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Minimally Invasive Lumbar Discectomy: Indications and Techniques
Introduction
The first accurate clinicopathologic descriptions of lumbar disc herniation, by Dandy and Mixter and by Barr, date as far back as 1929 and 1934, respectively. , The technique for open discectomy as practiced today is a modification of the technique described by Love in 1938. , The current “gold standard” for surgical intervention, microdiscectomy (MD), involving the use of the operative microscope to optimize visualization and minimize exposure, was described by Caspart and Yasargil in 1977.
The first minimally invasive approach to the lumbar spine was reported in 1987 by Kambin, who performed the first fluoroscopically assisted percutaneous lumbar discectomy. In 1991, Faubert and Caspar took the next step when they described tubular access to the spine using cannulae and guidewires, which paved the way for the development of tubular retractor systems. Foley and Smith further refined this technique in 1997 and introduced the microendoscopic discectomy (MED) procedure, which was essentially a muscle-splitting tubular approach employing a series of concentric dilators followed by a tubular retractor to remove the disc under visualization with an endoscope. More spinal surgeons adopted these techniques in 2003, as the microscope was adapted for use with tubular retractors.
The culmination of these efforts—to minimize exposure, avoid muscle injury, and hence reduce postoperative pain and discomfort—has led to the development of minimally invasive spine surgery (MISS) as we know it today, comprising essentially three broad categories : (1) percutaneous surgery, wherein access is established with a needle or cannula with visualization by fluoroscopy or computer-assisted navigation; (2) percutaneous endoscopic surgery, wherein access is obtained by a cannula while an endoscopic camera is used for visualization; and (3) tubular surgery, wherein a series of tubular dilators and retractors are used to establish access while anatomic structures are usually visualized by the naked eye, sometimes aided by adjunctive magnification (loupes or a microscope).
A clear distinction between different minimally invasive techniques remains unclear. A technique can be classified as MISS if it demonstrates certain generally accepted minimally invasive surgical principles including a smaller incision size and surgical corridor, less muscle manipulation, and lower perioperative morbidity. This chapter describe percutaneous endoscopic and tubular MISS techniques in reference to the treatment of lumbar disc herniation.
Endoscopic Lumbar Discectomy
Indications
Percutaneous endoscopic lumbar discectomy can be performed either through a dorsolateral or oblique transforaminal approach or a midline interlaminar approach. Although a majority of the disc herniations can be addressed through the transforaminal approach, the interlaminar approach can be utilized for paracentral disc herniations at L4–L5 and L5–S1. It necessitates puncturing the ligamentum flavum to access the disc just lateral to the thecal sac. Owing to technical difficulty of the latter, the transforaminal approach is most commonly employed and is the focus of discussion in this section. It allows relatively easy access to far lateral and foraminal pathologies, while central pathologies may be more challenging to remove.
Surgical Technique
The setup for the endoscopic discectomy requires an adequately sized room to accommodate the operating table, fluoroscopy unit and monitor, endoscope, and image processing equipment. The patient is typically positioned in lateral position or prone on a specially designed radiolucent hyperkyphotic carbon frame to enable 360-degree fluoroscopy. Although the lateral position is more comfortable for the patient and easier for the surgeon and anesthesiologist, since it enables easier airway control and lower venous bleeding risk, the prone position is recommended if bilateral access is planned. It also offers easier patient positioning and relative anatomic familiarity. Local anesthesia with mild sedation and pain control can be used. Although some providers prefer general anesthesia to avoid the need for medication titration and a shorter operative time, most surgeons recommend local anesthesia with mild sedation because of minimal anesthesia-related risk and real-time feedback from the patient in case of neural irritation, hence offering an additional layer of safety. It also makes it possible to perform the procedure on an outpatient basis.
Gaining optimal needle access to the disc under fluoroscopic guidance remains the most crucial step in the posterolateral endoscopic transforaminal approach. The standard site of access is the Kambin triangle, which is a hypothetical right triangle just lateral to the facet defined superiorly by the caudal border of the superior vertebra, anterolaterally by the traversing nerve root, and the posteromedial border formed by the exiting nerve. The access point at the skin is defined by fluoroscopic view of the anatomy and foraminal size and ranges between 6 and 14 cm from the midline, depending on the level, with higher distance required for more caudal lumbar vertebral levels. The distance from the midline also determines the optimal angle of trajectory, usually ranging between 25 and 50 degrees, to reach the anatomic center of disc through the foramen. Landmarks for entry can be modified depending on individual patient characteristics such as body habitus and size of neural foramen. After surface marking of the skin entry point, an 18-gauge spinal needle is introduced under fluoroscopic view with the trajectory aimed at the foramen. Yeung and colleagues advocate using an endoscopic system (YESS) featuring cannulas with beveled tips and Penfield-like extensions. The needle is advanced until bony resistance is encountered at the facet joint. The angle of trajectory is then increased to reach the neural foramen. The bevel is turned dorsally to expose the epidural space and protect the exiting nerve root while the tip is in the foramen. The location of the needle tip is confirmed on a lateral projection. Ideally, it should be closer to the posterior border of the foramen. Care must be taken to avoid violation of the facet joint capsule. While monitoring on a posteroanterior view, the needle is then advanced anteromedially to reach the midline and the location is confirmed on a lateral view. Needle-tip placement in the posterior third of the disc space is considered ideal. Optionally, at this point, a discogram can be performed to clearly delineate the site of pathology. A guidewire is inserted and the needle is withdrawn. The fascia and muscles are widened with a series of 6- and 8-mm dilators. Annular fenestration is then performed with blunt-tipped drills or an obturator with mallet to widen the foramen until a working port for the endoscope can be docked firmly at the annular window. The endoscope is then inserted to visualize the annulus and disc nucleus. The camera may be positioned either in the disc space or in the foramen outside the annulus depending on the location of the pathology. A low-pressure system is employed to repeatedly flush the field with saline in order to improve visualization. Removal of the herniated disc is then accomplished with micropituitary rongeurs; thorough decompression of the nerve root is thus performed. The working channel may be rotated to get a better view of the field in all directions. Bony decompression can be performed with micro-Kerrison rongeurs and osteotomes.
Achieving adequate decompression remains an important challenge of the technique and requires a thorough inspection of the working cavity. Clear visualization of the traversing nerve root and dural pulsations are important indicators. As mentioned, a prior discogram may aid in highlighting residual disc tissue, which can subsequently be removed.
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