The Combined Use of a Posterior Dynamic Transpedicular Stabilization System and a Prosthetic Disc Nucleus Device in Treating Lumbar Degenerative Disc Disease With Disc Herniations

DISCUSSION

Spinal instability caused by degenerative changes in the disc with subsequent spondylolisthesis, ankylosis, and neuroforaminal stenosis may induce back pain.² From among the causes of spinal instability, degenerative disc disease (with its characteristic clinical syndromes of disc herniation, spondylosis, and radiculopathy) is associated with vascular, biomechanical, and anatomic changes in the disc.

For many decades, stabilization, intended to fix the abnormal movement of the spine, has become the treatment of choice for instability. The goal of fusion was to provide motion stability in arthrodesis and deformity correction, thus controlling and even strengthening the movement of the segment.⁸ However, good fusion rates are not correlated with the clinical relief of the patient's back pain symptoms. The best results measured by different clinical scales show around 30% of suboptimal healing after instrumented fusion surgery despite a fusion rate of more than 90%.⁹ Many authors reported adjacent segment degeneration following lumbar and lumbosacral fusion with rigid instrumentation.¹⁰ Kumar et al. described degenerative changes in 49% of patients after 5 years of follow-up.¹¹ Because of these problems, less rigid stabilization systems have recently become more popular in spine surgery.

The PDN device has been in clinical application since 1996. It was developed by Charles D. Ray. This device is composed of a hydrogel core that can absorb fluid and expand when implanted.¹ The hydrogel core is covered with a polyethylene jacket. The core and the jacket are designed to assume the cushioning function of a healthy disc, while restoring and maintaining disc height and allowing a normal range of motion. The high-tenacity polyethylene jacket allows the device to absorb fluid, expanding only in height. This expansion restores disc height to relieve pressure on the spinal nerves. Biomechanical studies using a human cadaver model showed that PDN can effectively increase the heights of the intervertebral discs.¹² However, clinical observations showed us that in some cases the PDN, when used alone, slips to one side of the midline in the disc space.

Another promising system is the dynamic pedicular screwrod system. It is a non-fusion dynamic implant system that controls displacement in rotation and translation. It stabilizes while avoiding fusion. This system allows potential sagittal mobility at the hinge site between the screw head and the shank of the screw. Mobility occurs mechanically between the longitudinally oriented rod and the sagittally placed screw shank. This articulated connection between the rod and the screw is the reason for the lower rate of implant failure, because of a reduction in flexion strain. Part of the load sharing is transferred from the implant to the spine, ie, there is a reduction of stress-shielding effects on the bone.¹³ This reduced stiffness helps distribute load and minimize or prevent a motion discontinuity at the adjacent segment. Sharing of motion and stress load prevents deterioration of the neighboring superior disc level, slowing the process of degenerative progression at the adjacent levels, and tends to restrict mobility in flexion, extension, lateral bending, and axial rotation while sharing load and permitting movement.¹⁴ Also, the capability of a controlled load sharing between the implant and spine could reduce the frequency of postoperative damage to the joint segments.

The indication for PDN application is painful degeneration of the lumbar spinal disc, which has become sufficiently disabling despite 6 months of conservative medical care, rest, exercise, change in lifestyle, etc. The contraindications for PDN device use are spondylolisthesis greater than grade I, fractured or symptomatic degenerated facet joints, an incompetent annulus, or disc height at the affected level of < 5 mm. These criteria are all based largely on the history, examination, MRI, and, perhaps, provocative discography.¹ Also, the vertebral endplates must be free of significant defects such as Schmorl's nodules or fractures before PDN application.¹⁵ In our patient group, these features were taken into consideration for PDN application.

Dynamic stabilization using a hinged screw allows motion of disc and facet joints because of the hinged line just posterior to the facet joints. In contrast, posterior dynamic stabilization transfers the load through the posterior column and stabilizes the spine almost like a rigid stabilization technique, preserving motion.³ Preservation of motion makes this technique useful for conditions where PDN use might be contraindicated, such as degenerative spondylolisthesis. As a result, the use of these 2 instruments together minimizes their disadvantages and restores both anterior and posterior columns.

In this limited series, we observed slippage of PDN in 2 cases. However, slippage of PDN with Modic degeneration of adjacent vertebrae is consistent with findings in the literature related to the use of a standalone PDN. On the other hand, in 2 other cases we noticed severe inflammation of adjacent vertebrae due to fracture of upper and lower endplates. In these cases, the PDN was embedded into the upper and lower vertebrae. Surprisingly, there were no clinical symptomatic or neurologic findings in these cases.

We assumed that large size of the PDN device due to the excessively stiff high-tenacity polyethylene jacket enclosing the polymeric hydrogel caused the fracture of endplates, which then made contact with cartilage tissue and blood, provoking an immune response, and resulting in massive degeneration of the vertebral bone.¹⁶ On the other hand, the hydrolysis time of 3 weeks was too long; therefore a rigid material between the 2 vertebral corpuses carries the risk of fracture of the cartilaginous endplates.

Dynamic stabilization systems without fusion and PDN devices have been used sporadically in spinal surgery. Ideally, semi-rigid implants dedicated to dynamic stabilization should control movement and share the load with the disc. Constraining the movement of the dynamic screw is more for providing adequate stability and mobility of the segment, and has less to do with avoiding the hot spot pressure on the disc.

The reasons for using the dynamic screw system in combination with PDN are to restore and preserve the disc height with the PDN device, while providing dynamic transpedicular stabilization with the screw system.

Although the VAS and Oswestry scores were satisfactory in our limited series of patients, for those with migrated PDN and infection, we thought that we could eliminate the migration complication with the use of posterior dynamic stabilization. However, we could not. The surprising result was that we saw cartilaginous endplate fracture due to the PDN and associated infection findings on MR studies, but the clinical outcome was not negatively affected.

PDN alone is not soft enough to resemble disc tissue. More appropriate material resembling disc tissue should be developed, and a more dynamic posterior system will need to be found. Theoretically, this concept is superior, but practically we need more advanced technology.

Because this study examined the combination of the PDN and stabilization instrumention, the results cannot be compared with those reported in the literature for either PDN alone or dynamic screws alone.