History

Reports of wire and screw fixation of the thoracic and lumbar spine appeared in the medical literature in the late 1800s. In 1891, Hadra¹ described a procedure performed in 1887 in which Wilkins attempted fixation of T12-L1 in a neonate by using silver wire. Lange² contemporaneously described the (unsuccessful) use of nonfixed steel rods for the treatment of spinal deformity. Instrumentation of the thoracic and lumbar spine was restricted to wiring techniques and the occasional use of the facet screw until 1962, when Harrington introduced his spine instrumentation system. This system was the first that allowed for significant correction of spinal deformity and rigid fixation of the diseased spine.^3,4

In the early 1970s, Luque⁵ introduced segmental spinal instrumentation with sublaminar wires. The use of sublaminar wires provided multiple points of fixation, and when combined with closed loops instead of rods, or with the Harrington distraction system, provided significant resistance to flexion, extension, and lateral bending.⁶ Continued modification of the Luque and Harrington systems through the 1970s laid the groundwork for the introduction of universal instrumentation in the early 1980s.

Pedicle screw fixation devices were introduced by Roy-Camille⁷ in the 1980s. These devices use rods, plates, or fixators as longitudinal members. Pedicle screw fixation allows for the creation of rigid constructs. This rigidity has led to the advent of short-segment fixation. Because of the strength and the geometry of the systems, it is possible to allow for greater preservation of segmental motion at adjacent segments. Cotrel et al.⁸ introduced the first “universal” spine fixation system in the late 1980s. This system used pedicle screws as well as multiple hooks. The latter were specifically designed to engage the pedicle, lamina, or transverse process. This allowed the application of the device throughout the thoracic and lumbar spine. Furthermore, the use of a combination of components allowed for the application of a variety of forces (compression, distraction, three-point bending). This in turn allowed for the efficacious correction of spinal deformities.^4,6 Recently the advent of frameless stereotaxic techniques has led to an increased popularity in the use of thoracic pedicle screws. The use of thoracic pedicle screws allows for rigid fixation of the thoracic spine without the need for intracanalicular instrumentation.

Surgical Indications

The indications for thoracic and lumbar dorsal instrumentation are evolving. Zdeblick,⁹ Mardjetko et al.,¹⁰ and others have demonstrated that instrumentation improves the rate of fusion in traumatic and degenerative conditions. In addition to increasing fusion rates, the stabilizing effect of dorsal universal instrumentation allows for earlier mobilization of patients with traumatic or neoplastic spinal instability. Although no benefit regarding neurologic outcome has been demonstrated, the ability to allow patients to ambulate soon after injury or surgery substantially lowers morbidity and allows for a more rapid rehabilitation.^11,12 The most common current use for thoracolumbar universal instrumentation systems is in the setting of degenerative lumbosacral instability.

Several current models and point systems are available to determine acute traumatic instability.^13,14 Subacute and glacial instability may be objectively demonstrated with serial and dynamic radiographs. Regrettably, the great majority of patients with back pain do not exhibit such clear-cut indications for surgery. The role of fusion and instrumentation for the treatment of “dysfunctional motion segments” remains somewhat controversial.^15,16 For nonradicular low back pain, fusion shows some benefit over standard conservative management, but is no better than intensive rehabilitation for improvement in pain or function.¹⁷ The decision to use any of these systems for the treatment of back pain without clear radiographic evidence of instability is based solely on the clinical judgment of the surgeon.^6,18,19 Wide variation in surgical opinion due to the lack of consensus regarding indications for surgery has created a perception of overuse of instrumentation in many circles.²⁰

Biomechanical Forces Imparted by Thoracic and Lumbar Spinal Implants

The human spine is daily subjected to a variety of stresses. The upright posture necessitates significant load bearing by the thoracic and lumbar spine. In addition, normal activity results in flexion, extension, lateral bending, and axial rotation of the spine. Each of these maneuvers results in the application of forces to the spinal elements. The intact spine, to paraphrase White and Panjabi,¹⁴ resists these forces in such a manner as to avoid neural injury and deformation. When supraphysiologic forces are applied (e.g., in a motor vehicle accident), or when the integrity of the spinal elements is compromised (tumor or infection), deformation of the spine and possibly neural element damage results. A description of the forces acting on the spine is provided by clinical biomechanics. An understanding of these forces is helpful in planning corrective surgery.

Forces acting on the spine can be broken down into component vectors. A vector is a force that has both a magnitude and a fixed direction in three-dimensional space. A force vector may act directly on a point in space, causing translation (movement in the same plane as the vector). Alternatively, a force vector may act via a lever (moment arm), causing rotation about an axis. When a force vector acts via a moment arm, a bending moment is applied. The axis, or fulcrum, about which this bending moment causes rotation is termed the instantaneous axis of rotation (IAR). The IAR may be defined as the axis about which a given vertebral body rotates when acted on by a bending moment.^6,14 In the normal spine, the IAR is located in the region of the dorsal aspect of the vertebral body (middle column of Denis¹³). The bending moment (M) is defined as the product of the force (F) applied and the moment arm (D) or the perpendicular distance from the IAR (M = F × D). The neutral axis is defined as the longitudinal axis that encompasses the IAR of adjacent vertebral bodies. Forces transmitted along the neutral axis cause no significant bending moment¹⁴ (Fig. 150-1).

FIGURE 150-1 Biomechanical considerations. The forces acting on the thoracic and lumbar spine are depicted here as well as the effective kyphosis of the thoracic spine. An axial load (F) acts via a lever arm (D) to produce a bending moment (M) about the instantaneous axis of rotation (IAR). Forces that are transmitted within the neutral axis (dotted line) cause no bending moment.

(Copyright University of New Mexico, Division of Neurosurgery.)

Newton’s third law of motion, the law of conservation of momentum, states that interactions between objects result in no net change in momentum; thus for every action there is an equal (in magnitude) and opposite (in direction) reaction. In the present context, this implies that the spine (when at rest) exerts forces that are equal in magnitude and opposite in direction to the axial loads and bending moments applied. The ability of the normal spine to resist these forces depends on the material properties of the vertebral bodies and supporting bony, muscular, and ligamentous structures. When spinal instrumentation is applied, the construct may function simply as a replacement for a damaged spinal element (tension band fixation) or may apply forces to the spine in a relatively unusual and complex fashion (three-point bending).^6,14

Distraction

Dorsal distraction fixation, usually applied with sublaminar hooks, has been used for short-segment distraction for deformity correction and foraminal stenosis. This mode of application has not found widespread use historically, however, because of a tendency for exaggeration of kyphotic deformity (Fig. 150-2).⁶ Recently, Zucherman and others have introduced interspinous spacer devices that distract between the spinous processes of the lumbar spine.²¹ Although these devices relieve symptoms of lumbar stenosis compared with nonsurgical treatment, they do induce kyphosis, which has generally been considered to be a contributor to failed back surgery syndrome and flat back syndrome.^22,23

FIGURE 150-2 Dorsal distraction fixation may lead to kyphotic deformation, especially if used at or above the thoracolumbar junction. Use of distraction fixation in the lumbar spine may lead to a painful “flat back” syndrome.

(Copyright University of New Mexico, Division of Neurosurgery.)

Tension Band Fixation

Dorsal compression fixation, applied with hooks, cables, or pedicle screws, is used for tension band fixation in the case of dorsal ligamentous insufficiency. This technique depends on the integrity of the load-supporting capacity of the ventral elements as well as the preservation of the relevant dorsal bony elements. This type of fixation should never be applied without adequate ventral spinal canal decompression. Tension band fixation may be used as a short-segment fixator, since the applied bending moment is independent of construct length (Fig. 150-3). Tension band constructs do not, in general, resist translation and should not be relied on as stand-alone constructs when significant resistance to translation is required. When multiple segments are to be instrumented with this technique, multiple points of fixation should be used to prevent terminal bending moments.⁶

FIGURE 150-3 Tension band fixation of the dorsal spine is used most effectively when adequate ventral support is present. The bending moment applied is proportional to the distance between the implant and the instantaneous axis of rotation (d) and is independent of the length of the construct (L). Therefore, significant corrective forces may be applied with short-segment instrumentation.

(Copyright University of New Mexico, Division of Neurosurgery.)

Three-Point Bending

Three-point bending forces are applied when translational forces are applied at both ends of a construct that are equal in magnitude but opposite in direction to a translational force applied to the fulcrum of a pathologic curvature. These constructs are usually applied in a distraction or neutral mode. The prototypical three-point bending construct is the Harrington distraction rod, especially when augmented with sleeves. The application of three-point bending forces depends on the physical contact between the longitudinal member and the fulcrum of the kyphotic deformity. These constructs, when placed dorsally, result in a dorsally directed force at both termini and a ventrally directed force at the fulcrum of the kyphotic curve (Fig. 150-4).⁶ Three-point bending constructs must, by definition, traverse at least three spinal segments. Because the bending moments applied by a three-point bending implant are proportional to the length of the construct, multiple-segment instrumentation is frequently used to correct significant deformity. Because of the strong dorsally directed forces at the termini of the construct, three-point bending constructs are best applied by using multiple points of fixation. This maximizes the area of contact between the implant and bone. Laminar hooks are ideally designed to resist pull-out forces; however, sublaminar instrumentation placement carries a risk of injury to the neural elements. Pedicle hooks, transverse process hooks, and hook-screw combinations may be used in many cases to avoid sublaminar placement of hooks. A pedicle screw-hook construct at the same level provides substantial pull-out resistance and also contributes to load sharing (see later discussion) and is favored in many cases of significant instability. USI systems allow for the application of these constructs in a neutral mode by using hook “claws,” which are able to engage the lamina without the significant distractive forces required by the Harrington rod system. Use of the claw technique allows for shorter segment fixation, since greater stresses may be borne at the hook-hook-lamina junction.⁶

FIGURE 150-4 A and B, Three-point bending constructs rely on contact between the implant and the apex of a kyphotic curvature to produce forces perpendicular to the long axis of the spine. Forces are directed dorsally at the termini and ventrally in the center of the construct. These forces are equal in magnitude but opposite in direction. In the case of three-point bending implants, the bending moment (M_3PB) applied is directly proportional to the length of the construct (L). Therefore, since there is a limit to the dorsally directed forces that will be tolerated at the terminal implant-bone junctions, longer constructs are generally required for significant deformity correction. Alternatively, increasing the ability of the terminal interfaces to resist dorsally directed forces (e.g., using a claw configuration) allows for the application of greater corrective forces with shorter segment instrumentation.

(Copyright University of New Mexico, Division of Neurosurgery.)

Cantilever Beam Constructs

The final mode of application of dorsal universal instrumentation systems is cantilever beam fixation. A cantilever beam is simply a beam supported at one end, such as a balcony or awning support. These constructs are applied by using pedicle screws as the beams. Cantilever beams may be applied in one of three fashions. The great majority that are applied to the thoracic and lumbar spine are fixed-moment arm cantilever beams (Fig. 150-5A). A fixed-moment arm cantilever beam is one in which the pedicle screw is rigidly affixed to the longitudinal member. This type of construct allows for load bearing (when placed in a neutral or distractive mode) or load sharing (when placed in a compressive mode in conjunction with adequate ventral support).

FIGURE 150-5 There are three types of cantilever-beam constructs. A, Fixed-moment arm cantilever-beam constructs employ constrained linkages between pedicle screws and longitudinal members and allow significant load bearing by the implant. B, Nonfixed-moment arm cantilever-beam constructs do not allow load bearing and function poorly as tension band constructs because of problems related to toggling and screw pull-out. Currently, these types of constructs are rarely used in the thoracic or lumbar spine. C, Applied-moment arm cantilever-beam constructs allow for the application of significant forces to the lumbar spine through the use of long screws. Because of the forces involved, screw breakage is likely to occur in the absence of strong bony fusion.

(Copyright University of New Mexico, Division of Neurosurgery.)

Nonfixed cantilever beam constructs, in which the pedicle screw is not rigidly affixed to the longitudinal member, are rarely used in the thoracic and lumbar spine because of their inability to bear loads (like a hinged awning) and their poor performance as tension band constructs (caused by screw toggling and pull-out) (Fig. 150-5B). In the cervical spine, older lateral mass plate-screw systems are commonly applied nonfixed-moment arm cantilever beam constructs that work well. These systems take advantage of the anatomy of the cervical facet, which tends to resist translation. Due to recent tendencies to combine cervical and thoracic instrumentation systems and the extension of fixation techniques to the occipitocervical and atlantoaxial joints, fixed-moment arm cantilever beam systems have been developed for application in the cervical spine (Fig. 150-6). These systems allow for resistance to translation at C1-2 and seamless combination with thoracolumbar USI systems.²⁴

FIGURE 150-6 Combination of cervical and thoracic instrumentation systems with extension of fixation to the occipitocervical junction.

(Courtesy of Stryker Spine.)

The final cantilever beam construct is the applied-moment arm cantilever beam. This type of construct allows for the application of flexion or extension forces at the time of implant placement. Using long screws (Schanz type), a bending moment is applied to the spine. Once the desired corrective forces have been applied, the implant is fixed in place (Fig. 150-5C).⁶ The application of these forces places great stress on the implant, which may result in failure of the implant, particularly if osseous union does not occur in a timely fashion.

Biomechanical Properties of Universal Spinal Implant Systems

All universal spinal implant systems consist of screws, hooks, and longitudinal members. The composition, shape, and size of the implants vary to some extent. However, all conform to the constraints placed on them by the anatomic configuration of the bony spine. Some of the basic properties of the components and the effect that changes in these basic properties (e.g., the particular alloy of stainless steel used in a longitudinal member or the profile of the minor diameter of a screw) have on the performance of a given system are discussed in the following sections.