Lumbar disc rehydration after dynamic stabilization: A systematic review

Background:

Interbody fusion combined with posterior screw fixation is a traditional method used for treating lumbar degenerative disease (LDD). But in recent years, there have been more and more reports about its complications. Dynamic Stabilization Systems (DSS) are another method for the treatment of LDD, but the clinical results are still inconclusive. The objective of this study is to review, analyze, and discuss the probability of disc rehydration by DSS designed for LDD by systematically reviewing previous relevant studies.

Methods:

The Pubmed, Web of Science, and Embase databases were searched using keywords for articles published before June 2022. The following keywords were used: rehydration, rehydrated, lumbar, spine, disc, spinal, degenerative disc disease, degenerative spine disease, vertebrae, vertebral column, thoracolumbar, and lumbosacral. The included studies were printed in English. Two independent investigators compiled all data. For the quality assessment, the Newcastle–Ottawa Scale was used to evaluate case–control studies, while the Joanna Briggs Institute critical appraisal checklist was used to evaluate the case series studies.

Results:

This systematic review included 7 studies comprised of 5 case series and 2 case–control studies. Seven articles involving 199 cases were enrolled for the data extraction. Of the 199 cases, 55 cases observed rehydration, as evaluated by Pfrimann grading on magnetic resonance imaging. The rehydration rate was 27.6% (55/199). DSS can provide positive clinical outcomes. Both visual analog scale and Oswestry Dysfunctional Index scores were significantly improved at the final follow-up.

Conclusion:

DSS may promote disc rehydration and delay the development of LDD to some extent. Mechanical stretch may play an important role in the progress of intervertebral disc rehydration. It provides important evidence for the clinical application of DSS.

1. Introduction

The aging population has become a significant global social issue, having increased the prevalence of lumbar degenerative disease (LDD) and hence decreased the quality of life for those affected.^[1,2] Interbody fusion combined with posterior screw fixation is a traditional method used for treating LDD.^[3] Although it provides immediate postoperative stability and allows early functional exercise, there have been an increasing number of reports regarding its complications. Such complications include rod fracture, fusion failure, screw loosening, and adjacent segment degeneration (ASD).^[4,5] One important reason for this is that rigid metal alloys such as titanium alloys and stainless steels may cause abnormal changes in disc pressure, which can lead to disc degeneration.

In recent years, more compliant spinal stabilization systems were used for the lumbar spine to avoid these adverse effects. They are called dynamic stabilization systems (DSS). DSS has garnered attention as a surgical treatment for LDD. DSS has been shown to maintain physiological motion and reduce pressure by unloading the disc. However, several studies have produced contradictory findings regarding its clinical effectiveness, implant failure rates, and radiographic outcomes. Recently, several studies have shown that dynamic stabilization can achieve disc rehydration,^[6–8] and it may promote disc regeneration by providing suitable conditions for healing.

To enhance the current understanding of disc rehydration and regeneration, we conducted a systematic review. We aimed to compile previously published studies to assess whether DSS can effectively delay the progress of LDD by promoting disc rehydration.

2. Material and Methods

2.1. Search terms

The enrolled studies were selected through a systematic search of several electronic databases (from their inception until June 2022), including the Pubmed, Embase, and Web of Science databases. For the database search, the following keywords were used: “rehydration OR rehydrated” AND “lumbar OR spine OR disc OR spinal OR degenerative disc disease OR degenerative spine disease OR vertebrae OR vertebral column OR thoracolumbar OR lumbosacral.” Two separate researchers conducted the research.

2.2. Inclusion and exclusion criteria

The inclusion criteria were as follows: patients were diagnosed with LDD preoperatively based on their symptoms and radiography; case series or case–control studies were related to disc rehydration after dynamic stabilization; (3) all articles were published in English.

The exclusion criteria were as follows: comment, review, letter, and meeting abstract formats; the full-text articles could not be found.

2.3. Rehydration evaluation criteria

In this systematic review, all enrolled cases had preoperative and postoperative magnetic resonance imaging (MRI). In previous studies, many of the criteria for evaluating rehydration were mentioned. To unify the standard, we use Pfrimann grading on MRI as the gold standard for evaluating lumbar disc rehydration.

2.4. Clinical outcomes evaluation criteria

In this systematic review, the included articles provided clinical outcomes after dynamic internal fixation, with visual analog scale (VAS) and Oswestry Dysfunctional Index (ODI) scores being the most widely used criteria. We collected and analyzed clinical outcome data by carefully reading articles and adding them to a table.

2.5. Data extraction

The Cochrane handbook was used for data extraction. Duplicates were deleted automatically by Endnote X9 (Thomson Corporation, Stanford, CT) and were also manually eliminated by comparing the authors, publication years, journals, and titles. All duplicate-checked articles underwent a secondary search of their references. The enrolled studies were identified by reading the literature carefully. The following relevant parameters were extracted: first author, journal, location, published year, study design, implant types, number of cases, age, follow-up time, surgical levels, VAS, ODI, ASD, rehydration, and Pfrimann grading. Two reviewers gathered and evaluated the data independently, whereby any disagreements between the evaluations were settled through discussion.

2.6. Quality assessment

The quality of the case–control studies was assessed using the Newcastle–Ottawa Scale (NOS).^[9] We considered studies with a NOS score of 7 or more to be high quality. The quality of the case series was assessed using the Joanna Briggs Institute critical appraisal checklist.^[10] For the case series, there were 10 questions in the checklist with answers of “yes,” “no,” “unclear,” and “not relevant.” We determined that articles achieved the adequate standard of quality required for their inclusion if at least half of the questions were answered “yes.”

3. Results

3.1. Data selection process

A total of 499 articles were obtained from the electronic databases. Three hundred sixty-seven articles were retained after removing 132 duplicates. A further 348 irrelevant articles were removed by reading the titles and abstracts carefully. A total of 5 conferences, reviews, letters, and dissertations were then removed. Another 7 articles focusing on other methods to measure disc rehydration were excluded. Finally, a total of 7 studies were included in this systematic review. The process of the database search is shown in a flowchart (Fig. 1).

Figure 1.

Flow diagram of included studies.

3.2. Study characteristics

The baseline characteristics of the included studies are shown in Table 1. The enrolled studies included 5 case series and 2 case–control studies. We used the NOS and Joanna Briggs Institute critical appraisal checklist to assess the quality of the included literature for different research. Overall, the studies achieved an adequate level of quality to warrant their inclusion. A total of 211 cases (73 men, 138 women) were included in this systematic review. The mean age of the included cases is 49.1 years. The mean follow-up time is 46.7 months. The mean ODI scores decreased from 62.8 preoperative to 13.3 at final follow-up. The included studies were distributed globally, with 2 studies in China, 2 studies in Turkey, 1 study in Italy, 1 study in Korea, and 1 study in Mexico. Different implant types were used in the patients, including Cosmic, Safinaz, Wallis, ISOBAR, FPSS, BioFlex, and Accuflex (Fig. 2).

Table 1.

Characteristics of included studies.

First author	Journal	Location	Published year	Study design	Implant types	Number of cases (men/women)	Age (yr)	Follow-up time	Surgical levels	VAS (preoperative→ follow-up)	ODI (preoperative→ follow-up, %)	ASD (%)	Rehydration (%)	Pfrimann grading	Quality
Yilmaz[11]	Asian Spine J	Turkey	2017	Case series	Cosmic, Safinaz	26/33	46.5	6.4 yr	1 L2/3 4 L3/4 40 L4/5 14 L5/S1	7→1.8	68→16.3	5.1 (3/59)	34 (20/59)	17 single-grade improvement, 3 2-grade improvement Grade II: 0→7 Grade III: 7→19 Grade IV: 45→19 Grade V: 7→14	7 “yes”†
Canbay[12]	Turk Neurosurg	Turkey	2015	Case–control	Cosmic	11/16	38.7 ± 8.39	49.3 ± 18.35 mo	NA	7.74 ± 0.81→1.37 ± 0.88	75.19 ± 7.75→7.26 ± 4.58	7.4 (2/27)	14.8 (4/27)	Grade III: 11→15 Grade IV: 27→23 Grade V: 1→1	7*
Jiang[13]	J Clin Neurosci	China	2015	Case series	Wallis	12/14	47.6	66.8 mo	NA	Leg pain: 7.48 ± 0.86→3.49 ± 1.76 LBP: 5.20 ± 2.64→2.35 ± 1.54	62.6 ± 14.0→15.7 ± 13.4	0	15.4 (4/26)	Grade II: 0→2 Grade III: 8→10 Grade IV: 18→14	6 “yes”†
Gao[14]	J Orthop Surg Res	China	2014	Case–control	ISOBAR TTL	8/16	58.3 ± 13.5	28.7 ± 5.3 mo	4 L4/5 5 L5/S1 7 L3–L5 8 L4–S1	NA	60.36 ± 11.25→11.83 ± 5.68	NA	54.2 (14/24)	1 patient improved from Grade V to Grade III, 2 patients improved from Grade IV to Grade III, 2 patients improved from Grade IV to Grade II, 6 patients improved from Grade III to Grade I, 2 patients improved from Grade II to Grade I	8*
Zagra[15]	Eur Spine J	Italy	2012	Case series	FPSS	6/26	51.9	12 mo	2 L2–3 8 L4–5 22 L4-S1	Leg: 7→2 LBP: 5→1	49→6	NA	25 (8/32)	8 patients improved from Grade IV to Grade III	8*
Heo[16]	J Spinal Disord Tech	Korea	2012	Case series	BioFlex	6/19	59.0 ± 8.4	121.4 ± 21.8 wk	1 L2–3 10 L3–4 14 L4–5	NA	NA	18 (9/50)	15.4 (2/13)	Rehydration was seen in 2 patients	7*
Reyes-Sánchez[17]	Eur Spine J	Mexico	2010	Case series	Accuflex	4/14	44.05	24 mo	15 L4–5 2 L5/S1	Leg: 4.7→0.83 LBP: 7.9→2.8	55→24	NA	16.7 (3/18)	3 patients showed disk rehydration with 1 grade higher	6 “yes”†

ASD = adjacent segment disease, LBP = low back pain, NA = not available, ODI = Oswestry Disfunctional Index, VAS = visual analog scale.

* The quality of the case-control studies was assessed using the Newcastle–Ottawa Scale (NOS). † The quality of the case–control studies was assessed using the Joanna Briggs Institute (JBI) critical appraisal checklist.

Figure 2.

The patient’s number of different implants.

3.3. Levels distribution

A total of 5 studies, including 157 cases, collected information regarding stabilization levels. A total of 120 cases (76.4%) underwent 1-level stabilization, while 37 cases (23.6 %) underwent 2-level stabilization (Fig. 3). The L4/5 level accounted for the majority of them.

Figure 3.

The patient’s number of levels distribution.

3.4. Clinical outcomes

A total of 6 studies described the clinical outcomes. The results showed that both the VAS and ODI scores were significantly improved in the final follow-up, which is similar to the outcomes reported for rigid internal fixation reported in recent years.

3.5. Rehydration

A total of 211 cases were included in our study. However, only 199 cases took part in the research on disc rehydration. Of these 199 cases, 55 cases achieved rehydration, as evaluated by Pfrimann grading on MRI. The rehydration rate was 27.6% (55/199).

4. Discussion

LDD is one of the causes of low back pain (LBP),^[18] and LBP is the leading cause of disability in people over 55 years. In the US, the lifetime prevalence of LBP for adults is as high as 65–80%.^[19] In addition to generating high levels of pain in affected patients, it also causes a great economic burden to society. Lumbar fusion surgery is widely used to treat LDD. However, fusion surgery with rigid internal fixation limits the range of motion of the spine significantly, which increases the compensatory activity of adjacent segments, thereby leading to ASD. A meta-analysis of lumbar fusion by Hashimoto showed that the incidence of ASD was 26.6%.^[20] Michael et al found that the incidence of ASD was 13.4% after 2-level lumbar interbody fusion.^[21] However, Okuda et al found that the incidence of ASD was as high as 75% in a minimum of 10 years of follow-up.^[22]

In this systematic review, we found that the incidence of ASD after dynamic stabilization was lower than that of fusion surgery. Heo^[16] et al claimed that DSS may prevent the progression of ASD in patients experiencing early disc degeneration. However, DSS may not be superior to fusion surgery in patients with preoperative serious lumbar disease. Yilmaz^[11] discovered that ASD was more common in patients who had preoperative disc degeneration. However, the incidence of ASD in patients with postoperative improved conditions (rehydration) was 0. This is an interesting finding, suggesting that disc rehydration caused by DSS may delay the progression of lumbar degeneration. In Jiang’s study, ASD was not found. Thus, DSS can provide stabilization between 2 spinal columns and reduce the incidence of ASD by maintaining physiological movement while limiting pathological movement.^[13] Furthermore, DSS can provide positive clinical outcomes. Both VAS and ODI scores were significantly improved postoperatively. These clinical indicate that DSS is an effective treatment for LDD.

DSS maintains the stability of the intervertebral disc (IVD), thereby providing a suitable condition for disc rehydration and even regeneration. The elements of the lumbar, including the IVD, soft tissue, and neurovascular structures, are susceptible to different pressures caused by LBP. The IVD consists of 2 parts: the nucleus pulposus (NP) and the annulus fibrosus. The NP is rich in fluid and thus is sensitive to pressure. A decrease in the fluid content of the NP leads to a decrease in the hydrostatic pressure, which is often the cause of degeneration. Conversely, an increase in the fluid content of the NP may promote regeneration. Sato^[23] et al assessed the disc pressure in 36 patients and found that degenerated discs had a lower pressure than normal. The IVD possesses elasticity, but when the pressure on the IVD is greater than its own elastic force, the fluid in the IVD flows out and the pressure subsequently decreases. At low loading, the disc pressure increases. This suggests that fluid may reflow into the discs. Wilke^[24] et al measured 24-hour intradiscal pressure in a volunteer and found that the intradiscal pressure increased from 0.10 to 0.24 MPa during sleep, which may be related to disc rehydration. High loading can reduce the IVD pressure and the loss of disc height, and the loss of fluid in the IVD may be an important contributor to this. The reduced pressure may lead to fluid reflow in the disc, thereby providing a stable state of hydration.

Matsumoto^[25] et al conducted an animal experiment on rabbits to show that suitable cyclic mechanical stretch could increase collagenous protein synthesis and the growth rate of NP cells. However, Yang^[26] et al found that long-term excessive cyclic mechanical stretch leads to NP cell apoptosis in vitro. Functional IVD cells were decreased and promoted the development of IVDD. In another in vitro study, Wang^[27] et al found that cyclic mechanical stretch could increase the protein content of COL2A1 in NP cells by activating the ITGA2/PI3K/AKT signaling pathway. Cyclic mechanical strain is frequently associated with the regeneration of NP cells. In an animal experiment, Guehring^[28] et al compressed and then stretched rabbit discs to find the signal intensity on MRI was decreased after undergoing 28 days of compression (200 N), but the signal intensity was reestablished following a treatment involving mechanical stretch. This indicates that mechanical stretch can promote the rehydration of degenerative discs by increasing the expression of extracellular matrix proteins. Gene expression also implied corresponding protein expression. Thus, mechanical stretch can promote disc regeneration.

In this systematic review, we find that previous studies have shown that DSS leads to disc rehydration and can provide positive clinical outcomes, which is an encouraging finding. It is known that degenerated discs cannot regenerate themselves, but DSS may stop and reverse disc degeneration by providing a suitable environment for disc regeneration by reducing pressure and providing moderate stretch. This might be one of the most significant advantages of DSS.

A limitation of this study is that all the literatures included in this research are in English, which may exist a selection bias. Another limitation of this study is that the DSS included in this study are several different implant types, and this may have an impact on the results of this study potentially.

5. Conclusions

In conclusion, this systematic review provides evidence showing that DSS may promote disc rehydration and delay the development of LDD to some extent. Mechanical stretch may play an important role in the progress of IVD rehydration. It provides important evidence for the clinical application of DSS.

Acknowledgments

The authors are grateful to the researchers of the included studies in this systematic review.

Author contributions

Conceptualization: Weimin Huang.

Data curation: Wenqiao Wang.

Supervision: Xiuchun Yu, Lei Wang, Xiaoduo Xu.

Writing – original draft: Wenqiao Wang.

Writing – review & editing: Weimin Huang.