Modified eggshell procedure via posterior approach for sclerosing thoracic disc herniation: a preliminary study

The incidence of thoracic disc herniation was low, and the clinical manifestations were not typical. The diagnosis was easily delayed. The classification of thoracic disc herniation was determined by the segment and location of herniation. The classification was helpful for surgical method selection. According to the herniation parts, the thoracic disc herniation was divided into central type, paracentral type, lateral type, and intradural type. The main symptoms of central herniation type were spinal cord damage, and the main symptoms of lateral herniation type were radicular symptoms. The patients of intradural type were rare. The thoracic disc herniation was often accompanied with calcification and posterior marginal osteophyte formation, known as the rigid herniation [2]. In early time, it was reported [12] that 65 % of 82 cases of thoracic disc herniation occurred with intervertebral disc calcification and 7 % of 82 cases with intradural herniation. Hott et al. [13] reported that in a group (20 cases) of huge thoracic disc patients, there were 18 cases of rigid herniation, 1 case of soft herniation and 1 case of partial calcification.

The etiology and pathology mechanism of hardening thoracic disc herniation was not completely clear. The related imaging studies showed that the hardening thoracic disc herniation was often accompanied with ossification of posterior longitudinal ligament, yellow ligament ossification, the posterior edge transection and wedging of vertebral body, and so on. The clinical manifestations of hardening thoracic disc herniation were often severe. The herniation and hardening part usually deepened into the ventral spinal dural with adhesions, even causing mechanical compression on thoracic spinal cord and influencing spinal cord blood by affecting the anterior spinal central artery. Once the symptomatic thoracic disc herniation was confirmed, the patient had better receive surgical treatment as soon as possible [6]. The surgery indication [7, 8] included progressive spinal cord damage, lower limb weakness or paralysis, bowel and bladder dysfunction, segmental sensory disturbances, and persistent radiating pain.

There were several surgical methods to treat thoracic disc herniation [14], and the surgical procedures of posterior laminectomy, discectomy, and decompression are relatively simple. However, the intraoperative traction to the affected spinal cord was easy to cause spinal cord injury. Even the minor damage to the spinal cord can also cause irreversible damage of the nerve function, influencing the further functional recovery. Moreover, the surgical operation was difficult to perform because the affected spinal cord was under the inflammation, ischemia, and decompensated status for a long time, and the surgical risk was high [4].

Regarding surgical techniques for ossification of posterior longitudinal ligament (OPLL) in the thoracic spine, Prof. Kato and Murakami from Japan have done a lot of research and contributed much. In the year 2012, they firstly reported a novel surgical technique for OPLL in the thoracic spine [15]. Their maneuver included laminectomy and removal of the transverse processes and pedicles, which allowed space to be created bilaterally at the sides of the dural sac for the subsequent anterior decompression. The thoracic nerves at the levels of anterior decompression were ligated bilaterally and lifted up to manipulate the ossified ligament and the dural sac. An anterior decompression was then performed posteriorly. Complete removal of the ossified PLL was achieved in all cases. Recently, they reported that it was safe and effective for gradual spinal cord decompression through migration of floated plaques after anterior decompression via a posterolateral approach for OPLL in the thoracic spine [16]. Besides, they reported in another study [17] a case of severe anterior compression of the spinal cord by a calcified herniated thoracic disc at the T9/10 level in a 46-year-old woman. They performed resection of the calcified herniated thoracic disc and the integrated dura, using a microscopically assisted mini-open transthoracic approach in which the rib bone was partially drilled to widen the operative field, which was a successful attempt. The patient achieved an excellent recovery without any complications. Obviously, our modified eggshell procedure is different from what they reported before. However, the postoperative results are satisfying although we have used different methods.

The front side transthoracic approach for disc resection was used widely. In such procedure, the surgical field was very clear and direct, and the ventral spinal cord decompression had less interference to the spinal cord. However, the possibility of postoperative pulmonary complications increased significantly. The clinical effect was good to treat thoracic disc herniation by posterolateral approach through articular process, with shorter operative time and fewer complications in comparison with other treatment methods [18]. To our knowledge, eggshell procedure was firstly used for anterior decompression of spinal fractures via a posterior way and then used widely [9]. As eggshell procedure, vertebral body is firstly hollowed out via vertebral pedicle, and then the protruding fracture block is rammed into the hollowed vertebral body. In this study, we modified this procedure and applied it to the surgical treatment of sclerosing thoracic disc herniation. On the basis of the advantages mentioned above, we used the modified eggshell procedure, obviously decreasing the risk of damaging the spinal cord. In this study, the modified eggshell procedure had small interference to the spinal cord. No cases were found with postoperative neurological deterioration due to spinal cord traction.

The eggshell procedure was firstly used for spinal fracture with anterior decompression. The vertebral body was hollowed out through the pedicle, and the kyphotic fracture bones were inserted into the vertebral body. At present, the eggshell procedure was mainly used in spinal deformity correction. We modified this procedure and used it in the surgical treatment of hardening thoracic disc herniation. The articular process was removed so as to expose the side and rear side of intervertebral space through posterior approach. The affected disc tissue which was broken into the spinal canal was removed from the intervertebral space so as to alleviate the spinal compression through the anterior approach. Such procedure realized decompression through the anterior spinal cord by single posterior surgical operation with small disturbance for the spinal cord, and the operation was relatively safe.

The key points of modified eggshell procedure were as follows. First, the laminectomy and bilateral facet were removed to expose the anterolateral aspect of affected intervertebral space. Second, the osteophytes in posterior margin of vertebral body and intervertebral disc tissue were removed through the anterolateral aspect of the intervertebral space. Third, the adhesion between the herniated calcified disc and dura mater was separated in the anterior aspect of the dura mater. The nucleus pulposus pliers were used to completely decompress in the intervertebral space. If the adhesion between the ossified tissue and spinal dura mater was very close, the ossified tissue could be abrasively reduced so as to be free with surrounding tissue, making it “float,” thus effectively avoiding dura mater damage and reducing the occurrence of cerebrospinal fluid leakage.

The advantages of this procedure were as follows. First, the posterior surgery alone could reach full laminectomy, and remove the hypertrophy ligamentum flavum, bilateral facet joint and affected disc. Videlicet, it could completely alleviate the spinal stenosis, thus achieving 360° decompression. Second, the spinal disc which was broken into vertebral canal was removed from the ventral spinal cord. The traction on the dura mater should be avoided in order to decrease the risk of damaging the spinal cord, especially when there were difficulties in the anterior removal process due to the adhesion between the herniation disc and dura mater. Third, there was no important anatomical structure in the posterior section. The operation in this way was easy and the damage was less, generally with fewer incision-related serious complications. The transthoracic surgical approach should not be selected so as to avoid pulmonary complications and major vascular injury. The previous study [19] indicated that the lung function after thoracotomy at least decreased 10 %. Because of the obstruction by the diaphragm in the level of T10/L1, it was difficult to expose the structure through the anterior approach and was prone to complications such as pleural effusion [20]. Fourth, all of the surgical operations were under direct vision. There was enough space to remove the compressed tissue in the anterior approach, and the operation was easy and safe. Barbanera et al. [21] reported that the direct vision for the dura mater was very important in the process of operation while dealing with a huge thoracic disc herniation, especially with calcification. Unavoidably, it goes along with some limitations in this study. One limitation is the retrospective study design, with some medical records lost. Another limitation is lack of control group. That is, it would be better if a comparative study is performed, which is needed in the future.

The complication rate is 12 % in this study, including two cases of dural laceration and one subdural hematoma, all transient. Takahata et al. [22] reported a relatively high rate of surgical complications, which included a dural tear in 40 % and neurological deterioration in 33 % of the patients. Kato et al. [15] reported a minor dural tear in one of three patients that did not result in postoperative cerebrospinal fluid leakage. In another study from Kato et al. [16], two patients had operative complications (six cases in total), including a dural tear and a temporary neurological deterioration. Thus, our 12 % complication rate is relatively low, as compared to that reported before.

This surgical method removed the spinal posterior structure in operation session, influencing the spinal stability and leading to local curvature change. Especially for the lower thoracic, it was prone to instability and curvature changes due to the weakened thorax protection. Furthermore, after the removal of the disc, the anterior and middle column damage might worsen the instability of spine. Because of long-term compression, the spinal cord was rather fragile. Therefore, even a slight change of curvature might also cause serious damage to the spinal cord. The biomechanical study [23] indicated that after the removal of unilateral thoracic rib vertebral joints, transverse process joint of ribs or unilateral facet joints and bilateral facet joints, the local stability decreased. The anterior and middle column beared 80 % of the load, while the posterior column only beared 20 % of the load. The interbody fusion might provide a reliable support for spine. Therefore, we adopted the interbody fusion and pedicle screw with rod system which was fixed in situ fusion in order to enhance the stability of operating sessions, effectively avoiding the slight curvative changes after decompression, thereby reducing damages to the spinal cord. The application of autologous lamina and small joint bone for interbody fusion [24] could avoid the complications of iliac area removal and reduce the surgery time, reducing operative bleeding. According to the current follow-up data, the integration and stability effect was reliable, and the spinal damage caused by bone defluxion did not occur. The application of internal fixation was helpful for early ambulation of the patients to do functional exercise and helpful for providing support to the interbody fusion, effectively reducing postoperative bedridden complications, shortening recovery time, and improving the quality of life for patients.