Chapter 34 Anterior Cervical Stabilization in Tumor Surgery
INTRODUCTION
After a vertebral body tumor resection, various methods of interbody grafting are available to accompany anterior plate placement or posterior instrumentation. Ideally the plating system should facilitate placement of the interbody graft in distraction mode, allowing it to become load bearing. Anterior cervical plating provides immediate stability and prevention of graft dislodgement.1,2 It also decreases stress on the graft and eliminates the need for external orthosis.
TECHNIQUES OF PLATING
DIRECTION OF SCREWS
The screw is usually directed 10 to 15 degrees to the horizontal plane.3 However, pull-out tendency is greatest when the screw lies tangent to the arc of rotation around the instantaneous axis of rotation (IAR) and least when the screw lies perpendicular to the arc of rotation around the IAR. The IAR lies toward the ventral inferior aspect of the vertebra but is altered under the influence of a rigid load-bearing device. The screws that lay more perpendicular to the vector of pullout should have greater resistance to pullout (Fig. 34-1).
SELECTION OF THE DEVICE
The classification of devices is according to the motion at the plate-screw interface.4
Unrestricted Backout Device (Bicortical Non-Locked Bone Screw)
This system has unrestricted backout and is non-locking and non-rigid (Fig. 34-2). The screw angle is determined according to the surgeon’s preference. There is no fixed moment arm, allowing for subsidence of the construct because of the lack of fixation at the screw-plate interface.
Restricted-Constrained Device (Unicortical Locked Bone Screw)
In this system, the screw purchase is unicortical and the screw-plate interface is constrained (Fig. 34-3). There is a predetermined (rigid) screw trajectory in the plate. The locking mechanism prevents screw migration even if screw breakage occurs. The CSLP (cervical spine locking plate) and Orion plate (constrained type) are included in this category.
OTHER OPTIONS IN ANTERIOR CERVICAL PLATING
Telescopic Plate Spacer
The Telescopic Plate Spacer (TPS; Interpore Cross International, Irvine, CA) is a new option after corpectomy (Fig. 34-5). The device is a titanium cervical plate-interbody spacer hybrid, which can be used in either one-level or two-level corpectomy defects.5 The spacer portion of the device is hollow and may be packed with bone graft. The plate portion of the device can be fixed to the adjacent vertebral bodies with screws. Through the telescopic effect, the device can be expanded to fit corpectomy defects to restore anterior column height and correct kyphotic deformity. By applying distraction anterior to the IAR, the TPS restores lordosis in the cervical spine.
Junctional Plate
This is a technique in which a small anterior cervical plate is fixed at one end of the construct, usually at the lower vertebra graft because dislodgment often occurs at the inferior end of the construct (Fig. 34-6). It overlaps the end of the strut graft-vertebra junction and is able to block the end of the graft so that it does not dislodge anteriorly. An advantage of this system is that it does not create a tension band anteriorly, which would lead to large stresses on the strut graft. It allows load sharing by the graft and lets the graft settle gradually into the endplates without distraction force.
SCREW PURCHASE AT C2
The C2 vertebral body surface is more convex and narrower than the subaxial cervical spine. For plate implantation, the protruding portion should be drilled out. For better fitting of the plate to the vertebral body surface, a narrower and small radius plate may be adequate. It also is better for the screw to be directed less perpendicular to the cortical surface. The more obliquely the screw is directed, the longer is the length of the screw (Fig. 34-7).
SCREW PURCHASE AT T2
Using the usual anterior approach, the T1–2 interspace is the lowest level to reach without a sternotomy. Although the screw insertion angle is nearly horizontal, the angle relative to the disc plane is 20 to 30 degrees. With a horizontal insertion, the screw can be inserted with ideal angulation (Fig. 34-8).
