A Preliminary Observational Study of the Endoscopic Anatomical Structures of the Lumbar Spine Through the Foraminal and the Interlaminar Approach

1. Clinical Data and Methods

1.1. General Information

The study collected data from patients who underwent full spine endoscopic surgery at our hospital from September 2020 to September 2022, including their age, gender, chief complaint, diagnosis, surgical method, surgical approach, and surgical videos.

Inclusion criteria: patients aged between 18 and 80; underwent the first lumbar spine full endoscopic surgery; have no history of lumbar intervention or surgery.

Exclusion criteria: have a history of mental illness; have lumbar spondylolisthesis; have structural abnormalities such as congenital spine cleft or segmentation anomaly; have suspected spinal infections; have primary or secondary tumors in the spine; and have no high-definition video during surgery or do not cooperate with the research protocol.

1.2. Methods

According to the surgical approach, the enrolled patients were divided into two groups: interlaminar approach and transforaminal approach. The high-definition videos of operation were reviewed by 3 members of the research group independently. The names of the microscopic structures were approved if recognized consistently by the method, or the structure with different views were assigned to all study group members for discussion.

For these microscopic anatomical structures, the probability of occurrence, variation, origin position, function and other structural characteristics were further studied, so as to clarify their spatial position and clinical significance in spinal endoscopic surgery.

1.3. Statistics

SPSS 20.0 was used to calculate the probability of occurrence of the newly discovered specific structures and the probability of variation, and the value of P≤0.05 indicates that differences are statistically significant.

2. Results

A total of 290 patients who underwent lumbar endoscopic surgery were included in this study. After excluding 111 patients without complete surgical videos, a total of 179 patients who underwent lumbar endoscopic surgery with IVD herniation were finally included, with 108 males and 71 females, aged 19 to 65 years old, with a mean age of 41±16 years. There were no significant differences in general information between patients with the two approaches. The intervertebral foramen approach was the most commonly used approach for lumbar endoscopic surgery, followed by the interlaminar space approach, both of which are common approaches of minimal invasive spine surgery. In addition to some well-known anatomical structures, this study also found the distribution of some blood vessels on the IVD and annulus fibrosus, ligaments and blood vessels in the intervertebral foramen area, and the distribution of blood vessels on the dura mater and nerve roots (seen in Table 1).

Figure 1. Diagram of lumbar vessel system. (A) Diagram of lumbar artery system from sagittal view. (B) Diagram of lumbar venous system from sagittal view. (C) Diagram of lumbar artery system on the ventral side of dura sac (posterior vertebral body). (D) Diagram of lumbar venous system on the ventral side of dura sac. The use of these images has been approved by the copyright owner (https://clinicalgate.com/blood-supply-of-the-lumbar-spine/#bib3).

Figure 1. Diagram of lumbar vessel system. (A) Diagram of lumbar artery system from sagittal view. (B) Diagram of lumbar venous system from sagittal view. (C) Diagram of lumbar artery system on the ventral side of dura sac (posterior vertebral body). (D) Diagram of lumbar venous system on the ventral side of dura sac. The use of these images has been approved by the copyright owner (https://clinicalgate.com/blood-supply-of-the-lumbar-spine/#bib3).

ana, anastomosis over the surface of the lumbar IVD; ascb, anterior spinal canal branch; dr, branches accompanying dorsal ramus of spinal nerve; ia, posterior branch related to the pars interarticularis of the lamina; LA, lumbar artery; man, metaphysial anastomosis; ppa, primary periosteal artery; pscb, posterior spinal canal branch; spa, secondary periosteal artery; vr, branches accompanying ventral ramus of spinal nerve; NA, nutrient arteries; lumbar ascending branch (ab.); lumbar descending branch (desc.)

aevvp, elements of the anterior external vertebral venous plexus; pevvp, posterior external vertebral venous plexus; aivvp, elements of the anterior internal vertebral venous plexus; ALV, ascending lumbar vein; LV, lumbar vein;

medial AIVV; pivvp, posterior internal vertebral venous plexus, basivertebral vein; IVV, intervertebral vein.

After removing part of the bone from the ventral side of the SAP, the ligamentum flavum in the ventral side of the articular process can be seen (Figure 2A,B), and there are cross-linked ligaments in the annulus fibrosus and the posterior longitudinal ligament of IVD, which assist in fixing the dura and nerve roots (Figure 2C). The nerve root ventral to the ligamentum flavum and the ventral IVD to the nerve root form a disc-nerve root-ligamentum flavum, sandwich structure (Figure 2D). The sandwich structure is a unique mirror image seen under the foraminal approach, especially after the removal of part of the ventral SAP, it is more clearly visible, and is an important anatomical symbol under the microscope. The dorsal is the yellow ligament, the ventral is the posterior longitudinal ligament or IVD, and the nerve root is in the middle. After the removal of the ventral bone of the SAP, the ligamentum flavum was exposed.

2.1. Ligaments Attached to the SAP

The SAP is one of the most important anatomical symbols through the foraminal approach, and many ligaments attach to it, as well as MM. There are generally three muscular ligaments attached to the SAP, namely the MM, the medial intertransverse muscle and the lateral intertransverse muscle, which connect the articular process with the spinous process and the transverse process. In addition, there are also ligaments in the transverse process and the posterior vertebral body, namely, superior corporotransverse ligaments, inferior corporotransverse ligaments, superior-transforaminal ligament, mid-transforaminal ligament and inferior-transforaminal ligament (Table 1).

2.2. IVD and AF Under Endoscopy

In patients with degenerated IVD, the annulus fibrosus is usually porcelain white (Figure 2E, 2F), accompanied by inflammatory hyperplasia, and bulge. If the outer annulus fibrosus is perforated, there will often be serious local inflammatory tissue encapsulation, and protruding nucleus pulposus and annulus fibrosus can often be found after removing the inflammatory tissue (Figure 2G). A network of blood vessels can be seen on the surface of the microscopic ring (Figure 2F), which are usually distributed transversely and have branches that are often cross-linked with each other. The annulus fibrosus of the IVD is directly connected to the posterior longitudinal ligament, which cross-link to the ligamentum flavum (Figure 2C). The annulus fibrosus and the posterior longitudinal ligament are cross-linked ligaments, which are important anatomical markers, located between the ventral dural and the IVD, starting from the IVD, oblique, about 5-10mm in diameter, and their main function is to fix the dural and nerve roots. Sometimes, in order to show the ventral dural, this ligament has to be cut (Figure 2C).

2.3. Vascular System on the Dura and Nerve Root

After removal of the ventral bone of the SAP by a lateral-posterior approach, the walking root, dural sac, and ventral disc were visible (Figure 2D). The epidural fat on the traversing roots was stripped away to expose the nerve root. It can be seen that there are mainly 2 vascular bundles on the dorsal side of the walking roots, which are distributed longitudinally paralleling to the nerve roots, and there are vascular network connections between them (Figure 1 and Figure 2H).

The key anatomic marker in the interlaminal approach is the ligamentum flavum, and the removal of part of the ligamentum flavum is a landmark step in the surgical process (Figure 2I). After breaking through the ligamentum flavum, epidural fat and dural membrane can be seen, and the surface of the dural membrane is covered by a loose layer of disintegrated tissue (Figure 2K), which is composed of epidural fat and vascular network. This membrane may be the last protective barrier of the dural membrane and should be retained unless necessary. The dorsal central fold of the dural membrane can also be found in the approach of interlaminar space (Figure 2K). The nerve root sleeve and its vessel distribution (Figure 2J) can also be clearly found. It can be seen that these blood vessels are mainly distributed longitudinally along the dural membrane and nerve roots, and there are slightly smaller communicated blood vessels distributed among them. These blood vessels are the main blood supply to the dural membrane, arachnoid membrane and the superficial part of the nerve roots, and the protection of these blood vessel networks is very important in procedures. The central bulging disc can be seen after significantly pushing of the dura and nerve root, besides that, the vascular network distributed on the dura, and the protruding nucleus pulposus can be seen in the interlaminar approach (Figure 1 and Figure 2L).

3. Discussions

Familiarity with the above anatomical structures is the basis for carrying out surgery. From this, it can be discovered which points are most prone to bleeding, such as the nutrient blood vessels of muscles at the dorsal articular processes, the accompanying blood vessels of the dorsal ramus, the venous plexus on the ventral side of the lumbar canal, the accompanying blood vessels of the exit nerve roots, the cephalad side of the vertebral pedicles, and the base of the articular processes. Hemorrhage caused by injury to the venous plexus on the ventral side of the dura sac or accompanying blood vessels of the exit nerve roots can be particularly profuse and difficult to stop, often requiring the use of gelatin sponge packing for compression hemostasis. After becoming familiar with these anatomical structures, it is possible to timely identify them during surgery and avoid damage to blood vessels and nerves.

During the procedure, the author noted some specific structures under the endoscope, such as the small ligaments (not the flavum ligament) running obliquely from the ventral and apex of the SAP to the exit root above, the ventral annulus fibrosus, and the transverse process at the caudal end. These ligaments on the SAP are important fixation structures for the contents of the intervertebral foramen. They can fix the exit nerve roots and accompanying blood vessels from different directions, combined with fatty tissues, thus reliably fixing the exit nerve roots and accompanying structures in the intervertebral foramen. The space between the ligaments contains a lot fatty tissue, which acts as a good cushion and protection for the nerve roots. There is also a flat vein (distributed longitudinally) perpendicular to the IVD, located on the outer side of the traversing roots, along the edge of the intervertebral foramen, and the position is relatively constant. Sometimes, it is necessary to coagulate it when dealing with the bulging IVD. In addition, accompanying blood vessels of the nerve roots and the dura mater, as well the protective fat tissues, they both have a protective, cushioning, and lubricating effect on the nerve roots and dura mater. These structures can be clearly displayed under the endoscope. Excessive damage to them can cause problems such as dural degeneration, scar proliferation, adhesion, etc., which may probably affect postoperative recovery. Many of these structures have a submillimeter size. The magnifying effect of the endoscope allows us to clearly distinguish and protect them.

It should be emphasized that the presence of ligaments in all intervertebral foramina is not always guaranteed, but if present, the probability of all ligaments being present in all intervertebral foramina is approximately 47% [5]. The most common is the superior corporotransverse ligaments (27%), and the inferior corporotransverse ligaments connect two independent bones. Many studies believe that the anatomical position of the ligaments occupies the space of the intervertebral foramen, which is the main cause of nerve root compression and radiating pain. These ligaments can reduce the cross-sectional area of the intervertebral foramen by 30% [6,7]. However, there are also different opinions. Kuofi mapped the spatial position of the ligaments [8] and believed that these ligaments are continuously present in the intervertebral foramen, and their presence not only does not cause nerve compression or radiating pain, but also has a protective effect on the blood vessels and nerves in the intervertebral foramen [5,9]. This study found that the probability of the presence of ligaments (inferior corporotransverse ligaments) was highest in the lower intervertebral foramen (89%), followed by the superior corporotransverse ligaments (84%) and the middle corporotransverse ligaments (71%). These ligaments should be preserved as much as possible and generally do not affect microscopic operations. It should be stressed that the anatomical structures discovered in this study have some anatomical differences in occurrence rates, positions, sizes, and branching situations. However, these anatomical differences have a relatively low incidence and do not affect the clinical reference value of this study.

4. Conclusion

This study successfully explored the occurrence of specific structures under lumbar endoscopy to guide precise intraoperative operations. Secondly, after the structure is confirmed, it is necessary to transition from structural research to functional research, and there is still a lot of work to be done. Thirdly, understanding these endoscopic structures and their functions will promote further development of minimally invasive spine technology.