Medium and long-term radiographic and clinical outcomes of Dynesys dynamic stabilization versus instrumented fusion for degenerative lumbar spine diseases

Lumbar fusion has been considered as the gold standard for the treatment of lumbar degenerative pathologies over the last decades [1, 2]. However, instrumented fusion causes a series of complications, including instrumentation failure, pseudoarthrosis, and pain in donors [3‐5]. Moreover, the preservation of lumbar motion is less considered in fusion instrument, thus limiting the motility of stabilized segments and the increased load on adjacent segments, which may increase the risk of adjacent segment degeneration (ASDeg), adjacent segment disease (ASDis), and severe post-operative functional disabilities [6‐8].

In recent years, non-fusion systems have been applied clinically to prevent the incidence of adjacent segment pathology (ASP, including ASDeg and ASDis) after lumbar surgeries [9‐11]. The Dynesys posterior dynamic stabilization system (Zimmer Inc., Warsaw, IN, USA), which was first introduced in 1994, is a commonly used dynamic stabilization device [12]. This system consists of pedicle screws (Ti alloy), polyethylene–terephtalate (PET) cords, and polycarbonate–urethane (PCU) spacers for the stabilization of stabilized segments, restoration of normal segmental kinematics, and preservation of adjacent motion, and these parts aim to prevent the instability and decrease ASP incidence [5, 13, 14]. The Dynesys system (DS) has shown significant improvement in terms of visual analog scale (VAS) pain scores, Oswestry disability index (ODI) scores, trauma severity, and recovery time compared with the fusion method [4, 15‐18]. Moreover, the range of movement (ROM) at adjacent segments and along with the load across the intervertebral and adjacent discs has been reduced [4, 7, 13, 15, 19].

However, inconsistent results have been obtained in the advantages of DS over fusion method in terms of clinical outcomes, including the relief in post-operative pain status (VAS scores) and functional status restoration (ODI scores) [3, 5‐7, 20‐23]. Whether DS can retain ROM at surgical segment and reduce ASP in long-term follow-up remains controversial [5, 6, 23]. Furthermore, previous studies have mainly focused on short-term clinical efficiency, and limited reports have focused on medium and long-term outcomes with conflicting findings, especially trials involving fusion method as control [7, 15, 24‐26]. Thus, this meta-analysis aimed to compare the radiographic and clinical outcomes of dynamic DS and instrumented fusion for the treatment of degenerative lumbar spine diseases with a minimum follow-up period of 2 years.

Spine fusion is a primary therapy for spinal degenerative diseases, but this method is associated with several complications, especially the acceleration of ASP [1, 5, 13]. In response to the complications caused by fusion, dynamic stabilization techniques have been developed, including DS, which is a widely used non-fusion technique with the advantage of ASP prevention [3].

Many studies have confirmed the safety and clinical equivalence of DS to fusion method. [4, 21, 24, 36] DS relieves clinical symptoms and improves the functional status and fusion instruments with durations of follow-up ranging from 28.78 months to 70.14 months [7, 15, 16, 20, 26]. Furthermore, Bredin et al. [35] performed a study with a follow-up of 93.6 months and concluded that the DS group showed substantial improvements in VAS and ODI scores compared with the fusion method. The fusion method is likely to be chosen in the retrospective cases, in which a more severe disease state is present. Additionally, patients with far lateral discs or other pathology that required facetectomy and TLIF were not considered as candidates for DS because of the possibility of introducing confounding variables. Therefore, only the prospective studies that report the VAS and ODI scores were included for analyses, because these studies may effectively control the severity of diseases in this meta-analysis. In conclusion, DS remarkably improved the VAS scores for back and leg pain. No significant difference was observed in post-operative ODI scores between the two groups. The results may be explained as follows. Although the decompression of nerve roots was conducted in DS and fusion methods, the risk of nerve root injuries increased, because the latter not only dissects the bone and soft tissue but also requires the preparation of endplates and insertion of interbody devices or bone grafting, leading to back and leg pain after surgery. Additional surgical instrument and enlargement of surgical trauma in fusion method may increase the surgical time, blood loss, and in-hospital complications. Furthermore, the occurrence of non-fusion with fusion method would aggravate clinical symptoms in the later recovery process. Moreover, the Dynesys group did not differ from the fusion group in terms of screw loosening and breakage. These results can be attributed to the simplified operation of DS and the preservation of lumbar mobility. Hence, DS showed equivalent, or even better clinical measurements, compared with conventional fusion. The safety of DS has complied with the basic requirements for widespread clinical application in cases meeting indications, such as disc herniation, lumbar spinal stenosis, and grade I degenerative spondylolisthesis in lumbar.

DS is mainly used to minimize ASP. [6, 37] Hashimoto et al. [8] reported that the fusion of stabilized segments may increase the biomechanical stress on the adjacent levels, leading to ASDeg and ASDis. DS preserves the ROM at the stabilized segments and prevents hypermobility at the adjacent segments in the medium or long-term follow-up [4, 7, 15, 20, 21, 24, 36]. By contrast, Yang et al. [17] found that DS does not maintain the ROM at the adjacent segments compared with fusion method after a minimum follow-up of 24 months. Furthermore, Schaeren et al. [38] found no measurable motion at the stabilized segments after DS but reported signs of degeneration at adjacent segments in 47% patients after 4 years, thus supporting our findings from the use of fusion instrument. In the current meta-analysis, in the fusion group, the ROM at stabilized segments and the ROM of LL decreased remarkably, but the ROM at adjacent segments increased remarkably compared with those in the DS group. This result was obtained, possible because the DS pedicle screws are connected to the PET cords, thereby providing tension to limit excessive flexion and PCU spacers resisting compressive force to limit over extension; this phenomenon leads to reduced vertebral abnormal activity and preserved motion at the instrumental segments and hypermobility at the adjacent segments in the medium and long-term duration [39].

Another early radiographic manifestation of ASP in clinical practice is the narrowness and loss of the intervertebral space, and its height is commonly considered as an indicator to evaluate the degree of ASP [3, 15]. In the current meta-analysis, the Dynesys and fusion group did not differ in terms of disc heigh both at stabilized and adjacent segments. The results can be attributed to the natural degenerative progression of the disc at the stabilized and adjacent segments despite DS, and this phenomenon is not associated with the stabilization method [40, 41]. Thus, DS showed no advantage over fusion method in terms of prevention of disc degeneration after surgery.

Whether DS can delay the occurrence of ASP has not been confirmed. In the study of Bredin et al. [35], a mean follow-up of 5.5 years was obtained, and significantly less ASDeg was observed in the DS group than in the fusion group (12.1% versus 36%); Zhang et al. [24] confirmed the conclusion in their study with mean follow-up of 55.2 months. DS may not prevent ASDeg, with high rates ranging from 16 to 47% [37, 38, 42]. In the current meta-analysis, the Dynesys group showed a lower rate of ASDeg than the fusion group. Although DS showed no superiority in terms of preventing disc degeneration compared with fusion method, the former performed well in terms of sparing abnormal biomechanical load at the stabilized and adjacent segments, consequently preserving the physiological motion after stabilization; this finding may explain why DS can prevent ASDeg [10, 43]. Furthermore, the PET cords and PCU spacers were used to restrict flexion and extension. With increasing cord pretension, the flexion ROM at stabilized and adjacent levels increased, but the extension ROM decreased [44]. The realistic stiffness of DS varies with cord pretension and spacer length, which are decided based on surgeons’ personal experience; this phenomenon might partly explain the conflicting results in previously published works [7, 45, 46].

The prevention of ASDis is crucial because of the potential risk of revision surgery and adverse effect on health care outcomes and cost. Our meta-analysis demonstrated that the Dynesys group showed less ASDis than the fusion group, whereas the difference was not significant. The result revealed that DS can prevent ASDeg instead of ASDis compared with the fusion method. The occurrence of ASDis is relatively lower than that of ASDeg, because ASDeg does not always cause clinical symptoms. Furthermore, the lack of remarkable difference in disabilities (ODI) post-operatively between the two methods indicate the similar incidence of ASDis after surgery.

Screw loosening is a common complication of DS. The current study also showed that the Dynesys group did not differ from the fusion group in terms of screw loosening and screw breakage. Ko et al. [47] included 71 patients and found that the screw loosening had no adverse effect on the improvement of VAS and ODI scores after the surgery. Hu et al. [48] investigated the mid- and long-term outcomes of hybrid surgery that combined Dynesys fusion and non-fusion stabilization in the treatment of degenerative lumbar diseases. The results show that screw loosening mainly occurred at the end of the fixed segment, and old age was a risk factor for screw loosening. Screw loosening is usually asymptomatic and can be observed regularly. If the symptoms related to screw loosening or screw breakage occur, and the conservative treatment fails, revision surgery is required. Based on Chinese expert consensus on the treatment of lumbar degenerative disease by trans-pedicle dynamic rod fixation, the rate of screw loosening can be reduced by preserving the integrity of the bony structure and ligaments of the posterior column during decompression; thicker and longer screws are preferred, and repeated adjustment of depth and direction need to be avoided when placing screws [49].

The disadvantages of DS should be highlighted. DS is designed to preserve lumbar vertebral mobility and reduce the load on the intervertebral disc; if these two points cannot be achieved, the use of this technique has no advantage [49]. The Dynesys device is subjected to continuous stress during weight-bearing of the spine. Thus, fixation failure may occur in the cases of severe osteoporosis and severe decreases of lumbar stabilization. Besides, DS cannot effectively stabilize the lumbar spine when used for isthmic spondylolisthesis with a high risk of fixation failure [50]. Moreover, for patients with severe stenosis in intervertebral space and small range of motion before surgery, the range of lumbar motion is limited after surgery, thus rendering the use of DS unsuitable [51].

This meta-analysis has several limitations. First, only one RCT along with 16 comparative cohort studies was included, resulting in less powerful results compared with that obtained purely from RCTs. Second, the definitions of ASDed and ASDis in previous studies are inconsistent and ambiguous. No consensus criterion has been established to define ASDed and ASDis, leading to imprecise outcome measures. Third, the number of the included studies was limited in terms of the ROM of LL, disc heigh at proximal adjacent segments, and ASDis. Furthermore, the fusion method is likely to encounter screw loosening initially, while the dynamic stabilization is likely to fail in the latter part. This flaw is expected in studies that evaluate early outcomes. Thus, future work should employ more RCTs of high quality and patients with follow-up more than 10 years from different cultural contexts.

DS showed equivalent outcomes in terms of ODI, screw loosing, screw breakage, and ASDis compared with fusion method. Moreover, DS had better clinical measurements in terms of VAS scores for back and leg pain than fusion instrument. DS showed comparable clinical outcomes and provided benefits in preserving the motion at the stabilized segments, thus limiting the hypermobility at the adjacent segments and preventing ASDeg compared with the fusion method in degenerative disease with or without grade I spondylolisthesis.