Fully endoscopic cervical spine surgery: What does the future hold?

Abstract

Fully endoscopic cervical spine surgery is an emerging novel approach to address cervical spinal pathology. Techniques, both anterior and posterior have been adapted to address various cervical pathologies. The primary goal of these procedures like other open techniques is to surgically decompress the canal centrally and/or along the foramen. The narrative review aims to provide the reader an overview of the rapidly advancing field of endoscopic cervical spinal surgery and evaluate whether these newer approaches could potentially reduce the cost and the risk associated with instrumented cervical fusion.

1. Introduction

Cervical radiculopathy is a common and debilitating condition that affects adults, many of them in economically productive age groups. Sub-axial spine is involved in most cases and a conservative management is initially the treatment of choice. However, once an operative intervention becomes necessary, the treatment offered could be with either anterior or posterior approaches. Traditionally, these procedures have been performed by open approach with or without magnification. However, these procedures are increasingly being performed using fully endoscopic approaches. The purpose of this review is to examine the arguments for fully endoscopic surgery in decompressing the cervical spinal neural elements.

According to the properties of the endoscope used, there are three categories in endoscopic surgery: 1) full-endoscopic (or percutaneous endoscopic) surgery using a working channel endoscope; 2) microendoscopic surgery using a tubular retractor and attached endoscope system; 3) Biportal endoscopic spine surgery using an arthroscope with the separate working portal and viewing portal.^{1, 2, 3} A nomenclature of the cervical endoscopic surgery has recently been published by AO Spine for working channel endoscopic spinal procedures.⁴

A short review of the anatomy of cervical radicular compression is desirable in order examine the endoscopic decompression techniques.

1.1. Anatomy of cervical radiculopathy

Compressive cervical radiculopathy is caused by compression of the spinal nerve root at or beyond its exit from the spinal canal. The stenosis could result from an anteriorly placed disc herniation, degenerative osteophyte from uncovertebral joint or the facet joint. As the disc degenerates and the adjacent vertebral bodies settle on each other, the available vertical diameter of the exit foramen decreases. This coupled with the disc herniation itself and accompanying degenerative osteophytes leads to nerve compression with sensory and motor symptoms in the corresponding distribution. The primary goal be it anterior or the posterior approach is to increase the available diameter.

Bimodal age distribution is noticed in the clinical presentation of cervical stenosis. The younger population have the disc protrusion as the offending pathology. Whereas, in the older population osteophyte formation is usually the compressive component. Interestingly, the current standard management algorithms offer the same standard treatment ACDF for both the groups of pathologies.

1.2. What are the surgical options for cervical radiculopathy?

There are various techniques for addressing the stenosis. These range from the conventional microscopic approaches, both anterior and posterior, or the newer ones utilizing endoscopes. Each of these have their advantages and disadvantages.

Anterior cervical discectomy and fusion (ACDF) -Microscopic.

Anterior cervical discectomy and fusion with uncinectomy (ACDF-U).

Endoscopic anterior cervical discectomy.

Endoscopic transcorporeal cervical discectomy.

Fully endoscopic cervical foraminotomy (FECF).

Minimally invasive microscopic posterior cervical foraminotomy (MI-PCF).

Traditional posterior cervical foraminotomy (TPCF)- without magnification.

Anterior cervical discectomy and fusion using a spacer/cage not only decompresses the herniated disc but also restores the normal disc height between the vertebrae restoring the vertical height of the exit foramen. When combined with uncinectomy, this also addresses the bony component of the anterior compression. This approach involves accessing the plane between the carotid sheath and the tracheo-esophageal bundle. All these structures are thus potentially in harm's way’. Also, the patients sometimes complain of the prolonged dysphagia and dysphonia. It has also been argued that fusion results in accelerated adjacent level degeneration.⁵ Moreover, it does not directly address the bony osteophyte compression arising from the posterior elements.

More recently, anterior cervical discectomy has been performed purely endoscopically from anterior approach in young patients. Shen et al. reported their results of fully endoscopic approaches towards managing cervical compressive pathologies.⁶ They described their technique of anterior cervical discectomy performed through the disc annulus (19 patients) and through the transcorporeal route (6 patients). Both these groups of patients experienced statistically significant reduction in nVAS, aVAS and NDI scores at one year with only one patient in the first group experiencing recurrent disc herniation and requiring an ACDF. However, the series is a small one and further experience would be required before this becomes standard of care. Moreover, the technique of transannular endoscopic discectomy creates a new tear in the annulus in a patient who already has a posterior annular tear. We need to look carefully into the long-term outcome of these procedures as cervical discectomy in the past has led to localized kyphosis especially in these young patients.

The posterior approaches involve creating a small laminotomy, accessing the exit root foramen and directly addressing the compression by removing the posterior bony spurs. The manner in which the posterior elements are approached decides the invasiveness. However, the traditional and minimally invasive microscopic approaches are unable to safely decompress the anterior component of stenosis safely in many cases. The traditional approach (TPCF) involves mobilizing the paraspinal muscles laterally up to the facet joints. This leads to postoperative pain and delayed return to function. The minimal invasive microscopic approach using sequential dilator system lands directly on the facet-lamina junction and involves dilation of a small vertical parting in the paraspinal muscles and then proceeds to address the bony spurs by drilling and curetting. This leads to much less post-operative pain and has lesser rates of infection by minimizing tissue trauma. The endoscopic approach is even less invasive and tissue trauma as the incision is even smaller and muscle entry involves sequential dilatation instead of subperiosteal stripping. But, more importantly, it provides enhanced visualization of the anatomy with minimal bleeding, safer decompression of herniated disc potentially increasing the safety of the procedure. This is especially important in the younger age group who most often have soft foraminal stenosis from a disc herniation. This can be potentially be a day case procedure in the centers which have crossed the initial learning curve.

1.3. What surgical procedure is currently the procedure of choice and why?

Undoubtedly, ACDF procedures currently vastly outnumber the PCF of all types for cervical radiculopathy. In a recent study⁷ published from the Swedish registry in 2020, of the total of 4368 procedures, only 647 were PCFs. Another study published from a north American cohort, had 4852 patients with PCF and 46147 patients in ACDF group.⁸ Almost all the reviewed retrospective studies comparing ACDF to PCF for radiculopathy were skewed in favor of ACDF as far as the numbers were concerned.^{9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20} There may be several reasons for surgeon preference for ACDF over PCF. The purported instability caused by the bony decompression that is required for PCF and higher rates of revision are the ones most often quoted. However, these have been challenged from time to time and more trials are been proposed to conclude a definitive answer. ^21,22 The study by Leeds group from the UK reported by Selvanathan et al. also found similar rates of revision for ACDF and PCF while delivering similar level of pain relief.¹⁸ However, the study by the Swedish group did uncover a higher rate of revision for the PCF patients.⁷ The most common reason for revision surgery at the index level was restenosis in the PCF group. The authors thought that this could be due to differing levels of facet joint resections by different surgeons who participated in Sweden wide study. The other concern is seemingly higher rates of infection with PCFs.²³ But, all the studies invariably agree that the infections after PCF are superficial whereas the ones after ACDF are deeper requiring washout. A study based on one of the largest cohorts points out that outpatient procedures are associated with significant reduction in nosocomial infections.⁸ Introduction of minimal invasive approaches, and enhanced visualization with endoscopy, results in a thorough decompression of pathology and better preservation of normal anatomy. This coupled with a smaller incision and lesser postoperative pain as the potential of same day discharges. This seems to definitely overcome all the arguments against the PCFs.

1.4. What are the advantages of posterior approach and what have been the arguments against it?

Posterior cervical foraminotomy (PCF) has been offered as an alternative approach in appropriately selected patients and is the procedure of choice for many surgeons due to its simplicity, shorter operative times, near total avoidance of large vessel, esophageal and tracheal injury and avoidance of motion segment fusion thus potentially preventing accelerated adjacent segment degeneration. PCF has been performed with traditional open, minimally invasive microscopic (MI-PCF) and more recently fully endoscopic (FECF) approaches. The traditional arguments against PCF have been that it does not adequately address the anterior component of the compression viz. the herniated disc in an effective fashion and therefore the rates of re-operation remain high. Also, traditional posterior approaches involve mobilization of paraspinal musculature potentially leading to permanent compromise of its function as an effective supporting cable for the spine.

Kim et al. have described their experience of 30 patients discussing the effect of partial Pediculotomy and Partial Vertebrotomy for Posterior Endoscopic Cervical Foraminotomy (PPPV PECF) on cervical radiculopathy. The have provided a detailed description of the technique and the advantage of using endoscope to achieve adequate decompression by drilling out the pedicle and body adjacent to the exit foramen and without resecting the facet joint. This avoided destabilizing effect on spine while still achieving good decompression. They found an increase in area of decompression by 996 ± 266 mm² (p < 0.05) on postoperative 3D CT scan reconstruction. ODI, VAS and McNab scores showed excellent improvements.²⁴

1.5. Is there any real evidence of superiority of one approach over another?

The highest level of evidence that could be gathered from the available literature comprises the 3 RCTs^{25, 26, 27} and 2 meta-analyses.^28,29

Wirth et al. had published one of the earliest RCTs comparing the different techniques of managing unilateral single level cervical radiculopathy.²⁵ They compared ACDF with and without fusion and conventional open posterior cervical foraminotomy. A total of seventy-two patients were recruited and operated between 1984 and 1991. This early study also indicated that posterior cervical foraminotomy was as good as anterior approaches for single level unilateral radiculopathy. There was a higher rate of re-operations for recurrence of symptoms in the foraminotomy group for recurrent disc herniation at the index level. However, none of this reached a statistical significance.

A prospective randomised controlled study comparing ACDF to Full-Endoscopic Cervical Posterior Foraminotomy for the Lateral Disc Herniations Using 5.9-mm Endoscopes was conducted Ruetten et al. and published in 2008.²⁸ They recruited one hundred and seventy five patients with mono-segmental unilateral radiculopathy and randomised them to receive either conventional anterior cervical discectomy and fusion or posterior cervical foraminotomy by fully endoscopic approach. Eighty six patients received ACDF and eighty nine received endoscopic posterior foraminotomy. They did not encounter any significant difference in the rates of infection, revision or recurrence of symptoms in the two groups. The mean postoperative work disability in the posterior foraminotomy group was much lower (19 days), compared to 34 days in the ACDF group (P 0.01).This study clearly indicates that the fully endoscopic posterior foraminotomy is at least as effective as the ACDF for the right indications and results in much lower disability than the anterior approach.

A meta-analysis conducted by Fang et al. and published in 2020 which included 3RCTs and 12 retrospective studies came to the conclusion that here were no differences in the NDI between the ACDF and PCF groups.²⁹ But the PCF group had a slightly higher rate of reoperation for persistent or recurrent radicular pain at the same level. The authors did not theorize the reason behind the recurrence, but it could have been related to incomplete resolution of stenosis due to insufficient intraoperative visualization. However, none of the studies included the fully endoscopic foraminotomy approach.

One of the strongest studies favoring MI-PCF is a meta-analysis conducted by Sahai et al.²⁸ which included 1216 patients pooled from over 14 studies each of which had at least 10 patients. They came to the conclusion that in patients with unilateral single level cervical radiculopathy, MI-PCF lead to a significantly greater improvement in VAS-arm scores compared to ACDF.However, the VAS-neck and NDI scores were similar among the two groups. Reoperation rates were also similar, but the rates of complications related to wound, transient neuropraxia and durotomy were higher in MI-PCF group.

Zhang et al. .from the First Affiliated Hospital of University of South China, conducted a systematic review and meta-analysis of the studies done for percutaneous endoscopic cervical foraminotomy (PECF or FECF).³⁰ This was the only meta-analysis done specifically for endoscopic PCF and did not include other minimally invasive PCF approaches. It was interesting to note that in addition to indicating that VAS-arm scores were much better for PECF approach, the VAS-arm and NDI scores also showed a favorable trend over a median period of follow up of 20.9 months.

Could ACDF be completely avoided for the indication for cervical radiculopathy and be replaced with a less invasive approach? An answer in that direction was provided by the 5 year follow up study by Ji-Jun et al.³¹ in their study to assess the clinical outcomes of using the posterior full-endoscopic cervical discectomy (PECD) in comparison with the conventional anterior cervical decompression and fusion (ACDF) in treating patients with cervical radiculopathy. 38 patients underwent ACDF and the other 43 PECD.The blood loss and hospital stay were significantly less in patients treated with percutaneous endoscopic cervical discectomy(PECD) compared with those undergoing ACDF (p < 0.05∗). There were no significant differences in the VAS scores, the NDI, and the modified MacNab criteria between the two groups. The patients in the ACDF group obtained a better Cobb angle and had less operative time compared with those in the PECD group (p < 0.05∗). Only mild complications were observed in both groups, with no significant difference (p = 0.28).

1.6. Have the anterior approaches been adapted to the use of endoscope?

Kong et al.³² published their 2 year follow up for anterior percutaneous full-endoscopic transcorporeal decompression of the spinal cord for single-segment cervical spondylotic myelopathy. The study included 32 patients and the parameters noted included operating time, time of walking out of bed postoperation, length of hospitalization, complications, neck pain visual analog scale (VAS), and Japanese Orthopaedic Association Score (JOA). Radiological measurement included measurement of intervertebral height (HI) of surgical segments on cervical neutral X-ray, Harrison's method was used to measure cervical spine angle (CSA). The study concluded that endoscopic transcorporeal decompression was safe and effective atleast in the short term and avoided the problems of adjacent segment disease which occur with ACDF. However, the cohort would need to be followed up over much longer period in order for this technique to be truly competitive with ACDF.

Interestingly, the studies are being done to compare the various anterior endoscopic approaches to each other. One such study published in 2020 by Ren et al.³³ compared the difference in clinical and radiographic outcomes between anterior transcorporeal and transdiscal percutaneous endoscopic cervical discectomy (ATc-PECD/ATd-PECD) approaches for treating patients with cervical intervertebral disc herniation (CIVDH). At the end of 2 year follow up, there was no significant difference in the clinical outcomes between the 2 approaches. While the longer time was consumed in the ATc-PECD group, the lower rate of disc collapse and recurrence were notable. Additionally, when the central diameter of tunnel was limited to 6 mm, the bony defect could be healed without the occurrence of the collapse of the superior endplate, and ATc-PECD may be preferable in the endoscopic treatment of CIVDH.

1.7. Does the minimal invasiveness of endoscope accrue any benefit for medically compromised patient population?

Carr et al. shared their early experience on Full endoscopic unilateral laminotomy for bilateral decompression of the cervical spine.³⁴ They demonstrated the utility of cervical endoscopic unilateral laminotomy for bilateral decompression (CE-ULBD) in a series of elderly patients with severe central stenosis, significant medical comorbidity, and existing cervical deformity. Average clinical follow-up time was 22.0 ± 4.7 months; clinical outcomes at most recent follow-up were improved via both the Nurick grade (1.2 ± 0.4, P < 0.01) and modified Japanese Orthopedic Association (14.6 ± 1.0, P < 0.001) compared with pre-operative values. This study, in addition to describing a novel approach also indicated that some of the patients who would have been otherwise too medically unwell for open approach could be managed with endoscopic technique possibly due to its less invasive nature.

From the available literature, there does not accrue any evidence pointing us away from endoscopic approaches. Moreover, the more recent studies though insufficient in number compared to the large volume of literature for open approaches, are very encouraging from the point of view of patient satisfaction and early return to work.³⁵

2. Conclusion

Cervical degenerative pathology is no longer a condition only affecting elderly population. Younger cohort of patient are seen with disc herniation and associated effect in activities of daily living. It is essential that we look into addressing these soft stenoses using a technique that are less and less invasive and minimally intrusive on the normal anatomy of the spine. Arguably like in lumbar spine, these can be managed by simple discectomy rather than standard ACDF procedure. These patients are generally fit and would benefit from attempts at preserving the motion segments and earliest possible early restoration of ‘back to work’ status.

On the other hand, patients with hard stenosis due to anterior or posterior osteophyte complex can be managed with foraminotomy for the right pathology. Both in ACDF and TPCF or MI-PCF the principle remains the same to decompress the exiting foramen. However, better understanding of the type of foraminal narrowing is warranted in future to address this complex pathology.

The management of cervical radicular pain needs to be rethought based on age, pathology and available skillset. With advancements in optics and development of smaller endoscopes, the technique of posterior cervical foraminotomy has been re-invented and has become very minimally invasive, safe and effective for unilateral single level cervical radiculopathy. It seems likely that as more surgeons become proficient and adapt endoscopic foraminotomy in their practice, it has the potential to replace ACDF as a standard practice for unilateral single level cervical radiculopathy. This technique, if applied to this carefully selected group of patients could result in much higher rate of patient satisfaction and objective clinical improvement.

The anterior transdiscal and transcorporeal approaches further expand the armamentarium of the endoscopic spinal surgeons and open up new vistas for spinal decompression.

Understandably, due novelty of these techniques, there is a paucity of evidence in literature towards fully endoscopic cervical surgery. We hope with time as more surgeons verse themselves with this technique, stronger evidence will be published in near future.