Transforaminal Approach for Endoscopic Decompression

35 Transforaminal Approach for Endoscopic Decompression

Ralf Wagner

Summary

The transforaminal approach for endoscopic decompression gives different options for accessing a disc herniation or spinal stenosis. Different techniques should be chosen depending on the pathology. Diverse tools and instruments allow approaching the spinal canal in almost any possible direction, only limited by neural structures or anatomic borders, such as the retroperitoneal organs. Small changes in entry point and needle angulation make a difference in clinical outcome. Thorough preparation of the surgery is essential to avoid complications. The endoscopic surgeon should choose the approach with the least bone destruction to guarantee a safe and fast procedure. The key to success in endoscopic decompression surgery for degenerative lumbar spine disease is to work on the bone, not in the disc.

Keywords: endoscopic decompression migrated disc foraminal stenosis lumbar stenosis disc herniation transforaminal approach intracanal technique transpedicular technique

35.1 Introduction

In a lumbar motion segment, the neuroforamen consists of the superior articular facet (or superior articulating process = SAP, see Fig. 35.1), lower and upper pedicle, posterior wall of the cranial and caudal vertebra, the disc, the interarticular portion, and the flaval ligament. The posterior intervertebral disc, covered by the lateral expansion of the posterior longitudinal ligament, provides a major part of the ventral border of the neuroforamen. The joint capsule of the articular facet and the ligamentum flavum contribute as major components of its dorsal limitation.¹ Loss of the disc space leads to a subluxation and spondylophyte formation of the facet joint, changes in biomechanical forces, and hypertrophy of the ligamentum flavum. The loss of disc height in the intervertebral space and apposition of bone and ligamentous structures result in a narrowing of neural structures in the spinal canal or the neuroforamen.²^,³

Fig. 35.1 (a) Physiological foramen intervertebrale; (b) loss of disc height with hypertrophy of ligamentum flavum and narrowing; (c) Processus articularis stenosis, facet subluxation, and nerve impingement. IAP, inferior articular process; SAP, superior articular process.

Spinal stenosis is a wide term for different entities that decrease the available space for the neural structures in the spinal canal. Lateral spinal root stenosis occurs in 8 to 11% of surgical cases of lumbar degenerative disease. Lumbar spondylolysis, hypertrophy of the superior facet joints, hypertrophy of the ligamentum flavum, disc bulge, protrusion, and osteophyte formation can lead to an impingement of the exiting nerve root⁴^,⁵ (Fig. 35.1). According to Postacchini three different forms of degenerative lumbar spinal stenosis can be identified.⁶ The term central stenosis includes: (a) stenosis of the central part of the spinal canal, which also often involves the lateral recess; (b) lateral recess stenosis, which often affects the course of the traversing nerve root; and (c) foraminal stenosis leading to compression of the exiting nerve.

The intervertebral foramen and its elliptical shape are responsible for many of its relational problems. The vertical diameter of the foramen varies from 12 to 19 mm. A complete disc collapse rarely contributes to nerve compression; however, the sagittal diameter may be as little as 7 mm. Especially in L4/L5 and L5/S1, the tolerance for pathological alterations is limited.¹ Dynamic changes in the lumbar neuroforaminal dimensions were observed in positional magnetic resonance imaging (MRI) of young adults, showing an increase in flexion and decrease in extension of all motion segments except L5/S1. Lumbar angular motion showed an effect at L3/L4 and L4/L5 foraminal width only.⁷

Transforaminal ligaments are frequently found in the intervertebral foramen, often limiting the space of the exiting nerve root.⁸^,⁹ The dorsal root ganglion (DRG) may measure over 6 mm and is often located in the neuroforaminal space at the level of L5/S1. Width of pedicles and DRG increase from L1 to L5. The DRG is found more lateral while going from L1 to L5.¹⁰ Kikuchi et al determined the location of the DRG at L5 to be 19.2% intraspinal, 72.8% intraforaminal, and 8% extraforaminal, also potentially limiting the transforaminal approach at the level of L5/S1 and effecting the postoperative course.¹¹ Dynamic linear movement of the DRG of up to 5 mm with increasing movement during hip flexion of 30 degrees was observed in cadaver studies by Smith et al.¹²

The foraminal ligaments might limit distal displacement of the exiting nerve roots.¹³ A central or lateral recess stenosis leads to an anterior shifting of the traversing nerve root, which needs to be evaluated before surgery. Disc herniations may also displace the traversing nerve root laterally and therefore increase the risk of injury. Highly migrated disc herniations as well as hidden zone pathologies require a clear and structured preoperative planning.¹⁴^,¹⁵

The endoscopic approach highly depends on the location of the pathology. Preoperative evaluation of the patient’s anatomy and radiological and clinical findings have to be considered altogether to find the appropriate endoscopic approach to the lumbar spine.

35.2 Indications

The transforaminal endoscopic approach is used for disc herniations in all areas of the lumbar spine, lateral recess stenosis, central spinal stenosis, foraminal stenosis as well as extraforaminal pathologies (Fig. 35.2). This approach can be advantageous for recurrent disc herniations after microdiscectomy, adjacent level disease after fusion, facet joint cysts, discitis as well as resection of displaced bone fragments after fusion or arthroplasty.

Fig. 35.2 Pathology determined approaches.

35.3 Contraindications

Although there is no clear contraindication, some clinical findings make the surgery far more difficult: nerve root anomalies, such as low exiting nerve root or doubled nerve root; high iliac crest for level L5/S1; primary or secondary spinal stenosis in combination with hypertrophic facet joints; severe spinal deformity; and severe spondylolisthesis (Grade II Meyerding or higher).

35.4 Preoperative Planning

The physical examination of the patient helps in localizing the affected nerve root. The neurological examination and the nerve root tension sign are obligatory to further locate the compressed nerve root. In multiple degenerative lumbar pathologies, facet joint blocks or radicular blocks are used to further define the symptomatic level. The imaging diagnosis consists of (i) plain X-ray films in standing posture to detect a high iliac crest or scloliosis, (ii) flexion and extension X-ray to rule out instability, and (iii) MRI scans of the lumbar spine. A computed tomography (CT) scan might be useful to determine calcifications in chronic lesions. The clinical symptoms of the patient should have a correlate on the imaging diagnostics. In disc herniations, the exact location of the traversing nerve root, sequester size, and spinal canal anatomy should be evaluated preoperatively. In case of stenosis, the extent of compression determines the amount of bone removal.

35.5 Patient Positioning

The patient can be posititioned in the prone or lateral position (Fig. 35.3). Advantages of the prone position are an easier orientation for the surgeon, the possibility of bilateral surgery, and combined interlaminar and transforaminal surgery. The degree of required side bending or delordosing is less in prone position. The abdominal pressure on the epidural veins should be avoided by correct patient positioning. The lateral position has advantages in obese patients, in patients with deformity, and in surgeries under local anesthesia. The table can be angled to achieve changes of the iliac crest position and to open up the neuroforamina, without putting pressure on the epidural veins. Testing of the straight leg sign in patients under local anesthesia is possible. The lordosis can be straightened by hip and knee flexion of the patient in lateral position.

Fig. 35.3 (a) Operating room (OR) setup; (b) lateral and prone patient positions.

35.6 Surgical Technique

The procedure is performed under local or general anesthesia. The author prefers general anesthesia with short acting muscle relaxants. The correct disc level is identified with lateral and anteroposterior (AP) C-arm imaging. Disc level, iliac crest, midline, and trajectory of entry as well as the lateral facet joint line are marked on the skin (Fig. 35.4). The pathology determines the trajectory and the volume of bone removal for the transforaminal procedure.

Fig. 35.4 Lateral patient position and marking.

35.6.1 Surgical Steps

1.Needle insertion, needle placement, and skin incision:The needle insertion is performed under lateral X-ray. The needle tip first touches the superior articular process (SAP), then it is advanced to the neuroforamen. After AP and lateral X-ray control of the needle placement, a K-wire is inserted and then the skin incision is made. In case of foraminal or lateral recess stenosis a Jamshidi needle is docked on the bone of the SAP (Fig. 35.5). Refer to B. Different approaches section for other approaches.

2.Dilation of soft tissue:Soft tissue is gradually displaced using sequential dilators increasing in diamenter.

3. Widening of foramen with coronary reamers or bone drills:The neuroforamen is widened with coronary reamers with increasing diameters and side-cutting blunt bone drills. They also resect most of the yellow ligament. The medial pedicular line must not be crossed.

4.Insertion of working tube:The working tube is inserted with its lip first placed caudally (Fig. 35.6a), then turned into the direction of the exiting nerve root (Fig. 35.6).

5.Identifying the SAP and pedicle, arch of Wagner:A clear understanding of cranial/caudal and medial orientations of the endoscope (joimax^®) is necessary to identify the pedicle and superior facet (Fig. 35.6c and Fig. 35.7).

6.Preparation of epidural space:The lateral recess is prepared from caudal to cranial. The site of disc herniation is identified (Fig. 35.6d).

7.Resection of disc herniation:The disc herniation is resected. In cases of a contained disc, the posterior ligament has to be incised.

8.Control of decompression:The nerve root should be visualized and then decompressed starting at the lower pedicle to the site of the exiting nerve root.

9.Removal of instruments and skin closure:The instruments are removed and the skin is closed.