For patients with stage III KD, due to the compression to the neural elements by the retropulsed posterior wall of vertebral body, PVP or PKP surgery alone may not achieve adequate decompression and relieve neurological symptoms. Hence, open decompressive surgery is often required to achieve satisfactory decompression of the thecal sac and/or nerve roots and correction of local kyphosis3. However, current surgical methods of choice are accompanied by a high rate of intra- and perioperative morbidity.
Current surgical methods and their deficiencies
Previous studies reported that the anterior approach can remove the collapsed and compressed vertebral body and obtain good direct decompression, moreover, stabilization of the anterior and middle columns can be achieved through strut bone grafting13. Kanayama et al collected 31 patients who underwent anterior surgical treatment. The mean operative time was 193 min (150–285 min), the mean intraoperative blood loss 436 ml (100–1350 ml). The mean postoperative follow-up period was 57 months and satisfactory outcome was obtained13. However, most of the patients suffering from Stage III KD are elderly. The anterior approach is more invasive and has more intraoperative blood loss and risks internal organ injury. Patients recover slowly and stay bedridden for longer periods. The risks of the anterior approach are relatively greater8, 14. This suggests that the anterior approach may not be an ideal option for older patients with stage III KD.
The posterior approach is familiar to most spinal surgeons. With the continuous development of posterior surgical techniques and internal fixation instruments, many methods such as osteotomy and corrective maneuvers can be used to accomplish adequate decompression of neural elements deformity correction and obtain rigid spinal stability. However, this requires removal of the vertebral body and the posterior vertebral arch, so the stability of the spine is severely impaired, and the load on internal fixation is greatly increased. Especially for KD patients who usually suffer from serious osteoporosis, the internal fixation failure rate is high15–17. Zhang et al2 reported 12 cases of chronic spinal cord compression with neurological impairment in stage III KD treated with posterior surgery. The cranial half of the affected vertebral body and the adjacent intervertebral disc were removed through bilateral transpedicular approaches, the anterior and middle columns were reconstructed using a cage. The long segment was fixed. The average operation time of this surgical method is 148 min (100–220 min), and there was significant bleeding during the operation, with an average of 625 ml (450–850 ml) Furthermore, 2 out of 12 patients presented with neurological deficits postoperatively, questioning the safety of the procedure
Innovations and surgical techniques of this study
Compression to the thecal sac and the spinal cord in patients with stage III KD derives from a retropulsed posterior wall of the collapsed vertebral body. Furthermore, the retropulsed body fragment is severely osteoporotic, and the posterior wall of the vertebral body is weakened due to the presence of the vertebral basal vein foramen18, which makes the retropulsed body fragment easy to be compacted and pushed anteriorly, while the fragment of DH(Disc herniation) is the soft nucleus pulposus (with a hard bone shell when calcified), and OPLL(Ossification of posterior longitudinal ligament) is a hard cortical bone, which are not easily compacted and compressed19. Additionally, compared to compression of a DH or OPLL, the onset of patients with stage III KD is relatively short with no adhesion between the dural sac and the posterior bone fragment. The different characteristics of pathological compression mentioned contribute to the feasibility of the surgical technique presented in this study.
We hereby introduce a new surgical procedure for stage III KD: During the operation, under the real time guidance of ultrasonography, an angular compressor was used to squeeze and compact the posteriorly displaced vertebral body wall and collapse the fragment, which directly decompresses the neural elements and avoids the necessity of a more aggressive approach by corpectomy. Moreover, we used augmented pedicle screw fixation to restore the stability of the spine and bone cement to fill the affected vertebrae to obtain ideal anterior and middle column support. Ultrasonography was used to identify the location of the encroaching fragment from the sagittal plane and the axial plane during the operation20, 21 and to guide decompression in a real time way to minimize irritation of the dural sac and ensure the safe and complete decompression during the operation. An angular compressor is gently placed between the posterior vertebral body and the ventral side of the dural sac, compact and collapse posterior wall to the vertebral body. The neurophysiological monitoring needed to be closely observed to further ensure the safety of the nerve during the entire operation process. The results showed that Cobb angle, JOA score, and ODI score were significantly improved after surgery, which highlights the efficacy of our procedure.
This technique has obvious advantages compared with the previously reported surgical methods. The posterior approach can avoid interference and reduces complications. Bilateral facet joints can be spared if the local kyphosis is not significant or the lesion is located in the lumbar segment, reducing the damage to the spine’s posterior bony structures. The injured vertebral body and upper and lower intervertebral discs were not resected, which could avoid destroying the anterior and middle column structure and contribute to preserve the stability of the spine. Meanwhile, the bone cement placed into the injured vertebral body to provide anterior and middle column support, and screws were implanted before the bone cement hardened, so that the bone cement block and internal fixation integrated together to form a single load bearing frame. Therefore, the stability of the spine was restored to the maximum extent which may explain why no internal fixation failure was observed in our study. Additionally, squeezing and compacting the fragment at the posterior vertebral body by the compressor may theoretically close the vertebral basal vein foramen, reducing bone cement leakage into the spinal canal, and improving the safety of the operation22. There was no leakage into the spinal canal in our case series, which might illustrate the effectiveness of this technique in reducing leakage of bone cement. Through the application of ultrasound guidance, the location of the surgical instruments and the dura mater could be observed in real time, and compacting the retropulsed posterior vertebral wall could be achieved directly, thoroughly and accurately, minimizing the irritation of neural elements. Only one patient in this group had transient MEP signal changes during intraoperative neurophysiology monitoring, but the patient did not experience neurological deterioration after surgery, and neurological function gradually recovered during follow-up, suggesting the safety and effectiveness of this procedure. In addition, we designed special thin and hard compressor (angular compressor), minimizing encroachment into the spinal canal as much as possible, and it would not be deformed when squeezing and compressing the cancellous bone which also minimized irritating the dural sac. A delicate stripping instrument was employed to release the possible adhesion between dura and bone fragment, so to reduce the risk of a dural tear. Only one case in this group experienced a dural tear, which was lower than the incidence of previous studies23, 24. Various degrees (90 °, 120 °, etc.) are designed at the tip of the compressor, which was convenient for compressing and compacting the bone fragment. The operative time was relatively shorter, and bleeding and trauma were reduced and fewer neurological complications occurred compared to corpectomy for decompressive surgery. In this study, the average blood loss was 202 ± 40 ml (150 ~ 275 ml), which was less than that of Zhang et al.2 and Yang et al25 reported (625 ml (450–850 ml), 860 ± 130 ml, respectively). Jo et al.26 reported 13 patients who underwent osteotomy, with an average operation time of 275 minutes and intraoperative bleeding of about 1585 ml. One of the patients suffered neurological deterioration due a direct injury to the nerve root during surgery.
From our experience gained during this study, the following are our recommendations for the procedure: Firstly, if there are bridging osteophytes in front of the injured vertebrae with adjacent levels, compression between the pedicle screws of this segment should be avoided to prevent screws loosening. The articular process osteotomy at the other segments was recommended if the obvious kyphosis must be corrected. Secondly, if the residual kyphosis was acceptable or the lesion was located in the lumbar segments, facet joints should not be removed, thus reducing the destruction of the posterior column structure and preserve the bone grafting bed, especially when at the lumbar segments or thoracic ones where ultrasonography indicated the presence of the space between spinal cord and pathological compression when dural sac pulsation before decompression27. Thirdly, we recommend performing kyphosis correction first to indirectly release the nerve tissue before decompression, which can improve the safety of surgery28, 29. Fourth, when putting the compressor to the ventral side of dural sac, we recommend placing it at the adjacent intervertebral disc level first, then add saline for ultrasound visualization. The location of the compressor can be seen under ultrasonography to avoid interfering with the nerve tissue during placement. Fifth, the positional relationship between the nerve tissue and the device must be clearly observed under real time ultrasonography during decompression, and the device must not cause compression to the neural elements. Lastly, the L-shaped angular compressor should first be placed into the cephalic (or caudal) end of the apex of the encroaching bone fragment. After compressing part of the bone fragment, gently and gradually move towards the highest point. To avoid nerve injury, the soft cancellous bone could be compressed first to form a posterior bone culvert, then use a thin osteotome to cut the bone edge of the culvert from both margins outside the dural sac sides, and then press the cortex to the ventral side to complete the decompression.
As mentioned above, our study avoids complications related to anterior surgery approach and severe damage to spinal stability due to corpectomy surgery. Therefore, spinal stability was preserved in all patients, and the rate of instrumentation failure was significantly reduced. All patients in this group achieved bone fusion at last follow-up. Only one case had cerebrospinal fluid leakage, but it did not affect wound healing and neurological recovery. These indicates that this surgical procedure was safe and effective in our study population, but a larger number of cases and long-term follow-ups are still needed to confirm our findings.
Limitations of this study
This study also has certain limitations: the number of cases is small, and the follow-up time for some cases is short, especially considering the late onset of some complications like adjacent segment disease or delayed hardware failure. Because the incidence of stage III KD is relatively low, the analysis results of 11 patients are still valuable. This study is a retrospective study with a therefore impaired level of evidence.