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Symposium: Dynamic Stabilization of the Lumbar Spine

Issue: Issue 2 2006

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David L. Hamblen, PhD, FRCS: Historically, treatment options for symptomatic disc degeneration were limited. Options included conservative treatment with an emphasis on bed rest or a discectomy through a radical laminectomy. For patients with recurrent disease or multilevel disease, spinal fusion was the only surgical option.

Over time, surgeons became frustrated by the disappointing results of conventional spinal fusion. Surgeons tried to improve the fusion technique. There was a move away from a posterior approach to an anterior approach, which led to more morbidity in less-experienced hands. Surgeons responded with improved pedicle screw fixation, but again the results proved less than ideal, particularly in patients with degenerative disc disease.

In this era of successful joint replacement throughout the body, surgeons are now thinking about replacing spinal joints, and the field is moving toward artificial disc replacement and minimally invasive surgery.

Continuum of care and patient selection

Hamblen: The introduction of dynamic stabilization seems to offer a solution to bridge the gap between conservative care and spinal fusion. When treating patients with lumbar spine pain, which treatment options are preferred, and what characteristics are considered in patient selection?

Thomas Apostolou, MD: When first seeing a patient with low back problems without a major neurological deficit, I recommend rest, anti-inflammatory medication, physiotherapy and perhaps epidural injections as another option for a few weeks. I monitor the patient, looking for any improvement or deterioration of symptoms.

For pain that does not respond to conservative treatment after 3 months, I may recommend a more invasive treatment. I do not prefer to use decompressive laminectomy, however.

For a patient with a disc rupture, I may perform a discectomy, according to the indications. I also use the Wallis dynamic stabilization system (Abbott Spine), as well as other options, such as nucleus replacement.

I try to avoid total disc replacement and spinal fusion for several reasons. Total disc replacement requires an anterior approach and, especially when performed at level L4-L5, may result in adverse effects such as vessel thrombosis and retrograde ejaculation in men. Spinal fusion is a therapeutic end with high morbidity and does not solve adjacent segment disease.

Vincent Pointillart, MD, PhD: A surgeon must know the source of a patient’s pain and the patient’s background. If images of severe changes corresponding to the clinical pain are obtained, surgeons will already know that conservative care will fail and another treatment option should be pursued. At this time, there is still a place for a limited decompression if a patient has a narrow canal with radicular pain or neurological signs with no disc mobility. A posterior or anterior fusion without any hardware would also work if the surgeon needs to fuse in patients with spondylolisthesis or in patients with almost no residual disc.

It is important to simplify procedures, especially for elderly patients, and surgeons should try to maintain mobility of the disc. Preoperative dynamic X-rays are key. In my practice, we do not perform nucleus replacement, but we perform total disc replacement, specifically at the L5-S1 level, where the approach is the safest.

Michael Pfeiffer, MD: The primary goals of conservative treatment and operative treatment are to resolve neurological deficits, to reduce pain and to restore function. Surgeons should use a combined approach during inpatient treatment. Conservative treatment consists of injections for facet blocks, single-shot epidurals and physiotherapy. Operative treatment can include decompressive procedures. From a diagnostic approach, if surgeons perform facet blocks, they can obtain valuable information about a patient’s source of pain. The approach can also be combined with functional X-rays.

Approximately 20% of patients who come to my practice leave without any recommendation for surgery. In looking at surgical approaches in my practice, we perform many decompression procedures, but no total laminectomies. We also perform many minimally invasive nucleotomies, and we perform fenestrectomies and partial facetectomies.

I think dynamic stabilization systems closely relate to decompression procedures. In my part of Germany, many elderly patients have bony stenosis, and for these patients, surgeons often combine stabilization with decompression surgery.

Total disc replacement may benefit about 1% to 5% of my patients because many patients have facet joint degeneration. If a surgeon performs a facet block and the patient becomes pain-free, then total disc replacement is not indicated.

Spinal fusion is by no means the No. 1 option anymore. In my practice, we try to minimize anterior and posterior approaches if spinal fusion is performed. Next year, we plan to perform minimally invasive retroperitoneal anterior fusion with dedicated systems.

Francis Ch. Kilian, MD: My practice has a dedicated unit for conservative treatment. If a patient presents with low back pain, conservative treatment is the preferred option. It is important to know the pathology of the patient and the source of the pain. We sometimes obtain standard radiographs with dynamic pictures, but the most important tool is magnetic resonance imaging (MRI) because the disc is visible. A black disc or signs of facet hypertrophy can reveal information about a patient’s pathology. Clinicians then conduct a functional investigation with radiculography, facet joint infiltration and coagulation.

Provocative discography can be used to find out whether pain is generated by the disc, and myelography can illustrate a patient’s bending ability, inclination and reclination. We are usually able to determine whether the pain is coming from the front or from the functional structures of the spine, or whether it is a stenotic or facet pain. Epidural injection, a minimally invasive treatment, is performed, as well as radiculography or facet joint infiltration. If a surgeon finds the pathology is posterior, then interspinous implants are considered.

Removing a disc leaves fusion as the only option. Less invasive options are considered first, such as with the Wallis implant, to release the facet joints and to stabilize posteriorly with a tension band, giving support to the normal force transfer anatomy of the segment. If the source of the pain is anterior, as in patients with L5-S1 pathology, then surgeons might implant the Dynamic Neutralization System (Dynesys, Zimmer Spine). Minimally invasive microdiscectomy is performed for disc herniations. In my practice, we also perform less invasive laminotomy and interlaminar decompression with limited resection of the facet in stenotic cases. Total disc replacement is limited because the indication for it must be clear. Facet joint problems are contraindications for total disc replacement.

Nicholas Boeree, FRCS: Conservative care must remain the first-line treatment for patients with lumbar spinal pain.

A multidisciplinary approach to conservative treatment is important. Physiotherapists can form a bridge between physiotherapy treatment and the assessment of the spinal patient. Physiotherapists should be involved at the outset in assessment and can form a useful point of liaison with other members of the multidisciplinary team such as pain specialists, psychologists and the spinal surgeon.

After conservative treatment, there are various surgical options. Decompression and discectomy may be useful for patients with neuralgic leg pain, but surgery for back pain relies on properly addressing, in as physiologically way as possible, a patient’s pain source. Spinal fusion still has a significant role. Spinal fusion is appropriate for patients with more advanced single- or double-level degenerative disease and severe back pain. There are additional specific pathologies, such as spondylolisthesis, where spinal fusion may still represent the best option. I do not feel that surgical advances have yet reached a point where the need for spinal fusion can be obviated.

Total disc replacement, fitting into a particular niche, is indicated for patients with primarily discogenic back pain with moderate to more advanced disc degeneration but with a reasonably preserved motion segment in other respects.

Dynamic stabilization has an increasingly important role in treating patients, and the objectives are to support and protect the degenerative motion segment. Surgery should aim to restore the normal biomechanics of the motion segment, but to do this a motion segment that is capable of reasonably normal movement is required. With dynamic stabilization, surgery aims to offload the disc and the facet joints on the basis that excessive loading through those structures is responsible for the patient’s pain. Relief of these high areas of loading within the disc can be achieved through the loadsharing that is provided by dynamic stabilization in cases with mild or moderate change, but it may be overly optimistic to expect similar benefits if the disc is effectively worn away.

Patients who have a combination of back pain and leg pain, perhaps due to stenosis or lateral recess stenosis, may also benefit from dynamic stabilization. Dynamic stabilization offers the opportunity to restore the dimensions of the spinal canal and the neural foramina, and can act as an adjunct to procedures such as decompression or discectomy. Dynamic stabilization may also act as an alternative to decompression by restoring and distracting the motion segment. Interspinous devices, such as the Wallis implant and X-Stop (St. Francis Medical Technologies, Inc.), are increasingly used for treating patients without recourse to formal decompressive surgery.

Nonfusion technologies

Hamblen: Is dynamic stabilization appropriate for most spinal surgeons to perform, or should the procedure be restricted to a specialist?

Pointillart: Dynamic stabilization can be more widely used than the anterior approach. Perhaps a general orthopedic surgeon can be assisted by a general surgeon to perform a retroperitoneal or transperitoneal procedure. The anterior approach, however, has risks such as damage to the major pelvic vessels, venous tear or thrombosis after retraction and sexual dysfunction, which is underestimated, especially in women. For spine surgeons, posterior stabilization is simple to perform. Compared with the anterior approach, there is less risk of significant complications with posterior stabilization, although immediate results on pain may sometimes seem delayed.

Boeree: Dynamic stabilization, in all its various forms, is relatively simple to perform and does not need to be restricted to particular specialists. Dynamic stabilization techniques offload and protect the facet joints. From a surgeon and patient’s perspective, the better surgical option is one that is less invasive or destructive and that conserves anatomy and options for the future.

When choosing a procedure, surgeons must think about its risks, complications and comorbidities, as well as the consequences if the procedure does not work. For example, if a spinal fusion procedure does not result in a fusion, then another spinal fusion procedure will be necessary.

A surgeon must also consider the possibility of a patient developing adjacent-level disease after a successful spinal fusion.

A dynamic stabilization system, such as the Wallis system (Figure 1), does not require any rigid fixation to bone. There are no pedicle screws to fail or to loosen, and there is no adverse effect on the adjacent levels. Rehabilitation is quick, and anatomy is not destroyed. Surgeons essentially preserve a patient’s anatomy so all other options are preserved for the future. The Wallis system is a preserving and conserving procedure.

Hamblen: Can surgeons perform dynamic stabilization at more than one level?

Boeree: There are no particular technical challenges in stabilizing more than one level with most forms of instrumentation. Evidence from a long-term study of the first generation of Wallis suggests that contrary to our experience with multilevel spinal fusion, multilevel dynamic stabilization performs as well as or better than single-level stabilization (J. Sénégas, MD, and colleagues, unpublished data, 2006). If a patient has more than one level affected and a surgeon can stabilize and protect both levels, then the surgeon can expect good results. If a surgeon fused the same two levels, however, the surgeon would be rightly concerned about the risks of adjacent level deterioration.

Pfeiffer: Not every posterior stabilizing and distracting implant procedure allows multilevel stabilization. In my experience with anterior fusions, comorbidity occurs from the approach, not from the stabilization. We observed problems with the sympathetic nerves and with retrograde ejaculation. The anterior approach and disc replacement is beneficial for only a few patients. To perform this procedure, a surgeon must be able to perform the access and solve complications with, for instance, the ascending lumbar vein. A surgeon should have vascular surgical experience to solve complications such as those with the iliac vessels and have full visualization to place the prosthesis precisely into the midline to avoid problems later.

Evolution of the Wallis system

Hamblen: Please describe the design and material used for the Wallis implant and how the Wallis system has evolved.

Pointillart: The basic idea of the Wallis system was that by preventing hyperextension, the facets would be unloaded and, in turn, by preventing hyperflexion, offloading of the discs would be achieved. The Wallis system was conceived because surgeons face not only a mechanical problem, but also a biological one. Disc deterioration is due to modification of the mechanics of the spine. If surgeons can restore the stability of the spine, there is a chance to improve the biology of the discs. A key point of the first generation of Wallis was no rigid fixation to the bone. The implant is essentially a floating system because if something stiff is fixed in the bone and there is constant pulling and pushing on it, loosening will result. The Wallis system uses a woven Dacron ligament and a spacer between two spinous processes (Figure 2). Long-term follow-up of patients who received the first generation of the Wallis implant shows that the implants stay in place and patients are doing well (J. Sénégas, MD, and colleagues, unpublished data, 2006).

Pfeiffer: The Wallis implant design changed from titanium to a softer polyetheretherketone (PEEK) material for several reasons. When the spacer was made of titanium, MRIs showed artifacts that made it difficult to plan revision or additional surgery because parts of the spinal canal and facet joints were obscured. Patients were also reportedly sometimes concerned of seeing the large spacer on plain X-rays. The quality and elasticity of the spacer is different with PEEK compared with titanium. No written evidence thus far suggests, however, that the more rigid titanium device produced problems with the spinous processes or stress-related complications.

Boeree: Follow-up data are available on about 60% of the 240 patients implanted with the first-generation Wallis implant. Data are still being collected. Only three patients have required reoperation because of an implant-related problem. Two patients had a spinous process fracture.

Although the point cannot be proven, one must suspect intuitively that an interspinous device with a degree of springiness must be better for the intermittent loading that occurs between the spinous process and the implant. The PEEK implant is about 30 times more elastic than a solid block of titanium.

A significantly rigid device, such as the X-Stop, appears successful in spinal stenosis, but there are concerns that the implant’s rigidity as it is interfaced with bone may be a source of problems.

The elasticity of the current Wallis implant is an improvement over the first-generation implant, but a possible future development could allow surgeons to dial in a specific degree of elasticity for a young and active patient or an elderly patient. When considering spinous process loading, surgeons should bear in mind the possibility of osteoporosis in elderly patients, and it would be appropriate in those cases to obtain bone densitometry before surgery.

Hamblen: Why was the Wallis implant redesigned from a round Dacron cord to a flat band implant?

Pointillart: The manner of clicking or attaching a Dacron band to a spacer is easier with a flat band. By having a flat band instead of a round one, contact between the bone of the spinous process and the band improved. The redesign widened the contact area between the spinous process and the band.

Wallis vs. Dynesys

Hamblen: Currently, the Wallis implant is not indicated for use at the L5-S1 level. What are the other options in dynamic stabilization systems?

Pointillart: The Dynesys system includes pedicle screws and a ligament and a spacer, which limits hyperextension and flexion with the band.

The notable difference between the Dynesys system and the Wallis system is the stiffness of the screw in the bone with the Dynesys implant. With the Dynesys, a surgeon can place screws peripherally to prevent any contact between the screw and the lower facet joint of the upper vertebrae. The surgeon will also work far more laterally, disturbing the muscles much more than with the Wallis system. The latter system allows a surgeon to remain on the midline.

We are currently re-engineering the Wallis implant to achieve L5-S1 indication. We are developing a device that is fixed in the sacrum because the sacral spinous process is not strong or large enough to fix the Wallis implant on it, but a soft fixation between the Wallis implant will avoid screw loosening.

Hamblen: At the L4-5 level, is the Wallis system better than Dynesys in terms of biomechanics because of the spacer?

Pointillart: The Dynesys system has a spacer between the screws, so I do not think that there is a significant mechanical difference between the two systems.

Kilian: I believe the Dynesys has disadvantages because the length of the spacer determines the stiffness or flexibility of the spacer. The longer the spacer, the more flexible it is. The shorter the spacer, the stiffer it is. Choosing a small spacer will result in rigid fixation just as in a normal pedicle screw system.

Another disadvantage is if the pedicle screws are not completely parallel to each other, the spacer will change because the head of the screw will be in a different position. The spacer will be longer on one side, and, consequently, there will be no symmetrical force transferred to the segment, leaving it unbalanced. Surgeons must be careful to put the screws in completely parallel to each other in order to choose the same length of the spacer. Because the force transfer is not defined in implanting the Dynesys, surgeons must work intuitively.

Contrastingly, the Wallis system has different, defined implant sizes. The cord gives a tension band posteriorly in the midline to release the facet joints and limits inclination and reclination. This is important to maintain the height of the foramen and to release the posterior disc and the posterior ligament.

Pfeiffer: Implants that stabilize in flexion and extension include, among two others just released, the Dynesys, the Wallis and, to a certain extent, the Diam Spinal Stabilization System (Medtronic, Inc.), if the Diam is fixed together with a tension band. Fixing a Diam implant with a tension band is performed to secure the implant in the interspinous cavity. With other implants, the extension stabilization depends on the amount of distraction or preload given to the implant. For example, when implanting the X-Stop at L5-S1, which is feasible in about 60% to 70% of patients due to the shape of the spinous process at S1, the same result will not be achieved because it is an extension and distraction. Only interspinous blockers with tension bands can be placed under less pronounced kyphosing stress without the risk of loosening. Even if a surgeon does not over distract, some extension is maintained, provided a patient does not have excessive osteochondrosis at L5-S1.

Hamblen: Are there other biomechanical differences between the Dynesys and the Wallis systems?

Pfeiffer: An important biomechanical difference between the Dynesys and the Wallis is that the Dynesys gives stability, in respect to lateral bending. The lateral bending stability with the Dynesys is more significant than with the Wallis because the approach is more lateral. The more a surgeon works to the side with the screws, the more leverage the surgeon has, and lateral bending will be more stable.

Detachment of muscles is not a problem with the Dynesys. If a surgeon uses the posterolateral approach, the surgeon can work between the multifidus and the longissimus muscle, and the muscles are not torn apart. Objections against the Dynesys at L5-S1 can be made, however, because screws at the S1 pedicle must remain stable for many years in its relatively wide cancellous bone cavity. I prefer to use Dynesys in younger patients with good bone stock and a significant amount of primary mobility.

A problem could arise if a surgeon does not achieve some degree of fibrous ankylosis. Although I have not seen many loosenings with the Dynesys screws, the procedure to implant the device is demanding. If a patient has a wide iliac crest, a surgeon will face difficulties achieving convergence of the screws when trying to put them in at S1 in an appropriate position. When comparing the difficulties of inserting the implants, I believe that the Dynesys system is more difficult.

Kilian: Dynesys is an appropriate system for young patients with good disc conditions but not for older patients with stenosis combined with advanced disc degeneration. The Wallis system, however, is indicated for older patients with stenosis.

Boeree: The Dynesys incorporates the use of a plastic spacer that is stiff and rigid upon insertion but becomes flexible and more elastic when it warms to body temperature. The system, however, remains relatively rigid; there are studies that suggest it is comparable to spinal fusion in terms of its rigidity.

Surgeons should carefully consider the long-term results of implanting the Dynesys because there is a risk of screw loosening or breakage. Screw loosening or breakage may occur because the pedicle screws are constantly and repetitively loaded in the dynamic situation. Revision is difficult when screws break. They tend to break in the sacrum and at the upper level of multilevel Dynesys cases. A surgeon will either have to bypass the broken screw, which is difficult, or fuse the patient anteriorly.

Surgeons must consider the effects on the adjacent level of a relatively rigid motion segment. With a system that is almost as rigid as a spinal fusion, surgeons should expect the same problems to occur at an adjacent level. The Dynesys system is also different from other forms of dynamic stabilization because surgery is more invasive and disruptive.

Dynamic stabilization with X-Stop and Diam systems

Hamblen: How do the X-Stop and Diam implants compare with the Wallis implant?

Pfeiffer: The X-Stop implant, like the Wallis implant, evolved from its original design to its current design. The first-generation X-Stop was a full-titanium implant that resembled a torpedo. The implant was inserted into the interspinous interspace and locked with two wings to prevent shifting and tilting. The device has residual mobility inside the implant because it can turn slightly, and the wings will prevent it from slipping out. The wings do not prevent the implant from slipping backwards, however.

The current X-Stop design includes a sheath of plastic around the titanium core to reduce its stiffness and protect the spinous processes. The X-Stop is advocated as a tool only for distraction without decompressive surgery. It can be used as a posterior implant at L5-S1. The tips of the implant rest on the facet joints. I recommend surgeons reduce the hypertrophy of the facet joints via posterior decompression. Reducing the facet joints in this manner, as opposed to inserting the X-Stop in soft stenosis, should not be performed under local anesthesia.

In my experience, if a patient has bony stenosis, a surgeon should perform decompression because the patient has low primary mobility of the segment and will not benefit from the implant otherwise. I do not recommend implanting the X-Stop at L4-5 or above because other implants provide better stabilization of the segment in flexion and extension.

The X-Stop requires opening both sides of the spinous process. The Diam implant, however, requires only exposure on one side if used without a tension band.

The Diam implant has more resilience and elasticity than other implants because it is an H-shaped, elastic spacer with a cord attached. The cord is led through the implant differently than the cord led through the Wallis implant. This allows for some mobility of the cord inside the implant. Using a pair of special pliers or compressing forceps, a surgeon can slide the implant between the spinous processes without opening the other side of the spinous process. A spreader can also be used for this minimally invasive procedure.

Pointillart: I do not think surgeons should classify the X-Stop or the Diam implants as stabilizing devices. Although the implants prevent hyperextension, they induce a kyphosis deformity at the level at which they are implanted, unlike the Dynesys and the Wallis implants.

Kilian: Unlike the Wallis implant, the Diam’s lateral bending pressure on the facet joints and rotation of the segment are not limited because it is a soft device. If a patient has facet problems, they will return after a short period of time after implanting the Diam device. With the X-Stop implant, lateral inclination is not limited in any way.

Apostolou: Although the Wallis spacer is elastic, it is stiffer than the Diam implant. With the Wallis spacer, surgeons can reduce extension and with the stability of the ligament, surgeons can reduce kyphotic formation.

Boeree: The X-Stop implant was developed primarily for treatment of spinal stenosis. The implant was not specifically developed to address the instability present in the degenerative motion segment. The X-Stop implant was not primarily developed to treat back pain or instability-related back pain.

A degree of instability in the motion segment can lead to spinal stenosis due to bulging of the disc as it starts to collapse and hypertrophy of the facets as the motion segment tries to re-stabilize. The ligamentum flavum also bulges into the canal with loss of disc height. By implanting the X-Stop device and distracting into kyphosis, the dimensions of the spinal canal open, but the underlying pathology is not addressed.

Implanting the Wallis device, however, adds additional stability and resistance to flexion, which is an advantage and only requires 5 more minutes of operating time.

Indications and contraindications

Hamblen: What are the indications for using the Wallis system?

Apostolou: The Wallis implant can be used in every disc herniation because future options such as gene therapy and other modern techniques are left available. The biomechanic environment of the lumbar segment is restored, and surgeons can expect a regeneration of the disc.

I implant the Wallis in every patient with disc herniation from level L4-5 and higher. I do not use a curette to extract disc material from the endplate. I use forceps to extract the sequestrated parts of the disc. Disc regeneration may occur from the remaining disc cells, but patients who did not have disc regeneration are possible candidates for future options.

Hamblen: Would you implant the Wallis following a previous failed partial discectomy, where more disc must be removed?

Pointillart: My key indication is the degenerate disc. I use the Wallis system in these discs and for large midline herniations with a narrow canal when it is not known which side should be approached for a one-piece disc herniation. The Wallis system can be used for a one-piece disc herniation that is removed and the bone area is widened. If a patient is younger than 60 years and has a narrow canal, the Wallis implant may be beneficial. I would also implant the Wallis in patients with isolated low back pain.

Pfeiffer: I use the Wallis implant as an adjunct to various decompressive surgeries regardless of whether the surgery is a nucleotomy, decompression of the facet joints or laminotomy. My decision to use the Wallis implant is dependent upon my intraoperative measurements of the segment’s stiffness. I implant the Wallis in patients with massive prolapse. I also implant the Wallis in patients who have low back pain without spondylolysis and in patients with spondylolisthesis up to Meyerding grade 1. This is the limit for me. An interspinous implant is not sensible for patients with pars defects.

Kilian: A posterior device should not be implanted in patients with low-grade spondylolisthesis stemming from symmetric degeneration of the disc with malrotation and slippage. Degenerative spondylolisthesis at one level leads to additional degeneration at adjacent segments, which is a contraindication for using the Wallis implant.

Indications for Wallis are young patients with mild disc degeneration, black disc and facet problems. Elderly patients with stenosis are also good candidates for the Wallis system.

Boeree: In treating patients with primary low back pain, the Wallis dynamic stabilization system has an important role and is my first surgery of choice in mild to moderate degenerative disease. If the disc has been badly disrupted, however, then I am likely to use an alternative technology, such as disc replacement.

The Wallis implant may be beneficial for patients who present with sciatica and have a massive disc protrusion with potential for future dysfunction or who present with a recurrent disc protrusion. For example, a study of recurrent disc protrusion demonstrated that, compared with simple revision discectomy alone, discectomy with the device in place resulted with better outcomes for patients in terms of low back pain and leg pain.¹

I would also consider implanting the Wallis system in patients who present with sciatica with a significant history of back pain because discectomy alone will not fully address their back pain. The Wallis implant is a minor addition to surgery because the procedure does not have a significant downside or complications that add to discectomy alone.

Hamblen: What are contraindications to using the Wallis system?

Apostolou: Patients who have poor bone quality, a body mass index greater than 35 kg/m² or a previous laminectomy are not good candidates for the Wallis implant.

Pointillart: Certain types of disc degeneration are a contraindication for implanting the Wallis system. For example, a patient with grade-5 disc degeneration will not benefit from Wallis implantation. Also, spondylolisthesis should not be addressed with the Wallis system. Patients with retrolisthesis, however, may benefit from the Wallis implant.

Boeree: Spina bifida occulta and other conditions, such as lytic spondylolisthesis where posterior elements are not intact, are contraindications for the Wallis system.

The Wallis system can be useful as an adjunct to any sort of decompressive surgery for degenerative spondylolisthesis with slips up to 3 mm or 4 mm.

Pointillart: Patients with degenerative listhesis have frontal joints that become saggital due to slippage forces. To decompress the neural foramen, a surgeon will open these joints. I do not believe the Wallis system is able to prevent the continuous forces and slippage. The Wallis system would prevent extension and flexion, but slippage can continue, resulting in recurrent neural foramen compression due to increased listhesis.

Implanting the Wallis device

Hamblen: What is your surgical technique for implanting the Wallis system?

Pointillart: I position the patient on the table to ensure the procedure will not induce kyphosis. I perform the necessary decompression. It might be bony stenosis. I choose a spacer that fits perfectly with the spinous processes. Occasionally, I prepare the shoulder of the spinous process on the upper vertebra to ensure the spacer fits anteriorly as well as possible. It is important to have a lateral view using imaging during surgery to avoid overdistraction.

The interspinous ligament is completely removed to perform decompression. If the supraspinous ligament is strong, I leave it intact. If the supraspinous ligament is already destroyed, however, I remove it to improve control. Once I have prepared the space, I insert the spacer, preparing the space above and below the adjacent spinous processes with a 90º curved needle, which will allow the ligament to go above and below, and then I put the clip on. I introduce the band inside the clip, turn it 360º and clip it in the spacer (Figure 3).

Boeree: Implanting the Wallis is a simple procedure. Any orthopedic surgeon with familiarity with positioning other prostheses should have no difficulty with the technique.

There are various tips and tricks. For example, I prefer to pass the bands and then thread and attach the clips with the implant outside the patient before slotting it into the segment. Surgeons can check that the band is well-seated. If the band is not well seated, the clip may pop off during tightening. Ideally, surgeons should observe one or two procedures, or attend a workshop.

Pfeiffer: Surgeons should have good visualization of the adjacent spinous process because without it, they will not be able to move the cord around the adjacent spinous processes, and stability will be in jeopardy.

Pointillart: Surgeons must check that they are not oversizing the spacer because the spacer can be touching both spinous processes but still slightly floating, and the tension in the band will make it stiff. Orthopedic surgeons new to the technique tend to oversize the spacer because they want the spacer to be strong when, in fact, the band is strong enough.

Hamblen: While attempting not to destroy normal lordosis, is there any danger that a surgeon will insert a spacer that is too small?

Pointillart: No. Surgeons can select the correct size spacer using the trial spacers. Surgeons can also perform a fluoroscopy to ensure that kyphosis was not induced. A completely floating spacer will clearly not work properly. An appropriately sized spacer does not push the two spinous processes apart and is stiff after band tensioning.

Boeree: I have told surgeons watching the procedure that if the spacer is loose, then it is too small. The spacer should be snug, but if I have to force it in or if the spine seems to squeeze it back out like a bar of soap, then the spacer is too large.

Pfeiffer: I use a modified Novo device (Zimmer Spine), which makes it particularly easy to get an idea of how snug the spacer fits. I measure the distracting forces with this special tool, and I measure load displacement curves during the operation, but this is not part of the original protocol.

I also use a 16-mm implant to bridge one level with the missing spinous process, thus stabilizing two segments. This can only be a last line of defense, however. I do not recommend using the implant as an adjunct for patients who had laminotomy surgery at the level below because their spinous processes may be not sufficient.

Hamblen: Small apertures are made in both adjacent interspinous ligaments to permit passage of the bands. How should surgeons position the bands correctly?

Pointillart: Surgeons should position the bands as far anterior as possible. Surgeons must feel the anterior border of the adjacent spinous process. As soon as a surgeon touches the upper edge of the upper one, the surgeon will be as anterior as possible and can go through the ligament with a trocar.

Apostolou: Usually, I palpate the spinous process with a forcep. I then move anteriorly and penetrate the interspinous ligament with the forceps. After penetration, I pass the band with the special introducer trocar.

Tension

Hamblen: Achieving correct tension in the band is an important issue. What should surgeons know about it?

Boeree: The surgeon should remove any slack in the band (Figure 4A). The band, which passes from the implant around the spinous process of the adjacent level, must be pulled through properly. Surgeons should be cautioned that the band may catch on the spinous process, and then the band will not be properly pulled through when tightening. Surgeons should also ensure that the band is not twisted as it is passed around. After checking these things, surgeons should pull the excess slack through and tighten the band with the tightening device (Figure 4B). It is important for the surgeon to hold the implant in place with the tightener. Otherwise, a surgeon might twist the implant out or break the spinous process. Using the tensioner, surgeons must wind the ligament through. The tightener will gradually deform and bend towards the implant. As a surgeon tightens the tightener, it bends towards the corner of the implant. The tightener will move around the corner as a surgeon continues to tighten. At this point, surgeons should stop, unwind the tightener and pull the ligament through again, and then repeat the process.

I do not think there is a significant learning curve to implanting the Wallis. As with any new technique, however, it is sensible to observe a surgeon with experience with implanting the Wallis and to practice the procedure on dry bones.

Hamblen: Is there evidence that the tension may alter postoperatively? For example, with the Dynesys system, body temperature might affect the artificial ligament. Do the same things apply to the Wallis system?

Boeree: No, the band is pre-stressed. There is minimal further creep during physiological loading subsequent to implantation, with a miniscule fraction of a percent of further lengthening. The tightening device and the clips permit passage of the band in one direction but not the other. Consequently, no subsequent loosening of the band should occur. I have had only one band failure, which led to the return of the patient’s back pain. Looking into the case, I found I inadvertently scored the band with the scalpel blade, most likely when cutting off the excess ligament. The scoring marks on the band were visible under microscopy with the scoring extending part way through the band, which led to fatigue failure of the rest of the band. Three months after explantation, I reintroduced the Wallis into the patient and the patient’s back pain subsided.

Pfeiffer: The Wallis implant is easy to remove, and explanting it does not harm patients. An infection with the Wallis implant is easier to treat than an infection complicating a posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF) or pedicle screw instrumentation. I have had only one patient with an infection. I usually put collagen fleece, soaked in gentamycin, under the implant if I perform midline decompression. Surgeons will have excellent oversight when performing midline decompression with Wallis implantation. The entire dural sac and spinal canal are visible.

Apostolou: I do not place any collagen fleece under the implant. I have seen only one patient, who was elderly and had diabetes, with a minor infection. I easily cut the bands and removed the implant.

I removed the Wallis implant in only two other patients. One patient had a spinous process fracture. This patient was male and had a preoperative body weight of 100 kg. The patient’s weight rose to 130 kg postoperatively, and he sustained spinous process breakage. The other patient in whom I explanted the Wallis had an inadequate foraminal decompression. The patient did not completely recover from neurological deficits. Consequently, I performed decompression and removed the implant 7 months postoperatively. Now, 1 year after the second operation, the patient shows no signs of instability or any other complications. Although I recommended reimplanting the Wallis, the patient preferred to avoid reimplantation after considering the inadequate decompression related to the implant.

Hamblen: How would you rate the difficulty of implanting the Wallis after your first 6 months of performing the procedure?

Pointillart: The procedure is much easier to perform than spinal fusion.

Boeree: In a multicenter study, the operative time for implantation, after performing decompression, averaged slightly more than 15 minutes.² In my opinion, Wallis implantation is straightforward once surgeons learn the surgical techniques.

Postoperative management

Hamblen: Postoperatively, how do surgeons manage patients who received the Wallis implant?

Apostolou: My patients have no postoperative limitations if the only procedure I performed on them was Wallis implantation. Patients are able to stand up the next day.

Depending on the age of the patient, I may recommend that patients do not gain weight, bend or run.

Pointillart: Implanting the Wallis results in no more limitations than a discectomy.

Pfeiffer: I tend to be slightly conservative. I provide patients with light orthosis for 6 weeks. If the procedure was located at L4-5 or L5-S1, I advise patients against deep sitting for 3 weeks. I also advise against strenuous exercise and physiotherapy immediately after the operation.

Boeree: Patients tend to believe that all back operations are major procedures, often expecting long periods of bed rest, recuperation and time off work. With dynamic stabilization, surgeons need to correct this misperception.

I advise patients to rest for the day of their surgery. Patients are usually discharged within 1 to 4 days. I recommend they proceed with a regimen of physical therapy, particularly core stability exercises to strengthen muscles. I advise patients with sedentary or semi-sedentary occupations to return to work 2 to 3 weeks after surgery. I advise patients with physical occupations to take 3 to 4 weeks off from work.

Outcomes of using the Wallis system

Hamblen: Complication rates with the Wallis system are low. What are your patient outcomes from using the Wallis system?

Boeree: Few complications arise with the Wallis system. In 2002, the multicenter study began researching the safety and efficacy of the Wallis system.² The study is not controlled but is a prospective observational study with fairly strict inclusion and exclusion criteria. The current results of the study demonstrate excellent clinical and radiological outcome measures, including MRI scans (Figure 5). Generally, results are good. Ninety-one percent of patients at 1 year are classified as excellent or good, with 65% of those patients considered excellent. The SF-36 shows a steady improvement up to 1 year and, by that stage, patients are comparable to an age- and sex-matched normal population. Back pain, measured with the visual analog scale (VAS), improved, reducing from 7 to 1. Japanese Orthopaedic Association (JOA) scores improved from 6 to 13.

Pointillart: The ongoing, multicenter international study is checking postoperative long-term MRIs, as well as dynamic films. The quality and mobility of the discs are being monitored, which is important because no other dynamic stabilization device studies looked at disc quality and mobility.

Boeree: In many patients, paired MRI scans show that disc appearance improves. Improvement to the extent that a surgeon can show the improvement to a patient is impressive. This improvement is likely to reflect a degree of offloading of the disc. On a lateral view, improvement in disc height may be visible. Offloading of the disc allows some degree of rehydration, and possibly recreates good physiology and a good biomechanical environment for a disc. This, in turn, may result in improvement on the microscopic scale in terms of cell function within the disc and matrix turnover rates.

Pointillart: The basic idea of the Wallis system (Figure 6) is that biology is related to mechanics. The MRI scans might show that the Wallis system can restore some mechanics of the disc. The biology of the disc may improve with cell therapy suitable to assist the final healing of the disc.

Looking ahead

Hamblen: How can dynamic stabilization systems continue to improve?

Boeree: I think the future may hold exciting options. Surgeons may be able to alter the cellular biology within a disc, providing they can protect the biological environment and support it in a proper way. If treatment of lumbar spine pain moves toward genetics and cell biology, surgeons must first create a conducive biological environment. Treatment will fail if surgeons just insert cells into a damaged and unsupported disc.

Kilian: A company in Germany tested cellular biology within a disc. Researchers placed chondrocytes inside the disc, but, because the disc environment was disturbed, the treatment did not work. For future technologies, we must first resolve how to alter the environment correctly. With knee surgery, chondrocytes can be successfully implanted to recreate the surface of the knee. A spinal disc, however, is more complicated because the nutrition of the disc is disturbed in disc degeneration.

Hamblen: What would be the next ideal randomized trial for the Wallis system to prove its effectiveness in long-term follow-up results?

Pointillart: We could compare a simple discectomy with a discectomy and decompression stabilization. A future study looking at a disc at one level or one disc and discectomy vs. the Wallis system would reveal interesting results.

Hamblen: Will the Wallis system be able to address problems at the L5-S1 level in the future? How will the system be modified?

Pointillart: A strong fixation at S1 and the bone is necessary. The quality of the sacrum is variable in patients. The strongest part of it is the anterior upper corner of S1, which is usually difficult to reach. The Wallis system may incorporate transcortical screws to address this need. Also, stiff fixation is not desirable from the spacer and between the two screws because screw loosening will result. The Wallis’ floating system must remain fixed between the two screws and the spacer and the L5 spinous process.

Currently, the shape of the device is being improved, and a revised design could be released in the next few years.

Hamblen: Dynamic stabilization treatment options bridge the gap between conservative care and spinal fusion. For patients who experience lumbar spine pain, dynamic stabilization systems, such as the Wallis system, offer patients a surgical option with shorter recovery and rehabilitation times. Surgeons can incorporate dynamic stabilization systems into the continuum of care to offer patients greater benefits.

I would like to thank Orthopaedics Today International for organizing this discussion and Abbott Spine for its sponsorship. I would also like to extend my thanks to the faculty members for their participation in this symposium and monograph project.

References

1. Sénégas J. Mechanical supplementation by non-rigid fixation in degenerative intervertebral lumbar segments: the Wallis system. Eur Spine J. 2002;11:S164-S169.
2. Boeree N. Dynamic stabilization of the degenerative lumbar motion segment: the Wallis system. Presented at: Annual Meeting of the North American Spine Society; Sept. 27-Oct.1, 2005; Philadelphia, Pa.