Adjacent segmental degeneration following Wallis interspinous stabilization implantation: Biomechanical explanations and the value of magnetic resonance imaging.

Adjacent segmental degeneration (ASD) is a major issue after pedicular fixation. This study examined the degeneration of the adjacent levels due to the insertion of the Wallis interspinous stabilization system compared with discectomy, using magnetic resonance imaging (MRI).Thirty-eight patients diagnosed with lumbar degeneration disorders at L4-L5 were reviewed: 19 patients underwent discectomy and Wallis system implantation (group A), and 19 patients underwent discectomy (group B). The Visual Analog Scale (VAS) and Oswestry Disability Index (ODI) were assessed preoperatively and postoperatively. ASD was evaluated by MRI.There was no difference in the preoperative ODI scores between the 2 groups (non-normal distribution, median, 50 (40, 50) vs 50 (50, 50), P = .331), but the postoperative ODI scores were different (non-normal distribution, median, 0 (0, 32) vs 20 (20, 30), P < .005). Similar results were observed for VAS. In group A, ASD occurred in 4 patients (21.1%) in the disc and 8 (42.1%) in the facet joint at L3/4, and in 4 (21.1%) in the disc and 5 (26.3%) in the facet joint at L5/S1. In Group B, ASD occurred in 3 patients (15.8%) in the disc at L3/4, and in 4 (21.1%) in the disc at L5/S1. In general, there was no difference between the 2 groups (P > .05), except at L3/4 (P = .015).ASD of the facet joint in the cranial segment occurred after Wallis system implantation, suggesting that the Wallis system cannot prevent ASD of the facet joint, but could have some other benefits for the discs.

Introduction

Acute or progressive disc lesions lead to instability of the spinal segments.[ Currently, pedicular fixation (fusion) is the gold standard treatment in terms of increasing the biomechanical rigidity and clinical fusion rates because pedicle screws are the strongest component of spinal implants.[ Adjacent segment degeneration (ASD) is the development of a pathology at the mobile segment next to a lumbar or lumbosacral spinal fusion.[ Several reports revealed that ASD could be accelerated due to the relative immobility of fused spinal segments transferring stress to adjacent segments after fusion.[ Symptoms and signs of ASD include pain, stenotic lesions, and instability, leading to additional surgeries such as extended fusion and neural decompression.[ Unfortunately, there is currently no relevant literature about the prevention of ASD.

To reduce the incidence of fusion-related morbidity, non-fusion technologies have been developed, such as the Wallis interspinous stabilization system.[ Although the implant offers some advantages over fusion (e.g., motion of the involved levels and small operation wound), the efficacy of non-fusion implants in the prevention of ASD is now well established.[

ASD was first described using x-ray indexes such as disc height and segmental range of motion, [ but a previous animal study suggested that the changes in x-ray indexes were less sensible than those extracted from magnetic resonance imaging (MRI),[ as supported by a study in humans. [

Nevertheless, it is poorly known whether the use of the Wallis system could prevent ASD. Therefore, the aim of the present study was to compare the patients who underwent discectomy and Wallis system implantation with the patients who underwent discectomy only, based on MRI examinations.

Methods

Study design and patients

Patients diagnosed with lumbar disc herniation at L4-L5 and operated (by the same surgeon) at the Department of Orthopedic Surgery, Tongji Hospital affiliated to Tongji Medical University of HUST, in 2009 and 2010, were retrospectively reviewed after a 2-year follow-up. The project was approved by the institutional review boards and the ethics committee of Tongji Hospital affiliated to Tongji Medical University of HUST and followed the tenants of the Declaration of Helsinki. The need for informed consent was waived by the committee because of the retrospective nature of the study.

The inclusion criteria were: (1) history of lumbar disc herniation; (2) symptoms of sciatic and low back pain; and (3) failure of conservative treatment. The exclusion criteria were: (1) any other type of vertebral fracture; (2) patients without any indication for surgery or refused surgery; (3) adjacent segments with disc degeneration grade >5 and/or facet degeneration grade >2 according to MRI (Table 1[ and Table 2[); (4) history of cardiovascular or cerebrovascular diseases, trauma, or cancer; (5) lost to follow-up; or (6) missing data.

Table 1

Grading of intervertebral disc degeneration.

Table 2

Grading of the facet joint degeneration.

During the study period, 100 patients were treated at our center, but after excluding patients lost to follow-up and those with missing data, and after matching the 2 groups for age, gender, and occupation, only 38 patients remained.

Surgery

The treatment approach was decided by the surgeon in consultation with patients. After oral and written explanations on the details of the surgery, all participants signed a written surgical informed consent. After discussion, the patients underwent either discectomy and Wallis implantation (n = 19, group A) or discectomy only (n = 19, group B).

The indications for discectomy were: (1) symptoms of lumber spinal cord or nerve root compression; (2) conservative treatment did not produce satisfactory outcomes; and (3) willing to undergo surgery. The indications for Wallis system implantation were: (1) the sequence was stable and (2) no complications.

Data collection

Age, gender, body mass index (BMI), and duration of pain were collected preoperatively. The intensity of pain according to the visual analog scale (VAS) and Oswestry disability index (ODI) were collected preoperatively and postoperatively. The VAS ranged from 0 (no pain) to 10 (worst pain imaginable). The patients were asked to mark a point on the scale corresponding to their pain at that time. The ODI questionnaire contained 6 statements (denoted levels 0–5) in each of the 10 sections related to impairments such as pain and abilities such as personal care, lifting, walking, sitting, standing, sleeping, sex life, social life, and traveling. In each section, the patient chose the statement that best described his/her status. If the limitation fell between 2 levels, the higher point value was selected. The chosen statements received scores 0 to 5 corresponding to the level indicated. The total scores could range from 0 (the highest level of function) to 50 (the lowest level of function).

MRI

All patients had undergone magnetic resonance imaging (MRI) before and 6 months after operation. The lumbar spine MRI examination of each participant was done by the same clinical 1.5T system (Signa 1.5 T HD, GE Healthcare, Waukesha, WI) using a 4-channel Phased Array CTL Spine Coil. T1-weighted fast spin-echo sagittal images with effective echo time and repetition times (TE/TR) of 10/400 ms, T2-weighted fast spin-echo sagittal images with TE/TR of 102/3000 ms and T2-weighted fast spin-echo axial images with TE/TR of 120/3000 ms were included in the examination. The field of view (FOV) was 360 mm and the matrix was 128 × 128, whereas 5-mm sections with a 1-mm section gap was used. There were 6 averages and the echo train length was 72 seconds.

The visual grading of intervertebral disc degeneration and the facet joint degeneration were based on the T2-weighted images and adjacent levels. Two operators (8 and 5 years of experience in MRI of the spine, respectively) graded the disc and facet joint in L3/4, L4/5, and L5/S1. The G value, defined as a measure of segment (disc and facet joint) degeneration, was obtained by adding the grades of invertebral disc degeneration (Table 1) and facet joint degeneration (Table 2). The difference in the G-value after surgery was defined as ΔG = Gpostoperational – Gpreoperational of intervertebral discs and facet joints of L3/4, L4/5, and L5/S1. Positive ΔGdisc and ΔGfacet values indicate that the grade of the intervertebral discs and facet joints worsened after surgery and the segment was marked as ASD. Negative ΔGdisc and ΔGfacet values indicate that the grade improved after surgery. The interobserver reliability of image grading was assessed using the kappa score. The final results were determined according to the results by 1 neuroradiologist.

Statistical analysis

Interobserver analyses of all MRI measurements showed fair to excellent agreement. Changes in scores from before to after surgery were calculated. Normally distributed data are presented as mean ± standard deviation and were analyzed using the Student t test. Non-normally distributed data are presented as median (min, max) and were analyzed using the Mann–Whitney U test. SPSS 23.0 (IBM, Armonk, NY) was used for statistical analysis. Two-sided P-values <.05 were considered statistically significant.

Results

Table 3 presents the characteristics of the patients. There were no differences in age, gender, BMI, and pain duration between the 2 groups (all P > .05). The median preoperative ODI scores in groups A and B were 50 (40, 50) and 50 (50, 50), respectively (non-normal distribution; P = .331). The postoperative ODI scores were 0 (0, 32) and 20 (20, 30), respectively (non-normal distribution; P < .005). The median preoperative VAS scores in group A and B were 9 (9, 10) and 10 (9, 10) (non-normal distribution; P = .079). The postoperative VAS scores were 0 (0, 6) and 2 (2, 4) (non-normal distribution; P = .067).

Table 3

Characteristics of the patients.

Occurrence of ASD

For all patients (n = 38), ASD occurred in 7 patients (18.4%) in the disc and 8 (21.1%) in the facet joint at L3/4, and in 8 (21.1%) in the disc and 5 (13.2%) in the facet joint at L5/S1. For group A, ASD occurred in 4 patients (21.1%) in the disc and 8 (42.1%) in the facet joint at L3/4, and in 4 (21.1%) in the disc and 5 (26.3%) in the facet joint at L5/S1. For group B, ASD occurred in 3 patients (15.8%) in the disc at L3/4 and in 4 (21.1%) in the disc at L5/S1 (Table 4).

Table 4

Occurrence of ASD in the 2 groups.

Changes in G value during follow-up

The comparison of the Gpreoperational, Gpostoperational, and ΔG value of the discs and facets in the 2 groups are summarized in Table 5 and Fig. 1. There was no difference between the 2 groups for ΔGdisc (P > .05), but there was a difference for ΔGfacet at L3/4 (P = .015) but not at L5/S1 (P = .217). In Fig. 2, the ΔGdisc of the 2 groups were negative, and the changes in MRI were obvious. Detailed MRI examination of a patient from group B at the facet joints of L3/4, L4/5, and L5/S1 is shown in Fig. 3. Preoperatively, cartilage covers the surfaces of the articular processes with some erosion; the cortical bone of the articular processes is focally thickened with small/moderate osteophyte. After operation, regions of the underlying bone are exposed to the joint, with moderate/large osteophyte.

Table 5

Comparison of the Gpreoperational, Gpostoperational, and ΔG of the discs and facets in the 2 groups.

Figure 1

(A) ΔG in the discs at L3/4 and L5/S1 in groups A and B. At L3/4, there were 4 cases of ASD (ΔG > 0) of the discs in group A, whereas 3 cases of ASD were found in group B. At L5/S1, there were 4 cases of ASD, whereas 4 cases of ASD were observed in group B. (B) ΔG in the facets at L3/4 and L5/S1 in groups A and B. At L3/4, there were 8 cases of ASD (ΔG >0) of the facets in group A. At L5/S1, there were 5 cases of ASD in group A. ASD = adjacent segmental degeneration.

Figure 2

(A) A patient from group A before operation. The T2 signal at L4/5 is mildly hyperintense (slightly more than the outer fibers of annulus), and there is no distinction between the inner and outer fibers of annulus at the disc. The Gpreoperational is 4. (B) The same patient from group A after operation. The T2 signal at L4/5 is hyperintense (more than the outer fibers of annulus), and there is a distinction between the inner and outer fibers of annulus at the disc. The Gpreoperational is 3. (C) A patient from group B before operation. The T2 signal at L4/5 is mildly hyperintense (slightly more than the outer fibers of annulus), and there is no distinction between the inner and outer fibers of annulus at the disc. The Gpreoperational is 4. (D) The same patient after operation in group B. The T2 signal at L4/5 is hyperintense (more than the presacral fat and cerebrospinal fluid) and hypointense compared with the intranuclear cleft. There is a distinction between the inner and outer fibers of annulus at the disc. The Gpreoperational is 2.

Figure 3

A patient from group B. (A) Before operation, the T2 signal of the facet joints at L3/4 cartilage covers the entire surface of the articular processes but with erosion of the irregular region; the cortical bone of the articular processes is focally thickened with small osteophyte. The Gpreoperational is 2. (B) The facet joints at L4/5 before operation, the Gpreoperational is 2. (C) The facet joints at L5/S1 before operation, the Gpreoperational is 3. The cartilage incompletely covers the articular surfaces, with regions of the underlying bone exposed to the joint. Thickened cortical bone covers less than half of the articular processes, with moderate osteophyte. (D) After operation, the T2 signal of the facet joints at L3/4. Cartilage incompletely covers the articular surfaces, with regions of the underlying bone exposed to the joint. Thickened cortical bone covers less than half of the articular processes, with moderate osteophyte. The Gpostoperational is 3. (E) The facet joints at L4/5 after operation, the Gpostoperational is 3. (F) The facet joints at L5/S1 after operation, the Gpostoperational is 4. The cartilage is absent except for traces on the articular surfaces, dense cortical bone covers greater than half the articular process with large osteophyte.

Discussion

ASD after lumbar spinal fusion is a potential cause of further spinal surgery, which is disquieting to both patients and surgeons. The Wallis system can be used to stabilize the spine, but its effect on ASD is unknown. Therefore, the aim of the present study was to examine the degeneration of the adjacent levels due to the insertion of the Wallis interspinous stabilization system compared with discectomy, and using MRI. The results showed that in group A, ASD occurred in 4 patients (21.1%) in the disc and 8 (42.1%) in the facet joint at L3/4, and in 4 (21.1%) in the disc and 5 (26.3%) in the facet joint at L5/S1. In Group B, ASD occurred in 3 patients (15.8%) in the disc at L3/4, and in 4 (21.1%) in the disc at L5/S1. In general, there was no difference between the 2 groups (P > .05), except at L3/4 (P = .015). Therefore, ASD of the facet joint in the cranial segment occurred after Wallis system implantation, suggesting that the Wallis system cannot prevent ASD of the facet joint, but could have some other benefits for the discs, highlighted by the significantly lower ODI scores in group A compared to group B.

Biomechanical changes of ASD consist of increased intradisc pressure, increased facet load, and increased mobility after fusion.[ It is presumed that the motion is transferred from the fused level to the close free level, and therefore the incidence of proximal ASD is much higher than that of distal ASD.[ X-ray indexes such as disc height and segmental range of motion can describe ASD to some degree,[ but MRI indexes provide more reliable data.[ However, fusion surgery may cause artifacts with imaging. From the results of the present study, it seems that ASD occurs above the operated segment after implantation of the Wallis system, especially at the facet joint. Based on several studies, after spinal fusion, increased stress on the adjacent facet joints and a change in the load of the adjacent disc have been proved.[ In the studies of spinal fusion, several authors support the point of view that the load is shifted to the free and mobile cranial lumbar segments for compensation.[ Therefore, ASD always occurred in the facet joints above the reconstructed segment. Akamaru et al[ demonstrated that the highest increase in motion is the cranial segment (L3/4) to L4/5 after its hypolordotic floating fusion. In addition, the change in joint orientation is a major risk factor in the degenerative process of that segment.[ The Wallis implants can restrict the motion of the lumbar spine. The Wallis implant consists of an interspinous spacer that limits the extension and 2 bands that secure the implant in the interspinous space and limit flexion.[ Therefore, the motion and the load is shifted from L4/5 to the adjacent segments (L3/4 and L5/S1) after Wallis system implantation atL4/5, especially at the cranial segment (L3/4). The reason for ASD at the L5/S1 facet in this study could be due to damage to the posterior structure resulting from the implantation, but this requires further investigation.

In some studies, the intradisc pressure was strongly reduced in extension after the implantation of the Wallis system,[ but without difference in all other loading directions (flexion, lateral bending, and axial rotation), which has been observed in the present study. Nevertheless, the use of an interspinous implant could cause adjacent level facet pain or accelerated facet joint degeneration. [ At the implanted level, the mean peak pressure, average pressure, contact area, and force were significantly reduced, but there were no significant changes at the level above the implant. The implant appears to redirect a large portion of the load away from the intervertebral disc and to transfer that load to the spinous processes. In a study by Adams et al,[ there was a paradoxical decrease in posterior annular pressure during hyperextension at the tested level. They attributed this observation to the facet joints acting as a fulcrum and redirecting most of the force from the respective disc. When using the Wallis system, the lumbar spine is kept slightly flexed, meaning that the anterior part of the intervertebral disc is compressed, keeping the articular facets separated during movement of the lumbar spine.[ As superior-segment facet contact has been presumed to play a role in the onset of ASD, it is unclear why the Wallis system does not prevent ASD. Nevertheless, additional mechanical studies are necessary to characterize the spinal changes leading to ASD. Unfortunately, there is currently no relevant literature about the prevention of ASD and the present study does not allow drawing conclusions about ASD prevention. Additional studies are also necessary to address these issues.

The present study is not without limitations. The sample size was small, from a single center, and was operated by a single surgeon. The ODI scores were self-assessed and could be more severe than in reality. No patient with pedicular fixation (fusion) could be included as controls because the fixation affected MRI quality. Finally, the follow-up was short and was based on retrospective data.

In conclusion, ASD of the facet joint in the cranial segment occurred after Wallis system implantation, suggesting that the Wallis system cannot prevent ASD of the facet joint, but could have some benefits for the discs.

Acknowledgments

The authors thank Dr. Yuwei Liu for his help in data analysis, as well as Dr. Lingyun Zhao and Dr. Liang Qi for their help in dealing with the MRI and picture reporting in this study.