INTRODUCTION
Spinal fusion surgery is a well-established and widely utilized treatment for various spinal pathologies, including spondylolisthesis and segmental instability. By providing robust stabilization and neural decompression, fusion frequently results in substantial pain relief and functional improvement. However, the irreversible loss of motion at the fused segments inevitably increases biomechanical stress on the adjacent vertebral levels above and below the index segments [1,2]. This accelerated degenerative change at neighboring segments is a well-recognized risk factor for the development of adjacent segment disease (ASD) [3,4], which may ultimately necessitate additional surgical intervention and is regarded as a major long-term complication following fusion. Furthermore, complications such as hardware failure and pseudarthrosis remain potential concerns.
To address the intrinsic limitations of rigid fusion, a new concept known as spinal dynamic stabilization (SDS) has emerged [5]. This paradigm represents a shift from the traditional objective of “eliminating motion” to that of “controlling motion.” The primary goal of SDS is to provide sufficient stabilization to a structurally compromised spinal segment while preserving as much physiological range of motion (ROM) as possible. By maintaining controlled mobility, SDS aims to reduce stress concentration at adjacent levels and thereby lower the incidence of ASD. Additionally, concerns related to fusion-related failures are mitigated.
A variety of SDS systems have been introduced, with pedicle-based systems representing a prominent category [6-8]. These constructs utilize pedicle screws connected by dynamic, flexible rods rather than rigid ones. Such a design enables controlled motion and more favorable stress distribution, making them particularly suitable for cases of mild instability, in contrast to rigid fusion, which is more appropriate for severe instability. This study aims to evaluate the clinical and radiological efficacy of a novel pedicle-based dynamic rod system, Spine Gold (SG), by comparing its outcomes with those of decompression alone in patients with single-level central stenosis and grade 1 spondylolisthesis.
MATERIALS AND METHODS
1. Study Design and Patient Population
This retrospective, single-center comparative study included 262 patients who underwent spinal surgery between January 2013 and May 2024. Eligible patients were those diagnosed with single-level central lumbar stenosis and grade 1 spondylolisthesis who completed all required imaging, including preoperative and final follow-up x-rays, as well as preoperative, immediate postoperative (postoperative day 1), and final follow-up magnetic resonance imaging (MRI). Only patients who demonstrated early clinical success—defined as a 1-month MacNab criteria of "good" or "excellent"—and who showed radiologically confirmed adequate decompression on the immediate postoperative MRI were included. Patients were excluded if they had multilevel central stenosis, grade 2 or higher spondylolisthesis, a concomitant ruptured disc, or prior spinal surgery at the index level (Table 1). These criteria ensured a homogeneous cohort for evaluating the dynamic stabilization procedure. Patients were divided into 2 groups: the decompression with SG fixation group (unilateral laminectomy for bilateral decompression [ULBD]+SG, n=161) and the decompression-alone group (ULBD, n=101). This study was exempted from Institutional Review Board (IRB) review due to the use of strictly de-identified and anonymized data. Accordingly, the requirement for informed consent was waived as no personally identifiable information was accessed or utilized.
2. SG Dynamic Stabilization System
The dynamic stabilization system used in this study was the SG system (GS Medical, USA). All components are fabricated from Ti-6Al-4V alloy. This pedicle-based dynamic rod system consists of 3 primary components (Figure 1):
(1) Conical polyaxial screws (pedicle screws): featuring a conical polyaxial head design and available in diameters ranging from 4.0 to 8.5 mm and lengths from 20 to 100 mm;
(2) Flexible rods: designed to permit controlled motion and constructed using a braided and twisted structural configuration, available in 5.5-mm and 6.0-mm diameters with lengths up to 200 mm; and
(3) Caps: used to secure the flexible rod within the screw head.
3. Surgical Technique
In the SG group, surgery began with standard ULBD for central stenosis, followed by the application of the SG dynamic stabilization system. For SG fixation, a paramedian skin incision was made using the Wiltse approach, minimizing muscle injury by dissecting along the fascial plane between the multifidus and longissimus muscles. Pedicle screws were inserted under C-arm fluoroscopic guidance.
Screws were inserted and temporarily secured with the spine positioned in flexion to facilitate access and trajectory. An awl was used to prepare the screw path, and placement was confirmed using C-arm imaging. After all screws were inserted, the patient’s spine was carefully returned from flexion to a neutral physiological position. Following this critical positional adjustment, the SG flexible rods were seated and secured with caps. A final C-arm image was obtained to confirm accurate placement of the construct (Figure 2).
Because this technique does not require interbody fusion, it offers advantages in terms of the technical learning curve and cost-effectiveness compared with rigid fusion surgery.
4. Clinical Assessments
Clinical outcomes were assessed using preoperative and 1-month postoperative visual analogue scale (VAS) scores for back and radicular pain, along with the 1-month postoperative MacNab criteria. The MacNab criteria were also used to verify early surgical success as part of the patient inclusion criteria. Long-term outcomes were evaluated by documenting the incidence of reoperation and the interval from the index surgery to reoperation; the cause of reoperation was classified as either index-level failure or ASD.
5. Radiological Assessments
Radiological assessments were performed using standing lateral and flexion-extension radiographs obtained preoperatively and at the final follow-up (Figure 3). Key parameters included:
Static instability: measured on standing lateral radiographs as the horizontal translational distance between the posterior vertebral body lines of the superior and inferior vertebrae at the index level.
Dynamic instability: calculated as the absolute translational difference between maximal flexion and extension views at both the surgical and proximal adjacent levels, using the same measurement technique applied for static instability. Crucially, dynamic instability was determined based on the presence of absolute translation; any displacement of 1 mm or more was considered indicative of instability.
MRI parameters: evaluated for evidence of recurrence and ASD.
6. Cox Regression for Risk of Reoperation
Because the study was retrospective and surgeons selected SG fixation based on their assessment of instability, Cox proportional hazards regression was performed to evaluate whether baseline radiological instability at the index and proximal levels confounded the association between surgical strategy and reoperation. The event time was defined as the number of months from index surgery to reoperation; patients without reoperation were censored at their latest radiological follow-up (the maximum of either x-ray or MRI follow-up).
Preoperative static instability was defined as the degree of slippage on neutral lateral radiographs at both the index and proximal levels. Preoperative dynamic instability was defined as the absolute difference in translational movement between maximal flexion and extension views at each level. The Cox regression model included the surgical group (ULBD+SG vs. ULBD) and the 4 instability parameters. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated.
7. Statistical Analysis
Statistical analyses were performed using IBM SPSS Statistics ver. 25.0 (IBM Co., USA). Independent t-tests were used to compare differences between groups, and paired t-tests were applied for pre- and postoperative comparisons. A p-value<0.05 was considered statistically significant.
RESULTS
1. Patient Demographics
The average age of patients in the ULBD+SG group was 64.2 years, and that in the ULBD group was 66.5 years, with no statistically significant difference (p=0.07). The SG group had a higher proportion of female patients (78.9%) compared with the ULBD group (59.4%).
The mean follow-up duration for x-ray evaluation was 46.45±28.53 (median, 40.9; range, 1.4–130.6) months in the ULBD+SG group and 39.28±26.22 (median, 30.1; range, 1.0–111.8) months in the ULBD group. For MRI follow-up, the mean duration was 45.64±27.00 (median, 40.9; range, 4.9–123.1) months in the ULBD+SG group and 39.17±25.77 (median, 29.7; range, 1.0–111.8) months in the ULBD group (Table 2).
2. Clinical Outcomes
Preoperative VAS scores for back and radicular pain were comparable between the ULBD+SG and ULBD groups (back: 5.66±1.28 vs. 5.82±1.42, p=0.347; radicular: 6.68±1.49 vs. 6.68±0.98, p=0.968). Postoperative VAS scores at 1 month demonstrated significant improvement in both groups (p<0.001). The ULBD group showed a significantly greater reduction in back pain (p<0.001), while improvement in radicular pain was similar between groups, with no statistically significant difference (p=0.239) (Table 3).
At 1 month postoperatively, MacNab criteria outcomes did not differ significantly between the ULBD+SG and ULBD groups. The distribution of “excellent” and “good” outcomes was comparable (ULBD+SG: excellent 38.5%, good 61.5%; ULBD: excellent 34.7%, good 65.3%; chi-square test, p=0.619) (Table 4).
3. Radiological Outcomes
The SG group demonstrated a significant reduction in static slippage at the operative level, decreasing from 6.69 mm preoperatively to 5.50 mm at follow-up (p<0.01). Conversely, the decompression-alone group showed a significant increase in static slippage, from 4.54 mm preoperatively to 6.83 mm at follow-up (p<0.01). At the proximal adjacent level, the SG group maintained stability (preoperative: 1.99 mm; postoperative: 1.99 mm, p=0.98), whereas the decompression-alone group exhibited a significant increase in slippage (preoperative: 2.18 mm; postoperative: 2.72 mm, p=0.016) (Table 5).
Dynamic instability at the operative level significantly decreased in the SG group, from 3.02 mm preoperatively to 1.33 mm at follow-up (p<0.01). In contrast, the decompression-alone group demonstrated a significant increase in operative-level dynamic instability, from 1.83 mm preoperatively to 2.71 mm at follow-up (p<0.01). A notable finding was the significant reduction in proximal dynamic instability in the SG group (preoperative: 2.94 mm; postoperative: 2.29 mm, p=0.003). Meanwhile, the decompression-alone group showed a nonsignificant trend toward increased dynamic instability at the proximal level (preoperative: 1.84 mm; postoperative: 2.28 mm, p=0.061) (Table 6).
4. Reoperation Rates
The reoperation rate was significantly lower in the SG group (10.6%) compared with the decompression-alone group (21.8%) (Figure 4). Additionally, the mean time to reoperation was significantly longer in the SG group (40.73 months) than in the decompression-alone group (24.54 months) (Table 7).
All reoperations in the SG group were attributed to ASD (100%), with no reoperations performed for index-level failure. In contrast, the decompression-alone group underwent reoperations predominantly for index-level problems (14.9%), with fewer cases related to ASD (6.9%) (Table 8).
5. Cox Regression for Risk of Reoperation
Although the SG group exhibited more severe preoperative instability than the decompression-alone group—indicating a selection bias in surgical decision-making—the multivariable Cox proportional hazards model showed that none of the preoperative instability parameters was significantly associated with the risk of reoperation.
Preoperative static instability at the index level (HR, 0.95; 95% CI, 0.87–1.03; p=0.22), static instability at the proximal level (HR, 0.96; 95% CI, 0.85–1.09; p=0.53), dynamic instability at the index level (HR, 1.07; 95% CI, 0.84–1.35; p=0.59), and dynamic instability at the proximal level (HR, 0.91; 95% CI, 0.76–1.09; p=0.32) all demonstrated no statistically significant associations with reoperation risk.
In contrast, SG fixation showed a trend toward reducing the hazard of reoperation compared with decompression alone (HR, 0.49; 95% CI, 0.22–1.09; p=0.081). These findings indicate that the baseline differences in preoperative static and dynamic instability between the 2 groups are unlikely to fully explain the observed difference in reoperation rates (Table 9).
DISCUSSION
The SG system functions by providing controlled elasticity and damping through its unique flexible metallic coiled rod structure, effectively reducing the expanded neutral zone and limiting the ROM at the unstable index segment while preserving physiological motion [5,6]. This mechanism contrasts sharply with those of other pedicle-based dynamic stabilization systems. Dynesys primarily stabilizes the segment through axial tension and compression generated by its polycarbonate urethane tube and cord assembly, providing neutral position control [4,9]. The Isobar TTL system incorporates localized spring-damper elements between the screw head and the rod to absorb and attenuate shock loads [10]. In contrast, Graf Ligamentoplasty applies substantial posterior tension using ligamentous tapes to restrict flexion, a mechanism associated with a higher risk of ASD [9].
A novel and critical finding of this study is the significant reduction in dynamic instability observed at the proximal adjacent segment following SG fixation. This effect is likely attributable to the system’s ability to facilitate a smoother and more physiological transition in stiffness between the stabilized index segment and the adjacent level, thereby reducing compensatory hypermobility and stress concentration [9,10]. By stabilizing the index segment without the complete immobilization characteristic of fusion procedures, SG fixation prevents the pathological motion patterns that contribute to accelerated degeneration and functional instability at the proximal segment [5,9].
The outcomes observed in the decompression-alone group underscore an important clinical consideration: decompression without stabilization is insufficient in patients with preexisting instability [11]. Decompression alone appears to further destabilize the index segment, as evidenced by the significant increases in both static slippage and dynamic instability observed postoperatively. This progressive postoperative instability likely contributed to the higher reoperation rate in this group, with index-level failure being more common than ASD. These results highlight the inherent limitations of decompression-only strategies in patients with even mild underlying instability and support the rationale for incorporating stabilization in such cases.
The reoperation data further reinforce these findings. The SG group demonstrated a lower reoperation rate and a longer time to reoperation, with all reoperations attributable to adjacent-level pathology rather than index-level failure. In contrast, the decompression-alone group exhibited a substantial proportion of reoperations due to recurrent or progressive pathology at the original surgical site. When compared with published fusion surgery data [1-3], the reoperation rate in the SG group (10.6%) is markedly lower than the rates reported for fusion procedures (24.6%–27.8%), as shown in Table 10. This difference is likely related to the specific inclusion criteria of the present study, which focused on patients with single-level central stenosis and grade 1 spondylolisthesis—a population with less severe instability than typical fusion cohorts.
In terms of cost-effectiveness, the simplified surgical approach of this system reduces direct implant costs compared with more complex fusion devices and may alleviate the long-term economic burden by mitigating ASD and the need for revision surgery.
The primary limitation of this study is its retrospective design. The SG group demonstrated statistically more severe preoperative instability than the decompression-alone group, suggesting a selection bias in surgical decision-making. Surgeons likely selected SG fixation for patients they perceived as having greater instability, which makes the favorable outcomes in the SG group particularly noteworthy. Importantly, multivariable Cox regression analysis indicated that the elevated reoperation risk in the decompression-alone group persisted even after adjusting for preoperative static and dynamic instability parameters, suggesting that the observed group differences cannot be fully explained by baseline instability alone. Additionally, potential measurement error due to observer variability in radiographic and clinical assessments cannot be excluded. Future prospective, randomized controlled trials are warranted to address these limitations and validate the long-term benefits of SG fixation.
CONCLUSION
Although dynamic stabilization is not appropriate for patients with severe instability, the findings of this study suggest that, in carefully selected patients with single-level central stenosis and grade 1 spondylolisthesis, dynamic stabilization offers a safe and effective treatment option. This technique represents a promising alternative to fusion surgery, as it provides reliable stabilization of the index segment, reduces the need for reoperation, and may help prevent ASD by concomitantly stabilizing the adjacent level in this patient population.
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