The clinical features of, and microendoscopic decompression for, extraforaminal entrapment of the L5 spinal nerve

Abstract

Objective:  To evaluate the clinical results of, and surgical techniques for, microendoscopic (METRx) decompression of extraforaminal entrapment of the L5 spinal nerve at the lumbosacral tunnel.

Methods:  Five patients with extraforaminal entrapment of the L5 spinal nerve in the lumbosacral tunnel were treated in our department, including three men and two women. The average age was 65.6 years. All patients suffered severe leg pain and neurological deficits compatible with L5 radiculopathy. Minimally invasive decompression of the L5 spinal nerve was performed under METRx intertransverse decompression.

Results:  With an average follow‐up of 17.8 months, clinical results were assessed based on Nakai criteria and Visual Analogue scale (VAS). All patients experienced immediate pain relief postoperatively. Clinical outcomes were excellent in three patients and good in two. The average intraoperative blood loss was 59 ml, with an average operative time of 103 min. Average post‐operative stay in bed was 7 days, and average cost was $1860.

Conclusion:  Extraforaminal entrapment of the L5 spinal nerve in the lumbosacral tunnel can cause L5 radiculopathy. METRx partial resection of the L5 transverse processes, sacral ala and osteophytes of L5‐S1 vertebral bodies to relieve extraforaminal entrapment of the L5 spinal nerve is a very effective and minimally invasive surgical option.

Introduction

In 1984, Wiltse et al. first reported lumbar radiculopathy caused by L5 spinal nerve entrapment in the lumbosacral tunnel, and described the related clinical syndrome as ‘far‐out syndrome’ ¹ . Although extraforaminal entrapment of the L5 spinal nerve presents severe radicular symptoms, it is often missed or misdiagnosed because of its low incidence ² . The purpose of this study was to evaluate the clinical features of extraforaminal entrapment of the L5 spinal nerve, and describe minimally invasive techniques for its decompression under microendoscopic (METRx).

Materials and methods

From June 2003 to September 2007, five patients with extraforaminal entrapment of the L5 spinal nerve were surgically treated in our department, including three men and two women whose mean age was 65.6 years (range, 59–78). All patients had low back pain, severe unilateral leg pain and neurogenic claudication to some extent. The symptoms of sciatica presented on the right side in three patients and on the left side in the other two. Physical examination revealed a positive straight leg raising sign in all five patients, weakness of the extensor hallucis longus in four patients, numbness in the L5 dermatome in two patients and severe painful claudication in three patients. Radiographs and computed tomography (CT) showed obvious signs of lumbar spinal degeneration, including narrowing of the intervertebral space and osteophyte formation at the margin of the vertebral body. Central canal stenosis and facet joint hyperosteogeny were also seen in three patients, and calcified disc herniation and osteophyte formation in two. Three patients underwent selective L5 nerve block therapy and gained temporary leg pain relief. All the patients had received conservative therapy for 4.5–17 months before surgery. Decompression of the L5 nerve root was accomplished via an intertransverse approach using the METRx system.

Surgical technique

After epidural anesthesia the patient was placed in a prone position on the frame. Under anteroposterior fluoroscopy, two horizontal lines were marked over the centers of the L5 and S1 pedicles, and one vertical line just lateral to the L5‐S1 facet joint. The incision was made along the vertical line and between the two horizontal lines. Usually, the site was 2.6–2.8 cm long and 2.5–3 cm away from the spinal process. A K‐wire was placed though the incision and soft‐tissue stripping performed on the surface of the facet joint using the smallest dilator placed over the K‐wire. Stripping from the lateral surface of the L5‐S1 facet joint or deep to the intertransverse space should be avoided, as it may injure the nearby vessel and L5 nerve and cause uncontrollable bleeding or nerve root injury. Sequential dilations were used to split the lumbar sacral muscle, then gentle stripping of soft tissue from the lateral surface of the facet joint to the intertransverse space was guided by palpation with the forefinger. A set of dilators were placed sequentially, and finally a modified 2.6 cm tubular retractor was inserted between the transverse processes. The endoscope and camera system were set, and muscles attached to the lateral side of facet joint, L5 transverse process and sacral ala were dissected and removed by high speed drill. Resection of the lateral one third of the L5‐S1 facet joint was performed using a Kerrison rongeur, a high‐speed burr was also used to remove some of the sacral ala and inferior part of the L5 transverse process to enlarge the L5‐S1 intertransverse space (Fig. 1). After coagulation with bone wax and bipolar electrocautery, the intertransverse membrane was exposed, dissected and retracted caudally from the medial and inferior edges of the L5 transverse process, utilizing a microblade and nerve dissector. The exiting L5 nerve root was usually located in the lateral part of the neural foramina and covered by a thin layer of fat. The exiting L5 nerve root was identified and retracted laterally to the end of the L5 transverse process, taking care not to injury the radicular vessels while using monopolar coagulation near the nerve root. Gentle dissection was performed beneath the exiting nerve root to identify and remove possible bony osteophytes which may have been causing nerve impingement. Exploration along the nerve route laterally toward the pelvic cavity was also necessary to confirm satisfactory decompression of the lumbosacral tunnel. Careful measurement of the intertransverse space and cautious preoperative planning were always important, as the intertransverse space usually presents a narrow and deep surgical field and significant anatomic variations. Partial removal of the L5‐S1 facet joint, L5 transverse process, sacral ala and even lateral part of the pars interarticularis was particularly helpful for exposure and release of the L5 nerve root. The incision was normally closed with negative pressure drainage after thorough hemostasis.

Figure 1.

A typical patient, male, 64 years old, complaining of low back pain and left leg pain, presenting weakness of extensor hallucis longus, and straight leg raising sign positive at 40°. (A, B) Osteophyte formation at posterior and inferior edge of L5 vertebra; (C) Removal of inferior part of L5 transverse process; (D) Partial removal of sacral ala using high speed burr to enlarge the L5‐S1 intertransverse space; (E) Retraction of intertransverse membrane by nerve retractor to expose L5 nerve root; (F) Removal of adjacent osteophyte by high speed burr; (G) Dissection and release of L5 nerve root by nerve dissector.

Postoperative management

Antibiotics were used for 3–5 days. Strenuous exercise was avoided for at least two or three months.

Results

All the patients were followed up by regular clinic consultation and telephone interview or email interview. The mean follow up period was 17.8 months (range 9–47). Satisfactory leg pain relief was gained in all five cases on the day of operation with no recurrence by final follow up. Clinical outcomes were assessed by the Visual Analogue score (VAS) and Nakai criteria ³ . The VAS of five patients was 8.6 ± 2.1 preoperatively, 3.3 ± 1.4 at one week post operation, and 2.7 ± 1.2 at six months post operation. The rank sum test for multiple group models for ordinal variables indicated statistical significance in improvement of radiculopathy. The result was excellent in three cases and good in two according to the Nakai criteria. The operation time was 80–145 min (mean 103 min). Blood loss was 45–90 ml (mean 59 ml). Off‐bed ambulation started from 4–11 days after operation (mean 7 days). Admitting cost was 12–16 thousand Yuan (mean 13.4 thousand Yuan). At six weeks post operation, strength of the extensor hallucis longus muscle had been restored to grade V in four cases and grade IV in one case. No complication occurred in any of the cases (Table 1).

Table 1.

Clinical symptoms and treatment outcomes of extraforaminal entrapment of the L5 spinal nerve in five cases

Case	Age	Sex	Clinical Symptoms and Signs					Operation time (min)	Blood loss (ml)	Ambulation (d)	Cost (thousand yuan)	Nakai criteria
			SLR (°)	Kempt	SD	EHL	NC
1	62	Male	45	+	+	3	+	85	40	6	12	Excellent
2	59	Male	50	+	+	2	+	80	45	4	12	Excellent
3	78	Female	35	+	+	2	+	110	70	9	14	Good
4	64	Male	40	+	+	4	+	145	90	11	16	Good
5	65	Female	50	+	+	4	+	95	50	5	13	Excellent

EHL, muscle strength of extensor hallucis longus; Kempt, augmentation test; NC, neurological claudication; SD, sensory depression; SLR, straight leg raising (°).

Discussion

Recent studies have indicated that extraforaminal entrapment of the L5 spinal nerve has been an important cause of sciatica. Wiltse et al. reported 26 cases who had lumbar radiculopathy caused by extraforaminal entrapment of the L5 spinal nerve, and described it as ‘far‐out syndrome’ ¹ . The related anatomic study of Wiltse et al. showed L5 nerve root to be pinched between the L5 transverse process and sacral ala. In a cadaver study of 42 cases, Nathan et al. found the lumbosacral tunnel, consisting of osteophytes lateral and posterior to the L5 vertebra, lumbar sacral ligament (LSL) and sacral ala may cause entrapment of the L5 spinal nerve ⁴ . In the same study, Nathan et al. also revealed that among all these factors, hyperplasia or contracture of LSL and osteophyte formation lateral and posterior to L5 vertebra were the major contributors to impingement on the nerve root ⁵ , ⁶ . Diagnosis of extraforaminal entrapment of the L5 spinal nerve should be based on the following evidence: (i) low back pain and typical radiculopathy including weakness of extensor hallucis longus and numbness in the L5 dermatome; (ii) radiograph and CT revealing osteophyte formation at the lateral and posterior edge of L5 vertebra, with no bony central canal or foraminal stenosis, as well as far‐lateral disc herniation; and (iii) selective L5 nerve radiculography showing impingement of L5. Meanwhile, an L5 nerve block may significantly relieve the radicular symptoms.

Up to the present, surgical decompression has been the most effective therapy for extraforaminal entrapment of the L5 spinal nerve. Matsumoto et al. first reported decompression of the L5 nerve root in four cases with extraforaminal entrapment of the L5 spinal nerve ² . In the first case, osteophytes were partially resected via an anterior approach with the assistance of laparoscopy. In the other three cases, partial removal of the sacral ala and decompression of L5 nerve root were performed via a microscopic posterior approach, although an anterior approach with laparoscopy provides a field for partial resection of osteophytes and can give excellent early results. Reulen et al. recommended the posterior approach, as anterior methods make surgical manipulations difficult and cause additional trauma ⁷ . Some other researchers have indicated that the traditional posterior operation may be equally complicated, because the L5‐S1 extraforaminal area is located very deeply in the posterolateral aspect of the lumbar vertebra and surgery may be obstructed by the sacral ala ⁷ . Recently, the METRx system has been widely used for minimally invasive decompression of lumbar stenosis and microdiscectomy. This technique has also proven to be effective for treatment of far‐lateral lumbar disc herniation, with major advantages including a clear and legible surgical view as well as less trauma ⁸ , ⁹ . Matsumoto et al. reported their experience in decompression of L5 spinal nerve extraforaminal entrapment under METRx ¹⁰ . In this new procedure, removal of part of the sacral ala, release of the L5 nerve and decompression of the spinal canal are all accomplished, and the results are promising. In 2003, we treated our first case of extraforaminal entrapment of the L5 spinal nerve using the METRx system. We found that the standard 1.6 cm tubular retractor of the METRx system that Matsumoto employed offered insufficient work space, which lead to frequent orientation of the tubular retractor. Moreover the 1.6 cm tubular retractor tended to sink into the intertransverse space and subsequently cause injury to the L5 nerve root or radicular artery. So we modified the work canal by using a 2.6 cm tubular retractor. Although a larger retractor results in a longer incision, it brings a series of benefits such as satisfactory working space, no need for frequent retractor orientation and no worry about unexpected injury to existing nerve and vessels caused by sinking of the retractor, as the canal can be tightly fixed over the L5 transverse process and sacral ala. Our experience in the five cases described here shows that partial resection of the L5 transverse process, sacral ala and osteophytes of L5‐S1 vertebral bodies to relieve extraforaminal entrapment of the L5 spinal nerve can be effectively accomplished using the METRx system. Meanwhile, the clear and amplified image of the monitor system provides additional convenience and safety for the procedure.