Minimally Invasive Lumbar Interbody Fusion

Summary of Key Points

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Minimally invasive lumbar fusion is being increasingly performed with A growing list of possible indications.
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The goals of minimally invasive surgery are to decrease tissue disruption and thereby achieve similar or better clinical outcomes with decreased patient morbidity.
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Biomechanical studies have shown improved stability with minimally invasive decompression alone versus open decompression and with preservation of the supraspinous ligament.
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Minimally invasive transforaminal interbody fusion (MIS TLIF) can be done with unilateral or bilateral instrumentation, with evidence showing a higher rate of pseudarthrosis with unilateral instrumentation.
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MIS TLIF has been shown to have decreased operative time, anesthetic time, intraoperative blood loss, infection rates, postoperative narcotic use, reduced reoperation rates for adjacent segment disease and earlier time to mobilization compared to open TLIF.
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Patient-reported outcomes are similar or better with MIS TLIF compared to open TLIF.
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Multiple studies have shown an overall hospital cost benefit with MIS TLIF compared to open TLIF.

Lumbar fusion is an increasingly performed procedure in the treatment of spinal disease. This is especially so with the treatment of degenerative spine disease. The aim of lumbar fusion is to eliminate abnormal motion and instability, allowing for restoration or maintenance of spinal alignment and spinal load-bearing capacity. A variety of etiologies of spinal instability including iatrogenic bone removal, cases of spinal stenosis, spondylolisthesis and symptomatic degenerative disc disease can be treated with symptomatic relief through fusion. The indications for lumbar fusion continue to expand and evolve, as does the technology and techniques for performing fusion. Weinstein and colleagues examined U.S. Medicare data to determine a significant increase in the number of lumbar fusion surgeries in patients over age 65 from 0.3 per 1000 in 1992 to 1.1 per 1000 in 2003. They also found a 500% increase in the annual expenditure for lumbar fusion surgeries, at $482 million USD in 2003. Given the aging population, the presentation of degenerative spine problems to spine surgeons can only be expected to increase. In the latest guideline update for the performance of lumbar fusion surgeries, moderate evidence (grade B) was given for the option of an interbody device to enhance fusion rate. This was not definitively shown to translate to an improvement in clinical outcomes. It has, however, been difficult to demonstrate that an increased fusion rate leads to improved clinical outcomes. It is important to recognize that a posterolateral fusion is not recommended when an interbody fusion is placed, due to increased complication rates without an increased fusion rate (moderate, grade B).

These recommendations provide the background evidence for the performance of transforaminal lumbar interbody fusion (TLIF). The TLIF was first described in 1982 by Harms and Rolinger as a procedure that facilitated circumferential lumbar fusion through a single posterolateral approach. It gained popularity and has been technically performed for many years with good clinical results and efficacy as an open technique for a variety of procedures requiring stabilization. Increasing concern has been raised about the extensive muscle dissection and retraction required to expose the anatomic landmarks required for the procedure. Deleterious effects of extensive soft tissue dissection, retraction, and devitalization have been recognized and published, including greater pain, slower time to mobilization, and increased infection rate. Postoperative histologic and imaging studies have demonstrated that open surgical approaches to the spine are associated with additional scar tissue formation, and significant muscle stripping related to muscle retraction, which all serve to adversely affect patient outcomes and eventually lead to higher reoperation rates.

Minimally invasive spine surgery (MISS) is one of the most recent revolutions to occur in the treatment of the spine. Minimally invasive techniques, compared to “open” techniques, are considered to be those that use smaller skin incisions, spare the muscle and ligamentous complexes, and often utilize a tubular muscle retractor ( Fig. 171-1 ). Using these principles as guidelines, it is definitely possible to use minimally invasive techniques to achieve lumbar fusion, to decompress lumbar canal stenosis, and to correct or stabilize degenerative scoliosis and spondylolisthesis. The efficacy in comparison to open techniques will be discussed, as well as a brief description of techniques that can be implemented.

The notion of using smaller incisions and less tissue disruption has been present in all branches of surgery and in spine surgery has expanded rapidly since the 1960s. Most of the evolution in technique occurred concurrently with the rapid expansion in technology, allowing for intraoperative spinal imaging, improved microscopic and endoscopic video imaging techniques, and the development of the tubular dilation and retraction system. The majority of MISS techniques use a progressive muscle dilation and tubular retraction method. From the basic indication of the microendoscopic discectomy, the indications for MISS have rapidly expanded to a wide variety of procedures and pathologies. MISS has been developed for the cervical, thoracic, and lumbar spine for both anterior and posterior approaches. The field has been rapidly advancing as more surgeons train and become experts in these techniques to the degree that MISS approachable pathology now includes intradural spinal tumors and even spinal deformity. This chapter focuses on the literature and techniques as they pertain to transforaminal interbody fusion in the lumbar spine.

Minimally invasive modifications of the TLIF (MIS TLIF) is a development that aims to achieve the same goals of the open TLIF procedure while reducing complications and postoperative pain to help maximize patient recovery. We provide a summary of MIS TLIF technique as well as the literature outlining the efficacy and feasibility of MIS TLIF.

Technique

The MIS TLIF procedure, as described here, is preferred by the senior author. Based on current published evidence, the procedure can be done unilaterally with an adequate decompression and augmented with either bilateral or unilateral percutaneous pedicle screws; the choice of which remains a topic of discussion. Preoperative planning requires ensuring all appropriate equipment for the technique is available. At the minimum, C-arm fluoroscopy is required for tube docking and instrumentation during MISS approaches. Many surgeons advocate for the use of intraoperative imaging guidance for screw placement; this is an option but not a necessity. A tubular retractor is a basic equipment requirement for MISS. Many surgeons prefer to use expandable tubular retractors in the case of MIS TLIF. For visualization, the senior author prefers an endoscope, which also facilitates easy monitoring of surgeons-in-training in academic centers. Other visualization options include an operating microscope or loupe vision with a headlight or with fiber-optic lights that can be affixed within the tubular retractor. The majority of dissection and discectomy instruments remain the same as for open procedures, with a longer barrel and with bayonetted handles that allow easy manipulation and visualization while working through a long tubular retractor. The drill is required for bone removal during MIS TLIF, and the use of an adjustable drill guard can help to safely retract the thecal sac while drilling the underside of the contralateral lamina to achieve bilateral decompression from a unilateral approach, in those cases where such is deemed necessary.

The procedure is performed under general anesthesia, with the patient prone on a Wilson frame over a radiolucent table, or on a Jackson table. Care is taken to pad all pressure points and bony prominences. The patient’s arms are placed on arm rests, taking care to adjust the pressure and the amount of tension on the shoulders and therefore the brachial plexus. After routine sterilization and draping are performed, the surgical level is localized using lateral fluoroscopy. It is our practice to drape the fluoroscopy into the field. In cases where intraoperative imaging guidance is to be used, a percutaneous stab incision is usually made over the posterior superior iliac spine and an intraoperative localization array can be screwed into the posterior superior iliac spine (PSIS) until it is rigidly affixed, prior to obtaining the intraoperative computed tomography (CT) or 3D fluoroscopy.

An approximately 2.5 cm rostral-caudal incision is marked 4 cm lateral to midline, and it is infiltrated with local anesthetic prior to incision. Once the incision is made through the skin and into the subcutaneous tissue, a Kirschner wire (K-wire) is pushed through the fascia and docked to the facet joint of the appropriate level under fluoroscopic guidance. Once this is confirmed, the fascial incision is opened and then sequential muscle-splitting tubular dilators (METRx, Medtronic Sofamor Danek, Memphis, TN; see Fig. 171-1 ) are then passed over one another until the desired working channel size (24-mm to 26-mm fixed retractor or expandable retractor) is obtained. The retractor is affixed to the table using an adjustable arm. If an assistant is present, some surgeons use a bilateral approach to achieve bilateral decompression and perform the TLIF portion on only one side; however, this is not the authors’ preferred MISS approach.

Removing residual soft tissue with the aid of monopolar cautery and pituitary rongeurs exposes the bony surface through the tubular dilator. Once the bone is exposed and anatomic landmarks identified, a unilateral hemilaminectomy and facetectomy, respectively, are performed with the use of an osteotome, Kerrison rongeurs, and a high-speed drill. Careful use of an osteotome can allow for quick removal of the facet joint, facilitating the cuts and preservation of the pedicles with fluoroscopic guidance ( Fig. 171-2 ). With the facet removed and a laminectomy performed, both the exiting and traversing nerve root can usually be identified. In the case of spondylolisthesis, there may be scarring around the exiting root, and it may be compressed tightly against the rostral pedicle. Identification and mobilization of the nerve roots, when possible, allow for wide exposure of the disc space and annulus and a larger entry portal through which to work. The annulus is opened along each end plate with as medial and lateral a cut as can be safely achieved. The disc is removed in the usual fashion, removing as much disc as possible prior to turning toward end plate preparation. If the disc space is small, an osteotome can be used to cut Sharpey fibers where the annulus attaches at the superior and inferior end plate, and in the same maneuver a portion of any osteophyte or overhang of the end plates that may be limiting the exposure is also removed; it is critical to avoid violation of the cortical end plate when performing this maneuver. A variety of curettes and rasps can be used to remove cartilage and disc material to optimize fusion surface area. Interbody cage trials are then inserted from smallest to largest until the desired size is reached. The appropriate size can be confirmed with fluoroscopy and by checking manually for the pullout fit of the cage within the disc space ( Fig. 171-3 ). Autograft supplemented with allograft when necessary and recombinant human bone morphogenetic protein 2 (BMP, Infuse, Medtronic, Memphis, TN) can be split between the interspace and the interbody cage and then tapped into the center of the disc space. BMP has been noted to be associated with ectopic bone growth particularly in the neural foramina, in some cases leading to postoperative radiculitis; so although it is commonly used, it should be used sparingly.

Instrumentation with percutaneous pedicle screws is performed through the same incision on the ipsilateral side ( Fig. 171-4 ). Anteroposterior (AP) projected fluoroscopy visualizes the pedicles, and a Jamshidi needle can then direct the K-wire into the pedicle. The senior author favors the “bull’s-eye” technique for a straight on pedicle cannulation trajectory. The K-wire is then advanced to approximately two thirds of the vertebral body depth with lateral fluoroscopy. A cannulated bone tap is placed over the wire, followed by a cannulated screw attached to a screw extender with fluoroscopy obtained throughout screw insertion. Screws are then placed at the level below; if sacral, S1 screws are placed bicortically, with radiography used to confirm pedicle screw breach of the anterior sacral cortex. Extenders then join the screws and a rod is directed percutaneously through the screw heads, once more with lateral fluoroscopy performed to achieve desired placement ( Fig. 171-5 ). The screws are then compressed along the rod and set screws are tightened with a torque wrench. Placing the interbody cage as ventral as possible against the anterior longitudinal ligament combined with compression between the pedicle screws can promote fusion by applying compressive force on the graft and can also help to create lordosis, using the graft as a fulcrum. Screw placement is then similarly repeated on the contralateral side. Incisions are copiously irrigated with antibiotic solution, closed in multiple layers, and dressed appropriately.