Thoracic and Lumbar Spine Construct Design

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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.

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.

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

Construct Design Strategies

Multiple factors should be taken into account in designing a spinal instrumentation construct.4 Consideration should be given specifically to bony integrity, the location of the unstable spinal segment, the implant length with respect to the unstable segment, the need for cross-fixation, the need for dural decompression, the choice of ventral versus dorsal instrumentation, availability of specific instrumentation, metallic composition of instrumentation, and the familiarity of the surgeon with a particular technique. Each factor should be adequately addressed to achieve optimal outcome.

Bony Integrity

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