A Study of Unilateral Biportal Endoscopic Discectomy and Transforaminal Endoscopic Discectomy for Single-level Calcified Lumbar Disc Herniation

Calcified lumbar disc herniation (CLDH) is a degenerative disease of lumbar disc herniation caused by precipitation or conversion of substances in the body into calcium carbonate or other insoluble calcium salt complexes. It often occurs in patients with long-term conservative treatment of lumbar disc herniation, but the specific cause of the formation is not clear^1,2. The calcified protruding disc can occupy the space of the spinal canal and lateral recess, resulting in or aggravating lumbar pain and neurological symptoms of the lower extremities. Due to the hard texture of calcification and poor elasticity, it often produces persistent neurological symptoms after compressing the nerve root, which usually requires surgical treatment.

In the past, it was considered that it was difficult to deal with CLDH under endoscope and remove it completely, so open surgery was often used to remove it completely under direct vision³. However, the traditional posterior lumbar interbody fusion not only causes great muscle injury and bleeding, but also affects the stability and range of motion of the spine. With the development of spinal endoscopy technology and tools, CLDH is no longer a relative taboo in minimally invasive surgery^4,5.

Percutaneous endoscopic transforaminal discectomy (PETD) is the direct percutaneous discectomy of herniated intervertebral disc tissue through the intervertebral foramen under uniaxial endoscope to complete nerve decompression. With the application of endoscopic grinding drill, it is also gradually used in CLDH^6,7,8 . However, the learning curve of PETD is steep, and the operating space is narrow, which may lead to nerve root injury and dural tear, which is challenging for surgeon⁷.

Unilateral biportal endoscopic (UBE) discectomy has been especially popular in Asia recently because of its convenient access to instruments and simple operation⁹ . It shows a good curative effect in the aspects of lumbar disc herniation, lumbar spinal stenosis, and so on^10,11. However, there are a few reports about the application of UBE discectomy in the treatment of CLDH, and few studies have compared the difference of CLDH between PETD and UBE discectomy¹², so this study compared the efficacy and safety of PETD and UBE discectomy in the treatment of single-level CLDH.

This study was approved by the Ethics Committee of the Third Hospital of Hebei Medical University (K-2024-014-1). All patients' data collection was informed in advance, and informed consent was obtained from all subjects.

Study population

Patients with single-level CLDH were included based on the following criteria: 1) diagnosis confirmed by lumbar intervertebral disc CT; 2) presentation of typical symptoms, including low back pain with lower limb radicular pain, numbness, or muscle weakness; 3) failure to improve after a minimum of 3 months of regular conservative therapy. Exclusion criteria were defined as: 1) a follow-up duration of less than 12 months; 2) incomplete follow-up data; 3) a history of prior lumbar spine surgery; 4) presence of segmental instability; 5) diagnosis of spinal tumor, infection, or fracture. All surgical procedures were performed by two senior attending surgeons.

Clinical data and efficacy evaluation

Medical records were reviewed to collect demographic and perioperative data, including age, sex, body mass index (BMI), operative segment, operative time, length of hospital stay, estimated blood loss, incision length, and intraoperative fluoroscopy frequency. Preoperative imaging (X-ray, CT, MRI) and postoperative X-rays were reviewed. Based on preoperative disc CT, patients were classified into three morphological subtypes of CLDH: solitary type (calcification < 3 mm), semilunar type (calcification 3-10 mm), and continuous type (calcification > 10 mm)⁴. Clinical efficacy was assessed using the visual analogue scale (VAS) for low back and leg pain and the Oswestry Disability Index (ODI). These scores, along with records of complications, were collected preoperatively and at 1 day, 3 months, 6 months, and 12 months postoperatively. Modified MacNab criteria at the last follow-up were compared between the two groups.

Unilateral Biportal Endoscopic Discectomy

Anesthesia and patient positioning

General anesthesia with endotracheal intubation was administered. After successful anesthesia induction, the patient was placed in a prone position. Soft pillows were placed under the chest and bilateral iliac spines to ensure the abdomen was free, reducing intra-abdominal pressure and epidural venous bleeding during surgery. The operating table was adjusted to slightly flex the lumbar spine, helping to open the interlaminar space.

Preoperative localization and marking

The lamina on the side of the approach was located with a Kirschner needle (Figure 1A). Syringe needles were used to simulate the positions of the working channel and the observation channel, and the directions of the channels were observed under X-ray fluoroscopy ( Figure 1B,C). Two longitudinal incisions, approximately 1.5 cm each, were marked approximately 1.5-2.0 cm lateral to the midline (on the symptomatic side) at the level of the target disc space.

Establishing operational portals

The skin and superficial fascia were incised sequentially with a No. 11 scalpel. Subcutaneous tissue and the thoracolumbar fascia were bluntly dissected using haemostatic forceps. A series of muscle dilators were used to bluntly separate the paraspinal muscles along the direction of the interlaminar space until the lower edge of the lamina and the medial margin of the facet joint were exposed. The working cannula was inserted, and its position was secured.

Endoscopic procedure

A 30° or 0° arthroscope was introduced through the viewing portal cannula. Continuous saline irrigation was maintained for clear visualization and haemostasis. A radiofrequency probe was introduced through the working channel to clear soft tissue over the lamina and ligamentum flavum, exposing the surgical field. A high-speed drill or Kerrison rongeurs was used to perform a partial laminectomy and medial partial facetectomy of the responsible segment for decompression. The thickened ligamentum flavum was removed using punch or Kerrison rongeurs to expose the thecal sac and traversing nerve root. The nerve root was carefully identified and protected, and was gently retracted medially using a nerve retractor. The herniated calcified disc material was explored and exposed (Figure 1D). The posterior longitudinal ligament and annulus fibrosus were circumferentially incised. Disc forceps, pituitary rongeurs, or curettes were used to remove the calcified nucleus pulposus in pieces. For large or severely adherent calcifications, a micro-drill bit was used to break them down before removal. The ventral side, shoulder, and axilla of the nerve root were carefully explored to ensure no residual fragments were causing compression and that the nerve root was well relaxed and pulsating. Haemostasis was achieved thoroughly using the radiofrequency probe.

Wound closure

The endoscope and instruments were withdrawn. A negative pressure drain was placed through the working channel. The deep fascia was sutured with absorbable suture, and the skin incision was closed with suture and covered with a sterile dressing.

Percutaneous endoscopic transforaminal discectomy

Anesthesia and patient positioning

Local infiltration anesthesia (1% lidocaine) supplemented with sedation and analgesia was typically used. The patient was placed in a prone or lateral decubitus position (symptomatic side up). In the lateral position, the hips and knees were flexed, and a pillow was placed under the waist to increase the foraminal volume.

Puncture and localization

The surface projection of the target intervertebral space was marked under AP and lateral C-arm fluoroscopy. The skin entry point was calculated based on the "Yeung's safety triangle" principle, typically 8-14 cm lateral to the midline and at a 15-30° angle to the sagittal plane. After local anesthesia, an 18 G puncture needle was inserted. Under lateral fluoroscopy, the needle tip was ultimately located in the posterior third of the target disc space; under AP fluoroscopy, it was on the mid-pedicular line.

Foraminoplasty and cannula placement

Once the needle was correctly positioned, a guidewire was inserted, and the needle was withdrawn. A series of dilating tubes were sequentially rotated over the guidewire. If the patient had a narrow foramen or a large calcification, foraminoplasty was required. A smaller diameter protective cannula was placed near the foramen. Using a trephine, drill, or endoscopic forceps through the protective cannula, part of the ventral aspect of the superior articular process was removed to enlarge the foramen. After foraminoplasty, the final working cannula was placed (Figure 2A,B).

Endoscopic decompression

An endoscope system was introduced through the working cannula (Figure 2C). The annulus fibrosus was incised using a scalpel. Hard calcifications were broken down using an endoscopic drill. The herniated nucleus pulposus was gradually removed using Disc forceps, pituitary rongeurs, or curettes. The key step was adequate decompression of the nerve root. The endoscope angle was adjusted to carefully explore the shoulder, axilla, and ventral side of the nerve root, ensuring all compressive material was removed and the nerve was completely released (Figure 2D). Haemostasis was achieved with radiofrequency.

Completion of surgery

The endoscope was slowly withdrawn, checking for any active bleeding. The incision usually required only a single suture or was directly covered with a sterile adhesive bandage.

Postoperative

After the operation, patients were given mannitol and dexamethasone to relieve neuroedema. Ground movement was encouraged on the first day after the operation in the PETD group, and on the second day after the drainage tube was removed in the UBE group.

Data analysis

Statistical analyses were performed using SPSS software (version 26.0). Continuous data were compared between the two groups using independent t-tests or Mann-Whitney U tests. Categorical data are presented as numbers (percentages) and were compared using the chi-square test or Fisher's exact test. VAS scores and ODI scores were compared between the two groups using repeated-measures analysis of variance (ANOVA). All pairwise comparisons were adjusted using the Sidak method. The modified MacNab criteria were compared using the Mann-Whitney U test. A two-sided P value < 0.05 was considered statistically significant.

Apart from three cases of loss to follow-up, a total of 107 patients were enrolled in the study, including 45 patients with UBE and 62 patients with PETD. Table 1 shows that there is no significant difference in general information between the two groups, including sex, age, BMI, surgical segment, type of calcification, and so on (P ＞ 0.05). Table 2 shows that the amount of intraoperative blood loss, operation time, length of hospital stay, and surgical incision length in the PETD group are less than those in the UBE group, but the number of intraoperative radiation in the PETD group is higher than that in the UBE group (P ＜ 0.05). Figure 3 and Figure 4 illustrate typical cases in the UBE and PETD groups.

There was no significant difference in preoperative ODI score and VAS score of leg pain between the two groups (P＞ 0.05). The postoperative VAS and ODI scores of the two groups were significantly lower than those before the operation (P ＜ 0.05). The ODI score and VAS score of leg pain in each follow-up period after the operation were similar between the two groups (P > 0.05).

There were no serious complications related to the operation in both groups. Two patients (4.44%) in the UBE group had dural tears during the removal of calcified intervertebral discs. In the PETD group, one patient (1.61%) developed transient ipsilateral nerve root paralysis and limb numbness, which was gradually relieved after conservative treatment. On the basis of the modified MacNab criteria at the last follow-up, no significant difference was found between the two groups (P ＞ 0.05).

DATA AVAILABILITY

The data for metabolite expression profiles were uploaded to Zenodo (https://zenodo.org /records/18048510).

Figure 1: Intraoperative channel establishment and endoscopic images of UBE. (A,B,C) Surgical localization of L4-5. (D) Intraoperative endoscopic image showing a calcified intervertebral disc. Please click here to view a larger version of this figure.

Figure 2: Intraoperative cannula establishment and endoscopic images of PETD. (A,B) Positive and lateral X-ray images of the working cannula. (C) Intraoperative image of PETD to remove the calcified lumbar disc herniation. (D) Endoscopic image showing decompression of the traversing nerve root. Please click here to view a larger version of this figure.

Figure 3: A 20-year-old male patient with L5-S1 CLDH treated with PETD. (A) Preoperative MRI image. (B,C, D) Preoperative CT image showed a calcified lumbar disc herniation before the operation. (E,F,G) Postoperative CT and MRI showed that the calcifications were resected.(H) Postoperative sagittal CT scan showed the extent of L5-S1 intervertebral foramen formation. Please click here to view a larger version of this figure.

Figure 4: A 38-year-old male patient with L4-5 CLDH treated with UBE discectomy. (A) Preoperative MRI image. (B,C,D) Preoperative CT image showed a calcified lumbar disc herniation before the operation. (E,F,G) Postoperative CT and MRI showed that major calcifications were resected. (H) Postoperative sagittal CT scan showed the extent of L4-5 laminectomy. Please click here to view a larger version of this figure.

Table 1: General information in this study.

Table 2: Peri-operative outcome in this study.

In our study, the quality of life outcomes of patients showed that both UBE and PETD groups had a significant decrease in ODI and VAS scores after operation, which could improve the symptoms of CLDH patients. Compared with the UBE group, the PETD group showed less bleeding, shorter operation time, smaller incision, and shorter hospital stay, but more fluoroscopy times were needed during the operation. No obvious lumbar instability was observed in both groups one year after the operation.

CLDH is a specific form of disc herniation with unique surgical challenges. While posterior lumbar interbody fusion allows direct visualization and removal of calcified fragments, it carries risks of significant bleeding, tissue injury, and potential spinal instability^5,13. The advancement of spinal endoscopic techniques and instruments has made minimally invasive surgery a viable option for CLDH^8,14.

Foraminal endoscopy is a common minimally invasive technique widely used for spinal degenerative diseases. Compared with traditional open surgery, it causes less muscle and lamina damage, and results in less intraoperative blood loss and shorter hospital stays¹⁵. In treating CLDH, PETD has demonstrated safety, effectiveness, and a low complication rate^4,8. However, PETD requires specialized surgical instruments. When treating L5-S1 CLDH in patients with a high iliac crest, it is necessary to adjust the skin entry point medially, increase the angle between the working cannula and the coronal axis, and remove more of the superior articular process and the pedicle bone of the inferior vertebral body during foraminoplasty^16,17,18.

UBE discectomy is a recently developed endoscopic technique that is operated by arthroscopy, and the instruments are easy to obtain. Moreover, under the dual channels, the lens and the operation hole are separated, and the field of vision and operation range are wider. UBE discectomy from the rear approach is closer to the operation habit of the traditional lumbar posterior open surgery, and the learning curve is flatter¹⁹ . In addition, UBE discectomy does not rely too much on specialized surgical instruments, but can use open lumbar posterior surgical instruments.

In this study, the PETD group has more advantages in operation time, hospital stay, blood loss, and incision length. At present, the main mode of anesthesia in UBE discectomy is general anesthesia by endotracheal intubation, while local anesthesia is often used in PETD. Therefore, compared with PETD, UBE surgery needs more preoperative examination to ensure the safety of anesthesia and operation²⁰ . In addition to the time required for drainage tube removal and recovery, the difference in hospital stay between the UBE group and the PETD group in this study is also related to the need for more extensive examinations. UBE discectomy requires lamina fenestration to break the ligamentum flavum and enter the spinal canal to deal with the calcified intervertebral disc, which is not as straightforward as the PETD, so the operation takes a long time. In this study, the total operation time of the UBE group was slightly longer, but the efficiency of removing calcified intervertebral disc in the UBE group was significantly higher than that of the PETD group. This is because UBE discectomy can better expose CLDH, have a wider field of vision, and use efficient surgical instruments to remove calcified intervertebral disc. However, the field of vision of PETD is limited, and the tools are slightly inefficient.

In the process of creating surgical space, the extent of muscle and soft tissue peeling injury and lamina decompression in the UBE group was significantly wider than that in the PETD group, which resulted in more blood loss. Compared with other studies on the blood loss of LDH treated with UBE and PETD, the blood loss of the two groups in our study was slightly higher, which may be related to the larger decompression and longer operation time in the treatment of calcification²¹.

In this study, the VAS score of low back pain in the UBE group was significantly higher than that in the PETD group 1 day after the operation, which may also be related to a wider extent of muscle and soft tissue injury in the process of UBE cavitation. However, during the postoperative follow-up, the symptoms of low back pain could be relieved in both groups, and there was no statistical difference in the VAS of medium-and long-term low back pain. Both UBE discectomy and PETD achieved satisfactory decompression effects for lower limb radicular pain symptoms. Furthermore, the ODI assessment results further confirmed that both surgical techniques facilitate a substantial recovery in patients' daily living activities. For single-level calcified lumbar disc herniation, both UBE and PETD can provide stable and favorable mid- to long-term functional outcomes.

However, for large and continuous central CLDH, because UBE discectomy enters the spinal canal after decompression from the dorsal side of the lumbar spine, it is difficult to remove calcification completely. Although a reverse curette is used to decompress the contralateral calcified intervertebral disc, only the midline and part of the contralateral intervertebral disc can be removed, but the calcification near the contralateral intervertebral foramen is difficult to reach. On the other hand, by reducing the angle to the coronal axis and shaping the articular process on the lower vertebral body, the intervertebral foramen endoscope can fully adjust the angle of the channel through the intervertebral foramen. Reduce the tension of the nerve root, increase the visual field and operable range of the foraminoscope, and make it more convenient to deal with the nerve compression at the top of the continuous central CLDH and the contralateral side. It is similar to the rod adjustment technique and apex technique that we used in simple LDH in the past²². In the case of central CLDH, we adjusted the position of the guide rod from the bottom of the protruding nucleus pulposus to the top of the protruding nucleus pulposus by the rod adjustment technique, and gradually completed the bilateral decompression under the intervertebral foramina.

Dural tear is one of the intraoperative complications of UBE discectomy, especially in the case of this study, the adhesion between the calcified intervertebral disc and the dura often occurs, which can easily cause dural tears when separating the adhesion^23,24. Small dural tears can be cured by bed rest and conservative treatment, but larger tears can only be repaired by open surgery^25,26 . There were 2 cases of dural tear in this study. Clear exposure and avoiding violent stretching of the adhesion tissue are the key to reducing the tear of the dura mater. Secondly, if the calcification is heavily adhered to the dura mater, it is not necessary to completely peel off the calcification, just remove the main factors of oppression. In PETD, when the main factor of compression is the herniated intervertebral disc soft tissue, the removal range of calcification can be determined by the relief of symptoms during the operation, so as to reduce the damage to the dura mater during the peeling of calcification. For patients with foraminal stenosis complicated with CLDH, blind foramen shaping tools should be avoided. Endoscopic tools such as a visual ring saw and a grinding drill can reduce the probability of nerve root injury to a certain extent.

Our study also has some limitations, as it is a retrospective study with a short follow-up time and a small sample size, which may have failed to capture long-term complications, such as recurrence of intervertebral disc herniation. We need to conduct more prospective studies on sample size in the future.