Mini-open MIS fusions utilize the surgical corridor as described by Wiltse et al. (37) between the multifidus and longissimus paraspinal muscles (Fig. 25-1). This trajectory is ideal for both pedicle screw placement and decompression and fusion techniques.

Fluoroscopy is used to make the appropriate skin incisions. The skin incisions are made 2.5 cm in vertical length and 4 to 5 cm off midline. Sequential dilators are then passed through the fascia and docked onto the facet joint. A tubular retractor (typically 22 or 26 mm) is then docked and secured over the dilators. The use of an expandable retractor allows the blades to expand cephalad or caudad creating a corridor for pedicle screw placement (Fig. 25-2). Soft tissue is cleared to expose the standard pedicle screw entry points. Screws can be placed using a variety of methods including free hand, under C-arm guidance, or utilizing navigation depending upon surgeon preference. In addition, both posted and tulip-style pedicle screws can be used if working through the tubular retractor systems that are available.

Surgeon preference dictates the sequence of steps during a minimally invasive fusion. Early on in a surgeon’s experience, it may be easiest to place the pedicle screw tracts first prior to the decompression or facetectomy. This will preserve “normal” anatomy to help orient the surgeon to
the anatomic starting points. However, it is the author’s experience that it is more efficient to perform the transforaminal lumbar interbody fusion (TLIF) followed by appropriate decompression prior to screw tract preparation and screw placement. This minimizes the surgical exposure during the portion of the procedure that requires the most expansion of the modular retractor and thereby helps to limit muscle creep (Fig. 25-3).

The major advantage of this technique versus the percutaneous screw technique is the ability to directly visualize the bony anatomy, thus reducing the requirement of fluoroscopy. In addition, facet joint and posterolateral fusion can be accomplished through the same tubular retractor that the pedicle screws are placed.

Pedicle screws can be safely and effectively placed percutaneously, thus avoiding the additional dissection required for the placement of traditional and mini-open pedicle screws. Percutaneous pedicle screw placement has the advantage of less muscle damage and less potential damage to the medial branch nerve (innervation of the multifidus) and can be used effectively over long-segment fusions. The placement of percutaneous pedicle screws can require more operative time and more x-ray exposure for accurate placement. When using the fluoroscopic technique, the orientation of the C-arm beam is of critical importance. The anteroposterior (AP) images must be true AP images of each pedicle for screw placement (Fig. 25-4). The spinous process should be in the midline of the vertebral body, equally spaced between both pedicles. The superior and inferior endplates should be parallel, and the pedicles should be appropriately located at the caudal end of the ascending articular process. On the lateral view, the superior endplate should appear as one line, and the pedicles should overlap and thus appear as one (Fig. 25-5). True AP and lateral radiographs are of critical importance because small variance can produce erroneous placement of the pedicle screw.

The pedicle of interest is localized utilizing the AP fluoroscopic image. The skin is incised just lateral to the pedicle. The thoracolumbar dorsal fascia and muscle fascia are incised. A Jamshidi trocar is used to cannulate the pedicle. The ideal starting point is at the 10 o’clock and 2 o’clock positions on the left and right pedicles, respectively. The Jamshidi is slowly advanced a few millimeters into the pedicle. A lateral fluoroscopic image is obtained and should confirm that the Jamshidi is placed within the center of the pedicle. Under AP fluoroscopic imaging, the Jamshidi is advanced about 20 mm. The tip should stay lateral to the medial border of the pedicle. A lateral image is obtained and should show the tip of the Jamshidi at or past the neurocentral junction (Fig. 25-6). If so, the Jamshidi can be safely advanced to its desired depth. If the tip of the Jamshidi is at or medial to the medial border of the pedicle on the AP view and not yet passed the neurocentral junction on the lateral view, then it has likely breached the medial border of the pedicle.

After the Jamshidi is placed appropriately, a guidewire is passed. This is repeated at each pedicle. The pedicle screw is then placed over each guidewire, and then the appropriately sized rod is passed. It is critical to continue periodic lateral fluoroscopic visualization to avoid advancing the guidewire anteriorly. Avoid kinking of the guidewire, and remove any dried blood to minimize the risk of guidewire advancement while inserting the screw. In addition, remove the guidewire when the pedicle screw reaches the neurocentral junction.

When first performing these procedures, the authors encourage the liberal use of fluoroscopy or computer-aided navigation. With experience, radiographic exposure and operative time will diminish (Fig. 25-7).