. (50%) [23]. In fact, the structural stiffness provided by both rigid and semi-rigid fixation systems is significantly greater than that of the normal spinal unit, which further changes the ROM and IDP of adjacent segments [13, 41]. The ROM increases at the adjacent level may be derived from the non-physiological center of motion, which is a compensatory change to fixed segments [29, 42]. However, this adverse compensation makes the spine’s originally complex but regular coupling movement to become irregular, resulting in facet hypertrophy, an IDP change, and altered biomechanics [42]. Remarkably, PEEK rods reduce the ASD incidence owing to their low structural stiffness and physiological load sharing [14, 17]. Furthermore, the biomechanical effect of the rigidity of the rod on the adjacent segments seems to be more important than the amount of fusion mass [41]. Athanasakopoulos et al. reported a retrospective clinical study comprising 52 patients with posterior lumbar internal fixation using PEEK rods, and no ASD was observed after a mean follow-up period of 3 years [43]. However, although the disc stresses of the PEEK rod models at the adjacent level were better than those of the Ti rod models, the difference was not significant in our study. Generally, there is still a lack of high-quality clinical studies to verify the effect of PEEK rods on ASD.
The concept of flexible fixation is reflected not only in the partial restrictions on the ROM but also in reasonable load sharing [18]. Some researchers believe that low back pain is caused by abnormal load transfer rather than an abnormal ROM [17, 29]. As expected, the PEEK rods showed superior performance in terms of stress distribution. In our study, the peak stresses of screws, rods, and bone-screw interfaces in PEEK rod models were 0–1.2 times, 1.6–4.4 times, and 0–1.4 times lower than those of Ti rod models, respectively. Fan et al. showed that PEEK rods decreased the stress of pedicle screws by 12.0–36.7% and rods by 2.5–5.6 times compared to cases where Ti rods were used; however, the ratio of peak stress to yield stress of PEEK rods (10.2–15.7%) was greater than that of Ti rods (5.1–11.1%)[27]. The ratio in our study was 7.4% for PEEK rods and 2.8–16.5% for Ti rods, which also suggested that the fracture risk of PEEK rods was high. Theoretically, PEEK rods with low elastic modulus can reduce the structural stiffness and transfer loads to the anterior column. This could unload the stress of screws and bone-screw interfaces and is especially important for patients with osteoporosis [44]. Ahn et al. showed that the PEEK rod system transmitted 27.5% of the axial compressive load, while this ratio was 66.7% in the Ti rod system [18]. Gornet et al. proved that the PEEK rod loads were at least 6% less than the titanium rod loads under all loading conditions [14].
In addition, we found that PEEK rods increased the average strain of the bone graft by 0.5–61.6% and stresses of the cage by 0.9–44.1% and those of endplates by 2.1–52.9%. As noted earlier, this was because the PEEK rods transferred load to the anterior column. Furthermore, we found that the strain of the bone graft in the periphery of the cage was greater than that in the interior of the cage, suggesting that fusion may begin from the periphery. However, the bone graft of the anterior column experienced stress, which stimulated its growth and fusion according to Wolff's law. Wang et al. found that the PEEK rod group had better fusion than the Ti rod group after posterior bone graft fusion and internal fixation in canines [45]. However, PEEK rods simultaneously increased the risk of cage failure and endplate collapse. It is well known that cages are important than posterior fixation in maintaining spinal stability because the anterior column bears approximately 75% of the load [23].
The application of the cage is to overcome issues related to intervertebral space reduction caused by autogenous bone due to its weak strength [46]. However, given the high elastic modulus of the cage and the small contact area, which should be > 30%, events of cage subsidence and endplate collapse are still worthy of careful evaluation [47]. In our study, we found that compared to cage fusion, bone graft alone fusion increased the ROMs of the fixed segment, reduced the peak stresses of the endplates, and increased the average bone graft strain. However, bone grafting alone also increased the peak stresses of screws, rods, and bone-screw interfaces. In a clinical study involving 23 patients who underwent TLIF with autologous bone graft fusion alone, Sleem et al. found that autologous bone grafts alone could also achieve satisfactory clinical outcomes [48]. Interestingly, it was noticed that bone graft alone combined with PEEK rods had superior biomechanical characteristics, especially in terms of improving the bone graft strain, endplate stress, and rod stress. However, whether the model could provide sufficient stability and strength remains to be further studied because there is still no recognized ROM and load sharing.
This study has some limitations. First, muscles were not considered in the model. Undeniably, the role of muscles is complex and important. Second, our data were obtained from a healthy, young man. The human lumbar spine of each individual is unique and is also affected by the age, presence of disease, and other factors. Third, we only analyzed the biomechanical behavior of PEEK rods after rigid fusion without considering fusion failure. Fourth, we simplified the loading pattern, material properties, and interaction of the implants and the spine, but the actual situation was severely complicated.