Posterior Lumbar Interbody Fusion (PLIF)





Introduction


The technique of posterior lumbar interbody fusion (PLIF) is an important technique in the current spine surgeon’s armamentarium. That being said, the surgery is still defined by its level of technical difficulty and controversy exists regarding the safety of PLIF compared with other approaches to the intervertebral space.


The first in-depth description of a PLIF was described by Cloward, as an operation where the disk space is exposed from a posterior approach and a fusion is performed by directly grafting the intervertebral space. Classically, PLIF is performed via bilateral MEDIAL facet resection and exposure of the disk space; with retraction of the dura, the disk is visualized. Initially, some advocated that the procedure be maintained at the height of the neural foramen after diskectomy, preventing collapse and keeping the nerve root free of bony compression. The procedure also has the advantage of potentially increasing the fusion rate by direct graft-to-bone contact with compressive force (i.e., by placing the bone graft in the intervertebral space).


Posterior lumbar interbody fusion added extensively to the procedure of lumbar fusion, but it is not without its downsides. The procedure (see below) requires resection of the medial facet and retraction of the dura and traversing nerve root to visualize the disk space in order to perform the interbody fusion. At times, this retraction may be significant. As compared with transforaminal lumbar interbody fusion and posterior lateral fusion, there has been greater reports of dural tear, epidural fibrosis, nerve root injury, and chronic arachnoiditis.


Although more recent advances, such as transforaminal and direct lateral approaches to the disk space, have decreased the frequency in which PLIF is performed, PLIF remains an important staple for spine surgeons.




Surgical Indications


The literature still supports surgeon preference in regard to interbody fusion and its indication whenever a lumbar fusion is performed. The argument that the intervertebral space is biologically and mechanically superior for fusion compared with the intertransverse space because of its larger surface area of highly vascular bony endplate and because the interbody bone graft is subject to compressive forces, is a controversial one. Despite those theoretic advantages and the avoidance of large amounts of muscle damage that occur from exposure for posterolateral fusion, it has been difficult to show clinical superiority of interbody fusions over posterolateral fusions for most lumbar degenerative pathology. Most published studies comparing the techniques reveal similar outcomes regardless of what fusion technique is performed. That being said however, there are certain circumstances when interbody fusion offers definite advantages, such as adding an interbody fusion to a posterolateral fusion which has been shown to increase the rate of achieving successful arthrodesis. Additionally, placement of an interbody graft at the anterior column allows for restoration of optimal disk height and therefore maintenance or improvement of segmental lordosis and optimal sagittal balance.


Some authors have argued that interbody fusion should be combined with posterolateral fusion in patients at high risk for failed fusion, such as smokers. For patients with pseudoarthrosis after failed posterolateral fusion, an interbody technique is a good salvage operation to augment revision procedures. In patients with isthmic spondylolisthesis who undergo deformity reduction, it has been well documented that an interbody graft offers a biomechanical advantage, which protects pedicle screw instrumentation with an anterior load sharing graft and aiding in maintenance of alignment. Controversially, the procedure may be used in augmenting the ablation of the disk space in patients with discogenic pain with the thought process being only interbody fusion can completely remove motion at a painful disk and the nuclear material resection may eliminate the anatomic source of pain. It is known that the outer annulus is richly innervated with nociceptive fibers and that mechanical deformation or inflammation caused by a damaged nucleus can stimulate these nociceptors. One thought is that although posterolateral fusion can increase axial stiffness by 40%, interbody fusion can increase axial stiffness by 80% to minimize any micromotion even further and reduce any nociceptive stimulation of sensitized painful disks. Lastly, interbody fusion by technique allows for removal of much disk tissue along with any nociceptors involved in the generation of pain.




Open vs. Minimally Invasive


Open Technique


The patient is positioned in hyperextension to help create lumbar lordosis ( Fig. 7.1 ). The abdomen should hang free for unimpeded venous return, which will decompress the epidural venous plexus and help reduce bleeding. A radiolucent positioning frame is useful so that intraoperative fluoroscopic imaging can be obtained to confirm correct placement of screws and grafts. Attention should also be directed to the proper position of chest support, which will help to ensure adequate ventilation. Orbital pressure should always be avoided to prevent mechanical damage to the cornea or globe, which may ultimately cause visual loss. Some surgeons may choose to place the patient in a slightly reversed Trendelenburg position to help reduce the heightened intraocular pressure incurred by the prone position. Arms may be positioned outstretched or tucked next to the patient but done so as to prevent brachial plexus injury and allow for easy access to intravenous lines. If positioning the arms outstretched, they should be placed with shoulder abduction and elbow flexion to 90 degrees.




Fig. 7.1


Patient in the prone position on a Jackson table illustrating hyperextension of the lumbar spine.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 53-2b.


Using a combination of external landmarks, palpation of the spinous processes if possible, and/or the use of preoperative fluoroscopy, a midline incision should be planned over the levels of interest. A standard approach to the posterior lumbar spine is performed with dissection through subcutaneous tissue to the lumbosacral fascia. This is usually done with electrocautery as to provide hemostasis but may be done with sharp dissection of a scalpel. Sharp dissection is continued through the fascia. Once the correct levels are then identified using intraoperative fluoroscopy, exposure of the laminae above and below the level to be fused is completed using a subperiosteal technique. Further dissection is continued to the transverse processes of each level. It is important to remember which facet joint capsules may be sacrificed and which joint capsules should be preserved in order not to incur a potential for early adjacent segment disease by destabilizing the level above or below your proposed fusion construct. Once subperiosteal dissection is completed in regard to the laminae and transverse processes of each level, dissection is complete and laminectomy may be initiated ( Fig. 7.2 ).




Fig. 7.2


Operative view of the surgical anatomy and exposure just prior to laminectomy, with transverse processes visualized.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 53-4.


Pedicle screws may be placed before or after beginning the interbody fusion based on surgeon preference. The surgeon should be aware, however, that PLIF destabilizes the motion segment and should require adequate pedicle fixation.


A wide laminectomy and resection of the medial portion of the facet joints are essential to minimize retraction of the dura and nerve roots. A large amount of variation exists in regard to surgeon preference for the amount of bony resection; however, commonly it is carried laterally as far as the pedicle, removing at least half of the superior and inferior facets. The wide exposure will help avoid traction injury to the lumbar roots which may cause neuropathic pain and weakness ( Figs. 7.3 and 7.4 ). In general, the exposure of the disk space should extend lateral to the medial border of the pedicle below. Adequate exposure may require sacrifice of the facet joints, particularly in the upper lumbar spine, which will not destabilize the spine once pedicle screw fixation is implemented. The facet joints should always be removed when correcting deformity because this facilitates correction at that particular level. The superior edge of the upper lamina should be preserved to maintain the posterior ligamentous attachments to the vertebra above the fusion.




Fig. 7.3


Operative view of the surgical anatomy and exposure post laminectomy.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 53-7.



Fig. 7.4


Operative view of the surgical anatomy. (A) Sagittal view (B) axial view (C) coronal view with exposure and dura retracted to visualize the disk space.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 54-1b/c.


An abundant epidural plexus is usually encountered at the lateral border of the spinal canal. Bipolar electrocautery and packing with cottoned patties can prevent the excessive bleeding that may accompany an interbody fusion technique. Following cauterization, the venous plexus should be sharply divided to facilitate mobilization of the nerve roots and dura. A nerve root retractor is used to retract the dura and provide a degree of protection. The dura should not be retracted past the midline of the spinal canal in an attempt to limit nerve injury (personal experience). Additional cauterization may be required to clear the epidural tissue from the disk space. Once clearly identified and with assurance that the dura and nerve are protected, a rectangular opening is cut in the annulus with a scalpel. When extending the annular opening laterally, care is taken to protect the exiting nerve root. This is done by sweeping any soft tissue toward the root and then leaving the retractor in place to protect the root while working laterally.


The annular opening is then repeated in exactly the same fashion on the contralateral side. Intervertebral disk space spreaders are then used to distract the intervertebral space sequentially. These spreaders, which are flat bars of increasing width with rounded edges, are inserted into the disk space horizontally and rotated 90 degrees, which will distract the disk space. If the disk space is initially too narrow, sometimes it may be helpful to insert an elevator or curette to identify the path. It is vital not to use force so as to avoid inadvertent penetration into the vertebral body. Lateral fluoroscopy can be helpful to confirm that the starting tools are correctly placed in the interspace. The disk is then gradually distracted, working from side to side with increasingly large spreaders until resistance is met. The interspace should not be over distracted, or the endplates may collapse. It is not necessary to maximize distraction to achieve lordosis and decompression of the nerve roots should be accomplished by foraminotomy, not overzealous distraction. Placing a maximally tapered graft is the best way to increase lordosis. Placing a tall graft has many disadvantages; it increases the amount of dural retraction necessary, it increases the volume of bone graft needed to fill the interspace, and it increases the distance over which the fusion must occur.


After distracting the space to a comfortable working distance, usually 11 or 12 mm in the author’s experience, one of the spreaders is removed and a four-sided Collis curette 1 or 2 mm smaller than the largest spreader used is inserted horizontally, then rotated clockwise and counterclockwise to separate the cartilage from the bony endplate ( Fig. 7.5 ). This maneuver shaves the cartilaginous endplate from the bony endplate, which will permit rapid and complete removal of disk material and prepare the interspace for fusion. This step is repeated at various depths and angles to clean as much of the endplate as possible. The curettes can be sharp, and it is important not to violate the endplates. One may use a Kerrison rongeur to resect loosened annulus on the endplates.




Fig. 7.5


Examples of intervertebral shavers that will facilitate disk and cartilaginous endplate removal in preparation of the disk space for interbody placement.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 54-2a.


The diskectomy is completed with standard curettes and pituitary rongeurs. A reverse curette is very effective in removing any remaining cartilage from the endplates. It is important not to go too deep when using curettes because the anterior annulus is sometimes deficient, particularly in patients with spondylolisthesis. This will avoid both visceral injury as well as the potentially catastrophic occurrence of a great vessel vascular injury. After cleaning the interspace of disk material, the intervertebral spreader is reinserted, sometimes one size larger than previously used before removing the disk and cartilaginous endplate. The opposite side of the disk is now prepared in the same routine.


The interspace has now been grossly destabilized. Graft trials are then used sequentially to fit into the interspace ( Figs. 7.6 and 7.7 ). The graft should not be taller than the largest spreader used. It can be tapered to achieve the desired lordosis. Fluoroscopy is often utilized while placing graft trials. This will ensure adequate size and segmental angulation. After ensuring that the dura and exiting nerve root are protected, the trapezoidal graft packed with graft material is inserted horizontally in the disk space and then rotated into its final lordotic position (if a lordotic graft is used) ( Fig. 7.8 ). The intervertebral spreader is removed from the opposite side of the disk space.




Fig. 7.6


Examples of intervertebral body graft sizer trials.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 54-2b.



Fig. 7.7


Image of two posterior lumbar interbody fusion cages within the disk space.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 54-2c.



Fig. 7.8


Sagittal view of a carbon fiber cage appropriately positioned after successful preparation of the intervertebral body endplates.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 54-3a.


Cancellous bone graft is packed from the opposite side into the middle of the interspace. Finally, the second trapezoidal graft is inserted into position. Additional graft may be packed around the wedges to fill the disk space maximally with bone. The foramina and the midline under the dura are inspected to ensure no cancellous bone or disk material was displaced into the spinal canal or neural foramen.


Following placement of the intervertebral grafts, the pedicle screw instrumentation must be placed if not already done so. These screws are very easy to place following graft placement in that the rostral and caudal pedicles to be instrumented may be palpated with any angled instrument such as the Woodson elevator. Standard landmarks may be utilized. The entry point for the pedicle screw is at the junction of the transverse process, pars interarticularis, and superior articulating facet joint. Often the mammillary process is located at this junction. A pilot hole may be drilled with a high-speed burr to expose the cancellous bone of the pedicle. A pedicle probe (gear shift) may then be used to advance slowly through the pedicle and into the vertebral body. The medial, superior, and inferior walls may palpated during insertion to ensure there is no breech. The probe is advanced to the desired depth, often 40 mm. This hole may then be probed with a pedicle probe to ensure there are four walls and a floor of the pedicle hole. If a breech is palpated, the pedicle probe may then be redirected to enter the pedicle and vertebral body correctly. If difficult, fluoroscopy may be utilized to ensure correct trajectory.


Following placement of screws, rods are placed between screws and compression applied across each rod. Compression helps ensure a proper fit of the interbody graft and bony endplate and will also effect segmental lordosis.




Minimally Invasive Technique


Recently, there has been interest in developing surgical techniques to minimize the amount of dissection of the paraspinal musculature. As the paraspinal muscles are sacrificed, there is subsequent atrophy and possibly poorer outcome. Development of minimally invasive techniques (MIS) for PLIF have lagged somewhat behind other interbody techniques, mostly because of the requirement for a central decompression and bilateral placement of cages within the intervertebral space. However, the introduction of cortical bone screw trajectories for pedicle screw fixation from a medial-to-lateral trajectory has allowed for the development of these MIS approaches specifically for PLIF surgery.


The patient is positioned on a radiolucent operating table in conventional fashion (similar to an open PLIF technique). Once the correct operative level is confirmed, using anteroposterior (AP) and lateral fluoroscopy, a single posterior incision is made over the level of interest. Centering the tube over the facet and transverse process interface is usually preferred, as this will allow for optimal positioning when decompression and pedicle screw insertion are later required. The posterior musculature is then retracted down a muscle-splitting corridor that is extended to the lateral borders of the facet joints. Tubular retractor systems or stands can be used. Dissection can be limited with the use of the tube because it can be directed to access the appropriate anatomy for each step of the procedure ( Fig. 7.9 ). Laminectomies and partial or complete facetectomies are then completed with high speed burr and/or Kerrison rongeurs to allow for both access to the disk space and intended decompression of the neural elements. These steps are identical to that of the open procedure, but within the allotted space of the tube or stand used. As with the open procedure, it is imperative to perform proper decompression of the neural structures, as well as prepare adequate disk space with thorough removal of disk material, leaving bony endplates exposed on both sides. Bilateral interbody grafts are then placed on the lateral border of the apophyseal ring to allow for better load sharing on cortical bone as well as to obviate much of the need for dural sac retraction. As with the open technique, sagittally tapered lordotic cages can be placed using an insert and rotate technique. The insert and rotate technique involves preparing the endplates as previously discussed in the open technique section. The difference is this technique usually does not require overdistraction or cutting of any channel through the posterior endplates. The implant is inserted and then rotated into place, facilitating restoration of lordosis. Bone graft may then be afterloaded and placed to either side of the implants to help facilitate fusion. The main benefit is that this technique requires minimal neural retraction compared to that of an impacted cage technique, which involves over-distraction of the posterior disk space and usually far more neural retraction.




Fig. 7.9


Minimally invasive posterior lumbar interbody fusion (PLIF) dilation/tubular retraction.

Adapted from Benzel E. Spine Surgery: Techniques, Complication Avoidance, & Management . 3rd ed. Philadelphia: Elsevier Saunders; 2012: Fig. 60-1.


Medial-to-lateral cortical bone screws are then placed for pedicle screw fixation under fluoroscopic guidance. The technique for placement involves first identifying the superior most lateral edge of the pars; the entry point is 3 to 5 mm medial to this landmark. The trajectory is typically 15 degrees medial-to-lateral and 30 degrees inferior-to-superior. This trajectory may be less steep for the caudal screw to allow for a limited surgical exposure. Because of the medial entry point of these cortical bone pedicle screws, the size of the incision for MIS-PLIF can be limited to as little as 3 to 3.5 cm in length. If available, navigation may also be used to place cortical screws.


Traditional pedicle screws can also be used with the same landmarks as with the open technique. Biplanar fluoroscopy can be used to confirm both the proper starting point and that the pedicle screw is contained within the pedicle. A Jamshidi needle can be used with biplanar fluoroscopy to obtain the appropriate trajectory and place a guidewire. The tap and then subsequent pedicle screw may be inserted over the guidewire. If available, navigation may also be used to insert the pedicle screw.

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Nov 11, 2019 | Posted by in NEUROSURGERY | Comments Off on Posterior Lumbar Interbody Fusion (PLIF)

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