The clinical efficacy of hybrid surgery based on the Waveflex semi-rigid dynamic internal fixation system for the treatment of lumbar degenerative diseases: over three-year follow-up study

Abstract

Objective

To investigate the efficacy and safety of hybrid surgery based on the Waveflex internal fixation system for the treatment of lumbar degenerative disease.

Methods

A retrospective analysis was conducted on 31 consecutive patients who underwent hybrid surgery (fusion combined with non-fusion surgery) using the Waveflex internal fixation system for lumbar degenerative diseases between December 2019 and May 2021. The final follow-up assessments were completed by October 2024, yielding a follow-up period ranging from 41 to 58 months (mean ± SD: 48.1 ± 4.2 months). Postoperative outcomes were assessed using a comprehensive set of multidimensional indicators, encompassing both clinical and radiological evaluations. Clinical efficacy was evaluated based on pain intensity (measured by the Visual Analogue Scale, VAS), functional disability (assessed via the Oswestry Disability Index, ODI), incidence of postoperative complications, and patient satisfaction level. Radiological analysis included assessments of bone fusion rate, disc height measurements (anterior, middle, and posterior) at the non-fusion segment, range of motion (ROM) at the non-fusion segment, and dynamic changes in ROM at adjacent segments.

Results

All patients successfully completed the surgery and finished the follow-up. Three months, six months and the final follow-up VAS scores and ODI were significantly improved compared with the preoperative period, and the differences were statistically significant (P < 0.05). One case of postoperative complication occurred, involving delayed wound healing due to fat liquefaction at the incision site. The condition resolved completely after four weeks of conservative management, including regular dressing changes. At the final follow-up, the efficacy was excellent in 28 cases, good in 3 cases, moderate in 0 cases, and poor in 0 cases, with a satisfaction level rate of 100%. At the final follow-up, all index segments achieved bony fusion, with a fusion rate of 100%. There was no statistically significant difference between the disc height of the dynamically fixed segments compared with that of the preoperative period (P > 0.05). The ROM of the dynamically fixed segment decreased compared with the preoperative period but there was no statistical difference (P > 0.05). The ROM in adjacent segment of the dynamically fixed segment increased compared with the preoperative period, which was also not statistically different (P > 0.05).

Conclusion

Hybrid surgery based on the Waveflex internal fixation system for the treatment of lumbar degenerative diseases can achieve a favorable early clinical outcome. It is able to retain a certain degree in mobility of the dynamic fixed segment and effectively maintains the dynamic fixed segment intervertebral space height without affecting the adjacent segment.

Clinical trial number

Not applicable.

Background

Lumbar degenerative diseases are clinical disorders in which degenerative changes lead to intervertebral disc dysfunction, hypertrophy of the ligamentum flavum, and hyperplasia of the articular process. It mainly includes lumbar disc herniation, lumbar spinal stenosis and lumbar spondylolisthesis, and is one of the most common causes of chronic low back pain and lower limb pain in elderly patients [1,2,3]. Lumbar fusion surgery is a commonly used surgical treatment for lumbar degenerative disease. The biomechanical changes such as increased mobility of adjacent segment and stress concentration in the intervertebral discs and synovial processes after lumbar fusion may lead to degeneration of the adjacent segment. The literature reports [4, 5] that the occurrences for adjacent segment degeneration after lumbar fusion range from 8 to 100% on radiographic imaging, whereas the occurrences for adjacent segment degeneration on symptomatology range from approximately 5.2–18.5%.

To address these issues, hybrid procedures incorporating non-fusion techniques have emerged. Currently, the commonly used posterior non-fusion fixation techniques are divided into two main categories, namely transpedicular fixation systems and interspinous fixation systems [6, 7]. Hybrid procedures using interspinous fixation systems such as Coflex and X-Stop are more common, whereas transforaminal fixation systems are relatively less commonly used. Waveflex is a novel transforaminal semi-rigid fixation system that better integrates the advantages of traditional rigid fixation and transforaminal semi-rigid fixation. Unlike Coflex, which is limited to interspinous distraction, and Dynesys, which may overly restrict motion due to its tensioned cord structure, Waveflex provides dynamic, pedicle-based stabilization that preserves physiological mobility while maintaining segmental alignment. This design may better balance stability and motion, potentially reducing adjacent segment degeneration and implant-related complications [8,9,10]. In this study, we retrospectively analyzed 31 patients with degenerative lumbar spine disease who underwent hybridization and report the preliminary clinical outcomes.

Materials and methods

General materials

Retrospective analysis of 31 patients admitted to our orthopedic department from December 2019 to May 2021 with a diagnosis of lumbar degenerative disease who underwent hybrid surgery using the Waveflex system. The final follow-up assessments were completed by October 2024, with a follow-up duration ranging from 41 to 58 months (mean ± SD: 48.1 ± 4.2 months). There were 16 men and 15 women. The age was 23–76 (64.3 ± 9.5) years old. The body mass index was 22.0–33.0 (27.0 ± 2.9) kg/m² (Table 1).

Table 1 Demographic findings of the study patients (n = 31)

Inclusion criteria: (1) confirmed diagnosis of degenerative lumbar spinal stenosis, lumbar disc herniation, or lumbar spondylolisthesis, with clinical symptoms consistent with imaging manifestations; (2) ineffectiveness of systematic conservative treatments, with surgical indications; (3) imaging manifestations of degeneration in the adjacent segment of the responsible segment; (4) hybrid surgery with the use of the Waveflex system. Exclusion criteria: (1) patients with congenital deformities of the spine, the presence of infection, pregnancy, tumors and trauma; (2) history of previous lumbar spine surgery; (3) patients with severe osteoporosis or excessive BMI; (4) patients with incomplete follow-up data.

The study was conducted in compliance with the Declaration of Helsinki and was approved by the Ethics Committee of Beijing Friendship Hospital, Capital Medical University (Ethics No. 2024-P2-390-01). All patients provided written informed consent (Fig.1).

Fig. 1

Male patient, 62 years old, diagnosed with lumbar spinal stenosis and lumbar spondylolisthesis, the surgical plan was posterior lumbar decompression and fusion plus non-fusion internal fixation, The responsible segment, L3-5, was fused with an interbody fusion method, and L2-3 was in a degenerative state and was fixed with Waveflex elastic rods. The preoperative dynamic status image showed that the intervertebral mobility of the lumbar 2–3 segment was 3.7 degrees, and its adjacent L1-2 intervertebral mobility was 3.3 degrees; at the final follow-up, the elastically fixed L2-3 intervertebral mobility was 1.7 degrees, and the L1-2 intervertebral mobility was 5.0 degrees. The preoperative L2-3 intervertebral space heights were ADH 1.34 cm, MDH 1.68 cm, and PDH 0.88 cm, and the postoperative L2-3 intervertebral space heights were ADH 1.21 cm, MDH 1.59 cm, and PDH 0.71 cm, respectively

Surgical methods

After general anesthesia, the patient is laid prone on the spine bed, the abdomen is suspended, the compressed area is lined with a protective pad, the surgical space is determined and marked by fluoroscopy with a C-arm X-ray machine, a sheet is routinely disinfected and the operative area is exposed, a longitudinal incision is made in the posterior median, the skin is incised subcutaneously, and the paraspinal muscles are peeled off on both sides. After sufficient exposure, the pedicle screws were implanted with the “herringbone ridge” as the entry point, and the position was satisfied with the fluoroscopy of the C-arm. The responsible segment was partially bitten off the spinous process, vertebral plate, ligamentum flavum, and facet joints. Adequate decompression was performed, the intervertebral discs were removed and the endplate cartilage was processed, the implant bed was prepared, the intervertebral space was thoroughly flushed, the autogenous bone was implanted into the intervertebral space, and then the intervertebral fusion cage was implanted in the appropriate size. Longitudinal rods were installed (conventional cylindrical titanium rods for the responsible segment of the fusion, and multi-folded titanium elastic rods for the adjacent cephalic or caudal segments), transverse links were installed for the responsible segment, and fluoroscopy on the C-arm confirmed that the intervertebral fusion device and the internal fixation were well-positioned, and drainage was placed and sutured layer by layer after flushing.

Postoperative treatment

Postoperative patients in both groups were given orthopedic routine care, postoperative drainage tubes were placed for 24–72 h, postoperative prophylactic antibiotics were used for 24–48 h, and methylprednisolone was used for 3 days. Patients were instructed to carry out straight-leg raising and lumbar dorsal muscle functional exercises on the 2nd day of the postoperative period, and they wore lumbar braces for walking on the ground for about 3 days after the postoperative period. The patient was followed up regularly for imaging examinations after the operation.

Evaluation metrics

The VAS, ODI at preoperative, 3 months, 6 months, and final follow-up were recorded for both groups.

Recording of preoperative and postoperative dynamic fixation segment intervertebral space heights. The anterior disc height (ADH), middle disc height (MDH), and posterior disc height (PDH) were measured. The ROM of the fixed segment and its adjacent segments were recorded preoperatively and postoperatively. ROM is defined as the difference between the angles formed in hyperextension and hyperflexion positions of the lumbar spine as measured on lateral X-rays. The angle is measured between the inferior endplate of the upper vertebral body and the superior endplate of the lower vertebral body. An angle formed posterior to the vertebral bodies is considered positive, while an angle formed anterior to the vertebral bodies is considered negative. ROM = hyperextension angle– hyperflexion angle. The anterior, middle, and posterior disc heights are measured on the midsagittal plane (lateral radiograph) by marking points at the anterior, middle, and posterior margins of the intervertebral space on the adjacent vertebral bodies. The vertical distances between these corresponding points are then measured to obtain the respective disc heights. All imaging parameters were measured by two senior spine surgeons, and the average of the measurements was taken.

Bone fusion was defined as trabecular bone passing through the fusion device and connecting to the upper and lower vertebral endplates as observed on radiographs, and fusion failure was defined as a change in the angle of the intervertebral space of > 5° or loose displacement of the fusion device on dynamic radiographs.

The Fischgrund criteria were used to assess the postoperative satisfaction level at the final follow-up: excellent: complete or almost complete disappearance of low back and lower limb pain symptoms, daily activities were not affected; Good: significant improvement in symptoms after surgery, occasional low back and lower limb pain and numbness, occasional need for non-opioid analgesics; Moderate: improved symptoms after surgery, Intermittent episodes of low back and lower limb pain or with numbness in the lower extremities, Daily activities are affected, Occasional need for non-opioid analgesics; Poor: no improvement in symptoms after surgery, daily activities are significantly affected, need for non-opioid analgesics or occasional need for opioid analgesics.

Statistical analysis

Statistical analysis was performed using SPSS 25.0 software. Continuous data were presented as mean ± standard deviation (x̄±s). Prior to analysis, the normality of paired differences was assessed. Paired samples t-test was used for normally distributed data, while Wilcoxon signed-rank test was applied for non-normally distributed data. The significance level was set at α = 0.05 (two-tailed), with P<0.05 considered statistically significant.

Results

Clinical outcome

Comparing the postoperative time points with the preoperative time points, the VAS and ODI were significantly improved, and the difference was statistically significant (P < 0.05, Table 2). There was one case of postoperative complication, which was delayed healing of fat liquefaction of the incision, which was healed after 4 weeks of conservative treatment, including regular dressing changes. At the final follow-up, the efficacy was excellent in 28 cases, good in 3 cases, possible in 0 cases, and poor in 0 cases, with a satisfaction level rate of 100%. Among the patients with good results, oen patient had residual postoperative low back and lower limb pain and numbness, and two patients had low back and lower limb pain again about half a year after the operation, which was less severe than before, and occasionally required non-opioid analgesics, but had no obvious effect on daily life.

Table 2 Prevalance of VAS and ODI score

Imaging measurement

At the final follow-up, all index segments achieved bony fusion, with a fusion rate of 100%. The ADH, MDH, and PDH of the dynamically fixed segments [1.29 ± 0.28 (range, 0.70–1.98) cm, 0.96 ± 0.35 (range, 0.44-2.00) cm, and 0.66 ± 0.20 (range, 0.34–1.20) cm] were not statistically different from those of the preoperative period [1.38 ± 0.35 (range, 0.76–2.03) cm, 0.99 ± 0.31 (range, 0.29–1.53) cm, and 0.66 ± 0.21 (range, 0.22–1.12) cm, respectively] (P > 0.05, Table 3). At the final follow-up, intervertebral mobility of dynamically fixed segments [ 4.4 ± 1.7° (range, 1.8°-9.6°) ] decreased compared with preoperative [ 5.2 ± 2.3° (range, 1.3°-12.1°) ], but there was no statistically significant difference (P > 0.05, Table 4); A total of 17 cases of adjacent segments of dynamically fixed segments (except dynamically fixed L5-S1 segments) showed an increase in intervertebral mobility [ 4.6 ± 1.2° (range, 2.9°-6.5°) ] compared with the preoperative period [ 4.3 ± 1.6° (range, 1.1°-7.1°) ], but there was no statistically significant difference (P > 0.05, Table 4).

Table 3 Disc height of non-fusion segment

Table 4 ROM of non-fusion segment and adjacent segment

Discussion

Lumbar fusion surgery has been the “gold standard” for the treatment for lumbar degenerative disease (LDD) since the concept was first introduced by Hibbs and Albee in 1911 [11]. With the increasing maturity of lumbar internal fixation devices, the success rate of lumbar spinal fusion has been significantly improved, but the problem of adjacent segment degeneration (ASD) brought about by fusion has aroused increasing attention. The pathogenesis of ASD is not yet fully understood, and it is generally believed that postoperative changes in the biomechanics of adjacent vertebrae are an important cause of it [12,13,14]. Scholars continue to explore interventions for ASD, and fixed non-fusion techniques are gradually becoming more widely used in clinical practice. Compared with conventional fusion, it is characterized by the maintenance or reconstruction the stability of the spine without the need for intraoperative implant fusion, partial preservation of the mobility of the operated segment, and prevention the ASD [15,16,17].

Selecting appropriate indications is a prerequisite for good clinical outcomes for non-fusion technique. In cases when the responsible segment requires fusion, non-fusion technique alone do not solve the problem. Especially in cases when the adjacent segment of the responsible segment have already degenerated, fusion of the responsible segment alone results in accelerated progression of the adjacent segment on top of the existing degeneration. If adjacent segment are fused at the same time, it increases surgical trauma and prolongs the patient’s recovery time, as well as exacerbates the risk of degeneration of the adjacent segment due to the increase in the number of fused segments. In response to these problems, hybrid surgery combining the advantages of fusion and non-fusion techniques has emerged. This technique is characterized by the use of fusion in the responsible segment and non-fusion in the adjacent segment, thus creating a transitional buffer zone that theoretically reduces the mobility of the adjacent segment and slows down the incidence of degeneration [18,19,20].

With the development of material and biomechanical technologies, non-fusion fixation techniques have also made significant progress. Various non-fusion fixation modalities have been applied in clinical practice, providing new treatment options for lumbar spine surgery [21,22,23]. Currently, the clinically conducted hybridization techniques reported in China mainly involve the implantation of an interspinous fixation device in the upper segment of the fused segment, with Coflex being the most widely used. Li et al. [24] used fusion surgery of the inferior segment and Coflex fixation of the superior segment in a two-segment lumbar degenerative disease. The stability of the Coflex-placed segments was confirmed to be good after follow-up, preserving some of the motor functions of the segment and its neighboring segments and reducing the possibility of degeneration of the neighboring segments. Early clinical use of dynamic stabilization devices was limited by the mismatch between the nail tail of the pedicle screw and the conventional screw. Herren et al. [25] used the DTO (dynesys transition optima) device, which consists of a hybrid procedure by connecting a titanium rod to a dynesys polymer cord.

In this study, the Waveflex semi-rigid fixation system has the characteristics of both strong and dynamic fixation. The strong fixation part is a conventional cylindrical titanium rod, allowing effective fusion fixation; the dynamic fixation part is a pre-bent rod with an elastic structure, and the instantaneous axis of rotation of the elastically fixed segment is located posterior to the articular process, close to the normal physiological state. Elastic fixation limits the excessive anterior-posterior flexion and extension range of motion of the spine while preserving some of the motion of the vertebral body in situ, and uniformly reduces the load on the intervertebral discs during lumbar flexion and extension motions. After a one-year follow-up, the patient’s VAS and ODI of lumbar and leg pain effectively improved, with an excellent rate of 100% at the last follow-up. Previous biomechanical study [26] on hybrid fixation (combining rigid fixation with non-fusion dynamic stabilization) demonstrated that under simulated physiological loading conditions (flexion, extension, axial rotation, and lateral bending), compared to models with L4-5 rigid fixation alone, those incorporating L3-4 non-fusion dynamic stabilization showed significant reductions in annulus fibrosus stress at the L3-4 level by 41.6%, 77.3%, 41.7%, and 47.9%, respectively, along with corresponding decreases in range of motion (ROM) by 63.5%, 85.2%, 46.0%, and 63.4% across the four loading conditions. Our current findings are consistent with these finite element analysis results. Compared with the preoperative period, there was no significant change in the intervertebral space height of the elastic fixed segment after surgery, but the intervertebral range of motion appeared to decrease to some extent. This indicates that the semi-rigid fixation with elastic rods both limits the intervertebral mobility of the segment to a certain extent and reduces the load on the intervertebral disc, thereby slowing down the degeneration of the intervertebral disc in the dynamic segment. After all, the mobility of dynamically fixed segments is limited to a certain extent, and it is necessary to consider whether the adjacent segments will be affected. Among the 31 patients enrolled in this study, 14 underwent non-fusion dynamic stabilization at the L5-S1 segment. Due to anatomical constraints that compromised reliable evaluation of adjacent segment parameters, these cases were excluded from radiographic analysis. The final study cohort for adjacent segment evaluation therefore consisted of 17 patients who received dynamic stabilization at L2-L5 segments. Comparative analysis demonstrated a marginal increase in adjacent segment range of motion (ROM) postoperatively (4.6 ± 1.2°) versus preoperative values (4.3 ± 1.6°), though this difference was not statistically significant (p > 0.05). We postulate that this phenomenon may reflect compensatory biomechanical adaptation when dynamic stabilization partially restricts segmental mobility, adjacent segments may exhibit increased motion to preserve overall spinal kinematics during physiological movements (e.g., flexion-extension), consistent with the principle of motion compensation in spinal biomechanics.

At the same time, we should acknowledge the limitations of this study. First, although the inclusion of 31 patients provides valuable insights for an exploratory analysis, the relatively small sample size may limit the generalizability of the findings. Second, as a single-arm study, the lack of a control group consisting of patients treated with traditional rigid fusion makes it difficult to directly and systematically compare the clinical efficacy of hybrid fixation with that of other surgical techniques. It is important to note that the primary aim of this study was to introduce the hybrid fixation technique and evaluate its short- to mid-term clinical outcomes, thereby demonstrating its feasibility and initial effectiveness in clinical practice, rather than to conduct a comparative analysis of different surgical methods. Future research will aim to expand the sample size, extend the follow-up duration, and incorporate control groups to more comprehensively assess the long-term outcomes and potential advantages of this technique compared to conventional surgical approaches.

Conclusion

Hybrid surgery based on the Waveflex system can preserve a certain degree of mobility of dynamic fixed segment and effectively maintain the intervertebral space height of dynamic fixed segment without affecting the re-adjacent segment. Obtaining good early clinical efficacy in the treatment of lumbar degenerative diseases.