Minimally Invasive Posterior Cervical Fusion

Minimally invasive spinal surgery (MISS) has undergone a renaissance over the past decade. Although current adoption has been limited to less than 10% because of its technical difficulty and learning curve challenges, its advantages compared with traditional, open surgical approaches are becoming obvious. Minimizing the damage to dorsal muscles, ligaments, and joints of the spine has the potential to reduce postoperative pain, narcotic medication use, hospitalizations, and medical costs.

Cervical nerve root decompression via the dorsal approach was the first modern application of minimally invasive surgery in the cervical spine. Initial cadaveric studies confirmed that an endoscopic-assisted foraminotomy could accomplish the same decompressive effect as a traditional open approach, ¹^, ² but widespread adoption was limited by the significant learning curve, surgeon disorientation, and lack of facility with the surgical endoscope. The advent of tubular dilator retractors compatible with a variety of visualization methods, including the operating microscope, surgical loupes, and the endoscope resulted in a resurgence of interest in this approach.

In 2001 Adamson published excellent results achieved in his first 100 consecutive minimally invasive endoscopic cervical foraminotomies. ³ This report did not prove superiority over open foraminotomy, but it demonstrated the safety and efficacy of the procedure. After the Adamson report, other case series were published, ⁴ including a report by Fessler and Khoo, comparing in a nonrandomized fashion the outcomes achieved with minimally invasive versus open surgical technique. ⁵ That report compared 25 MISS patients with a historical cohort of 26 open patients, showing an equivalence on clinical outcomes but an average of 48 hours shorter hospitalization, less blood loss, and less narcotic consumption. In support of the technique, a recent position paper from Adamson related his experience with more than 900 cervical endoscopic foraminotomies, describing the procedure as safe and effective, although no quantitative outcomes data were reported. ⁶

21.2 Minimally Invasive Cervical Stabilization

The ability to perform a decompression through limited access ports has also resulted in an exploration into techniques for accomplishing a bony arthrodesis and fixation. Because direct visualization of the dorsal spinal column can be easily accomplished through a tubular port, this was a natural extension of previously innovated decompression techniques.

The first case series, reported by Wang et al, showed the safety and effectiveness of lateral mass screw fixation in three patients treated for facet dislocations with a minimally invasive procedure. ⁷ The technique was similar to the exposure for minimally invasive posterior foraminotomies, with the screw-rod constructs placed through a single tubular dilator retractor inserted in the midline and directed laterally ( ▶ Fig. 21.1). Long-term follow-up of 2 years or longer for patients treated in this manner was subsequently reported in 2006 by Wang and Levi. ⁸ The series consisted of 18 patients who underwent the procedure at the C3 to C7 levels, and 16 of the patients had successful treatment. The remaining two required conversion to the open procedure because of a lack of adequate fluoroscopic visualization to aid screw placement in the lower cervical spine. A total of 39 levels were instrumented, and cases were limited to a maximum of three vertebral levels (two intersegmental levels). Whereas no comparison data with open surgery were reported, there were no complications or pseudarthroses in the series. Similar findings were described in a case series of two patients treated by Fong and DuPlessis. ⁹

Fig. 21.1 Intraoperative view showing (a) drilling of a pilot hole and (b) placement of standard lateral mass screws through a 14-mm tubular dilator retractor.

21.3 Patient Selection

The selection of patients for minimally invasive surgical stabilization is not dramatically different from that for the open procedure. As discussed already, the area to be stabilized is generally restricted to three vertebral levels (two segments). In addition, consideration should be given to the likely adequacy of intraoperative imaging. As such, some patients with short necks may be poor candidates for this technique without intraoperative computed tomography (CT)-guided neuronavigation.

21.4 Operative Procedure

Posterior surgery was performed with the patient in the prone position, and lateral fluoroscopy was used in all the procedures to compensate for the reduction in visual cues. A 2.0-cm midline skin incision was made to introduce a set of tubular dilator retractors after infiltrating the skin with local anesthetic. This maneuver, along with stretching of the skin using the retractor, results in a postoperative scar of approximately 1.5 cm. Sharp incision of the muscular fascia allows easier placement of the retractor.

The skin entry point was chosen so that the tube trajectory would be parallel to the facet joint in the sagittal plane and dock on the levels to be treated. This typically placed the entry point approximately two spinal segments below the level of interest ( ▶ Fig. 21.1). The tube trajectory was also directed laterally so as to dock on the posterolateral elements. In this manner, the tube trajectory approximated the ideal screw orientation using the Magerl technique. ¹⁰ A tubular retractor measuring 20 mm in diameter was used. The surface of the lateral mass was then exposed with monopolar cautery and pituitary rongeurs to remove any intervening muscle or soft tissues. The synovium of the facet joint to be fused was removed with a curette and packed with autograft bone. For cases without an anterior approach, a small amount of cancellous bone was harvested from the posterior iliac crest. Illumination was provided with either a small fiberoptic cable placed down the bore of the tubular retractor or with the operating microscope ( ▶ Fig. 21.2).

Fig. 21.2 Artist’s rendition of an axial cutaway view of screw placement. Note that one central incision allows for the trajectory for both the right and left lateral mass screws given the fact that the screws are directed laterally.

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