A Novel Paraspinal Approach With Unilateral Biportal Endoscopy for Hidden Zone Lumbar Disc Herniations: A Technical Note and Outcomes

Article information

J Minim Invasive Spine Surg Tech. 2025;10(1):45-51

Publication date (electronic) : 2025 April 30

doi : https://doi.org/10.21182/jmisst.2025.02040

Malcolm Darayes Pestonji, Sharvari Gunjotikar

Golden Park Hospital and Endoscopic Spine Foundation India, Vasai, India

Corresponding Author: Malcolm Darayes Pestonji Golden Park Hospital and Endoscopic Spine Foundation India, Sai nagar, Vasai West, India Email: malcolmpestonji@yahoo.com

Received 2025 January 7; Revised 2025 February 16; Accepted 2025 February 17.

Abstract

INTRODUCTION

Lumbar disc herniations are broadly classified based on their axial location into 3 categories: central, para-central, foraminal and far lateral [1]. Apart from the more commonly seen para-central disc herniation, different patterns of disc herniation have been described based on the direction of migration of the sequestrated fragment and the level of the pedicles at the involved level [2]. The lateral lumbar spinal canal can be divided into several regions: the subarticular (lateral recess), foraminal (pedicle), and extraforaminal (far lateral) zones. Within these areas lies the “hidden zone” of MacNab known for its challenging surgical access (Figure 1) [3]. The disc herniations in the hidden zone imply those that typically lie in the area behind the posterior wall of the cranial vertebra just medial to the medial wall of the pedicle [2,3]. It has been estimated that approximately 10%–20% of all disc herniations migrate in a cranio-lateral direction, potentially placing them in the preforaminal and foraminal regions of the "hidden zone." Due to local anatomy, these lesions can affect both the traversing and exiting nerve roots [4]. Surgical treatment options for upmigrated lumbar disc herniation in the hidden zone of MacNab include conventional open laminotomy and discectomy, microscope-assisted tubular discectomy, full-endoscopic and biportal translaminar discectomy and transforaminal endoscopic discectomy. Standard inter-laminar approach necessitates hemilaminotomy or hemilaminectomy of the cranial vertebra and this often includes partial or complete removal of facet joints, increasing the risk of lumbar segmental instability [4-6]. Multiple reports have described a more direct approach to the herniated disc via fenestration of the cranial lamina [7,8]. However, there are concerns about the practicality of these techniques at higher lumbar levels due to the potential for iatrogenic injury to the pars interarticularis and the risk of developing instability after surgery. In the recent years, various techniques of endoscopic spine surgery, particularly unilateral biportal endoscopy (UBE) have become popular due to its several advantages such as reduced soft tissue trauma, minimal blood loss, minimal bony resection and excellent visualization [9]. In this article, the authors describe a novel paraspinal approach with UBE for surgical treatment of lumbar disc herniations in the hidden zone of MacNab and present their postoperative outcomes.

Figure 1.

The red arrow points to the hidden zone of MacNab. The hidden zone (in red), which is bordered laterally by the pedicle, ventrally by the dorsal aspect of the vertebral body, and dorsally by the pars interarticularis of the hemilamina. The hidden zone includes both the preforaminal (in yellow) and the foraminal zone (in green).

MATERIALS AND METHODS

We retrospectively reviewed hospital records for patients who underwent paraspinal UBE for up migrated lumbar disc herniations in the “hidden zone” of MacNab between January 2022 to January 2023. All patients were operated by a single surgeon (MP) and hospital records were evaluated by 2 spine consultants (MP and SG). Institutional Review Board approval was obtained prior to conducting this study. After reviewing the records, the following patients were included: 1. Patients who had symptomatic disc herniations in the hidden zone of MacNab confirmed on magnetic resonance imaging (MRI) evaluation. 2. Patient having type 1 and type 2 migrated disc herniation as classified by Lee et al. [10] (Figure 2). 3. Patients who failed to have relief of symptoms following a trial of conservative treatment for a period of 6 weeks. 4. Patients with history of prior surgical intervention at the level of upmigrated disc herniation. The authors excluded the following patients: 1. Type 3 and type 4 disc herniations classified as per Lee’s classification [10]. 2. Patients who underwent surgical treatment options other than paraspinal UBE approach. 3. Patients having segmental spinal instability as confirmed by dynamic x-rays. 4. Patients with central or lateral recess stenosis on MRI.

Figure 2.

Lee’s classification of migrated disc herniations: Zone 1: from the inferior margin of the upper pedicle to 3 mm below the inferior margin of the upper pedicle (far-upward). Zone 2: from 3 mm below the inferior margin of the upper pedicle to the inferior margin of the upper vertebral body (near-upward). Zone 3: from the superior margin of the lower vertebral body to the center of the lower pedicle (near-downward). Zone 4: from the center to the inferior margin of the lower pedicle (far-downward).

1. Surgical Procedure (Supplementary video clip 1)

All surgical procedures were performed with the patient positioned prone over a radiolucent Wilson’s under general anesthesia. The authors prefer to use a Wilsons frame in order to achieve appropriate reversal of the lumbar lordosis which aids in achieving maximal opening up of the neural foramen. After the spinal level of interest was identified under fluoroscopic guidance, the viewing and instrument portals were created using stab incisions according to the fluoroscopy guided markings illustrated in Figure 3. The area of the pars interarticularis just below the pedicle of the cranial vertebra is chosen as the starting point of the surgery. From here on, a 90° plasma radio frequency probe (Endovision UBE pro) is used to strip of the paraspinal muscles from dorsal surface of the lamina caudal to the starting point in order to create the initial working space for the procedure. A limited resection of 2–3 mm of the lateral border of the pars interarticularis is performed using a 3-mm diamond burr. After identification of the transverse process and the inferior articular process (IAP), further dissection is carried out along the facet joint. At this point, the neuromuscular bundle comprising of the dorsal branch of the primary segmental artery and the artery supplying the multifidus and the other paraspinal muscles were frequently encountered. After achieving haemostasis with the help of the plasma radiofrequency probe, the intertransverse ligament was released. The inferior corner of the cranial pedicle and the tip of the superior articular process (SAP) was identified with a blunt ball tipped nerve hook. At this point, the extension of the ligamentum flavum and its attachment to the SAP were easily identified. Above this flavum, and parallel to the tip of the SAP, the artery to the exiting nerve root was visualized and coagulated. The release of the ligamentum flavum, along with the transforaminal ligament complex embedded within it, was performed to open up the foraminal canal around the tip of the SAP. Upon opening the exiting nerve root canal, further descent into Kambin triangle and onto the disc surface was possible. Osteotomy of the SAP was performed if the disc height was collapsed and there was difficulty in entering Kambin triangle. The extent of osteotomy was limited to less than 1/3rd of the SAP after confirmation with fluoroscopy. In most cases, the traversing nerve root could be visualized after removal of the sequestrated disc fragment which was causing the axilla to be elevated. During the initial phase of the surgery, a 120° ball-tip nerve hook was utilized to access the superior vertebral body surface to mobilize and tease of disc fragments into Kambin triangle. Subsequently, an angled pituitary rongeur was used to extract the remaining fragments. Following this, thorough inspection was done to identify and remove any remaining disc fragments before concluding the surgery. After achieving homeostasis, surgical wounds were sutured and the sutures were removed after 10–12 days. The surgical steps are outlined in Figure 4.

Figure 3.

Magnetic resonance imaging (MRI) and intraoperative fluoroscopic images of 54/F with left L3–4 zone 1 upmigrated lumbar disc herniation. (A) T2-weighted sagittal MRI showing L3–4 zone 1 upmigrated lumbar disc herniation. (B) T2-weighted axial MRI showing large sequestrated upmigrated lumbar disc fragment on the left side. (C) Initial targeting on the pars below the left L3 pedicle. (D) Triangulation below the left L3 pedicle. (E) Intraoperative fluoroscopic image showing presence of the nerve hook medial to the left pedicle at the site of the sequestrated disc fragment. (F) Intraoperative lateral fluoroscopic image showing the position of the nerve hook in the neural foramen.

Figure 4.

Endoscopic images of the same patient (54/F) showing removal of the upmigrated disc herniation. (A) Endoscopic view after removal of the intertransverse ligament. A black arrow denotes the superior transforaminal ligament complex. SAP, superior articular process; IAP, inferior articular process. (B) Osteotomy of the tip of the superior articular process. (C) Endoscopic view showing retrieval of the disc fragment from under the axilla of the exiting nerve root. (D) Endoscopic view showing the sequestrated disc fragment. (E) Endoscopic view after decompression showing the exiting and traversing nerve roots.

RESULTS

Twelve patients with a mean age of 50.8 were included in this study. The study group included 7 males and 5 females with a mean follow-up of 12.9 months. 4 patients underwent paraspinal UBE at L4–5 and L3–4 each. 3 patients underwent the procedure at L2–3 and 1 patient at L1–2. Seven patients had zone 1 disc herniation and 5 patients had zone 2 disc herniation. The average surgical time taken for the procedure was 94.2 minutes (range, 92–115 minutes) and the mean length of hospital stay was 3.7 days. The mean values of VAS score for leg pain decreased from 7.33 preoperatively to 2.25 (p<0.05) in the immediate postoperative period. One patient experienced transient exiting nerve root irritation that subsided in 8 weeks with medication. All 12 patients had resolution of radicular pain at final follow-up after 12 months. The average ODI score decreased from 84.5 preoperatively to 9.33 (p<0.05) at final follow-up. Eleven out of the 12 patients has excellent outcomes as per modified MacNab criteria. One patient was reported to have good outcome due to transient exiting nerve root pain which subsided with conservative management over 8 weeks. The summary of results has been displayed in Tables 1-3.

Table 1.

Distribution of cases across different lumbar levels

Table 2.

Summary of results

Table 3.

Summary of results

DISCUSSION

In this study, the authors have described a novel paraspinal approach for treatment of hidden zone lumbar disc herniations using UBE. To the authors best knowledge, this is the primary report of application of paraspinal UBE to treat disc herniations in the hidden zone of MacNab.

Ever since the first description of this challenging area by MacNab in 1971 [3], several descriptions of this zone have been published by many authors each of them acknowledging the surgical difficulties posed by local anatomy [4,11-12]. It has been described as the medial hidden zone bordered laterally by the pedicle, ventrally by the dorsal aspect of the vertebral body, and dorsally by the pars interarticularis of the hemilamina (Figure 1) [3,4]. Schulz et al. [11] designate sequestration in the hidden zone as "craniolateral lumbar disc herniation." Papavero et al. [12] characterize this condition as a "fragment extruded upward into the spinal or nerve root canal, impinging on the exiting nerve root.” Soldner et al. [8] designated these entities as “canalicular and cranio-postero-lateral” lumbar disc herniation. Despite the varied terminology, all authors have recognized the difficulties associated with surgical exposure in this clinical situation and the surgical challenge in treating these disc herniations cannot be emphasized enough.

The optimal surgical treatment for hidden zone lumbar disc herniations has often been debated. The conventional treatment options range from traditional hemilaminectomy, translaminar fenestration over the sequestrum, a cross over translaminar fenestration, microscopic extralaminar sequestrectomy and transforaminal endoscopic discectomy [4,13,14]. Conventional hemilaminectomy when performed often necessitates removal of a significant portion of the facet joints and the pars interarticularis and, thus compromising spinal stability requiring the surgeon to resort to spinal fusion. [6,15,16]. Some authors suggest approaching such herniations from the cephalad interlaminar space. However, this requires great caution as it may sometimes result in loss of the inferior facet [17]. Di Lorenzo et al. [7] suggested a microsurgical pars interarticularis fenestration at the level of the herniated fragment. Though this approach involved minimal bony resection, there are concerns about the integrity of the pars interarticularis and thus spinal stability in the postoperative period. The pars interarticularis is one of the most vulnerable regions of the lumbar spine [18], and research by Ivanov et al. [19] has shown that injuring the lateral part of the pars interarticularis significantly raises the risk of stress fractures. Additionally, the average width of the lumbar isthmus is narrower from L1–3 compared to the lower lumbar regions (cranio-caudally), while the distance from the lateral margin of the pars interarticularis to the outer border of the vertebral body increases (Figure 5) [20]. This further makes the translaminar approach at higher lumbar levels technically difficult whilst maintaining spinal stability. Another limitation of this technique is that via this approach, access to the intervertebral disc space is often not possible [4].

Figure 5.

Anatomic variation in the lumbar spine from L1 to L4, showing a gradual increase in the width (X1) of the isthmus from L1 to L4 and greater dimensions of the working space (i.e., distance between the hemilamina and the vertebral body [X2]) from L1 to L4. ‘Y’ is the height of the lamina.

Several other translaminar facet sparing approaches that also preserve the borders of the lamina have also been described [2,8,11]. Reinshagen et al. [21] introduced a translaminar crossover approach to the lumbar hidden zone in recurrent disc herniations via a small fenestration over the base of the spinous process on the contra-lateral side. However, this approach too could not access the intervertebral disc [21]. Wang et al. [13] described their microscopic extralaminar sequestrectomy technique which the herniation is removed after resecting a 2- to 3-mm crescent shaped portion of the lateral lamina under microscopic guidance. An endoscopic approach accessing the craniolateral disc herniation from outside the spinal canal through the neuroforamen has previously been described. This technique may require additional widening of the neuroforamen (foraminoplasty) and carries a risk of iatrogenic injury, as the endoscope must be positioned very close to the nerve root passing through the neuroforamen [4]. Even when the SAP and the lower edge of the superior pedicle are partially removed using an endoscopic approach to enlarge the foramen cranially and retract the nerve root for easier manipulation, the transforaminal approach does not provide complete visualization of the relevant area [6].

With our technique, we were able to achieve excellent visualization of the nerve root and the herniated fragment after limited bony resection of a few millimeters of the lateral pars and in some cases the super articular process of the caudal vertebra. The extent of removal of the SAP was carefully monitored under fluoroscopic vision to avoid any excessive bony resection. Postoperative instability has not been encountered in our patient group as demonstrated by post operative computed tomography scan images showing the rate of violation of the facet joint (Figure 6) and none of our patients had any instances of recurrence. Our technique is similar to that described by Wang et al. [13], however, we believe that our endoscopic approach has superior visualization and a more detailed understanding of local anatomy. One of our patients did experience transient irritation of the exiting nerve root, however, there were no sensory or motor deficits and the pain eventually subsided with over-the-counter analgesics within 8 weeks.

Figure 6.

(A, B) Comparative computed tomography scan images of patient with L4–5 upmigrated disc herniation. The total area of the red line in panel B is 26 mm, and the area in yellow indicates the area of resection (i.e., 6 mm). The total percentage of bony resection is 23%, not affecting the stability of the facet joint.

There are some limitations in this study. Our sample size consisted of a relatively small cohort of 12 patients. However, our aim was to introduce this novel technique. Our results have been comparable to other conventional approaches and satisfactory outcomes have been achieved. We believe that comparative studies evaluating the outcomes of conventional techniques and our technique will better clarify results in the future.

CONCLUSION

The paraspinal UBE procedure ensures adequate visualization and removal of the herniated disc with minimal bony resection preserving the integrity of the pars interarticularis and the facet joints and thus maintaining spinal stability. In conclusion, paraspinal UBE is a safe and effective surgical procedure for management of disc herniations in the hidden zone of MacNab, particularly at higher lumbar levels.

Supplementary Material

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