Raj Rao and Satyajit V. Marawar
SURGICAL MANAGEMENT
Operative intervention in the posterior subaxial cervical spine is frequently carried out for decompression or stabilization.
Fusion and instrumentation of the posterior cervical spine may be required for unstable fractures or after extensive decompressive procedures.
Instrumentation reduces the need for postoperative immobilization and orthosis wear, augments fusion success, and allows better maintenance of sagittal alignment of the cervical spine.
Interspinous Wiring
Interspinous wiring can be an alternative to lateral mass or pedicle screw fixation in stabilization of the posterior cervical spine.
Although it resists flexion reasonably well, it is generally not as strong in resisting extension, axial load, rotation, and lateral bending.
The most commonly used implants are 18 or 20-gauge stainless steel wire or 1to 1.2-mm titanium braided cable.
Alternatives include braided stainless steel or polyethylene cable. Multistrand braided steel, titanium, or polyethylene cables show superior fatigue resistance, greater flexibility, and improved stability on flexion–extension testing compared to a single-filament stainless steel wire.31,34
In modern spine surgery, wiring techniques are generally limited to cases in which biomechanically superior techniques such as lateral mass fixation cannot be used, a somewhat less invasive midline-only exposure is desired, or the additional rigidity of lateral mass fixation is not necessary (eg, for posterior repair of relatively stable pseudarthroses, or to provide a tension band effect as an adjunct to anterior instrumentation).
Techniques of wiring include simple interspinous wiring (eg, Rogers), Bohlman triple wiring (can be used also for occipitocervical fixation), and oblique wiring.
As a result of the direction of its stabilizing forces, oblique wiring may counter rotational instability better than simple interspinous wiring.
Lateral Mass Screw Fixation
The lateral mass of the subaxial cervical vertebra is a quadrangular column of bone formed by the complex of the superior and inferior articular processes and the intervening bone.
Lateral mass screws are the implants most commonly used at present for posterior fixation of the subaxial cervical spine.
They are versatile in that they can be used when the posterior elements are deficient (eg, from trauma, tumors, or surgical resection for decompression).
Lateral mass screw and rod–plate fixation provides superior flexion and torsional stiffness compared to posterior wiring.9,32
The improved strength of fixation allows instrumentation to be limited to the levels of fusion. When wiring techniques are used, the construct occasionally needs to be extended proximally or distally to obtain additional points of fixation.
A lower incidence of postoperative kyphosis can be achieved with lateral mass screws versus wiring techniques.13
Lateral mass screws have a low incidence of complications and are much easier to insert than cervical pedicle screws.
The Magerl technique of lateral mass screw fixation has been shown to have superior pullout strength and higher load to failure when compared to screws inserted with the RoyCamille technique.26
This may be related to the longer screw length generally obtained with the Magerl technique (18 mm with Magerl technique versus 14 mm with the Roy-Camille technique).6,16
Pullout strength is significantly greater with a bicortical screw than with a unicortical purchase.
Because bicortical purchase engenders potential risk to nerve roots and the vertebral artery, however, unicortical purchase is used in most cases.
Pedicle Screw Fixation of the Cervical Spine
Pedicle screw fixation allows superior, simultaneous stabilization of all three columns of the cervical spine.
The risk of neurovascular injury from penetration of the small cervical pedicle restricts the widespread use of this technique.
Pedicle screws are most commonly used at C2 and C7, where the pedicles are largest in the cervical spine.
They are often used at the cephalad or caudal ends of long instrumented constructs.
At C7, most patients do not have a vertebral artery in the foramen transversarium, making pedicle screw fixation feasible.
At C2, the vertebral artery is generally lateral to the insertion site and trajectory of the pedicle, making pedicle screw fixation feasible.
From C3 to C6, the proximity of the vertebral artery and the small diameter of the pedicles make pedicle screw fixation challenging and not feasible for routine use.
Whenever pedicle screw fixation in the cervical spine is contemplated, careful scrutiny of preoperative CT and MRI scans is essential.
The cervical pedicle is generally taller than it is wide, with the mean height of all cervical pedicles around 7 mm (range 6 to 11 mm).27
The width of the pedicle is the critical determinant for feasibility of pedicle screw placement.
Pedicle outer diameters less than 4 mm may preclude pedicle screw insertion.12
Multiple morphologic studies have found that the mean cervical pedicle outer width varies from 4 to 7 mm, with significant variation in width at different levels (Table 1).12,24,27
The pedicles of C2 and C7 are generally large enough to accommodate either 3.5 or 4-mm screws.
The length of the pedicles from C3 to C6 ranges from 12 to 18 mm.12 Screw lengths are generally slightly longer to obtain purchase within the vertebral body.
The axial angle of the pedicle (medial angle to the sagittal plane) is the least at C2 (25 to 30 degrees)10 and increases to a mean of 44 degrees (25 to 55 degrees) at C3. From C3 to C7 it gradually reduces to a mean of 37 degrees (33 to 55 degrees).24
TECHNIQUES
POSTERIOR CERVICAL FUSION
Although it is tempting to focus on instrumentation techniques, performing a meticulous fusion technique is just as important, if not more so, to the success of surgery.
In virtually all cases, posterior fusion is supplemented by some form of instrumentation.
To maximize the surface area for fusion, all posterior bony surfaces that do not need to be resected for the decompression should be left intact for fusion.
The main priority, however, should always be to first achieve adequate neural decompression.
After exposure, all soft tissues, including the interspinous ligaments and muscles, facet joint capsules, and paraspinal soft tissue, are meticulously resected so that the cortical surfaces lie exposed.
Facet joint cartilage is removed with a curette or 3-mm burr within the facet joint.
The posterior cortical surfaces of the laminae and spinous processes are decorticated with a 3-mm burr to expose bleeding subcortical bone (TECH FIG 1).
Bone graft obtained from the iliac crest is morselized into small cancellous and corticocancellous chips and onlaid over the bleeding bone.
Cancellous chips of bone are directly inserted into the facet joint.
Bone placed between the spinous processes has the additional benefit of being readily visualized on postoperative lateral radiographs obtained to radiographically assess the presence of fusion.
TECH FIG 1 • Decortication of the laminae and facet joints at the levels selected for fusion. Onlay iliac crest bone graft is placed over the decorticated areas.
INTERSPINOUS WIRING
Simple Interspinous Wiring
The spinous processes and laminae at the level to be instrumented should be confirmed to be intact and instrumentable on preoperative imaging studies.
Closed or operative reduction of spinal fracture-dislocations should be carried out before instrumentation if possible.
In some cases of flexion–distraction injury, sequential tightening of the wires can be done to reduce the spine.
Twoto 3-mm drill holes are made through the cortex at the junction of the spinous process and laminae bilaterally, at the levels to be included in the fusion.
Attention should be paid to the ventral location of the dural sac, and the drill should be directed coronally to minimize the risk of inadvertent spinal cord injury (TECH FIG 2A).
The drill holes should be positioned at the proximal aspect of the cephalad spinous process and the distal aspect of the caudal spinous process to provide the widest margin of safety against the wire cutting through the spinous process.
TECH FIG 2 • Simple interspinous wiring. A. Safe position of the drill hole for passage of spinous process wire is dorsal to the spinal laminar line. B. The wire or cable selected is passed through and around the base of the cranial and caudal spinous processes at the levels selected for fusion, so that both ends of the wire are on the same side of the spine. C. Ends of wire are twisted together after releasing any cervical traction.
The tips of a towel clip or a tenaculum clamp are placed in the cortical holes on either side of the base of the spinous process.
A gentle side-to-side rocking movement is used to create a continuous tract in the cancellous bone at the base of the spinous process.
The wire or cable selected for use is passed through and around the base of the cranial spinous process (TECH FIG 2B).
One end of this wire is similarly passed through and around the base of the caudal spinous process so that both the ends of the wire end up on the same side of the spine.
A plate of corticocancellous bone graft is harvested from the iliac crest and divided into halves.
The graft should be long enough to extend from the superior edge of the cephalad lamina to the inferior edge of the caudal lamina within the fusion levels.
The cancellous surface of this bone is placed on the decorticated laminae on either side of the spinous process.
The free ends of the wire are then tightened over the graft (TECH FIG 2C).
Additional cancellous bone graft is placed over the decorticated laminae and spinous processes and within the facet joint at the fusion levels.
Triple Wire Technique
The wire or cable selected is passed through and around the spinous processes at the cephalad and caudal ends of the fusion levels, as with routine interspinous wiring.
TECH FIG 3 • Triple wiring technique. After simple interspinous wiring, additional wires are passed through the cranial and caudal spinous processes at the levels selected for fusion. These wires are used to firmly hold corticocancellous plates of bone graft against the decorticated laminae at the fusion levels.
A pair of corticocancellous plates of bone graft including the full thickness of the cancellous bone of the iliac crest but excluding the inner cortical table is harvested from the posterior iliac crest.
The length of the bone block should be adequate to span the fusion construct and wide enough to cover the decorticated laminae within the fusion levels.
Twoto 3-mm drill holes are created in the proximal and distal portions of the harvested bone grafts.
Two additional 22-gauge wires are passed through the holes in the proximal and distal spinous processes.
These wires are then passed on either side through the holes made in the bone graft.
The wires are tightened over the grafts on both sides to hold the bone graft rigidly against the decorticated lamina and spinous process (TECH FIG 3).
Oblique Wiring
A periosteal elevator is carefully inserted into the facet joint to slightly distract and clearly identify the plane of the facet joint.
A 2-mm drill bit is used to make a channel in the sagittal plane through the midportion of the inferior articular process, exiting through the articular surface into the joint.
The periosteal elevator within the joint confirms penetration by the drill and prevents overpenetration by the drill (TECH FIG 4A).
A 20-gauge wire or cable is passed through this drill hole and is guided distally through and out of the facet joint using a periosteal elevator in a “shoehorn” fashion.
One end of the wire is then passed either around or through a hole in the intact spinous process of the vertebra one or two levels caudal to the level of injury.
This procedure is done bilaterally, and the free ends of the ipsilateral wires are twisted together to the appropriate tension (TECH FIG 4B). The absence of laminae or spinous processes is often a reason to consider oblique wiring over interspinous wiring.
TECH FIG 4 • Facet wiring techniques. A. A channel is drilled in the sagittal plane through the midportion of the inferior articular process, exiting through the articular surface into the joint. A periosteal elevator held within the joint space prevents overpenetration by the drill and can be used to guide the wire out through the joint space. B. Facet wires may be obliquely looped around the spinous process when the lamina is deficient at a level.
Supplemental midline interspinous or triple wiring is frequently added when the bony anatomy permits.
Multilevel Buttress Facet Wiring
Posterior stabilization after multilevel laminectomy can also be obtained by posterolateral facet fusion with multilevel facet wiring.8
Oblique facet wires are passed bilaterally through the inferior articular processes at all facet joints included in the fusion.
Two wires are passed through a hole in the spinous process of the most caudal vertebra.
Rib grafts, iliac crest strut grafts, or metal rods have all been used with the multilevel facet wires (TECH FIG 5).8,14
The graft or rod is placed over the decorticated lateral masses and in between the free ends of the wires, and the wires are twisted together at each level to the appropriate tension.
Postoperative Immobilization
Rigid external bracing is recommended in all posterior cervical wiring procedures until solid bony fusion is obtained. Six to 12 weeks of halo vest or rigid cervicothoracic bracing should be used after interspinous or oblique wiring, depending on the stability of the construct and the number of levels included in the fusion.
TECH FIG 5 • Facet wires may be tightened over rib grafts bilaterally in cases of multilevel laminectomy.
Radiographs should show a continuous fusion mass and absence of mobility in flexion and extension before immobilization is discontinued.
LATERAL MASS SCREW FIXATION
The quadrilateral posterior surface of the lateral mass is clearly exposed.
A ridge between the lamina and lateral mass identifies the medial border.
The lateral edge of the lateral mass can be easily palpated.
The joint lines above and below delineate the superior and inferior borders.
The center of the quadrilateral posterior surface of the lateral mass is identified.
Several techniques have been described for lateral mass screw insertion.
Roy-Camille et al30 proposed an entry point for the lateral mass screw at the center of the posterior surface of the lateral mass.
The screw is directed perpendicular to the posterior surface of the lateral mass, angled laterally 10 degrees to the sagittal plane.
This trajectory aims to exit lateral to the vertebral artery and inferior to the exiting nerve root (TECH FIG 6A–C).
Magerl et al25 proposed an entry point 1 mm medial to the center of the posterior surface of the lateral mass.
The screw is directed parallel to the plane of the facet joint with 25 degrees of lateral angulation in the axial plane.
Magerl et al recommended inserting a needle into the facet joint to determine the plane of the joint.
We use lateral-plane fluoroscopy to determine the direction of the screw in the sagittal plane, aiming to keep the screw parallel to and between the articular surfaces of the lateral mass.
This trajectory aims to exit lateral to the vertebral artery and superior to the exiting nerve root (TECH FIG 6D,E).
A modification of the Magerl et al technique by An et al5 uses a similar starting point but recommends angling the screw 30 degrees laterally in the axial plane and 15 degrees cranially in the sagittal plane.
This trajectory again aims to exit lateral to the vertebral artery and superior to the exiting nerve root at the junction of the transverse process and the lateral mass.
Lining up the screw heads for subsequent fixation to the rod is easier if the most proximal and distal screws are inserted initially, followed by the screws in between.
Most current instrumentation systems use a rod to connect the screws after they have been precisely positioned and inserted into the lateral mass.
Polyaxial screw heads compensate for minor variations in insertion or anatomy.
The rods can be contoured in multiple planes and allow for application of compressive, distractive, and rotatory forces for correction of deformity.
A rod–screw construct can easily be extended to the occipital and thoracic region.
TECH FIG 6 • Lateral mass screw insertion techniques. A–C. In the Roy-Camille method, the entry point is at the center of the posterior surface of the lateral mass, with the screw directed perpendicular to the posterior surface of the lateral mass and angled laterally 10 degrees to the sagittal plane. D,E. In the Magerl technique, the entry point is 1 mm medial to the center of the posterior surface of the lateral mass, and the screw is directed parallel to the plane of the facet joint and angled laterally 25 degrees to the sagittal plane.
Bicortical screws should be considered in certain cases:
Patients with rheumatoid arthritis or metastatic bone tumors in whom bone quality may be suboptimal.
Longer fixation constructs extending to the occipital or thoracic regions, to reduce the chances of implant pullout.
PEDICLE SCREW FIXATION OF THE CERVICAL SPINE
Insertion of Pedicle Screws from C3 to C7
Preoperative radiographs and CT images should be reviewed to assess pedicle dimensions and orientation and to confirm the feasibility of obtaining intraoperative radiographs; this is especially important in patients with short, stocky necks.
Inserting pedicle screws before decompression allows better identification of morphologic landmarks and reduces the risk of inadvertent injury to an exposed spinal cord during the insertion process.
The most commonly used technique relies on identification of topographic landmarks combined with fluoroscopy.1
The entry point to the pedicle is 1 to 2 mm inferior to the caudal edge of the inferior articular process and 2 to 3 mm lateral to the midline or 2 to 3 mm medial to the lateral edge of the lateral mass.
Occasionally, degenerative changes at the joint may obscure true landmarks.
The dorsal cortex of the lateral mass is penetrated using a high-speed burr.
The cancellous bone of the pedicle in many cases can be visualized in this pilot hole.
A blunt, fine pedicle probe is advanced through this cancellous bone to find the medially angled pedicle (TECH FIG 7A).
Fluoroscopy is used to guide the trajectory in the sagittal plane.
In general, the screws should be parallel to the superior endplate of the vertebral body from C5 to C7 and angled slightly rostral to the endplate from C2 to C4.
Some authors recommend that a keyhole laminoforaminotomy be performed after locating the entry point.4
The superior and medial walls of the pedicle are directly palpated through this foraminotomy with a right-angled nerve hook to direct the trajectory of the pedicle probe (TECH FIG 7B).
The pilot hole is tapped before inserting the screw.
Size 3.5 or 4-mm screws are generally used, based on preoperative imaging of pedicle dimensions.
Small pedicle diameters may require a 2.7-mm screw.19
The length of the screw ranges from 18 to 26 mm, depending on the length of the pedicle as determined on preoperative CT scans.
The screw should be inserted to a depth no longer than two thirds of the anteroposterior width of the vertebral body, as confirmed on the lateral fluoroscopy image.
Since the C7 pedicle is longer, a screw up to 30 mm can usually be inserted at this level.
Computer-assisted image guidance systems have been used for pedicle screw insertion in the cervical spine.
Preoperative CT data are used by the computerassisted system to prepare a three-dimensional model of the vertebra.
After registration of surface landmarks during surgery, a registered probe or drill bit can be used to locate the entry point and guide a fine drill bit through the pedicle into the vertebral body.
C2 Pedicle Screw Insertion
The entry point for the C2 pedicle is located on the superior medial quadrant of the posterior aspect of the lateral mass of C2, 3 mm lateral to the medial edge of the isthmus, and in line with or slightly distal to the superior margin of the C2 lamina.
The cortex is penetrated with a 3-mm burr.
The underlying cancellous bone is probed with a fine curette or pedicle probe to locate the pedicle channel.
The entry point and trajectory for subsequent drilling are confirmed by palpating the medial and superior margins of the C2 pedicle with a Penfield probe, and with fluoroscopy to determine sagittal angulation.
The drill is generally angled 15 to 25 degrees medially and 20 to 30 degrees cranially.
The integrity of the drilled hole is verified with a blunt probe and tapped, and a 3.5to 4.0-mm screw is inserted.
Twenty to 22-mm screw lengths are generally used. C2 pedicle screws longer than 24 mm are likely to penetrate the anterior surface of the vertebral body and may provide superior fixation in some situations.28
Using a polyaxial screw head allows easier compensation for the difference in medial angulation between the C2 and other subaxial pedicles when connecting to a rod.
TECH FIG 7 • A. Comparative trajectories of cervical pedicle screw and lateral mass screw in the axial plane. B. Palpation of the superior and medial pedicle walls through the laminoforaminotomy window helps determine the trajectory of the pedicle probe.
OUTCOMES
Posterior Wiring Techniques
Long-term successful fusion rates of 94% to 96% have been reported with interspinous wiring techniques when used for trauma, degenerative conditions, and tumors of the cervical spine.22,29
Weiland and McAfee33 reported a fusion rate of 100% when a triple-wire technique was used for subaxial posterior cervical fusion in 60 patients.
Two of the 60 patients required halo vest immobilization, while the rest fused with a two-poster orthosis.
Cahill et al7 reported stable fusion and acceptable alignment in all 18 patients with facet dislocations treated using bilateral oblique wiring.
Fusion generally occurred within 3 to 4 months. No patients developed neurologic deterioration after wiring.
Callahan et al8 reported solid fusion in 50 of 52 cases with multilevel facet fusion done after, using iliac crests or rib graft for fixation along with facet wires.
Two patients who failed to achieve solid fusion were followed up with regular assessments and did not require any further management.
Fusion rates with interspinous wiring have been found to be comparable to those obtained from lateral mass plating.23
Lateral Mass Screw Fixation
Ebraheim et al11 retrospectively reviewed the radiographic and clinical outcomes in 36 patients treated with lateral mass plate–screw fixation for traumatic instability, post-laminectomy instability, or metastatic disease. Fusion occurred at an average of 3 months in all patients. One patient demonstrated postoperative neurologic deterioration, but this resolved with subsequent decompression.
Fehlings et al13 reported successful arthrodesis in 39 (93%) of 42 patients treated with lateral mass plate–screw fixation for cervical instability at a mean follow-up of 46 months. Revision of posterior plating was required in two patients for a screw pullout. Another patient required supplementary anterior plating for progressive postoperative kyphosis.
Cervical Pedicle Screws
Screw loosening or pullout has not been an issue with cervical pedicle screw use.
Abumi et al3 used pedicle screw–rod fixation after correction of cervical kyphosis in 30 patients and reported excellent correction and no adverse mechanical or neurovascular sequelae related to the pedicle screws.
COMPLICATIONS
Wiring
The most common complication reported with interspinous wiring is loss of reduction and recurrence of the deformity.
Loss of reduction is more common when posterior wiring is done across a level with fractured posterior elements by bypassing that level.22
Osteoporosis or excessive tensioning of the wires may result in intraoperative or postoperative fracture of a spinous process.
Wire breakage can occur with use of a single-strand wire.
Use of multistrand cable reduces the risk of wire breakage.18,34
Inadvertent passage of spinous process wire through the spinal canal can lead to spinal cord injury.
Appropriate placement of drill holes at the spinolaminar line and avoiding a ventrally placed tract between the holes on either side should avoid this complication.
Lateral Mass Screw Fixation
In a cadaveric comparison of different screw placement techniques, Xu et al35 found that violation of either the dorsal or ventral nerve root was least likely using the modification of the Magerl technique described by An et al.5
Clinical studies with lateral mass screw insertion have reported a 6% incidence of nerve root injury17 and a 6% incidence of screw malposition.15
Three percent of the patients required screw removal for radiculopathy.15
Screw loosening is reported to occur with a incidence ranging from 2% to 6%.13,15,17
In addition to direct contact of the nerve root by the screw, radiculopathy can also occur from foraminal stenosis as the lateral mass gets pulled up to the rod during final tightening of the construct.
Precise screw length and placement and appropriate contouring of the rod should minimize the incidence of this problem.
Vertebral artery injuries have not been reported after lateral mass plating.
Cervical Pedicle Screws
The medial pedicle wall is the thickest, making medial perforation and spinal cord injury less likely.
The lateral pedicular wall is thin, increasing the risk of lateral perforation during pedicular screw insertion.
There is little to no space between the superior border of the pedicle and the superior nerve root, while there is a mean gap of 1.4 to 1.6 mm between the inferior border of the pedicle and the inferior nerve root.36
Thus, cortical perforations of the pedicle walls by the pedicular screws are more likely to damage the vertebral artery or superior nerve roots.
Abumi et al2 reported a 6.7% (45/669 pedicle screws) incidence of cortical perforation by the screw in 180 consecutive patients.
Three of the 180 patients developed screw-associated neurovascular complications, with two patients developing radiculopathy that resolved with nonoperative management.
One patient developed vertebral artery injury without neurologic sequelae.
Kast et al20 reported lateral cortical perforation with more than 25% narrowing of the vertebral artery foramen in 4 of 94 pedicular screws implanted in 26 patients.
No vascular or neurologic sequelae occurred with these breaches.
Three screws encroached on the intervertebral foramen; one of these screws was revised for a sensory radiculopathy.
Kotani et al21 reported reduced pedicle perforation when an image-guided system was used, while other authors24 have not shown significant improvement in safety or accuracy with these systems.
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