Lumbar spinal fusion is an increasingly performed surgical procedure for degenerative pathologies of the lumbar spine. Stenosis, degenerative spondylolisthesis, and disc degeneration are typical indications if associated with symptomatic instability. Various techniques are available to achieve fusion, such as 1) uninstrumented posterolateral fusion by autologous bone grafting on transverse processes, 2) instrumented posterolateral fusion with added pedicle screws, or 3) interbody fusion, in which the disc is mostly removed and an interbody graft is placed. The aim of these surgical procedures is to obtain (in instrumented cases) an immediately rigid construct resulting in a segmental spinal fusion by the addition of fusion material. At present, the most commonly performed stabilization technique to achieve fusion is the combination of pedicle-based rigid stabilization with an interbody cage.
Typical complications of lumbar fusion involve implant-associated problems such as implant malposition or secondary failure and an increased rate of adjacent-segment degeneration (radiological as well as symptomatic), potentially resulting in secondary surgery.
An alternative to rigid instrumented fusion is dynamic stabilization by pedicle-based constructs, that is, a dynamic, partially mobile construct using pedicle screws to reduce painful pathological motion while maintaining some residual nonpathological motion.1,2 While targeting patients’ clinical symptoms by reduction of segmental movement, this technique is thought to be associated with reduced surgical complexity, as an interbody approach is not necessary. It is a faster, less-invasive surgical procedure with a reduced rate of secondary progressive adjacent-segment degeneration, as residual motion of the index segment should reduce biomechanical forces at the adjacent segment in comparison with fusion.
Different systems for pedicle-based dynamic stabilization and techniques of dynamization or motion preservation are approved for degenerative instabilities in many countries. So far, noncontrolled case series assessing clinical outcomes of dynamic pedicle-based stabilization have reported clinical results that are similar to those of fusion.3–6 One study compared lumbar pedicle-based nonfusion with rigid fusion within a small, prospective, randomized study and found no differences in clinical outcome.7 So far, no high-quality study has assessed whether dynamic pedicle-based stabilization is superior to fusion with regard to patient symptoms. Therefore, the aim of this study was to assess whether dynamic pedicle-based lumbar stabilization is associated with the same degree of functional improvement, reduced surgical complexity and duration, and perioperative complications as lumbar fusion for mono- or bisegmental degenerative instability.
Methods
The study followed the World Medical Association’s Declaration of Helsinki and was approved by the local responsible ethics committee of each participating center. The trial was registered at ClinicalTrials.gov (identifier no. NCT01365754).
Study Design and Treatments
A multicenter, double-blind, prospective, randomized, 2-arm superiority trial was designed to compare lumbar spinal fusion with dynamic lumbar spinal stabilization for patients with degenerative instabilities with or without spinal stenosis. Patients fulfilling the inclusion and exclusion criteria (Table 1) were randomized to undergo mono- or bisegmental lumbar fusion or dynamic stabilization with or without decompression. The indication for fusion or dynamic stabilization and the necessity of additional decompression of the spinal canal for stenosis were based on clinical data, MRI findings, and flexion-extension radiographs of the lumbar spine. The presence of 1) ≥ 5 mm spondylolisthesis or segmental instability of ≥ 3 mm or ≥ 10° on flexion-extension imaging or 2) predominant back pain with Modic changes was defined as the indication for fusion or stabilization.
Inclusion and exclusion criteria
| Inclusion | Exclusion |
|---|---|
| Mono- or bisegmental symptomatic lumbar degenerative disease w/ or w/o canal stenosis | Spondylolisthesis >Meyerding grade I, spinal deformity (scoliosis >20°, sagittal imbalance) |
Indication for fusion/stabilization
| Spondylolysis |
| Failure of conservative therapy for >3 mos | Previous lumbar stabilization or fusion |
| Age ≥18 yrs | Significant comorbidities w/ potential influence on surgical success (e.g., advanced osteoporosis, rheumatoid arthritis, mental disease) |
Treatment assignment was performed at the study site using randomization envelopes. Randomization was 1:1, stratified by center and need for decompression, with a block size of 2 due to the multitude of study centers.
Patients randomized to fusion underwent mono- or bisegmental rigid pedicle-based fixation and interbody fusion with a PEEK cage via transforaminal lumbar interbody fusion or posterior lumbar interbody fusion techniques, with or without additional posterolateral fusion and decompression of the spinal canal where necessary. Patients randomized to dynamic stabilization underwent mono- or bisegmental dynamic stabilization with the pedicle-based dynamic stabilization system cosmicMIA (ulrich medical GmbH & Co. KG) without interbody or posterolateral fusion with or without decompression. The cosmicMIA system is a pedicle-based dynamic system with a hinged joint between the screw head and threaded part of the screw, allowing residual movement between the rod and the screw in flexion-extension (Fig. 1). The implant allows load sharing between the spinal segment and the implant.4,6,8–10
Pedicle screw and rod of the cosmicMIA top-loading system. Hinged joint between thread and screw head allowing motion in flexion and extension (1). Self-cutting thread with hydroxyapatite coating for increased bone ongrowth (2). © ulrich medical.
The trial was performed at 18 centers in Germany and Austria (see Appendix). Surgeons involved were experienced in fusion and dynamic stabilization. The method of decompression (laminectomy or less-invasive techniques such as hemilaminectomy or laminotomy) was the surgeon’s choice.
Data Collection and Endpoints
Patient data were collected prior to surgery; before discharge; and at 3, 12, and 24 months after surgery. Data gathered included the Oswestry Disability Index (ODI) score, the SF-36 questionnaire Physical Component Summary (PCS) score, leg and back pain on rest and spinal loading on a visual analog scale (VAS), and the Patient Satisfaction Index (PSI). The PSI is a 4-point rating scale as follows: 1, "Surgery met my expectations"; 2, "Surgery improved my condition enough that I would go through it again for the same outcome"; 3, "Surgery helped me, though I would not go through it again for the same outcome"; and 4, "I am the same or worse compared to before the surgery." A neurological examination was performed at all time points, and information on adverse events and changes in medication was gathered. Patients were blinded to their treatment; the 12- and 24-month outcome data were assessed by an evaluator blinded for the treatment arm as well.
The primary endpoint of the study was to show between-group differences in ODI 24 months after intervention. Secondary endpoints included an overall success rate defined as 1) at least 15% improvement in ODI score from baseline to month 24, 2) maintenance or improvement of neurological status, 3) no second surgery at the treated level, and 4) no serious adverse event associated with the instrumentation at 24 months. Secondary endpoints were between-group differences at 24 months of low-back pain on the VAS, health-related quality of life measured by the PCS of the SF-36, PSI, and ODI. Adverse events were classified as serious/not serious and related/not related to the procedure or product. Adverse events were coded according to MedDRA version 19.1 (German).
Health Economic Evaluation
An economic evaluation alongside the clinical trial was conducted to evaluate costs from a payer’s perspective according to German recommendations.11 Costs of hospitalization, outpatient treatment, drugs, and further direct and indirect costs such as reduction of working hours, quitting work, and days off due to the lumbar disease were assessed based on a patient questionnaire examining the 3-month period prior to inclusion in the study and after treatment.12,13 The EQ-5D score was assessed at baseline and 12 and 24 months.
Statistical Analysis
Sample size calculation was performed for the primary endpoint using the two-group t-test for fold change assuming log-normal distribution, results from previous data, and nQuery Advisor version 7.0 (Statsols). It was assumed that the coefficient of variation would be approximately 0.5 within the treatment groups. Thus, a sample size of approximately 185 patients per group was required to detect an n-fold difference of at least 1.15 in the ODI with 80% power on a 2-sided level of significance of 0.05. Adding a dropout rate of 15% increased the sample size to 440 patients (220 per group).
All analyses except safety were performed on the full analysis set (FAS), which was based on the intention-to-treat (ITT) principle. It contained all randomized patients with results attributed to their treatment group who had complete preliminary examinations and started surgery. Safety analyses were performed “as treated” on the safety set, which consisted of all patients in the FAS who started the surgical procedure.
Missing baseline ODI values did not occur due to the FAS definition. The last observation carried forward (LOCF) approach was employed to analyze the primary efficacy endpoint. Additionally, complete case and Markov Chain Monte Carlo multiple imputations were applied. The results did not differ substantially from the primary analysis. No other missing data were imputed.
The 2 treatment arms were compared and tested on the FAS for superiority of the experimental over the comparator arm in terms of ODI scores 2 years after surgery.
As randomization was stratified by center and decompression, the primary endpoint analysis needed to include those variables as factors. Unfortunately, several centers had very low recruitment, which justifies the exclusion of center from the analysis. An ANCOVA including ODI baseline score, two-level factor decompression as adjustment covariates, and intervention group as the two-level factor variable was used to compare treatment groups with respect to ODI 2 years postintervention. A 2-sided evaluation of the adjusted group contrast regarding ODI level 2 years postintervention (proof of efficacy) was performed at a 0.05 significance level. Secondary endpoints were analyzed in an exploratory manner at a 2-sided 0.05 significance level. Between-group differences were tested depending on the underlying distribution using the chi-square test, Fisher’s exact test, independent-samples t-test, or the Wilcoxon rank-sum test.
Statistical analyses were performed using SAS, version 9.3 (SAS Institute Inc.) or IBM SPSS Statistics, version 21.0 (IBM Corp.).
Role of the Funding Source
The study was supported by a grant from ulrich medical GmbH & Co. KG. The funding source was not involved in trial design, patient recruitment, data collection, data analysis, or interpretation. It had no access to the study data until final analysis; the database was hardlocked, and the funding source was not involved in any aspect pertinent to the study. The writing and submission of the manuscript were not influenced by the funding source. The authors have not been paid to write this article. The corresponding author had full access to all data and final responsibility for the decision to submit for publication.
Results
Trial Participants
From August 2011 to October 2015, 293 patients were randomized. As the study stopped early due to slow recruitment, a post hoc power analysis revealed that the actual power of the study using the original assumptions was 67.6%. Of 293 patients, 24 were excluded from the FAS, as 19 had missing baseline data and 5 did not undergo surgery. Finally, 269 patients (137 in the fusion group and 132 in the dynamic stabilization group) were available for an ITT analysis (Fig. 2). Four of the patients assigned to dynamic stabilization underwent fusion. Those were included “as randomized” in the ITT analysis and “as treated” in the safety analysis. Baseline characteristics of the ITT population are given in Table 2 without significant differences between groups. The indications for stabilization were not different between groups, as well as the baseline pain and health-related quality of life.
Flowchart showing enrollment, randomization, treatment, 2-year follow-up, and analyses. AE = adverse events. Figure is available in color online only.
Patient characteristics at baseline in the ITT population
| Parameter | FU (n = 137) | DY (n = 132) | p Value |
|---|---|---|---|
| Female sex | 70 (51) | 76 (58) | 0.255* |
| Age, yrs | 64 ± 11 | 64 ± 12 | 0.842† |
| BMI | 28 ± 5 | 27 ± 4 | 0.186† |
| Indication for stabilization | |||
| Spondylolisthesis | 53 (39) | 60 (45) | 0.261* |
| Instability | 28 (20) | 20 (15) | 0.258* |
| Predominant back pain | 68 (50) | 63 (48) | 0.754* |
| Other | 17 (12) | 14 (11) | 0.643* |
| Patient-reported outcome data | |||
| VAS back (rest) | 4.8 ± 2.9 | 4.6 ± 2.9 | 0.754† |
| VAS leg (rest) | 4.2 ± 3.1 | 3.8 ± 2.8 | 0.351† |
| VAS back (load) | 6.5 ± 2.5 | 6.1 ± 2.8 | 0.312† |
| VAS leg (load) | 6.1 ± 3.0 | 5.6 ± 3.1 | 0.207† |
| ODI | 44.7 ± 17.6 | 43.4 ± 17.4 | 0.534† |
| SF-36 MCS | 45.0 ± 14.6 | 43.8 ± 13.5 | 0.554† |
| SF-36 PCS | 28.2 ± 6.8 | 28.3 ± 6.7 | 0.891† |
FU = fusion; DY = dynamic stabilization; MCS = Mental Component Summary.
Values are presented as the number of patients (%) or mean ± SD unless stated otherwise.
Chi-square test.
Independent-samples t-test.
For the final follow-up, 101 (73.7%) patients were available in the fusion group and 96 (72.7%) in the dynamic stabilization group (Fig. 2). A total of 190 patients had ODI scores at final follow-up (97 in the fusion group and 93 in the dynamic stabilization group), which means that 79 scores had to be imputed for the primary endpoint analysis.
Surgical Parameters
Levels treated and surgical parameters are summarized in Table 3. Levels ranged from L1–2 to L5–S1 with a maximum at L4–5 for both groups. Ninety-two (67%) patients were treated monosegmentally and 45 (33%) bisegmentally in the fusion group, versus 96 (73%) and 36 (27%), respectively, in the dynamic stabilization group. Decompression in addition to fusion or dynamic stabilization was performed in 130 (95%) fusion patients and 124 (94%) dynamic stabilization patients. In the fusion group, 58% of decompressions were less-invasive fenestrations, while 42% were laminectomies versus 73% fenestrations and 27% laminectomies in the dynamic stabilization group. The mean duration of surgery was 211 ± 89 minutes for fusion and 184 ± 66 minutes for dynamic stabilization (Wilcoxon rank-sum test, p = 0.005). The mean estimated blood loss was 506 ± 475 ml for fusion and 380 ± 349 ml for dynamic stabilization (Wilcoxon rank-sum test, p = 0.003); a transfusion was necessary in 7 and 3 patients in the fusion and dynamic stabilization groups, respectively.
Levels treated and surgical parameters, ITT population
| Parameter | FU (n = 137) | DY (n = 132) | p Value |
|---|---|---|---|
| Monosegmental | 92 (67) | 96 (73) | 0.319* |
| L1–2 | 1 (0.7) | 0 | >0.99‡ |
| L2–3 | 2 (1.5) | 2 (1.5) | >0.99‡ |
| L3–4 | 39 (28) | 34 (26) | 0.617* |
| L4–5 | 109 (80) | 99 (75) | 0.372* |
| L5–S1 | 31 (23) | 33 (25) | 0.648* |
| Decompression | 130 (95) | 124 (94) | 0.734* |
| Duration of surgery, mins | 211 ± 89 | 184 ± 66 | 0.005† |
| Blood loss, ml | 506 ± 475 | 380 ± 349 | 0.003† |
| Transfusions | 7 (5) | 3 (2) | 0.335‡ |
Values are presented as the number of patients (%) or mean ± SD unless stated otherwise. Boldface type indicates statistical significance.
Chi-square test.
Wilcoxon rank-sum test.
Fisher exact test.
Endpoints (ITT analysis)
Regarding the primary endpoint (ODI score at 24 months after intervention), there was no significant difference between the 2 treatment groups (ANCOVA, p = 0.128). The mean difference in ODI at 24 months was 1.4 (95% CI −3.7 to 6.5). The estimated marginal means for the ODI values at 24 months reported by the ANCOVA model were 27.8 ± 18.7 (95% CI 24.6–31.0) for fusion and 31.4 ± 19.5 (95% CI 28.1–34.7) for dynamic stabilization (Fig. 3). The mean ODI scores for the fusion and dynamic stabilization groups were 45 ± 18 and 43 ± 17 at baseline, 30 ± 21 and 27 ± 20 at 3 months, 28 ± 22 and 28 ± 22 at 12 months, and 29 ± 22 and 30 ± 21 at 24 months, respectively.
Primary and secondary study endpoints. ODI scores per group (A), difference between baseline (BL) and 24-month (24M) postintervention ODI scores within groups (B), SF-36 PCS score (C), and pain during rest and on load in back and legs (D).
The between-group difference in ODI change from baseline to 24 months showed no significant difference (p = 0.890), with changes of 17 ± 20 and 12 ± 21 for fusion and dynamic stabilization, respectively.
The overall success rates were 46% (63/137) in the fusion group and 39% (51/132) in the dynamic stabilization group, with no statistical significance (p = 0.327) (Table 4).
Overall success rate including components in the ITT population (LOCF)
| Parameter | No. of Patients (%) | p Value* | |
|---|---|---|---|
| Fusion (n = 137) | Dynamic Stabilization (n = 132) | ||
| Overall success (LOCF) | 63 (46) | 51 (39) | 0.327 |
| Component | |||
| ODI improved ≥15% (LOCF)† | 102 (74) | 84 (64) | 0.053 |
| Neurological worsening | 21 (15) | 21 (16) | 0.738 |
| Related SAE | 20 (15) | 15 (11) | 0.430 |
| Reop | 9 (7) | 11 (8) | 0.581 |
SAE = serious adverse event.
Chi-square test.
One patient in each treatment group had a baseline ODI value of 0. No calculation of percent change was possible.
The mean intensity of low-back pain at rest improved from 4.8 ± 2.9 at baseline to 2.0 ± 2.2 at 24 months in the fusion group and from 4.6 ± 2.9 to 2.2 ± 2.0 in the dynamic stabilization group. Likewise, back pain on loading improved for both groups at all points, as well as leg pain at rest or loading. Pain intensity improved within groups but did not differ between groups. The SF-36 PCS scores for the fusion and dynamic stabilization groups improved from 28.2 ± 6.8 and 28.3 ± 6.7, respectively, at baseline to 39.3 ± 10.7 and 38.1 ± 9.9 at 24 months and were not statistically different. The mean PSI scores at final follow-up were 1.7 ± 1.0 and 1.9 ± 1.0 for the fusion and dynamic stabilization groups, respectively. At the 24-month follow-up, 39 patients per group reported ongoing or worsened pain.
Secondary Procedures and Adverse Events
Until final follow-up, 141 additional procedures (mostly minor) were necessary; 121 additional infiltration therapies of facet joints, iliosacral joints, and the epidural space were performed (65 cases in the fusion group and 56 cases in the dynamic stabilization group). Twenty reoperations (9 in fusion and 11 in dynamic stabilization patients) were performed for redecompressions (3 fusion and 6 dynamic stabilization patients) and screw-related problems (4 fusion and 3 dynamic stabilization patients), complete replacement of instrumentation (1 fusion and 3 dynamic stabilization patients), or other reasons (5 fusion and 2 dynamic stabilization patients). A total of 106 spine-associated complications occurred (Table 5). Fifty-three medical complications were reported for fusion (n = 27) and dynamic stabilization (n = 26). The overall frequency as well as the frequencies of procedure- or product-related and unrelated adverse events were not different between groups. Likewise, outcomes of the adverse events and serious adverse events were not different.
Spine-associated adverse events; safety set; actual treatment
| Adverse Event | No. of Events | |
|---|---|---|
| Fusion | Dynamic Stabilization | |
| Durotomy | 6 (0 reops) | 4 (0 reops) |
| Neurological worsening | 6 (1 reop) | 3 (1 reop) |
| Screw malposition | 1 (1 reop) | 4 (3 reops) |
| Restenosis/disc herniation | 2 (2 reops) | 2 (2 reops) |
| Facet/ISJ problems | 13 (0 reops) | 10 (1 reop) |
| Screw loosening | 5 (4 reops) | 4 (3 reops) |
| Adjacent-level degeneration | 4 (2 reops) | 5 (3 reops) |
ISJ = iliosacral joint.
Health Economic Evaluation
Within 3 months prior to surgery, the mean number of physician consultations per patient in the fusion group was 2.7 (range 0–12), and that for the dynamic stabilization group was 3.1 (range 0–13). Some patients specified that they had lost days of work (15.0% of fusion and 15.8% of dynamic stabilization patients) due to lumbar degenerative disease, with a mean of 33.2 days (33.8 days and 32.2 days in the fusion and dynamic stabilization groups, respectively; range 1–90 days) during the previous 3 months.
The length of inpatient stay due to surgery was not different between the two groups (fusion: 9.6 ± 30.1; and dynamic stabilization: 9.3 ± 24.8, resulting in mean costs for fusion of €6720 ± €21,070 and for dynamic stabilization of €6510 ± €17,360), and drug costs were not different between groups. However, the duration of surgery was significantly different between groups, resulting in a cost difference. The mean duration of surgery was 211 ± 89 minutes for fusion and 184 ± 66 minutes for dynamic stabilization (p = 0.005), resulting in operating room costs of €7792.12 ± €3075.84 for fusion and €6478.84 ± €2280.96 for dynamic stabilization. Additional costs were €2192 for mono- and €3656 for bisegmental fusion implants versus €2288 and €3377 for mono- and bisegmental dynamic stabilization, resulting in mean implant costs of €2673 ± €690 for fusion and €2585 ± €487 for dynamic stabilization. The mean in-hospital costs were €17,185.12 for fusion and €15,574.84 for dynamic stabilization, a difference of 10% between groups.
At the 12-month follow-up, 5 patients from the fusion and 2 from the dynamic stabilization groups reported still being out of work. At that point, the mean number of days lost was 17.8 (25.6 ± 37.5 for fusion and 10.0 ± 0 for dynamic stabilization). At the 24-month follow-up, 2 fusion and 4 dynamic stabilization patients reported days off work because of back problems (14.0 ± 9.9 days for fusion and 15.8 ± 10.27 days for dynamic stabilization), and 7.1% of fusion patients and 5.2% of dynamic stabilization patients reported that the spinal disease had delayed their career development. Additionally, 40.5% and 38.1% of fusion and dynamic stabilization patients, respectively, did not see the disease as a reason for any change in their working status.
Discussion
This multicenter, double-blind, prospective, randomized, 2-arm superiority trial revealed no difference in functional outcome after 2 years for patients with degenerative lumbar instability undergoing fusion or dynamic pedicle-based stabilization of 1 or 2 segments. In addition to the lack of difference in functional outcomes, the number of procedure-related or implant-related adverse events and secondary procedures was not different. Secondary endpoints such as overall success and patient satisfaction did not differ between treatment groups. However, there were differences in the duration of surgery and intraoperative blood loss, with shorter duration and less blood loss for dynamic stabilization, resulting in lower hospital treatment costs.
The rate of lumbar fusion surgeries for degenerative disease has increased rapidly during recent years.14,15 Various surgical techniques of instrumented and noninstrumented fusion are used in these sugeries. In parallel with the increasing rate of fusion surgeries, indications in degenerative disease have been assessed in recent studies identifying subgroups of degenerative instabilities necessitating fusion for adequate functional outcome.16–20 While degenerative spondylolisthesis per se is no longer accepted as an indication for fusion,16,18 for the present trial, accepted inclusion criteria for lumbar fusion were ≥ 5 mm spondylolisthesis, ≥ 3 mm or ≥ 10° mobile spondylolisthesis, or predominant back pain with Modic changes.17 For this subgroup of patients with degenerative instabilities, fusion was compared with dynamic pedicle-based stabilization.
The ideal technique for fusion remains unclear, and many variations are available. While motion reduction and fusion rates with the use of the older noninstrumented techniques are potentially low, more modern instrumented pedicle-based stabilization together with interbody fusion by different approaches has become an established technique.21 Various technical approaches to interbody fusion have been compared in many studies. While the complication rates and types depend on the approach, functional patient outcomes and fusion rate do not depend on whether interbody fusion is anterior, posterior, transforaminal, or lateral.22 However, all approaches necessitate additional soft-tissue manipulation with removal of disc material from the interbody space and implantation of fusion material. Side effects of interbody fusion relate to the surgical invasiveness based on the approach to the anterior column and of fusion itself, with the potentially associated increased adjacent-segment strain that can lead to radiological or symptomatic degeneration.23–25
In contrast, pedicle-based dynamic stabilization is based on the reduction of pathological painful motion and preservation of residual nonpathological painless motion.8 Theoretical advantages are reduced surgical invasiveness and adjacent-level strain.26,27 Different types of pedicle-based stabilization devices have been assessed in uncontrolled and controlled lower-quality trials with contradictory results and a high rate of screw failure for some of the assessed devices.28–31 The present trial is the first high-quality, controlled, blinded study revealing a similar clinical efficacy of dynamic pedicle-based stabilization in comparison with fusion with reduced surgical invasiveness, with the same number of procedure- and implant-related complications. The rate of symptomatic adjacent-segment degeneration was not different between groups.
In a biomechanical analysis, the dynamic stabilization of the present study reduced motion of an intact segment by 53% for lateral bending, 21% for axial rotation, 68% for flexion, and 40% for extension.8 So far, the in vivo long-term effect of this motion reduction and preservation of residual motion remains ambiguous. However, 2 uncontrolled studies revealed that motion reduction leads to a functional outcome comparable to that of fusion with a mean ODI score improvement of 26.4–30, a VAS pain score improvement of 4.4–4.8, and a SF-36 PCS score improvement of 5 points, associated with implant-related reoperation rates of 2.8% and 2.9% after mean follow-ups of 15 and 24 months, respectively.4,6 Whether the residual motion leads to any change in adjacent-segment strain, is just sufficient to avoid pedicle screw loosening, or is lost by slow delayed fusion remains undetermined.
Screw loosening or implant breakage is a common concern in dynamic pedicle-based stabilization and has been reported for the Dynesys system in several series. A previous retrospective study reported screw loosening in 19.7% of 71 patients as assessed by CT scans obtained 18–24 months after surgery.32 The same group evaluated a larger cohort of patients at an average follow-up of 51.1 months and detected screw loosening in 20.4% of patients.33 These data demonstrate that radiographic screw loosening seems to occur within 2 years after surgery, mostly without a relevant increase during later follow-up. Furthermore, radiographic screw loosening was not associated with clinical decline.32,33 The rate of radiographic screw loosening in the present study remains largely unclear, as no follow-up CT scans were obtained within the protocol, and radiographs allow a limited evaluation of screw loosening. However, the hydroxyapatite coating of the dynamic system used in the present study is intended to avoid screw loosening, and the incidence of symptomatic implant-related complications was not increased in dynamic stabilization in comparison with fusion, thereby showing no increased risk of symptomatic implant failure. In addition to screw loosening, previous retrospective studies on lumbar dynamic stabilization with the Dynesys device have revealed that the rationale of maintaining motion was not achieved in some cases, where dynamic stabilization induced slow fusion. When evaluating postoperative segmental motion in previous studies, several cases showed segmental motion below 3° on flexion-extension radiographs.34,35 In another study, CT scans obtained 18–24 months postoperatively demonstrated an unintended facet joint arthrodesis in more than 50% of cases with Dynesys stabilization.36,37 Whether slow fusion was associated with dynamic stabilization in the present study remains unclear, as fusion in the dynamic stabilization and fusion groups was not systematically assessed by CT scans as recommended for the evaluation of fusion.38 However, irrespective of a potential slow fusion in dynamic stabilization, the benefit of reduced surgical invasiveness while achieving the same clinical results in comparison with fusion remains.
To elucidate the aspects of further symptomatic screw loosening, slow fusion, and adjacent-segment degeneration in the present series of patients, further long-term observation and a thorough analysis of further follow-up imaging by CT are necessary. Regarding adjacent-segment disease, the present follow-up of 2 years was too short to establish differences in symptomatic or asymptomatic adjacent-segment degeneration between groups.
However, a benefit of the less-invasive dynamic technique was proven in terms of reduced operating room time and intraoperative blood loss, with no functional outcome differences in comparison with fusion.
The cohort assessed in the present study showed marginal differences in baseline values without any significant differences. After treatment, minimal clinically important differences (MCIDs) for lumbar fusion are 1.2 for back pain, 1.6 for leg pain, 12.8 for ODI, and 4.9 for SF-36 PCS scores.39 These values were reached or exceeded for all time points except the final 2-year ODI of the dynamic stabilization arm. With a value of 12, the MCID for ODI was marginally not reached, while the MCID was exceeded for VAS back and leg pain and SF-36 PCS for this time point. Likewise, the proportion of patients reaching an ODI improvement of ≥ 15% at 2 years was slightly lower in the dynamic stabilization group, as well as the overall success rate. However, these differences did not reach significance, and overall ODI scores at different time points showed no differences. Therefore, these marginal differences between groups might be attributable to the sample size decreasing due to patient dropout.
While outcome assessment of the primary endpoint was performed by a blinded assessor and patients were blinded to the treatment arm to reduce bias, blinding of the surgeons was not possible. Therefore, surgeons were not involved in the final evaluation to reduce bias. Despite structured study monitoring, the patient dropout rate during follow-up was higher than estimated when calculating the initial sample size, weakening the results of the study. However, dropout rates showed the same proportion in both arms, and baseline values of lost patients as well as ODI scores were not different between groups before dropout and inclusion criteria. The analysis of the data using the LOCF approach compensates for these patients but does carry some risk of bias. To reduce this risk, LOCF was compared with multiple imputations, which did not reveal different results.
While the present study shows equivalent functional outcomes of fusion and dynamic stabilization with reduced surgical complexity and invasiveness as well as reduced costs in the dynamic group, these results cannot be generalized without limitations to other pedicle-based dynamic implants, since methods of dynamization are variable between devices. However, dynamic pedicle-based systems with similar mechanisms could lead to comparable results to those found in the present study.
The study follow-up duration of 2 years does not exclude any changes in functional outcome between groups beyond this time point. Further advantages and disadvantages of fusion or dynamic stabilization might occur during later follow-up as adjacent-segment problems or late implant failure. To elucidate for these potential problems, further long-term follow-up is necessary.
Conclusions
The present study shows that dynamic pedicle-based stabilization for lumbar degenerative mono- or bisegmental instability can have functional results similar to the standard of transforaminal lumbar fusion. While procedure- and product-related complications as well as overall complications were not different between groups, surgical invasiveness and associated costs were reduced in the dynamic stabilization group. Therefore, dynamic pedicle-based stabilization is a beneficial alternative to fusion for selected cases.
Appendix
DYNORFUSE Study Group
Bernhard Meyer, MD,1 Florian Ringel, MD,1 Michael Behr, MD,1 Haiko Pape, MD,1 Michael Putzier, MD,2 Marc Schuerings, MD,2 Claudius Thomé, MD,3 Sebastian Hartmann, MD,3 Peter Vajkoczy, MD,4 Marcus Czabanka, MD,4 Veit Rohde, MD,5 Kajetan von Eckardstein, MD,5 Wolfgang Börm, MD,6 Stefan Zausinger, MD,7 Rafael Sambale, MD,8 Michael Stoffel, MD,9 Marcus Richter, MD,10 Mirko Arp, MD,11 Richard Bostelmann, MD,12 Frerk Meyer, MD,13 Fritz Weber, MD,14 Tobias Schulte, MD,15 Uwe Spetzger, MD,16 Andreas Müller, MD,17 Rainer Wirtz, MD,18 Richard Dodel, MD,19 Janis Evers, MA,19 Elisabeth André, PhD,20 Alfred Zollner, PhD,20 and Viktoria Kehl, PhD21
1Department of Neurosurgery, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany (96 patients); 2Department of Orthopedic Surgery, Charité Medical Center, Berlin, Germany (37 patients); 3Department of Neurosurgery, University of Innsbruck, Innsbruck, Austria (31 patients); 4Department of Neurosurgery, Charité Medical Center, Berlin, Germany (29 patients); 5Department of Neurosurgery, University of Göttingen, Göttingen, Germany (21 patients); 6Department of Neurosurgery, Diakonissenkrankenhaus, Flensburg, Germany (18 patients); 7Department of Neurosurgery, University of Munich, Munich, Germany (14 patients); 8Orthopädische Klinik Hessisch-Lichtenau, Hessisch-Lichtenau, Germany (10 patients); 9Department of Neurosurgery, Helios Klinikum Krefeld, Krefeld, Germany (6 patients); 10Department of Spine Surgery, Josephs Hospital Wiesbaden, Wiesbaden, Germany (6 patients), 11Department of Neurosurgery, University of Mannheim, Mannheim, Germany (6 patients); 12Department of Neurosurgery, University of Düsseldorf, Düsseldorf, Germany (6 patients); 13Evangelisches Krankenhaus Oldenburg, Oldenburg, Germany (4 patients); 14Department of Neurosurgery, Klinikum Köln Merheim, Köln, Germany (3 patients); 15Department of Orthopedic Surgery, University of Münster, Münster, Germany (2 patients); 16Department of Neurosurgery, Klinikum Karlsruhe, Karlsruhe, Germany (2 patients); 17Department of Neurosurgery, RWTH Aachen, Aachen, Germany (1 patient); 18Department of Neurosurgery, University of Ulm, Ulm, Germany (1 patient); 19Department of Geriatrics, University of Duisburg-Essen, Duisburg and Essen, Germany (health economic evaluation); 20Münchner Studienzentrum, Technical University of Munich, Munich, Germany (monitoring and data management); and 21Institute for Medical Informatics, Statistics and Epidemiology, Technical University of Munich, Munich, Germany (statistical analysis)
Disclosures
This study was supported by a grant from ulrich medical GmbH, Germany, to B.M.
Dr. Meyer: consultant for ulrich medical, Sonovum, Medacta, Icotec, Spineart, and Medtronic; honoraria from ulrich medical, DePuy Icotec, Spineart, Medtronic, and Brainlab; royalties from Spineart and Medacta; clinical or research support for this study from ulrich medical; and support of non–study-related clinical or research effort from Icotec, Medtronic, and Relievant. Dr. Thomé: consultant for DePuy Synthes, Intrinsic Therapeutics, and Signus Medical; clinical or research support for the study described from Technical University Munich; support of non–study-related clinical or research effort from Brainlab, DePuy Synthes, Icotec, Intrinsic Therapeutics, Pfizer, Signus Medical, and TETEC; and honoraria from Brainlab, DePuy Synthes, Icotec, Intrinsic Therapeutics, Medtronic, NuVasive, Siemens, Signus Medical, and Zeiss. Dr. Ringel: consultant for Spineart, Stryker, and Icotec; royalties from Spineart; and speakers bureau for Stryker and Icotec.
Author Contributions
Conception and design: Ringel, Meyer, Thomé, Vajkoczy, Dodel. Acquisition of data: Ringel, Meyer, Thomé, Vajkoczy. Analysis and interpretation of data: Ringel, Meyer, Kehl, Dodel. Drafting the article: Ringel, Kehl, Dodel. Critically revising the article: Ringel, Meyer, Thomé, Vajkoczy. Reviewed submitted version of manuscript: Ringel, Meyer, Thomé, Vajkoczy, Dodel. Approved the final version of the manuscript on behalf of all authors: Ringel. Statistical analysis: Ringel, Kehl. Study supervision: Ringel, Meyer.
Supplemental Information
Previous Presentations
Portions of the paper were presented orally at the 13th Annual Meeting of the German Spine Society (DWG), Wiesbaden, Germany, December 6–8, 2018; and the Global Spine Conference 2019, Toronto, Ontario, Canada, May 15–18, 2019.
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