Spinal fusion is commonly performed in spine surgery to improve the pain and clinical outcomes of patients with failed conservative treatment for degenerative lumbosacral disease by reducing segmental instability, which is recognized as a major cause of low back pain. Currently, pedicle screw fixation or posterior interbody fusion is the gold standard for surgical management of lumbar spinal instability1,22)
. It has proven to be effective, with reported lumbar fusion rates as high as 96%4,6,19,23)
. However, lumbar fusion with instruments does not always lead to favorable results.
Transpedicular fixation is associated with significant risks as followings; 4% cerebrospinal fluid leakage, 2% transient neurapraxia, 2% permanent nerve root injury, 4 to 5% deep tissue infection, and 3 to 12% hardware failure5,9)
. Recently, numerous complications and problems after fusion surgery have been reported, with ASD being one of the most important. As a cause of ASD, Cunningham et al.3)
reported a significant increase in the intervertebral disc pressures by destabilization of the adjacent lumbar spine followed by stabilization with instrumentation. Still, the exact etiology is uncertain but alteration in facet loading, hypermobility, and increased intradiscal pressure at the segments adjacent to fusion mass is believed to play certain key roles7,8,12,14,16)
Spinal fusion alters the biomechanics of the spine and the loss of motion at the fused levels is at least theoretically compensated by increased motion at other unfused segments resulting in ASD11)
. Superior segment facet violation or laminectomy has recently shown in vitro
to destabilize the adjacent level in pedicle screw fixation2)
In contrary, a recent comparative biomechanical study showed that the IFD reduced the ROM and load on the disc and articular processes stresses, while it increased loads transmitted through the spinous processes13)
. And the IFD can preserve normal anatomy and does not violate adjacent facet joints, making it less likely to cause hardware-related pain or to accelerate adjacent facet degeneration20)
. In the present study, it tended to affect the adjacent segment less than a pedicle screw. A recent clinical study showed that the incidence of ASD in the cephalad adjacent segment 10 years following 360° instrumented lumbar fusion averaged 21%17)
. In the present study, the incidence of ASD was 36.1% in the patients who received pedicle screw fixation, significantly higher incidence compared with the IFD group in which it was 12.5% (p
We showed that ROM at the instrumented level was significantly decreased in both the IFD and pedicle screw group compared with the preoperative state. This means that the IFD has some stabilizing effect at an unstable segment. Wang et al.21)
have studied the biomechanical characteristics of the IFD in an in vitro
test of 109 cadaveric specimens and resulted that the greatest mean limitation of flexion-extension ROM was 4.14° for IFD with lumbar interbody fusion whereas it was 5.03° for pedicle screw fixation with lumbar interbody fusion, and 10.13° for the intact spine. With an interbody device, the IFD provided the greatest limitation in flexion and extension over pedicle screw constructs. In short, the IFD provides enough segmental rigidity to make it a viable alternative to pedicle screw fixation.
However, the IFD has some weak point which cannot effectively control the axial rotation and lateral bending. Recent paper by Karahalios et al.10)
demonstrated, through cadaveric study, that the capacity controlling ROM such as axial rotation and lateral bending is lack compared with transpedicular screw fixation when only interspinous device used as following; ROM of lateral bending was -3.83±1.47 in left and 3.92±1.26 in case used only interspinous device. These ROM was superior to normal motion, but inferior to transpedicular screw fixation (ROM of lateral bending was -0.57±0.14 in left and 0.57±0.09). However, their papers revealed that lumbar interbody fusion gave the control of ROM to the IFD. Via cadaveric study, there was no statistically significant difference of ROM between IFD with lumbar interbody fusion and pedicle screw fixation with lumbar interbody fusion.
Also, the IFD implant have several advantage compared with pedicle screw. It is less invasive and presents no risk of dural or neural injury and cerebrospinal fluid leakage because it is placed on only the spinous process. In addition to the easy maneuverability of the IFD implantation, the operative time for cases involving the IFD was shorter than that for cases involving pedicle screw fixation. Less time in the operating room also leads to reduced hospital and healthcare costs. Similarly, intraoperative EBL was much less in cases involving the IFD; it was 50% lower than that in patients who underwent open pedicle screw fixation. The minimal bone exposure, easy implantation, and short operative time required for implantation will contribute to decreasing EBL. Because many patients undergoing lumbar fusion are elderly, a lower EBL may help prevent anemia and other postoperative complications. Additionally, the IFD needs a smaller incision and requires less muscle and soft tissue retraction than the pedicle screw fixation. So, it may lead to less postoperative discomfort and we showed that immediate postoperative VAS scores in the IFD group were much lower than pedicle screw group in this study.
Although postoperative radiographs in the IFD group showed less improvement of instability at the instrumented level compared with in pedicle screw group, patients showed a similar clinical improvement as represented by K-ODI, and VAS scores. If more long-term studies confirm this clinical outcome, such techniques that preserve much of normal anatomy and biomechanical function of the lumbar spine as interspinous implant, will be highly indicated in the surgical treatment of spinal stenosis with various instability. It is attractive to alter the pedicle screw fixation for selected patients requiring instrumentation augmented fixation. In particular, the IFD may be well suited implant for augmenting the lumbar interbody fusion.
It is important to note that the application of the IFD is contraindicated in patients with pars interarticularis defects or any incompetence in the bone between anterior and posterior spinal columns21)
. In addition, patients with advanced degenerative spondylolisthesis (≥grade II) are not good candidates for interspinous fixation.
This study has some limitations. As a retrospective unrandomized design, it couldn't exclude all the factors that affect the results. And, because the incidence of ASD was investigated during short period of follow up, ASD was likely to occur after follow up period. So, a prospective, randomized trial with long-term follow up period is needed to confirm the efficacy of the SPIRE® plate as IFD.