41 Maximum Access Surgery Posterior Lumbar Interbody Fusion (MAS PLIF)
Summary
Keywords: maximum access surgery posterior lumbar interbody fusion (MAS PLIF) minimally invasive posterior lumbar interbody fusion (MIS PLIF) minimally invasive spinal surgery spondylolisthesis lumbar stenosis degenerative disc disease
41.1 Introduction
The maximum access surgery posterior lumbar interbody fusion (MAS PLIF) has been developed as an alternative to minimally invasive lateral or posterior approaches. MAS PLIF uses a well-known medialized approach, resulting in less lateral retraction and dissection of the muscles, as opposed to open PLIF, where wide dissection is required for fusion cage placement and pedicle fixation in treating spinal pathologies. Minimally invasive PLIF may confer several advantages, including smaller incision, reduced scarring, reduced blood loss, decreased need for transfusion, shorter hospital stay, faster recovery time and return to ambulation, as well as reduced facet joint manipulation.1,2,3,4,5 We also propose that decrease in subsidence would be an added benefit, as cages are placed laterally along the cortical bone. Finally, there is increasing economic data that suggests both direct and indirect cost-savings in favor of minimally invasive surgery (MIS) fusion as compared to open fusion.2
41.2 Indications
Similar to other minimally invasive procedures, MAS PLIF is applicable to many general deformity and degenerative conditions, including degenerative disc disease, lumbar stenosis, recurrent disc herniation, and Grades 1 and 2 spondylolistheses. An ideal patient has mid- to low-grade spondylolisthesis in the lower lumbar spine without severe osteoporosis, and bilateral symptoms without significant deformity. The most common surgical levels remain L4–L5 and/or L5–S1, but it is appropriate for all levels in the lower lumbar spine. It has the advantage of being a single-position surgery, combing bilateral decompression along with interbody fusion. The main drawbacks include difficulties in treating high-grade spondylolisthesis or significant hyperlordosis in the lower lumbar spine, both of which can complicate the surgery. MAS PLIF does require a change in pedicle screw instrumentation, namely, through the use of cortical bone screws with an alternative trajectory. MAS PLIF is relatively easy to learn, and it is also well-suited for navigation systems that can provide teaching and/or training tools. The cortical bone screws compare very favorably in biomechanical testing with pedicle fixation and are important in maintaining the medialized approach that provides much of the benefit.
41.3 Contraindications
One primary contraindication is multiple prior posterior procedures with significant scar tissue. Often in this case, a TLIF and/or lateral approach to avoid scar tissue may be a more appropriate choice. High-grade spondylolisthesis or spondylosis can be completed but should not be your initial case series.
41.4 Preoperative Planning
As with any surgical procedure, a complete history and physical examination should be completed. Preoperative radiographs can include anteroposterior and lateral views to determine sagittal alignment, disc space height, and often full-length films to evaluate for scoliosis. In suitable patients, flexion and extension views can provide further information of instability. Magnetic resonance imaging (MRI) can be performed to evaluate for canal or foraminal stenosis. In select patients with specific contraindications (aging pacemaker, metallic implant) a computed tomography (CT) myelogram may be used. Perhaps most importantly, a thin-slice CT scan for evaluation of soft tissue and bony architecture can be obtained for preoperative planning. Using preoperative imaging, the site of decompression and fusion should be determined and a plan for instrumentation should be discussed. Intraoperative neuromonitoring with electromyography (EMG) is a useful adjunct and may offer some protection of neural structures. For this purpose, discussion with the anesthesiologist to avoid long-acting muscle relaxants during induction and maintenance is important.
41.5 Patient Positioning
The patient is positioned on a radiolucent operating table in the prone position that allows for fluoroscopy and/or navigation. A bedrail on the table is necessary in order to attach an articulating arm (Fig. 41.1). After draping the patient in the usual manner, adjust the table so that the C-arm provides an anteroposterior (AP) image when the orbital angle is at 90 degrees. Adjust the cranial/caudal angle of the C-arm to obtain a true AP image of the targeted vertebra, and a true lateral image when the orbital angle is at 0 degree.
41.6 Surgical Technique
a)Incision:
1.Locate the affected level by using fluoroscopy in the AP and lateral views. Using the targeting tool, obtain a true AP image to identify and mark the starting location.
2.The incision is localized by extending a midline marker from the inferior one-third of the cranial spinous process down to the middle of the caudal spinous process, as defined by the targeting tool.
b)Technique:
1.Exposure:
i)Dissect bilaterally down the spinous process to the lamina and elevate muscles laterally to the facet capsule (Fig. 41.2). Place the distal end of the blade measurement tool on the pars and select the appropriate retractor blade length using the scale on the blade measurement tool.
ii)Use the blade attachment handle to retract the muscle and place the blades directly over the facet capsule (Fig. 41.3). Insert light cable tips and attach the other end of the light cable to a light source. Attach the articulating arm to the retractor body, and lock the articulating arm.
2.Identify shank entry points:
i)Visualize and identify the superior-most lateral edge of the pars and move 3 to 5 mm medial for shank entry points (Fig. 41.4).
ii)The starting point of S1 screws will be superior and medial to the S1 foramen, and the trajectory will be parallel to the sacral end plate at a slight 10 degrees medial to lateral trajectory. This trajectory is also commonly used for the inferior shank placement.
3.Pilot hole preparation:
i)Starting with the cephalad level, perforate the proximal cortex using a high-speed burr. Cephalad shanks should be placed in a medial to lateral and inferior to superior trajectory.
ii)Under lateral fluoroscopy, drill through the pedicle in a medial to lateral and inferior to superior trajectory to the posterior, superior-most aspect of the vertebral body. Ensure that the drill guide insulator is properly attached. Attach the stimulation clip onto the drill and activate the stimulation to obtain dynamic monitoring readings while drilling.
iii)Use a ball tip probe to inspect the pilot hole for perforations.
iv)Under lateral fluoroscopy, tap through the pedicle to the posterior, superior-most aspect of the vertebral body. Ensure that the tap insulator is properly attached and attach the stimulation clip onto the tap and activate the stimulation to obtain dynamic monitoring readings while tapping.
4.Shank insertion:
ii)Use a ball tip probe for inspection and measure for appropriate shank length. Insert the shank head into the distal tip of the shank driver (Fig. 41.5).
ii)Introduce the screw shank into the pilot hole and advance until the distal end of the shank tip reaches the posterior, superior-most aspect of the vertebral body. Release the shank driver and ensure that the shank driver insulator is properly attached. Attach the stimulation clip onto the shank driver and activate the stimulation to obtain dynamic monitoring readings while driving in the screw.
5.Decompression:
i)Perform bilateral decompression, removing the inferior articular processes and superior two-thirds of the superior articular processes (Fig. 41.6).
6.Disc removal and sizing:
i)Make an ipsilateral annulotomy and perform a conventional discectomy. Using the paddle sizers, sequentially distract the disc space prior to trial placement (Fig. 41.6). Insert the final trial size on the ipsilateral side. Repeat disc prep and sizing sequence on the contralateral side. Once the contralateral disc space is prepared, use the paddle shavers to prepare the end plates.
7.Interbody grafting:
i)Place the appropriate size implant onto the Inserter. Prior to implant placement, pack the anterior third of the disc space with graft material. Insert the contralateral implant. Once the implant is in its final orientation, ensure the markings on the inserter are in the proper orientation.
ii)Use the paddle shaver to prepare the end plates and pack the center of the disc space with graft material using the bone graft funnel (Fig. 41.7). Insert the ipsilateral implant and ensure the markings on the inserter are in the proper orientation.
8.Screw head attachment:
i)With the head inserter fully threaded into the screw head, align with the long axis of the screw shank and attach by applying downward force. Rotate the head inserter in a 360 degrees orbital motion while applying downward force to facilitate screw head engagement.
ii)Release the head inserter from the tulip and repeat the previously described process to attach the other tulip heads.
9.Rod insertion:
i)Use the head adjuster to position the tulips in the proper orientation and select the desired rod length and place in tulips. Load up to four lock screws directly from the caddy (Fig. 41.8).
10Final tightening:
i)Place the counter-torque over the superior tulip and tighten until the T-handle breaks free and repeat on remaining screws (Fig. 41.8).
c)Closure:
1.Closure is very standard and is one of the advantages of working through well-known anatomy, with incisions and closure of the dorsal fascia subcutaneous layers and skin closure. A drain can often be avoided, as the patient is often discharged within 24 hours.
Fig. 41.2 Exposure. Dissect bilaterally down the spinous process to the lamina. Place the distal end of the blade measurement tool on the pars and select the appropriate retractor blade length using the scale on the blade measurement tool. (Used with permission from NuVasive, Inc. Mas Plif Surgical Technique. San Diego, CA: Nuvasive, Inc; 2013, 4.)
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