The Smith-Robinson approach is the principal surgical approach for the perfor- mance of anterior cervical fusions. A left-sided exposure is used primarily because the course of the recurrent laryngeal nerve is more predictable and protected in the left tracheoesophageal groove. The presence of a kyphotic deformity necessitates consideration of preoperative and/or intraoperative cervical traction. Preoperatively, cervical range of motion must be assessed by the anesthesia and surgical teams.
Hyperextension of the cervical spine should be avoided during intubation and patient positioning. In patients with myelopathy, awake fiberoptic intubation and neurophysiologic monitoring of transcranial motor evoked potentials (tcMEPs) and somatosensory evoked potentials (SSEPs) should be considered. OPLL beyond the margins of the disk space causing retrovertebral compression warrants either a partial or complete corpectomy.
The level of the skin incision can be assessed using the palpable subcutaneous landmarks corresponding to the adjacent vertebral bodies (Figure 3-4). The hyoid
A B
FIGURE 3-3 Sagittal and axial MRI images of OPLL effacing cerebrospinal fluid signal and compressing the spinal cord.
corresponds to C3, the thyroid cartilage to C4-C5, and the cricoid to C6. In addition, the carotid tubercle of C6 can often be palpated. An oblique incision paralleling the anterior margin of the sternocleidomastoid is often utilized for multilevel procedures because it grants access to more levels than does the more cosmetically appealing transverse incision that corresponds to Langer lines. Sharp dissection is performed down to the platysma, and it is divided transversely. Flaps are raised proximally and distally deep to the platysma. The sternocleidomastoid is identified. Dissection is performed bluntly, anterior and medial to the anterior edge of the sternocleido- mastoid, through the deep cervical fascia where the omohyoid is encountered. Care must be taken to keep the carotid sheath and its contents lateral to the plane of dissection (Tips from the Masters 3-1). The middle layer of the deep cervical fascia between the omohyoid and the sternocleidomastoid is bluntly dissected. The omo- hyoid may be divided if necessary to extend the exposure over multiple levels. The deep layer of the deep cervical fascia is incised vertically over the midline of the ver- tebral bodies and disks. Radiographic documentation of the appropriate diskectomy or corpectomy level is obtained by placing a spinal needle or marker within either a disk or vertebral body. The longus colli is elevated for 3 to 4 mm off the adjacent disk spaces and vertebral bodies.
Tips from the Masters 3-1 • The carotid sheath and its contents should be kept lateral to the plane of dissection, which is toward the anterior cervical spine.
The disk material is extricated and the uncinate processes are identified to delin- eate the lateral limits of the decompression or corpectomy. The decompression
SCM
C1 C2
C3 C4
C5 C6
C7 T1 Longitudinal
incision
Transverse incision
FIGURE 3-4 Palpable landmarks to identify the appropriate level of surgical incision for the anterior approach to the cervical spine. The hyoid bone is at C3, the thyroid cartilage corresponds to C4-5, and the Chassaignac tubercle and cricoid cartilage correspond to C6.
includes removal of the posterior disk-osteophyte complexes, identification and removal, or “floating,” of the ossified PLL, and foraminal decompression via removal of uncovertebral osteophytes.
If the surgical plan involves a corpectomy, the disks cephalad and caudad to the planned vertebrectomy level, along with any intervening disk in the case of a multi- level procedure, are completely excised before the vertebrectomy is performed. The decompression should be extended so as to completely alleviate elements leading to spinal cord deformation and compression.
When OPLL is the source of compression, caution must be exercised in removal of the ossification, and corpectomies are often necessary (Tips from the Masters 3-2).
The presence of the double layer sign (a rim of ossification surrounding the hypodense ligament) on radiographic workup of OPLL is suggestive of dural penetration.1 The two layers represent ossification of the ventral PLL and ossification that invests the dura with an intervening space of less dense PLL. This increases the risk of neurologic injury and iatrogenic durotomy if complete débridement of the OPLL is performed. An alternative to complete resection of the OPLL is the anterior floating method, which involves a transverse decompression of 20 to 25 mm to the joints of Luschka and release of the OPLL around the region that invests or replaces the dura to allow sufficient 4 to 5 mm of anterior migration of the ossification.2 In the event of a dural defect, a primary repair should be attempted, and adjuncts such as dural grafts and fibrin glue sealant can be applied to prevent cerebrospinal fluid (CSF) leaks. The patient can be kept in an upright position to diminish the buildup of CSF pressure across the anterior cervical spinal cord. Persistent leaks have been successfully treated with lumboperitoneal shunts and lumbar drains to prevent CSF fistulas.3
Tips from the Masters 3-2 • Electrocautery exposure of anterior osteophytes facilitates complete removal, which improves visualization of the posterior vertebral body and allows for anatomic placement of anterior plates.
Orientation to the midline must be maintained during the decompression. As demonstrated by An and colleagues4, the risk of vertebral artery injury increases as one moves rostrally in the subaxial cervical spine. At C3 a 15-mm-wide cen- tral decompression, and at C6 a 19-mm-wide decompression, yields a 5-mm mar- gin of safety for the transverse foramen and vertebral artery5 (Figure 3-5). Goto and associates6 reported in 1993 that the central decompression should be at least 16 mm. This can be achieved by maintaining C3 15-mm and C6 19-mm-wide central
Carbide- tipped bur
17-18 mm FIGURE 3-5 A depiction of a corpectomy trough that has been widened poste- riorly to provide adequate decompres- sion of the spinal cord. (Truumees E HH:
Anterior cervical corpectomy. In Haher T, Merola A, editors: Surgical techniques for the spine, New York, 2003, Thieme, pp 29–35.)
decompressions at the level of the vertebral artery and expanding the decompres- sion dorsally as the canal is approached. After the decompression is completed, the focus shifts to reconstruction and grafting of the corpectomy defect. Fibular strut allografts and cages filled with local autograft are principally used to reconstruct the corpectomy defects. Before anterior cervical instrumentation became standard, postoperative graft dislodgment was a significant risk. Several authors have recom- mended that the treatment of OPLL involving one- or two-level corpectomies consist of allograft strut grafting and anterior plating.7-9 Multilevel OPLL involving three or more levels should be addressed with multilevel corpectomies, allograft strut graft- ing, anterior plating, and supplemental posterior fixation and fusion. This approach has been associated with few, if any, graft-related complications.7-9