26 Deformity Prevention and Correction: Component Strategies

10.1055/b-0035-106401

26 Deformity Prevention and Correction: Component Strategies

This chapter addresses deformity and its correction and prevention. To convey and portray the fundamentals and their clinical techniques, a “building block” approach is undertaken here. First, commonly used strategies are discussed. Next, clinical applications are addressed, with a discussion of deformity-specific principles that build on these commonly used strategies. This is followed by a region-specific approach to deformity prevention and correction. Finally, factors that are not region-specific are addressed.

26.1 Commonly Employed Strategies

One or a combination of two fundamental techniques can be used for deformity correction: (1) implant force and bending moment application that “brings the spine to the implant” and (2) implant force and bending moment application with in vivo implant configuration alteration techniques. A fundamental understanding of these techniques provides the surgeon with a broadened surgical latitude and allows an individualized and customized implant selection process for the patient in each case. Finally, strategies that maintain the acquired correction must be used.

Implant forces, when “bringing the spine to the implant,” can be applied along any of the three axes of the Cartesian coordinate system. They are usually applied in the sagittal plane of the spine (e.g., lordosis or kyphosis correction or prevention). They can also be applied in the coronal plane (e.g., scoliosis correction or prevention; Fig. 26.1).

**Fig. 26.1** In “bringing the spine to the implant,” forces that are oriented along any axis or plane may be used: (A) the long axis, (B) the sagittal plane, and (C) the coronal plane. Arrows depict forces applied by the implant.

Implant force and bending moment application by means of in vivo alteration of implant configuration first involves application of the implant to the spine (insertion), followed by adjustment of the implant shape. This is achieved by one or a combination of three fundamental types of implant manipulation: (1) implant contouring, (2) derotation, (3) or the application of an intrinsic implant bending moment (Fig. 26.2).

**Fig. 26.2** Implant force application by in vivo alterations of implant configuration with (A) implant contouring, (B) intrinsic implant bending moment application about the long axis of the spine (i.e., derotation), and (C) intrinsic implant bending moment application about an axially oriented axis of the spine. Straight arrows depict forces; curved arrows depict bending moments.

26.2 Component Strategies for Deformity Prevention and Correction

26.2.1 Bringing the Spine to the Implant

Various techniques can be used to bring the spine to the implant. As mentioned previously, this is accomplished via the application of forces to the spine along one or a combination of the three axes of the Cartesian coordinate system. Forces applied along the long axis of the spine (e.g., distraction) can be used to correct compression deformations, as well as coronal and sagittally oriented translational deformations (Fig. 26.3). Bending moments applied in the sagittal plane are of a three- or four-point bending or applied moment arm cantilever beam type (Fig. 26.4).

**Fig. 26.3** Distraction (a force applied along the long axis of the spine) can be used to correct (A) compression deformations, (B) coronal plane translational deformations, and (C) sagittal plane translational deformations if enough ligamentous integrity is present. Arrows depict applied forces.

**Fig. 26.4** Bending moments applied in the sagittal plane by (A) a three-point bending mechanism and (B) an applied moment arm cantilever beam mechanism. Straight arrows depict forces; curved arrows depict bending moments.

Three- or Four-Point Bending Force Application

Three-point bending constructs were discussed in Chapter 17. The forces that they apply to the spine are common and, for the most part, well understood. Three- and four-point bending implants are a classic example of the strategy that “brings the spine to the implant.” They can be applied to reduce subluxations (Fig. 26.5). Crossed-rod techniques can be applied in the sagittal and coronal planes (see below). They are primarily used to correct angular (kyphotic) spine deformities via a three-point bending mechanism applied dorsally. Regardless, three-point bending techniques can be employed dorsally to correct deformity by essentially “bringing the spine to the implant” (Fig. 26.5a).

**Fig. 26.5** (A) A three-point bending construct that “brings the spine to the implant” via a dorsal approach. (B) This is further illustrated by an example of cervical spine deformity and subsequent ventral deformity correction by “bringing the spine to the implant” by sequentially tightening the intermediate screws of a ventral cervical plate, thus applying three-point bending forces. (C) Terminal three-point bending constructs simply have one long and one short moment arm. Arrows depict forces applied.

A ventral approach can be applied in the cervical spine to correct kyphotic deformities via a “bringing the spine to the implant” strategy, as well (Fig. 26.5b). Such a technique provides an advantage for both deformity correction and maintenance of fixation. The latter is achieved via the application of three-point bending forces (see Chapter 27).

Terminal three-point bending techniques can be used to “bring the spine to the implant” as well as to prevent the spine from “falling away from the implant” (Fig. 26.5c). This technique can be applied to any spinal level. It is most commonly used in the cervical region because of the lesser loads accepted by the implant and the relatively insubstantial design of the construct. It is most useful for the prevention or reduction of translational deformation (see Chapter 17).

Note that if a ventral translation deformation is to be corrected or prevented, the long arm of the construct must be situated caudal to the site of translation, whereas if dorsal translation deformation is to be corrected or prevented, the long arm of the construct must be situated rostral to the site of translation (Fig. 26.6).

**Fig. 26.6** For terminal three-point bending constructs to be effective in reducing translational deformation, they must be applied properly. The long arm of the construct must be placed (A) caudally with a ventral translational deformation and (B) rostrally with a dorsal translational deformation.

Four-point bending of the spine, as defined by White and Panjabi, involves loading a long structure (i.e., the spine) with two transverse forces on one side and two on the other (Fig. 26.7a). The bending moment is constant between the two intermediate points of force application if all forces are equal, whereas in three-point bending, the bending moment peaks at the intermediate point of force application (see Chapter 17 and Fig. 26.7a, b). If the forces applied by a three- or four-point bending construct are oriented in the opposite direction, the technique is termed reversed three-point or reversed four-point bending fixation (Fig. 26.8). This technique may be used to reduce lumbar spondylolisthesis.

**Fig. 26.7** (A) Four-point bending and (B) three-point bending construct forces and associated bending moments. In the four-point bending construct depicted here, all forces (F ₄ _PB) and the distance from the intermediate and terminal points of force application (1/3 × D ₄ _PB) are equal. In this situation, the maximum bending moment, which is constant between the two intermediate points of force application, is defined by the following equation: M ₄ _PB = F ₄ _PB x 1/3 D ₄ _PB. D ₄ _PB is the length of the entire construct. Because the forces (F ₄ _PB) are applied at points dividing the construct into three equal sections, the moment arm defining the bending moment is one-third of the entire construct length. In the three-point bending construct depicted here, the intermediate force is applied halfway between the terminal points of force application. Therefore, as demonstrated in Chapter 17, the maximum bending moment occurs at the point of intermediate force application and is defined by the following equation: M ₃ _PB = 1/4 F ₃ _PB x D ₃ _PB. However, because F ₄ _PB is the force applied at the terminal hook–bone interface and F ₃ _PB is the force applied at the fulcrum, at the outset, F₄ _PB and F₃ _PB by definition vary by a factor of 2. The force applied at the terminal hook–bone interface in this example is thus 2 x F ₃ _PB. This is defined here as F _terminus3 _PB and is equal to 2F ₃ _PB. Therefore, M ₃ _PB = 1/4 × 2 × F _terminus3 _PB × D ₃ _PB. To compare three-point and four-point bending constructs, the following derivation is performed. Assume that a three-point and four-point bending construct are of similar length and that the bending moments applied are equal. The following derivation, thus depicts the comparison between the constructs; because D ₄ _PB = D ₃ _PB, and M ₄ _PB = M ₃ _PB, then F ₄ _PB × 1/3 × D ₃ _PB – 2 × F _terminus3 _PB × D ₃ _PB; F ₄ _PB = 3/2 × F _terminus3 _PB. The forces applied at the terminal hooks by each construct are depicted by the above equations. The closer the intermediate forces are applied to the terminus of the four-point bending construct, the greater the numerator of the right half of the equation and the greater the forces required to achieve an equivalent bending moment (compared to a three-point bending construct of similar length). Conversely, the closer the intermediate forces of a four-point bending construct are placed to the middle of the construct, the more it biomechanically approximates a three-point bending construct (i.e., F ₄ _PB = F _terminus3 _PB).

**Fig. 26.8** (A, B) Reversed three-point bending forces (dorsally directed force at the fulcrum) can be used to reduce a spondylolisthesis. This subjects the screw to significant pullout stresses.

The forces applied to the various components of an implant must be carefully considered. For example, a screw can be exposed to significant three-point bending forces. This must be considered during the implant design decision-making process (Fig. 26.9).

**Fig. 26.9** A depiction of three-point bending forces applied to a nonfixed moment arm screw that traverses potentially mobile media or media of different densities. (A) Solid arrows depict the three-point bending forces applied to the screw. (B) Similar forces can be applied when a screw passes across a fracture, such as a transodontoid screw used for a dens fracture. (C) A screw fractures at the point of maximum stress (maximum bending moment if the inner diameter is constant).

Crossed-Rod Deformity Correction

The crossed-rod technique is a well-established method of thoracic and lumbar kyphotic deformity correction. It was first used with Harrington distraction rods (Fig. 26.10a); it was later employed more effectively with multisegmental sublaminar wiring (Luque) techniques (Fig. 26.10b). ¹ Most recently, it has been most effectively used via the sequential hook insertion (SHI) technique with universal spinal instrumentation systems (Fig. 26.10c). ² It can also be used to correct coronal plane deformities ( Fig. 26.10d). Creative adjuncts may also be used (Fig. 26.10e). Regardless of the construct type, the technique involves the simultaneous application of deformity reduction forces to the spine by means of moment arms (longitudinal members; i.e., rods). Gradual reduction is thus achieved via a three-point bending force application mechanism (Fig. 26.11).