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Mid-term Clinical Results of Microendoscopic Decompression for Lumbar Foraminal Stenosis

Introduction There have been several reports on surgical techniques involving microendoscopy or percutaneous endoscopy for treating lumbar foraminal stenosis (LFS). However, no studies have assessed the mid-term clinical results of endoscopic techniques in spite of their relatively long history. In this study, we report 20 consecutive cases of LFS treated by our microendoscopic technique focusing on clinical results with a follow-up of at least two years. Methods Twenty consecutive cases of LFS treated with microendoscopic decompression were followed up at 1, 2, 6, and 12 months postoperatively and annually thereafter. The patients were 14 males and 6 females, and the mean age at the time of surgery was 64.7 years. The Japanese Orthopaedic Association (JOA) score was used as the clinical outcome index. Results Of the 20 patients, 16 were monitored successfully for more than 2 years. The follow-up rate was 80.0%, and the mean follow-up period was 66.3 months. The JOA score improved from 13.8 points before surgery to 24.6 points at final follow-up. Revision fusion surgeries were performed in two cases for LFS recurrence. Conclusions The microendoscopic technique effectively treats LFS.


Introduction
There is no standardized diagnostic imaging technique for lumbar foraminal stenosis (LFS), so this disease can be easily overlooked and result in failed back surgery syndrome. In many cases, the whole length of the nerve root, from the spinal canal to the extraforaminal zone, must be decompressed surgically since it is difficult to identify the nerve compression site 5 before surgery; as a result, preserving the posterior elements is difficult. Therefore, total facetectomy combined with spinal fusion using spinal instrumentation is normally performed.
Microendoscopic discectomy (MED), developed by Foley and Smith, 1 is used to treat lumbar disc herniation. Because of recent advances in surgical techniques and instruments, spinal microendoscopy is also used for various conditions such as lumbar spinal stenosis (LSS) and cervical myelopathy. 2,3 For lumbar lateral lesions, especially far-lateral lumbar disc herniation 10 and far-out syndrome, spinal microendoscopy, which can reach deeper into the body less invasively, is becoming the standard procedure. 4,5 We previously reported a microendoscopic technique to decompress the entire length of the nerve root, from the spinal canal to extraforaminal zone, while preserving the posterior elements in LFS patients. 6 Similar techniques with microendoscopy or percutaneous endoscopy for treating LFS have been reported by several authors. [7][8][9] However, during our 15 literature search using PubMed, we found no studies that have assessed the mid-term clinical results of such endoscopic techniques, in spite of their relatively long history. The mid-term results might suggest problems, such as deterioration of clinical results, because of intervertebral foramen restenosis. In this study, we report 20 consecutive cases of LFS treated by our method focusing on clinical results with a follow-up of at least two years.

Patient Data
Microendoscopic decompression for LFS was introduced in Sapporo Medical University Hospital in 2008. The first 20 5 consecutive patients treated with this technique between 2008 and 2016 were followed up at 1, 2, 6, and 12 months postoperatively and annually thereafter. The patients were 14 males and 6 females and the mean age at the time surgery was 64.7 years (range 52-80 years). Seven patients who had degenerative lumbar scoliosis (DLS) with Cobb angles of more than 10 degrees were included in this series. Three patients had undergone posterior decompression for central stenosis previously, but their leg symptoms persisted. 10 All patients had intermittent claudication with unilateral radicular pain. The preoperative diagnoses were based on detailed neurological examinations, selective nerve root blocks, lumbosacral roentgenographies, and magnetic resonance imaging (MRI) or computed tomography (CT) studies after radicurography. 6 Cases of far-lateral lumbar disc herniation and far-out syndrome were excluded. Patients were candidates for surgery if conservative therapies such as administration of prostaglandin E1 and analgesics, epidural block, and selective nerve root block, for at least three months were ineffective. The 15 most frequently decompressed level was L5-S1 (16 patients), followed by L5-L6 (2 patients) and L4-L5 (2 patients).

Surgical Procedure
All procedures were performed according to the method described by Yoshimoto et al. 6 Surgery was conducted under general For patients where L5-S1 is affected, the iliac crest may become an obstacle, limiting lateral approaches to the lesion.
In such cases, the tubular retractor is inserted from the cranial side, avoiding the iliac crest. CT or MRI should be used to make pre-surgical decisions regarding the direction of insertion of the tubular retractor and the point of skin incision. 5

Clinical Evaluations
The Japanese Orthopaedic Association (JOA) score (−6 ~ 29 points) (Appendix 1) was used as a clinical outcome index. The rate of improvement in JOA score was calculated as follows: (postoperative score-preoperative score) / (29-preoperative score) × 100 %. Cases where JOA score improvement was less than 50% were recorded as "poor outcome" cases. Required revision 10 surgeries were also recorded. Postoperative DLS progression was evaluated using anteroposterior view radiographs at final follow-up. All clinical and radiographic data were collected prospectively, and this study is a retrospective review of that data.

Statistical Analysis
Changes in JOA scores were analyzed by paired Student's t-test. A p-value of less than 0.05 was considered statistically 15 significant.

Ethics
This study was conducted in compliance with the Declaration of Helsinki. Written informed consent was obtained from all 6 Microendoscopic Decompression for Lumbar Foraminal Stenosis patients before treatment, and the study protocol was approved by the hospital Institutional Review Boards.

Results
Seven patients underwent the combined procedure because slight stenosis of the lateral recess was seen on CT-myelography.

Follow-up Status
Of the 20 patients who underwent the procedure, 4 patients stopped coming to their scheduled clinic visits and did not respond to our letters. The remaining 16 patients, who were successfully monitored prospectively for more than two years, 10 were enrolled in further clinical studies. The follow-up rate was thus 80.0%, and the mean postoperative follow-up period was 66.3 months (range, 24−105 months). Of the 16 patients, 5 had DLS with Cobb angles of more than 10 degrees (Table 1).

Clinical Outcomes
JOA scores improved significantly after surgery in all cases. The mean JOA score was 13.8 points (range, 7 − 22) before 15 surgery and 24.6 points (range, 14−29) at final follow-up ( Figure 2). The mean improvement rate at the final follow-up was 63.9 % (range, −42.9−100). The mean improvement rates by procedure were 60.1 % in patients who underwent L5-S1 surgery and 67.4 % in patients with different surgeries (i.e., L4-5 or L5-L6). This difference was not significant.  (Table 1).

Revision Surgeries (Patients 3 to 7)
Revision surgeries were performed in two cases for LFS recurrence at the operated level (patients 3 and 4), and in three cases 10 for lumbar disc herniation or LSS at other levels (patients 5, 6, and 7). The two patients with LFS recurrence underwent transforaminal lumbar interbody fusion (TLIF) 30 and 34 months after surgery, respectively, and the patients with lumbar disc herniation or LSS at other levels underwent microendoscopic discectomy or decompression 19, 30, and 55 months after surgery.

Patient Three
A 64-year-old woman underwent microendoscopic decompression for LFS on the right side at L4-5. Immediately after surgery, the pain in her right leg was greatly improved, and the JOA score improved from 13 points before surgery to 22 points six 8 Microendoscopic Decompression for Lumbar Foraminal Stenosis months after surgery. However, leg pain recurred, the JOA score deteriorated to 9 points, and she underwent TLIF 34 months after decompression surgery. A coronal image of reconstructed three-dimensional CT showed recurrence of foraminal stenosis at the operated level because of DLS progression; the Cobb angle increased from 15.9 to 22.5 degrees (Figure 4).

Patient Four
A 64-year-old man underwent microendoscopic decompression for LFS on the right side at L5-S. Immediately after the operation, the pain in his right leg was greatly improved and the JOA score improved from 13 points before surgery to 26 points 6 months after surgery. However, leg pain recurred, the JOA score deteriorated to 9 points, and he underwent TLIF 30 months after decompression surgery. Anteroposterior radiographs did not show DLS progression with 12 degree of Cobb 10 angle (L2-L4), but a coronal image of reconstructed three-dimensional CT showed recurrence of foraminal stenosis at the operated level because of bone regrowth of the L5 pedicle ( Figure 5).

Discussion
LFS is defined as compression of a nerve at a site between the medial and lateral borders of the pedicle. 10  Also, using the tubular retractor minimizes muscle invasion.
Clinical results of the microendoscopic technique used in this study were favorable and similar to those of TLIF and posterior lumbar interbody fusion (PLIF), which are conventional LFS treatment methods (Table 2). [12][13][14] However, two patients experienced deterioration of clinical outcomes, and revision surgery for LFS recurrence was required in two other cases.

15
DLS, which is common among the elderly population, is a major cause of LFS. 15 Actually, of the 16 patients who were successfully followed up more than two years in this study, 5 (31.3%) had DLS with Cobb angles of more than 10 degrees. Of these five patients, two showed increases in Cobb angle; one underwent TLIF for recurrence of LFS at the Theoretically, compared to decompression alone, spinal fusion requires more aggressive intervention, requires longer operative times, and necessitates spinal implant insertion. All of these may increase the risk of complications. Therefore, it is reasonable to choose decompression alone when the patient's primary symptom is neurogenic claudication, especially for 15 elderly patients with comorbid conditions. Development of adjacent segment disease is another major issue encountered after spinal fusion. The current microendoscopic procedure seems to offer the potential for avoiding unnecessary fusion surgery in LFS cases. We recommend the current minimally invasive microendoscopic method for patients who have LFS with neurogenic claudication as a primary symptom without or with mild DLS.    A. Before surgery.

Figure Legends
14 Microendoscopic Decompression for Lumbar Foraminal Stenosis B. Six months after surgery. Clearance between the L4 and L5 pedicles was enlarged by removing the caudal aspect of the L4 pedicle (circle).
C. Thirty-four months after surgery. Clearance between the L4 and L5 pedicles decreased because progressed degenerative scoliosis (circle).