Summary of Key Points
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Anterior subaxial cervical fixation is still relatively new.
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Current fixation techniques involve the use of titanium plates and screws.
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Polyetheretherketone and carbon fiber interbody devices can be used in lieu of structural autograft of allograft in ventral spine reconstruction.
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Multiple plate options are available, allowing for constrained or semiconstrained constructs.
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Prior to implant placement, appropriate preparation of the spine including proper decompression and gardening of the spine must be completed to allow for a better fit of the implants.
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Ventral fixation is used as an adjunct to promote fusion.
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Proper and adequate review of preoperative imaging allows for anticipation of the implants necessary for ventral cervical fixation.
History
Historically, cervical spine surgery was mostly done posteriorly. In the 1950s, anterior cervical surgery was pioneered and popularized by Smith, Robinson, Cloward, Bailey, and Badgley. Cloward used a cylindrical graft for an anterior fusion after a discectomy, whereas Smith and Robinson used a tricortical bone graft. At this time, anterior surgeries were being performed without the plating. However, in the 1970s Orozco and Senegas released the first reports on anterior cervical plating. The addition of a ventral cervical plate prevents the anterior migration of the bone graft used after a discectomy. The plates also decrease the compression and shear forces that the spine is exposed to. Some of the earlier plates included the Caspar plate in the 1980s and the Morscher plate, also known as the cervical spine locking plate (CSLP).
Cervical Spine Anatomy
The cervical spine has two main regions: the upper (C1 and C2) and the lower (C3 through C7) cervical regions. The upper cervical spine is unique because of its distinct anatomic arrangements, compared with the rest of the cervical spine. C1 is a bony ring without an actual body and as a result, it allows for the intrusion for the dens of C2 between the lateral masses of C1. It is held in place by a ligamentous complex. The dens articulates with the dorsal aspect of the ventral portion of the ring of C1. The lateral masses of C1 join with the occipital condyles and C2 by kidney-shaped articulations. The C2 vertebral body is more closely related to the rest of the subaxial spine than the C1 vertebra. It has a rostral extension knows as the dens or the odontoid process. The pars interarticularis of C2 connects the posterior bony elements of C2 to the anterior bony element. It projects from the lamina to attach to the lateral masses and is the part of C2 that is compromised in a hangman’s fracture. The atlanto-occipital joint allows flexion-extension (25 degrees), as well as a minimal degree of lateral flexion (5 degrees) and minimal rotation (5 degrees). The atlantoaxial joint allows 20 degrees of flexion-extension, 5 degrees of lateral bending, and 40 degrees of axial rotation. The failure strength of the alar ligament is about 200 N, whereas that of the transverse ligament is 350 N. The vertebrae of the middle and lower cervical spine are fairly uniform and the overall alignment allows for a lordotic curve to the cervical spine, which helps to prevent spinal cord injury because most axial loads are imparted symmetrically to the spine rather than with a significant flexion component. Because the addition of a flexion component to an axial load greatly increases the chance of vertebral body failure and the retropulsion of bone and disc fragments into the spinal canal, the lordotic posture thereby helps to prevent catastrophic injury.
Anterior Instrumentation
Anterior cervical instrumentation is used to treat a variety of abnormalities of the cervical spine, including traumatic injuries, neoplastic processes, infectious processes, and degenerative disorders of the cervical spine. These techniques involve placing a ventral cervical plate to the spine to promote fusion, maintain alignment, and prevent graft or cage dislodgment after a cervical discectomy or corpectomy. As previously stated, Caspar developed a semiconstrained (semirigid or dynamic) plate that uses a bicortical screw purchase in the vertebral body. Today the options for ventral cervical fixation are almost limitless, but in order to use the appropriate system, an in-depth understanding of biomechanics is required. It is important to determine what type of construct is required to achieve a particular goal. For example, in trauma, a rigid construct or implant is preferred to a dynamic construct or implant.
Biomechanics of Ventral Anterior Spine Constructs
During ventral cervical spine fixation, the biomechanical principles employed in creating a sound construct include ventral distraction, central compression (tension band), and ventral cantilever beam fixation. In addition to the cervical plates helping to achieve these biomechanical principles, they are also attained through the use of Caspar posts/pins.
Ventral Distraction
Ventral distraction can be achieved by placing a Caspar post in the most cephalad and caudad vertebral bodies in the construct with a distractor placed over the posts with a craniocaudal distraction force applied. Ventral distraction may also occur from the placement of a neutral construct with the expectation that the construct will bear the axial load and thereby distract the spine by resisting compression. Ventral distraction implants come in two fundamental types: interbody struts and cantilever beams. They both use screws in a fixed-moment arm, nonfixed moment arm, applied-moment arm, or dynamic mode. The interbody struts may be composed of bone, polymer (polyetheretherketone or carbon fiber), or metal implants; the cantilever beams are generally of a screw-plate construct type.
Ventral Compression (Tension Band) Fixation
Unlike ventral distraction techniques, ventral compression techniques do not employ interbody struts that apply compression forces to the spine. In general, it is difficult to use rods to provide significant compression or distraction in the ventral cervical spine, as can be easily achieved in the thoracic and lumbar spine. However, with the development of implants such as the DOC VSS (DePuy Spine, Raynham, MA: discussed later in this chapter), such use has been facilitated. This device allows the application of a compression force on the bone graft by preloading it, which is consistent with Wolf’s law. This compression and preloading is achieved using a cantilevered screw-rod system.
Ventral Cantilever Beam Fixation
There are three types of cantilever beam fixation constructs: fixed-, nonfixed-, and applied-moment arm constructs. The fixed- and applied-moment arm constructs provide what has been termed constrained or rigid spinal fixation. The nonfixed moment arm fixation provided what is termed as semiconstrained, semirigid, or dynamic fixation.
Dynamic Versus Constrained Plates
Lowery and McDonough, retrospectively, compared nonconstrained (dynamic) and constrained plates, and they reported fewer complications with the constrained system. Dynamic plates are now in use to compensate for graft subsidence, and more recent biomechanical studies have reported the improved outcomes with these translational plates over the constrained devices. Brodke and colleagues compared two dynamic plate designs and two constrained designs in a corpectomy model. They discovered that locked constrained plates provided excellent load-sharing fixation in the absence of graft resorption and subsidence, whereas dynamic plates provided superior load sharing with 10% graft subsidence. DiAngelo and coworkers noted significant loading of the graft in extension with constrained plates. This was not observed in translational plates. It is up to the surgeon to determine what type of construct is required. In the case of trauma, for example, most surgeons agree that a more rigid nonconstrained construct is required.
First-Generation Plates
The first-generation plates allowed motion at the screw-plate interface and as such were considered nonrigid implants. They also had limited fixation at the screw-plate interface, allowing the graft to be exposed to greater compressive forces thereby promoting fusion. Examples of the first-generation plates include the Caspar plate (Aesculap, Center Valley, PA) and the Orozco plating system (Synthes, Paoli, PA). These plates had poor screw backout prevention mechanisms and were abandoned due to a high rate of screw backout and breakage. These plates also required fixation with bicortical screw purchase, which can sometimes be challenging as overpenetration of the vertebral body can lead to spinal cord injury and underpenetration can lead to a weaker construct causing failure of the construct or screw pullout before a fusion is achieved.
Caspar Plate System
This is a nonconstrained titanium plate where the screws are not locked to the plate ( Fig. 62-1 ). It can be affixed to the spine using both unicortical and bicortical screws. The unicortical screws come in a variety of lengths ranging from 10 to 28 mm. Screws have a constant outer diameter of 3.5 mm and an inner diameter of 2.2 mm. The bicortical screws are self-tapping and come in lengths from 14 to 19 mm. The outer diameter is 4 mm and the inner diameter is 2.2 mm at the tip and 2.7 mm at the head; creating a tapered configuration. The screws are made of a titanium alloy with a corundum-blasted surface over a third of the length of the tip.
Techniques for Placing Caspar Ventral Cervical Plates
Once the spine is exposed, distracting pins are placed into the rostral and caudal vertebral bodies. This allows for the discectomy or corpectomy to be performed with relative ease. The distracting pins are placed using a mallet and a screwdriver. The length of the plate to be used may be measured intraoperatively or it could be measured on preoperative radiographs. Once the plate is placed, it is important to make sure that the screw holes are not over holes caused by the distracting pins or over soft tissue. Prior to plate placement, ventral osteophytes should be drilled off to allow the plate to sit flush on the ventral aspect of the spine. The plate should also be placed in the midline. If the contour of the spine does not allow for a perfect fit, one may consider contouring the plate as a last resort option. It is important to avoid multiple bends, as this may weaken the plate. It is also important to place the plate greater than 5 mm from the adjacent disc space to decrease the incidence of adjacent segment disease. To determine screw length, intraoperative imaging may be used, or this could have been measured out on preoperative imaging. The distance between the ventral cortical margin and the back end of the vertebral body will guide screw length selection. The drill guide should be set to 3 mm less than the distance from the ventral to the dorsal aspect of the vertebral bodies to prevent spinal cord injury. The plate is held in place with a temporary plate holding pin and the drill guide is used to drill the screw holes to the preset depth. Next the screw holes are tapped and the first screw is placed. It is not completely tightened. Next the screw that is diagonal to the first screw is placed and once the final screw is placed, they are all tightened, as initially tightening a screw prior to placement of the other screws may cause the plate to move.
Related posts:
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Ventral and Ventrolateral Subaxial Decompression
Complex Lumbosacropelvic Fixation Techniques
Timing of Surgery Following Spinal Cord Injury
Spinal Wound Closure
Explant Analysis of Wear, Degradation, and Fatigue in Motion Preserving Spinal Implants
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