Chapter 141 Thoracic and Lumbar Spine Construct Design

Construct design is a process that formulates a specific blueprint for an orderly and thoughtful assembly of implantable spinal instrumentation, designed to correct instability or deformity or both of the spinal column.¹ Construct design requires specific understanding of the deformity or instability and the biomechanical forces acting on the pathologic alignment. An understanding of corrective forces and where they must be applied also is required. A keen knowledge of the anatomy and pathoanatomy is required to preserve neuralgic function and avoid adjacent segment injury. Although skillful assembly of the mechanical construct is a definite prerequisite, ultimate success is determined by the orderly thought process for designing the construct, based on personal experience, experience of others, and laboratory data. Creating a preoperative plan, or blueprint, can focus this design process. Spinal instrumentation surgery must not be assumed to be strictly “mechanical” or “routine”; rather, it requires serious and meticulous planning to ensure success.

Construct Types (Modes of Force Application)

The six fundamental construct types are simple distraction, three-point bending, tension band fixation, fixed-moment arm cantilever-beam fixation, non–fixed-moment arm cantilever-beam fixation, and applied-moment arm cantilever-beam fixation (Fig. 141-1).

FIGURE 141-1 The application of forces to a construct can be broadly categorized into simple distraction (e.g., Knodt rod; A); tension band fixation (e.g., spinous process wiring; B); three-point bending using the middle point as a fulcrum (e.g., Harrington rods; C); non–fixed-moment arm cantilever beam design, in which axial load cannot be resisted because of nonrigid fixation between the screw and the longitudinal member (e.g., lateral mass plates; D); fixed-moment arm cantilever beam design, in which axial loading forces can be resisted because the screws are rigidly fixed to the longitudinal member (e.g., locking cervical plate systems; E); and applied-moment arm cantilever beam design, in which complex forces can be applied (e.g., universal spinal instrumentation; F).

Development of Construct Blueprint

The preoperative development of a blueprint for implant placement, based on the composite information obtained from clinical assessment and imaging studies, ensures a definitive plan and saves time in the operating room. Some flexibility in this plan may be required after surgical exposure of the bony spine because of unexpected findings. For instance, minor fractures at the implant-anchor site may necessitate deviation from the original plan.

A simple scheme should be used that provides (1) information about the level of the lesion or the level of the unstable segment or segments, (2) the types of implants to be used (anchors, longitudinal members, and cross-connectors), (3) the length of stabilization required on either side of the lesion, and (4) the mode of load bearing by the construct. The scheme guides selection of the appropriate implant components in advance, improves the intraoperative communication between surgeons and assistants, and enhances the chances of success.

Although the concept of construct design encompasses similar principles in all anatomic regions of the spine, designing a thoracolumbar construct poses more challenges than most cervical constructs. Various constructs, using a variety of anchors in different bony landmarks, each used in various mechanical modes (i.e., compression, distraction, neutralization, distraction followed by compression, or distraction and compression at different segmental levels), may be used in a successful strategy. In consideration of these complex decision-making dilemmas, this chapter focuses on thoracic and lumbar fixation design strategies.

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. 141-2).² This drawing can be easily obtained from a CT scan or from radiographs.

FIGURE 141-2 A blueprint of the proposed surgery is helpful in clarifying the treatment plan for both the surgeon and the operating team.

(From Benzel EC: Construct design. In Benzel EC, editor: Spinal instrumentation, Chicago, 1994, AANS Publications, pp 239–256.)

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 at the present time in the thoracic region, should incorporate three spinal levels above and two spinal segments below the limits of the lesion (3A-2B rule). For the past decade or so, hooks have mainly been reserved for situations in which the pedicles are very small or for additional support in osteoporotic patients along with pedicle screws. 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.

In the lower thoracic spine (T8-10), the thoracolumbar junction (T11-L1), and the lumbar region (L2-5), pedicle screw constructs are often preferred. The size of the pedicles and the increased stiffness these screws provide make them an ideal choice. Short-segment fixation, with the inclusion of only one vertebra immediately above and below the lesion, is appropriate if the anterior, load-bearing column is intact and kyphotic deformity is not present. The bone structure must also be of sufficient strength. With increasingly sophisticated fixation choices available, it must be remembered that a rigid, stable construct is the goal, and biology and biomechanics surrounding the implants must be taken into account.

Both segmental pedicle screw and hook instrumentation techniques are successful in obtaining global balance safely. Balance can be accomplished without neurologic injury, even in adolescent idiopathic scoliosis, where neurologic risk is greatest. Segmental pedicle screw instrumentation offers a significantly better overall major and minor coronal curve correction and maintenance without neurologic problems and 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.³

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.

Mechanical Attributes of Spinal Implants: Construct Type

The mechanism that the construct uses (types of construct) to bear loads is also designated. Six methods of load bearing are associated with the six construct types: distraction, three-point bending, tension band fixation, fixed-moment arm cantilever-beam, non–fixed-moment arm cantilever-beam, and applied-moment arm cantilever-beam. It is difficult to depict this information on the line drawing. This information may be recorded in the space provided at the bottom of the drawing.

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