and J. Alex Thomas2
Keywords
Lumbar fusionPosterior lumbar interbody fusionTransforaminal lumbar interbody fusionRadiation reductionPedicle screw placementDecompressionCage placementSpondylolisthesis reductionIntroduction
Lumbar fusion is an effective treatment for low back pain secondary to degenerative lumbar pathology [1]. The posterior lumbar interbody fusion (PLIF), described by Cloward in the 1952, has long been considered the most popular technique of achieving lumbar fusion. Indeed, these methods of interbody fusion, now typically supplemented with posterior instrumentation, are still routinely performed today. Unfortunately, both of these techniques are known to have high complication rates: ALIF with its visceral, vascular, and male reproductive complications and PLIF with its complications associated with bilateral neural retraction [2].
Unilateral PLIF, first described by Blume and then popularized by Harms as the TLIF, reduced the risks of PLIF associated with excessive retraction of the neural elements. Compared to PLIF, TLIF allows for lateralized access to the disc space and foramen with less exposure and retraction of the neural elements all with preservation of the contralateral structural anatomy. Despite these potential advantages of TLIF over PLIF, open TLIF, like other open spinal procedures performed via midline incisions, is still quite destructive. The midline incisions and prolonged retraction time seen with open TLIF are associated with significant iatrogenic injury to supporting anatomical structures and thus may result in poor clinical outcomes [3–8].
The MIS-TLIF was introduced by Foley et al. in 2003 as a way to mitigate the collateral damage to supporting anatomical structures seen in open TLIF [9]. Despite an initial steep learning curve, experience with MIS-TLIF grew rapidly, and the procedure has become widely accepted [10–13]. Like other minimally invasive, muscle-sparing techniques, MIS-TLIF is associated with less blood loss, decreased risk of infection, faster return to ambulation, and shorter hospitalization [14–16]. Also, especially important in the era of value-based care, MIS-TLIF has been shown to be more cost-effective when compared to open TLIF [17, 18]. Certainly, circumferential fusions have improved clinical outcomes and are more cost-effective when compared to posterolateral fusions only. [19, 20]
Today, technologies such as image guidance, surgical robotics, expandable interbody spacers, and advanced spacer materials have only increased the ease and effectiveness of MIS-TLIF. Here we discuss indications, technical nuances, and outcomes of MIS-TLIF.
Indications
MIS-TLIF has the same indications as its open analog, namely, spondylolisthesis/instability, unilateral foraminal stenosis, recurrent disc herniation, focal kyphotic deformity, and discogenic pain [21]. Due to the significant reduction in wound/incision size, it is arguably a superior choice for obese and healing-challenged patients (i.e., diabetes mellitus, rheumatologic disease, etc.) [22–24].
Surgical Management
Positioning
This procedure is typically performed under general endotracheal anesthesia and, due to the brevity of the procedure, a Foley catheter is seldom necessary for one and two level cases. Preoperative antibiotics are given and serial compression devices placed. Neural monitoring is commonly used and significantly improves the safety and predictability of the procedure.
After induction the patient is placed in the prone position on the operating table (we prefer an open Jackson table) with the hips extended and the knees slightly flexed in order to maximize lordosis.
Radiation Reduction
MIS-TLIF is a fluoroscopy-intensive procedure. There are numerous studies in the literature documenting the health risks of excessive radiation exposure to the patient as well as the treatment team [25]. ALARA (As Low As Reasonably Achievable) is the practice of adopting methodologies to lower the radiation exposure in medical procedures as much as possible. With most modern C arms, there are some simple actions can dramatically lower the radiation exposure for both the patient and the team. Simple measures such as turning off auto-contrast, activating low-dose mode, and going to pulse mode (decreasing the number of pulses down to the lowest number where the image is still diagnostic and usable) can achieve 90–95% dose reduction. New technologies such as LessRay® can further help reduce the radiation exposure and improve image quality. Procedurally, standing on the image intensifier side of the table and using predominately AP flouro (and more sparing use of lateral imaging) further help to lower exposure for the team.
Pedicle Screw Placement
The skin is then prepped widely as the paramedian incisions are at times farther from the midline than is initially expected, particularly in obese and wide girth patients. The C-arm is then brought in, and using AP imaging the boundaries of the pedicles are marked. The bilateral, paramedian incisions are then marked out between 3 and 5 cm lateral to the midline. This is variable and is largely dependent on patient girth, with larger patients requiring more lateral incisions to achieve the necessary lateral-to-medial trajectory for pedicle cannulation. The Jamshidi needles are then docked on the 9 and 3 o’clock positions of the left and right pedicles, respectively. There are common variations on how this step is done. For some, there is no dissection and the needles are passed right after incision. Others perform a Wiltse-type dissection to get to the junction of the lateral facet and the transverse process. K-wires are then passed and after removal of the Jamshidi needle, serial dilation follows. Next, the holes are typically tapped and then the screws are placed. Triggered EMG is often used during Jamshidi needle placement, tapping, and screw placement to lower the risk of screw malpositioning and neural impingement.
There are two basic variations of MIS retractor systems used in TLIF: tubular retractor systems (e.g., Quadrant®, Medtronic Corp.) and pedicle-based refractor systems (e.g., MAS TLIF®, Nuvasive Corp.). With tubular retractor systems, the pedicle screws are typically placed after decompression and cage placement in order to avoid interference with proper docking of the tubular dilators. With pedicle-based retractor systems, pedicle screws are placed prior to decompression in order to serve as anchor points for the cephalad and caudad retractor blades. If needed, medial and lateral retractor blades are then placed to facilitate wider exposure of working corridor to the disc space.
Decompression
At this point, the operating microscope is typically brought in, and the remaining soft tissue is removed with a combination of electrocautery and pituitary rongeurs to expose the lamina and facet complex. First, the inferior articulating process is removed using the high-speed drill or bayoneted osteotomes by making a series of cuts: (a) horizontally across the pars interarticularis, (b) longitudinally along the lamina just medial to the facet complex, and (c) separating the facet joint articulation. This releases the inferior articulating process which, after careful dissection away from the synovium and ligamentum flavum, is then removed en bloc with a pituitary rongeur. The superior articulating facet is then drilled away or removed in a piecemeal fashion using a Kerrison rongeur. Morcellized bone may be collected for later use as autograft. Finally, the ligamentum flavum is removed to expose the thecal sac and neural elements. While not necessary, we advocate exposure of both the exiting and traversing nerve roots so that they can be clearly seen and avoided during disc preparation and cage placement. This exposure of the two nerve roots, and the disc space within Kambin’s Triangle, is only possible after a complete bony decompression from the inferior edge of the cephalad pedicle and the superior edge of the caudad pedicle. In cases of severe central stenosis, a contralateral decompression can also be achieved after angling the retractor across midline.
Cage Placement
The approach corridor to the disc space occurs within Kambin’s Triangle with its lateral boundary of the exiting nerve root, its medial boundary of the traversing nerve root, and its inferior boundary of the caudal pedicle. A generous annulotomy is performed and the disc space is prepped. Pituitary rongeurs, rasps, curettes, and rotating paddle shavers are used to help accomplish this. Meticulous care must also be taken to avoid violating the endplates to mitigate the risk of subsidence as the cages are typically placed on the weakest part of the endplate. At the same time, as these surfaces are the primary fusion surface, the endplates must be thoroughly debrided of cartilaginous disc material. One of the most common causes of non-union or cage malpositioning in MIS-TLIF is poor disc space preparation. Thus, the surgeon must take time to perform a complete discectomy and adequate endplate preparation. The space is then sized with either the paddle shavers or interbody trials.
Bone graft is typically packed into the prepped disc space and tamped to the contralateral side so as not to impede interbody graft placement. Most MIS TLIF systems have a funnel which can be packed with graft and introduced into the disc space, greatly facilitating adequate graft volumes. We typically aim for delivery of 12 cc or more of grafting material. The graft is then tamped to the contralateral side of the disc space. The interbody graft or cage is then packed with grafting material and impacted into the disc space.
As with open TLIF, a variety of sizes and shapes of intervertebral spacers exist. The so-called bullet cages may be the easiest to place via an MIS corridor. Banana or boomerang cages, while more technically demanding to place, may offer two theoretical benefits: (a) decreased risk of subsidence as the graft abuts the more compact of the apophyseal ring anteriorly, and (b) greater potential restoration of segmental lordosis due to the more anterior location of the spacer. Finally, expandable intervertebral spacers can be quite advantageous in tight spaces where excessive retraction might be necessary to place a static graft. These devices may also allow for greater correction of foraminal height and segmental lordosis. Back-filling the space and or interbody device with grafting material is then an option.
Rod Placement
Rods are then sized with calipers. The rods are contoured. A tissue blade is then passed to facilitate rod passage. The rods are passed with particular attention being paid to staying subfascial on the rod pass. It is also important to avoid over-sizing the rods to avoid suprajacent facet impingement. Proper rod contouring can help maximize lordosis.
Lordotic Restoration
As stated previously, positioning has a significant influence on preservation and restoration of lordosis. The Jackson table and similar frames facilitate hyperextension of the lumbar spine. Using pillows to extend the hips and flex the knees further exerts a lordotic force on the lumbar spine. Maximizing the fulcrum effect of the interbody device is accomplished in two ways: anterior placement of the interbody device and avoiding oversizing (as the intact, taught anterior longitudinal ligament will resist lordosis). Existing MIS system compressors have some utility but often fail to provide maximal, angular compressive force.
Multilevel Cases
It is possible to perform multi-level MIS TLIF. Two level cases are quite common, and, with the tubular retractor method, the surgeon simply dilates and places the tube over each of the facets for the levels to be fused. With the pedicle screw-based systems, the blades on the pedicles are simply rotated 180° to treat each level. Three or more levels are possible but not commonly done. Frequently, MIS-TLIF will be done at L5/S1 as a second phase while placing the pedicle screws to back up lateral lumbar interbody fusions (LLIF) of L4/5 and more cephalad levels.
Spondylolisthesis Reduction
Spondylolisthesis correction on single level cases can be challenging. Distraction of the disc space by the interbody graft will often at least partially correct the listhesis. Similarly, prone positioning can influence the listhesis. Posterior translation of the pedicle screw towers on the cephalad screws while deploying the cage (particularly expandable cages) can be helpful on single-level cases. On multi-level cases, underbending the rod and sequentially reducing the middle vertebral body are an effective method for correction of spondylolisthesis.
Grafting
While the primary fusion by design occurs within the disc space and is outlined above, contralateral facet and laminar fusion are popular adjuncts. Dilators are typically docked on the facet or laminar surface and the microscope is used for visualization. The high-speed drill is used to decorticate the surfaces, and grafting material is packed onto them.
Outcomes and Complications
MIS-TLIF has been proven to be a safe and effective alternative to open TLIF [26, 27]. Reduced blood loss, infection rate, hospital stay, postoperative narcotic usage, and return to work have been demonstrated in the literature [14–16]. Fusion rates have been shown to be comparable to traditional fusion techniques [28].
The challenging learning curve associated with minimally invasive spinal procedures in general is particularly relevant for MIS-TLIF [29, 30]. The extended working distance, constricted field of view, paucity of orienting structures, and the disparity in screw placement technique (vs percutaneous versus open screw placement) can create a barrier to adoption. With experience, the surgeon experience is typically felt to be less physically demanding with this minimally invasive technique.
The limited incision size and muscle sparing nature of this procedure help minimize wound complications. This is particularly relevant to healing challenged treatment populations, especially the obese and diabetic [22–24].
There is a pervasive current trend toward shifting surgical treatment to the outpatient setting. MIS-TLIF has been proven to be a safe, effective, and lower-cost procedure in the outpatient model in contradistinction to open fusion [31].
While initially spinal deformity was felt to be a relative contraindication, increasingly MIS-TLIF is being employed in corrective strategies. It can be a useful adjunct in the minimally invasive treatment of spinal deformity when implemented along with lateral interbody fusion and long-segment percutaneous constructs. This is particularly true at L5/S1 and even L4/5 in cases of anterior psoas anatomy precluding the lateral approach.
Complications of MIS-TLIF are in general similar to those of its open analog and include pseudoarthrosis, hardware failure, cerebrospinal fluid leak, subsidence, neural injury, and vascular/visceral injury. Due to the limited exposure of MIS-TLIF, some of these potential complications take on a unique character and deserve special attention. Pseudoarthrosis is a concern in this operation as the grafting surfaces are inherently more limited than those afforded by an open procedure. As stated earlier, meticulous care must be taken during disc preparation so that the endplates are clean and abraded but remain intact. The disc space is the sole fusion surface in this operation in most cases. Advances in biologics cannot make up for poor carpentry. Facet/laminar fusion is another adjunct to help achieve solid arthrodesis but the disc space remains the primary fusion surface. While a thorough direct decompression is part of MIS-TLIF, there is an indirect component that comes from distraction of the interspace. Subsidence can result in recurrent stenosis and is best avoided by appropriate patient selection (avoiding patients with poor bone quality), careful endplate preparation, and avoidance of graft oversizing (which includes over expansion of expandable grafts). Cerebrospinal fluid leak, while thankfully uncommon in MIS-TLIF, can be challenging to address due to the narrow and deep working corridor. Repair techniques are the same as those used in open procedures except that primary closure is not always feasible due to the aforementioned working corridor. The suboptimal suturing ergonomics increase the risk of ensnaring neural elements. Placing a small piece of Gelfoam® just inside the dura can stem the flow of CSF and displace the neural elements away from the suture line. This technique is useful even when suturing is not possible as it gives dural sealant (DuraSeal®) a surface to adhere to. A lumbar drain is usually not necessary but meticulous closure of the fascia up to the skin is paramount. In the authors’ experience, pseudomeningocele has not been an issue.
Tips, Pearls, and Bailouts
Measuring pedicle screw lengths preoperatively on the MRI or CT can be very helpful and allows consideration of facet pathology for screw placement. Maximizing lordosis by hyperextending the hips and flexing the knees with pillows is paramount as most of the MIS devices used for compression are not as effective as their open iterations.
Rotating the table away from the surgeon can be extremely helpful in enhancing visualization, particularly of the contralateral side. If contralateral decompression is necessary, leaving the ligamentum flavum intact until bone removal is complete facilitates thecal retraction and lowers the risk of cerebrospinal fluid leak.
Again, the point of adequate discectomy and end plate preparation cannot be stressed enough. The most common obstacle to proper cage insertion and positioning is inadequate discectomy. Especially early on in the learning curve, the surgeon must be sure to take enough time to remove as much disc material as possible from within the disc space. Special care must be taken to remove the disc material from the contralateral, dorsal quadrant of the disc space as this material is poorly visualized.
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