Short‐term clinical observation of the Dynesys neutralization system for the treatment of degenerative disease of the lumbar vertebrae

Abstract

Objective:  To explore the safety and short‐term efficacy of the posterior approach of the Dynesys dynamic neutralization system for degenerative disease of the lumbar vertebrae.

Methods:  From March 2008 to March 2010, 32 cases of degenerative lumbar vertebral disease, 19 men and 13 women, (mean age 58 ± 5.2, range, 43–78 years), were treated with posterior laminectomy and Dynesys internal fixation. All patients had a history of over 3 months waist or leg pain that had not been relieved by conservative treatment. There were 10 cases of single lumbar intervertebral disc protrusion, 14 of degenerative lumbar spinal stenosis, 5 of degenerative lumbar isthmic spondylolisthesis, and 3 of recurrent lumbar disc protrusion after surgery. A visual analogue score (VAS) was used for pain assessment, and the Oswestry disability index (ODI) for functional evaluation of clinical outcomes.

Results:  All patients were followed up for 6–23 months (mean, 16.4 ± 5.5 months). Forty‐one segments in 32 patients were stabilized; 23 cases (71.9%) underwent single‐segmental stabilization, and 9 (28.1%) two‐segmental stabilization. VAS of leg pain, root and low back pain was significantly improved postoperatively. The ODI improved from preoperative 69% ± 12.6% to postoperative 28% ± 15.7% (P < 0.001). On the stabilized segment and adjacent segments above and below, the range of movement showed no statistical difference; no loosening of screws, cord and polyester spacer occurred.

Conclusion:  The Dynesys dynamic neutralization system combined with decompression can achieve satisfactory short‐term clinical results in lumbar degenerative disease. This procedure system not only reduces back and leg pain, but also preserves the mobility of fixed segments, minimizes tissue injury and avoids taking bone for spinal fusion.

Introduction

Spinal fusion is the traditional surgical approach to spinal degenerative disease, for which conservative treatment is ineffective ¹ . In recent years, although according to radiographic findings the success rate of spinal fusion using newly developed spinal fusion techniques is over 95%, clinical satisfaction has not increased accordingly. Besides, spinal fusion can lead to loss of lumbar spinal mobility, pseudarthrosis, chronic lower back pain, accelerated degeneration of adjacent segments and other related symptoms ² . In the past 10 years, some researchers have used dynamic stabilization systems to treat low back pain. The theoretical advantages of such approached have attracted much attention from physicians, although the clinical effect of these methods still needs further investigation. Dynesys (Zimmer Spine, Minneapolis, MN, USA), one of the new non‐fusion, dynamic stabilization devices for lumbar degeneration and instability, is also described as a dynamic neutralization system ³ , ⁴ . It keeps the intervertebral disc and zygapophysial joint intact, maintains or restores intersegmental mobility and the load transfer mode of motion segments without adversely affecting the adjacent segments, all of which is beneficial for the physiological stability of the spine ⁵ , ⁶ . From March 2008 to March 2010, we used the Dynesys system to treat 32 patients with lumbar degenerative back and leg pain and achieved satisfactory short‐term clinical outcomes.

Materials and methods

General data

A retrospective review, based on a database search, was performed to identify 32 cases of lumbar degenerative disease treated by posterior decompression combined with Dynesys internal fixation at our department from March 2008 to March 2010. There were 19 men and 13 women aged from 43 to 78 years with a mean of (58 ± 5.2) years. Inclusion criteria were typical symptoms of nerve root compression because of degenerative lumbar disease. There were 10 cases of single lumbar intervertebral disc protrusion (2 at L₃‐L₄,5 at L₄‐L₅, 3 at L₅‐S₁,), 14 of degenerative lumbar spinal stenosis (5 single‐segmental: 1 at L₂‐L₃, 2 at L₄‐L₅, 2 at L₅‐S₁; two‐segmental: 2 at L₂‐L₄, 3 at L₃‐L₅, 4 at L₄‐S₁), 5 of first degree degenerative lumbar isthmic spondylolisthesis (3 at L₄‐L₅, 2 at L₅‐S₁), and 3 of recurrent lumbar disc protrusion after surgery (2 at L₄‐L₅, 1 at L₅‐S₁, 1, 2). All patients had a history of over 3 months waist or back and leg pain which was resistant to conservative treatment. Lateral and dynamic X‐ray films, CT and MRI scanning of lumbar vertebrae were taken preoperatively. These showed straightened lumbar spine curves, lumbar degeneration and dynamic instability of the lumbar spine. MRI demonstrated lumbar disc protrusion associated with signal changes and disc protrusion of varying degrees at adjacent segments. The preoperative Oswestry disability index (ODI) was 69% ± 12.6% (30%–87%). The visual analogue scores (VAS) of preoperative leg and root pain were 7.6 ± 3.2 (6.2–9.4), and of preoperative waist pain 7.3 ± 3.7 (6.0–9.2).

Figure 1.

Distribution of patients according to disease type.

Figure 2.

Lumbar vertebrae stabilized by the Dynesys system. Horizontal axis, number of patients.

Surgical procedure and perioperative treatment

Anesthesia and patient position

General intravenous anesthesia with tracheal intubation was performed. The patient was placed in a prone position with the chest and ilium elevated to suspend the abdomen to avoid compression to reduce blood loss. Both lower extremities were placed at 30° of knee or hip flexion, to expand the laminar space. Normal physiological lumbar lordosis was maintained. Preoperatively, a “C”‐arm X‐ray machine was used to determine the lumbar degenerative space.

Operative procedure

The entrance point was lateral to the conventional entrance point of pedicle screws and at 35°–45° abduction to the crossing point between the median line of the transverse and superior articular processes. Under the guidance of C‐arm X‐ray, pedicle screws were inserted with a funnel technique ⁷ , so as to prevent their penetration into the vertebral canal or outside the vertebral body, thus avoiding accidental damage to the spinal cord, blood vessels or nerve tissue. Large‐sized screws were selected and their roots placed close to the bone surface to keep elastic fixation elements in the plane of, or anterior to, the facet joints. The vertebral plates were partially resected above and below, the lateral recess and root canal enlarged and the lateral facet joints preserved. The yellow ligament below the lamina was separated, cut open and severed to expose the dura mater and nerve roots. A nerve root retractor was used to protect the dura mater of the spinal cord and nerve root, and the protruded disc removed with a nucleus clamp.

After decompression, translation of the nerve roots was carefully examined and the intervertebral foramen probed until full decompression had been confirmed. Then, maintaining normal physiological lumbar lordosis and mild separation of the spine, the distance between the upper and lower pedicle screws was measured on both sides and the appropriate length of spacer selected accordingly. In order to obtain better clinical outcomes, the surgeon should follow this operative procedure closely because an overlong spacer may result in local kyphosis, which can affect the prognosis ⁴ . Finally, a polyethylene terephthalate (PET) cord was used to link the polyester spacer and upper and lower pedicle screws. The cord was tightened and locked with screws. The wound was rinsed, a negative pressure drainage system installed and the incision sutured.

Postoperative care and rehabilitation

Antibiotics, hormones and dehydrating agents were used prophylactically postoperatively. Two days after surgery, rehabilitation exercises for the lower limbs were initiated. Three days to six weeks after surgery, the patients were permitted to get out of bed, sit, stand and walk with the protection of abdominal belt, but advised to avoid bending, rotation, and weight bearing activities. Seven weeks after surgery, they began a gradually return to normal life and work under the guidance of their doctors. At 3 months, 6 months and 1 year follow‐up, anterior, lateral and dynamic digital X‐ray images of the lumbar vertebrae were taken to check the movement of the lumbar vertebrae and determine whether any loosening of screws or cord had occurred.

Results

The mean operative time was 160 minutes (130–200 minutes), mean blood loss 600 mL (400–1500 mL). All patients were followed up for 6 to 23 months (mean: 16.4 ± 5.5 months). Forty‐one segments were stabilized in 32 patients; 23 (71.9%) underwent single segment stabilization, and 9 (28.1%) two‐segment stabilization. The most commonly treated segments were L₄‐L₅ (19/41, 46.3%) and L₅‐S₁ (12/41, 29.3%), followed by L₃‐L₄ (7/41, 17.1%) and L₂‐L₃ (3/41, 7.3%). The VAS score (0–10) for leg pain and root pain improved from 7.6 ± 3.2 (6.2–9.4) preoperatively to 2.8 ± 3.7 (0–5.4, P < 0.001) postoperatively and for waist pain from 7.3 ± 3.7 (6.0–9.2) preoperatively to 3.5 ± 2.2 (0–5.1, P < 0.001) postoperatively. The ODI improved from 69% ± 12.6% (30–87%) preoperatively to 28% ± 15.7% (0–60%, P < 0.001) postoperatively (3, 4). In the stabilized segment and adjacent vertebrae above and below, the range of movement showed no significant difference. No loosening of screws, cord or polyester bolster was observed (5, 6).

Figure 3.

The results of follow‐up showing that the postoperative VAS scores for waist and leg pain are significantly decreased compared with preoperative VAS. Vertical axis, VAS score; horizontal axis, duration of follow‐up. 
m, months; w, weeks.

Figure 4.

The results of follow‐up showing that the postoperative ODI is significantly improved compared with the preoperative ODI. Vertical axis, ODI; horizontal axis, duration of follow‐up. 
m, months; w, weeks.

Figure 5.

A 66 year male patient (a, b) Preoperative anterior and lateral X‐ray images showed degenerative changes in the lumbar vertebrae; (c–e) Preoperative MRI showing degeneration and protrusion of vertebral discs at L₃/L₄, L₄/L₅, hypertrophy of ligamentum flavum and significant spinal canal stenosis at L₃/L₄, L₄/L₅; (f, g) Postoperative anterior and lateral X‐ray films showed good internal fixation with the Dynesys system.

Figure 6.

A 45 year female patient (a, b) Preoperative anterior and lateral X‐ray images showing that the physiological lordosis of lumbar spine has been lost; (c, d) Preoperative MRI showing disc protrusion at L₅‐S₁ compressing the epidural capsule and left intervertebral foramen, leading to disc degeneration of adjacent segments L₄/L₅; (e) Decompression by fenestration on the left vertebral plate of L₅‐S₁ plus discectomy at L₅‐S₁ and Dynesys stabilization at L₄‐L₅; (f, g) Postoperative lateral X‐ray films showing good alignment after Dynesys stabilization; (h–k) Range of movement (anterior flexion and posterior extension, left lateral flexion and right lateral flexion) of the lumbar spine 3 months after surgery has almost returned to normal.

Discussion

It is generally believed that fusion of a motor unit causes an increase in biomechanical pressure on the adjacent segments, leading to accelerated degeneration of vertebrae and related clinical symptoms. Spinal fusion increases the stress and strain of adjacent segments ⁸ . Accordingly, new methods of spinal stabilization for achieving stability without fusion have been explored, resulting in the new concepts of “dynamic stability” and “soft stability” ⁶ . Dynesys, a posterior non‐fusion spinal fixation technique, was developed by the French scholar Gilles‐G. Dubois in 1991, and was clinically introduced in 1994 ⁷ . Currently Dynesys, constituting titanium alloy pedicle screws, polyester spacer and PET cords, is the most frequently reported clinical posterior non‐fusion system. The pedicle screws are linked with a cord and spacers instead of rigid metal rods. The Dynesys system restores the stability and internal relationships of the spine and inhibits over‐activity of the affected segments. The dynamic “push‐ pull” relationship in the system of screws, cord and spacers stabilizes the joint, maintains the vertebrae in their normal position, reduces movement of the vertebral body, results in a physiological curve, and decreases the stress of flexion, rotation and shearing. The system as a whole reduces pressure on the vertebral discs.

Analysis of clinical efficacy of the Dynesys system

There have been few reports on clinical experience of the Dynesys system from the USA, most clinical reports being from Europe. The Dynesys system can be used alone for lumbar degenerative disease, recurrent disc protrusion and mild lumbar instability, especially in patients with significant waist pain that has not been relieved by conservative treatment ⁹ . Stoll et al. reported that indications for the Dynesys system include lumbar spinal stenosis, spondylolisthesis, disc protrusion and spinal revision surgery ⁴ . It is believed that the indications for the Dynesys system are mild degenerative spondylolisthesis or degenerative disc disease with spinal instability, disc protrusion with spinal flexion instability and degenerative spinal stenosis requiring extensive laminectomy for decompression ⁹ . Stoll et al. conducted a multi‐center retrospective study on the safety and efficacy of the Dynesys dynamic stabilization system for lumbar instability. The Oswestry system was used to assess pain and function of the lumbar spine; the ODI improved from 55.4% preoperatively to 22.9% postoperatively ⁴ . They concluded that the Dynesys system for the treatment of degenerative lumbar disease is safe and effective.

Schnake et al. studied 26 cases of first or second degree degenerative spondylolisthesis treated by laminectomy decompression and Dynesys internal fixation and performed a corresponding study on vertebral stability ¹⁰ . Twenty‐four patients were followed up for a minimum of 2 years; the VAS scores averaged 80 preoperatively and 23 postoperatively and the mean walking distance 250 meters preoperatively and over 1000 meters postoperatively. The clinical efficacy of the Dynesys system for first and second degree degenerative spondylolisthesis is similar to that of spinal fusion and pedicle screw fixation, but the Dynesys system avoids the complications of taking bone for grafting. Putzier et al. used the Dynesys system to treat patients with disc protrusion undergoing discectomy and found that the short‐term result was similar to that of discectomy, but the medium term result was superior to the latter ¹¹ . In addition, it effectively prevents vertebral disc degeneration of the operated segments. The Dynesys system can also achieve good clinical outcomes in the treatment of degenerative lumbar spondylolisthesis and lumbar scoliosis ¹² , ¹³ . The former mainly involves spinal cord decompression; the latter decompression and stabilization. Single nerve decompression is inadequate for the treatment of degenerative lumbar scoliosis because associated neurological symptoms and deformity can be further aggravated ¹³ .

In the present study, the ODI improved from 69% ± 12.6 preoperatively % to 28% ± 15.7% postoperatively. With regard to the VAS score, the VAS of leg and root pain was 7.6 ± 3.2 preoperatively and 2.8 ± 3.7 postoperatively and the VAS of waist pain 7.3 ± 3.7 preoperatively and 3.5 ± 2.2 postoperatively, indicating good clinical results. Back and leg pain was relieved in all patients. Three months after surgery, they all returned to work with no pain medication, but with restricted physiological function at the waist. Schmoelz et al. performed in vitro biomechanical tests and found that the Dynesys system provides the same stability as fixation with better flexibility and remarkably reduces disc pressure on lateral bending and posterior extension without affecting the adjacent segments ¹⁴ , ¹⁵ . They demonstrated that, compared to non‐fixation, Dynesys stabilization and conventional pedicle screw fixation reduces the disc pressure. Secondly, internal fixation with the Dynesys system provides sufficient stability and mobilization of the stabilized segment, particularly with regard to the range of movement in posterior extension. Previous studies have concluded that the Dynesys system prevents degeneration of adjacent segments ⁸ , but Schnake et al. believed that this was not the case when the Dynesys system was used for degenerative lumbar spondylolisthesis ¹⁰ . They found that, after a 2‐year follow‐up, 29% of cases showed degeneration signals in segments adjacent to the fixed segments; however, long‐term follow‐up results are needed to further confirm their conclusion. In this study, a black disc of an adjacent segment turned to white in one case. However, because of the limited sample sizes and short periods of follow‐up so far reported, there is not enough evidence to prove that the Dynesys system can reverse disc degeneration in adjacent segment.

Vaga et al.'s study suggested that the Dynesys system can prevent or partially reverse severe degeneration of adjacent vertebral discs ¹⁶ . Kumar et al. concluded that, with the Dynesys system, degeneration of the intervertebral disc of the stabilized and adjacent segments was ongoing ¹⁷ . Continuing disc degeneration may be due to natural progression of the original condition. Grob et al. recently reported a retrospective study which showed that, among patients with degenerative lumbar disease who underwent Dynesys stabilization and 2 years follow‐up, only half of them considered the surgery helpful, less than half showed improved function, and 19% of patients needed reoperation ¹⁸ . The overall result was inferior to that of spinal fusion. In a word, a multi‐center, prospective randomized clinical study is needed to further evaluate the safety and efficacy of the Dynesys system.

Surgical treatment with the Dynesys system

To obtain satisfactory clinical outcomes, the surgeon should not only abide by the indications for surgery, but also pay close attention to detail during surgery. The 32 patients in our study achieved satisfactory short‐term outcomes, which may be due to the following.

• 1For single‐segment stabilization, the PET cord was guided from distal to proximal; whereas for two‐segmental or multiple‐segmental stabilization, the PET cord went from the screw hole in the middle to the screw holes at both ends. This avoided the wear of the cord that occurs when it repeatedly goes through the screw holes; wear which may lead to decreased biomechanical properties.
• 2100 mm PET cord was used only for one or two segment stabilization and 200 mm PET cord for 2–5 segment stabilization. The Dynesys system is not recommended for stabilization of more than four segments, however there have been reports of using it for stabilization of more than five segments ¹³ .
• 3The pedicle screws were inserted bilaterally symmetrically, so that the length of elastic tube on each side was equal.
• 4For single‐segment stabilization, a scale reading of “2” was used to measure the distance between the two screws, whereas “1” was used for two‐segment or multiple‐segment stabilization because a scale reading of “2” results in longer elastic tubes. In two‐segmental or multiple‐segmental stabilization, if the scale reading is “2”, the longer tube may cause loss of lumbar lordosis, thus decreasing the therapeutic effect.
• 5Pedicle screw loosening and rupture has been reported in Dynesys stabilization ¹⁰ . Such screw loosening is commonly seen at S₁, mainly because the S₁ pedicle screw is shorter than other screws and it is difficult to maintain a proper inclination angle while blocking the lumbosacral muscle during screw insertion, thus reducing the screw's holding force. Therefore, the Dynesys system is not recommended for stabilization of L₅‐S₁, especially in obese patients. If L₅‐S₁ vertebrae are stabilized by the Dynesys system, it is important to keep the inclination angle of the S₁ screw head as small as possible, with the tip of the S₁ screw pointing to the sacral promontory. This technique increases the length and anti pull‐out force of the screws. It also keeps the ends of the S₁ and L₅ screws in the same plane, so the cutting surface of the PET spacer can better match up with the end of S₁, because the PET spacer can only be cut into a plane, not a slope.
• 6Strictly abiding by the appropriate surgical indications. In our study, the patients had mild lumbar degeneration and instability, which had been resistant to 3 months of conservative treatment. Their mean age was 58 ± 5.2 years. The difference in reported clinical results may be related to the wide range of indications for which the Dynesys system has been used for the treatment of lumbar degenerative disease.
• 7During surgery, it is important to protect soft tissue in the surgical area, decompress by fenestration, remove the protruded disc or perform chisel‐type decompression to relieve nerve root canal stenosis symptoms, as well as pay attention to the protection of bony structures posterior to the lumbar spine.
• 8The Dynesys system can maintain a good range of movement and regional lordosis of the stabilized segments and restrain abnormal movement of the unstable segments. Therefore waist pain is significantly relieved by surgery with little change in the range of movement.
• 9The section of PET spacer should be smooth, not rough. Otherwise it will affect the mobility of the replaced segments.
• 10Early postoperative ambulation and active functional rehabilitation exercise are beneficial to functional recovery of muscle in the waist and back.

Analysis of complications after Dynesys stabilization

Dynamic non‐fusion stabilization has the advantages of less trauma, less bleeding, shorter operative time and avoidance of fusion‐related complications. Deyo et al. reported that the postoperative complication rate of spinal fusion was 1.9 times that of non‐fusion surgery in senile patients over 78 years old ¹⁹ . Since elderly patients often have associated medical disorders, the surgical procedure and operative time should be as short as possible and intraoperative blood loss carefully controlled ¹³ . In a study conducted by Schnake et al., four patients (17%) had implantation failure; there were four screws with lucency visible on X‐ray films and one with screw rupture (6%, a total of 96 screws) ¹⁰ . The two patients with screw loosening had degenerative scoliosis, one of them having multiple sclerosis and significantly abnormal gait and lameness in the right lower extremity. His postoperative X‐ray films revealed a screw “with lucency” and a ruptured screw. Schnake et al. therefore suggested that the Dynesys system not be used in patients with seriously abnormal gait ¹⁰ . All screws were ruptured at about 1/3 to screw head because of metal fatigue. Stoll et al.'s study showed that, of 280 screws, the screw loosening rate was 4% ⁴ . Therefore, the screw loosening rate with the Dynesys system is very similar to that reported for solid fixation ²⁰ , ²¹ , whereas the revision rate is similar to, or lower than that, of solid fixation ⁴ .

In Dynesys non‐fusion stabilization, the mean operation time and blood loss are lower than that of posterior spinal fusion ²² , ²³ . In this study, the mean operative time was 160 minutes (130–200 minutes), and mean blood loss 600 mL (400–1 500 mL). There were no implant‐related complications such as screw loosening or rupture. Because there are no metal components which can be visualized on X‐ray films in the Dynesys system, during follow‐up it cannot be determined whether there is rupture, loosening or breakage of the elastic tube and PET cord. Therefore we suggest that some metal components be added to the elastic tube or PET cord.

The Dynesys system, a new stabilization method for lumbar spine degeneration and instability, improves clinical symptoms and preserves the range of movement of the stabilized segments. Nowadays, it is widely applied in the clinic, but the clinician should adhere strictly to the indications and carefully protect the bony structure behind the lumbar spine during surgery. Additionally, its long‐term efficacy and kinetics still need to be clarified by long‐term follow‐up.

Disclosure

The authors did not receive any outside funding or grants in support of this research for, or preparation of, this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity.