Introduction
Lumbar interbody fusion (LIF) surgery is widely employed in orthopaedic procedures to treat various spine-related diseases, such as degenerative diseases, spondylolisthesis, lumbar spinal stenosis (LSS) and spinal instability (1). The primary aim of this surgery is to alleviate pain and increase spinal stability by fusing two or more vertebrae (2). This is achieved by implanting bone fusion materials (such as autologous bone, polyether ether ketone or metal) and reinforcing the spinal structure with metal devices (such as screws and rods) (3).
Full endoscopic spinal surgery represents an innovation in the surgical field, allowing physicians to perform spinal operations using an endoscope through smaller incisions (4). This technique provides clearer and broader visual fields, finer neural decompression and improved intervertebral space handling. Using the trans-superior articular process approach under endoscopy allows direct decompression of the affected foramina, lateral recess stenosis and central canal (5). Under endoscopic observation, the cartilage endplate can be scraped without excessive treatment of the bony endplate. This improvement in surgical anatomy can minimize surgical trauma, reduce recovery time and decrease the incidence of complications (6). This technique has been widely applied in various spinal surgeries, such as decompression, spinal fixation and spinal fusion surgeries. Despite its numerous advantages, it faces some challenges. First, full endoscopic surgical techniques require highly skilled surgeons, which can only be achieved through specialized training. Second, for some complex conditions, such as severe spinal scoliosis (7) or severe spinal stenosis, this technique may not provide a sufficient surgical view or operating space. Additionally, the long-term efficacy and success rate of full endoscopic techniques in spinal surgery require further research for confirmation.
Trans-superior articular process LIF surgery under full endoscopy is an innovative surgical method designed to overcome the limitations of traditional spinal surgical techniques. Traditional spinal surgical methods, especially LIF surgery, typically require large surgical incisions, extensive tissue dissection and wide exposure of spinal structures (8). Furthermore, uncertainties in the bone fusion process and potential failures of fixation devices could affect the long-term success rate post-surgery (9). These factors may lead to significant surgical trauma, prolonged recovery time and a high risk of complications. Current studies almost unanimously agree that endoscopic lumbar fusion surgery has advantages over traditional surgical methods, such as smaller surgical incisions, reduced muscle and soft tissue dissection, less bleeding and faster postoperative recovery (10-16). Therefore, developing a new surgical method is crucial to reduce surgical trauma, shorten surgery and recovery times and lower the risk of complications. In the present study, the primary goal of transforaminal upper facet joint LIF (TSAP-LIF) under full endoscopy was to use endoscopes and specialized surgical tools through smaller incisions and with less tissue dissection. Unlike the currently popular approaches, this technique enables LIF surgery to be performed by only removing the superior articular process, without removing the inferior articular process, resulting in fewer steps and a reduced surgical duration for patients.
Discussion
Lumbar degenerative disease is a common condition in spinal orthopaedics, often causing back and leg pain with restricted movement. Lumbar fusion surgery has become the standard procedure for treating such diseases. The origin of spinal endoscopy dates back to the early 1930s when Burman used arthroscopic instruments to perform the first ‘spinal endoscopy’ on a cadaver, successfully displaying the spinal cord and nerve roots (20). Soon after, Pool (21) began performing spinal endoscopy through incisions ≤2.5 mm long, providing detailed observation of the nerve roots. With the advancement of optical lens systems and fiber optic technology, and the continuous development and expansion of surgical techniques, Cloward (22) first proposed posterior LIF (PLIF) in 1953. This technique offered clear surgical field exposure, high neural decompression and a stable three-dimensional spine structure, restoring the normal lumbar curvature. However, PLIF also had significant drawbacks, such as damaging posterior spinal structures (e.g. spinous processes, lamina and bilateral facet joints) and causing nerve root traction injuries (22). In 1983, Kambin and Zhou (23) performed the first percutaneous arthroscopic discectomy, and in 1991, Kambin (24) introduced the concept of a triangular safety zone, a triangular area formed by the upper edge of the lower vertebra, the outer edge of the dural sac or traversing nerve root and the inner edge of the exiting nerve root. This area is relatively safe for surgical operations and is the pathway for endoscopic transforaminal neural decompression and interbody fusion. To reduce iatrogenic injuries, Leu and Hauser (25) first reported the use of percutaneous endoscopic lumbar fusion in 1996, although this surgery had a high overall complication rate, including postoperative nerve root pain with dysesthesia, symptomatic interbody cage displacement and the need for salvage surgery. Since then, surgical methods and tools have gradually improved, allowing for adequate discectomy, endplate preparation and the use of appropriate lumbar interbody cages to avoid nerve injuries.
The indications for TSAP-LIF under full endoscopy are similar to those for conventional spinal fusion surgery, especially when spinal instability causes neural compression (26). With regard to approach selection, the SAP reshaping should ensure the safety of the exiting nerve root, typically by gradually reshaping the dorsal side of the SAP to create sufficient safe space and reduce the probability of nerve root injury. The rational use of foraminoplasty tools, such as power systems, trephine systems and protective sleeves under full endoscopy, can effectively prevent injury to the exiting nerve root and dural sac (27).
According to the location and characteristics of stenosis, a reasonable decompression method should be selected. The TSAP approach can cover most of the intervertebral foramen's internal and external ranges and the affected-side lateral recess. It is also suitable for severe central canal stenosis dorsal decompression and L5/S1 segment decompression (28), but cannot perform bilateral decompression through a unilateral approach, which is a limitation of this technique (29). Therefore, the indications of the patient should be clarified preoperatively.
Proper graft bed preparation is a prerequisite for achieving bony fusion, which can be performed under direct vision or endoscopic vision. The former is similar to traditional methods, using endplate chisels and curettes to scrape the cartilage endplate, but it is prone to damaging the bony endplate. High-quality endoscopic treatment of the cartilage endplate can avoid excessive damage to the bony endplate, although it is less efficient. Previous reports (30-32) used modified instruments, such as specially designed endplate chisels and L-shaped reverse curettes, to handle the intervertebral space safely and efficiently, preparing the graft bed through a combination of direct and endoscopic vision (33).
After years of clinical screening and verification, PEEK cages are the most widely used clinically (34). Although smaller cages can safely pass through the working path, they may not effectively restore intervertebral height, and PEEK is an inert bone-inducing material, unfavorable for LIF. Clinical refinements to the shape and size of the cages have been made to address this problem, and their hollow centers have been designed to accommodate osteoinductive material implants to promote inward growth of bone to induce fusion where rigid stabilization is required. Graft material selection includes autogenous bone from decompression, autogenous iliac bone, allograft bone and BMP-2. In theory, autogenous bone from decompression is the best choice. If affected by bone quantity, composite grafting with autogenous bone from local decompression and other graft materials can be used (35).
Bilateral pedicle screw fixation is currently advocated, providing a more stable biomechanical environment than unilateral fixation, bilateral lamina facet screws or facet screws, reducing complications such as cage displacement, graft non-union and internal fixation failure (36).
TSAP-LIF has significant advantages in terms of a shorter operation time compared with other minimally invasive LIF surgeries. Zhang et al (37) conducted a retrospective study of 62 patients, where the operation time for endoscopic transforaminal LIF (Endo-TLIF) was 202.6±31.4 min, and that for minimally invasive transforaminal LIF was 192.1±18.9 min. In the present study, the operation time was 113.3±13.9 min. Unlike Endo-TLIF, which requires the removal of both superior and inferior articular processes, TSAP-LIF only involves the removal of the superior articular process. The use of ring saw tools allows for rapid excision of the superior articular process, thus shortening the operation time. Fan et al (38) retrospectively analyzed the data of 69 patients with LSS, finding that the operation time for unilateral lateral interbody fusion was 112.78±19.29 min and that for Endo-TLIF was 174.58±18.41 min. According to a recent meta-analysis, percutaneous endoscopic lumbar interbody fusion (PE-LIF) compared with unilateral biportal endoscopic transforaminal lumbar interbody fusion (UBE-TLIF), the TSAP-LIF procedure showed similar operative time; however, TSAP-LIF exhibited advantages in reduced tissue trauma (supported by lower intraoperative blood loss and postoperative CRP levels), which accelerated wound healing (39). Nevertheless, due to limitations in instrument maneuverability, the working and viewing channels of TSAP-LIF are integrated into a single portal, precluding simultaneous bilateral decompression of the intervertebral foramina and lateral recesses. The mean hospital stay for patients in this study was 12.7±3.2 days, influenced by China's universal healthcare system (40), which reduces hospitalization costs, thereby enabling patients to discharge upon clinical improvement or to undergo in-hospital rehabilitation prior to discharge.
Additionally, the preoperative and postoperative ODI and VAS scores from the present study were compared with those reported in other studies on endoscopic LIF techniques. Brusko and Wang (10) reported a significant improvement in the average ODI score by 12.3 points at the 1-year follow-up of 100 patients. Morimoto et al (28) showed improvements in VAS scores for back pain to 4.4-6.0 and for leg pain to 4.3-6.9, with ODI score improvements ranging from 19.5 to 41.5. In the study by Xie et al (41), the VAS scores decreased from 7.43±0.50 preoperatively to 3.20±0.48 at the first postoperative week, 2.97±0.41 at 1 month and 2.80±0.41 at 1 year. The improvements in ODI and VAS scores from the present study are generally consistent with those achieved by other minimally invasive lumbar spine surgical techniques (42-45), indicating significant relief of preoperative symptoms of lower back and leg pain, as well as favorable outcomes in interbody fusion.
There are certain limitations to the present study. As TSAP-LIF is a novel surgical technique evaluated in a single-center institutional study, the retrospective analysis is limited by a relatively small sample size. Although paired sample tests revealed significant differences (P<0.05) in the changes in VAS scores for back pain and ODI indices in TSAP-LIF patients, the limited sample size could result in broad confidence intervals, increasing uncertainty in the outcomes and potentially affecting inferences about the effects of TSAP-LIF. A small sample size may lead to unstable effect size estimates, potentially resulting in an inaccurate assessment of the superiority of TSAP-LIF over other techniques. Additionally, selection bias may influence the results due to factors such as patient selection criteria and willingness to participate, limiting the generalizability of the findings. Future prospective studies should employ strategies such as randomization and stratified sampling, and involve larger multicenter cohorts to verify these results and enhance their applicability to a broader population.
The primary aim of this retrospective study was to evaluate the initial safety and short-term efficacy of TSAP-LIF. A 6-month follow-up period is commonly used in similar studies to assess early postoperative recovery and functional improvements. In clinical practice, significant recovery and symptom improvement typically occur within the first 6 months post-surgery, providing sufficient data to evaluate the direct effects of the surgery. Research by Uçar et al (46) showed that patients could be allowed to engage in full activity and return to work 6 months after lumbar spine fusion surgery. Woods et al (47) conducted a retrospective study on 137 patients who underwent LIF, and the CT fusion rate at all fusion levels was 97.9% at 6 months after surgery. A 6-month follow-up period is adequate for early efficacy and safety assessments, and it can provide some insight into long-term outcomes, such as fusion durability and sustained clinical improvement. However, it is essential to extend the follow-up duration beyond 12 months to thoroughly evaluate the long-term success of TSAP-LIF. This is particularly important due to the progressive nature of LSS and the potential for recurrence of symptoms over time.
The technical and instrumentation limitations of TSAP-LIF are important considerations, particularly for patients with unilateral or central stenosis. The lumbar spine features substantial upper and lower laminar spaces, which allow for the removal of the upper articular process to decompress the contralateral affected side. Due to the specific anatomy of the lumbar spine, there is often ipsilateral blockage from the upper and lower articular eminences or lamina, making it difficult to decompress the lateral recess or foramen on both sides simultaneously via the superior articular eminence. Consequently, the intervention primarily addresses the affected side and the central region through this approach. In cases where there is bilateral compression in the lumbar spine, it may be necessary to remove portions of the upper and lower articular processes or even the vertebral plate on the same side for ipsilateral decompression. This demands a high level of skill on the part of the surgeon in manipulating the instrumentation. Furthermore, the option exists to perform simultaneous surgeries using a dual- or multichannel system, which facilitates enhanced decompression of both sides of the intervertebral foramen and lateral recess. The integration of more flexible and maneuverable endoscopes and surgical instruments can better accommodate diverse anatomical structures, thus overcoming some limitations in instrument maneuverability. Additionally, the utilization of real-time imaging and navigation technologies, along with robotic-assisted systems, can enhance the precision and adaptability of surgical instruments. This advancement not only aids surgeons in accurately localizing the surgical site but also enables them to execute more complex procedures effectively.
In conclusion, TSAP-LIF represents a promising minimally invasive technique for addressing lumbar spine pathologies. The technique offers significant advantages, including reduced operative time, minimized intraoperative blood loss and the ability to achieve effective decompression of the affected and central regions through a unilateral approach. This allows for the efficient completion of discectomy and endplate preparation. Despite the challenges posed by the limited maneuverability of current instruments, advancements in surgical tools and the growing expertise of surgeons in endoscopic spinal procedures enhance the potential for improved clinical outcomes. Future efforts will focus on extending patient follow-up to 12 months, employing postoperative imaging techniques such as plain films and CT scans to thoroughly evaluate long-term fusion success. By addressing the current limitations and with ongoing innovations in surgical technology, TSAP-LIF has the potential to deliver superior clinical and radiological outcomes, thereby extending the capabilities of the Kambin approach.