Minimally Invasive Transforaminal Lumbar Interbody Fusion

Vishal C. Gala and Regis W. Haid Jr.

Low back pain is among the most common reasons individuals seek medical attention in the United States.1 Although the etiology of low back pain is multifactorial and most often managed with conservative, nonsurgical treatment, some patients require more intensive treatment in the form of surgery. Lumbar spinal fusion is sometimes employed in the treatment of degenerative conditions of the spine, such as facet arthropathy, degenerative disk disease, and spondylolisthesis, as well as in selected cases of spinal fractures, scoliosis, and tumors. Several studies over the past 2 decades have shown that the ability to achieve successful arthrodesis of spinal segments is enhanced by the use of instrumentation.² The use of spinal instrumentation, such as interbody spacers, pedicle screws and rods, and spinous process plates, has become widely accepted as the standard of care in lumbar spinal fusion surgery.

The use of spinal instrumentation has traditionally required wide and extensive exposure of the osseous anatomy of the lumbar spine to provide adequate exposure and visualization of anatomical landmarks. As a consequence, significant muscle dissection is required, resulting in denervation of the paraspinous musculature, bleeding, and retractor-induced muscle injury due to ischemia.^3–5 In addition, the dissection of the supporting ligamentous structures of the spine can often result in iatrogenic instability at adjacent levels.

Technological advancements in equipment and instrumentation, such as digital fluoroscopy, image guidance, high-resolution endoscopy, and microscopy, along with tubular dilators and working channels, have allowed for the development of minimally invasive surgical (MIS) techniques for the surgical treatment of degenerative spinal disorders. With the use of small portals of entry or working corridors, injury to the surrounding muscles and ligaments of the spine is minimized. Once appropriate access is obtained, the same surgical objectives of decompression of the neural elements and/or stabilization of the spine may be accomplished. Therefore, perhaps the term minimal access is a more accurate description. The array of modern MIS procedures for the spine are distinguished from previous “fad” procedures for the spine in that they are not limited to treating a single pathological process, nor are they based upon a single, heavily marketed device or technology. Rather, they are a concept that may be applied to several different clinical scenarios throughout the length of the spine.

With respect to the lumbar spine, the development of tubular retractor systems along with percutaneous pedicle screw systems has allowed for the application of minimally invasive techniques for conditions requiring fusion in the lumbar spine. This chapter describes the technique of minimally invasive transforaminal lumbar interbody fusion (MI-TLIF) along with the indications, decision-making process, and potential complications.

Preoperative Evaluation

All patients should undergo a detailed and through history and physical examination. Radiologic evaluation must include either magnetic resonance imaging (MRI) or postmyelogram computed tomography (CT) to define the patient’s specific pathoanatomy. Anteroposterior (AP), lateral, and dynamic flexion-extension plain radiographs are also critical to evaluate the patient for instability. Electromyography and nerve conduction studies may also provide supporting evidence to localize a patient’s specific radiculopathy.

Operative Technique

Patients are placed under general endotracheal anesthesia. A Foley catheter is placed. Use of an arterial line is discretionary based upon the patient’s comorbidities. Sequential pneumatic compression devices are placed on the lower extremities. Leads are placed for somatosensory evoked potentials, and free run electromyography and baselines are obtained. Neurophysiological monitoring allows for continuous assessment of the integrity of the involved nerve roots and allows for the option of screw stimulation to ensure that the pedicle has not been breached during instrumentation.

The patient is placed in the prone position on a Jackson flat-top table outfitted with gel chest rolls. The flat-top table facilitates easy movement of the fluoroscopic C-arm during the operation. Gel rolls allow for maintenance of normal lordosis as opposed to a Wilson frame, which may place the patient in some degree of kyphosis or flat back. Prophylactic antibiotics are administered and standard surgical prep is performed. The surgeon is positioned on the side of the patient’s most significant pathology. In cases of bilateral pathology, it is the surgeon’s preference. The base of the fluoroscope is placed on the side opposite the surgeon.

With the assistance of AP fluoroscopy, the midline is marked. From there, a parallel line is drawn 4 to 4.5 cm laterally. A lateral image is obtained to localize the disk space of interest. In two-level cases, the incision is localized over the intervening vertebral body. Local anesthetic (0.25 to 0.5% bupivacaine) is injected into the skin, subcutaneous tissues, and musculature. The needle is also used to confirm the trajectory into the disk space. A small puncture incision is then made with a #11 blade. A Steinmann pin or K-wire is then advanced in a slightly medial trajectory toward the ipsilateral facet complex. Fluoroscopic guidance should be utilized during this process of docking and dilation to ensure that the spinal canal is not entered. Once confirmation is obtained by fluoroscopy that the K-wire is docked on the facet, the incision is lengthened symmetrically to match the diameter of the tubular retractor, typically 20 to 25 mm (depending on surgeon preference). The first dilator is then placed over the K-wire and the K-wire removed. Utilizing a rotatory motion, sequential dilators are then placed to split the musculature until the diameter of the tubular working channel is reached. Several different proprietary systems are available; some offer expandable blades as well. The working channel is then secured with a table-mounted flexible arm based on the side opposite the surgeon. At this point, the operation may proceed under loupe magnification with a headlight or with the assistance of an operating microscope.

The working channel should be docked over the facet of interest. It is paramount to carefully identify the midline spinous process and ipsilateral facet joint prior to any bone removal. With the oblique orientation of the tubular retractor, it is possible to inadvertently cross to the contralateral side of the spinal canal, especially in an obese patient. The facet should be visualized in the lateral half of the working channel, with the laminofacet junction and lateral portion of the lamina in the medial half of the working channel. Monopolar cautery is then used to dissect off any remaining soft tissue. The entirety of the working channel should be cleared of soft tissue to optimize visualization of the relatively small working corridor. The prominence of the top of the facet facilitates identification of the bony elements, and dissection should begin here to avoid entry into the spinal canal. The inferior edge of the superior lamina is then identified and the sublaminar plane developed with a curette. Utilizing curettes, rongeurs, and a high-speed drill, the surgeon performs a hemilaminotomy, extending rostrally to the superior pedicle and then caudally to the inferior lamina to the level of the inferior pedicle. A generous laminotomy and facetectomy must be performed to safely access the disk space and insert an interbody graft. Fluoroscopy should also be utilized to ensure that the pedicle is not entered with the drill.6 The ligamentum is left in place to allow safer removal of the bone. To avoid potential dural injuries, all bony decompression should be performed prior to excision of the ligamentum flavum and exposure of the dura and neural elements. With a small working corridor and a narrow working angle, primary repair of a dural rent may be challenging but is possible with the use of microsurgical instruments. For small tears, onlay dural substitute may be placed over the durotomy and then a coat of fibrin glue or commercially available dural sealant placed over the synthetic pledget. Only in cases where persistent cerebrospinal fluid (CSF) leakage is observed after this maneuver is the option of a lumbar drain considered. Converting the procedure to an open one will typically serve to enlarge the area for a potential pseudomeningocele to form; therefore, it is best to maintain a smaller potential space. If the durotomy occurs prior to the interbody portion of the procedure, consideration may be given to proceeding with a posterolateral fusion alone.⁷

Bone fragments can be harvested for use as autograft. After bony decompression is completed, the subligamentous plane is developed and the ligamentum flavum excised. The disk space, lateral thecal sac, and traversing nerve root should be visualized, whereas the exiting nerve root may or may not be visualized depending upon the extent of bony removal. The thecal sac and nerve root should be gently retracted medi-ally and all epidural veins over the disk space coagulated. If an adequate exposure has been obtained, minimal retraction is required. In rare instances of large exiting nerve roots or conjoined nerve roots that cover the disk space, interbody fusion may not be feasible. In these instances, consideration should be given to performing a posterolateral fusion alone.

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