Cervical Sympathetic Chain

The cervical sympathetic chain is located posteromedial to the carotid sheath and runs over the longus muscles. It extends longitudinally from longus capitis to longus colli over the muscles and under the prevertebral fascia. From superior to inferior, longus colli muscle diverges laterally, whereas the cervical sympathetic chain converges medially at C6. The average distance between the cervical sympathetic chain and medial border of the longus colli muscle at C6 is 11.6 (±1.6) mm. The superior ganglion of the cervical sympathetic chain in all dissections was located at the level of the C4 vertebra.

Vertebral Artery

The vertebral artery originates from the subclavian or innominate artery and enters the transverse foramen (TF) of the sixth cervical vertebrae. Before this, it is anterior to the transverse foraminae of C7. The artery passes through a series of foraminae until it reaches the base of the axis. It then turns posteriorly to enter the foramen transversarium in the posterolateral part of the ring of atlas and perforates the posterior atlantoaxial membrane to enter the foramen magnum. It then joins the contralateral vertebral artery to form the basilar artery, which supplies the brain stem and cerebellum.

The vertebral artery is intimately associated with the lateral border of the uncovertebral joint. Pait and colleagues reported that the distance from the tip of the uncinate process to the vertebral artery averaged 0.8 mm at C2 to C3 and 1.6 mm at C4 to C5. In a magnetic resonance imaging (MRI) study, Eskander and colleagues reported that the smallest distance between the uncovertebral joint (measured from the medial aspect of the uncovertebral joint to the medial aspect of the vertebral artery) was 4.15 mm at C3. Caution should be exercised when the uncinate process is removed in an attempt to remove osteophytes. In a review of 10 cases of vertebral artery injury, Smith and colleagues found that the use of a high-powered drill laterally was the most frequent cause of arterial injury. The surgeon must always check preoperative imaging for vertebral artery anomalies. There are 3 types of vertebral artery anomalies: (1) intraforaminal anomalies, (2) extraforaminal anomalies, and (3) arterial anomalies.

Recurrent Laryngeal Nerve

Injury to the recurrent laryngeal nerve (RLN) is a commonly reported complication following anterior cervical spine surgery. The RLN innervates all the intrinsic muscles of the neck with the exception of the cricothyroid. Almost all RLN injuries resolve over time; however, permanent paralysis rates have been reported in up to 3.5% of patients. Several causes for RLN injury have been suggested, including postoperative edema, stretch-induced neuropraxia, direct nerve ligature, direct retractor-induced nerve trauma, and nerve impingement between the endotracheal tube and retractor.

The right RLN branches from the vagus nerve at the level of T1 to T2. After looping around the subclavian artery, the right RLN becomes invested in the tracheoesophageal fascia (TEF). The RLN then travels superiorly and slightly anterior to the tracheoesophageal groove (TEG), before coursing between the trachea and the thyroid. In 82% (9 of 11) of right-sided dissections, the RLN entered the larynx at or inferior to C6 to C7. After looping around the aortic arch, the left RLN becomes invested in the TEF inferior to the T2 level. The nerve then travels slightly anterior to the TEG and within the TEF before coursing between the trachea and thyroid. In all the left-sided dissections, the RLN entered the larynx at or inferior to C6 to C7. The investigators concluded that both the left and the right RLNs had similar anatomic courses and received similar protection via surrounding soft tissue structures. From an anatomic perspective, the investigators did not appreciate a side-to-side difference superior to this level that could place either nerve at greater risk for injury.

Carotid Sheath

The carotid sheath contains the common and internal carotid arteries, the internal jugular vein, the vagus nerve, deep cervical lymph nodes, carotid sinus nerve, and sympathetic nerve fibers. The carotid sheath is a fascial investment that blends anteriorly with the investing and pretracheal layers of fascia. During surgical dissection, the carotid sheath is protected by the anterior border of the SCM. Care must be taken when placing and removing retractors to avoid vascular injury.

Esophagus

The esophagus is a thin ribbonlike structure that is located anterior to the retropharyngeal space. To prevent injury, it is critical to identify the esophagus during the approach. Placing an orogastric tube preoperatively can aide in esophageal identification.

Sagittal Alignment

Cervical sagittal alignment is an important consideration, because it affects the surgical treatment options and approach. The normal range of cervical lordosis is 40° (±9.7°) with most occurring between C1 and C2 while the occipital cervical junction is in kyphosis. The average lordosis between C4 and C7 is 6°, making this one of the flatter parts of the cervical spine. The presence of a cervical kyphosis poses a challenge for posteriorly based procedures. Because cervical laminectomy or laminoplasty are dependant on the posterior drift of the spinal cord, they are less effective in the kyphotic spine.

Restoring sagittal alignment should also be one of the goals of surgery and this may be more difficult to achieve from a posterior-only approach. Some of these difficulties may be overcome with an anterior approach or a combination anterior-posterior approach.

Disease Type and Location

The type and location of spinal cord compression dictate the surgical approach. The compression can be either soft or hard. Soft compression results from noncalcified soft tissues, whereas calcified or hard compressive disorders need to be identified because their presence may alter the surgical approach used to achieve decompression. Ventrally located compressive disorders can be localized to the level of the disk, dorsal to the vertebral body, or both. It can involve 1 or multiple levels. The addition of a dorsally located compressive disorder increases the complexity of decision making. Simple ventrally localized degenerative changes, such as anterior osteophytes or disk herniations, should be approached anteriorly to ensure adequate decompression. Dorsal compression, such as from ligamentum flavum hypertrophy or multilevel ossification of the posterior longitudinal ligament, should be approached posteriorly to achieve direct cord decompression. Sagittal alignment needs to be verified with a standing lateral radiograph. Because dorsal compression tends to be more generalized than focal, a posterior approach can more easily decompress multiple levels than anterior decompression. Generally, 1-level to 3-level ventral compressive disorders can be addressed from an anterior-only approach, whereas 4-level disorders should be addressed from a posterior approach or a combination anterior-posterior approach ( Fig. 2 ).

( A ) An 82 year-old diabetic male presented with painless progressive lower extremity weakness and long tract sings. His cervical T-2 weighted MRI was significant for spinal cord compression at C6-C7 with myelomalacia secondary to a large disc osteophyte complex; posterior ligamentum flavum buckling and spinal cord compression at C7-T1; Anterolisthesis of C7 on T1. ( B ) Sagittal CT scan demonstrates moderate disc calcification and osteophyte formation at C5-C6 and marked disc calcification and osteophyte formation at C6-C7. The patient has circumferential cervical spinal cord compression in addition to the C7-T1 anterolisthesis. This type of spinal cord compression would best be addressed with an anterior-posterior decompression and fusion. ( C ) Lateral x-ray demonstrating C6 corpectomy and anterior instrumented fusion from C5-C7 with posterior decompression of C7-T1 and posterior instrumented fusion from C5-T2. ( D ) Post decompression T-2 weighted MRI demonstrating a fully decompressed spinal cord.

Motion Preservation

The incidence of adjacent segment degeneration following anterior cervical fusion has been reported to be approximately 3% per year. Motion preservation technology has gained popularity because it is intended to reduce abnormal stresses transferred to adjacent levels, thus theoretically reducing the chance of developing adjacent segment degeneration. Clinically, the risk of developing adjacent segment degeneration has been shown to be equivalent to both fusion and arthroplasty. There is also a concern that motion preservation may continue to result in microtrauma to the spinal cord, thus negatively affecting clinical outcomes. Buchowski and colleagues reported that 2 years after surgery, cervical arthroplasty outcomes were equivalent to arthrodesis for the treatment of cervical myelopathy for a single-level abnormality localized to the disk space. Hybrid surgery in which 1 level is fused and the second level is treated with cervical arthroplasty has been reported to be superior to 2-level anterior cervical diskectomy and fusion (ACDF) in better Neck Disability Index recovery, lower postoperative neck pain, faster C2 to C7 range of motion (ROM) recovery, and less adjacent ROM increase. Cervical arthroplasty is an option for the treatment of uncomplicated CSM. Whether adjacent level degeneration is the result of the fusion, or simply the expression of the natural deterioration of the motion segments, is unclear.

Corpectomy Versus ACDF for 2-Level Disease

When dealing with multilevel disease, the surgeon has to decide whether to perform an ACDF, a corpectomy, or a hybrid procedure ( Fig. 3 ). Both have unique advantages and disadvantages. The location of the disorder plays a dominant role in the decision-making process. Compression localized to the level of the disk can be approached with either procedure, whereas a disorder localized dorsal to the vertebral body needs to be treated with a corpectomy. The main advantage of a corpectomy in treating a multilevel disorder is the decreased number of graft-bone surfaces that need to fuse. In a 2-level ACDF, there are 4 bone-graft interfaces, compared with only 2 for a corpectomy. The theoretic risk of pseudarthrosis is therefore generally less with corpectomy than with multilevel ACDF. In a recent meta-analysis Fraser and Härtl reported that, for 2-disk–level disease, there was no significant difference between anterior cervical diskectomy with a plate system and corpectomy with a plate system. However, for 3-disk–level disease, the investigators reported that corpectomy with plate placement was associated with higher fusion rates than diskectomy with plate placement. Park and colleagues compared 2-level diskectomy with single-level corpectomy and reported comparable results for sagittal alignment, cervical lordosis, graft subsidence, and adjacent level ossification. Wang and colleagues compared 32 patients undergoing a 2-level ACDF with autograft with 20 patients with a 1-level corpectomy and anterior instrumentation. The investigators reported similar fusion and complication rates between the 2 groups. In contrast with these studies, Guo and colleagues reported that ACDF was superior in most outcomes to anterior cervical corpectomy and fusion and anterior cervical hybrid decompression and fusion. The investigators concluded that, if the compressive disorder could be resolved by diskectomy, then an ACDF is the treatment of choice.

( A ) A 62 year-old diabetic female presented with burning hands and gait instability. T-2 weighted MRI of the cervical spine demonstrates spinal cord compression secondary to spondylosis at C5-C6 and C6-C7. ( B ) Sagittal CT scan demonstrates calcification of both the C5-C6 and C6-C7 discs. Both the MRI and the CT scan confirm that the compression is localized to the level of the disc space. This particular case can be treated with either a C6 corpectomy or a 2 level ACDF. ( C , D ) AP and Lateral post-operative x-rays demonstrate a 2 level ACDF with a semi-constrained cervical plate.

ACDF

ACDF is commonly used in the treatment of CSM. This approach allows for spinal cord and nerve root decompression secondary to herniated disks and spondylotic bone spurs. With the use of an operating microscope, excellent visualization of the disk space, uncovertebral joints, and spinal cord is possible. A thorough diskectomy and meticulous end-plate preparation are critical for neural decompression, disk space distraction, and interbody fusion. This surgical approach uses intermuscular plane and is usually better tolerated than posterior surgery. Nerve root decompression can also be performed by resecting the uncovertebral spurs and foraminal decompression. Correction of sagittal plane deformities can be achieved by using lordotic intervertebral spacers. In addition, restoring anterior disk height increases the foraminal space and indirectly decompresses the nerve root. Single-level ACDF is associated with high fusion rates, particularly when used in conjunction with anterior plating. In a study of 2-level and 3-level ACDF with anterior plating, Samartidis and colleagues reported a fusion rate of 97.5% with either autograft or allograft. Wang and colleagues reported a high percentage of good to excellent clinical outcomes and a low incidence of segmental kyphosis and pseudarthrosis using anterior cervical plating.

Corpectomy

Cervical corpectomy is performed by removing the disk above and below the vertebral body to be resected. This removal is followed by resection of the vertebral body itself. Anterior osteophytes, ossification of the posterior longitudinal ligament (PLL), and disk herniations located behind the vertebral body can be addressed with this approach. Preoperative kyphosis can also be corrected. The procedure involves the creation of a trough in the middle portion of the vertebral body. It is critical to identify the lateral cortical wall of the vertebral body, which is located at the junction of the transverse process and the vertebral body. Identification of this landmark keeps the trough centered and symmetric, thereby avoiding injury to the vertebral artery. The wide trough permits extensive visualization of the disk, PLL, and osteophytes, thereby allowing a safe resection of all compressive disease. The strut graft for reconstruction following corpectomy is typically either structural iliac crest autograft, fibula allograft, or a cage construct packed with autograft. The incidence of surgical-related complications was associated with increasing patient age. Potential technical issues with these grafts include difficulty in restoring normal lordosis, graft dislodgment, graft migration, and, more commonly, graft subsidence. Anterior cervical plating is recommended for cervical corpectomy.

Multilevel Corpectomy

Multilevel disorder may require more complex surgical management. For corpectomy involving 3 or more levels, an anterior-posterior fusion has been recommended. Stand-alone anterior constructs have a high rate of graft migration, dislodgment, and pseudarthrosis. Construct motion has been shown to be lowest in a combined anterior-posterior instrumentation model. Circumferential cervical fusion has been shown to maximize cervical stability and decrease the incidence of graft migration or dislodgment. Accosta and colleagues reported that multilevel instrumented corpectomy offered a biomechanically stable fixation and allowed for significant correction of preexisting kyphosis. Supplemental posterior instrumentation was thought to limit delayed cage subsidence and loss of sagittal balance after this procedure. In the absence of posterior instrumentation, a large moment arm is created with an anterior-only construct. This construct can lead to loss of stability and pistoning or toggling of the graft and its eventual dislodgment, even with the use of an anterior plate or supplemental external fixation in a halo device. Vaccaro and colleagues treated 45 patients with 2-level or 3-level corpectomies and anterior plating. No supplemental posterior instrumentation was used. The investigators reported a 9% dislodgment rate in the 2-levelcorpectomy and 50% dislodgment rate in the 3-level corpectomy. Graft migration occurred despite the use of a halo in 10 of 12 patients who underwent a 3-level instrumented corpectomy. Sasso and colleagues reported a 71% catastrophic failure rate involving graft-plate dislodgment that occurred within 2 months of instrumented 3-level corpectomy. An additional advantage of supplemental posterior instrumented fusion is the ability to perform a posterior laminectomy, if needed, to further decompress the neural elements.

Graft Choice: Allograft Versus Autograft

When cervical plating is used, using allograft versus autograft has not been shown to make a significant difference in the rate of pseudarthrosis with up to a 3-level ACDF. A longer time to union was seen with allograft, although this difference was not clinically significant. Fusion rates can also be increased by burring the end plates down to bleeding bone to augment graft incorporation. This technique was shown to be associated with minimal graft subsidence and decreased the pseudarthrosis rate to approximately 4.4% per level.

Anterior Cervical Plating

Anterior cervical plating has commonly been used to stabilize ACDF and corpectomies, and many studies have found a higher fusion rate with the use of a cervical plate. Some of the other benefits of plating include improved initial stability; decreased need for orthotics; and reduction in complications from graft dislocation, end-plate fracture, and kyphotic collapse. The original anterior cervical plate designs provided rigid fixation between segments, acting as a tension band system in extension and as a buttress plate in flexion. These plate types are commonly referred to as constrained devices. It is hypothesized that these rigid fixation devices are load bearing and may eliminate the mechanical loads on the bone graft, which are thought to be important for fusion. These static designs may also prevent any settling from occurring, thus leaving a gap between the bone graft and the end plate. To overcome these disadvantages, dynamic anterior cervical plates have been introduced that allow for graft loading by axial settling. In biomechanical cadaveric cervical corpectomy studies comparing dynamic and static plates, Brodke and colleagues showed that more load sharing occurred in dynamic cervical plates, because these plate designs allowed for axial settling to make up for graft resorption during the postoperative period. In the experimental situation of a collapsed bone graft, less motion was found with dynamic plates. Fogel and colleagues reported that the static plate had significantly higher graft loads in extension and significant loss of graft load in flexion, whereas the dynamic plate maintained a reasonable graft load throughout the ROM even when graft contact was imperfect. The design of the plate does not affect the outcomes in single-level fusions but statistics indicate that multiple-level fusions may have better clinical outcomes when a dynamic plate design is used.

Anterior Cervical Plate Location and Adjacent Segment Degeneration

Cervical plate location has implications for developing adjacent level degeneration. Koller and colleagues reported that progression of adjacent segment disease after instrumented ACDF was significantly linked to a plate to adjacent disk distance of less than 3 mm. Park and colleagues recommended that anterior cervical plates be placed at least 5 mm away from the adjacent disk spaces to decrease the likelihood of moderate-to-severe adjacent level ossification. Based on their experience with static and dynamic anterior cervical plating, DuBois and colleagues suggested that placing instrumentation at least 3 mm from the adjacent level disk would decrease the potential for adjacent level changes. These studies suggest that extension of plates toward adjacent healthy disks, and perhaps extensive surgical dissection and damage to the anterior longitudinal ligament, are contributing factors in the development of adjacent segment disease.

Decompression

The causes of postoperative deterioration in function include spinal cord injury during surgery, malalignment of the spine associated with graft complication, and epidural hematoma. The extent of spinal decompression remains controversial. Removal of posterior osteophytes and resection of the PLL increase the space available for the spinal cord. However, this is more technically challenging, adds time to the procedure, and increases the risk of neural injury and postoperative epidural hematoma. Recently, Wang and colleagues reported that removal of PLL was generally safe, although more technically demanding. More importantly, the investigators reported a significant improvement in Japanese Orthopaedic Association (JOA) scores in patients in whom the PLL was resected.

It has been suggested that the posterior osteophytes resorb once the spine is fused and segmental motion is eliminated. Stevens and colleagues showed that there was no evidence of remodeling or resorption of osteophytes, and persistent osteophytes continued to deform the spinal cord for up to 12 years after fusion. This finding suggests that osteophyte removal should be performed whenever possible.