INTRODUCTION
Lumbar foraminal stenosis (LFS) mainly occurs in the elderly population, accounting for approximately 8%–11% of degenerative diseases of the lumbar spine. It is known to result from a complex degenerative mechanism involving herniated discs in the foraminal, extraforaminal, and central zones, leading to a decrease in disc height, foraminal height due to pedicle impingement, hypertrophy of facet joints and soft tissues, syndesmophytes, and other factors [1,2]. Therefore, the pathology of LFS is not confined to the foraminal zone but can also occur from the central and subarticular zones to the extraforaminal zone, and even to MacNab hidden zone located between the foraminal zone and lateral recess zone [3].
Surgical treatment for LFS can be broadly divided into foraminal decompression and fusion. In the past, microscopic foraminotomy via the Wiltse approach was considered the gold standard for treating LFS. However, since the concept of percutaneous decompression for herniated lumbar discs was introduced by Kambin and Sampson, full endoscopic foraminotomy for LFS has been widely performed due to advancements in endoscopic equipment and techniques. This approach can be classified into transforaminal, interlaminar contralateral, and paraspinal approaches. Among them, transforaminal endoscopic lumbar foraminotomy (TELF) using Kambin triangle has been reported to have shorter surgical times, less blood loss, and good clinical outcomes and stability compared to microscopic foraminotomy [4-6]. Additionally, favorable clinical outcomes of TELF have been reported for unilateral LFS accompanied by spondylolisthesis [7,8]. However, difficulties in surgery via the transforaminal approach are common, especially in the L5–S1 segment due to the presence of anatomical barriers.
Efforts have been made to overcome these difficulties, and this study aims to introduce the overall clinical outcomes and complications of lumbar foraminotomy via TELF. It also aims to discuss the limitations of TELF for L5–S1 foraminal stenosis and efforts to overcome them, as well as alternative approaches. Through a thorough understanding of these factors, appropriate techniques, instruments, and optimal approaches can be selected for planning TELF for L5–S1 foraminal stenosis, leading to favorable patient outcomes and long-term success.
SURGICAL TECHNIQUE
The basic surgical procedure for transforaminal approach to L5–S1 foraminal stenosis is not significantly different from that of other segments. Patients are administered midazolam (0.05 mg/kg) and intravenous fentanyl (0.8 μg/kg) for preoperative sedation, and surgery is performed under conscious state with transforaminal local or epidural anesthesia. Patients are placed in a prone position on a radiolucent surgical table, with knees and hips slightly flexed to reduce the lumbar lordotic curve, and surgery is performed using a uniportal endoscopic system. Apart from the commonly used 6.3-mm outer diameter with a working channel diameter of 3.7 mm and optical angle of 30° endoscopic system, various other endoscopic systems can also be utilized.
TELF involves setting the superior articular process (SAP) base near Kambin triangle as the target landing point and proceeding with surgery after passing a working channel to this point. Bone work is performed on the SAP and isthmus using endoscopic instruments and a burr, and in cases requiring more extensive decompression of the exiting nerve root (ENR), additional bone work can be extended to the pedicle of the upper body and the upper endplate of the lower body, applying a full-scale foraminotomy [9]. After removing bone and soft tissue, the working channel is further advanced to decompress the ventral portion of the ENR, allowing for root retraction followed by radiofrequency annuloplasty or consideration of protruding disc and osteophyte removal. Interbody discectomy may also be considered by the surgeon to prevent disc re-protrusion [10-12]. However, in cases of L5–S1 foraminal stenosis, it is often challenging to advance the working channel to the target landing point. In such cases, the use of a bone reamer or performing bone work on structures such as the transverse process of L5, isthmus of L5, and SAP of S1 using an endoscopic burr can help position the working channel and endoscope at the appropriate target landing point (Figure 1).
To achieve sufficient decompression of the ENR, careful identification of anatomical portions causing foraminal stenosis via preoperative imaging is necessary. Efforts and techniques to minimize irritation of the ENR by the working channel or equipment are essential during sufficient foraminal decompression (Figure 2). Particularly in cases of L5–S1 foraminal stenosis, where anatomical barriers are often present, it is presumed that there is a higher likelihood of irritation to the ENR during the entry angle or process of the working channel. Various efforts have been made to safely and effectively address circumferential moderate to severe foraminal stenosis [13,14] while minimizing unnecessary irritation to the ENR. Techniques such as the bone reamer technique [15], floating technique [10], mobile out-in technique [11], outside-in-outside technique [12], and large endoscopic trephine system using the outside-in technique [16] have been introduced. By appropriately utilizing these methods, surgery can be conducted more safely and effectively. This surgical method is similarly applied to L5–S1 far lateral or foraminal disc herniation, and can decompress the 5th lumbar nerve root (Figure 3, Supplementary video clip 1).
DISCUSSION
1. Development of the Full Endoscopic Transforaminal Approach
Traditional surgical approaches for LFS can be categorized into microscopic foraminotomy via the Wiltse approach and interbody fusion. Among these, the traditional microscopic foraminotomy via the Wiltse approach has limitations in visualization and field of view. It is difficult to address multiple neural compression points simultaneously, especially in LFS cases with foraminal/extraforaminal/hidden zones. Additionally, minimizing facet violation while performing neural decompression poses a challenge. Consequently, postoperative dysesthesia (POD) and long-term instability are common, with approximately 80% of cases reporting only satisfactory clinical outcomes [17-21]. Hence, in the past, interbody fusion surgery was often chosen as the initial treatment for LFS. However, with the increasing human lifespan and rising patient preference for nonfusion procedures, there has been a recent trend towards selecting foraminotomy as the initial treatment option.
Full endoscopic spine surgery has evolved in this regard. Since the concept of percutaneous decompression for herniated lumbar discs was introduced by Kambin and Sampson, significant advancements have been made in transforaminal approaches for foraminal decompression. TELF is less invasive compared to microscopic foraminotomy and allows deeper access to the foraminal region [16,22], proving to be a useful tool for exploration of extraforaminal and foraminal zones [23,24]. Subsequent studies have demonstrated the safety and efficacy of TELF in treating LFS, with several reporting favorable clinical outcomes (approximately 85%) [5,6,25,26]. Studies have also confirmed favorable clinical outcomes in cases involving spondylolisthesis [7-9].
However, severe degenerative changes such as degenerative lumbar scoliosis or kyphosis, or LFS at the L5–S1 level, often present challenges due to hypertrophied SAP causing anterior and superior subluxation and reduced foraminal and disc height, resulting in a relatively narrow intervertebral foramen. Nevertheless, utilizing bone reamers or various techniques in TELF allows for a stable transforaminal approach [10-12,16,27,28]. Overall, considering the combined evidence, TELF demonstrates slightly superior clinical outcomes compared to microscopic foraminotomy (85% vs. 80%). It also shows favorable clinical outcomes even in cases occurring at the L5–S1 level or accompanied by stable spondylolisthesis, and can be applied in cases of severe degenerative changes in the lumbar spine.
2. Limitations of the Full Endoscopic Transforaminal Approach
The TELF, which is the standard and classic approach of FELF, involves the use of small-diameter endoscopic instruments, resulting in a narrow surgical field and making it a procedure with a challenging surgical difficulty level. Complications that may occur after TELF surgery include motor weakness, incidental durotomy, instability, recurrence, segmental artery injury, dorsal root ganglion injury, POD, and postoperative vertebral slippage [10,29-32]. The overall clinical outcomes of TELF surgery are slightly superior to those of the past microscopic foraminotomy, but to assess its reliability as a surgical procedure, it is necessary to also consider postoperative complications such as POD, instability, and recurrence. Kim et al. [32] reported that the incidence rate of POD grade 1 or higher in the TELF group was 26%, and grade 2 or higher was 12.8%, indicating that POD is quite common. Also, even in cases where satisfactory clinical outcomes were observed with microscopic foraminotomy via the Wiltse approach for LFS, some degree of persistent lower back pain or neurogenic symptoms in the legs may still occur postoperatively. Similarly, despite the good clinical outcomes demonstrated with endoscopic foraminotomy via the transforaminal approach, residual pain in the lower back and legs may persist to some extent after surgery. In such cases, it is believed that symptoms can be managed by adjunctively administering medications such as nonsteroidal anti-inflammatory drugs, muscle relaxants, neuroleptics, and selective serotonin reuptake inhibitors. Additionally, while LFS frequently occurs at the L5–S1 segment, the narrow space for the working channel compared to other segments due to the presence of the transverse process of L5, sacral ala, and SAP of S1 makes it difficult to access Kambin triangle deeply through the transforaminal approach, often requiring additional procedures such as transverse process resection, sacral alar resection, and bone reaming.
Various techniques and instruments have been introduced to minimize the risk of POD by minimizing irritation to the ENR and to facilitate safer and easier transforaminal approaches, even in cases of foraminal stenosis with anatomical barriers at the L5–S1 level. Ahn et al. [27] and Schubert et al. [28] have proposed methods for performing the transforaminal approach using bone reamers, Cho et al. [10] have presented a technique where the working sheath is floated over the ENR during surgery, Kim et al. [11] have introduced a mobile outside-in technique similar to the floating technique, and Fiorenza and Ascanio [12] have introduced the outside-in-outside technique. With advancements in instruments and techniques, it is indeed possible to perform TELF adequately even in the presence of anatomical barriers at the L5–S1 level. However, the surgical difficulty may be slightly higher, and the operating time may be longer. For these reasons, it is speculated that complications such as POD may occur more frequently in the transforaminal approach for L5–S1 foraminal stenosis compared to other segments. However, studies so far have not shown significant differences in POD incidence rates based on the level [32]. Furthermore, while favorable clinical outcomes have been reported for the transforaminal approach in cases of foraminal stenosis accompanied by spondylolisthesis, conversely, unfavorable clinical outcomes have been noted for cases of foraminal stenosis accompanied by disc wedging and consequent degenerative scoliosis [33]. In summary, advancements in instrumentation and technology have allowed TELF to overcome anatomical barriers even at the L5–S1 segment, minimizing damage to the ENR while achieving sufficient neural decompression. However, it is speculated that TELF may not entirely replace microscopic foraminotomy via the Wiltse approach and fusion surgery. This is because although TELF shows slightly superior clinical outcomes compared to microscopic foraminotomy, it presents challenges such as increased surgical difficulty and longer operation times, particularly at the L5–S1 segment where anatomical barriers exist. Moreover, there is a higher POD rate of around 26%, and a higher likelihood of recurrence in cases with severe degenerative changes.
In recent years, interlaminar contralateral endoscopic lumbar foraminotomy has been introduced as an alternative to the transforaminal approach, with promising clinical outcomes and techniques [2,34,35]. This approach allows smooth foraminal decompression at the L5–S1 level regardless of anatomical barriers and enables access to both the central canal and bilateral lateral recess stenosis using a single skin incision and channel in channel technique, even in cases of double crush syndrome [35]. For foraminal stenosis at the L5–S1 segment, selecting the appropriate approach based on a thorough understanding of these techniques and utilizing various tools to overcome the limitations of the transforaminal approach can lead to optimal outcomes.
CONCLUSION
When performing a transforaminal approach for L5–S1 foraminal stenosis, it is crucial to selectively utilize appropriate techniques and instruments. If overcoming anatomical barriers and concerns about irritation of the ENR during surgery are challenging, it may be necessary to consider alternative approaches that can reduce postoperative complication rates and overall surgical time instead of relying solely on the transforaminal approach.
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