17.1 Simple Distraction Fixation

Simple distraction fixation can be applied from either a ventral interbody or a dorsal approach. Ventral distraction constructs generally apply forces that are in line with the IAR—that is, in the interbody region. This allows the ventral distraction implant to effectively resist axial loads without applying a bending moment (see Fig. 17.1 b). Ventral interbody distraction can cause extension of the spine if the distraction forces are applied ventral to the IAR (ventral to the neutral axis; Fig. 17.3a). The application of a dorsal distraction force as an isolated entity is uncommon. This is so because of its propensity to pathologically exaggerate or cause a kyphotic deformity. The location of the point of force application dorsal to the IAR creates a bending moment that results in flexion (Fig. 17.3b). The combination of distraction and three-point bending instrumentation application eliminates this pathologic situation by applying a ventrally directed force at the fulcrum (Fig. 17.4).

Distraction applied to the spine at a finite perpendicular distance from the IAR results in a force application similar (but opposite in direction) to that achieved with tension-band (compression) fixation. This distraction force application might be termed tension-band (distraction) fixation (see Fig. 17.3b).

17.2 Three-Point Bending Fixation

A springboard is a common example of a three-point bending force application. It consists of a fulcrum that directs a force vector in a direction opposite the direction of the terminal force vectors (Fig. 17.4a). Three-point bending spinal instrumentation constructs apply similar force vectors (Fig. 17.4b), usually with an accompanying distraction or compression force application (Fig. 17.4c). Three-point bending constructs commonly involve instrumentation application over multiple (five or more) spinal segments, with accompanying dorsally directed forces at the upper and lower construct–bone interfaces and a ventrally directed force at the fulcrum that is equal to the sum of the two dorsally directed forces (see Fig. 17.4b). This technique can be used to decompress the ventral dural sac following trauma by distracting the posterior longitudinal ligament (ligamentotaxis or annulotaxis). The desired resultant force is the pushing of the offending bone and/or disc fragments ventrally and away from the dural sac (see Chapter 8).^1,​2 Because of the relative weakness of the posterior longitudinal ligament and/or the fixed nature of the retropulsed fragments, however, this technique may not always succeed (see Chapter 8).

Dorsal distraction force vector application is rarely “pure.” It is frequently used in combination with the application of a three-point bending force to the spine (see Fig. 17.4c). The application of sufficient dorsal distraction so that the implant makes contact with the spine at the level of the site of pathology (at an intermediate point along the construct; i.e., at a fulcrum) results in three-point bending force application.^1,​3 The application of a distraction force between two adjacent spinal levels where a fulcrum is not present is an exception (e.g., the use of a Knodt rod in distraction that spans only one motion segment). In this case, no intermediate point of fixation at a fulcrum is available. With longer constructs, flexion occurs before engagement of the fulcrum because of the application of the distraction force at points dorsal to the IAR. This is most common in the lumbar region, where a lordotic posture is present (Fig. 17.5).

The bending moment at the site of pathology from three-point bending construct application is defined mathematically by the following equation:

Equation s. Kap.

in which M is the bending moment, D₁ and D₂ are the distances from the fulcrum to the terminal hook–bone interfaces, D_3PB is the sum of D₁ and D₂, and F_3PB is the ventrally directed force applied at the fulcrum (see Fig. 17.4).^1,​4

17.2.1 Terminal Three-Point Bending Fixation

A three-point bending construct can be used to correct a deformity near the termini of the construct, as opposed to the midportion of the construct (the more common situation). This is termed terminal three-point bending fixation. Usually, the implant is positioned so that the sagittal deformation is at the rostral end of the construct, if ventral subluxation is present (Fig. 17.6).

In reality, terminal three-point bending fixation is simply a three-point bending construct in which the fulcrum is situated near one end of the construct—that is, D₁ is short and D₂ is relatively long. In light of this, the springboard previously discussed is more appropriately considered a terminal three-point bending structure. Of note is that the moment arm attained by a terminal three-point bending construct is less than that attained if similar ventrally and dorsally directed forces are applied when the fulcrum is in the midportion of the construct. This must be taken into consideration clinically.

17.3 Tension-Band (Compression) Fixation

Dorsal spinal compression (tension-band fixation) is usually applied by wires, clamps, springs, or rigid constructs in compression. These techniques apply spinal compression forces at dorsal sites (Fig. 17.7a). Ventral tension-band fixation constructs, however, may also be applied (Fig. 17.7b). Although one may not think of a ventral cervical plate as a compression device, it resists extension of the motion segment when the spine is extended, thus functioning as a compression fixator.

It is emphasized that implants function differently under different loading conditions. A ventral cervical fixed moment arm cantilever (i.e., constrained plate) functions as a cantilever if an axial load is applied. If, however, the patient extends the neck, the implant will limit extension at the instrumented segment via a tension-band fixation (in extension) mechanism (see Fig. 17.7b). Tension-band fixation applies compression forces at a perpendicular distance from the IAR (e.g., from the plate to the IAR; see Fig. 17.7b).

By the nature of the tension-band fixation construct, an extension (dorsal) or flexion (ventral) bending moment is applied to the spinal segments that are “compressed.” For tension-band fixation techniques, the bending moment applied at the site of pathology is defined mathematically by the following equation:

Equation s. Kap.

in which M_TBF is the bending moment, F_TBF is the compression force applied at the upper and lower termini of the construct at the instrument–bone interface, and D_{IAR – TBF} is the perpendicular distance from the IAR to the applied force (Fig. 17.8).¹

Ventrally positioned extradural masses (bone and/or disc fragments) may be thrust dorsally into the spinal canal during the application of dorsal compression forces. Therefore, ventral decompression procedures may be appropriate before the application of dorsal instrumentation constructs, particularly if ventral compression via retropulsed bone and/or disc fragments exists (Fig. 17.9a).³ Additionally, tension-band fixators do not, in and of themselves, bear axial loads. They simply apply compression via a tension band. Therefore, if axial load–bearing ability is inadequate, it must be restored (Fig. 17.9b).