Minimally invasive scoliosis surgery: an innovative technique in patients with adolescent idiopathic scoliosis

Three two-inch-long midline skin incisions are made for instrumentation of eleven to thirteen segments. Three to four segments (6-8 pedicle screws) are instrumented per skin incision. Intraoperative fluoroscopy is used to determine the length and location of these incisions. Although a single straight midline incision can be made, three smaller incisions are often more cosmetically appealing to patients. The skin is undermined to either side of midline in order to allow for the placement of bilateral pedicle screws. In the lumbar spine the facet can be manually palpated. A stab incision in the fascia is made directly over the facet. Thereafter, the muscle fibers are split bluntly in line with their fibers using a small cobb elevator, or with an insulated tipped electrocautery, to expose the joint (Figure 1). A small Gelpi retractor provides excellent exposure and a fiber optic light source is used for illumination. The Gelpi retractor also allows for adjustment of tissue tension, which decreases muscle necrosis and spasm. Once the facet joint is exposed, facetectomy is performed with a high speed burr. Stab incisions are made for placement of pedicle screws at all levels. At times, the stab wounds become contiguous, rendering the fascial and the muscle exposure akin to a Wiltse approach [11].

In the thoracic spine the inferior facet usually lies at the level of the tip of the spinous process of the superior vertebra (Figure 2). On the concave side, we make a stab incision approximately one centimeter lateral to the midline at the level of the spinous process tip, whereas on the convex side we make a stab incision approximately 1.5 cm lateral to the midline. When in doubt, we use C-arm to confirm the location of the pedicle. We take advantage of the thoracic spine anatomy in locating the facet joints at the level above and at the level below - the overlapping laminae allow for contiguous exposure of these facets. Under direct visualization and illumination, the facet joint is osteotomized using a combination of a quarter-inch osteotome and a high-speed burr. Adequate excision of the facet joint is carried out to ensure a solid fusion. The facet joint of the uninstrumented intervening segment between skin incisions is also exposed in the manner previously described. The facet joint is osteotomized and prepared for fusion. This area is left uninstrumented, but bone graft is placed to facilitate arthrodesis. Exposure of this joint can usually be accomplished by undermining the skin and extending the fascial incision.

Freehand anatomic placement technique is utilized for pedicle screw insertion, as has previously been described [12]. The facet osteotomy and the exposure allow for easy identification of the anatomic entry point. The entry point in the thoracic spine is usually the point of intersection between the midline of the facet joint and the upper-third of the transverse process. In the lumbar spine the entry is the intersection point between the midline of the transverse process and the midline of the facet joint (Figure 3). This is often at the base of the superior facet. Complete exposure of the transverse process is usually not needed for identification of this landmark, but may be carried out if desired. Alternatively, the pedicle screw can be placed using fluoroscopy guidance. We prefer to place all screws on one side first. This allows for easy mobilization of the skin. It is typically not possible to place both screws in each vertebra simultaneously due to the limited skin incision.

We utilize the DePuy 5.5 EXPEDIUM^® (5.5 mm rod, stainless steel) pedicle screw instrumentation system. Our preference is to use two uniaxial reduction screws and one MIS screw with an open connector per skin incision (Figure 4). The reduction screws have significant advantages. They allow for easy reduction of the rod and/or the spine to achieve correction of scoliosis deformity in two planes. Since the extended tabs stick out above the fascia, they allow for easy passage of the rod into the screw heads. Thus, the rod can be passed under direct visualization and the screw heads can be manipulated for easier passage. The extended tabs also serve as soft tissue retractors. Prior to the insertion of the pedicle screw, the fusion site is prepared. Local autograft (from the facet joint osteotomy) as well as a small BMP-enriched sponge or other bone graft substitute is placed in the fusion bed. We usually do not perform EMG stimulation of the pedicle screws, but SSEP and transcranial MEP monitoring are employed throughout the case.

Two rods, cut to appropriate length, are contoured in the normal sagittal plane to reproduce desired thoracic kyphosis and lumbar lordosis. The rod is inserted cephalad to caudad (Figure 5). This is an important safety step, as the overlapping laminae in the thoracic spine prevent an inadvertent entry into the spinal canal. The reduction screws are capped as the rod advances, and the screw heads can be manipulated for rod passage. The open MIS connectors serve as a post against which the rod can be maneuvered. A vise-grip pliers is utilized to hold the rod as it provides a strong grip and easy manipulation. A rod pusher can be utilized to translate the rod onto the screw heads. Alternatively, the assistant can directly apply translation force to the chest wall to push the spine over to the rod. We prefer to utilize rod translation maneuvers, however, a CD rod derotation technique can also be utilized with the help of a vise-grip. As the rod passes from one wound to the next, one must confirm that the rod is lying beneath the fascia. The rod can be manually palpated in the distal wound (Figure 6), which helps to properly direct the rod.

Once the rod is seated in all pedicle screw heads, the rod is manipulated to restore the sagittal contour of the spine (Figure 7). Caution must be given to prevent over-rotation of the rod, as this has the potential to reverse the thoracic kyphosis. This mistake is more likely to occur with a rod derotation maneuver, as the sagittal contours of the rods are difficult to appreciate through small skin incisions. The set screws are now sequentially tightened, and the rod is formally seated. The same maneuver is now carried out on the opposite side following the placement of pedicle screws. Appropriate compression and distraction force can also be carried out. This can be easily accomplished through the midline skin incisions. In situ rod bending can also be carried out, but is not our preferred technique. A direct vertebral rotation maneuver is then carried out off the concave-side screws. Triangulation technique is difficult to achieve, as the skin incision limits access to both sides simultaneously. This can, however, be easily carried out with a straight midline incision approach. Closure is fairly rapid, and is carried out in a layered fashion. We do not utilize drains. In our more recent cases, we excise the wound edges to prevent hypertrophic scar formation (Figure 8). While the patient is still on the operating table, anteroposterior and lateral X-rays are taken to confirm adequate correction in both planes. Our postoperative protocol is fairly routine. We do not routinely use a brace. Patients must be able to walk up a flight of stairs prior to hospital discharge, and are to remain out of school for six weeks to three months. At two weeks post-op, patients may go outside their residence, and at three weeks post-op, they may travel short distances. We allow activity as tolerated within the limits of pain. Patients can return to gym class between four and six months post-op, and may participate in contact sports and perform heavy lifting after six months. We prefer to use Ketorolac (Toradol, Roche Laboratories, Nutley, NJ) rather than morphine for analgesia purposes.