18.1.1 Indications for Spinal Instrumentation

Indications for spine surgery often depend on the extent and type of spinal instability present. The quest to quantitate the extent of spinal instability in order to optimize its management should lead the surgeon to ask two fundamental questions: What is expected from the implant? Is this expectation reasonable? If these questions are answered appropriately, the foundation of the construct design process has been properly established. ¹

18.1.2 Choice of Implant Construct

The use of a spinal implant involves several choices: (1) the longitudinal member (rod or plate), (2) the method of anchoring to bone (wire, hook, or screw), and (3) the mechanism of cross fixation.

18.1.3 Mode of Application of the Implant

The mode of application of the implant is a critical element in the construct design process. The surgical placement of the implant in distraction, compression, neutral, translation, flexion, extension, or lateral-bend mode affects the extent of the exaggeration or correction of deformity and the extent of the exaggeration or relief of neural compression. ¹

18.1.4 Mechanism of Load Bearing

As was outlined in Chapter 17, there are six fundamental construct types: (1) simple distraction, (2) three-point bending, (3) tension-band fixation, (4) fixed moment arm cantilever beam fixation, (5) nonfixed moment arm cantilever beam fixation, and (6) applied moment arm cantilever beam fixation. This implies that there are six fundamental mechanisms of load bearing. These are associated, respectively, with corresponding construct types: (1) simple distraction, (2) three-point bending, (3) tension-band fixation, (4) fixed moment arm cantilever beam fixation, (5) nonfixed moment arm cantilever beam fixation, and (6) applied moment arm cantilever beam fixation. An implant almost always functions differently under different loading conditions (see Chapters 17 and 29).

18.2.1 Line-Drawing Framework

Simple posterior–anterior and lateral line drawings of the spine provide a framework for the clear definition of the operative plan (Fig. 18.1). Often, only a posterior–anterior drawing is necessary, unless the operative plan includes the reduction of a deformity in the sagittal plane (e.g., a kyphotic deformity) or the placement of both dorsal and ventral implants. Hence, redundant information should not be depicted on the lateral view. The line drawing provides the blueprint for surgery. It should be clear and concise. It should also permit a specific focus on either the cervical and cervicothoracic region (Fig. 18.1a) or on the thoracic and lumbar region (Fig. 18.1b). Occasionally, a blueprint of the entire spine may be required (Fig. 18.1c).

The convention for the posterior–anterior line drawing is that the left side of the drawing portrays the left side of the patient—that is, the drawing portrays the patient as viewed from behind. This is in accordance with the most common surgical approach and, as such, reduces the chance of confusion.

18.2.2 Level of Pathology and Level of Fusion

The designation of the level of pathology or spinal instability, the levels to be fused, and the type of fusion should next be placed on the line drawing. The level(s) of instability or pathology are designated by Xs and the fusion by a hatched outline of an anatomically correct depiction of the fusion (Fig. 18.2).

An accurate delineation of the unstable motion segment(s) is important regarding the definition of the number of spinal levels to be spanned, both above and below the level of pathology. For example, the instability consists of a loss of integrity of only the T12–L1 motion segment, the instrumentation of three levels above places the upper end of the implant at T10, and the instrumentation of two levels below places the lower end of the implant at L2 (Fig. 18.3a). This is designated by the nomenclature 3A–2B, which describes an implant extending from three spinal levels above to two levels below the region of pathology.

If, however, the L1 vertebral body is fractured and its juxtaposed disc interspaces are disrupted, the T12–L1 and L1–L2 motion segments are structurally disrupted. In this case, the same implant design designation described above (3A–2B) results in an implant extending from T10 (three levels above the upper extent of the pathology) to L3 (two levels below the lower extent of the pathology; Fig. 18.3b). In the former case, the implant extends from T10 to L2 (the lower extent of the pathology being the upper aspect of T12); in the latter, it extends from T10 to L3 (the lower extent of the pathology being the lower aspect of L1).

The mechanical effect of immobilizing any motion segment may be unnecessarily significant. Therefore, a clear definition of the level of instability is critical regarding the surgical decision-making process.

18.2.3 Type 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 interface (anchor) is a wire, hook, or screw. The convention used here is to designate a hook by a right-angled arrow, with the arrowhead pointing in the direction of the orientation of the hook (i.e., toward the bone purchase side of the hook). Each screw is designated by an X surrounded by a circle. Wire is depicted as a loop. ¹ The insertion sites of these components are indicated by placement of the previously described symbols at the appropriate levels of the spine on the line drawing, with accompanying designations to specify anatomical sites of purchase: P for pedicle, L for laminar or sublaminar, T for transverse process, and I for iliac (Fig. 18.4). ¹

Implant components that function as anchors to bone include screws, hooks, and wire. Hooks may be placed in a sublaminar, transverse process, or pedicle location. Wires can be placed in a sublaminar, interspinous, and a variety of other locations. Care must be taken with sublaminar placement of hooks or wires to prevent neural compression or injury. This is particularly relevant in the midthoracic region, where the spinal cord blood supply is relatively tenuous and the spinal canal relatively small.

18.2.4 Mode of Application at Each Segmental Level

The mode of axial load application (distraction, compression, or neutral) at each implant–bone interface is indicated by an arrow pointing in the direction of force application for distraction and compression, or by a horizontal line for neutral application.

Bending moments are difficult to depict accurately on the line drawing; hence, they are described in the notation. For example, the rods are placed in a concave left configuration, which is then followed by a 90-degree counterclockwise rotation (derotation maneuver) to convert the scoliotic deformity to a kyphotic deformity.

The modes of application at each segmental level are depicted with the arrows and lines, as described previously. These are drawn lateral to the designations for implant type (Fig. 18.5a). If force applications in the sagittal plane are planned, they are depicted on the lateral line drawing (Fig. 18.5b). Finally, cross member (cross fixator) locations can be designated by elongated rectangles with circles (see Fig. 18.5a).

18.2.5 Mechanical Attributes of Spinal Implants: Construct Type

The mechanism by which a construct bears loads is also specified. There are six methods of load bearing associated with six construct types (see Chapter 17): (1) distraction, (2) three-point bending, (3) tension-band fixation, (4) fixed moment arm cantilever beam, (5) nonfixed moment arm cantilever beam fixation, and (6) applied moment arm cantilever beam. Because this information is difficult to depict on the line drawing, it is simply recorded in the space provided at the bottom of the page.

18.3.1 Osteoporosis

Osteoporosis creates a surgical dilemma in the form of reduced integrity of the implant–bone interface. Hooks and sublaminar wires resist pullout better than screws and therefore are advantageous in the patient with osteoporosis. Hooks and sublaminar wires apply forces to the spine at a considerable perpendicular distance from the instantaneous axis of rotation (IAR). In general, it is optimal to use as many anchors as possible in the patient with osteoporosis. This strategy allows the surgeon to “share” the load between components of the construct, thus making individual single-component failure less likely.