49 Minimally Invasive Dynamic Stabilization
of the Lumbar Motion Segment with an Interspinous Implant
J. S´en´egas
49.1
Terminology
The principle of dynamic stabilization consists in both increasing the stiffness of the intervertebral segment and limiting the amplitude of mobility to stop the oth- erwise inexorable course of degenerative disc disease, and possibly, in some cases, to foster the healing of the least severe lesions.
As in any dynamic system, a mobile intervertebral segment undergoes acceleration inversely proportional to the moment of inertia when it is submitted to a force.
The rigidity of the system limits the displacement. This braking action preserves a margin of security and helps protect against tissue lesions involving the disc or the intervertebral ligaments. “Rigidity” is a mechanical pa- rameter defined in terms of load for a given displace- ment. It corresponds to the slope of the load/deformity curve.
Laxity or a diminution in the rigidity of an interver- tebral segment is constant in the degenerative process, as demonstrated by Ebara et al. [1] and Mimura et al.
[7]. This is true regardless of the stage of degeneration.
At the beginning of the degenerative process, before al- teration of the disc height, an increase in the range of motion is observed on bending studies because of the greater laxity. When the disc lesions are more severe, intervertebral mobility is reduced because of the nar- rowing of the disc space. However, mechanical testing shows that the system is still less rigid than normally observed, the decrease being reflected by an increase in the neutral zone.
49.2
Surgical Principle
Depending on the indication, the Wallis implant is placed either subsequent to a conventional decompres- sive procedure or in isolated fashion through a midline incision. The supraspinous ligament is detached from the two spinous processes of the degenerative lumbar segment and retracted intact with the underlying para- vertebral muscles. The interspinous ligament is re-
moved and, if necessary, a decompressive procedure is performed. After determining the size with a trial im- plant, a spacer is placed in the interspinous space. Two attached tension bands are then secured and tightened around the spinous processes on both sides of the spac- er stiffening the segment. The supraspinous ligament is reattached by single sutures through a hole in each spi- nous process and the wound is closed over a suction drain.
49.3 History
Prompted by progress achieved in similar techniques in other joints of the locomotor system, we began study- ing and developing dynamic stabilization of lumbar segments. Research in this field had been hindered by the fact that the multiarticular spinal system compen- sates relatively well for damage to a single segment, re- gardless of whether such lesions result from a degener- ative process or surgical fusion.
The stretching of the elements of articular union leads to a force resisting the displacement. The dissipa- tion of kinetic energy in the form of heat is mediated by the viscoelastic properties of the connective tissue (passive damping). This damping phenomenon would, in fact, be quite insufficient to protect the disc if it were not constantly supplemented by a much more effective active damping provided by the reflex contraction of the powerful paravertebral muscles. Although the dy- namic equilibrium of the intervertebral articular sys- tem is dependent on a combination of muscle activity and tension of the passive elements of union, the active system constantly protects the passive elements, which consequently are never submitted to the limits of their elasticity under normal conditions.
Under these specific mechanical conditions, the in-
tervertebral disc cells that produce the extracellular
matrix exhibit normal activity. These cells are, in fact,
mechanodependent, as demonstrated by Lotz and Chin
[6]. They function normally only under a precise range
of mechanical loading. Outside of this range, they ini-
tiate apoptosis. When loading is excessive or the active
system of damping is deficient, the passive system rep- resented by the disc and intervertebral ligaments can be overloaded and rupture. If these lesions are not ex- cessively severe, or if the lesional process takes place over time analogously to stress fractures, cell activity can repair the damage, as is the case in any connective tissue. However, when the stresses persist, the repara- tive process can be overwhelmed, and irreversible de- generative lesions develop if the loss of rigidity persists.
Nonetheless, the disc tissue, notably the annulus, has healing capacity, as do all connective tissues. In fact, an indisputable healing process can be observed in the intervertebral disc, with a fibroelastic reaction and neovascularization, at least at the beginning of degen- erative lesions. However, the persistence of excessive mechanical loading leads to the failure of this healing process, similar to that observed in pseudarthrosis of long bones or in meniscal lesions.
Fig. 49.2. A Wallis implant in place
Fig. 49.1. The Wallis dynamic stabilization implant
We performed animal studies to test this hypotheti- cal healing capacity of the intervertebral segment un- der propitious mechanical conditions. Encouraged by these preliminary unpublished results, we carried out biomechanical cadaver studies, mechanically testing various dynamic systems of stabilization of lumbar in- tervertebral segments, from 1984 to 1986. The system that we developed and first implanted in 1986 included a titanium interspinous blocker and a woven polyester cord. The results of an initial observational study were published in 1988 [12, 14]. This was followed by a pro- spective controlled study from 1988 to 1993 [15]. The results of these studies were quite promising.
Subsequently, more than 300 patients were treated for degenerative lesions with the first generation im- plant, with clinical and radiological follow-up. Assured of the complete innocuousness of the technique by the absence of serious complications in the latter patients over a 10-year period and after careful analysis of the points that could be improved, we developed a second generation implant called the “Wallis” system (Figs.
49.1, 49.2), which was marketed in 2002 [13].
49.4 Advantages
49.4.1
Technical Advantages
No bony fixation, as we think it is illusory to hope for durable purchase of bone screws in a dynamic intervertebral construct.
Due to its shape and its composition in polyether- etherketone (PEEK), the interspinous spacer is 30 times more elastic than the first generation (tita- nium) spacer to reduce the risk of spinous process stress fractures.
Flat bands of woven polyester replace the former woven polyester cord to avoid stress concentration on the spinous processes and thus reduce the risk of osteolysis.
Anatomical angles of the grooves for better fit in the interspinous process space.
New system of attachment between the spacer and the bands eliminating the torque present in the former system at final tightening.
The established long-term biocompatibility of PEEK and woven polyester.
Completely radiolucent materials compatible with MRI.
Radiodense markers for convenient radiographic follow-up.
Reduced number of dedicated instruments for rapid placement of the implant.
49.4.2
Clinical Advantages
Increases stiffness and restores stability to the treated segment relieving low back pain due to degenerative instability.
Limits but does not eliminate movement in the treated segment (Fig. 49.3).
Fig. 49.3. Lateral films showing flexion (left), neutral position (middle), and extension (right). As shown by the lines drawn for the second level of this two-level procedure, extension appears to retain a larger range of motion than flexion in this patient. The ra- diodense markers of both implants are visible as are the clipping rings added to prevent fraying of the bands at each extremity
Based on finite element analysis modeling the im- plant’s effect on an uninjured lumbar segment and the intradiscal pressure, Wallis appears not to alter the mechanical behavior of the adjacent segment, possibly limiting the domino effect of degenerative disc disease.
An interspinous spacer displaces mechanical load- ing dorsally reducing the load upon the disc and facet joints (by as much as 50 % for a spacer 12 mm in thickness [8]).
With the exception of the interspinous ligament and occasional slight trimming of the spinous pro- cesses, preservation of the vertebral and interverte- bral structures.
No pedicle screws, meaning less invasive procedure with fewer complications.
Blood loss involving implant placement is negligi- ble.
Rapid placement reducing operative duration.
The procedure is fully reversible, leaving all subse- quent surgical options open, including fusion, disc replacement, and potential future cell therapeutic interventions [3].
49.5
Disadvantages
The disadvantages of the first generation implant have been identified and taken into account when develop- ing the second generation implant (see above).
We believe that it is not possible to rigidify all joint elements of the intervertebral segment with a simple system. In our system of dynamic stabilization, only flexion/extension and rotation are damped.
To avoid problems involving screw fixation in a dy-
namic construct, the Wallis system is not provided with
implants for the L5-S1 segment.
49.6 Indications
The Wallis mechanical normalization system treats low back pain that accompanies degenerative lesions of grades II, III, and IV (among the five grades in the MRI classification proposed by Pfirrmann et al. [10]) in lumbar segments down to L4-5 in the following indica- tions:
Voluminous herniated disc in young adults Recurrent herniated disc
Herniated disc accompanying an L5 sacralization transitional anomaly
Degenerative disc disease at a segment adjacent to fusion
Modic I degenerative lesions
Lumbar canal stenosis treated by undercutting (“recalibrage” in French).
49.7
Contraindications
Grade V degenerative lesions in the MRI classifi- cation of Pfirrmann
Spondylolisthesis Osteoporosis
Non-specific low back pain
Patent constitutional or acquired spinous process insufficiency
L5-S1 Litigation
Patent psychological disorders
49.8
Patient’s Informed Consent
It is important for surgeons to inform patients of the in- herent complications of the spinal procedures per- formed under the same anesthesia as the implant. Re- garding the Wallis dynamic stabilization procedure it- self, the surgeon need only provide information on po- tential complications of anesthesia and spine surgery, in general (e.g., allergy to anesthesia, infection, deep venous thrombosis, pulmonary embolism).
It is just as important to inform the patient of the possibility of persistent low back pain due to concomi- tant degenerative lesions in other discs or facet joints, unless this possibility has been eliminated by methodi- cal diagnostic procedures performed over several days prior to the operation [9, 11].
49.9
Surgical Technique
The procedure to insert a Wallis implant is typically as- sociated with minimally invasive unilateral decom- pression consisting in discectomy, undercutting to en- large the spinal canal, or both. Only the implant place- ment will be described here. The decompressive proce- dures will not be addressed.
49.9.1
Preoperative Planning and Preparation of the Patient
Plain films
Bending films MRI
49.9.2 Anesthesia
The operation is performed with the patient under gen- eral anesthesia, but with no precautions specific to the Wallis procedure alone.
49.9.3 Positioning
The patient is placed in the prone position on a frame of foam padding. A neutral position of physiological lumbar lordosis is best to optimize the effect of the im- plant. All efforts should be made to avoid subsequent lumbar kyphosis.
49.9.4 Surgical Steps 49.9.4.1 Localization
Carefully locate the segment requiring the implant with an image intensifier.
49.9.4.2
Skin to Interspinous Space
After a short midline incision, the supraspinous liga- ment is detached from the two spinous processes at the level involved and retracted laterally without section- ing.
After the initial incision, it is recommended to tem- porarily suture a small sterile surgical drape over the wound edge on both sides to prevent contact between the skin and the implant.
The interspinous ligament is resected. Rough edges
of the inferior aspect of the upper spinous process and
of the superior aspect of the lower spinous process are
trimmed, if necessary, to facilitate insertion of the in- terspinous spacer.
The bony junction between the laminae and spinous processes may also be trimmed to position the implant as anteriorly as possible and ensure a stable fit against the laminae. Note: If the procedure includes enlarge- ment of a stenotic lumbar canal by resection of the up- per portion of the laminae, make sure to preserve suffi- cient spinous process thickness.
49.9.4.3
Placement and Securing of the Implant
To preserve physiological lordosis, when hesitating be- tween two implant sizes, the surgeon should choose the smaller implant. Given the prone position of the patient during the operation, substantial primary implant sta- bility is not necessary.
The spacer is positioned in the interspinous space and the bands are passed through the interspinous liga- ment as close as possible to the instrumented spinous process. On each side, the system is secured by posi- tioning and snapping onto the spacer a clip, through which the band is inserted. The bands are then tight- ened and a ring is crimped onto each band to avoid fraying after cutting off the excess band.
49.9.4.4
Reinsertion of the Supraspinous Ligament
The supraspinous ligament is returned to its original position and reinserted onto each spinous process with a single silk passed through a hole made in the spinous process using a Backhaus towel clamp.
49.9.4.5
Final Operative Procedures
To permit proper drainage of possible postoperative epidural bleeding related to a decompressive proce- dure, the operative wound should be closed over a suc- tion drain on one side of the spacer.
49.10
Postoperative Care
The suction drain is removed the following day in the absence of abnormal discharge.
External lumbar support is recommended for 1 month.
Patients should refrain from rehabilitation exer- cises for 1 month.
49.11 Results
A prospective single-arm open study of the second generation Wallis implant is underway, in which to date 220 patients (37 % women, 63 % men; age range 18 – 82 years; average age 44 years) have been included. Thirty- six percent of the patients presented with degenerative disc disease without disc herniation, 30 % with volumi- nous disc herniation, 18 % with canal stenosis, and 16 % with recurrent disc herniation.
The mean operating time for implant placement was 19 minutes and the mean time for surgery skin to skin was 79 minutes. The average blood loss was 190 cc. For- ty-eight percent of the patients received a 12-mm Wal- lis implant and 38 % received a 10-mm Wallis implant.
The most frequently operated level was L4-5 (92 % of the patients).
Among the 220 patients, there were six (2.72 %) postoperative complications. Three patients developed deep infection leading to removal of the implants. One patient with psychological problems had the implant removed by another surgeon 10 months after the inter- vention. Two implants were replaced by a second Wallis implant, one after immediate recurrence of herniated disc and the other after implant displacement. Al- though these were considered as adverse events, they confirmed the straightforwardness of Wallis implant replacement and that all options remained for other surgical solutions.
Preliminary, 12-month results have been obtained for 52 patients involved in the ongoing multicenter in- ternational study.
Preoperative evaluation of the functional and pain status of the patients was performed using the Japanese Orthopedic Association assessment of lumbar pain management (JOA score) [16], a visual analog scale for lumbar pain (VAS), and two quality-of-life question- naires, the first version of the medical outcomes score short form 36 (SF-36) [4] and the Oswestry low back pain disability questionnaire [2].
In the preoperative period, 65 % of the patients had pain scores between 70 and 100 on the VAS and 90 % of the patients had pain scores between 0 and 30 1 year af- ter surgery.
The result of the straight leg raising test was negative in 92 % of the patients. Sensory and motor disturbances were present in 75 % and 61.5 % of the patients before surgery and in 10 % and 4 % of the patients after 1 year, respectively.
The therapeutic success as assessed by Odom’s crite-
rion at 1 year follow-up found 89.6 % of the patients
with excellent or good clinical results, 6.25 % with satis-
factory results, and 4.15 % with poor results.
49.12
Critical Evaluations
The Wallis system is only applicable above L5 and does not include L5-S1. Although much work has been done and solutions have been found, none have been suffi- ciently satisfactory. Consequently, we have intentional- ly limited the application not to include the lumbosa- cral junction. A bilateral approach is necessary for the moment in order to secure the implant with the bands, but work is in progress to develop an implant that can be inserted through a unilateral approach. There is also a small risk of recurrent disc herniation due to the re- duced, but persistent mobility of the segment.
Next to these points of criticism, the present system has many substantial advantages. It is technically sim- ple and straightforward, with a short learning curve and reduced perioperative and postoperative morbidi- ty. The early mobilization and rehabilitation of operat- ed patients are also appreciable. One of the major in- centives prompting the development of dynamic stabi- lization was its potential protective effect against accel- erated changes in adjacent discs seen after fusion. Years of follow-up will be necessary to establish whether this theoretical advantage is actually achieved. Further- more, although it is still too early to determine whether the treated disc tissue actually heals or undergoes scar- ring, there is MRI evidence of disc rehydration at fol- low-up. Many initially black discs are coming back with normal, white signal. Examples are shown in Figs. 49.4 and 49.5.
Fig. 49.4. Patient with MRI evidence of disc rehydration at follow-up
Among the notions to retain concerning dynamic lumbar stabilization, foremost are the good clinical re- sults being observed. Follow-up evidence has con- vinced us that, to date, the efficacy of the new implant is at least as good as that of fusion.
Wallis is an operative alternative for many young pa-
tients with degenerative disc disease who suffer, but for
whom fusion, or even a disc prosthesis, might seem too
radical. Surgery rarely affords definitive solutions, and
spinal surgery is no different. If the threshold of sur-
gery is low and if a surgical procedure leaves other op-
tions open, a step-by-step strategy to treat low back
pain without compromising future solutions should be
considered. Through the Internet, more and more pa-
tients are aware of the advent of biological methods of
treating degenerative discs with the patients’ own stem
cells or fibroblasts [3]. This perspective is at our door-
step. Stem cell therapy has already begun for tendon
and ligament lesions [5]. For patients and surgeons,
Wallis is a failsafe solution because the procedure is
completely reversible. If relief from low back pain is not
achieved or if, after years of relief, the degenerative pro-
cess regains the upper hand, surgeons will still have all
the original options to treat their patient. Consequent-
ly, this method should rapidly assume a specific role
along with total disc prostheses in the new stepwise
surgical strategy for early forms of degenerative disc
disease.
Fig. 49.5. Another patient with MRI evidence of disc rehydration at follow-up
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