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.

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.)

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 Sénégas ¹⁰ 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).