This surgical technique was studied and practiced on cadavers before it was applied in living patients. The goal was to achieve direct and effective anatomical decompression of the spinal cord with maintenance of spinal stability, thus eliminating the need for bone fusion and immobilization. The surgical technique of microsurgical anterior foraminotomy has been reported elsewhere.[4]
The operation is performed after general endotracheal anesthesia has been attained in the patient. Somatosensory evoked potentials (SSEPs) are obtained by stimulating the upper and lower extremities immediately after induction of general anesthesia. Once the baseline SSEPs are obtained, SSEP monitoring is used continuously until the end of the operation. Positioning of the patient is similar to that for the conventional anterior approach to the cervical spine. With the patient supine, a bolster is placed behind both shoulders to maintain gentle extension of the cervical spine. When the patient is properly positioned, the baseline spinal cord function is reconfirmed with SSEP monitoring. The head is positioned with the midline upright. Both shoulders are gently pulled and fixed caudally with tape to facilitate a lateral view of the cervical spine on intraoperative roentgenogram. A cervical traction device is not used. The entire anterior neck is prepared with antiseptic solution and draped.
A 3- to 6-cm long transverse incision is made at the anterior neck along a skin crease that is similar to the incision made for an anterior approach to the cervical spine. The skin incision is made ipsilaterally to the radiculopathy or to the narrower side of the spinal canal. The first two-thirds of this incision is made medially to the sternocleidomastoid muscle and the remaining one-third is kept lateral to the medial border of the sternocleidomastoid muscle. The subcutaneous tissue and the platysma muscle are incised along the line of the skin incision. The loose connective tissue layer under the platysma muscle is cleanly undermined to provide space to operate. A combination of sharp and blunt dissection is used to access the anterior column of the cervical spine to keep the carotid artery and the sternocleidomastoid muscle lateral and the strap muscle, trachea, and esophagus medial. The prevertebral fascia is opened, and the anterior column of the cervical spine is exposed. The correct level is then confirmed with a radiographic lateral view of the cervical spine. Up to this point, the procedure is similar to that for an anterior approach to the cervical spine.
An anterior cervical discectomy retractor system is then applied; only smooth-tipped retractor blades are used. Retraction naturally exposes the ipsilateral longus colli muscle rather than the midline anterior disc surface. An operating microscope is used at this stage. The medial portion of the longus colli muscle is excised to expose the medial parts of the transverse processes of the upper and lower vertebrae. The vertebral artery (VA) is located anterior to the C-7 transverse process and beneath the longus colli. Therefore, when operating at the C67 level, care must be taken not to injure the VA while removing the medial portion of the longus colli. Because the VA occasionally enters the transverse foramen at another level, the longus colli is incised carefully under the operating microscope. For operations above the C67 level, the VA is not exposed purposefully at this point.
Once the medial portions of the transverse processes of the upper and lower vertebrae have been identified, the ipsilateral uncovertebral joint between them can be seen; however, advanced spondylosis may obscure the anatomical landmark of the uncovertebral joint and transverse processes. Anterior spondylotic spurs at the intervertebral disc can act as a guide, leading to the uncovertebral joint superolaterally. Although the interface of the uncovertebral joint will be angled approximately 30° cephalad from the horizontal line of the intervertebral disc in the normal cervical spine, advanced spondylotic changes may obscure the normal anatomy. The uncovertebral joint is drilled between the transverse processes using a high-speed microsurgical drill attached to an angled hand piece (Fig. 1). To prevent injury to the VA, a thin layer of cortical bone is left attached to the ligamentous tissue covering the medial portion of this artery. Drilling continues down to the posterior longitudinal ligament. As drilling advances posteriorly, the direction of the drill is gently inclined medially. When the posterior longitudinal ligament is exposed, a piece of thin cortical bone is left attached laterally to the periosteal and ligamentous tissue covering the VA. This lateral remnant of the uncinate process is dissected from the ligamentous tissue and fractured at the base of the uncinate process. It is further dissected from the surrounding soft tissue and removed, which enables identification of the VA by its pulsation between the transverse processes of the vertebrae. It is necessary to proceed cautiously with drilling at the base of the uncinate process because the nerve root lies just adjacent to it. After the uncinate process becomes loosened at its base, it is safer to remove the thin layer of remaining bone of the uncinate process by fracturing it rather than by continued drilling. When the remaining piece of the uncinate process is removed, the posterior osteophytes are drilled by crossing the midline diagonally toward the opposite margin of the spinal cord dura mater. The size of the hole made by the drilling at the uncovertebral joint is usually approximately 5 to 6 mm wide transversely and 7 to 8 mm vertically.
The posterior longitudinal ligament is incised and resected to achieve decompression of the ipsilateral nerve root and spinal cord. The beginning of the contralateral nerve root is identified for adequate decompression of the spinal canal in the transverse axis (Fig. 2). Multiple anterior foraminotomies are performed as needed. Using the holes of anterior foraminotomies, the spinal cord canal is enlarged in the longitudinal axis by removing the posterior portion of the vertebral bodies with Kerrison rongeurs and a long-armed up-biting curet. The bone bleeding is controlled with the application of bone wax. Epidural bleeding from the posterior longitudinal ligament can be controlled with bipolar coagulation. Hemostatic agents are not used in the epidural space.
Finally, the platysma is closed with interrupted No. 3-0 absorbable stitches, and the skin is approximated with subcuticular sutures. To minimize postoperative incisional pain, a local anesthetic (a few milliliters) is injected subcutaneously. A cervical collar is not used. Although microsurgical anterior foraminotomy for cervical radiculopathy has been performed as outpatient surgery, this group of patients with myelopathy stayed in the hospital overnight to observe their spinal cord function clinically; they were discharged home the next morning.
The surgery was performed with patients supine, and a prevertebral surgical exposure of the affected cervical disc level was accomplished on the affected side as described by Cloward.1 Figures 1 and 2 (A to E) illustrate the site of surgical decompression and summarize the steps of the procedure. The anterior cervical retractor naturally tends to maintain exposure centered over the medial border of the longus colli muscle. A long segment of the colli muscle was mobilized laterally to expose the transverse processes above and below the affected disc space without removal of the muscle (Figure 2A). Dissection around the circumference of the vertebral body between the transverse processes and lateral to the uncus with a curet or Freer type instrument was accomplished. Fluoroscopic imaging (Figure 3) was helpful to guide the placement of a 1 to 4-in. or 3 to 8-in. malleable blade retractor inserted between the vertebral body and vertebral artery that maintained retraction of the colli muscle and protected the artery. The malleable retractor was attached to a table-mounted (Greenburg type) retractor to maintain the exposure (Figure 2B). The lateral view fluoroscopic image showed correct placement of the retractor inserted to the midvertebral body to avoid compression of the nerve root posteriorly. The remainder of the procedure was then completed with microscopic magnification. The lateral portion of the uncovertebral joint was drilled (Figures 1 and 2C) until a thin posterior cortical rim was left posteriorly that was removed with curettage and Kerrison rongeurs to expose the lateral posterior longitudinal ligament overlying the exiting nerve root. Further exploration for removal of any compressive ligament, herniated disc, and osteophytes on the adjacent endplates allowed removal (Figure 2D). The small remaining portion of the uncus was then removed to complete the anterior foraminal decompression (Figure 2E).