Numerous intervertebral fusion techniques have arisen as a result of the ongoing upgrading of internal fixation techniques. There are several intervertebral fusion techniques, since internal fixation techniques are always being updated. After short-segment fixation, ASD may emerge, necessitating the selection of yet another surgical strategy. The traditional trajectory for PS requires significant tissue dissection and muscle retraction, whereas the cortical bone trajectory (CBT) screw has the advantages of minimal muscle damage and the preservation of the superior facet joint (Qiu et al., 2022) (Figure 1). Many biomechanical studies have proved that the CBT technique provides greater pull-out strength, rigid insertion torque fixation, and a stable screw structure similar to traditional PS (Matsukawa et al., 2016; Sansur et al., 2016). Less research has been carried out on the impact of various screw implantation strategies on the stability and flexibility of the lumbar spine. The effects of numerous spinal fusion methods on spinal stiffness are still unclear, and research on the benefits and drawbacks of each method is still lacking. Physicians are curious to see whether combining traditional PS and CBT has benefits and how the unique mechanical features function. How to select the correct screw fixation technology to treat degenerative spinal conditions is the problem of the greatest concern for clinicians. For this reason, FEA and IVEs are frequently available.

For the treatment of lumbar instability and ADS surgery, a precise biomechanical analysis of the various surgical modalities is necessary to choose the best option for effective lumbar internal fixation. To define the intricate biomechanical characteristics of the lumbar spine, complementary methods include FEA and IVEs (Xu et al., 2013; Lu and Lu, 2019). In this study, the distribution pattern of the internal fixation system was explored, and the data were analyzed to explore the mechanical references for procedure selection in clinical practice. FEA and IVEs of the T12-S1 vertebral body were performed using different combinations of screw placement techniques to reveal the biomechanical differences between different implantation techniques. The main goals of this study were as follows: 1) to create a workable finite element model (FEM) of the lumbar spine; 2) to simulate screw placement techniques on the models; 3) to compare the stress distribution of the posterior screw–rod system under various screw placement techniques; 4) to create specimen models of various posterior screw–rod systems; and 5) to test the differences in joint mobility and stiffness values between the FEMs and the IVEs.

The research was approved by the Science and Ethics Committee of the School of Biological Science and Medical Engineering at Beihang University (protocol code: BM20220087).

In this study, the following parameters were evaluated: 1) compressive stiffness in the T12-S segment; 2) overall ROM; 3) ROM of the adjacent segment; 4) ROM of each segment of T12-L5; and 5) Von Mises stress distribution of the screw–rod system.

A frequent and essential surgical approach for the treatment of spinal problems such as degenerative spine conditions is the internal fixation technique. Long and six screws both demonstrated improved lumbar spine sagittal stability. According to Spiegl et al. (2021), older individuals with mid-thoracic spinal instability treated with extended segmental stabilization had a much lower risk of subsequent vertebral fractures over time. Santoni et al. (2009) first introduced the CBT screw internal fixation approach in 2009, using screws along the caudal head sagittal and lateral paths. Later, Takata et al. (2014) presented a novel surgical technique that eliminates soft tissue stripping and shortens the incision length by fusing the upper portion with CBT screws and the lower segment with traditional pedicle screws. Numerous authors have also looked into the bone purchase of CBT screws. Using a human lumbar spine model, Santoni et al. (2009) examined the uniaxial tension of CBT screws and compared the axial pullout force of CBT screws with standard pedicle screws. Although lumbar pedicle screw fixation has the benefit of improving biomechanical stability, screw loosening and fracture can still happen as ASD progresses. The biomechanical examination of the various surgical procedures is crucial for successfully fusing the spine since it enables us to choose the best surgical strategy. There is no consensus regarding the evaluation of different spinal fusion methods leading to spinal stiffness. Additionally, little research has been conducted on the benefits and drawbacks of different fusion procedures (Park et al., 2009). In this experiment, a combination of FEA and IVE was used, and the experimental data were then cross-checked to increase their accuracy. This study’s findings provide guidance for the decision-making process in terms of the best pedicle screw therapy for ADS.

The stiffness results of the FEM group and the IVE group were similar: L34CBT/L45PS > L3CBT-L45PS > L345PS > L4CBT/L5PS > L45CBT > L45PS > Intact. This study demonstrates that hybrid screw placement and CBT may both produce greater stiffness and improved stability. The researchers also discovered that compared with short-segment fixation, long-segment fixation produced higher stiffness (Kahaer et al., 2022b). The overall stress is decreased by lengthening the internal fixation system, and several biomechanical studies have demonstrated that the use of hybrid screws enhances the biomechanical stability of the joint. The spine’s flexibility is greatly decreased by posterior fixation, and its stiffness is significantly increased. The longer the fused segment, the stiffer the spine becomes; hence, the dual-segment internal fixation strategy is stiffer than the single-segment approach. Excessive stiffness may result in excessive spinal motion and may cause ASD (Goto et al., 2003; Dai et al., 2007). At present, the term stability is misused. A stable system is one that does not undergo a large displacement under small perturbations (Liang et al., 2020). Clinically, an ROM less than 5° was considered to be a successful fusion in terms of the FDA definition (Boustani et al., 2012). Focusing only on more stiff constructions is not scientific. In order to reduce the incidence of ASD, changes in ROM must be taken into account.

In this study, the results are consistent between the FEM group and IVE group when comparing the three groups of single-segment internal fixation techniques, as determined by the total ROM loss rate. For fused segments, L45PS has high rotational resistance, whereas L45CBT has high flexion, extension, and lateral bending resistance. Comparing the three groups of two-segment internal fixation modalities, the FEM group had stronger rotational resistance with hybrid screws and similar results in the three groups for both flexion and extension and lateral bending resistance. Due to the domino connection of the screw–rod system, which leads to instability of the fused segment in the rotating state, L34CBT/L45PS and L3CBT-L45PS have greater resistance to flexion and extension as well as lateral bending in the IVE group. According to this study, L34CBT/L45PS is more effective than L3CBT-L45PS and L345PS when ASD develops as a result of single-segment fixation. Zhang et al. (2022) came to the conclusion that while the fused segment’s flexion and extension mobility were relatively low in the CBT screw group, its rotational ROM was higher than that of the PS screw, and comparable findings were found in the current investigation. Spinal flexibility is greatly decreased by posterior fixation. In a study by Elmasry et al. (2017), a posterior fixation system was discovered to have greater stiffness. According to the adjacent-segment ROM, there was no difference between the single-segment internal fixation groups and barely any difference between the dual-segment internal fixation groups. Dual-segment internal fixation had higher stability than single-segment internal fixation, but it also resulted in a substantial loss of adjacent-segment ROM. The hybrid screw design decreased the rate of ROM loss in flexion and extension and lateral bending, whereas the pure CBT treatment better maintained rotation better.

The L345PS screws were substantially more stressed than the other two groups in rotation for the dual-segment internal fixation, but the L34CBT/L45PS and L3CBT-L45PS screws were significantly more stressed than the L345PS in flexion-extension and lateral bending. For PS and CBT screws, the head and tail of the screw should be reinforced for stiffness since the stress distribution was focused in these areas where the screw made contact with the cortical bone. The upper screw stress is significantly greater than the lower screw stress, and extending the nail bar system can effectively reduce the total stress. The authors of various investigations on this subject came to the conclusion that most broken screws (78%–90%) happen in the caudal area (Chen et al., 2005; Kwon et al., 2006). Studies on the lumbar spine by Natarajan et al. revealed that the screw’s caudal area experiences the highest degree of von Mises stress, which is around 5–6 times larger than that in the screw head location and rod. (Natarajan et al., 2018). Therefore, it may be said that the rod in the same location has a higher likelihood of failing than the caudal position of the screw in posterior internal fixation.

Bone density is a risk factor for the development of postoperative ASD (Yuan et al., 2022). There is a greater surgical failure rate in patients with osteoporosis (Wu et al., 2012; Kim et al., 2015); however, osteoporosis has little effect on the ROM of the lumbar spine (Yang et al., 2009; Liu et al., 2022), so better stability should be considered when internal fixation is performed in osteoporotic patients. Our results suggest that long-segment fixation results in greater stiffness but is concomitant with a greater loss of ROM. Therefore, short-segment fixation is recommended as the first clinical option whenever possible. On this basis, CBT should be further chosen because it can provide higher stability, avoiding the occurrence of secondary operations due to the failure of screw–rod systems. Although PS is still the dominant technique for spinal internal fixation in clinical practice, CBT has a distinct advantage of reducing the incidence of ASD, thereby effectively avoiding screw–rod systems and the prolongation of surgery (Sakaura et al., 2016). Furthermore, even if a patient has developed an ASD, CBT should also be prioritized by clinicians as extended screw–rod systems, because it can provide a better retention of rotation and higher resistance to flexion, extension, and lateral bending compared with other surgical procedures. By reviewing the literature (Kahaer et al., 2022a), different scholars have conducted similar experiments with the hybrid screw and also pointed out that the hybrid screw can provide greater stability, but none of them studied the adjacent-segment ROM and did not point out the relationship with ASD development, which is also an essential highlight of this paper. The hybrid screw approach in particular has been poorly studied, and although its long-term efficacy still requires clinical validation, it is admittedly an innovation.

Both FEM and IVE methods have advantages and disadvantages. FEA has been used to analyze the biomechanical parameters within the spine and connective soft tissues that are difficult to capture by experimental techniques (Alizadeh et al., 2013; Chang et al., 2020). The use of FEA can solve some practical issues and play a significant role in clinical practice because of its relative simplicity (Wang et al., 2020b). However, due to the complexity of human structures, FEA approaches have their limitations. Different material properties and model simplifications could cause experimental results to be inaccurate. IVE can yield relatively realistic results, but its widespread use is limited by the fact that its physical specimens are scarce, expensive, and not reusable. Many scholars are currently using FEA to conduct studies, despite the inevitably great limitations of simple FEA studies; IVE is a crucial research method that will make the experimental results more accurate. Biomechanical characteristics obtained by IVE are closer to in vitro biomechanical characteristics (Iliescu et al., 2021). FEA and IVE are still the dominant methods for studying spinal biomechanics. In this study, FEA and specimen experiments were used to verify each other for lumbar internal fixation, and the experimental design was complete and scientific.

In this study, there are a few issues: 1) Muscles and paravertebral soft tissues were not included in the FEM and IVE, and body weight loads and muscle forces were used to calculate the loading torque. 2) The sample size should be increased to account for statistical analysis. 3) Due to the limited experimental settings, significant stress indicators were not examined in the specimen experiments. 4) The bone quality of the specimens was not taken into account for bone quality, and there were individual differences between specimens.

In this study, the biomechanical responses of the lumbar spine with PS and CBT fixation were investigated. Our results show that long-segment fixation produces greater stiffness than short-segment fixation, but is more likely to lead to ASD. Using a hybrid screw combination technology can considerably increase the spine’s biomechanical stability. For non-fused segments, the CBT approach provides the better retention of rotation and higher resistance to flexion, extension, and lateral bending compared with the PS technique. However, there are differences between single-segment and dual-segment internal fixation, and the ROM of adjacent segments is lost more strongly reduced by dual-segment internal fixation. Thus, the hybrid screw is an approach to consider, and is perhaps a better surgical option after clinical validation. In summary, an alternative to take into account is the hybrid screw, if internal fixation lengthening is carried out. This study can provide a better understanding of the biomechanical response to single-versus dual-segment internal fixation by different surgical procedures. A direction for future work could be to carry out statistical analysis on a larger sample of clinical data and verify the biomechanical results of this study.