A Radiographic Measurement of the Anterior Epidural Space at L_4–5 Disc Level

Abstract

To observe the morphology character of the anterior epidural space at the L_4–5 disc level and to provide an anatomical basis for safely and accurately performing a percutaneous endoscopic lumbar discectomy (PELD). Fifty‐five cases with L₅S₁ lumbar disc herniation were included in this study, and cases with L_4–5 disease were excluded. When the puncture needle reached the epidural space at the L₅S₁ level, iohexol was injected at the pressure of 50 cm H₂O during the PELD, then C‐Arm fluoroscopy was used to obtain standard lumbar frontal and lateral images. The widths of epidural space at the level of the L₄ lower endplate, the L₅ upper endplate, as well as the middle point of the L_4–5 disc were measured from the lumbar lateral X‐ray film. Epidural space at the L_4–5 disc plane performs like a trapezium chart with a short side at the head end and a long side at the tail end in the lumbar lateral X‐ray radiograph, while the average widths of epidural space were 10.2 ± 2.5, 12.3 ± 2.3, and 13.8 ± 2.6 mm at the upper, middle, and lower level of the L_4–5 disc. Understanding the morphological characteristics of epidural space will contribute to improving the safety of the tranforaminal percutaneous endoscopy technique.

Introduction

Lumbar disc herniation (LDH) is an extremely common cause of low back pain affecting the general population. In 5% of patients, LDH is due to lumbar disc protrusions1. Currently, a doctor is able to provide surgical intervention or initial conservative treatment for LDH. Conservative treatments for LDH include physical therapy, corticosteroid injections, and anti‐inflammatory medication.

Over the past few decades, minimally invasive spine surgery (MISS) has become widely used around the world because it is safe, effective, and reproducible2, 3, 4. Percutaneous endoscopic transforaminal lumbar discectomy (PELD) is a popular and effective MISS technique used to treat lumbar disease. The technique involves inserting an endoscope into the spinal canal through the intervertebral foramen, and performing decompression of nerve roots with live imaging; it requires precise placement of the lens in the interspace of the rear of disc and the front of the dura mater–anterior epidural space5, 6. Therefore, PELD is considered a safe, minimally invasive, and effective technique for LDH surgical treatment7, 8. The work channel for PELD is approximately 7  mm.

The spinal epidural space is an anatomic space between the dura and the ligamentum flava and periosteum of the vertebral bodies, pedicles, and laminae. Doctors and anatomy experts have done little research on the anatomy and structure of this interspace. Although researchers of epidural anesthesia‐related anatomy have focused on the posterior epidural space, existing literature lacks anatomic study of the anterior epidural space9, 10.

‘For this reason, during transforaminal endoscopic surgery, we would observe and measure the morphology of the epidural space at the L_4–5 disc level, to help accurately and safely place the lens in the discs during PELD. Moreover, this provides anatomical data for the anterior epidural space.

Materials and Methods

Patients

The present study focuses on 50 adult patients who were hospitalized between March 2012 to June 2014 due to simple L₅S₁ dorsal disc prolapse. The inclusion criteria for patients in this study were: (i) MRI investigation showed a single level of posterolateral disc herniation at L₅S₁ that correlated with the clinical findings; and (ii) no previous surgery at the same level of the lumbar spine. The exclusion criteria included: (i) L_4–5 dorsal disc prolapse; (ii) lumbar spinal stenosis; and (iii) other diseases discovered by by X‐ray, CT, and MRI examination. A total of 29 men and 21 women were selected and constituted our study population. Information on patients’ age, height, weight, and other general information are presented in Table 1.

Table 1.

Gender differences in general information (mean ± standard deviation)

Gender	Age (years)	Height (cm)	Weight (kg)
Male (29 cases)	43 ± 8	177 ± 25	72 ± 17
Female (21 cases)	48 ± 6	165 ± 18	58 ± 20

Epidural Space Contrast

The anterior epidural space is a potential gap; that is, the dura mater and the wall of the spinal canal are attached to each other under physiological conditions. The anterior epidural space can be formed by injecting a certain pressure into the epidural space. Therefore, it is impossible to measure the anterior epidural space in fresh or fixed cadaver specimens. In this study, we used the technique of injecting contrast medium into the epidural space and forming a subdural space. To perform PELD to remove the protruded segments of L₅S₁, when the 18 G puncture needle reached the epidural space at L₅S₁ level, iohexol was injected at the pressure of 50 cm H₂O. After the filling of contrast media, and maintaining a pressure of 50 cm H₂O, C‐Arm fluoroscopy was used to obtain standard lumbar frontal and lateral images. The images were collected and stored using the Philips image workstation and then used to carry out morphometric measurements.

Measure Parameters

The width of the epidural space at the L₄ inferior endplate (A point), the L₅ inferior endplate (point B), and the midpoint M of line AB was measured using lumbar lateral images (Fig. 1).

Figure 1.

Measurement of the width of the epidural space. A, the width of the epidural space at L₄ inferior endplate; B, the L₅ inferior endplate (point B) and midpoint M of line AB were measured on lumbar lateral images; M, midpoint of line AB was measured on lumbar lateral images.

Results

On the images of lumbar lateral C‐Arm fluoroscopy after injection of iohexol at the pressure of 50 cm H₂O, the side view of L_4–5 epidural space was a trapezoid with a slightly narrower head end and a slightly widened tail end (Fig. 2). The average width of L_4–5 epidural space at the superior border was 10.2 ± 2.5 mm, with a minimum value of 8.1 ± 0.5 mm; the average width of L_4–5 epidural space at the middle point was 12.3 ± 2.3 mm, with a minimum value of 10.1 ± 1.7 mm; and the average width of L_4–5 epidural space at the inferior border was 13.8 ± 2.6 mm, with a minimum value of 11.3 ± 2.4 mm.

Figure 2.

A zoom display: the head end of the dura mater was narrow; at narrow A zone (the superior edge of L_4–5 disc), the reamer tines may adversely damage the dura mater.

Discussion

Present Situation of Transforaminal Percutaneous Endoscopy

Transforaminal percutaneous endoscopy is used in laparoscopy in spinal surgery9. The working area of the endoscope is often a cavity, such that the laparoscope works in the abdominal cavity, the arthroscope operates in the joint cavity, and the work area of the endoscope of PELD is the lumbar epidural space9. The exact position is the space between the vertebral body and the dura mater. After placing the endoscope during surgical operation, the injected fluid will have enough pressure to push away the dura mater around the lens to form a larger spatial extent of dura mater. In this region, by adjusting the direction, angle, and depth of the lens, we can observe nerve roots, dura mater, and the herniated disc in every direction; and with a special microsurgical instrument, we can perform the removal of the herniated nucleus pulposus, nerve root decompression and release, and disc annulus radiofrequency ablation repair11, 12.

Percutaneous Endoscopic Lumbar Discectomy Technology is a Popular and Effective Technique Used to Treat Lumbar Disc Herniation

After having determined the working area of “the intervertebral epidural space,” PELD technology needs to be familiar with the anatomy of the dura mater before conducting minimally invasive surgery. Anatomical research on epidural space has focused on the cavity of the dorsal dural sac. The purpose has been to puncture the epidural space more safely during epidural anesthesia or to avoid injury to dorsal dural mater during traditional spinal decompression surgery13, 14. However, for the work area of transforaminal endoscopic surgery (i.e. the epidural area between the rear vertebral body and the anterior dural sac), relevant morphological study is deficient. Therefore, during the PLED, on removal of the herniated segment of L₅S₁, we selected cases with no lesion at the L_4–5 disc, by injecting iohexol at the pressure of 50 cm H₂O, to expand the lower lumbar epidural space, then using X‐ray images to make radiological measurements on their morphological characteristics15, 16.

Significance of Measurement of the Anterior Epidural Space at L_4–5 Disc Level

The results showed that after the injection of contrast agent iohexol at the pressure of 50 cm H₂O, the epidural space at the L_4–5 disc plane resembled a trapezoid with a narrow head end and wide tail end; namely, the epidural space at the inferior edge of L₄ level was relatively narrow, but at the superior edge of the L₅ level, it was wider. The reason may be that for the epidural space at the L_4–5 disc space plane, the head end is fixed by the L₄ nerve root (travel root), limiting the dura backward shift, and causing the head end of the epidural space at L_4–5 to be narrow17, 18. In addition, the intradural coccygeal nerve root becomes more scarce at the tail end; at the pressure of 50 cm H₂O, the distal dural has greater potential to shift backward, which may also be the reason why the epidural space at the L_4–5 disc plane was relatively narrow at the head end and relatively broad at the tail end19.

This measurement shows that the mean width at the superior edge of the L_4–5 disc was 10, and the minimum was 8 mm, which was close to the diameter of three grade reamer and working channel. Therefore, for the transforaminal endoscopic spine system (TESSYS) technique target at this location, care should be taken in using the reamer to expand the intervertebral foramina. This is especially true when the anteroposterior view reveals that the drill had passed through the inner edge of the pedicle of the vertebral arch, and was about to enter the spinal canal. In this case, the method promoted by Zhou et al. should be applied20. This involves using a bone hammer to gently knock the tail of the reamer slowly, rather than using the method of rotating the reamer, to avoid the sharp tines of the reamer twisting the dura mater during rotation and tearing the dura mater, resulting in cerebrospinal fluid leakage, and even leading to the need for open surgery in serious cases. The width of the middle and the lower edge of the dura mater at the L_4–5 disc reach 12 and 13 mm, respectively; they are wide enough for a 7.5‐mm reamer to rotate and cut, as long as the X‐ray demonstrates that if the guide rod is placed in the center of the epidural space it will not damage the dura. However, a limitation of this study is not having considered the sex difference for the pressure formed widths of the anterior epidural space. We understand that sex difference in anterior epidural space could help in more accurately and safely place the lens in the discs during PELD, and, thus, will consider this in our further study.

Endoscopic TESSYS technology is applied in disc herniation surgery, as the herniated disc itself plays the role of pushing the dura and expanding the epidural space. If the operation is in accordance with the target technology to puncture and cut using reamers, under normal circumstances, when entering the spinal canal and cutting the herniated disc tissue, the reamer would not damage the dura mater. However, for spinal stenosis and nerve root canal stenosis, using TESSYS technology to depress may tear the dura because of the twisting of the dura mater or Hofmann ligament on the sharp saw. The epidural space may not expand, and the Hofmann ligament is connected between the dura mater and the posterior longitudinal ligament or posterior wall of the vertebral body21. If the reamer is rotated to enter the epidural space, especially at the superior edge of the disc, the Hofmann ligament may be in danger. Therefore, when using transforaminal endoscopic TESSYS treatment for spinal stenosis or nerve root canal stenosis, following the target puncture, it is appropriate to cut using the reamer after pushing away the dura and completing expansion of the epidural space by injecting a contrast agent at a certain amount of pressure. When the reamer enters the spinal canal, and the epidural space, which has been expanded by the water pressure, the sharp saw will not touch the dura mater, thus avoiding damage to the dura, and the situation will be conducive to safely applying the TESSYS technology.

Conclusions

In the present study, we analyze the morphological characteristics of epidural space, which will contribute to safely carrying out the tranforaminal percutaneous endoscopy technique.