A Modified Eggshell Technique for Sclerosing Thoracic Disc Herniation

Abstract

Sclerosing thoracic disc herniation refers to a condition in which the intervertebral disc in the thoracic region protrudes and becomes calcified, causing compression on the spinal cord and/or nerve roots. Sclerosing herniation of the thoracic disc poses a significant danger as it can lead to serious complications like paraplegia during or after surgery. Iatrogenic spinal cord injury is a common risk for individuals diagnosed with sclerosing thoracic disc herniation due to the inflexible protrusion of the sclerosing disc into the spinal canal and its adhesion to the ventral side of the dural sac. The challenging and crucial aspect of the surgery is how to safely and efficiently eliminate the hardened tissue. The eggshell method is a surgical procedure that addresses the kyphosis abnormality of the spinal column by excavating the vertebral body via the pedicles and subsequently inserting the kyphotic fracture block into the excavated vertebral body. In this article, a revised surgical method using the eggshell technique will be presented for the treatment of sclerosing thoracic disc herniation. The surgical procedure briefly involves hollowing out the anterior intervertebral space of the hardened disc tissue to create an eggshell-like structure, with the sclerotic tissue forming the posterior wall. Subsequently, the sclerotic disc tissue is pushed into the hollow intervertebral space to achieve complete decompression of the ventral spinal cord. The safety and effectiveness of this approach for treating sclerosing thoracic disc herniation have been confirmed.

Introduction

In medical situations, thoracic disc herniation is categorized into three types: central, paracentral, and lateral. Spinal cord injury (SCI) symptoms are predominant in central protrusion, whereas radicular symptoms are commonly observed in lateral protrusion. Central and paracentral protrusions account for approximately 70%. Predominant segments affected are T11 and T12 (26%), with 75% of thoracic disc herniation occurring between T8 and T12, and the subsequent thoracic vertebrae exhibiting the highest incidence^1,2,3.

When nerve compression occurs, the thoracic spinal canal typically becomes smaller than the cervical and lumbar spinal canal, resulting in reduced space for spinal cord escape^4,5,6. Once patients exhibit symptoms, many are accompanied by signs of spinal cord injury^7,8. The effectiveness of conservative treatment is generally poor, necessitating eventual surgical intervention.

The primary objective of thoracic disc herniation surgery is to alleviate pressure on the spinal cord⁸. Surgical approaches include two types - anterior and posterior approaches. Anterior surgery directly alleviates compression on the spinal cord, while posterior surgery indirectly relieves this compression. Even for skilled surgeons capable of directly removing compression from the anterior sclerotic disc, anterior surgery is technically demanding and poses a high risk to patients. Complications, including spinal cord injury, worsening symptoms, leakage of cerebrospinal fluid (CSF), and infection, were observed in a range of 9.6% to 40.8%^9,10,11.

A major factor contributing to the elevated surgical risk is the relative fragility of the dorsal subarachnoid space and the ventral spine's dura. Even slight increased backward traction on the spinal cord can lead to disastrous outcomes¹². Furthermore, detaching the dural sac from the thoracic spine and applying pressure often results in electrophysiological warning occurrences, significantly increasing the chances of spinal cord damage^13,14,15. Additionally, anterior thoracic spinal stenosis surgery often requires thoracotomy and is more traumatic.

Posterior surgery, achieved by removing the posterior structure of the spinal canal, allows the spinal cord to have a certain backward movement space, indirectly relieving compression from the sclerotic disc to the spinal cord^9,16,17. Both surgical approaches can yield certain surgical effects, but anterior surgery, influenced by the presence of lungs, blood vessels, and nerves, increases the surgical difficulty¹⁸. In contrast, posterior surgery's effect on spinal cord decompression is limited for patients^9,16,17, and complete relief of compression may not be achieved. However, the advantage lies in the absence of vital vascular nerves and organs at the back, making it easily exposed and convenient for surgical operations¹⁵. Nevertheless, there is still debate on which surgery constitutes the gold standard for thoracic disc herniation.

Severe symptoms can manifest when the spinal cord or nerve roots are compressed due to the herniation and calcification of the thoracic intervertebral disc, a condition known as sclerosing thoracic disc herniation¹⁹. Because of the typical positioning of the hardened disc on the ventral side of the spinal cord, direct visual removal of the hardened disc is often unfeasible. Our team previously reported a posterior modified eggshell surgical technique for treating sclerosing thoracic disc herniation²⁰. This modified eggshell technique, performed under direct vision, enables complete spinal cord decompression in all directions. The modified eggshell approach can completely remove the sclerotic disc, thereby reducing the risk of SCI. Treating sclerosing thoracic disc herniation with this surgical method is both safe and efficient. This article introduces and demonstrates the surgical procedure.

Protocol

This protocol has received approval and adheres to the guidelines established by The Ethics Committee of Hebei Medical University Third Hospital. Patient data were collected after obtaining informed consent from them. The inclusion criteria for patients were as follows: patients suffering from symptoms of spinal cord damage with ineffective conservative treatment, the presence of hardened tissue observed on X-ray, computer tomography (CT), and magnetic resonance imaging (MRI) on the ventral side of the spinal cord, and complete clinical data with regular follow-ups. Patients with coexisting contraindications such as infection or tumor, or those unable to tolerate the operation, were excluded from the surgery. The surgical tools and equipment used for this study are listed in the Table of Materials.

1. Preoperative preparation

1. Allow the patient to fast for 6 h and proceed with anesthesia via endotracheal intubation²⁰ following institutionally approved protocols.
2. Position the patient in the prone position.
3. Perform a double disinfection of the surgical area using povidone-iodine, followed by two rounds of alcohol. Place sterile sheets to cover the surgical site.
4. Mark the positions for surgical incisions with a marker. Place a Kirschner needle horizontally and perpendicular to the spine on the surgical area. Verify the needle placement using a C-arm fluoroscope.

2. Exposing the surgical site

1. Make an incision along the posterior midline of the designated surgical site, approximately 10 cm in length. Successively expose the skin, subcutaneous tissues, and paraspinal muscles. Completely expose the upper and lower vertebral segments²⁰.
2. Carefully peel the soft tissue away from the bone surface to prevent damage to the intercostal nerve, arteries, and veins. Expose the bony protrusion of the spine, both above and below, as well as the joints.

3. Inserting pedicle screws

1. Insert pedicle screws into the decompression section of the spinal column. Identify the intersection of the vertical line of the lateral margin of the pedicle and the midline of the transverse process as the location for inserting pedicle screws.
2. Use a rongeur to remove part of the cortical bone from the articular process where a pedicle screw is to be inserted. Establish the pathway of a pedicle screw using an awl, then use a probe to confirm the integrity of the pedicle pathway and measure the length of the pedicle screw pathway. Insert the pedicle screw into this pathway.
3. Verify the suitability of the pedicle screw instrumentation using intraoperative fluoroscopy²⁰. Ensure that the pedicle screws remain within the pedicle pathways, avoiding extension beyond the anterior edge of the vertebrae and surpassing the central axis of the spinal column. This indicates the appropriate location of the pedicle screws.

4. Posterior decompression

1. Use lamina forceps to extract the bone situated at the intersection of the vertebral lamina and the inner side of the pedicle. Employ the uncover technique to eliminate the spinous process and lamina²⁰.
2. Remove the bilateral facet joints using a rongeur to fully expose the dura mater and spinal cord (see Figure 1A and Figure 2A).

5. Modified eggshell technique

1. Use a clamp to remove the vertebral body corner and cut open the annular fibrosis with a No. 11 scalpel blade.
2. Hollow out the intervertebral disc by extracting the disc tissue from the intervertebral space, retaining only the anterior and a portion of the lateral section of the annulus (see Figure 1B and Figure 2B).
3. Employ a nerve retractor crook or hook blade to detach the adhesion between the herniated calcified disc and dura mater. Subsequently, use a reverse-angled curette to push the hardened disc tissue into the anterior intervertebral space (see Figure 1C and Figure 2C).
   NOTE: This procedure is known as the "modified eggshell procedure," and the surgical tools used are depicted in Figure 3.
4. Utilize a curved clamp to remove the hardened disc tissue.
5. To detach the hardened disc from surrounding tissues, use a grinding drill to remove the hardened tissues. If the adhesion between the hardened tissues and spinal dura mater is severe and poses a risk, consider making it 'floating' rather than attempting a risky removal.

6. Strengthening the stability of the spine

1. Fill the cages with bone harvested from the spinous, lamina, and articular processes.
2. Implant a cage vertically on both sides into the intervertebral space, maintaining a minimum distance of 0.5 cm between the back edge of the cage and the back edge of the vertebral body.
3. Secure the pedicle screw-rod system in its current position. Press the pedicle screws longitudinally along the rod and secure the rods with nuts. Prevent any minor alteration in curvature following decompression and minimize harm to the spinal cord (see Figure 1D and Figure 2D).

7. Suturing the incision

1. Check for bleeding and completely stop it using electrocautery.
2. Thoroughly clean the incision with saline. Insert a drainage set.
3. Suture the incision layer by layer. Use an absorbable suture No.7 for the fascia layer, an absorbable suture No.4 for the subcutaneous tissues, and a silk suture No.4 for the skin.

8. Postoperative procedures

1. Administer preventive antibiotics to patients for 2 days following the operation.
2. Prescribe bed rest for 1 week.
3. Initiate ambulation with a lumbar brace for patients starting at 1 week after surgery.

Representative Results

As reported in our previous study²⁰, the modified eggshell surgery was successfully performed on 25 patients. The sclerotic tissues of four patients were extensively adhered to the dural sac, making it impossible to fully remove the tissues. However, the compression force on the spinal cord caused by the anterior sclerotic tissue was completely relieved. In two patients, dural sac rupture occurred due to the intraoperative separation of sclerotic tissue and the dural sac, but CSF leakage stopped after suturing. In one patient, lower-limb nerve symptoms worsened 8 h after surgery due to hematoma compression caused by blocked drainage. After correcting the drainage, 200 mL of non-coagulated blood was drained out, and the patient's symptoms were quickly relieved with hormones, dehydration, nutritional nerve, and other medications.

Figure 4 illustrates representative radiological images for the modified eggshell procedure. Before surgery, an MRI scan displayed disc herniation at the T9/10 level, causing compression on the dura mater (Figure 4A). A preoperative CT scan in the sagittal plane revealed calcification and disc herniation at the T9/10 level (Figure 4B). The axial plane of the preoperative CT scan showed calcification and disc herniation at the T9/10 level (Figure 4C). Postoperative X-ray exhibited satisfactory placement of internal fixation, with intervertebral bone grafting observed at the T9/10 level (Figure 4D). Sagittal and axial CT scans after the surgery demonstrated complete removal of the sclerosing herniated disc through 360° decompression (Figure 4E,F).

All patients were followed up for 5-8 years, with a median of six years. Complete intervertebral fusion was achieved in all patients without thoracic kyphosis deformity and regional instability. Patients exhibited varying levels of neural function recovery. At the final follow-up, the JOA score²⁰ before surgery was 5 (with an interquartile range (IQR) of 1), while the JOA score after surgery was 8 (with an IQR of 2). There is a notable distinction (P < 0.001, Table 1).

Figure 1: Schematic representation of the modified eggshell technique. (A) Removal of the bony projection and thin plate on both sides of the joint in the impacted area reveals the protective covering and protruding disc between the vertebrae. (B) Extraction of disk material from the intervertebral space creates a cavity resembling an "eggshell." (C) Use of a hook knife or nerve retractor crook to detach the connection between the dura mater and the herniated calcified disc, followed by the use of a reverse-angled curette to move the hardened disc tissue towards the front intervertebral area. (D) For fusion purposes, a cage and autologous bone graft are inserted into the interbody. Secure placement of the pedicle screw-rod system in its original position minimizes curvature alterations following decompression, reducing harm to the spinal cord. This figure has been adapted from Yang et al.²⁰. Please click here to view a larger version of this figure.

Figure 2: Surgical procedure for modified eggshell technique. (A) Removal of the bony projection and thin plate on both sides of the joint in the impacted area reveals the protective covering and protruding disc between the vertebrae. (B) Extraction of disk material from the intervertebral space creates a cavity resembling an "eggshell." (C) Use of a hook knife or nerve retractor crook to detach the connection between the dura mater and the herniated calcified disc, followed by the use of a reverse-angled curette to move the hardened disc tissue towards the front intervertebral area. (D) For fusion purposes, a cage and autologous bone graft are inserted into the interbody. Secure placement of the pedicle screw-rod system in its original position minimizes curvature alterations following decompression, reducing harm to the spinal cord. This figure has been adapted from Yang et al.²⁰. Please click here to view a larger version of this figure.

Figure 3: Surgical Tools required for the modified eggshell technique. The tools employed in a modified eggshell procedure include fasteners, direct opposite scoop, and slanted opposite scoop. This figure has been adapted from Yang et al.²⁰. Please click here to view a larger version of this figure.

Figure 4: Representative radiological images for the modified eggshell procedure. (A) MRI scan before surgery revealed disc herniation at the T9/10 level, causing compression on the dura mater. (B) CT scan in the sagittal plane before surgery revealed calcification and disc herniation at the T9/10 level. (C) A preoperative CT scan in the axial plane revealed calcification and disc herniation at the T9/10 level. (D) Postoperative X-ray revealed satisfactory placement of internal fixation, with intervertebral bone grafting observed at the T9/10 level. (E,F) Sagittal and axial CT scans after surgery revealed complete removal of the sclerosing herniated disc through 360° decompression. This figure has been adapted from Yang et al.²⁰. Please click here to view a larger version of this figure.

Male	Female	Age range	Mean age	Operative time	Mean operative time	Blood loss	Mean blood loss	JOA score before the operation	JOA score after the operation
14	11	32-70 years	51.7 years	150-310 min	222 min	400-1100 mL	722 mL	5 (IQR=1)	8 (IQR=2)

Table 1: Clinical data of patients. JOA score between pre- and post-operation, Mann-Whitney U test, Z = −4.891, P < 0.001. This table has been adapted from Yang et al.²⁰.

Discussion

The primary application of this surgical procedure is to achieve thorough decompression of the spinal cord from all angles, utilizing the posterior pathway of the thoracic spine. Serious complications are common in patients with thoracic disc herniation, primarily due to the anatomy of the thoracic spine. According to Min et al.¹⁸, anterior decompression using an anterior method has a definite impact but requires a challenging procedure. Furthermore, the extensive trauma it induces, substantial disruption of pulmonary function, and a notable frequency of intraoperative spinal cord and nerve damage greatly restrict its clinical utilization¹⁸. Many reports in the literature suggest that for patients with mild thoracic disc herniation, simple posterior laminar decompression can also achieve good results. However, the decompression is not complete, and for the front compression of severe patients, the effect is limited^9,16,17. There are also reports on the method of anterior decompression through posterior surgery. Still, it is imperfect, each with its advantages and disadvantages, and a unified standard cannot be reached^21,22,23.

Narrowing of the thoracic spinal canal, with minimal room for the spinal cord, can lead to severe complications like paraplegia due to mild traction on the spinal cord. Because the dural sac has the most prominent tissue adhesions, it is challenging to separate, which increases the possibility of surgery for spinal cord injury. Safely and effectively removing the compression in front of the spinal cord during surgery poses a challenge in terms of avoiding spinal cord damage. According to the literature²¹, it has been documented that the combined surgery of the front and back is effective in treating thoracic disc herniation. However, this procedure is associated with significant surgical trauma, and numerous complications and poses a challenging operation. The key to the operation is ensuring the safety and effectiveness of relieving complete compression in front of the spinal cord through posterior surgery. There is no important anatomical structure at the back of the thoracic vertebra, which is easy to operate and has few complications. If the decompression can be completed entirely by simple posterior surgery, the difficulty of the surgery can be greatly reduced, and it has high application value.

The Eggshell technique was initially developed to address vertebral body compression fractures and correct kyphosis deformity. To avoid intraoperative traction injury to the spinal cord, the surgical procedure evolved to target anterior compression of the spinal cord, allowing for a 360° decompression solely through posterior surgery.

The modified eggshell technique primarily involves procedures away from the spinal cord. Using a standard posterolateral intervertebral space approach, the annulus is cut, and anterior disc tissue, specifically sclerotic tissue, is emptied to create a hollow structure in front of the sclerotic tissue. This results in an eggshell-like structure with sclerotic tissue as the posterior wall. The sclerosing tissue is then isolated from the dural sac and pushed into the vertebral space to relieve compression on the spinal cord. Caution is crucial during lamina exposure to prevent spinal cord injury, and the operator must examine for dura mater adhesion or resistance during dura mater traction. All procedures are performed under direct vision, significantly reducing surgical risks. The fixation of pedicle screws and interbody cage enhances spine stability, reduces the incidence of thoracic kyphosis, and improves interbody fusion outcomes.

The advantages of the modified eggshell technique include the ability to achieve complete spinal canal decompression through posterior surgery alone. Additionally, the postoperative complication incidence is only 12%, significantly lower than reports from other teams. For instance, Takahata et al. reported a 40% dural tear rate and a 33% neurological deterioration rate in surgical complications²². Kato et al. reported a 33% rate of surgical complications, including temporary neurological deterioration and a dural tear in two out of six patients²³.

In conclusion, the modified eggshell technique is a safe and effective treatment for sclerosing thoracic disc herniation.

Materials

Name	Company	Catalog Number	Comments
Bipolar electrocoagulation tweezers	Juan'en Medical Devices Co.Ltd	BZN-Q-B-S	1.2 mm x 190 mm
Bone wax	ETHICON	W810T	2.5 g
Curette	Qingniu	20739.01	300 x Ø9 x 5°
Curette	Qingniu	20739.02	300 x Ø9 x 15°
Curette	Qingniu	20739.03	300 x Ø9 x 30°
Curette	Qingniu	20739.04	300 x Ø9 x 45°
Double jointed forceps	SHINVA	286920	240 mm x 8 mm
High frequency active electrodes	ZhongBangTianCheng	GD-BZ	GD-BZ-J1
Laminectomy rongeur	Qingniu	2054.03	220 x 3.0 x 130°
Pedicle screw	WEGO	800386545	6.5 mm x 45 mm
Pedicle screw	WEGO	800386550	6.5 mm x 50 mm
Rod	WEGO	800386040	5.5 mm x 500 mm