Posterior Cervical Fusion with Lateral Mass Screws
DEFINITION
The lateral mass is a quadrangular area of bone lateral to the lamina and between borders of superior and inferior facets as seen from posteriorly. Seen in three dimensions, the lateral mass is a skewed box. Its appearance on lateral radiographic projection is that of a parallelogram whose anterosuperior corner points cephalad (FIG 1).
Roy-Camille first described the use of lateral mass screw fixation for cervical spine stabilization. Posterior cervical fusion with rigid constructs provides better deformity correction and stability for arthrodesis.
Cervical instability and/or deformity are indications for posterior cervical arthrodesis in the pediatric population.
ANATOMY
Each segment from C3 to C7 is made up of a centrum (body) and two posterior arches that form from mesenchymal tissue migrating around each side of the neural tub.
The arches fuse posteriorly by age 2 or 3 years, and they fuse to the body between 3 and 6 years.
Secondary ossification centers for the superior and inferior ring apophyses ossify during late childhood and fuse to the vertebral bodies by 25 years of age.
Other ossification centers for the transverse and spinous processes generally fuse by 3 years of age.
The facet joints initially are relatively horizontal and, during growth, gradually become more vertical, which enhances stability in flexion and extension. The vertebral bodies initially have an oval or wedge shape but
gradually become fully ossified in a more rectangular configuration.4 The spinal nerve exits the canal through the interpedicular foramen. The ventral ramus travels anterolaterally on the transverse process. The spinal nerve sits anteromedial to the anterior aspect of the superior facet. The dorsal ramus runs posteriorly against the anterolateral corner of the base of the superior articular process. When the lateral mass is divided into quadrants, the superolateral quadrant is away from the spinal nerve.
 |
FIG 1 • Sagittally reformatted CT scan through the lateral masses of a 12-year-old boy with short stature bone dysplasia and paraparesis due to cervical stenosis. Note the templating lines for preoperative planning (see text).
The vertebral artery enters the transverse foramen of the sixth cervical vertebra and courses cephalad through the foramina. The vertebral artery is anterior to the lateral mass but is also anterior to the spinal nerve (FIG 2).
A study by Al-Shamy et al1 of computed tomography (CT) scans of the pediatric cervical spine found that in children age 4 years and older, the size of the lateral mass would allow for lateral mass screw fixation from C3 to C7.
PATHOGENESIS
Trauma
Although spine injuries are rare in children, cervical spine injuries are the majority of spine injuries in children.
Children younger than 10 years of age are more likely to sustain upper cervical injuries (occiput to C2), whereas older children have a preponderance of lower cervical injuries (maximally at C4 or C5).
Tumor: Aneurysmal bone cyst, osteoblastoma, and osteochondroma involving posterior elements are the tumors most likely to be encountered in the first and second decades of life.
Iatrogenic: posterior instability after tumor decompression or postlaminectomy
 |
 |
FIG 2 • Preoperative CT angiogram of an 11-year-old girl with Down syndrome and paraparesis due to C1-C2 instability. Note the relationship of the vertebral artery to the lateral masses.
P.716
 |
FIG 3 • Lateral radiograph of an 11-year-old boy with neurofibromatosis. Despite the extent of the kyphosis, he was neurologically intact except for lower extremity hyperreflexia.
Congenital/syndromic: Anomalous cervical spine anatomy can be seen with VACTERL (vertebral, anal atresia,
cardiac, tracheoesophageal fistula, renal anomalies, and limb defects) and Klippel-Feil syndrome. Cervical kyphosis
This can be posttraumatic, postlaminectomy (tumor decompression), or syndromic.
Congenital or developmental cervical kyphosis is associated with Larsen syndrome, diastrophic dysplasia, chondrodysplasia punctata (Conradi syndrome), camptomelic dysplasia, and neurofibromatosis (FIG 3).
NATURAL HISTORY
Instability of the subaxial spine can present with or develop myelopathy.
There is risk of progressive neurologic symptoms and/or progressive deformity of the cervical spine. Paralysis or death are rare but may occur.
PATIENT HISTORY AND PHYSICAL FINDINGS
A thorough history for traumatic injury, conditions that are associated with cervical instability, or history of posterior cervical surgery
Patients may present with complaints of neck pain or decreased range of motion. Patients may have neurologic symptoms on presentation, which may be radicular in nature from cervical nerve root compression or myelopathic from spinal cord compression.
Examination should include a thorough neurologic evaluation including motor and sensory examinations and assessment for myelopathy (hyperreflexia, clonus, pathologic reflexes).
Assess for possible associated abnormalities including a cardiac examination (for congenital/syndromic etiologies).
IMAGING AND OTHER DIAGNOSTIC STUDIES
Standard radiographs including anteroposterior (AP) and lateral views in neutral, flexion, and extension. An open mouth view should also be taken to ensure that concomitant O/C1/C2 instability is not present.
Lateral views allow for assessment of true instability from pseudosubluxation, which occurs most commonly between the second and third cervical vertebrae and between the third and fourth cervical vertebrae.
The spinolaminar line of Swischuk is helpful to differentiate between pseudosubluxation and true subluxation. This line is drawn along the posterior arch from the first cervical vertebra to the third. It should pass within 1.5 mm of the anterior cortex of the posterior arch of the second cervical vertebra during forward flexion. As long as Swischuk line is maintained, as much as 4 mm of vertebral body subluxation can be accepted.
Fine-cut CT scan of the cervical spine to assess for anomalies of the cervical anatomy including location of the vertebral artery and for preoperative planning of screw lengths and trajectories
Magnetic resonance imaging (MRI) to evaluate for spinal cord or nerve root involvement
DIFFERENTIAL DIAGNOSIS
Posttraumatic instability
Infection/tumors involving the posterior elements
Postlaminectomy instability/kyphosis (post-tumoral decompression) Congenital/syndromic cervical abnormalities
Cervical kyphosis (Larsen syndrome, diastrophic dysplasia, chondrodysplasia punctata [Conradi syndrome], camptomelic dysplasia, and neurofibromatosis)
Cervical abnormalities of segmentation/formation (VACTERL, Klippel-Feil syndrome)
NONOPERATIVE MANAGEMENT
Stable cervical spine deformity with no neurologic involvement can be treated with observation and serial imaging.
Unstable cervical spine, neurologic involvement, or progressive deformity would be indications for surgical management.
SURGICAL MANAGEMENT
Many techniques of posterior cervical fusion including wiring, plate and screw, and rod and screw constructs have been described.
Lateral mass screw fixation provides benefits compared to nonrigid fixation including fixation of patients with posterior laminar element deficiency, providing immediate stability, and allowing for greater deformity correction.
Preoperative Planning
Fine-cut CT evaluation to preoperatively plan screw lengths and trajectories, note any anomalous anatomy and assess location of the vertebral artery.
CT angiography can be performed at the same time as assessment of the bony anatomy. Flexion and extension CT can also be helpful.
The measured angle of 35.9 degrees seen in TECH FIG 1A is the most extreme lateral angulation that can be achieved with a more medial starting point in this particular patient. This maximally protects the vertebral artery but impinges against the spinous process in drill placement. In practice, a compromise between vertebral artery safety and easy access of drilling is ideal.
P.717
MRI evaluation for any spinal cord or nerve root compression
Neuromonitoring, including somatosensory evoked potentials and transcortical motor evoked potentials of upper and lower extremities
Fiberoptic intubation minimizing cervical motion.
Positioning
While supine, placement of halo or Mayfield tongs followed by careful turning of patient into a prone position with surgeon stabilizing the cervical spine (FIG 4)
Fixation of Mayfield device to operating room (OR) table with careful attention to cervical spine flexion-extension
Padding of all bony prominences, positioning of upper extremities to avoid nerve stretch or compression, ensuring patient is secure on table
Take AP and lateral fluoroscopy images to ensure all anatomy can be seen and assess cervical alignment. Prepare autograft donor site.
Ensure that good baseline neuromonitoring is present once positioned.
 |
FIG 4 • A halo vest has been placed in this patient with Down syndrome and paraparesis to maximize spinal control during the turn to prone position. The posterior half of the vest is removed for surgical access.
Approach
A standard posterior cervical approach is used.
A midline approach posteriorly, dissecting midline through the muscle down to the spinous process Subperiosteal dissection to expose the posterior vertebra to the lateral border of the lateral mass
TECHNIQUES
-
Lateral Mass Screw Placement
Starting Point Identification
Define the medial and lateral borders of the lateral mass as well as superior and inferior borders as defined by the facets. Outline the lateral mass as a rectangular box.
Divide the rectangular box of the lateral mass into quadrants and identify the center point. Entry point2, 5, 6
Roy-Camille technique: Entry point is at the center point of the four quadrants. Magerl technique: Entry point is 1 mm superior and 1 mm medial to the center point. Anderson technique: Entry point is 1 mm medial to center point.
Our technique: In smaller children, often the simplest starting point is in the center of the lateral mass (Roy-Camille). The smallest screws (usually 3.5 mm) fill much of the lateral mass, and maximal containment is the most desirable technical goal.
Lateral Mass Screw Hole Creation
Make a small burr hole at your entry point. Confirm this starting point on AP fluoroscopy if there is much congenital deformation or bone dysplasia.
Use a hand-powered drill with a stop mechanism at desired depth to create screw hole. Start with a conservative depth stop and deepen the hole after probing for bone integrity.
Roy-Camille technique: Direct the drill 10 degrees laterally and perpendicular to the posterior cortex of the lateral mass.
Magerl technique: Direct the drill 30 degrees laterally and parallel to the facet joint.
Anderson technique: Direct the drill 10 degrees laterally and parallel to the facet joint.
Our technique: Direct the drill in the sagittal plane parallel to the facets on lateral fluoroscopy projection and angled away from the midline at an angle approaching the preoperative CT scan templating lines (TECH FIG 1A). This usually results in a larger lateral angulation (more than 10 degrees) (TECH FIG 1B).
 |
 |
TECH FIG 1 • A,B. Same patient as FIG 1. A. Axial CT scan. The depth and angle of screw insertion are derived from these two CT formats and the live fluoroscopic lateral projection intraoperatively. B. There has been an occipital plate placed and translaminar screws with lateral connectors at C2. (continued)
P.718
 |
 |
TECH FIG 1 • (continued) C,D. Same patient as in FIG 1. C. This is the lateral fluoroscopic projection of the left C3 lateral mass being drilled. D. The probe has bottomed out in the anterior wall of the drilled hole. Depth measurement followed by screw placement ensues.
The head is to the right. Note the measured depth stop sleeve and drill guide. The drill is angled laterally
greater than 10 degrees, approaching the preoperative templated angle of 36 degrees. A more medial starting point necessitates greater lateral angulation, whereas a more lateral starting point allows a more “straight back” approach but puts the vertebral artery at greater risk (TECH FIG 1C).
Check trajectory with fluoroscopy in the lateral view to confirm your position (see TECH FIG 1).
Once you have drilled, probe the screw hole to ensure there are intact walls on all sides (TECH FIG 1D).
Screw Placement
Tapping the dorsal cortex can be helpful in hard bone, followed by placement of a 3.5-mm polyaxial screw.
Usual screw lengths are between 10 and 14 mm. Given the trajectory of the screw hole, the screw head will not be flush to the bone but rather angled. The polyaxial nature of the head allows rod placement despite screw head angulation.
 |
TECH FIG 2 • The left rod has been bent and placed in two lateral mass screws at C3 to C4, a translaminar screw at C2 and attached to a lateral connector at C2 occipital plate. The right rod is being manipulated into position. Final tightening follows.
Check screw position with fluoroscopy in the lateral view.
Rod Placement
Measure and cut rods to appropriate length.
Precontour rods with rod benders, then place rods and set screws (TECH FIG 2). Perform any compression or distraction then final tightening.
Fusion Bed Preparation
Decorticate in area of fusion with a small burr.
Obtain posterior iliac crest autograft and place along fusion. Perform standard layered closure.
|
PEARLS AND PITFALLS |
|||||
|
|
|
|
|||
|
|
Positioning |
|
|
||
|
|
fluoroscopy.
Approach ▪ Avoid lateral dissection which will increase bleeding.
Screw ▪ Studies have supported both unicortical and bicortical screw purchase. Although length bicortical has a biomechanical advantage, screw lengths must be carefully assessed to avoid too long screws that may place neurovascular structures at risk. Oblique views can help assess screw length (FIG 5).
Rod ▪ Perform careful rod precontouring to avoid screw loosening or pullout during rod placement placement. |
|
|
|
||
P.719
 |
 |
FIG 5 • A 15-year-old female suffered a flexion distraction injury at C4-C5. Lateral mass instrumention and fusion
was performed achieving bicortical purchase over just one level. Note the tips of the screws safely away from neurovascular structures.
POSTOPERATIVE CARE
Postoperative CT scan is obtained to assess screw positioning (FIG 6). Immobilization in halo vest or rigid cervical orthosis until bony union is achieved.
Long-term follow-up with radiographs to assess for any development of junctional sagittal imbalance
OUTCOMES
Radiographic and cadaveric studies of the different techniques of screw placement have shown that the Magerl technique allows for longer screw lengths and has a larger margin of safety than the Roy-Camille technique. Studies have shown that a modified Anderson technique with 20 to 30 degrees of lateral angulation is safer with regard to avoidance of the nerve root and artery. Our technique approximates to this. The standard Roy-Camille technique did pose risk to neurovascular structures at C4-C7 levels, whereas Magerl posed a risk to the spinal nerve at C7.
 |
 |
FIG 6 • Same patients as TECH FIGS 1 and 2. A. Postoperative axial CT scan. Two lateral mass screws have been placed at C3 with a significant lateral angulation. They are unicortical. The vertebral artery is safe. B. Sagittal reformatted CT scan. Note the cephalad trajectory of the lateral mass screws at C3 and C4. The polyaxial attributes of the screws allow for accommodation of the contoured rod without pullout stresses. Note again the occipital plate and C2 laminar screws.
Hedequist et al3 reported on 36 patients that had undergone posterior cervical fusion with lateral mass screw fixation. On postoperative CT scans, it was noted that 100% of the screws were contained in the lateral mass. (Our technique occasionally results in bicortical purchase but with anterolateral penetration safely away from the nerve root and vertebral artery; see FIG 5.)
Complications included one infection, one seroma, and one pseudarthrosis. There were no neurologic complications and no surgery revisions.
Studies reporting results from lateral mass cervical fixation in adults have shown 85% to 100% fusion rates.
P.720
COMPLICATIONS
Neurologic injury to spinal cord or spinal nerves Injury to vertebral artery
Screw penetration of the facet joint Infection
Implant failure (screw loosening or pullout) Pseudarthrosis
Junctional sagittal imbalance (kyphosis/lordosis)
REFERENCES
-
Al-Shamy G, Cherian J, Mata JA, et al. Computed tomography morphometric analysis for lateral mass screw placement in the pediatric subaxial cervical spine. J Neurosurg Spine 2012;(17):390-396.
-
Ebraheim N. Posterior lateral mass screw fixation: anatomic and radiographic considerations. Univ Penn Ortho J 1999;12:66-72.
-
Hedequist D, Proctor M, Hresko T. Lateral mass screw fixation in children. J Child Orthop 2010;4(3):197-201.
-
Lustrin ES, Karakas SP, Ortiz AO, et al. Pediatric cervical spine: normal anatomy, variants, and trauma. Radiographics 2003;23(3): 539-560.
-
Merola AA, Castro BA, Alongi PR, et al. Anatomic consideration for standard and modified techniques of cervical lateral mass screw placement. Spine 2002;2(6):430-435.
-
Stemper BD, Marawar SV, Yoganandan N, et al. Quantitative anatomy of subaxial cervical lateral mass: an analysis of safe screw lengths for Roy-Camille and magerl techniques. Spine 2008;33(8): 893-897.
