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Posterior Thoracic and Lumbar Universal Spine Instrumentation
Summary of Key Points
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Dorsally applied universal spinal instrumentation (USI) systems increases the rate of bony fusion after surgery for traumatic, neoplastic, or degenerative conditions of the spine.
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Various biomechanical forces and properties imparted by thoracic and lumbar spinal implants are fundamental aspects of USI systems.
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Multiple systems are now available that differ in coupling mechanisms, metallurgic composition, screw and hook design, and even placement technique.
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The surgeon must be wary of potential complications of hook fixation, as well as transpedicular fixation.
Universal spinal instrumentation (USI) systems have been used for treating traumatic, neoplastic, congenital, and degenerative disorders of the thoracic and lumbar spine for many years. The term universal refers to the versatility of these spinal systems with regard to fixation techniques and the ability to span the extent of the spine. In this chapter we will discuss the history of posterior thoracic instrumentation and the biomechanical principles of constructs achieved using these systems.
History
Reports of wire and screw fixation of the thoracic and lumbar spine appeared in the medical literature in the late 1800s. In 1891, Hadra described a procedure performed in 1887 in which Wilkins attempted fixation of T12‒L1 in a neonate using silver wire. Lange 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. Harrington used a combination of hooks and rods to correct deformities in scoliosis. Instrumentation facilitates deformity correction through application of various forces upon the spine. Harrington’s system was designed upon these principles and was the first that allowed for significant correction of spinal deformity and rigid fixation of the diseased spine. ,
In the early 1970s, Luque 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. 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 screws were first described by Boucher in the 1950s and made popular by Roy-Camille et al. later in the 1960s and 1970s. These constructs also use rods, plates, or fixators as longitudinal members. Pedicle screw fixation allows for the creation of rigid constructs. This rigidity facilitated the development of short-segment fixation techniques. Because of the strength and the geometry of the systems, it is possible to allow for greater preservation of segmental motion at adjacent segments that would have been incorporated into constructs using less rigid instrumentation.
Cotrel et al. 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 either the pedicle, lamina, or transverse process, allowing significant flexibility in bony purchase. These constructs were able to be applied throughout the thoracic and lumbar spine and were able to apply a variety of forces (compression, distraction, three-point bending) for the treatment of traumatic, degenerative, congenital, neoplastic, and infectious diseases.
Although early authors proposed spinal instrumentation to be an effective means for treating spinal instability and deformity, the biomechanical forces behind Cotrel’s techniques were not well understood until decades later.
Surgical Indications
The indications for thoracic and lumbar dorsal instrumentation are evolving. Zdeblick, Mardjetko et al., and others have demonstrated that instrumentation improves the rate of fusion in traumatic and degenerative conditions. In the setting of trauma, in addition to facilitating spinal fusion and healing, the stabilizing effect of dorsal universal instrumentation allows for earlier mobilization of patients with spinal instability. Although no benefit regarding neurological 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. ,
Several current models and point systems are available to determine acute traumatic instability. , The three-column model described by Denis has guided surgeons for decades, since its initial publication in 1983. Currently, a number of classification and point systems exist , ; particularly, the Thoracolumbar Injury Classification and Severity (TLICS) scale has emerged as a useful tool for surgical intervention for thoracolumbar injuries based on initial neurological status, spinal stability, and disruption of the posterior ligamentous elements as established from plain films or computed tomography (CT). Controversy still exists for the treatment of thoracolumbar burst fractures without neurological compromise. Additionally, subacute and glacial instability may be objectively demonstrated with serial and dynamic radiographs that may not be reflected by the TLICS scoring system. Therefore, although the TLICS score is a very useful tool, the decision to operate should not be based on these three factors alone.
The most common current use for thoracolumbar universal instrumentation systems is in the setting of degenerative spinal stenosis with evidence of spondylolisthesis and instability. The role of fusion and instrumentation is for the treatment of “dysfunctional segmental motion,” defined as instability related to disc interspace and vertebral body degenerative changes. , For axial low back pain alone, fusion shows some benefit over standard conservative management, but may be no better than specialized intensive rehabilitation (if available) for improvement in function. Some authors have advocated for the use of dynamic stabilization with polyaxial pedicle screws and elastic rod systems for patients with axial pain, with limited evidence of efficacy. The decision to use any of these constructs for the treatment of back pain without clear radiographic evidence of instability is based solely on the clinical judgment of the surgeon. , , Wide variation in surgical opinion as a result of the lack of consensus regarding the indications for surgery has created a perception of overuse of instrumentation in many circles.
Other common applications of thoracolumbar universal instrumentation systems include deformity correction in patients with idiopathic scoliosis, where surgical intervention is typically indicated in patients with severely restricted lifestyles and/or poor quality of life who have failed conservative therapies. The most common complaints are related to spinal imbalance and disabling back or leg pain. The goal of surgical intervention in these cases is to decompress the spinal canal to stabilize spinal motion segments and correct sagittal/coronal imbalances to ameliorate gait and back pain syndromes. Likewise, cases of infection or a neoplastic lesion in which surgery would cause anterior/middle/posterior column instability may be treated with thoracic instrumentation.
Biomechanical Forces Imparted by Thoracic and Lumbar Spinal Implants
Giovanni Borelli is considered by some to be the founding father of spine biomechanics because of his descriptions from the 1700s; however, modern understandings of spinal biomechanics were popularized by Francis Denis. In his 1983 paper, Denis described the three-column model for spinal fractures. These theories sparked interest in spinal biomechanics and have led to more sophisticated and refined treatment modalities.
USI techniques use the principles of spine biomechanics to treat a broad range of spinal pathology. These systems can be used for degenerative, traumatic, infectious, congenital, or neoplastic causes of spinal destabilization. The theory behind using USI in spine surgery is to provide stability over a spinal pathology through load-sharing and force dispersion, thus facilitating recovery and preventing further injury.
The human spine is subjected daily to a variety of stresses. Upright posture necessitates significant loadbearing 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, resists these forces in such a manner as to avoid neural injury and deformation. When supraphysiological 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. , In the normal spine, the IAR is located in the region of the dorsal aspect of the vertebral body (middle column of Denis ). 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 moment ( Fig. 121.1 ).
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). ,
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. 121.2 ).
Recently, interspinous spacers have been introduced, either as stand-alone devices or in conjunction with conventional decompression procedures, for the treatment of lumbar stenosis with or without spinal deformity. Interspinous spacers are devices that distract between the spinous processes of the lumbar spine to open up the intervertebral foramen. Interspinous spacers have been found in biomechanical studies to decrease the range of flexion and extension movement, increase the dimensions of the neural foramen and spinal canal, and decrease the axial loading forces on the facets. These devices have been shown to be more effective than nonsurgical options in the lumbar stenosis population. The most commonly cited study in recent times is the Coflex study. The initial study, published by Davis et al. in 2013, compared decompression and Coflex interlaminar device and decompression and posterolateral fusion for spondylolisthesis. Such a study with an interlaminar device is almost irrelevant, as decompression alone for grade 1 nonmobile spondylolithesis is very much an acceptable approach. More recently, Schmidt et al. performed a randomized trial that compared decompression and Coflex versus decompression alone for symptomatic lumbar stenosis. Unfortunately, this industry-sponsored study is fraught with biased data, such as the exclusion of reoperation rates. Furthermore, the results of non–industry-funded studies are mixed and suggest potential complications such as postoperative kyphosis, which has generally been considered to be a contributor to failed back surgery syndrome and flatback syndrome. ,
Tension Band Fixation
Dorsal compression fixation, applied with hooks, cables, or pedicle screws, is used for tension band fixation in the case of posterior 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 because it redirects certain axial loading forces upon the anterior and middle column. Tension band fixation may be performed in a short-segment procedure because the applied bending moment is not dependent on construct length, but rather on the distance from the IAR ( Fig. 121.3 ). 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 and construct failure.
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 pathological 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. 121.4 ).
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 to provide adequate stability and prevent construct failure. 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, thereby reducing screw pullout.
Hook constructs are ideally designed to resist pullout forces and therefore are often used as augmentation devices at the termini of three-point bending constructs. Sublaminar instrumentation placement carries a risk of injury to the neural elements; however, 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 pullout resistance and also contributes to load-sharing (see later discussion), and is useful 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, because greater stresses may be borne at the hook-hook-lamina junction.
Cantilever Beam Constructs
The final mode of application of dorsal universal instrumentation systems is cantilever beam fixation. A cantilever is any rigid projecting structure that provides support at one end while being fixed at the other. These structures can be beams, rods, screws, or any combination that form a structural framework and are used to carry a load. In spine surgery this load refers to forces applied to screws that are rigidly fixed to a longitudinal rod or plate.
Cantilever fixation systems are typically broken down into three types: fixed moment, non–fixed moment, and applied moment arms. Each of these types presents a different biomechanical advantage and drawback; the indications for each modality depend on the pathology, as well as patient comorbidities.
The great majority of constructs applied to the thoracic and lumbar spine are fixed moment arm cantilever beams ( Fig. 121.5A ). 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 loadbearing (when placed in a neutral or distractive mode) or load-sharing (when placed in a compressive mode in conjunction with adequate ventral support).
An illustrative case for use of fixed moment constructs would be utilizing pedicle screws to stabilize the lumbar spine after a laminectomy and interbody fusion at the L5‒S1 level for spinal stenosis and spondylolisthesis. The pedicle screws are rigidly connected to longitudinal elements, facilitating load-sharing between the construct and the interbody graft.
Nonfixed moment arm techniques are similar to fixed moment arm systems; however, biomechanically they lack an immobile longitudinal connection, which leads to ineffective load bearing transmission by the construct. Nonfixed moment arms do not effectively support axial loading forces, and necessitate additional support systems such as additional instrumentation or grafts for load-sharing and resistance to translation along a spinal pathology.
Nonfixed moment cantilever beam constructs traditionally have rarely been 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 pullout) (see Fig. 121.5B ). 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. Because of 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. 121.6 ). These systems allow for resistance to translation at C1‒C2 and seamless combination with thoracolumbar USI systems.
An illustrative case would be using a dynamic anterior plate system in conjunction with a ventral bone graft to stabilize the spine after an anterior cervical discectomy or corpectomy. In these cases, the use of a non–fixed moment arm construct results in loadbearing by the bone graft. Again, this form of cantilever construct is rarely used in the thoracic or lumbar 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 (see Fig. 121.5C ). 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.
The most common clinical indication for applied moment arms is for spinal deformity correction. Various forces are applied after assembly of the construct to maintain spinal reduction. An illustrative case would be stabilization of the spinal elements using pedicle screws with a fixed moment arm construct after pedicle subtraction osteotomy in the lumbar spine.
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