Full-Endoscopic Decompression for Thoracic Ossification of the Ligamentum Flavum Combined with Dural Ossification

Summary

We describe a minimally invasive surgery using a full-endoscopy system for the single segmental thoracic ossification of the ligamentum flavum, combined with dural ossification, at Hebei General Hospital.

Abstract

Ossification of the ligamentum flavum (OLF) can result in spinal stenosis. Thoracic spinal cord compression due to spinal stenosis is a common cause of progressive thoracic myelopathy in Asian countries. The incidence of complications is high in open decompression surgeries for thoracic OLF. With dural ossification (DO), the risk of complications is even higher in thoracic OLF. We introduce a full-endoscopic decompression surgery for thoracic OLF combined with DO under local anesthesia. Hemilaminectomy is performed using a high-speed burr under the endoscopy first, and then decompression of the contralateral spinal canal is completed using an “over the top” technique. DO resection uses the eggshell technique; after the base of the DO is cut from the lamina, forceps or lamina rongeurs are typically used for removal. The dural defect left after resection does not need repair. Neurological function was improved, and no complications such as hematoma or neck pain occurred. On imaging, no pseudodural cyst, cerebrospinal fluid leakage, or wound complications were observed after the operation. Endoscopic surgery causes less damage to the posterior ligament complex, so no cases of persistent back pain complaints or secondary internal fixation requirements were found in this study. Full-endoscopic decompression can achieve good imaging and clinical effects in the treatment of thoracic OLF with DO.

Introduction

Ossification of the ligamentum flavum (OLF) is one of the most common causes of thoracic myelopathy or radiculopathy in Asian countries, especially in Japan^1,2. The prevalence has been reported to vary from 3.8% to 26% in East Asia^1,3,4. Posterior decompression is recommended for the treatment of thoracic OLF according to the compression location and its aetiologies⁵. Dural ossification (DO) is a difficult problem in surgical procedures⁶; when the dura mater is ossified, the incidence of surgical complications such as cerebrospinal fluid (CSF) leakage and spinal cord or nerve root injury increase.

Endoscopic surgeries have been widely used in the treatment of lumbar spinal stenosis, with satisfactory clinical outcomes^7,8,9. Endoscopic surgeries have many advantages. First, preservation of the posterior ligament complex prevents lumbar instability during these operations. Second, wound healing complications related to cerebrospinal fluid leakage rarely occur in endoscopic surgeries because of thick muscle coverage. As the thoracic and lumbar spine have similar anatomical features, we transplanted this technique to the thoracic spine.

OLF is mostly located in the lower thoracic vertebrae, where the posterior ligament complex is particularly important because of the maximum tensile force¹⁰. In cases with dural ossification, CSF leakage resulting from dural defects is inevitable^11,12. The incidence rate is 32% in thoracic OLF¹¹. Prevention of the wound problem caused by CSF leakage is a troubling problem for spinal surgeons.

We introduce a full-endoscopic decompression surgery for thoracic OLF combined with DO. In this surgery, bilateral decompression of the spinal canal is achieved by an "over the top" technique. The DO can be completely removed without wound complications. The long-term clinical outcome is satisfactory.

Protocol

This study was approved by the Ethics Committee of Hebei General Hospital. Informed consent was obtained from all individual participants.

1. Preoperative preparation

1. Have the patient simulate the operation position. Have the patient lay in the prone position on the sickbed 3-5 days before the operation. Cushion the thorax and ilium with soft pillows.
2. Plan the operation on the picture archiving and communication system (PACS). Measure the thickness of the lamina and OLF on the computerized tomography (CT) and magnetic resonance imaging (MRI) scans, so that the depth of the bone window is roughly sized. The distance of ossification on the sagittal CT images can help the surgeon determine the upper and lower range of the window (Figure 1).

2. Skin marking and anesthesia

1. Have the patient lay prone on a Jackson spinal table, with ECG, blood pressure, and oxygen saturation monitoring.
2. Under C-arm fluoroscopic guidance, determine the segmental lesion and puncture path. Confirm the segmental lesion by anatomical features such as the ribs and osteophyte. The puncture target is the center point of the triangle formed by the upper and lower pedicles and spinous process. The path is 10°-15° to the head inclination (Figure 2).
3. Use local anesthesia.
   1. Use dexmedetomidine at a concentration of 4 µg/mL for continuous venous inflow at a dose of 0.1-0.5 µg/kg/h by a micro-pump. The proportion of the local anesthetic is 0.5% lidocaine and 0.25% ropivacaine.
   2. In order to relieve the pain of the patient during the procedure, inject more anesthetics into the following sites: skin, fascia, paraspinal muscle, and the surface of the vertebral lamina. The dose of local anesthetics is 5 mL for skin, 8 mL for fascia, 5 mL for paraspinal muscle, and 8 mL for the surface of the vertebral lamina.

3. Working sheath establishment

1. Make a surgical incision approximately 40-60 mm from the midline with a no. 11 surgical blade. Incise the fascia sharply. Use the guide rod with a blunt head for puncture. Confirm the target by C-arm fluoroscopy (Figure 3).
2. Insert the working sheath to the lamina surface along the guide rod. Nail a 2.5 mm diameter Kirschner wire into the lamina softly with the hammer as a marker (Figure 4).

4. Endoscopic operation

1. Hang an irrigation bag containing 2 L of 0.9% saline solution approximately 1-1.5 m above the patient. Connect the bag to the endoscope with a transfusion tube. Continuous saline is transfused to the endoscopy, which can provide clear vision for the endoscopic operation.
2. Use the bipolar electrocoagulation probe to peel off the soft tissue covering the surface of the vertebral lamina, and take it out using forceps to expose the cortical bone of the vertebral lamina. Look for the small hole left by the Kirschner wire under the endoscopy as a mark (Figure 5).
3. Find the lateral boundary of the articular process. Grind off the cortical bone of the vertebral lamina to be removed with a high-speed burr to mark the boundary of the bone window. Take the diameter of the burr as a scale (Figure 6).
4. Grind off the medial half of the facet joint and the ipsilateral lamina with a high-speed burr. During the grinding process, the layers of the vertebral lamina can be observed as cortical bone, cancellous bone, and inner cortical bone. After this step, ossified ligamentum flavum or soft hypertrophic ligamentum flavum with different colors to the cortical bone can be seen (Figure 7).
5. Grind the ossification away softly with a high-speed burr. During the grinding process, use the probe or hook intermittently to check whether the ossification is loose. When the ossification is loose, try not to press down the burr, but slide it on the surface of the ossification to thin it.
   1. Separate the ossification from the spinal cord with a hook, and remove with the forceps or rongeurs (Figure 8).
6. Perform contralateral spinal canal decompression using the "over the top" technique.
   1. Grind the inner layer of bone under the dorsal side of the spinal cord, the base of the spinous process, and gradually push the burr to the contralateral side. Grind the contralateral vertebral lamina to achieve the contralateral articular process and pedicle wall.
      NOTE: Sufficient space around the spinal cord and pulsation of the spinal cord are signs of adequate decompression. These decompression operations are performed over the top of the spinal cord (Figure 9).
      NOTE: The DO resection is the most difficult part of this operation.
7. Expose the boundary of the DO from its cranial and caudal sides. When grinding and separating the OLF, there is adhesion between the dura and the ossification. The part where the adhesion occurs is the boundary of the DO.
   1. To take out the ossification easily, grind the massive ossification into pieces using the eggshell technique. Start grinding from the center and gradually approach the spinal cord, so that the whole block becomes an eggshell¹³.
   2. Isolate the ossification at the junction between the ossification and the lamina (Figure 10).
      NOTE: The junction between the ossification and the lamina is broken in the above step. The DO adheres to the spinal cord and floats in the spinal canal.
8. Separate the eggshell-shaped ossification with a hook. Grab it in pieces with forceps or a lamina rongeur, and remove the ossification from the gap between the DO and the spinal cord. The dural defect left after resection of the DO does need repair (Figure 11).
9. Confirm that the range of decompression is large enough. There are some signs under endoscopy that indicate sufficient decompression. First, there is space between the spinal cord and vertebral lamina at the cranial, caudal, ipsilateral, and contralateral sides of the spinal canal. Second, the spinal cord can pulsate freely under endoscopy (Figure 12).
10. Use the bipolar electrocoagulation probe to coagulate the bleeding points. Hemostatic materials are not recommended. Check that there is no active bleeding in the operation field. Remove the endoscopy slowly to avoid negative pressure caused by rapid withdrawal, which may cause dural herniation.

5. Postoperative care

NOTE: The patient stayed in bed for 24 h after the operation.

1. Disinfect the wound 24 h after the operation and replace the dressing. Monitor muscle strength and sensation of the lower extremities within 48 h. Administer celecoxib orally for 1 week.
   NOTE: The patient could get out of bed with a brace 24 h after the operation.
2. Avoid back exercise for 6 weeks.

Representative Results

From June 2017 to December 2020, full-endoscopic decompression surgeries were performed on four patients in our hospital, including one male and three females aged 46-72 years with an average age of 64.3 years. The average operation time was 185.3 min. The mean follow-up was 16 months. Patients experienced relief of their myelopathy symptoms. The modified Japanese Orthopedic Association (mJOA) score improved at the last follow-up. The Patient Satisfaction Index (PSI)¹⁴ at the last follow-up was 75%, which can be seen in a previous study¹⁵ and Table 1.

No complications such as hematoma or neck pain occurred. No imaging pseudo dural cyst, cerebrospinal fluid leakage or wound complications occurred after operation (Figure 13).

Figure 1: Operation plan in the picture archiving and communication system (PACS). (A) Measurement of the thickness of the ossification and lamina. Line a represents the thickness of the ossification. Line b represents the thickness of the lamina. Line c represents the midline. Line d represents the operation path. (B) Measurement of the range of the ossification on the sagittal CT scan. The line represents the length of the ossification. This figure has been modified from Li et al.¹⁵. Please click here to view a larger version of this figure.

Figure 2: The puncture path. (A) The puncture path on the C-arm fluoroscopic anteroposterior imaging. (B) The puncture target on the fluoroscopic imaging: the center point of the triangle formed by upper and lower pedicles and spinous process. (C) The puncture path marked on the skin surface. Please click here to view a larger version of this figure.

Figure 3: The puncture. (A) Puncture with the blunt head rod. (B) Position of the guide rod under fluoroscopy. This figure has been modified from Li et al.¹⁵. Please click here to view a larger version of this figure.

Figure 4: Working sheath establishment. (A) Nail the Kirschner wire into the lamina (A). (B) Position of the wire under fluoroscopy (B). This figure has been modified from Li et al.¹⁵. Please click here to view a larger version of this figure.

Figure 5: The "bone hole" left on the lamina surface by the Kirschner wire as shown by the arrow. Please click here to view a larger version of this figure.

Figure 6: The extent of the bone window to be removed. The distance can be evaluated by the diameter of the burr. Please click here to view a larger version of this figure.

Figure 7: Exposure of the ossification of the ligamentum flavum. ○ represents the lamina. Δ represents the ossification of the ligamentum flavum. □ represents the soft hypertrophic ligamentum flavum. Please click here to view a larger version of this figure.

Figure 8: Separationg of the ossification from the spinal cord. (A) Separate the ossification from the spinal cord with the hook. (B) Take the ossification out with forceps. → represents the spinal cord. Δ represents the ossification of the ligamentum flavum. Please click here to view a larger version of this figure.

Figure 9: Contralateral spinal canal decompression using the "over the top" technique. (A) → represents the spinal cord. Δ represents the ossification of the ligamentum flavum. ○ represents the lamina. (B) Axial CT image showing the contralateral decompression technique. Please click here to view a larger version of this figure.

Figure 10: Isolate the ossification. (A) Grind the ossification into a thin eggshell.(B) The isolated ossification. → represents the spinal cord. Δ represents the ossification of the ligamentum flavum. Please click here to view a larger version of this figure.

Figure 11: Resection of the dural ossification. (A) The thin and floating dural ossification. (B) Separate the ossification by a hook. (C) Take the ossification out with forceps. (D) The dural ossification taken out from the spinal canal. Please click here to view a larger version of this figure.

Figure 12: Endoscopic view of the decompressed spinal cord. → represents the spinal cord. represents the arachnoid. Please click here to view a larger version of this figure.

Figure 13: Images of a patient with thoracic ossification of the ligamentum flavum combined with dural ossification before, after, and 3 months after surgery. (A) Preoperative CT scan; (B) Postoperative CT scan; (C) Preoperative MRI; (D) Postoperative MRI; (E) MRI 3 months after surgery. Please click here to view a larger version of this figure.

No.	Sex	Age	Operating segment	Operation time (minutes)	Preoperative mJOA	Last follow-up mJOA	Patient satisfaction
		(Years)			(11points)	(11points)	index
1	Female	46	T10-11	172	3	7	1
2	Female	67	T10-11	158	1	5	2
3	Female	72	T10-11	191	4	6	2
4	Male	72	T9-10	220	3	4	3
Overall		64.3		185.3
mJOA=modified Japanese Orthopedic Association
Patient satisfaction index: 1. Surgery met my exception; 2. surgery improved my conduction enough so that I would go though it again for the same outcome; 3. surgery helped me but I would not go though it again for the same outcome; 4. I am the same or worse compared to before surgery. 1or 2: satisfied. 3or 4: unsatisfied

Table 1: Clinical information of the four patients in the study.

Discussion

Although endoscopic thoracic surgery has achieved satisfactory clinical outcomes^10,15,16, there are still debates on some operation details, such as the operation sequence, surgical instruments, and decompression skills. The operation sequence and the resection of the DO are the key steps of the surgery. Some surgeons prefer to perform contralateral decompression first and then remove the ipsilateral ossification¹⁷, while others perform ipsilateral decompression first and then perform contralateral decompression^15,16. If contralateral decompression is performed first, ipsilateral ossification can restrain the floating of the spinal cord so as not to affect the contralateral operation. If ipsilateral decompression is carried out first, the dura can be exposed quickly after the spinal canal breaks through, and the locations of the spinal cord and ossification can be confirmed as soon as possible. At the same time, the pressure of the spinal canal can be released, which is helpful for subsequent operations. In our experience, for the fused type¹⁸, decompression from the ipsilateral side first is recommended.

Some surgeons prefer to use a circular saw¹⁶, but we prefer to use a diamond burr throughout the process. The circular saw technique is highly efficient, but it requires good hand sensation and multiple X-ray confirmations, and the risk of nerve injury is also higher. Although the efficiency of the drill is slightly lower, the whole procedure is under view, so the operational safety is higher.

DO resection is an intricate problem. When the base is still connected with the OLF, it can be ground into pieces. Once the DO floats, it should be removed en masse by forceps or rongeurs. After removal of the DO, there will be dural defects and an increased risk of spinal cord injury. In all cases in this study, we resected the DO completely and preserved the arachnoid membrane as much as possible. The defective dura was not repaired during the operation, and there was no spinal cord injury or CSF leakage. This could be due to the magnifying effect of endoscopic surgery and the small damage to muscles. To obtain clear vision, endoscopic hemostasis is very important. The measures to do so include raising the infusion bag, dealing with the small bleeding points before it gets out of control, and vessel electrocoagulation before bleeding. Try not to raise the height of the bag while keeping the operation view clear.

For the evaluation of spinal stability after surgery, the follow-up time in this study was 16 months. No case patient complained of persistent back pain or required secondary internal fixation in this study. This could be related to the supporting function of the thorax and ribs, and the fact that endoscopic surgery causes less damage to the posterior ligament complex.

Full-endoscopic decompression surgery has the following technical advantages: the surgical trauma is significantly less than that of open surgery; endoscopic surgery is safer because the enlarged field of vision can identify anatomical structures more clearly; the patients under this anesthesia method were fully conscious during the surgery; when the spinal cord is stimulated, the patient can give good feedback to the operator; and good muscle coverage can avoid cerebrospinal fluid leakage.

The long operation time is a disadvantage of this surgery. Other disadvantage is the steeper learning curve as the operational cases are rarer. We believe that with the optimization of surgical procedures and the improvement of skills and instruments, the operation time will gradually shorten.

Materials

Picture Archiving and Communication System (PACS)	Neusoft Co., Ltd.	Neusoft PACS	Image requirement: DICOM; Running system: Windows 7 or Windows 10
Endoscope system	SPINENDOS GmbH	SP081430.030	Inner diameter:4.3mm; Outer diameter:7.0mm; Field angle: 80 °; Visual angle: 30 °; Working length: 181 mm.
Working sheath	SPINENDOS GmbH	SP082615.265	Φ7.2mm×178mm
Puncture needle	SPINENDOS GmbH	SP082016.150	1.6mm×150mm
Guide rod	SPINENDOS GmbH	SP082616.300	Φ7.0mm×225mm
Endoscopic rongeur	SPINENDOS GmbH	SP082700.040L	Φ4.0mm×360mm
Bipolar electrocoagulation probe	ELLIQUENCE	DTF-40	40cm
Endoscopic forceps	SPINENDOS GmbH	SP082781.835	Φ2.5mm×330mm
Endoscopic hook	SPINENDOS GmbH	SP082628.351	Φ2.5mm×310mm
High-speed burr	XIYI	MQZ	Φ3.2mm×328mm