Thoracic and Lumbar Spine Construct Design

Line Drawing of Proposed Construct

A simple dorsoventral or lateral line drawing of the spine provides a framework for the clear definition of the operative plan. Often only a dorsoventral drawing is necessary, although clear consideration of any sagittal plane deformity is vital. The line drawing provides the blueprint for surgery ( Fig. 115.2 ). This drawing can be easily obtained from a CT scan or from radiographs.

The convention used in this chapter with regard to a dorsoventral line drawing dictates that the left side of the drawing portrays the left side of the patient (i.e., the drawing portrays the patient as viewed from behind). This portrayal is in accordance with the most common surgical approaches for complex instrumentation constructs and decreases the chance for confusion.

Level of Lesion and Level of Fusion

The designation of the level of the lesion or location of instability, the levels to be fused, and the type of fusion should be placed next on the line drawing. The level of instability or lesion is designated by an “X,” and the precise extent of proposed bony fusion is designated by a hatched outline. The number of unstable motion segments should be assessed carefully, as should associated deformity. These factors determine the number of levels to be spanned with the construct. The choice of implants also affects this decision.

Hook constructs, often used in the past throughout the spine and still used occasionally in the thoracic region, should incorporate three spinal levels above and two spinal segments below the limits of the lesion (3A-2B rule). Since the early 2000s, hooks have mainly been reserved for situations in which the pedicles are very small or for additional support along with pedicle screws in osteoporotic patients. If the patient has a marked angular kyphotic deformity, and if three-point bending is considered in an attempt to reduce the deformity, inclusion of four or more spinal levels above the lesion is common (4A-2B rule) and may provide a more functional lever arm. Such long constructs are suited mostly for lesions in the middle and upper thoracic regions, although thoracic pedicle screws are widely used even in these regions. With the use of uniplanar and fixed-head pedicle screws, correction of such kyphotic deformities can be achieved through multilevel dorsal approaches, as long as the bone quality can maintain the stresses applied to the screws during such corrective maneuvers. As such, hooks are being used less in current instrumentation systems and designs.

In the lower thoracic spine (T8‒T10), the thoracolumbar junction (T11‒L1), and the lumbar region (L2‒L5), pedicle screws are the workhorse for fixation, stabilization, and correction of deformity. The size of the pedicles at these levels and the increased stiffness of these screws provide make them an ideal choice for stabilization. In the thoracic spine, surgeons can take advantage of the costovertebral articulation with “in-out-in” technique maximizing the width of implanted screws, not relying on the width of the actual thoracic pedicles themselves. With this “in-out-in” technique, the screw traverses several cortical borders in a more lateral to medial trajectory into the thoracic vertebral body, therefore allowing not only the benefits of triangulation of the instrumentation in the axial plane, but also greater pullout strength. Short-segment fixation, meaning instrumentation of only one vertebra immediately above and below the lesion, is appropriate if the anterior, loadbearing column is intact, the kyphotic deformity is not present, and the bone structure shows sufficient strength. With increasingly sophisticated fixation choices available, it must be remembered that a rigid, stable construct is the goal with the consideration of biology and biomechanics surrounding the implants.

Both segmental pedicle screw and hook instrumentation techniques are successful in safely achieving local and global alignment goals. Balance can be accomplished without neurological injury, even in adolescent idiopathic scoliosis, where neurological risk is greatest. Segmental pedicle screw instrumentation offers significantly better overall major and minor coronal curve correction and maintenance, without neurological problems and with slightly improved pulmonary function values, for the operative treatment of adolescent idiopathic scoliosis. Pedicle screw constructs may also allow for a slightly shorter fusion length than segmental hook instrumentation.

Depiction of Implant Components

The type of implant components used in the instrumentation construct should be delineated clearly on the blueprint. The implant component at each implant–bone juncture may be a cable, hook, or screw. The convention used is to designate hooks by a right-angled arrow, with the arrowhead pointing in the direction of the orientation of the hook. The purchase site—and the type of hook—is designated further by “P” (for pedicle), “L” (for laminar or sublaminar), or “T” (for transverse process). Screws are designated by an “X” surrounded by a circle and placed over pedicles. Cable (or wire) is depicted as a loop.

The mode of axial load application (distraction, compression, or neutral) at each implant–bone juncture is depicted as an arrow. The arrow points in the direction of force application for distraction and compression or a horizontal line for neutral. Bending moments are difficult to depict accurately on the line drawing and are described.

The modes of application of each segmental level are depicted with arrows and lines, as previously described. The arrows and lines are drawn lateral to the designations of implant types. If sagittal plane forces are to be applied, they are depicted on the lateral line drawing. Finally, cross fixator locations can be designated by rectangles with circles.