Stabilization of the Subaxial Cervical Spine

The cervical spine is the most mobile portion of the spinal column, and its stabilization has unique features. Cervical spine stabilization may be performed using anterior, posterior, or combined techniques.

Fusions by anterior approaches have been widely used in cervical spine injuries, allowing anterior decompression of the spinal column. Anterior fusion techniques were first introduced in 1955 by Smith and Robinson and then popularized by Cloward.

However, if there is a posterior column injury in addition to disruption of the anterior column, an anterior stand-alone bone graft will not be sufficient for fixation. This is due to the possibility of graft extrusion, resulting in a kyphotic deformity and significant risk of neural injury. To avoid dislocation and graft extrusion in cases where the posterior column is damaged, options include supplemental posterior fixation, rigid external orthosis with a halo vest, or the use of anterior plating.

The first application of a metal plate as a supplement to an anterior bone graft in cases of cervical dislocation was performed in 1975 by Herman. In 1980, Böhler also used small plates as proposed by Orozco and Llovet. In 1980, Caspar subsequently popularized the use of anterior cervical plates, resulting in more widespread use of Caspar plating ^, in the mid-1980s in both Europe and the United States. Stabilization of the cervical spine requires a clear understanding of the biomechanical benefits and limitations of cervical fixation, its indications, and associated complications. Ideal cervical instrumentation must provide an immediate stability to the motion segment, allow high rates of fusion, correct deformity in any plane, and be low profile and easy to apply. The risk of hardware failure must also be minimal, and instrumented constructs should ideally be radiolucent and not ferromagnetic, so as to cause minimal artifact on magnetic resonance imaging.

This chapter summarizes anterior and posterior stabilization techniques of the subaxial cervical spine. Table 162.1 summarizes the advantages and disadvantages of a variety of anterior and posterior cervical fixation techniques (see Table 162.1 ).

TABLE 162.1

Summary of Advantages and Disadvantages of Anterior and Posterior Subaxial Cervical Fixation Techniques

	Advantages	Disadvantages
Anterior cervical plate	Augment stabilization and solid fusion. Resist kyphosis. Reduce the need for external bracing No graft extrusion. Less nonunion	Increase the cost of surgery. Require special instruments Screw loosening or fracture, infection, and neural injury. Rate of complication is as high as 23% in some series. Dysphagia
Interbody cage	Prevent subsidence of the disc height	Increase the cost of surgery
Vertebral body cages	Replace the body Less subsidence than bone graft	Increase the cost of surgery Metal artifacts in magnetic resonance imaging
Spinous process wiring (Rodgers)	No neurologic complication	Need for intact posterior elements Weaker construct especially in rotation
Sublaminar wiring	Easy application	May cause neurologic deficit Need for intact posterior elements
Bohlman triple wiring	Easy application No neurologic complication	Need for intact posterior elements
Laminar hooks	Efficient in osteoporosis	Difficulty with application, higher rate of hook dislocation
Lateral mass screw fixation	More rotational stability No need to preserve the posterior elements Lower risk of neural injury More fusion area and little bone graft	Semirigid fixation Screw loosening
Cervical pedicle screw fixation	More rigid fixation Ability to reduce subluxation and kyphosis	Technically demanding Injury to nerve root, dura, and vertebral artery
Facet distraction cages	Technically easy	Efficient decompression questioned

Anterior Stabilization Techniques

Anterior cervical plates have significantly changed since their early application in cervical trauma. They are currently commonplace in anterior cervical decompression and fusion (ACDF) procedures, especially in cases requiring decompression of two or more levels. Routine use for the treatment of cervical spondylosis has caused plate design to change significantly in recent years. The first anterior cervical plates were unlocked and required bicortical purchase. Anterior cervical plates with constrained designs and locking plate–screw head connections then came into favor. The latest plates are semiconstrained dynamic plates that allow some movement in rotation and translation ( Fig. 162.1 ).

FIGURE 162.1, Evolution of plates. First-generation Caspar plate (A) and H plate (B) had nonconstrained screw–plate connections. Second-generation plates or constrained plates have locking mechanisms to restrict screw backout (C and D). Third-generation plates, or semiconstrained plates, allow graft subsidence by movements of the plate-screw connections (E).

Biomechanics of Cervical Plates

Anterior cervical plates are supposed to achieve the following goals: they must hold the interbody graft in place and provide immediate rigidity, optimize the fusion environment and increase the fusion rate, and improve clinical outcomes.

The plate-graft relationship is another important factor to consider during anterior cervical surgery. A satisfactory amount of graft loading is necessary to achieve bony fusion. A very rigid plate can cause the bone graft to resorb, or it can result in pseudarthrosis owing to inadequate graft loading. In the case of weaker constructs, anterior column height can decrease owing to graft subsidence into the adjacent end plates.

Plates bear and share loads and behave like a ventral tension band mechanically, thereby building a barrier that limits vertical and horizontal translation of the spine. This is particularly the case in extension. However, in the case of three-column injuries, anterior cervical plates provide little stability in flexion and rotation, so either external fixation in a halo jacket or combined posterior supplemental fixation is required to achieve adequate spinal stability.

Although plate-screw constructs increase the rigidity of the injured spinal segment, they cannot restore the strength of a normal healthy spine. In other words, an uninjured spine is stronger than an injured and internally fixated one. Therefore surgeons should not rely completely on the strength of internal fixation alone. In the case of excessive loading, instrumentation can fail through fracture or screw pullout. For these reasons, consideration should be given to external bracing or additional supplemental fixation to provide additional load sharing with the anterior construct.

Screw choice and insertion technique also affect the biomechanical properties of anterior plating. For instance, hollow screws with small holes on the shaft were developed to allow improved osteointegration at the screw-bone interface. They were removed from the market because of high screw fracture rates and increased difficulty of removal.

Medial or lateral angulation of anterior screws during insertion results in a triangulation in the axial plane, whereas cranial and caudal angulation results in sagittal plane triangulation. Varying the angle of trajectory provides improved construct strength and lessens the risk that the screw will back out ( Fig. 162.2 ).

FIGURE 162.2, Plate construction after corpectomy. Screw purchase may be bicortical (A) or unicortical (B). A shorter screw to the graft may be used (C). However, use of a shorter screw is criticized because of weakening of the graft. Cranial and caudal angulations are recommended to increase the strength.

Types of Plates

Nonconstrained Plates

First-generation plates are nonconstrained plates. These plates provide a weak interaction between the plate and the screw heads. Types include Caspar plates and H plates. Screw backout is unrestricted in these models.

Static Plates

Second-generation plates are constrained plates (static plates). Constrained plates provide strong fixation between the plate surface and the screw heads. Examples include Synthes cervical spine locking plates (CSLPs), Orion plates, and Atlantis plates. These plates use a fixed moment arm cantilever beam design. Screw backout is restricted in these models. ^, The CSLP is an example of a second-generation anterior cervical plate and was first introduced by Morscher with fixed-angle screws. Small set screws are placed into the main screw heads, widening the screw head and locking the head to the plate. The CSLP variable-angle plate is a modification that allows up to 20 degrees of variability in the plate-screw angle. Other anterior cervical locking plates in this category use a special screw head design that expands when it incorporates into the plate.

Dynamic Plates

Third-generation plates are semiconstrained (dynamic) plates. These plates have designs that allow a variable amount of graft subsidence. Subsidence is observed during aging and after spine surgery and is accepted as a naturally occurring process. Although anterior cervical plates help to stabilize the spine, they also constrict subsidence. For that reason, an anterior plate that carries most of the axial load instead of sharing it with the bone graft has a high rate of failure. Dynamic plates were developed to avoid the late complications of rigid plates.

Screw loosening and screw and plate fracture are more common in cases of multilevel fusion with either a corpectomy or ACDF grafts. The main reason is graft absorption resulting in subsidence. Although it is a gradual process, if the loss of graft height cannot be accommodated by the plate-screw angle, the screw has increased risk of fracture.

Bone density is another important factor that must be considered during anterior cervical plating. If the bone is too dense, the screw will fracture instead of rotating within its hole. Alternatively, if bone density is low, screw pullout is another failure mode of anterior cervical constructs.

Understanding these failures and the mechanisms responsible for them has led to the development of dynamic plating systems. During initial designs, spine surgeons had failed to consider the biology of bone healing and its relationship to anterior cervical plating. When a problem arose, such as settling, screw breakage, or plate fracture, they responded with stronger plate designs and thereby set the stage for additional failure modes, as well as delayed union and nonunion. Through a better understanding of the biology of bone healing, plates now exist that allow stronger and quicker fusion with lower failure rates while still achieving the additional goals of restoration or preservation of lordosis and protection of neural elements.

Such dynamic plates now restrict screw backout while also allowing some variability in translational and rotational movements. There are two main two kinds of dynamic anterior cervical plates manufactured by the spinal device companies, rotational and translational. In the semiconstrained rotational design, variable-angle screw systems allow the screws to toggle inside the bone. This rotational movement can also lead to instrumentation failure ( Fig. 162.3 ). Examples include anterior cervical plates from Codman, Blackstone, Acufix, Zephir, and Atlantis (hybrid and variable). The semiconstrained translational design allows translational motion that is provided by the plate-screw interface. Examples include the ABC plate, DOC system, and Premier plate.

FIGURE 162.3, Static plates aim no movement (A). However, dynamic plates have motion provided by rotational movements of the screws (B) or translational movements at the plate-screw interface (C).

Dynamic implants allow natural subsidence to occur ( Fig. 162.4 ) while effectively stabilizing the spine by preventing excessive movements in translation and rotation. Load sharing helps to improve normal bone healing, resulting in earlier fusion. Decreased rates of construct failure have been reported with dynamic implants. Adding a titanium mesh cage to dynamic plates did not prevent some degree of subsidence.

FIGURE 162.4, Behavior of the translational dynamic plate before (A) and after (B) subsidence of a corpectomy graft.

Advantages and Disadvantages of Anterior Cervical Plates

Advantages

Anterior cervical plates have the following advantages:

They augment stabilization, enhancing the likelihood of a solid fusion.
They resist kyphosis.
They reduce the need for external bracing or halo vest placement, and they allow mobilization of the adjacent spinal segments.
They reduce the risk of graft extrusion.
They significantly reduce the rate of nonunion. Nonunion rates range from 11% to 63% in multilevel interbody fusion cases and from 25% to 45% in corpectomy and strut graft applications.

Based on these advantages, anterior plating has become the standard treatment for one-level cervical discectomy procedures to avoid complications of graft collapse and loss of lordosis.

If a cervical kyphosis is not severe, an anterior plate can also be used to reduce it. This is achieved by spreading the disc space with a vertebral body spreader, followed by lordotic graft and plate placement ( Fig. 162.5 ).

FIGURE 162.5, Kyphosis reduction and plate fixation. Vertebral body spreader inserted (A) and distracted (B) for reduction.

Disadvantages

Anterior cervical plates have the following disadvantages:

They increase the cost of surgery.
They require special instruments and training for application.
Plate-specific complications can occur, such as screw loosening or fracture, infection, and neural injury. It has been reported that the rate of complication is as high as 23% in some series.

Indications

Anterior cervical plates have been widely used in cases of trauma and after anterior corpectomy for cervical spondylotic myelopathy. ^, ^, Anterior cervical plate placement is indicated in the following conditions:

Cervical spine trauma with anterior column injury
Cases of cervical spondylotic myelopathy requiring anterior decompression via ACDF
After cervical corpectomy
In anterior surgery in patients who have been previously treated with a cervical laminectomy
Following decompression and stabilization of cervical spine tumors involving the anterior column
In postlaminectomy kyphosis following anterior decompression

In his first series, Caspar used plates only in cases of cervical trauma. This has given way to widespread use in cervical tumors and following decompressive surgery for cervical disc disease. In the case of plating following cervical corpectomy, vertebral body reconstruction can be performed using bone autograft or allograft, polymethyl methacrylate, or nonexpandable or expandable cages.

Indication for anterior plating in degenerative problems is decision of an instability. However, the diagnosis of cervical instability requires a subjective evaluation. White and Panjabi have developed a scoring system to easily determine spinal instability.

Contraindications for anterior cervical plating are few and include severe osteoporosis and osteomyelitis or discitis.

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