Interspinous Bumpers

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Chapter 166 Interspinous Bumpers

Basem I. Awad, Thomas E. Mroz, Michael P. Steinmetz

Lumbar stenosis is defined as the reduction in the diameter of the spinal canal, lateral recess, and/or neural foramina. Stenosis is most frequently a sequela of the degenerative process of aging, but recently genetic factors have been demonstrated to play a significant role.1–3 The degenerative changes of disc desiccation with anular bulging, osteophyte formation, ligamentum hypertrophy, facet hypertrophy, and/or facet joint cyst all contribute to reducing the space available to the cauda equina and causing the classic symptoms of neurogenic claudications. In particular, compression and ischemia of nerve roots are the main source for this type of pain.4–6

Patients with the classic presentation of intermittent neurogenic claudication have buttock and leg pain when they stand and walk and have relief of symptoms when they flex forward, as when pushing a shopping cart, or when they sit or lie down (Figs. 166-1 and 166-2). This results mainly from the buckling of the ligamentum flavum and a decrease in the size of the central canal, subarticular region, and foraminal area with standing and walking.

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FIGURE 166-1 A, With upright posture, the spine is in extension, resulting in buckling of the ligamentum flavum and further narrowing of the spinal canal (B) with resultant symptoms.

(Copyright Cleveland Clinic Foundation.)

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FIGURE 166-2 A, Bending forward, as with pushing a shopping cart, often relieves the symptoms of spinal stenosis. B, Flexion of the spine results in a tightening of the ligamentum flavum and an increase in the dimensions of the spinal canal. Symptoms are relieved in this position.

(Copyright Cleveland Clinic Foundation.)

The presence of canal stenosis on radiographic imaging does not itself define the syndrome, as there is a poor correlation between the degree of stenosis and the severity of symptoms.7,8

Surgery for lumbar spinal stenosis is generally accepted when conservative treatment has failed. The aim of surgery is to improve the quality of life and reduce claudication and its attendant radiating neurogenic pain. The use of wide decompression procedures for spinal stenosis, regardless of the integrity of the lamina and facet joints and without preservation of the spinous processes and interspinous ligaments, may lead to mechanical failure of the spine and chronic pain syndrome.

Recently, attention has been directed toward less aggressive surgical techniques. Options include fenestration, laminotomy, and selective decompression. These appear to provide adequate decompression with less postoperative morbidity. Moreover, these techniques may be done with minimal invasiveness. These considerations are mainly relevant for elderly patients.

The lumbar interspinous spacer devices provide a new minimally destructive alternative treatment for lumbar canal stenosis. These devices aim to unload the facet joints, restore foraminal height, and provide some stability, especially in extension, but still allow motion. The spacer is inserted between adjacent spinous processes and is placed dorsal to the neural elements. These techniques allow for the maintenance of spinal stability, minimal tissue disruption, and decompression. With some devices, there is no violation of the integrity of the lamina and/or facet joints.

This chapter reviews current concepts in the different types of interspinous spacer devices, Wallis, DIAM, and X-STOP.

Interspinous Spacers: Indications and Contraindications

Interspinous devices are generally indicated for patients age 50 years or older who have neurogenic intermittent claudication: buttock and/or leg pain relieved when the patient’s spine is flexed or bending forward. The diagnosis must be confirmed via MRI and/or CT (myelography) evidence of a thickened ligamentum flavum, a narrowed lateral recess, and/or central canal narrowing. For most devices, stenosis must be limited to one or two levels. Low-grade spondylolisthesis (grade I) with neurogenic claudication is generally not a contraindication for device placement. The device is ideal for patients who have moderate to severe symptoms of neurogenic claudication and are not considered good candidates for decompressive surgery for whatever reason (e.g., excessive comorbities).

However, interspinous spacers are contraindicated in patients with spinal anatomy or disease that would prevent implantation of the device or cause the device to be unstable in situ. This includes significant instability of the lumbar spine, acute fracture of the spinous process or pars interarticularis, high-grade spondylolisthesis, an ankylosed segment at the affected level, significant scoliosis, or a diagnosis of severe osteoporosis by dual-energy x-ray absorptiometry (DEXA) scan, and/or active infection that is either systemic or localized to the site of implantation. In addition, patients with intermittent claudication associated with significant dermatomal weakness or cauda equina syndrome are not proper candidates for these procedures; they should have an open surgical decompression.

It should be noted that in the United States the primary use of these interspinous devices is for the treatment or stabilization of the lumbar spine to correct neurogenic claudication caused by canal stenosis. There is biomechanical evidence of unloading the facet joints and disc space,9 which theoretically may limit back pain associated with a degenerative lumbar segment. In other areas of the world, these devices are used primarily to treat lumbar axial pain, as opposed to neurogenic claudication.

Wallis Interspinous Spacer

In 1986 Sénégas10 developed the first lumbar interspinous implant designed to stiffen unstable operated degenerative segments without eliminating mobility. At first called the Mechanical Normalization System, it included a titanium interspinous blocker and an artificial ligament made of Dacron.

This device was designed to stabilize the intervertebral axis of extension and flexion and reduce the mobility of the instrumented segment. The spacers placed between the dorsal arches produced an unloading effect, reducing pressure in the facet joints and dorsal portion of the intervertebral end plates in lordosis.

Between 1988 and 1993, more than 300 patients were treated for degenerative conditions with this first-generation device. It was concluded that many patients demonstrated significant resolution of residual low-back pain.11

The Wallis System (Abbott Spine, S.A., Bordeaux, France) was the second-generation interspinous spacer. The crucial change was in material properties. The Wallis spacer is made of polyetheretherketone (PEEK), a strong, completely radiolucent material compatible with MRI. This plastic-like polymer is more elastic, which reduces the risk of spinous process stress fractures. In addition, the Wallis device has notches that fit the physiologic shape of the lumbar spine, which minimizes the need for bone resection and avoids constraint on bone.

The Wallis interspinous device was designed as an alternative treatment for neurogenic claudication and the pain attributed to facet joint disease. By keeping the spine in a rather flexed position, the Wallis device increases the total canal and foraminal size and decompresses the cauda equina responsible for neurogenic claudication.

Surgical Technique

The surgical technique for the Wallis interspinous spacer was initially described by Sénégas.12 The procedure is typically performed under general anesthesia with the patient in a prone position on a radiolucent operating table. The patient’s lumbar midline should be marked before he or she is positioned. A neutral position of physiologic lumbar lordosis is best to optimize the effect of the implant.

Fluoroscopy is used to localize the affected level; localization is at the interspinous level, not the disc space level. A 4- to 6-cm midline skin incision is used, and the correct interspinous level is verified with fluoroscopy. A spinal needle may be used to aid in this identification. The paraspinal muscles are then separated from the spinous processes and the lamina of the treated level.

The supraspinous ligament is detached from the two spinous processes at the level involved and retracted laterally without sectioning. The interspinous ligament is then resected. An interlaminar distractor is used to facilitate insertion of the trial spacer, making sure that the gap between the spinous processes has not been enlarged to prevent local kyphosis.

The appropriately sized spacer is placed in the interspinous space; the bands are passed through the adjacent interspinous ligaments, as close as possible to the spinous process edges. A primary tension is then obtained with a forceps before inserting the bands in the clips.

Apply the clip holder and tab against the clip; both layers of the band are grasped and the clip is positioned next to its lodging in the spacer. The clipping forceps is inserted in the opening of the spacer closest to the clip being attached. The clipping forceps must be inserted in the spacer opening as deeply as possible to ensure introduction of all four clip stubs into the corresponding slots in the spacer. A snapping sound lets the surgeon know that the clip has been properly placed onto the spacer. Follow the same procedure for the other clip.

Primary tension ensures that the bands are optimally placed before the use of the final tightening device. The surgeon cuts the excess band with a scalpel, being careful not to damage the band that remains on the implant.

The supraspinous ligament is returned to its original position and reinserted onto each spinous process with a single silk suture passed through a hole made in the spinous process (Fig. 166-3).

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FIGURE 166-3 Depiction of the Wallis device placed in the interspinous space. The polyetheretherketone implant is held in position by notches in the implant and a strap placed around the spinous processes. The supraspinous ligament must be elevated to place the device; it may be reapproximated with suture after placement.

(Copyright Cleveland Clinic Foundation.)

X-STOP Interspinous Spacer

The X-STOP (Kyphon, Medtronic, Sunnyvale, CA) interspinous process decompression (IPD) device is a two-component oval titanium implant that fits between the spinous processes of the lumbar spine. A newer version has a PEEK sleeve over the titanium insert. Several clinical studies have shown the efficacy of the X-STOP device in treating neurogenic claudication secondary to lumbar stenosis.13–16

Surgical Technique

The implant procedure for this device can be done after induction of light sedation or general anesthesia. The patient is placed on a radiolucent table in a right lateral decubitus position; afterward the patient is asked to flex or assume a knee-chest position (Fig. 166-4). The level to be treated is identified by fluoroscopy; localization is at the interspinous level, not the disc space level.

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FIGURE 166-4 We prefer to place the implant with the patient positioned in the lateral decubitus position.

(Copyright Cleveland Clinic Foundation.)

After administration of a local anesthesia, a 4- to 6-cm midline skin incision is made over the spinous processes of the stenotic level. The incision is opened to the fascia, which is incised 2 cm to the right and to the left from the midline. The correct interspinous level is verified again with fluoroscopy.

The paraspinal muscles are lifted subperiosteally from the spinous processes using a Cobb retractor and/or electrocautery, and all midline structures are left intact. The small curved dilator is inserted across the interspinous space through the most ventral margin of the interspinous ligament (Fig. 166-5). The curved dilator is removed. A sizing distractor is then inserted between the spinous processes and expanded until the supraspinous ligament is taut, which is verified by palpation of the ligament (Fig. 166-6). The correct implant size is indicated on the sizing dilator.

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FIGURE 166-5 Following subperiosteal dissection, a dilator is first placed through the interspinous ligament between the spinous processes. The correct level is determined by fluoroscopy.

(Copyright Cleveland Clinic Foundation.)

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FIGURE 166-6 Following dilation, a sizing distractor is placed. Under fluoroscopic guidance, distraction is placed until the interspinous ligament is taut (inset). The size of the implant is determined at this step.

(Copyright Cleveland Clinic Foundation.)

The appropriate X-STOP device is then placed between the spinous processes to the point where the wing is flush with the right side of the spinous process (Fig. 166-7). Fluoroscopy should verify the correct level and ventral placement. If the device appears to be placed too far dorsally, the facet will need to be trimmed. The two wings are approximated medially and the universal wing screw is secured (Fig. 166-8). The incision is then irrigated and closed in standard fashion (Fig. 166-9).

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FIGURE 166-7 The correct-size implant is now placed between the spinous processes. The implant is placed until the wing is flush against the spinous process and the hole for the screw in the opposite wing is well visualized (inset).

(Copyright Cleveland Clinic Foundation.)

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FIGURE 166-8 The opposite wing is placed. Compression may be placed against the wings, and the screw placed and torqued.

(Copyright Cleveland Clinic Foundation.)

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FIGURE 166-9 Final lateral view of the spine with the X-STOP implant in place.

(Copyright Cleveland Clinic Foundation.)

DIAM Interspinous Spacer

The DIAM Spinal Stabilization System (Medtronic, Sofamor Danek, Memphis, TN) is a soft interspinous spacer. The core is made of silicone, which is covered by a polyethylene coating. The DIAM implant was developed by Jean Taylor.17 It is a dorsal interspinal dynamic stabilization or balancing device. The DIAM is thought to work by reducing loading of the disc, restoring the dorsal tension band, realigning the facet joint line, and increasing foraminal height. But, few articles on the efficacy of this product have appeared in the literature.18,19

Surgical Technique

The patient is placed prone, as is standard for interspinous spacer procedures. Fluoroscopic capability is indicated to detect the interspinous level of interest. There are two methods of implanting the DIAM; the first sacrifices the supraspinous and intraspinous ligaments and the second preserves them, which we prefer.

In the ligament-sacrificing procedure, the supraspinous and interspinous ligaments are removed at the level of interest for DIAM implantation. The interspinous space is then prepared by squaring it off with a rongeur. The space is then distracted, and measuring trials are used to determine the size of the DIAM. The implant is inserted and seated with a tamp and mallet. It is secured in place with two laces, one around the spinous process above, and another around the one below, the DIAM is then tightened and crimped. The wound is irrigated and closed in the standard fashion. No postoperative immobilization is required (Fig. 166-10).

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FIGURE 166-10 The DIAM implant is made of silicone that is covered by a polyethylene coating. The implant may be placed either by removing the supra- and intraspinous ligaments or by preserving them. The laces secure the device if the ligaments are removed. They do not need to be placed if the ligaments are preserved.

(Copyright Cleveland Clinic Foundation.)

In the ligament-sparing procedure, the supraspinous and interspinous ligaments are preserved. The exposure can be unilateral or bilateral. A space is created between the spinous processes. The distractor is then placed and the space is sized. The appropriately sized DIAM is folded and deposited. The DIAM cords are removed before insertion, because they are not necessary if the ligaments are preserved. The area is irrigated and closed in the standard fashion.

Outcomes

Anatomy and Biomechanics

Richards et al.20 published the effect of the X-STOP spacer on the spinal canal, subarticular diameter, and foraminal height with and without the implant. With the device in place, the canal area was found to be increased by 18%, the diameter by 10%, and the subarticular diameter by 50%. The foraminal area was found to be increased by 25%, and the width increased by 41%.

Wilke et al.21 evaluated in vitro the biomechanical segmental stability and intradiscal pressure achieved with the four different interspinous implants (i.e., Coflex, Wallis, DIAM, and X-STOP). Interestingly, the effect on the flexibility of all the treated segments is similar, despite the variations of implant design—referred to the neutral position of the intact segments, each device significantly stabilized and unloaded the disc in extension, but had little effect on range of motion (ROM) and intradiscal pressure in flexion, lateral bending, and axial rotation.

In 2008 Schulte et al.22 studied biomechanically the effect of spinal decompression alone, as well as in conjugation with semirigid stabilizing implants (Wallis, Dynesys) on the ROM of lumbar spine segments. Undercutting decompression leads to a significant segmental instability in all planes. Implantation of Wallis or Dynesys spacers after decompression leads to a significant restriction of segmental ROM. The Wallis device limits only the flexion-extension plane; the Dynesys limits both flexion-extension and lateral bending. Axial rotation is minimally restricted by both implants. The observed effects on the neutral zone correspond well with the effects on ROM.

Clinical Outcomes

X-STOP

One- and 2-year outcomes utilizing the X-STOP device were published by Zucherman et al.14,15 There were multicenter, randomized controlled studies for the treatment of lumbar canal stenosis with the X-STOP implant. The implant was directly compared with the best conservative care, usually involving epidural steroid injection. At 1 year, the success rate was 59% with X-STOP compared with 12% for nonoperative therapy. Six patients in the X-STOP group and 24 in the control cohort underwent decompressive laminectomy. Interestingly, the results following laminectomy were similar to those of the remainder of the X-STOP patients. At 2 years, 73.1% of patients were satisfied with their outcome compared with those who underwent conservative therapy (35.9%).

It also appears that the device is effective for treating spondylolisthesis. Anderson et al.23 published a randomized controlled study in patients with neurogenic claudication secondary to degenerative spondylolisthesis. They concluded that the X-STOP device was more effective than nonoperative treatment. There was no increase in the degree of spondylolisthesis postoperatively. These data are appealing for the treatment of those patients who are not felt to be candidates for a fusion following decompression.

The durability of the X-STOP device has not been well defined. Kondrashov et al.24 demonstrated the success rate among patients treated with X-STOP devices at one and two levels was 78% at a mean of 4.2 years postoperatively, which is consistent with the intermediate-term (1 and 2 years) results of Zucherman et al.14,15 This indicates that the long-term durability is at least 4 years; unfortunately not many people were followed out to 4 years for the purpose of this study.

A recent study has shown slightly less promise than the aforementioned manuscripts.25 Although 71% of patients reported satisfaction with the procedure, only 54% of patients reported clinically significant improvement in their symptoms. Twenty-nine percent of patients required caudal epidural steroid injection 12 months after surgery for recurrence of symptoms.

Wallis

The Wallis device has not been approved by the FDA for use in the United States. Despite this, long-term analysis has been reported regarding its use outside the United States. In 2009 Sénégas et al.26 published a long-term retrospective clinical survey on 107 patients with canal stenosis or herniated disc, or both, who underwent lumbar dynamic stabilization with the Wallis system. One hundred thirty-three were alive at the time of analysis, but only 107 filled out the questionnaire. The average follow-up was 13.5 years. Twenty-three patients underwent a subsequent lumbar procedure, with 20 having the implant removed and fusion performed. The other three had surgery at adjacent levels. At an average of 13 years postprocedure, 95% of patients reported they were very satisfied or satisfied with the implant, and the same was seen in only 65% following fusion. There are many weaknesses of this manuscript, but it does lend credence to the long-term durability and clinical utility of the implant. It remains unclear, however, who is the most appropriate patient to utilize the device.

DIAM

Similar to the Wallis device, the DIAM implant is not currently FDA approved for use in the United States. It has been used extensively in other countries for the treatment of a variety of lumbar pathologies. One comparative study was published evaluating the use of DIAM following simple lumbar discecotmy.27 Sixty-two patients underwent lumbar laminectomy and/or discectomy; 31 of these patients underwent concomitant placement of the DIAM device (33 devices total). At an average of 12 months following surgery no significant differences were noted between groups, using the VAS and McNab outcomes scales. There were three device failures noted in the DIAM group, two transverse process fractures and one infection. It appears that the device did not alter the sagittal alignment or disc height following placement; however, it did not improve outcome as well. What remains unclear in comparative studies is how the device may affect the treatment of other lumbar degenerative diseases, such as mechanical pain and pseudoclaudication. Certainly further research is warranted at this time.

Conclusion

The lumbar interspinous spacers provide a new minimally destructive alternative treatment for neurogenic claudication attributed to lumbar canal stenosis. Several interspinous implants are available on the market today, although only the X-STOP is available in the United States. They have different design strategies and are made of varying materials, but all have similar biomechanical effects. These devices permit neural decompression with only a minimal amount of tissue resection; also they make the procedure less invasive (destructive).

These devices may be implanted without a laminectomy and function through indirect decompression, thus avoiding the risk of epidural scarring and cerebrospinal fluid leakage. Of note, X-STOP cannot be placed following laminectomy as it requires intact supra- and infraspinous ligaments. Furthermore, these devices limit extension of the spine, unload the facet joints, and potentially relieve the pain attributed to facet disease.

In addition, it appears that they do not alter the mechanical behavior of the adjacent segment, possibly limiting the domino effect of degenerative disc disease.

Key References

Anderson P.A., Tribus C.B., Kitchel S.H. Treatment of neurogenic claudication by interspinous decompression: application of the X-STOP device in patients with lumbar degeneration spondylolisthesis. J Neurosurg Spine. 2006;4:463-471.

Kirkaldy-Willis W.H., Wedge J.H., Yong-Hing K., et al. Pathology and pathogenesis of lumbar spondylosis and stenosis. Spine (Phila Pa 1976). 1978;3:319-328.

Lindsey D.P., Swanson K.E., Fuchs P., et al. The effects of an interspinous implant on the kinematics of the instrumented and adjacent levels in the lumbar spine. Spine (Phila Pa 1976). 2003;28:2192-2197.

Sénégas J., Vital J.M., Pointillart V., et al. Long-term actuarial survivorship analysis of an interspinous stabilization system. Eur Spine J. 2007;16:1279-1287.

Sénégas J., Vital J.M., Pointillart V., et al. Clinical evaluation of a lumbar interspinous dynamic stabilization device (the Wallis system) with a 13-year mean follow-up. Neurosurg Rev. 2009;32:335-342.

Zucherman J.F., Hsu K.Y., Hartjen C.A., et al. A prospective randomized multi-center study for the treatment of lumbar spinal stenosis with the X-STOP interspinous implant: 1-year results. Eur Spine J. 2004;13:22-31.

Zucherman J.F., Hsu K.Y., Hartjen C.A., et al. A multicenter, prospective, randomized trial evaluating the X-STOP interspinous process decompression system for the treatment of neurogenic intermitent claudication: two-year follow-up results. Spine (Phila Pa 1976). 2005;30:1351-1358.

References

The complete reference list is available online at expertconsult.com.

References

1. Kirkaldy-Willis W.H., Wedge J.H., Yong-Hing K., et al. Pathology and pathogenesis of lumbar spondylosis and stenosis. Spine (Phila Pa 1976). 1978;3:319-328.

2. Yong-Hing K., Kirkaldy-Willis W.H. The pathophysiology of degenerative disease of the lumbar spine. Orthop Clin North Am. 1983;14:491-504.

3. Battie M.C., Videman T., Parent E. Lumbar disc degeneration: epidemiology and genetic influences. Spine (Phila Pa 1976). 2004;29(23):2679-2690.

4. Dejerine J. La claudication intermittente de la moelle épinière. Presse Med. 1911;19:981-984.

5. Blau J.N., Logue V. Intermittent claudication of the cauda equina. An unusual syndrome resulting from central protrusion of a lumbar intervertebral disc. Lancet 1:. 1961:1081-1086.

6. Wakamatsu E. Clinical features of degenerative lumbar stenosis and indication of operation. J Jpn Orthop Assoc. 1984;58:S42-S43.

7. Boden S.D., McCowin P.R., Davis D.O., et al. Abnormal magnetic-resonance scans of the cervical spine in asymptomatic subjects. A prospective investigation. J Bone Joint Surg [Am]. 1990;72:1178-1184.

8. Boden S.D., Davis D.O., Dina T.S., et al. Abnormal magnetic-resonance scans of the lumbar spine in asymptomatic subjects. A prospective investigation. J Bone Joint Surg [Am]. 1990;72:403-408.

9. Lindsey D.P., Swanson K.E., Fuchs P., et al. The effects of an interspinous implant on the kinematics of the instrumented and adjacent levels in the lumbar spine. Spine (Phila Pa 1976). 2003;28:2192-2197.

10. Sénégas J. Mechanical supplementation by non-rigid fixation in degenerative intervertebral lumbar segments: the Wallis system. Eur Spine J. 2002;11(Suppl 2):S164-S169.

11. Sénégas J., Vital J.M., Pointillart V., et al. Long-term actuarial survivorship analysis of an interspinous stabilization system. Eur Spine J. 2007;16:1279-1287.

12. Sénégas J. Dynamic lumbar stabilization with the Wallis interspinous implant. Interact Surg. 2008;3:221-228.

13. Hero A., Saari T., Suomalainen O., et al. The degree of decompressive relief and its relation to clinical outcome in patients undergoing surgery for lumbar spinal stenosis. Spine (Phila Pa 1976). 1999;24:1010-1014.

14. Zucherman J.F., Hsu K.Y., Hartjen C.A., et al. A prospective randomized multi-center study for the treatment of lumbar spinal stenosis with the X-STOP interspinous implant: 1-year results. Eur Spine J. 2004;13:22-31.

15. Zucherman J.F., Hsu K.Y., Hartjen C.A., et al. A multicenter, prospective, randomized trial evaluating the X-STOP interspinous process decompression system for the treatment of neurogenic intermitent claudication: two-year follow-up results. Spine (Phila Pa 1976). 2005;30:1351-1358.

16. Kuchta J., Sobottke R., Eysel P., et al. Two-year results of interspinous spacer (X-Stop) implantation in 175 patients with neurologic intermittent claudication due to lumbar spinal stenosis. Eur Spine J. 2009;18(6):823-829.

17. Mariottini A., Pieri S., Giachi S., et al. Preliminary results of a soft novel lumbar intervertebral prosthesis (DIAM) in the degenerative spinal pathology. Acta Neurochir Suppl. 2005;92:129-131.

18. Minns R.J., Walsh W.K. Preliminary design and experimental studies of a novel soft implant for correcting sagittal plane instability in the lumbar spine. Spine (Phila Pa 1976). 1997;22:1819-1827.

19. Christie S.D., Song J.K., Fessler R.G. Dynamic interspinous process technology. Spine (Phila Pa 1976). 2005;30(Suppl 16):S73-S78.

20. Richards J.C., Majumdar S., Lindsey D.P., et al. The treatment mechanism of an interspinous process implant for lumbar neurogenic intermittent claudication. Spine (Phila Pa 1976). 2005;30:744-749.

21. Wilke H.J., Drumm J., Haussler K., et al. Biomechanical effect of different lumbar interspinous implants on flexibility and intradiscal pressure. Eur Spine J. 2008;17:1049-1056.

22. Schulte T.L., Hurschler C., Haversath M., et al. The effect of dynamic, semi-rigid implants on the range of motion of lumbar motion segments after decompression. Eur Spine J. 2008;17:1057-1065.

23. Anderson P.A., Tribus C.B., Kitchel S.H. Treatment of neurogenic claudication by interspinous decompression: application of the X-STOP device in patients with lumbar degeneration spondylolisthesis. J Neurosurg Spine (Phila Pa 1976). 2006;4:463-471.

24. Kondrashov D.G., Hannibal M., Hsu K.Y., et al. Interspinous process decompression with the X-STOP device for lumbar spinal stenosis: 4 year follow-up study. J Spinal Disord Tech. 2006;19:323-327.

25. Siddiqui M., Smith F.W., Wardlaw D. One-year results of X-Stop interspinous implant for the treatment of lumbar spinal stenosis. Spine (Phila Pa 1976). 2007;32:1345-1348.

26. Sénégas J., Vital J.M., Pointillart V., et al. Clinical evaluation of a lumbar interspinous dynamic stabilization device (the Wallis system) with a 13-year mean follow-up. Neurosurg Rev. 2009;32:335-342.

27. Kim K.A., McDonald M., Pik J.H.T., et al. Dynamic intraspinous spacer technology for dorsal stabilization: case-control study on the safety, sagittal angulation, and pain outcome at 1-year follow-up evaluation. Neurosurg Focus. 2007;22(1):E7.

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