The aim of surgical treatment for a lumbar disc herniation is sufficient decompression and minimizing operation-induced trauma. A faster recovery is needed in modern society, allowing patients to resume their normal activities sooner. Technical developments in the past decades have made treatment of herniated discs safer and less invasive. By using microsurgical or microendoscopic techniques through small incisions, nerve root decompression is performed with minimal complication risk and preserves normal anatomy.
Endoscopic lumbar discectomy (ED) is growing in popularity for treating disc herniation, and the range of indications has expanded with advances in instruments including endoscopes. ED has several theoretical advantages over conventional open surgery. Most current percutaneous endoscopic discectomy techniques are based on the Kambin's transforaminal approach and offer favorable outcomes for soft disc herniation. The most important point of successful endoscopic treatment is an accurate approach for the proper indication.
Transforaminal endoscopic discectomy has many advantages over conventional open discectomy such as shorter hospital stay and less postoperative pain medication [11-14
]. Transforaminal endoscopic discectomy is not indicated for patients with severe spinal stenosis, cauda equina syndrome, or disc herniations with highly migrated sequestrated fragments. But, operating on a disc herniation between the fifth lumbar and first sacral segment using an endoscopic technique in individuals with a high iliac crest may be difficult. Sometimes endoscopic discectomy may not be performed because of the anatomical relationship between the foramen and the root and root irritation [15
The transforaminal endoscopic discectomy approach begins at the skin entry point at the posterolateral aspect of the lower back (8 to 14 cm from the midline) and proceeds through the critical triangular window of the intervertebral foramen, which is bordered superolaterally by the exiting nerve root, posteriorly by the superior articular process of the facet joint, and caudally by the inferior pedicle. During this procedure, the patient should be kept continuously awake under local anesthesia and instructed to report any pain, numbness, or electrical shock sensations to prevent nerve injuries.
The available angle to reach around the superior boundary of the foramen is much larger than that around the inferior boundary of the foramen. Therefore, this anatomical characteristic allows endoscopic instruments a sufficient angle to pass through the foramen into the upper portion of the spinal canal and easily remove cephalad-migrated disk material. In contrast, the inferior boundary of the foramen limits reaching the spinal canal below the disk level in cases of a caudal-migrated disk herniation. Over-manipulation of spinal canal content and compression of the exiting root and ganglia by a horizontally positioned cannula may cause neural injury, as well as postoperative pseudocausalgic pain in the index extremity. The surgical approach has evolved from the traditional posterolateral approach into a transforaminal approach. Thus, confirmation of the dimensions of the working zone appropriate for the transforaminal approach is required [10
Min et al. [10
] concluded that the mean diameter of the working zone base was 11.6 ± 4.6 mm and this value increases going down the level of the spine. In the same study performed on cadaveric spines, the L2-L3, L3-L4, and L4-L5 intervertebral foramens were similar in multiple comparison tests. Similarly, in the current study, foraminal area of the right L3-L4 foramen was 140.07 ± 38.19 mm2
and was 137.94 ± 35.59 mm2
on the left. The foraminal area of the right L4-L5 foramen was 121.00 ± 32.88 mm2
and was 121.89 ± 32.42 mm2
on the left. In our study, comparisons between both sides were performed, and no significant difference was observed between these parameters. So, there would be no technical difference when performing ED at these levels.
The proportion of root area to foraminal area can be used to predict the efficacy of the endoscopic discectomy procedure for each particular case. This can be easily performed by assessing a preoperative MRI.
The working cannula should be inserted into the foramen as close as possible to the facet joint and not directly target the disc. Ahn et al. [16
] and Min et al. [10
] have confirmed this method.
The superior and inferior borders of the working zone are formed by a line from the posterior margin of the spinal nerve root to the superior articular process parallel to the inferior and superior endplate. The anterior border is formed by the exiting root and is the oblique side. In a study performed on cadaveric spines by Min et al. [10
], the average angle between the anterior border and the base was 79.1° ± 7.6°. This value decreased going down the spine level. In that study, they analyzed the working zone dimensions at the lateral exit zone of the intervertebral foramen, but the working zone is actually a three-dimensional structure. That study was limited, because they analyzed the working zone in only two dimensions. Additionally, deformation of the spinal nerve roots in cadaveric spines may have had a negative effect. In our study, the root angles were calculated in both axial and coronal planes. Although a significant difference was observed in the coronal L3 root angles and axial L4 root angles of the right and left sides, no difference was found in the root angles at these two disc levels. In contrast to cadaveric studies [9,10
], the values we found were small. But, our study showed no significant differences between individuals that may create technical difficulties for a transforaminal endoscopic lumbar discectomy.
Transforaminal endoscopic lumbar discectomy may be used for upper lumbar disc herniations [17
]. A radiological analysis of the upper lumbar region before the operation may be useful to perform a safer procedure.
One of the limitations of this study was the absence of intraoperative data. A correlation between preoperative MRI measurements and intraoperative data would be more valuable. Although the current study was a radiological study, the data given here may help surgeons plan the procedure by evaluating endoscope diameter and foraminal and root area.