Discussion
The incidence of LDH is greater than 90% in L4-5 and L5-S1 intervertebral disks, because they are located at the junction of the spine and pelvis and more prone to degenerative changes and injuries than the other lumbar disks [
6]. L5-S1 LDH tends to be severe in clinical presentation [
7], and surgery is a choice when conservative treatment is ineffective. Traditional open surgeries, such as total laminectomy and lamina fenestration, have a wide field of view and result thorough decompression, but the incidence of spinal structural injury and symptomatic epidural scarring has been reported in the literature to exceed 10% [
8]. PETD is a more classic minimally invasive treatment for LDH. However, the L5-S1 segment has peculiar anatomical features in which the transverse process space is generally narrow, the L5 vertebral arch isthmus is more lateral than that of the upper lumbar spine, and the height of the intervertebral foramen is minimal in the lumbar range [
9]; additionally, other factors, such as a high iliac crest and articular hyperplasia osteophytes, make the PETD operation challenging [
10]. In recent years, some surgeons have treated L5-S1 LDH with modified PELD methods, achieving satisfactory results [
3], but some scholars are concerned that bony manipulation of the intervertebral foramen may increase the risk of lumbar instability after surgery [
11]. The posterior anatomy of the spinal canal of the L5-S1 segment is simple; the laminar space is large, the S1 nerve root runs almost vertically, and there is only the sacral plexus nerve in the horizontal dural sac, providing a good anatomical basis for the treatment of L5-S1 LDH with the UBE technique. As an emerging minimally invasive technique in recent years, the UBE technique has many advantages in the treatment of LDH, such as wide surgical vision, flexible operation, high efficiency and satisfactory clinical efficacy [
12,
13].
The OSE technique, similar to the UBE technique, has working and observation channels that can be accessed through interlaminar operations, and many scholars in China have noted its advantages when applied to clinical operations. In this study, intraoperative blood loss, incision length, and bone resection area in the OSE group were smaller than those in the UBE group, and all the patients could move the day after surgery if there were no special circumstances, thereby reducing the incidence of postoperative bedding-related complications. Excessive radiation can cause varying degrees of damage to both patients and surgeons. Ann concluded that without radiation shielding, surgeons who perform PELD 291 times per year would be exposed to the maximum permissible radiation dose [
14]. In this study, the fluoroscopy times were low in both the UBE group and the OSE group, and the amount of radiation was greatly reduced. Both groups had significant improvement in VAS and ODI, and the excellent-good rates were high, indicating that the clinical efficacy of the OSE technique is comparable to that of the UBE technique. A finite element model study showed that the spinal range of motion, facet joint load, and intervertebral disk pressure increased with 30% facial resection [
15]. In this study, the destruction of the facet joint was minimized during surgery. The facet resection rates in both groups were significantly less than 30%, and there was no statistical significance in the lumbar ROM and ST before and after surgery, which demonstrated that the two surgical methods protected the stability of the lumbar spine. Height loss of the intervertebral space is one of the most common radiographic findings of lumbar disk degeneration, with the intervertebral disk space narrowing progressively at an annual rate of 3.2% in women, with an average age of 50 years [
16]. In this study, the change of DH before and after surgery indicated that both techniques may have a certain impact on lumbar segment degeneration.
Most scholars believe that imaging examination showing herniated disk tissue in excess of 50% of the sagittal diameter of the spinal canal can be considered giant LDH [
17]. This type of herniation is large and is often combined with an inflammatory response; moreover, it easily adheres to the ligament flavum, so that the treatment by endoscopic surgery is challenging. Moreover, this type of herniation often causes severe low back and leg pain, and some patients experience bilateral neurological symptoms and even cauda equina signs [
18]. It has been reported that the surgical failure rate of central LDH with giant herniation is high (15%), and percutaneous endoscopic techniques should be carefully chosen for LDH that protrudes into more than 50% of the spinal canal area [
19]. In this study, there were 21 cases of giant LDH in the UBE group and 27 cases in the OSE group, all of which were thoroughly decompressed. For giant herniations, sneak decompression is performed to remove part of the ligamentum flavum, clean up the surrounding tissue of the herniation, and leave enough space to address the herniation, which can reduce the risk of nerve root or dural injury. In this study, patients with bilateral symptoms underwent “over the top” decompression treatment, and the postoperative results were satisfactory.
Migrated LDH is a more serious type of LDH and is reported to account for approximately 35–72% of LDH cases [
20]. The general clinical symptoms of migrated LDH are severe, and this type is often combined with nerve root function damage or abnormalities, therefore surgery is often required when conservative treatment is not effective [
1]. Lee et al. divided the prolapse site into four zones: the dissociation of the nucleus pulposus to zones two and three was called the low-grade migration type, and zones one and four were called the high-grade migration type [
21]. According to the literature statistics, the high-grade migration type accounts for approximately 30% of patients with migrated LDH [
22]. The UBE technique has significant clinical efficacy and is a flexible operation in the treatment of migrated LDH [
9,
12]. The failure rate of PELD in high-grade migrants was reported to be as high as 15.7% [
19]. We have reported that OSE has been used for different types of migrated LDH, with satisfactory clinical efficacy [
23]. In this study, there were 31 cases of LDH migration in the UBE group, including 12 cases of high-grade migration type, and 22 cases in the OSE group, including 6 cases of high-grade migration type. The free nucleus pulposus tissue of all cases was completely removed during the operation, and the postoperative clinical effect was excellent, with no recurrence during the follow-up period.
The incidence of calcific lumbar disk herniation (CLDH) is approximately 4.7–15.9% [
24]. PETD has some deficiencies in the treatment of L5-S1 CLDH, in which the adhesion between calcification and nerve roots or the dura mater not only increases the difficulty of PETD surgery but also may lead to iatrogenic injury [
25]. Yu reported that 25 patients with CLDH were treated with PETD; the patients’ symptoms were relieved, but seven patients had postoperative dysesthesia, and one patient experienced relapse [
26]. In this study, in cases of CLDH, the UBE technique and OSE technique could fully reveal and remove the calcification that compressed the nerve root from behind, isolate the nerve root, and release the stenosis caused by peripheral degeneration, with clear surgical vision, extensive exploration range, complete decompression and little postoperative nerve root injury.
Postoperative burning radicular pain or dysesthesia is a common complication after lumbar spine surgery; it is mostly transient and generally believed that it may be related to nerve root adhesion, excessive traction or compression during the surgery, or obvious hyperemia and edema of nerve roots with excessive use of bipolar radiofrequency knife. In this study, the incidence of postoperative transient hypoesthesia in the UBE group and the OSE group was 1.43% and 1.58% respectively, and the symptoms disappeared after nutritional neurological treatment, considering the possible adhesion to the nerve root. Incomplete decompression is mostly caused by deviation in preoperative judgment or deviation in the range of intraoperative decompression; it is characterized by persistent low back and leg pain and other related symptoms after surgery, which is a common reason for poor postoperative effects and affects patient satisfaction [
19]. Choi et al. retrospectively analyzed 10,228 patients with LDH who underwent intervertebral foraminal surgery and found that 283 patients had decompression insufficiency [
27]. There were no cases of decompression insufficiency or relapse at postoperative follow-up in this study. We think that it is important to plan reasonably, select the appropriate surgical method, and perform bilateral lateral crypt decompression if necessary. In addition, perioperative education and postoperative rehabilitation guidance are also very important to avoid recurrence. Dural tear or nerve injury is a common complication in endoscopic surgery [
28]. In this study, there were no cases of dural injury in the OSE group, and there was a mild tear of the dura mater in one patient with giant LDH in the UBE group, which was considered that the large protrusion squeezed the dura chronically and made the membranous vertebral ligament adhere to the ligament flavum. Compared with the UBE technique, the microscopic field of view in the OSE technique did not have a "V" shaped angle, and the reduction of visual error may reduce the risk of dural injury.
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