Patient Positioning and Room Setup

The setup for a minimally invasive lumbar microdiscectomy requires an adequately sized room that can comfortably accommodate the operating table, the fluoroscopic unit and monitor, and the operating microscope. Any operating table may be used, but we prefer a Jackson table with a Wilson frame. The Jackson table allows free passage of the fluoroscopic unit, without the hindrance of the pedestal. Regardless of the table used, the patient should be positioned prone with the spine flexed to optimally open the interlaminar space. The various minimally invasive sets have a table-mounted clamp, which should be placed opposite to the side being operated upon. For example, for a right L4–5 microdiscectomy, the clamp should be placed on the left at the base of the Wilson frame. This facilitates placement of the retractor arm, which is used to secure the tubular retractor (Fig. 162-1).

FIGURE 162-1 Operating room setup. A, The operating room should comfortably accommodate a fluoroscopic unit and operating microscope. We prefer a Wilson frame atop a Jackson table to facilitate fluoroscopy. B, The clamp for the table mounted retractor is secured at the base of the Wilson frame opposite to the side of the operation.

Technique

Incision

To work through a small incision, precise planning is essential given the limited field of view through the tubular retractor. A critical component of this planning is the trajectory of the tube. The optimal location of the incision is at a point where the tubular retractor lies in a trajectory parallel to the disc space (Fig. 162-2).

FIGURE 162-2 Intraoperative lateral fluoroscopic images demonstrating the importance of trajectory. A, This image confirms the L5–S1 level with an 18-gauge spinal needle. Note the trajectory of the needle veering away from the disc space. An incision based on this mark will be too low and adjustments will need to be made to optimize visualization of the nerve root and the disc. The visualization will remain suboptimal because of the trajectory required to access the disc space from this incision. B, The trajectory of the needle in this image is parallel to the disc space. Once the muscle is dilated and the retractor is placed, no adjustments will be required to optimize visualization.

The patient’s lower lumbar area is draped widely because no preoperative imaging is performed in order to minimize fluoroscopy time. The midline is marked by palpating the spinous processes, the level is approximated by palpating the bony landmarks, and a mark 2 cm from midline is made. After all the drapes are placed, the fluoroscopic unit is brought into the field and remains in position until the tubular retractor is secured to the table-mounted arm. Prior to making an incision, a 20-gauge spinal needle is placed to confirm the level. Once again, the needle is placed at a point on the skin that allows a trajectory parallel to the disc space, which is typically 2 cm lateral from the corresponding interspinous process space. This is confirmed with the first lateral fluoroscopic image.

Once an optimal point has been identified, a vertical incision is marked. To minimize trauma to the skin edges, the incision should be 2 mm longer than the diameter of the tubular retractor that will be used. For example, if a 16-mm tubular retractor will be used, an 18-mm incision should be planned, centered around the entry point of the spinal needle. we prefer to infiltrate the incision with a lidocaine-bupivacaine local anesthetic containing epinephrine, both superficially and deep within the muscle layer. This minimizes bleeding from the muscle and provides excellent postoperative pain control.

Dilator Insertion

Despite the numerous publications that detail the use of a K-wire for puncturing the fascia and docking onto the laminofacet complex, we refrain from using this technique out of concern that the wire might advance into the interlaminar space and enter the canal. Nevertheless, several publications have reported the safe use of this technique.^6,9 Instead, we prefer to sharply divide the fascia with either Metzenbaum scissors or Bovie cautery. Once the fascia has been divided, the first dilator may be safely passed and docked onto the laminofacet junction (Fig. 162-3).

FIGURE 162-3 A, After dividing the fascia with either Bovie cautery or Metzembaum scissors, the first dilator is passed and docked onto the inferior aspect of the rostral lamina. Careful palpation with the dilator is used until the medial aspect of the facet is determined. B, Subsequent dilators are used with intermittent fluoroscopic guidance to ensure a satisfactory trajectory.

To optimize the position of the first dilator, we palpate the bony anatomy in the immediate vicinity with the dilator itself. Once fluoroscopic guidance has been used to confirm the level, the inferior aspect of the lamina is determined with careful probing and, in a similar fashion, the medial aspect of the facet is identified. It is important to keep in mind while probing with the first dilator that it is small enough to enter the canal when the spine is flexed. Once the position of the first dilator is anchored into position with one hand, sequential dilators of increasing size are passed until the desired diameter is reached (Fig. 162-4). As each dilator is passed, the risk of inadvertently entering the canal with the dilator decreases.

FIGURE 162-4 Sequential dilation of the paraspinous muscles. Gentle wanding of the dilator minimizes the subsequent need for tissue resection. The retractor is secured with downward pressure (see Fig. 162-5), and a final fluoroscopic image is obtain to ensure the trajectory has remained unchanged.

The term wanding has been used in the minimally invasive vernacular to describe the use of the dilator to expand the field of view for the tubular retractor. This involves placing the dilator against the lamina and clearing the muscle from the bone. With each sequential dilator, we attempt to wand only in the vicinity of the surgical site, that is, the inferior aspect of the rostral lamina and the medial facet. Excessive wanding outside of the operative field, namely, the lateral facet or superior to the lamina, can result in increased postoperative discomfort for the patient. Conversely, inadequate wanding results in the need for excessive tissue resection for adequate visualization of the requisite anatomy. The depth of the working channel is determined by a scale on the outside of the final dilator, and the tubular retractor is selected. We prefer a 14-mm diameter retractor when the depth of the working channel is less than 5 cm. For working depths greater than 5 cm, a 16-mm diameter tube is more appropriate. All reoperations are performed through an 18-mm diameter tube.

When anchoring the tubular retractor, it is helpful to exert a downward pressure against the lamina while securing the retractor to the table-mounted arm because this will minimize migration of the retractor off of the lamina (Fig. 162-5). A final fluoroscopic image is taken to confirm that the trajectory of the tube is parallel to the disc space. The fluoroscopic unit is then moved to the head of the bed and the operating microscope is brought into the field.

FIGURE 162-5 The dilation and placement of the retractor is done with the fluoroscopic unit in place. Once the retractor is placed into position, it is docked against the lamina and given a slight medial angulation. With downward pressure against the lamina, the retractor is secured.

Orientation and Landmark Identification

The tubular retractor is secured so that the stem of the retractor is orthogonal to the spine. This will aid in rostral-caudal orientation when under the microscope. With the microscope in position, part of the inferior aspect of the lamina should be immediately visualized with the appropriate amount of wanding. If not, a large straight bayoneted curette is used to palpate the inferior aspect of the lamina and medial facet. At this point, it is crucial to be oriented with the anatomy, either with direct visualization or with palpation of the bony elements. A straight curette may be used to scrape the underside of the rostral lamina and to again confirm the medial extent of the facet. With certain knowledge of the bony anatomy, Bovie or bipolar cautery may be used to clearly expose the inferior aspect of the lamina and the medial facet (Fig. 162-6). Particular care is taken to prevent entry into the facet joint and any significant disruption of the facet capsule. Now, with the bony anatomy clearly exposed, the inferior aspect of the rostral lamina and the medial facet are removed with a drill and cutting bur until the ligamentum flavum is encountered.

FIGURE 162-6 Intraoperative photograph demonstrating the inferior aspect of the lamina and medial facet. Note the orthogonal position of the stem of the retractor to help the surgeon maintain rostral-caudal orientation. In this figure, the medial facet is being palpated by a Penfield 4 dissector.

A small cuff of muscle remains over the ligamentum flavum; this cuff requires resection. Then the inferior aspect of the lamina and medial facet are removed, providing access to the lateral aspect of the canal for removal of the herniated disc. We have noted that a well-positioned 14- or 16-mm diameter tube provides an excellent guide to the extent of bone work required for the procedure. Bone removal encompassing approximately 75% of the field of view of the retractor is typically adequate. The thin layer of muscle over the ligamentum flavum that is caudal to the bone work is removed. In this way, the ligamentum flavum overlying the lateral thecal sac and affected nerve root is exposed. At this point in the operation, the tenets of microdiscectomy are identical the open procedure.

Ligamentum Flavum Removal

The fibers of the ligamentum flavum are divided longitudinally with a Penfield 4 dissector until the canal is entered, evidenced by visualization of epidural fat and the thecal sac. A right-angle nerve hook is then passed on the underside of the ligamentum to ensure that the dura is adequately freed from the ligamentum. The ligamentum flavum is then resected by passing a 1-mm or 2-mm Kerrison rongeur into the divided ligamentum and undercutting the lamina. Using this technique, the ligamentum is resected caudally to the extent of the field of view provided by the tubular retractor. The ligamentum flavum should be resected laterally until the lateral border of the thecal sac is clearly visualized (Fig. 162-7).

FIGURE 162-7 Intraoperative photograph of the exposure of the thecal sac and nerve root through a 16-mm tubular retractor.

Postoperative Care

After adequate recovery from anesthesia, patients are transferred to the same-day surgery ward. With the preinjection of bupivacaine and lidocaine, patients seldom have significant postoperative pain and are able to ambulate within an hour or two of surgery. Patients are discharged home the day of surgery after tolerating fluids, voiding, and ambulating.

Complication Avoidance and Lessons Learned

Because the minimally invasive technique is distinct from the traditional midline approach, there are nuances to this technique that help the surgeon avoid complications. Nevertheless, the complications of a minimally invasive microdiscectomy are similar to the open approach and can include dural tears, recurrent disc herniation, and wound infections. Infections may be less of a risk in the minimally invasive approach.^10–12

The first principle of avoiding complications with this technique is orientation within the field of view of the tubular retractor. The consequence of the minimally invasive exposure is the absence of clearly visualized bony landmarks. The ability to follow the spinous process to the lamina and then the facet for orientation is replaced by direct palpation of the bony anatomy from a more lateral position in a more constrained working area. Initially, this can be disorienting. When beginning, additional fluoroscopic images, both anteroposterior and lateral, may be of value when uncertain about the anatomy. It is imperative that the trajectory of the tubular retractor be parallel to the disc space being operated upon. This facilitates passage of instruments into and out of the tube, thus minimizing risks to the neural elements. It is preferable to remove the retractor and extend the incision, rather than proceeding with a suboptimal trajectory.

Another factor to consider is the length of the tubular retractor with regard to the medial-lateral trajectory. The same medial-lateral angle for a patient who requires a 4-cm tube compared with a patient who requires an 8-cm tube places the field of view in two very separate areas. Whereas the former allows the surgeon to dock exactly on the lamina-facet junction, the later places the field of view far too medial because of the length of the tube. Thus it is important to account for this when positioning longer tubes.

In our estimation, the initial placement of the K-wire (as described in some techniques) is where the greatest risk of cerebrospinal fluid leak lies.^6,9 Although several authors have described the safe implementation of this technique, forgoing this step eliminates this risk. After exposing the neural elements, it is vital to use the bayoneted instruments in a manner that optimizes the line of sight and thereby minimizes the risk of dural tears and injury to the neural elements. In the event of a dural tear, repairing it primarily may be challenging given the constraints of the tubular retractor. In light of the absence of a potential space with the minimally invasive technique, some authors have advocated the placement of dural graft matrices and fibrin glue or other sealants in lieu of primary repair.¹³ Nevertheless, we prefer to repair a dural tear primarily whenever possible. Under these circumstances, we have modified a small needle (on 6-0 Prolene) into the shape of a check mark. This needle is then pushed through the dural defect with a Castroviejo needle driver. This prevents the need to rotate the needle, which is untenable in a constrained cylindrical opening. The suture is secured with the assistance of a knot pusher. The use of a self-closing clip for managing incidental durotomies has also been described.¹⁴

There is a learning curve that appears to plateau after 15 to 20 cases. Review of the operative records in our first 100 cases demonstrates a marked decline in surgery times after 20 cases. The first 20 cases averaged 132 minutes, and our last 20 cases averaged only 54 minutes (unpublished data). A similar trend has been identified by other authors.^15,16 During the initial part of the learning curve, it is best to start with at least an 18-mm tube. A larger aperture allows greater ease in establishing the bony landmarks. As greater facility develops with the anatomy from a paramedian approach and with working through a narrow and more-limited field of view, the size of the retractor may be incrementally decreased. The vast majority of our cases are performed through either a 14-mm or 16-mm tubular retractor (Fig. 162-8).

FIGURE 162-8 Photograph of a healed minimally invasive lumbar microdiscectomy incision performed through a 14-mm tubular retractor. The incision measures approximately 16 mm.

The literature has demonstrated a very low rate of surgical site infections for patients undergoing minimally invasive decompressions. O’Toole and colleagues reported their experience with 1338 minimally invasive surgeries and identified a surgical site infection rate of 0.10%. Compared with other large series of open spinal surgery, this is a tenfold decrease in postoperative wound infections. Within our own series, we have not encountered an infection with well over 100 cases.

Conclusion

The minimally invasive microdiscectomy is a safe and effective technique for the management of a unilateral radiculopathy secondary to a herniated lumbar disc. With precise surgical planning and optimal placement of the retractor over the relevant anatomy, a smaller paramedian incision does not compromise the exposure needed to safely accomplish the operation. The minimally invasive microdiscectomy represents the next advancement in limiting the disruption of the normal anatomy for removing a disc fragment and decompressing a nerve root.