Introduction

Posterior dynamic stabilization of the lumbar spine is an evolving field. At the core is the concept of incremental stabilization or progressive control of motion of the spine. This contrasts with more traditional fusion techniques where stabilization entails rigid fixation with abolition of motion. These goals are achieved with surgical implantation of posteriorly placed devices, often following surgical decompression where indicated. These devices exert varying degrees of motion control at varying anatomical regions of influence, based on design.

It is a rapidly expanding field with a broad spectrum of devices, approaches, materials, and degree of imparted stabilization. Proper discussion of dynamic stabilization can only occur with organizational hierarchy, considering anatomical location, functional impact, intensity of intervention, and targeted goal of intervention. Each will be considered with relationship to the others.

Devices

The devices can be largely grouped into three categories, based on anatomical location of implantation. These categories are interspinous, facet, and pedicle. These discrete anatomical insertion points allow for a targeted intervention with very specific actions and indications. There is sufficient overlap such that a broad spectrum of disease can be treated.

Interspinous Spacers

The spinous processes and the interspinous space are the sites used with increasing frequency in treating spinal conditions. Though few conditions directly affect the spinous process itself, this anatomical location has utility in implant attachment for distraction and stabilization to a significant degree.

The rationales for interspinous devices are several. They distract the neural foramen to reduce nerve root compression. They also share load with the posterior disc, unload the facets, and assist with stability at the operated level. For the most part they are minimally invasive and easily revisable.

These devices are surgically implanted between the spinous processes or within the interlaminar space. In this location they act to control extension, unload the facets, tension the posterior annulus, as well as tension the posterior ligaments. This influence can be enhanced with surgical positioning in more flexion, or after loaded distraction. The resulting kyphosing moment can increase overall volume for neural elements.

X-Stop is a titanium alloy device that is designed to stop extension (Figure 56-1). The oval spacer conforms to the interspinous space, and the wings prevent lateral migration. It is minimally invasive and inserted laterally, thus preserving the supraspinous ligament. It is designed to be implanted under local anesthesia. In clinical trial, X-Stop was significantly better than nonoperative treatment of lumbar spinal stenosis at 1 and 2 years post-op. The observed success rate was comparable to published reports for decompressive laminectomy, but with considerably lower morbidity.^1,⁵ X-Stop is currently approved by the U.S. Food and Drug Administration (FDA) for use in patients with claudication symptoms from lumbar stenosis. Patients with relief in flexion and who are otherwise comfortable sitting respond well.

FIGURE 56-1 X-Stop device interspinous spacer (Kyphon).

Wallis (Zimmer Spine)

The Wallis device is a PEEK interspinous spacer secured to the spinous processes using PET bands (Figure 56-2). It is designed to block extension and control flexion. Indications include isolated lumbar intervertebral instability, such as herniated discs, Modic I degenerative lesions, degenerative disc disease at a level adjacent to a previous fusion, and spinal stenosis treated without laminectomy. In long term (13 year) OUS follow-up, the device obviated the need for arthrodesis in 80% of patients.⁴ The Wallis device is currently undergoing clinical evaluation and is not FDA approved.

FIGURE 56-2 The Wallis device interspinous spacer (Zimmer Spine).

Diam (Medtronic)

Diam is an interspinous stabilizer composed of a silicone bumper, encased in polyester mesh, and secured with polyester sutures (Figure 56-3). It is inserted with minimal access and minimal tissue disruption. It is designed to resist extension and reduce intradiscal pressure. Indications include restoration of early segmental degeneration, correction of misalignment often seen in discectomy, and stenosis. Diam is currently undergoing clinical evaluation and is not FDA approved.

FIGURE 56-3 The Diam interspinous stabilizer (Medtronic).

Coflex (Paradigm)

Coflex is a titanium alloy device whose U-shaped body conforms to the interspinous and interlaminar space (Figure 56-4). It acts as a stiff spring, dynamically stabilizing extension. Lateral migration is prevented by wings compressed onto the spinous processes. Indications include lumbar stenosis, adjacent segment disease, recurrent HNP and early symptomatic disc degeneration. In OUS implantation after decompressive laminectomy in degenerative lumbar spinal stenosis, Coflex was found to be less invasive and provided similar clinical outcome in comparison with instrumented fusion.² Coflex is currently undergoing clinical evaluation and is not FDA approved.

FIGURE 56-4 The Coflex device (Paradigm).

ExtenSure (NuVasive)

ExtenSure is a PEEK interspinous spacer secured by geometric conformity to the anatomy and the suturing of the supraspinous ligament (Figure 56-5). It is designed to alleviate pseudoclauditory symptoms and radicular symptoms by enlarging the central canal, lateral recess, and foramina. It also decreases or eliminates low back pain by decreasing pressure on arthritic facet joints, degenerative discs, or both. It also restores disc height and foraminal volume. It is indicated in moderate to severe spinal stenosis, degenerative spondylolisthesis, and mild to moderate degenerative scoliosis. ExtenSure is currently undergoing clinical evaluation and is not FDA approved.

FIGURE 56-5 The ExtenSure interspinous spacer (NuVasive).

In-Space (Synthes)

In-Space is a laterally placed PEEK cylindrical device, secured by deployable wings (Figure 56-6). It is intended to stop segmental extension and to distract the symptomatic interspinous space. In doing so, maintenance of foraminal height, opening of the area of the spinal canal, reduction of stress on the facets, and relieving of pressure on the posterior annulus results. It is indicated in lumbar spinal stenosis, disc protrusions with discogenic low back pain, facet syndrome due to face osteoarthritis, degenerative spondylolisthesis up to grade 1, and degenerative disc disease. In-Space is not FDA approved.

FIGURE 56-6 The In-Space device (Synthes).

Superion (Vertiflex)

Superion is a titanium alloy interspinous device with deployable wings (Figure 56-7). It is designed for percutaneous implantation in the treatment of moderate degenerative lumbar stenosis at one or two levels. Superion is currently under clinical investigation in the United States and is not FDA approved.

FIGURE 56-7 The Superion interspinous device (Vertiflex).

Facet Devices

The facet poses unique challenges. The anatomy and functional interrelationships are complex. The 3-D sliding synovial articulation is difficult to replicate. Physiologically, the facets have nociceptive and proprioceptive input that is vital to the proper function of the motion segment. The resulting chemical and mechanical pain generation is difficult to treat surgically. Kinematically complex, facet joints engage in coupled shear and sliding, as well as rotational load sharing, all of which can be specific to certain levels.

This results in a very complex continuum of disease. Painful inflammation, osteoarthritis, stenosis, abnormal loading, and total failure of the functional spinal unit can all originate with facet disease. These challenges are not easily overcome, addressed, or surgically treated. Facet arthroplasty, a rapidly evolving subspecialty in motion preservation, strives to address these issues in the most ergonomic manner.

The overarching goal is reduction of pain and the return of function. The design rationales for the facet-based devices are to clinically correlate intensity of intervention to severity of disease, to allow or restore more physiological loading, to allow or restore the physiological center of rotation, and to control the range of motion of the motion segment.

The emerging technologies are thoughtfully engineered. They are well studied and modeled as the result of extensive research and analysis. The facet-based devices vary from resurfacing, through augmentation and partial replacement, to total replacement. These devices are designed to complement residual kinematics of the motion segment, or to duplicate native motion in total replacement.

Zyre (Quantum Orthopedics)

Zyre is an interpositional arthroplasty device (Figure 56-8). It consists of a cobalt chromium intraarticular spacer, through which passes a PET cord with chromium retainers. It is minimally invasive, requires no bone resection, maintains capsular integrity, and can be implanted with or without decompression. Indications include painful degeneration of the facet with failed CMM. Advantages include minimal disruption of anatomy and multiple revision options. This concept is early technology with a paucity of clinical data. Zyre is not FDA approved.