Correlation of the single-segment dynamic stabilization with different segmental mobility and zygapophysial (facet) joint degeneration: a retrospective study in northern China

Abstract

Objective

To compare the clinical and radiographic outcomes of single-segment posterior decompression combined with two different non-fusion dynamic stabilization systems, Isobar EVO and Isobar TTL, in the context of facet joint degeneration and segmental mobility.

Method

A retrospective study was conducted on 47 patients who underwent single-segment surgery at the L4/5 level using either the Isobar EVO (n = 23) or Isobar TTL (n = 24) systems. We assessed facet joint degeneration on both sides of the fixed (L3/4, L4/5) and superior adjacent (L2/3) segments using the Fujiwara MRI grading system. Clinical outcomes were evaluated using the Oswestry Disability Index (ODI) and visual analog scale (VAS) for back and leg pain at baseline, 12 months, and 24 months postoperatively.

Result

Both groups exhibited significant facet joint degeneration at the fixed segments (L3/4 and L4/5) at 24 months. The TTL group also showed significant degeneration at the superior adjacent segment (L2/3), whereas the EVO group did not. Restoration of lumbar lordosis was significantly better in the EVO group. Pain and disability scores improved more in the EVO group than in the TTL group at both 12 and 24 months postoperatively.

Conclusion

The Isobar EVO system, with its enhanced mobility, may delay facet joint degeneration in the superior adjacent segment compared to the Isobar TTL system. However, both systems result in degeneration at the fixed segment, indicating a need for further improvements to mimic the natural biomechanics of the spine more closely.

Introduction

The facet joint (FJ) plays a crucial role in maintaining the stability of the lumbar spine. Over time, increased pressure from aging, disc degeneration, and muscle atrophy can lead to physiological and pathological changes within the FJ, causing facet joint degeneration (FJD). These changes, including hypertrophy, arthritis, and osteophyte formation, are now widely regarded as major contributors to lower back pain. [1, 2] Additionally, the rigid fusion fixation of the lumbar spine using pedicle screws can result in increased stress on the adjacent vertebral segment due to the loss of normal segmental motion. This further exacerbates degeneration in the adjacent segmental disc and facet joint (FJ), leading to the development of adjacent segment degeneration (ASD). Most research has indicated that the greatest improvement in mobility occurs in the superior adjacent segment, which is the primary joint where ASD develops following posterior fixation with pedicle screws [3,4,5].

The utilization of non-fusion dynamic stabilization systems for the treatment of lumbar degenerative diseases is growing in popularity, as these systems aim to address the numerous complications associated with fusion. Several studies have demonstrated favorable clinical and imaging outcomes [6,7,8,9]. Despite the theoretical benefits of dynamic stabilization compared to rigid fixation, the long-term impact of dynamic stabilization on ASD remains a topic of debate. [10, 11] Previous studies have demonstrated that the non-fusion dynamic stabilization technique has a preventative effect on FJD compared to lumbar fusion [12]. However, the underlying cause of this effect, whether it be due to the preservation of mobility or the mechanical properties of the device, has yet to be established through relevant studies. The Isobar TTL system is a semi-rigid dynamic stabilization device designed to stabilize the spine while allowing limited motion at the treated segment. It consists of pedicle screws connected by rigid rods with a fixed articulation joint that provides a controlled range of motion. The fixed articulation joint in the Isobar TTL system restricts motion to a predefined range, providing semi-rigid stabilization. This helps in distributing loads more evenly and reducing stress on the adjacent segments compared to rigid fusion. However, the limited range of motion can still result in some stress transfer to adjacent segments, potentially leading to ASD over time. The Isobar EVO system represents an advancement in dynamic stabilization technology, offering greater flexibility and mobility than the TTL system. Its design aims to provide a more physiological range of motion, closely resembling the natural biomechanics of the spine. (Fig. 1) These differences impact how mechanical stresses are distributed across the spinal segments, potentially influencing the development of ASD and FJD. In a previous study, our team compared the clinical outcomes of the Isobar TTL and the Isobar EVO devices and found that dynamic stabilization systems with increased mobility yielded better clinical outcomes and prevented disc degeneration in the superior adjacent vertebral segment. [13, 14]

To further understand the relationship between mobility and facet joint degeneration (FJD), our current study compares the imaging outcomes of the FJ after non-fusion dynamic stabilization using systems with varying levels of mobility.

Methods

Study design and participants

We conducted a retrospective, observational study on 47 patients diagnosed with lumbar disc herniation and spinal stenosis with neurogenic claudication. The study included patients who underwent posterior decompression combined with either the Isobar TTL (March 2014 - June 2016) or the Isobar EVO (June 2016 - July 2018) dynamic non-fusion systems at the L4/5 segment.

Inclusion Criteria: ①Patients with lumbar disc herniation and spinal stenosis with neurogenic claudication, including Meyerding I degenerative spondylolisthesis. ②Patients who underwent posterior decompression with either Isobar TTL or Isobar EVO at the L4/5 segment. ③Follow-up MRI of the lumbar spine at least 24 months post-surgery.

Exclusion Criteria: Patients with spinal instability due to lumbar spine trauma, previous lumbar fusion surgery, lumbar spondylolysis, or severe osteoporosis treated with medication.

The study was approved by our Institutional Review Board Dongzhimen Hospital and Honghui-hospital, Xi’an Jiaotong University. The approval number is 2022DZMEC-085-04.

Isobar simi-rigid internal fixation device (Scient’x-Alphatec, France). The differences between the TTL and EVO are the increase in flexion and extension mobility from ± 2° to ± 4.5°, the increase in longitudinal displacement from ± 0.4 mm to ± 0.8 mm, the increase in dynamic bar flexion from 8° to 12°, and the 25% reduction in titanium ring profile for the EVO system compared to the TTL system. And The flexion angle of EVO is 12° (160 mm radius of curvature) compare with the 8° (240 mm radius of curvature) of TTL system. (Figures 1 and 2C and D)

Fig. 1

Isobar dynamic stabilization devices (Right: Isobar TTL, Left: Isobar EVO) [14]. A The 25% reduction in titanium ring profile for the EVO system (8.9 mm) compared to the TTL system (11.5 mm). B The increase in flexion and extension mobility from ± 2° to ± 4.5° and the increase in longitudinal displacement from ± 0.4 mm to ± 0.8 mm. C The flexion angle of EVO is 12° (160 mm radius of curvature) compare with the 8° (240 mm radius of curvature) of TTL system

Surgical Procedure

All patients underwent general anesthesia while positioned in the prone position and autologous blood transfusion was utilized throughout the entire operation. Isobar internal stabilization was performed using an open procedure with a midline skin incision. The procedure involved making a midline incision and dissecting the erector spinae muscles subperiosteally to expose the facet joints and the entry points for the pedicle screws. Subsequently, spinous processes, laminae, hyperopic osteophytes, and hypertrophic ligamentum flavum were removed from the stenotic segment of the lumbar spine using rongeurs and lamina forceps. Submerged decompression of the lateral recess was employed until the compression of the nerve root canal and central spinal canal was completely alleviated, after which the prolapsed nucleus pulposus tissue was explored and removed. If the herniated disc was not compressing the nerve root, it was left untouched to avoid disrupting the intervertebral space of the surgical segment. Following sufficient decompression, TTL or EVO dynamic rods and locking bolts were implanted in accordance with the established fixation criteria. (From March 2014 to August 2016, the Isobar TTL was utilized for the enrolled patients. In August 2016, the surgical team transitioned to performing dynamic non-fusion surgery using the Isobar EVO, which was used for all enrolled patients from August 2016 to July 2018.) The Isobar titanium alloy pedicle screws were then inserted transpedicular without damaging the facet joints, and the positioning of the screws was verified using a C-arm. (Figure 2A and B) Subsequently, the incision was closed layer by layer after ensuring adequate hemostasis, flushing, and the placement of an indwelling epidural drainage tube.

Moreover, in this study, decompression was performed at the L4-5 level for all patients prior to dynamic stabilization with either the Isobar EVO or Isobar TTL systems. The decompression procedures varied in terms of the extent of bone removal and approach. Here, we provide a detailed breakdown. (Table 1)

Postoperatively, to prevent infection, antibiotics were administered routinely 24 h after the surgery. The drainage tube was removed within 24 to 48 h of installation, depending on the drainage volume. To aid patients in getting out of bed and gradually retrain their lower back muscles, they were instructed to wear a brace for three to five days. After surgery, the brace is typically worn for a period of one month. Upon removal of the brace, patients received functional exercise recommendations and were encouraged to regularly exercise their lower back.

Table 1 The detail of decompression techniques between Isobar EVO and TTL groups

Fig. 2

Images A and B depict the radiographic results after the intraoperative placement of pedicle screws, serving to assess the direction and accuracy of the screw placement. This helps to ensure that the facet joint capsule is not damaged during surgery and that the screw tip is oriented towards the superior endplate of the vertebral body as much as possible. Images C and D are X-ray images after Isobar system

Clinical evaluation

The study was designed to detect a minimum change of five time points in the clinical index from pre-operation to the final follow-up. Patients underwent review by an independent surgeon, with a minimum follow-up period of 24 months, who was blind to the patient’s device or surgical procedure. Clinical outcomes were evaluated using the Oswestry Disability Index (ODI) and visual analog scale (VAS) for back and leg pain, with VAS scores recorded on a scale from 0 (no pain) to 10 (worst imaginable pain). The assessments were performed by two highly experienced and specialized surgeons who had no prior knowledge of the surgical instrument used for each patient. The minimal clinically important difference (MCID) for VAS and ODI were considered to be five points.

Radiological evaluation

Preoperatively and at each follow-up visit, X-ray radiographs in both anteroposterior and lateral positions as well as functional views with flexion and extension lateral were obtained using Picture Archiving and Communication System (PACS). On X-ray radiographs, the segmental range of motion (ROM) of fixed segments and the upper adjacent segments was measured. Disc degenerations were assessed using MRI, with the Pfirrmann grading system determining the grade of degeneration in the fixed segment and the upper adjacent segment [15]. Given that the final follow-up period for this study was limited to 24 months, it was not sufficient to observe significant changes in the bone structure of facet joints in all patients. We chose to perform MRI for assessing facet joint degeneration because of its superior soft tissue contrast, which aligns with our research objectives. As a result, we conducted the Fujiwara scale assessment of facet joints using MRI. Facet joint degeneration was categorized into 4 grades using the method proposed by Fujiwara et al. and axial spin-echo T2-weighted images. Normal joints corresponded to grade 1, while joint space narrowing, or mild osteophyte corresponded to grade 2. Sclerosis or moderate osteophyte corresponded to grade 3, and marked osteophyte corresponded to grade 4 [16].(Fig. 3) In accordance with previous research indicating the occurrence of facet joint degeneration in the fixed segment and upper adjacent segment, our study evaluated facet joint degeneration exclusively in the fixed segment FJ (L3/4 FJ and L4/5 FJ) and the upper adjacent segment FJ (L2/3 FJ). In our study, superior adjacent segment FJ refers to L2-3 facet joint because it is the spinal segment immediately above the fixed segments. In addition to the primary fixation at the L4-5 level, the L3-4 segment FJ is also often included in the fixation process to enhance overall spinal stability. Thus, both the L3-4 FJ and L4-5 FJ have their facet joints immobilized by the fixation hardware which means fixed segments FJ refer to L3-4 FJ and L4-5 FJ.

Adjacent segment degeneration is influenced by the proper restoration of lumbar lordosis. To evaluate this restoration, pre-operative and post-operative lateral radiographs were analyzed for each patient. Lumbar lordosis was measured as the Cobb angle between the superior endplate of L1 and the superior endplate of S1. The degree of lordosis restoration was assessed by calculating the difference between the pre-operative and post-operative Cobb angles.

The aforementioned radiological parameters were evaluated by two experienced and specialized surgeons, who were unaware of which surgical instrument was utilized for the patient.

Fig. 3

The Fujiwara Grading System for the lumbar facet joint was as follows: Grade 1 corresponded to normal, Grade 2 corresponded to joint space narrowing or mild osteophyte, Grade 3 corresponded to sclerosis or moderate osteophyte, and Grade 4 corresponded to marked osteophyte

Statistical analysis

All data were analyzed using SPSS 19.0 statistical software (IBM, Chicago, IL, USA). The study was designed to detect at least a five-point change in the VAS and ODI score from pre-operation to the final follow-up, and baseline standard deviation of them was estimated to approximately five points. Sample size calculation was performed before the study. The type I error α was set at 0.05 and the type II error β at 0.1. Based on these assumptions and adding 10% for possible dropouts, sample size calculations indicated that a total of 52 patients were required for the study. Parametric data was compared by independent t-test and categorical variables were compared by χ2-tests or Fisher exact tests depending on the sample size. (The chi-square test was used when the sample size was greater than or equal to 40, and the Fisher exact test was used when the sample size was less than 40) A P-value of less than 0.05 was considered significantly statistically different. Ranked variables were determined using the Mann–Whitney U signed-rank test. Statistical significance was set at a P < 0.05.

Results

A total of 129 patients were obtained for this retrospective study, there were 26 patients who underwent posterior decompression combined with Isobar TTL dynamic stabilization and 26 patients who underwent posterior decompression combined with Isobar EVO dynamic stabilization. The patient selection process is shown in Fig. 4. Of the 52 patients, 5 were lost to follow-up; 1 in the TTL group died of glioblastoma, 1 in the TTL group and 3 in the EVO group lost contact. Thus, 47 patients (18 males and 29 females, mean age 50.63 ± 8.32 years) were finally included by inclusion and exclusion criteria. There were 24 patients who underwent Isobar TTL and 23 patients who underwent Isobar EVO. The patient selection process is shown in Fig. 4. The mean follow-up time was 37.01 ± 9.97 months (24–57 months). No significant differences were detected between groups in terms of age, sex, BMI, disease type, preoperative segmental and superior adjacent segmental Fujiwara classification for facet joint degeneration, intervertebral disc Pfirrmann classification and segmental mobility, postoperative lumbar lordosis, operative time, and intraoperative bleeding between the two groups (P > 0.05). (Table 2)

Fig. 4

Flow diagram of patients’ selection process selection process

Clinical outcome

Before surgery, there was no significant difference in the VAS and ODI scores between the two groups. (p > 0.05)

During follow-up, there was a significant improvement in VAS values for both back and leg pain (p < 0.05). At 1, 3, 6, and 12 months postoperatively, there was no difference in back and leg pain scores between the two groups. However, at 24 months postoperatively, back and leg pain scores were significantly lower in the EVO group compared to the TTL group. (p = 0.012 and p = 0.004; Table 3; Fig. 5A and B) Of the 47 patients who completed the follow-up, 36 achieved an MCID.

During the follow-up period, ODI scores were significantly lower in both groups. However, after the 12-month follow-up, they gradually increased. (Fig. 5C) During the follow-up, there were no statistically significant differences in ODI scores between the two groups at 1, 3, and 6 months postoperatively. However, at 12 and 24 months postoperatively, ODI scores were significantly higher in the TTL group compared to the EVO group. (p = 0.001 and p = 0.002, Table 3; Fig. 5C) In the TTL group, 13 patients took NSAIDs and antidepressants, while 5 patients in the EVO group mainly took NSAIDs as analgesics. Of the 47 patients who completed the follow-up, 40 achieved an MCID.

Radiological outcome

Before the surgery, there was no significant difference in the mobility of the fixed segment (L4/5) and the upper adjacent segment (L3/4) between the two groups. (p = 0.496 and p = 0.360, Table 2). Two years postoperatively, there was a significant difference in the mobility of the fixed segment (L4/5) and the upper adjacent segment (L3/4) between the two groups. The L4/5 mobility was significantly higher in the EVO group compared to the TTL group, while the L3/4 mobility was significantly lower in the EVO group than in the TTL group. (p < 0.05, Table 4; Fig. 6) Before the surgery, there was no significant difference in the Pfirrmann grading of the fixed and upper adjacent discs between the two groups. At 2 years postoperatively, there was also no significant difference in the Pfirrmann grading of the fixed segments between the two groups. However, the grade of disc degeneration in the upper adjacent segment was significantly higher in the TTL group compared to the EVO group. (p = 0.0032, Table 4) Before the surgery, there was no significant difference in the Fujiwara degeneration grade of the fixed segments FJ (L3/4 FJ and L4/5 FJ) and the superior adjacent segment FJ (L2/3 FJ) between the two sides of the facet joint. (p = 0.062, p = 0.356 and p = 0.172, Table 2) Two years postoperatively, the Fujiwara grade of the FJ on both sides of the fixed segments in both groups was significantly higher compared to before the surgery. There was no significant difference between the two groups. Additionally, the Fujiwara grade of the upper adjacent segment on both sides in the TTL group was significantly higher than before the operation. (p = 0.041 and p = 0.014, Fig. 7) In the EVO group, there was no significant difference in the Fujiwara classification between the two sides. (p = 0.07 and p = 0.182, Fig. 8)

Both groups had similar pre-operative lumbar lordosis angles, with no significant differences (p > 0.05, Table 5). Both groups showed an improvement in lumbar lordosis after surgery. The Isobar EVO group achieved a slightly higher mean post-operative Cobb angle (49.8° ± 5.9°) compared to the Isobar TTL group (48.5° ± 6.4°). The mean increase in lumbar lordosis was 7.5° ± 3.2° for the Isobar EVO group and 6.6° ± 3.5° for the Isobar TTL group. The difference in the degree of lordosis restoration between the two groups was statistically significant (p = 0.047, Table 5), indicating that the Isobar EVO system was marginally more effective in restoring lumbar lordosis.

Table 2 Patient demographics

Table 3 Clinical outcomes of TTL and EVO patients

Fig. 5

The scores on clinical outcome measures of VAS-leg (A), AS-back (B), and ODI (C) showed significant differences between the two groups. At 24 months postoperatively, the TTL group had significantly higher VAS-leg (A) and VAS-back (B) scores compared to the EVO group. Additionally, the ODI scores (C) were significantly worse in the TTL group at both 12 and 24 months postoperatively

Table 4 Radiological outcomes of TTL and EVO patients

Fig. 6

At both preoperative and final follow-up, there was a significant decrease in fixed segment mobility in both the TTL and EVO groups, while there was a significant increase in upper adjacent segment mobility. Furthermore, at the final follow-up, both fixed and upper adjacent segment mobility were significantly lower in the TTL group compared to the EVO group

Fig. 7

In the TTL group, MRI evaluation showed changes in the left and right facet joints, with degeneration present in both the upper adjacent segment FJ (L2/3 FJ) and fixed segment FJ (L3/4 FJ and L4/5 FJ) at 24 months postoperatively. The degeneration was statistically significant on both the right and left sides of both the upper adjacent and fixed segments at 24 months postoperatively

Fig. 8

MRI evaluations in the EVO group revealed degeneration in the left and right facet joints of both the upper adjacent segment FJ (L2/3 FJ) and surgical segment FJ (L3/4 FJ and L4/5 FJ), with statistically significant degeneration observed only in the fixed segment at 24 months postoperatively

Table 5 Comparison of lumbar lordosis restoration between Isobar EVO and Isobar TTL groups

Case1

Posterior compression combines with Isobar EVO dynamic stabilization.

Case 1

A 52-year-old female was diagnosed with L4/5 lumbar disc herniation combined with lumbar spinal stenosis. Preoperative MRI images are shown in Figures A-D, which indicate L3/4 Pfirrmann grade III, L4/5 Pfirrmann grade IV in Figure A, both bilateral FJs as Grade 2 in L2/3 FJ in Figure B, left FJ as Grade 1 and right FJ as Grade 2 in L3/4 FJ in Figure C, and left and right FJs as Grade 3 in L4/5 FJ in Figure D. Two years after surgery, MRI images are shown in Figures a-d, which indicate L3/4 Pfirrmann grade IV, L4/5 segmental Pfirrmann grade V in Figure a, both bilateral FJs as Grade 2 in L2/3 FJ in Figure b, left FJ as Grade 2 and right FJ as Grade 3 in L3/4 FJ in Figure c, and left and right FJs as Grade 4 in L4/5 FJ in Figure d. Consent for publication was obtained from patients

Case 2

Posterior compression combines with Isobar TTL dynamic stabilization.

Case 2

A 48-year-old male was diagnosed with L4/5 lumbar disc and lumbar spinal stenosis. Preoperative MRI images (Figures A–D) show L3/4 and L4/5 Pfirrmann grading III. Figure B shows Grade 3 for the left L2/3 FJ and Grade 2 for the right FJ; Figure C shows Grade 2 for the left L3/4 FJ and Grade 3 for the right FJ; Figure D shows Grade 3 for both left and right L4/5 FJ. Postoperative MRI images at 2 years (figures a–d) show L3/4 and L4/5 Pfirrmann grading III. Figure b shows Grade 3 for the right L2/3 FJ and Grade 4 for the left FJ; Figure c shows Grade 4 for bilateral L3/4 FJ; Figure d shows Grade 3 for the left L4/5 FJ and Grade 4 for the right FJ. Consent for publication was obtained from patients

Discussion

Dynamic stabilization aims to maintain stability in the fixed segment while enabling some degree of segmental mobility, potentially reducing compressive stresses in the disc and facet joints of both the fixed segment and adjacent segments. [7, 17] In theory, the non-fusion dynamic stabilization technique facilitates better biomechanical conduction of the fixed segment and adjacent segments, potentially resulting in superior clinical outcomes. Most current research has concentrated on comparing the clinical outcomes of dynamic stabilization techniques and titanium rod fusion procedures, as well as investigating the preventive benefits of non-fusion on ASD. [7, 10, 11, 18] Nonetheless, the factors contributing to the positive outcomes of non-fusion techniques have received limited research attention. This is largely due to the substantial surgical differences between non-fusion and fusion devices, leading to numerous confounding factors that are challenging to analyze accurately. Additionally, certain devices, like Dynesys, offer not only flexion and extension mobility but also lateral bending mobility, and the degree of mobility varies across studies, presenting a significant complicating factor in investigating the impact of mobility on the segment. [10] Considering the above, we contend that the semi-rigid fixation device, Isobar, can effectively address these concerns. The Isobar TTL and Isobar EVO exhibit minimal differences in contour volume. Their primary distinction lies in the degree of flexion and extension mobility, with no clinical studies reporting device-related complications. Consequently, we believe that the Isobar semi-rigid dynamic stabilization system has demonstrated significant advantages in examining the impact of flexion and extension mobility on the fixed segment and adjacent segment.

Prior biomechanical investigations have demonstrated that the Isobar system offers flexion and extension mobility and biomechanical transfer closer to that of a healthy spine than rigid fixation. [19,20,21] Our team has previously published on the clinical effectiveness of Isobar for a minimum of four years, and found that the EVO system [13], which provides greater segmental mobility, outperformed the TTL system in several clinical outcomes. This implies that increased device mobility may lead to better outcomes, but the exact spinal structure responsible for this difference in clinical symptoms remains unknown. Earlier studies have demonstrated a strong clinical association between facet joint degeneration and lower back pain [22, 23]. Consequently, we wondered whether the variation in mobility led to a varying degree of facet joint degeneration in the fixed and upper adjacent segments, thereby resulting in clinical symptom discrepancies.

The study is the first clinical investigation to compare the impact of non-fused dynamic fixation systems with different segmental mobility on the radiological and clinical outcomes of facet joints in fixed (L3/4 and L4/5) and adjacent (L2/3) segments, to the best of our knowledge. However, before we discuss the results, let us explain why patients with disc herniation and spinal stenosis, who did not have any pre- or post-operative instability, were treated with non-fused dynamic fixation systems. First, the included patients in our study prefer dynamic stabilization over non-instrumented discectomy or decompression due to perceived benefits, such as reduced pain, enhanced stability, or quicker recovery. Secondly, our team are more experienced with dynamic stabilization and believes it can achieve the desired outcomes effectively and safely. Thirdly, each patient’s condition is unique, and a detailed pre-operative assessment takes into account factors like the extent of disc herniation, the severity of spinal stenosis, the patient’s age, overall health, and other individual characteristics. These factors lead to the choice of dynamic stabilization for included patient. Then, dynamic stabilization, in included cases can provide stability and reduce the risk of future issues, even in the absence of pre- or post-operative instability. Our team believes dynamic stabilization can prevent or delay future degeneration of adjacent segments, justifying its use. Finally, the choice of dynamic fusion for these patients are in accordance with clinical guidelines and our previous research suggesting its effectiveness in specific scenarios, even without overt instability [13, 21].

Our results revealed that the EVO group had a higher fixed segment mobility than the TTL group, whereas the EVO group had lower mobility in the upper adjacent segment. These findings suggest that increasing the fixed segment mobility can effectively decrease the compensatory mobility of the upper adjacent segment. Both the TTL and EVO groups demonstrated a gradual decrease in VAS leg pain from 1 to 12 months post-surgery, with no significant difference observed between them at these four time points. However, at 24 months post-surgery, the average VAS leg pain score for both groups were significantly higher than that at 12 months after surgery. The VAS back pain score of both groups also showed a gradual decrease from 1 to 6 months post-surgery, but at 12 months after surgery, both groups experienced an upward trend in scores, with no significant difference between them. At 24 months post-surgery, the back pain VAS score continued to increase for both groups, but the EVO group’s score was significantly lower than that of the TTL group. Regarding the ODI score, at 12 and 24 months after surgery, the EVO group’s score was significantly lower than that of the TTL group. Notably, the ODI scores for both groups began to rise again after 12 months post-surgery, with significant differences between the two groups. Although both groups experienced FJ degeneration at the fixed segment, only the TTL group demonstrated significant degeneration in the upper adjacent segment, while the EVO group did not. These results suggest that FJ degeneration may be a contributing factor to persistent postoperative and recurrent leg and low back pain. Previous studies have suggested a correlation between disc degeneration and FJ degeneration [24]. In our study, we compared the Pfirrmann grading of intercalated discs in the fixed and upper adjacent segments and found significant differences in the upper adjacent segment at the last follow-up, indicating that the higher mobility EVO system can provide better prevention of both FJ and disc degeneration in the upper adjacent segment compared to the TTL system. Although the EVO group also showed FJ degeneration at the fixed segment, our precise control and radiological confirmation of intraoperative screw placement in each patient demonstrated that the fixed segment degeneration may be related to the pedicle screw system itself causing FJ degeneration. This finding is consistent with previous finite element model results that show the Isobar system distributes stresses in the fixed segment to the anterior and middle columns of the vertebral body compared to rigid fixation, while still increasing compressive stresses in the FJ compared to the normal spine. [19, 25] (Fig. 1) The restoration of lumbar lordosis also plays a critical role in the biomechanical environment of the spine, influencing the stress distribution on adjacent segments. In our study, patients treated with the Isobar EVO system demonstrated a statistically significant improvement in lumbar lordosis compared to those treated with the Isobar TTL system. The increased mobility provided by the Isobar EVO system may facilitate better alignment and dynamic adaptation of the lumbar spine, promoting a more physiological restoration of lordosis. While effective, the more rigid stabilization provided by the Isobar TTL system may limit the extent of lordosis restoration, potentially leading to less optimal biomechanical outcomes. Proper restoration of lumbar lordosis is crucial for reducing the risk of adjacent segment degeneration. The improved lordosis observed in the Isobar EVO group suggests that this system may offer better long-term preservation of adjacent segment health by optimizing spinal alignment. Surgeons should consider the potential advantages of dynamic stabilization systems like the Isobar EVO, which not only stabilize the operated segment but also support the restoration of natural spinal curvature.

Numerous studies have examined the compressive stresses on facet joints in both fused and non-fused dynamic stabilization. [26,27,28] The findings demonstrated that both fusion surgery and dynamic fixation generated higher FJ contact forces compared to those of a normal spine under all loading conditions. Biomechanical investigations have indicated that FJ compressive stress during spinal flexion and extension rises with motion amplitude, and when spinal unit mobility is constrained, FJ pressure is expected to increase significantly, leading to degeneration of the fixed segment FJ, which is comparable to the degeneration caused by elevated FJ stress following artificial disc replacement in the lumbar spine. In addition to its effects on FJ pressure, several biomechanical studies of the Isobar system have revealed non-physiological effects, such as reduced mobility and abnormal stress transfer patterns compared to the intact spine. [29, 30]. This suggests that the dynamic system has its own limitations when attempting to simulate the normal spinal motion of the intact spine, but still offers significant advantages over conventional fixed fusion. [31, 32] In our study, the Isobar systems used were identical in surgical approach and indication, with the only differences being in mobility and subtle contouring. Additionally, the effect of aging on FJ degeneration was partially avoided by limiting follow-up to 24 months. [33] The small differences in confounding factors between the two patient groups selected for this retrospective study allowed for the conclusion that mobility was the primary reason for the differences between the two Isobar systems, and that the EVO group had less significant FJ degeneration in the fixed and upper adjacent segments than the TTL group. This suggests that increasing the mobility of the non-fusion stabilization system can reduce or prevent FJ degeneration in adjacent segments to some extent, highlighting the importance of mobility in the non-fusion stabilization system. Additionally, this finding may indicate a potential link between degeneration and osteoarthritis of the fixed and adjacent segmental FJ and recurrent postoperative low back pain in patients.

Clinical implications and limitations

Our study underscores the comparative effectiveness of the Isobar EVO and Isobar TTL systems in preventing adjacent segmental facet joint degeneration following decompression and dynamic stabilization at the L4/5 level. While the Isobar EVO system exhibited superior clinical outcomes and a reduction in adjacent segment degeneration, it is imperative to address certain considerations and limitations to fully contextualize these findings. Firstly, it is important to note that facet joint degeneration at the L4/5 level, which serves as the fixed segment in our study, could potentially emerge as a new source of pain post-surgery. Our short-term follow-up data indicated similar facet joint integrity between the EVO and TTL systems. However, the possibility of facet degeneration evolving into a significant clinical issue over time highlights the necessity for extended follow-up. This extended monitoring is crucial to track the progression of facet joint pathology at the fixed segment and assess its potential impact on patient outcomes. One significant limitation of our study is the relatively short follow-up period. Given the degenerative nature of spinal conditions and the use of dynamic fixation systems, long-term follow-up is essential. This is needed to comprehensively understand the durability of the observed clinical benefits and identify any delayed complications. Longer-term monitoring will provide valuable insights into the progression of adjacent segment degeneration, the emergence of new pain generators, and the longevity of the benefits provided by the Isobar EVO system. Additionally, dynamic fixation systems like the Isobar EVO, which do not involve spinal fusion, carry inherent risks of metal failure over time, such as screw loosening, rod fracture, or failure of the dynamic components. Although our current follow-up did not reveal instances of metal failure, this does not eliminate the potential for such complications in the future. Surgeons should be aware of these risks and ensure diligent post-operative monitoring to promptly identify and manage any hardware-related issues.

Conclusion

The findings of this retrospective study propose that the Isobar EVO system is more advantageous in forestalling adjacent segmental facet joint (FJ) degeneration than the Isobar TTL system. In other words, any rise in the mobility of the dynamic stabilization system can help prevent proximal FJ degeneration. Furthermore, an enhanced dynamic stabilization system mobility can also mitigate and delay the onset of persistent postoperative low back pain and recurrent low back pain by forestalling degeneration of the adjacent segmental FJ. Nonetheless, both the EVO system and the TTL system produce degeneration of the fixed segment FJ, highlighting the need for more detailed biomechanical studies of the Isobar system to improve its physiological compatibility.