Principles of effective Principles of effective dynamic stabilizationsdynamic stabilizations

George SapkasGeorge SapkasAsc. ProfessorAsc. Professor

--Medical School Medical School

Athens UniversityAthens University

InterspinousProcess Spacers

The concept of using the vertebral spinous process to secure an implanted device is not new.The Knowles device, introduced in the 1950s, consisted of a steel cylinder designed for temporary insertion between adjacent lumbar spinous processes in the patient with acute disk herniation.

Whitesides TE Jr, Spine 2003

Subsequent interspinous process devices have been designed for longer-term implantation for managing various conditions, including:

spinal stenosis,

disk herniation,

segmental instability,

degenerative disk disease.

In some patients, the devices are intended for use in conjunction with more traditional spinal fusion surgery.

Tsuji H et al, J Spinal Disord 1990

Senegas J, Eur Spine J 2002

Rationale for the Use of Interspinous Process Spacers

The interspinous process spacer is a motion-preserving spinal implant designed to provide symptomatic relief to selected patients without the need for spinal fusion

Theoretic indications for interspinous process spacer devices include:

spinal stenosis with and without degenerative spondylolisthesis, as well as chronic discogenic low back pain.

Lindsey DP et al. Spine 2003Lindsey DP et al. Spine 2003Zucherman J, et al. Spine 2005Zucherman J, et al. Spine 2005Richards JC, et al. Spine 2005 Richards JC, et al. Spine 2005

These implants have been proposed as a “dynamic stabilization” alternative to rigid instrumented fusion,

with the advantages of :a more limited and less morbid surgical procedure that may confer less risk of adjacent segment degeneration.

Minns RJ,et al, Spine 1997

Spinal stenosisSpinal stenosis

The pathophysiology of spinal degeneration remains a matter of controversy; however, a popular hypothesis suggests that the spondylotic sequence begins with:

progressive disk desiccation, bulging, and collapse.

Low-grade segmental instability may subsequently result in:

facet joint subluxation and hypertrophy, as well as in progressive thickening of the ligamentum flavum.

The risk of developing

symptomatic stenosis, typically in the sixth decade of life or later, is increased in the patient with pre-existing:

developmental stenosis or

a trefoil-shaped spinal canal.

Neural dysfunction has been attributed:

to direct compression of the cauda equina and

lumbosacral nerve roots as they travel within the:

canal,

lateral recesses, and

neuroforamen.

Presumably,

compression results in:

disruption of the vascular supply,

neural metabolism, and

axonal processes.

The postural dependency of:

neurogenic claudication and stenosis related symptoms is the result of the anatomic effects of :

flexion and

extension

on the spinal canal and foraminal dimensions.

During lumbar

extension, the ligamentum flavum buckles anteriorly, while the posterior annulus bulges posteriorly;

Both contribute to further reduction in the size of the central canal and lateral recesses

Neuroforaminal narrowing occurs:

as the facet capsule is pushed anteriorly

by the superior articular facet of the caudal vertebra

Mayoux-Benhamou MA ,et al, Surg Radiol Anat 1989

Conversely,

flexion is associated with:

a relative increase in the area of the spinal canal

as buckling of the ligamentum flavum is relieved.

Interspinous process spacer technology is designed to take advantage of the marked postural dependence of symptoms that exists in many patients with spinal stenosis.

The device is interposed between

adjacent spinous processes following limited surgical exposure of the posterior lumbar spine.

The implant maintains the treated level :

in modest flexion and

limits extension without limiting either:

axial rotation or

lateral bending.

Lindsey DP, et al, Spine 2003

In general,

normal cross-sectional area of the dural sac in the lumbar region is 150 to 200 mm2

stenotic symptoms may be associated with a decrease in area to <100 mm2.

Ullrich CG, et al, Radiology 1980

Computed tomography studies suggest that lumbar flexion increases the area of the spinal canal by 11%. By comparison, in vivo magnetic resonance imaging evaluation of patients following implantation of an interspinous process spacer has suggested a mean 22.3% increase in cross-sectional area of the dural sac.

Inufusa A,, et al, Spine 1996

Lee J, et al, J Spinal Disord Tech 2004

Low Back PainLow Back Pain

Interspinous process spacer implants also are being promoted for use in managing low back pain caused by degenerative disk disease.

The mechanism of pain generation associated with disk degeneration

remains unclear, and

surgical treatment of this condition remains controversial.

The patient with chronic severe low back pain unresponsive to nonsurgical management is commonly treated:

with spinal fusion, usually with rigid implant fixation systems, including pedicle screws and interbody cages.

Interspinous process spacer implants have been proposed:

as a dynamic stabilization alternative to rigid instrumented fusion, with the advantages of a more limited and less morbid surgical procedure that may confer less risk of adjacent segment degeneration.

Minns RJ, et al, Spine 1997

Initial biomechanical studies in cadaveric spines indicate that the interspinous process spacer reduces:

intradiskal pressure and

posterior annular pressure at the implanted level.

Lee J, et al, J Spinal Disord Tech 2004

In neutral sagittal alignment, posterior annular pressure is reduced by 38%, while nuclear pressure is reduced by 20%.

Swanson KE, et al, Spine 2003

With extension, pressure reduction is 63% and 41%, respectively.

Pressures at adjacent levels do not appear to be significantly affected.

Swanson KE, et al, Spine 2003

1.1. Reduction ROMReduction ROM

2.2. Increase stabilityIncrease stability

3.3. Reduction of the Reduction of the neutral zoneneutral zone

4.4. Reduction of Reduction of displacementdisplacement

Senegas J, Eur. Spine 2002

Biomechanincs of the interspinous spacer

DIAM

I n t a c tF a c e t e c t o m y a t L 4 - 5D i s c e c t o m y a t L 4 - 5D i s c e c t o m y w i t h D I A M a t L 4 - 5

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Low back pain originating in pathologic facet joints (facetogenic pain) is another controversial topic.Some investigators have suggested that more than 15% of chronic low back pain originates from pathologic facet joints; others are skeptical that the facet joints are a significant pain generator.

Dreyer SJ, et al, Arch Phys Med Rehabil 1996Berven S, et al, Semin Neurol 2002

Biomechanically, depending on

position and

the presence of associated arthrosis, the lumbar facet joints are thought to transmit 25% to 47% of axial load.

Shirazi-Adl A, et al, J Biomech 1987YangKH, et al, Spine 1984

In cadaveric studies, interspinous process spacer implants reduced:

facet joint contact area by 46%, and mean pressure by 39%, at the implanted level, with no significant effect on pressures at adjacent levels.

Wiseman CM, et al, Spine 2005

Consequently, some proponents of interspinous process spacer technology have suggested a potential role for these implants in managing facetogenic pain.

Functional Anatomy ofthe Posterior Column

In terms of potential sites for implant attachment, the spinous process has been identified as the weakest component of vertebral anatomy.

Coe JD, et al, Spine 1990Shepherd DE, et al, Spine 2000

The mean load to fracture has been reported to be between

339 to 405 N and is one half to one fifth that of spinal laminae.

Spinous process bone strength has been found to:

correlate linearly with bone mineral density.

Coe JD, et al, Spine 1990Shepherd DE, et al, Spine 2000

Anatomically, the interspinous ligament is composed of three distinct regions:

dorsal,

middle, and

ventral.

Of these, the middle region is the area in which ruptures typically occur.

Heylings DJ, et al, J Anat 1978Rissanen PM, et al, Acta Orthop Scand Suppl 1960

Histologically, the ligament consists of multiple fibrous cords composed of intermingled collagen and elastic fiber bands arranged in parallel and zig zag fashion. In many individuals, the supraspinous ligament is completely absent at the L4-5 and L5-S1 levels.

Coe JD, et al, Spine 1990Shepherd DE, et al, Spine 2000Barros EM, et al, Spine J 2002

Controversy and Concerns

Numerous concerns exist regarding interspinous process spacer technology.Some concerns are theoretical and involve the potential of

interspinous process spacer implants to cause local pain and contribute to segmental destabilization.

Others involve the true clinical efficacy and durability of benefit from these devices.

Interspinous process spacer implants are designed to:

produce increased segmental kyphosis (spinal process flexion) at the treated level.

Concern has been raised regarding the potentially deleterious effect of local kyphosis on adjacent segments.

Pre - opPre - op

6mts Post - op6mts Post - op

The spinous process normally serves as:

an origin and

insertion site for muscles and ligaments;

it is designed to resist tensile forces.

It does not normally function as a compressive load-bearing structure.

In the patient with: advanced spondylosis and

disk degeneration,

adjacent spinous processes can abut one another:

with formation of a bursa and

the potential for local pain generation.

It is possible that:

compression loading of the spinous processes and

cyclic device motion

may lead to: local tissue changes and pain generation.

Placement of interspinous process spacer implants may:

disrupt and potentially weaken the interspinous ligament and further destabilize the implanted level, particularly in terms of its ability to resist flexion-associated tension forces.

Lumbar segmental stability is maximized by locking of the facet joints, which has been demonstrated to occur with approximately 50 to 100 N of compression.

Papp T, et al, Spine 1997

By maintaining these joints in relative distraction, there is concern that interspinous process spacers may decrease overall stability.

Although biomechanical studies have suggested:

no significant effect on segmental range of motion in terms of

rotation and lateral bend

at the instrumented level,

these cadaveric studies were performed at low and controlled loads and may not accurately reflect in vivo forces.

Lindsey DP, et al, Spine 2003

A study of interspinous process spacer placement following graded facetectomy demonstrated a marked increase

in lateral bending motion at the implanted level.

Fuchs PD, et al, Spine 2005

Conclusions Conclusions

1. Disc deloading

2. Facets deloading

3. Reduction of the extension

4. Increase dimensions of the foramens

Disc decompression

Senegas J, Eur. Spine 2002

Intespinous spacers succeed in :

The rationale behind interspinous process spacer devices appears to be :

sound and

is well-supported by biomechanical studies.

Surgery to implant an interspinous process spacer is less invasive than standard laminectomy.

Early clinical reports suggest promising short-term results when these devices are properly applied in appropriately selected patients.

Overall, clinical efficacy appears to be moderate, with most patients experiencing measurable improvement

in symptoms and function

?? Disc rehydration Disc rehydration

However, a large minority of patients

fails to experience adequate relief, and concern remains regarding the durability of clinical improvement in those experiencing short-term symptomatic improvement.

Currently, it seems likely that there is a role for interspinous process spacer technology in specific sub-populations of patients, such as those with:

persistent symptoms despite nonsurgical treatment and with borderline anatomic stenosis, or those who are severely debilitated by medical contraindications that prohibit more definitive decompressive surgery.

Appropriate candidates for these devices are:

patients with neurogenic claudication symptoms that are relieved by:

forward flexion of the spine and who have no significant pain at rest.