Applied Radiology

Dr. Rojas is an Associate Professor of Radiology, Dr. Hamide is a 2nd-year Radiology Resident, and Dr. D'Antonio is an Assistant Professor of Radiology in the Department of Radiology, Section of Neuroradiology, Louisiana State University, New Orleans, LA. Dr. Castillo is Chief of Neuroradiology and a Professor of Radiology in the Department of Radiology, Section of Neuroradiology, University of North Carolina School of Medicine, Chapel Hill, NC.

Correct interpretation of imaging studies of the postoperative cervical spine requires knowledge of the underlying pathology and of the surgical procedures performed. This paper is divided into two sections: the upper cervical spine (defined as the base of the skull to C2) and the lower cervical spine (C3­C7). ¹ Surgical techniques used in these regions are divided into anterior and posterior approaches. Most patients require radiographs, plain computed tomography (CT) or post-CT myelography, and magnetic resonance imaging (MRI). MRI has not been widely used in these patients because of potential artifacts; however, artifacts can often be reduced using imaging sequences that decrease magnetic susceptibility. ^1,2

Upper cervical spine

Fixation and fusion of the upper cervical spine is difficult due to its location and unusual biomechanics. The posterior approach affords the surgeon a better view of the spinous processes, laminae, facets, and spinal cord than the anterior approach. Advantages of this approach include safety and greater facility to apply bone grafts and hardware. ^1,2

Anterior approaches

Anterior transoral approach-- The indications for this approach include C1­C2 abscess drainage, biopsy of tumor, and congenital neck deformities. Risks of this approach include infection and limited surgical exposure.

The procedure is performed as follows. The patient is placed in a supine position with the neck fully extended, and the face and oropharynx are prepped with retraction for good exposure. The surgeon palpates the anterior tubercle of C1 with the anterior body of C2. Fluoroscopy and plain radiographs can aid the surgeon as needed. A midline incision is made through the posterior pharyngeal wall, including the retropharyngeal tissue and anterior longitudinal ligament from C1 to C3, providing access to the vertebrae and disk spaces. Proper closure of the pha-ryngeal layers is essential.

Anterior lateral retropharyngeal approach­­ The indications for this approach include C1­C2 fusion when the posterior ring is hypoplastic or absent. For this procedure, the patient is placed in the supine position with neck extended and the head turned. Traction is provided via tongs and a halo device. An incision is made along the anterior border of the sternocleidomastoid muscle crossing the temporal bone base. The sternocleidomastoid is mobilized and everted with external jugular vein and occipital artery ligation. The dissection is continued to the retropharyngeal space laterally and posteriorly to the carotid artery sheath. The alar and prevertebral fasciae are dissected to expose the vertebral bodies with the anterior longitudinal ligament incised in the midline.

Posterior approaches

Posterior occipitocervical fusion-- The indications for this approach are instability, intractable neck pain, cervical osteoporosis, severe rheumatoid arthritis, and trauma (ie, unstable C2 teardrop fracture). The techniques and devices used in this approach include the following.

Hartshill-Ransford loop : This device is contoured and shaped for the individual patient (Figure 1). It provides occipitocervical and atlantoaxial stability by preventing vertical migration of the dens. It can be used for rheumatoid arthritis, congenital malformations, tumors, complicated fractures, spondylosis, and osteogenesis imperfecta.

Luque rectangles: These are smooth rods that are shaped to accommodate the occipitocervical junction (Figure 2). These rods are fixed to the occiput with wires placed in the outer table of the calvarium. These rectangles are then fixed to the cervical spine with sublaminar wires. They may also require bone grafting.

Occipitocervical plates: The occipitocervical plates are metal plates that are preformed to accommodate the base of the skull. The plates help maintain an occipitocervical angle of ~105š and are fixed with screws to the occiput superiorly and with screws at the articular masses of C1/C2 inferiorly. There is increased morbidity from penetrating the inner table of the skull.

McLauren fusion: This procedure addresses atlantoaxial instability that is >2.5 mm between the dens and the atlas in adults and 4.5 to 5.0 mm in children. This fusion of C1­C2 requires passing a stainless steel wire around the posterior arch of C1 and under the spinous process of C2.

Brooks fusion: Songer cables are passed under the arch of C1 and around the lamina of C2 (Figure 3). This procedure prevents subluxation of C1 and C2.

Posterior transarticular screw fixation: This technique is a conventional means for wiring of C1­C2. Screws are placed through the articular pillars of C2 and into the lateral masses of C1.

Laminar clamps (Halifax clamps): This technique is used for atlantoaxial fixation. Two C-clamps grip the laminae with threaded screws that keep the clamps in place. Fusion is acquired in 12 weeks in ~80% of patients. ^1,2

Lower cervical spine

In this segment, the anterior approaches are the most direct method for decompression of the cervical canal. These methods are also used to decompress the cervical nerve roots or spinal cord compression secondary to osteophytes, ossification of the posterior longitudinal ligament, congenitally narrowed canal, and herniated disks. The indications include cervical myelopathy (gait disturbance, parasthesias, neck pain) and radicular signs. Once the cause for the symptoms has been identified, it is important to recommend surgical therapy, anterior diskectomy/fusion, or subtotal vertebrectomy/fusion. ^1,2

Anterior approaches

Smith-Robinson procedure-- The diskectomy with fusion is performed with the patient in a head-holder traction or in a neutral position via a left-sided neck approach with incision of the prevertebral fascia exposing the vertebrae and disk spaces. After radiographic confirmation, the disk is removed to the level of the posterior longitudinal ligament and laterally to the unconvertebral joints. Osteophytes are removed and the vertebral bodies are then spread. An iliac crest graft is placed in the disk space, and the graft is locked into place (Figure 4). This method is considered the gold standard for anterior cervical fusions. Six to 12 weeks after surgery, the graft becomes incorporated. The variation to this procedure is the Bailey-Badgley procedure that utilizes bone strut grafts; the patient wears a brace for approximately 6 weeks ^1,2 (Figure 4).

Anterior cervical vertebrectomy/ fusion-- This procedure is performed via a transverse neck incision with mobilization of the deep cervical fascia. A diskectomy is performed and the central portion of the vertebral body is removed. End plates and cortical lips are prepared and grafts locked into place. Postoperative patients are placed into a halo vest for 8 to 12 weeks of immobilization. Physical therapy and rehabilitation are required. Placement of metallic plates and/or screw fixation are required for burst fractures, traumatic disk herniation, tumor resection, and unstable injuries. Following corpectomy and removal of disk material below or above the level of corpectomy, bone grafts are placed. Afterward, a cervical plate is placed anteriorly across the bone graft. Screws secure adjacent vertebral bodies. Different plates are available for fusions.

The Caspar plate has three rows of holes and is concave anteriorly. The plate is trapezoidal and bicortical screws are used for greater stability (Figure 5).

The Morscher plate is an H-shaped titanium plate with self-locking screws. (Figures 6 and 7). These screws have spiral fenestrations that allow the ingrowth of cancellous bone. Titanium hardware has great tissue compatibility and stealthiness over stainless steel especially when imaged with MR.

Posterior approaches

Laminotomy/diskectomy-- This type of posterior approach is used in the decompression of the posterior cervical disk herniations and osteophytes at one level. The patient is placed in a prone position with appropriate traction applied. A 3- to 4-cm incision is followed by retraction of the paraspinous muscles. This process exposes the spinous processes. The laminae are drilled with removal of ligamentum flavum and epidural venous plexuses and nerve root retraction. This facilitates the exposure of the lateral disk space. ^1,2

Laminectomy/foraminotomy-- This is a treatment for multilevel spondylitic radiculopathy. The patient is placed in a prone position and a midline incision is made. The spinous processes are removed to adequately expose the soft tissues and ligaments. ^1,2

Laminoplasty-- This procedure is performed on cases of myelopathy secondary to canal stenosis, multilevel spondylosis, and ossification of the posterior longitudinal ligament. This technique increases the canal diameter. The spinous processes and laminae are partially resected with a bony gutter drilled laterally, and the thinned border of the laminae are excised. The second bony gutter is drilled into the contralateral laminae and sutured to prevent the laminar door from closing. The canal diameter is increased to 5 mm, which may necessitate the need for posterior stabilization via interspinous wiring, wrap-around interspinous wiring, facet wiring with bone graft, interfacet wiring, or interspinous wiring with methylmethacrylate. ^1,2

Interspinous wiring-- Holes are drilled into the outer cortex of the spinous processes near the laminae. Wires are then passed through the holes into the adjacent vertebrae. The ends of these wires are twisted.

Facet wiring-- Holes are drilled into the posterior pillars of the facets. Wires are passed through the holes and then tied around the bone graft struts. This promotes strong fusion.

Sublaminar wiring-- In this procedure, wire loops are placed around both sides of the laminae using 16- to 18-gauge Luque wires or braided Songer steel cables; then bone grafts are placed over the fusion site.

Complications of cervical spine surgery

Complications associated with anterior approaches

Intraoperative complications associated with the use of plates include arterial/venous laceration, spinal cord injury, esophageal/tracheal injury, and recurrent laryngeal nerve injury. Injury to the recurrent laryngeal nerve is the most common complication with plate placement anteriorly. The symptoms of such an injury include temporary/permanent hoarseness. ^1,2

Postoperative infection/abscesses are also common. Imaging modalities commonly employed for these conditions include MRI with gadolinium and contrast-enhanced CT. T1-postcontrast images demonstrate an area of thickened epidural enhancement with mass effect and posterior displacement of the thecal sac. There is occasional spinal cord compression. The necrotic center will not enhance.

Within a few days of the surgical intervention, MRI reveals discrete areas of signal abnormality at the superior and inferior corpectomy sites. The vertebral bodies demonstrate low T1 and high T2 signal intensities that are consistent with edema. These changes often persist for a few weeks with signal intensity abnormalities within the adjacent vertebral bodies that vary from isointense to hypointense on T1-weighted images and from isointense to hyperintense on T2-weighted images (Figure 8). Graft sites that are not stable will have prolonged edema while stable graft sites tend to have fatty infiltration and sclerosis. The graft always remains rectangular in shape. These findings should not be confused with infection. ^1,2

Autologous bone graft imaging often depends upon the state of the marrow, ie, fatty vs. cellular. The cortex of the graft is low in signal on both T1- and T2-weighted spin-echo sequences. The marrow cavity is usually of intermediate signal intensity on T1- and T2-weighted imaging. Bone graft struts without bone marrow have no specific signal centrally. ^1,2

Cervical canal stenosis is defined as an anteroposterior canal diameter of less than 1 cm. Significant abnormal postoperative findings result from hypertrophic bone formation at the level above and below the fusion sites. Due to increased biomechanical stress at these levels, there is increased propensity for degenerative changes. New bone is isointense to the vertebral body on T1-weighted images and tends to cause mass effect on the subarachnoid space and the spinal cord. Osteophytes on T2-weighted images and gradient spin-echo sequences produce a low signal intensity. Graft extrusion can produce functional canal stenosis and cord compression. Graft material hypertrophy can also lead to spinal canal stenosis.

Complications associated with posterior surgical approaches

Complications from this technique include arterial/venous injury, tracheoesophageal tears, and recurrent laryngeal nerve injury. Other complications such as air embolism and wound infections are less common.

Special imaging considerations

Multiple metallic substances may preclude MR imaging because of patient safety considerations and imaging quality. ^3,4 Metallic fragments that originate from the drill bit can cause image degradation (Figure 9). These artifacts are described as producing a blooming artifact on MRI. ^3,4 MR is vital in the evaluation of the postoperative patient. ⁵ Fast-spin-echo sequences demonstrate increased amounts of T2 information in the presence of metallic artifact because of decreased magnetic susceptibility. ⁶ Magnetic susceptibility can cause field inhomogeneity and image distortions created by dephasing of spins. This effect increases with the length of time the spins have to dephase. ^7,8 Factors that influence magnetic susceptibility include field strength, chemical structure, density, spatial resolution, echo spacing, and sampling bandwidth. ^8,9 Ferromagnetic substances have a greater effect than paramagnetic substances. Fast-spin-echo is a sequence that is performed at a faster rate than the standard spin-echo pulse sequence with improved signal-to-noise ratio and contrast. Use of short-spin-echo spacing allows for decreased magnetic susceptibility effect. ¹⁰ A reduction of magnetic susceptibility may be a disadvantage for detecting hemorrhage; however, this technique has a great advantage in the postoperative spine. Half-Fourier acquisition single-shot turbo-spin-echo (HASTE) images are also advantageous with regards to decreasing metal-induced artifacts at the expense of lower contrast resolution. ^9-11

Conclusion

Correct imaging evaluation of the postoperative spine requires an understanding of the surgical principles, different surgical approaches, and recognition of the various available fixation devices. As most fixation devices are metallic, they often preclude interpretation secondary to image degradation. A reduction of these artifacts produced from the hardware is important when evaluating the postoperative spine. The use of short-spin echo, fast-spin echo, or HASTE is effective when imaging the postoperative patient. AR