Those of you paying attention while reading the article describing the steps of the XLIF procedure probably noticed that I really only covered the first half of a full XLIF procedure: I only discussed the placement of the intervertebral spacer. As I’ve discussed previously, supplemental posterior fixation is typically also used in conjunction with the intervertebral spacer (some surgeons will just perform a “standalone” fusion with just the spacer but this isn’t that common in the U.S.) This posterior fixation acts an internal brace to immobilize the treated motion segment of the spine to facilitate bony growth across the intervertebral spacer (i.e. the fusion.)
The only form of posterior fixation that I use in my lumbar fusion cases is pedicle screw fixation. Pedicle screws (some patients have heard them referred to as “pins”) are by far the most common form of posterior instrumentation inserted during spinal fusion cases. These screws get their name from the fact that they traverse the pedicle of the vertebral body which is like a bridge between the facet joints and lamina posteriorly and the vertebral body anteriorly (see image 1). When inserted properly, the threads of the screw will be surrounded by the hard cortical bone of the pedicle which provides significant pull out strength to the screw (see image 2).
Image 1: Lateral (side) view of the lumbar spine. The pedicle is the bridge of bone (outlined in red) that connects the posterior elements to the anterior elements of the spine.
Image 2: Axial (cross-sectional) CT scan showing pedicle screws traversing L4 pedicles (the right pedicle is outlined in red) to terminate in the vertebral body anteriorly (“VB” on image). The bone at the edges of of the pedicle is hard cortical bone. The screw should be sized such that the threads of the screw capture this hard cortical bone so that the pull-out strength of the screw is increased. The triangulation of the screws with the lateral-to-medial trajectory also helps resist pull-out.
A step-by-step description of how I place pedicle screws is as follows:
Step 1: I first have to plan where I’m going to make my incisions for screw insertion. After the flank incision of the XLIF is closed I’ll then take AP and lateral fluoroscopy shots to mark the boundaries of the pedicle on the skin of the lumbar region (see image 3). Typically the incision will be about 1.5cm lateral to the lateral edge of the pedicle on the fluoro image (see image 4). These Wiltse incisions are well off midline and thus spare the damage to midline ligamentous structures often seen with traditional open incisions (devoted Spinal (con)Fusion readers know this as one of the basic tenets of minimally-invasive spine surgery). Since I always place bilateral pedicle screw fixation (some surgeons settle for unilateral screws but the gold standard is bilateral fixation) there will be one incision on each side of the spine to allow placement of all of the screws on that side. Thus, a typical 1-level XLIF patient will have three incision when they’re done: the flank incision and two lumbar incisions.
Image 3: Image on left shows me using a K-wire on the skin of the lumbar region to mark the lateral radiographic border of the pedicle on the AP fluoroscopic image (image on right shows the wire at the lateral border of the left sided pedicles. The red oval is marking the left L3 pedicle.)
Image 4: Hatch marks on the line marked previously (tough to see on this image, may need to zoom in) mark the center of the pedicle as marked on a lateral fluoroscopic image. I’ll then plan a small incision one finger-breadth lateral (outside of) the mark made along the lateral border of the pedicle. I typically make one incision on each side of the spine for the screws on that side (so in this case involving pedicle screws at L4 and L5, the two screws on each side will be inserted through the one small incision.)
Step 2: I’ll then make the incision and carry it down through the fascia. I rely heavily on tactile feedback and don’t want the tough fascia to interfere with that feedback. Once that’s done I’ll then use a large needle called a Jamshidi needle to dock on the bony anatomy of the starting point for pedicle cannulation. The starting point for insertion of a pedicle screw is classically defined as the junction of the transverse process (TP) with the lateral border of the facet joint (see image 5). Often there’s a small protuberance of bone there called the mamillary process that is like a bullseye for the entry point into the pedicle. Recall that because I primarily use minimally-invasive techniques (rather than open surgery) I’m not able to actually see this starting point. Here’s where that tactile feedback is so important: the feel of the tip of the needle on the compact bone at this starting point is unmistakable. Once I’m on that bone I’ll walk the tip of the needle medially (towards midline) until I hit the facet joint and boom, I’m there. Of course, I’ll confirm that I’m where I’m supposed to be by checking a fluoroscopy image (see image 6).
Image 5: posterior (top) an lateral views of the starting points (red dots) for pedicle cannulation at the junction of the transverse process(TP) and facet joint (FJ). On lateral view the pedicle is outlined in blue.
Image 6: AP (eft) and lateral fluoroscopic images showing Jamshidi needle docked at starting point for pedicle cannulation. The image on left shows needle docked at left L5 pedicle (after a K-wire was placed already at L4) and the image on right shows docking at L4 pedicle. The right L4 transverse process (TP) and facet joint (FJ) are marked.
Step 3: Once I know I’m at the correct starting point I’ll start to hammer the Jamshidi needle into the pedicle. Once you penetrate the hard bone at this junction point it’s almost like you fall into the soft bone within the pedicle. I then hammer in the needle at a 25-30-degree angle to a depth of at least 25mm to enter the soft cancellous bone of the vertebral body. How do I know I’m where I’m supposed to be? Of course I can check fluoroscopy to be sure I’m heading in the correct trajectory. Also, remember all those neuromonitoring leads we hooked up to the legs prior to starting the XLIF? We use their feedback during placement of the screws as well. The tip of the Jamshidi needle is electrically stimulated as I’m impacting it through the pedicle. If I inadvertently breach the inner wall of the pedicle where the nerve passes by, the needle will stimulate that nerve and we’ll be able to pick up that stimulation in muscle of the leg supplied by that nerve (see image 7). This monitoring definitely provides one extra layer of safety to prevent any malpositioned screws.
Image 7: Intraoperative monitoring feedback during pedicle cannulation with Jamshidi needle. Image on left shows tip of needle medially breached and in close contact with nerves within spinal canal. This stimulates the nerve at a low threshold and thus gives the surgeon a red “warning” indication. The image on the right shows the tip of the needle completely within the bone of the pedicle and thus only stimulates the nerve at a much higher threshold, thus the “safe” green indication. Adapted from Gupta et al, 2019.
Step 4: The Jamshidi needle that I passed into the vertebral body via the pedicle is a hollow-bore needle with a stylet within. Once the needle is properly inserted into the pedicle I remove the inner stylet and then pass a Kirschner wire (or K-wire) through the needle into the vertebral body and carefully remove the needle. This will serve as a placeholder within the pedicle to guide the screw into place via the proper trajectory. After all the wires have been inserted I will check a confirmatory AP and lateral fluoro shot to confirm proper placement (see image 8).
Image 8: Intraoperative (left) and AP fluoroscopic image showing all 4 wires in place within L4 and L5 pedicles.
Step 5: Now it’s time to place the pedicle screws. Unlike standard pedicle screws used in open spine surgery, the screws used here are cannulated so that they can be passed over the K-wires (see image 9). Again, I know from from the previous fluoro image that the wires are where I want the screws to be so I just pass the screw over the wire and follow it right down the pedicle (I use a power driver to make this easier on my wrists; see video below.) I typically remove the wire once I’ve inserted the screw about half way into the pedicle (if you’re not careful the screw will start to catch the wire and can advance it right out the front of the vertebral body into the abdomen. Not good.) Typically for a fusion at L4/5 these screws are 6.5mm in diameter and 45mm in length (see image 10). The screws are made out of titanium so no, they won’t set off the metal detector at the airport (this is one of the top three questions I get in clinic.) Just like after the wires I’ll check fluoro shots here to confirm proper positioning of the screws (see image 11).
Image 9: the cannulated pedicle screw is passed over the wire into the pedicle.
Video: insertion of cannulated pedicle screw over a K-wire using power driver.
Image 10: pedicle screws with rods and locking caps in place. Courtesy: Nuvasive corp.
Step 6: Now that the screws are in place in each pedicle I now have to connect the screws on each side of the spine to one another so that they can collectively act as an internal brace. This connection is made by securing a rod into the tulip heads of each pedicle screw (see image 10). In open spine surgery the surgeon can see all of the heads of the screws and just drop the rod in place and lock it down with locking caps. Percutaneous pedicle screws, on the other hand, typically have a reduction tower attached to facilitate passage of the rod via the small minimally-invasive incisions. After screw insertion I then line up the apertures of the towers and then pass the rod through each tower and down into the screw heads (see image 11). This rod usually is curved a bit to match the desired curvature (lordosis) of the lumbar spine. Once I’ve confirmed the rod is properly seated I then screw a locking cap in place into the tulip head (the inside of the tower is also cannulated to get the locking cap started in the correct trajectory and also to help reduce the rod if necessary.) This is a little tricky to master and can be one of the most frustrating parts of the case, particularly when passing a rod through several towers as in a multi-level fusion. With a little patience, though, and some fluoro shots, it typically goes smoothly. Once I’ve final tightened the locking caps in place, the reduction towers are removed. That’s it! Now the cage is in place in front of the spine and the screws and rods are place in the back of the spine (see image 13).
Image 11: AP fluoroscopic image showing pedicle screws, with attached reduction towers, in proper position within L4 and L5 pedicles.
Image 12: pedicle screws (with attached reduction towers) are in place. Here, we begin to pass a rod through the reduction towers and down into the heads of the screws where they can be locked in place with locking caps. It takes a bit of practice to do this via such a small incision.
Image 13: AP (left) and lateral standing X-ray images showing properly positioned cage and pedicle screws after L4/5 XLIF procedure.
In an upcoming post I’ll discuss what can happen if the above process doesn’t go as planned.
Thanks for reading!
J. Alex Thomas, M.D.
Source: Gupta M., Taylor S.E., O’Brien R.A., Taylor W.R., Hein L. (2019) Intraoperative Neurophysiology Monitoring. In: Phillips F., Lieberman I., Polly Jr. D., Wang M. (eds) Minimally Invasive Spine Surgery. Springer, Cham.
Spinal (con)Fusion 