Dynamic Stabilization Surgery in Patients With Spinal Stenosis

Long-Term Outcomes and the Future

Jong-myung Jung, MD; Seung-Jae Hyun, MD, PhD; Ki-Jeong Kim, MD, PhD; Tae-Ahn Jahng, MD, PhD

Disclosures

Spine. 2021;46(16):E893-E900. 

In This Article

Discussion

Most studies have reported favorable clinical outcomes for patients treated with the Dynesys system.[7,13,14,18–20] A meta-analysis comparing the Dynesys system and posterior lumbar interbody fusion reported that there were no significant differences in the radiologic and clinical outcomes between the two techniques.[21] However, only a few studies focusing on the long-term outcomes of DS for LSS with or without spondylolisthesis have been published. The present study showed that decompression and DS for LSS with or without spondylolisthesis yielded favorable long-term outcomes. The radiographic and clinical results were similar in both groups at the final follow-up evaluation.

Almost all radiographic parameters showed good results in both the LSS group and the spondylolisthesis group. However, global lordosis decreased from 36.5° ± 8.2° preoperatively to 32.6° ± 6.0° at the last follow-up in the spondylolisthesis group (P = 0.039). This phenomenon seems to be the result of the natural course of aging, not a secondary change caused by surgery. A previous study reported that, even in asymptomatic patients, global lordosis was steady until the age of 70, but after 70 it significantly decreased (P = 0.002).[22]

Preservation of ROM at the stabilized segment could avert ASD by preventing hypermobility and reducing stress.[20] Nonetheless, it remains controversial whether DS can prevent the occurrence of ASD. Several studies reported unfavorable radiographic outcomes with long-term follow-up.[15,23] However, the patients included in these studies were relatively old, and their degenerative changes were also severe. Several studies have reported that the Dynesys system can preserve the ROM of the stabilized segments and prevent radiographic ASD.[14,24] Another study with a long-term follow-up duration (minimum 5.5 years) reported that radiologic ASD was significantly less common in the Dynesys group than in the fusion group (12.1% vs. 36.0%, P = 0.01).[14] A systematic review reported that DS surgery was effective in protecting against ASD.[25] The present study showed that DS partially preserved the ROM of the index level, which changed from 7.8° ± 2.0° to 4.9° ± 2.0° in the LSS group and from 6.6° ± 1.3° to 4.2° ± 1.5° in the spondylolisthesis group.

In this study, global ROM decreased continuously from the preoperative examination to the last follow-up in both the LSS group and spondylolisthesis group (P = 0.135 and P = 0.012, respectively). This phenomenon seems to have resulted from spontaneous joint facet fusion or facet joint degeneration. In a previous study on DS-related facet arthrodesis, spontaneous facet joint fusion occurred in 54.3% of cases, with an average ROM of 3.7° ± 3.3°.[26] In addition, 71.1% of patients had ROM <3°. However, this spontaneous fusion did not affect clinical outcomes. In another study, the Dynesys system caused facet joint degeneration at the index and proximal adjacent level, but less degeneration than occurred after fusion surgery.[27]

Theoretically, if part of the spine is fused, the remaining nonfused segments must do more work to preserve spinal motion.[1–3,9] Limited motion at the operated segments may increase stress on the nonoperated upper levels by inducing a hypermobile state; therefore, stress on adjacent segments is likely to increase after stabilization.[2,9,28] A cadaveric study reported that the proximal adjacent level had to work more frequently toward the extremes of its ROM after fusion surgery.[28] The repetitive extreme motion may exceed physiologic limits with time, potentially causing ASD.[29] It was reported that clinical ASD occurred in 20% of patients who underwent spinal fusion surgery.[1–3,30] Theoretically, DS may prevent or slow ASD and permit motion of the stabilized segment.[1,3,19] ASD after stabilization with Dynesys was initially observed in a multicenter study, and ASD was the most common reason for reoperation.[20] However, degeneration at an adjacent segment may be a natural process rather than something caused by DS.[19,31] In cadaveric studies, adjacent segment motion was not found to be influenced by DS, and intradiscal pressure did not increase.[10,31] In the present study, radiographic ASD and symptomatic ASD occurred in four patients (14.8%) and two patients (7.2%) in the LSS group and in five patients (13.2%) and three patients (7.9%) in the spondylolisthesis group, respectively. The surgical outcomes seem to be better than those of the previous study because of strict patient selection, excluding relative contraindications such as previous ASD and spinal surgery.

Screw loosening is one of the most common complications of DS surgery.[32] In several retrospective studies, the incidence of screw loosening was 18.0% to 19.8%.[33,34] Another long-term follow-up study showed that screw loosening occurred in 20.5% (22/107) of patients, and three patients (2.8%) underwent revision surgery.[35] Decreased bone mineral density is known to be a risk factor for screw loosening.[36] In this study, screw loosening occurred in 12.3% (8/65) of patients, none of whom underwent re-operations. The mean BMD of the LSS group and the spondylolisthesis group seems to have shown good results (−1.2 ± 0.7 and −1.3 ± 0.8, respectively).

How to improve

To improve surgical outcomes, we would like to make several suggestions. Appropriate patient selection should be considered. DS surgery has provided favorable surgical outcomes to relatively young patients (particularly <60 years) with painful degenerative disc disease, LSS, and grade I degenerative spondylolisthesis.[18]

The DS system should replace the fused segments, not extend the stabilization segments. The radiographic and clinical outcomes of the hybrid system (DS and fusion surgery) in long-term follow-ups are still not definitive.[37]The spacer shape should be modified into a trapezoid rather than a rectangular shape. A finite element study reported that the maximum von Mises stresses were markedly decreased with increased angle of the spacer up to 20° (Figure 3A–C).[38]

Figure 3.

(A) Three-dimensional finite element model of the Dynesys spinal system implanted L4–5 spinal motion segment with three loading conditions. (B) The maximum von Mises stresses were markedly decreased with an increased angle of the spacer up to 20°.

The magnitude of cord pretension should be decreased. A biomechanical study reported that for the implanted level, increasing the cord pretension from 100 to 300 N increased flexion stiffness, a facet contact force, the annulus stress, and the high-stress region of the pedicle screw.[39]

The DS system should be compatible with the change of the distance between two pedicle screws. Although the NFlex system, which improves upon this shortcoming, seems effective in improving pain and functional scores with sustained clinical improvement after 2 years, long-term evidence is needed.[40]

The center of rotation of the DS system should be similar to that of a normal intact spine. A finite element analysis demonstrated that the center of rotation and stress distribution differed according to the design and materials used.[41]

Limitations of This Study

This study has several limitations; it has a retrospective observational design, it is a single institutional study, and it includes a small number of patients. Furthermore, radiographic parameters, including disc height, segmental lordosis, and segmental ROM at index level and proximal adjacent level, are different at each level. Despite these limitations, the present study analyzed the long-term radiologic and clinical outcomes in patients with LSS with or without spondylolisthesis treated with DS and suggested how we can improve the results. An additional long-term comparative follow-up study of the DS system and fusion surgery is required to more accurately evaluate the clinical efficacy of the Dynesys system for lumbar degenerative diseases.

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