Animal models revealed that the intradiscal pressure might be reduced when continuous axial pressure was applied to the spinal column, resulting in degeneration of the intervertebral disc, while traction of the spine could stabilize the intradiscal pressure and reverse or prevent disc degeneration to some extent22. According to above theory, conservative traction therapy has been applied to alleviate the clinical symptoms of LDH23. However, the spine is under compressive loads due to patient body weight, which may result in reducing the effect of traction therapy when the patients are standing. The retraction effect of ISD or interlaminar device is similar to that of conservative traction therapy, which can maintain the sustained effect on the spine by the time the patients are standing. That is, ISD or interlaminar device converts conservative in vitro traction therapy into sustained in vivo traction24.
Open lumbar discectomy (OLD) is the early technique for LDH with reduced disc height, segmental instability and retrolisthesis, which may be a source of postsurgical pain25.Percutaneous endoscopic lumbar discectomy (PELD) is a minimal invasive spinal technique for LDH, which has gained popularity and shown successful results. PELD has many advantages such as less paraspinal muscle trauma, preserving facet joint, which minimize postoperative instability to some extent26. However, both OLD and PELD are the direct intervention of discectomy, which causes a decrease of the disc space inevitably.11 Extended removal of a disc and the consecutive reduction of the disc height may lead to spinal instability and chronic facet joint pain27. To prevent the reduction of the disc height after discectomy is necessary for the treatment of LDH.
The dynamic stabilization device as an alternative to fusion technique in the treatment of lumbar degenerative diseases has been widely applied. In related literature, Dr. Knowles was recognized as the pioneer of interspinous process devices in the 1950s28. Interspinous spacers were typical representative of the dynamic stabilization devices. However, the related complications, including the intraoperative spinous process fractures, spinous process fatigue fracture, supra-spinous ligament ruptured were gradually reported in recent years.16,17 Even if Xu et al. made attempts to apply Coflex(a “U”-shaped titanium alloy spacer) in L5-S1 lumbar degenerative diseases, he had to admit that the prosthesis was prone to shifting and couldn’t offer enough supporting strength by a conventional method due to the short S1 spinous process29. What’s more, the device-related complications of Coflex caused by excessive load on spinous process and the patient’s degree of osteoporosis, including spinal process fracture, device loosening, and fixed-wing breakage, were not unusual in literatures30,31. Thereby, it is commonly recognized that interspinous spacers were not generally suitable for the application in L5-S1 level.15,18
IntraSpine as a new interlaminar dynamic stabilization device for application in lumbar degenerative diseases is closer to the lamina and deeper than ISD when implanted. Interlaminar spacer could compensate the physiological load on lumbar spine from the facet joints, provides direct neurological decompression, and reestablishes the foraminal height32. Our hospital developed this technique since August 2017.The designer initially assumed that the indications were low back pain caused by disc degeneration, lumbar instability, young patients after lumbar discectomy, chronic low back pain caused by the syndrome of zygapophyseal joints, etc33. The related clinical reports about the application of IntraSpine interlaminar device are rare at present, and the indications need verifying and probing.
In our study, IntraSpine interlaminar device could be regarded as the easily applied and minimally invasive technique for less operative time, blood loss, and postoperative volume of wound drainage, which was conformed with previous study19. The previous study indicated that the IntraSpine could significantly improve functional status in patients with chronic low back pain34 and relieve the chronic low back pain due to Baastrup disease35. In the current study, patients’ back and leg VAS, ODI had significant improvement after the operation. The physical pain was relieved, lower limb function and quality of life were improved, with 93.3% of patients achieving a successful outcome. Hence, the improvement of symptoms was similar to the above studies.
In Carrabs’s small sample, ten IntraSpine interlaminar devices were implanted in 10 single-level procedures with a 6-month follow-up period. In their report, the radiological results revealed that IntraSpine interlaminar device was able to reduce the workload on adjacent levels compared with interspinous spacers36. Furthermore, the percentage composition of cortical bone in lamina is higher than spinal process, which admits lower influence of osteoporosis. Therefore, we consider that the above feature is the advantage for the use of IntraSpine interlaminar device. In our study, the post-operative SD, PDH, LFH, RFH, LFW, and RFW increased by 33.0%, 24.6%, 13.3%, 12.2%, 18.9%, 19.6% when compared with pre-operative data. That is, IntraSpine was verified to significantly enlarge the spinal canal and neuro foramen by distracting the intervertebral space, reducing the workload of the intervertebral disc and facet joint. Meanwhile, there was no significant difference between follow-up ROM and pre-operative ROM, indicating that IntraSpine interlaminar devices can maintain the physiologic movement of the spinal motion segment.
It's important to note that the improvement of symptoms mainly because of the lumbar discectomy. The function of IntraSpine interlaminar device is to enlarge the sagittal diameter of the lumbar spinal canal, expand the foramina, increase the posterior disc height, and then prevent the reduction of the disc height after discectomy, playing a positive role in alleviating the symptoms.
What’s more, Imaging data of SD, PHD and RFH decreased at final follow-up by 3.4%, 4.2%, 3.4%, when compared with postoperative data (P < 0.05). While the data of LFH, LFW, RFW at final follow-up decreased without significantly difference when compared with postoperative data (P > 0.05). This statistical phenomenon may be caused by the small sample size in our study. A retrospective study revealed that patients with X-STOP had the most significant improvements in FH, FW, and FA; however, PDH, FH, and FA during follow-up were decreased by 17.9%, 5.6%, and 11.4%, respectively, compared with those immediately after surgery37. Another two studies found similar results15,38. That is, the slight decrease of PHD is ubiquitous during follow-up when compared with those immediately after operation. However, though there was a loss in SD, PHD and RFH in this study, the values of IntraSpine were still better than those in previous studies on ISD devices. In terms of IntraSpine, the decrease of imaging data was understandable due to the material of IntraSpine interlaminar device. We speculate that flexible medical silica gel as the primary material, which occurred slight deformation due to pressure imposed on the prosthesis. But the decreases in imaging data were slight and acceptable, which didn’t mean a significant and continuous decline.
In our follow-up period, we found that the position of the IntraSpine interlaminar device shifted to the edge of the S1 spinous process in one case. During the operation of this patient, we found that 10 mm-size of the device seemed to be a little smaller while the 12 mm-size was larger when measuring the spacing. 10 mm-size of the device was chosen finally, and we speculated that the mismatching selection might be the risk actor of device shifting. Hence, we tended to choose the larger size of devices in our subsequent cases while encountering similar situation. There was no recurrence of devices shifting subsequently. Moreover, the the less resection of lamina in laminectoy might prevent the device from invading into the spinal canal. The reservation of supraspinous ligament might limit the device from shifting continuously even falling out.
Furthermore, we noticed that position of the IntraSpine interlaminar device rearward shifting slightly in initial series of cases. But these patients did not complaint relative discomforts such as significant back and leg pain. Besides, the patients’ clinical symptoms were still satisfactorily relieved without recurrence. We presume that the small size of the S1 spinous process leads to slightly rearward shifting of the device. In addition, we just implanted the device into the intralaminar without any other additional fixation in initial cases. Therefore, we firmly fixed four corners of the IntraSpine to supraspinous ligament by 10# silk thread after inserting device. Few shifting of the device recurred in subsequent cases after we modified the procedure.
In our study, we noticed that the IntraSpine interlaminar device couldn’t be clearly observed by X-ray at the L5-S1 level due to the unique structure. Therefore, both CT and X-ray were indispensable in the clinical follow-up period.