Bony Anatomy

The thoracolumbar spine is divided into five regions: the cervicothoracic junction, the mid-thoracic spine, the thoracolumbar junction, the mid-lumbar spine, and the lumbosacral junction. Each region has distinct anatomic and biomechanical characteristics that must be considered when planning reconstructive and instrumentation surgery. The anatomy typically serves the biomechanics. For example, facet orientation predicts motion segment direction and range of motion. Facet orientation also “couples” motion so that flexion necessitates translation.⁷

Placement of a thoracolumbar implant affects the spinal ligaments directly or indirectly. Direct anterior lumbar interbody approaches require sacrifice of the anterior longitudinal ligament. In disc replacement procedures, to achieve more parallel distraction, the posterior longitudinal ligament may need to be resected. The long-term effect of such ligamentous resection on motion preservation kinematics is unknown.⁸

The thoracic spine can be divided into two subregions: the upper thoracic spine to T8 and lower thoracic spine (T8-thoracolumbar junction). The articulation of the thoracic spine with the rib cage makes the thoracic spine the most rigid portion of the spine. Extending the three-column concept of Denis, an intact rib cage and sternum complex functions as a mechanical fourth column, preventing flexion and extension above T9.⁹

The upper thoracic spine allows significant segmental rotation (10 degrees). Below T8-9, the major plane of motion is flexion and extension.⁹ Before proceeding with a transthoracic approach and spinal reconstruction, the surgeon must be familiar with the articulations between the rib and vertebral body. There are two sets of demifacets: one at the disc level and the other at the transverse process. Radiate ligaments stabilize the articulation further. The rib attaches to the transverse process and the superior aspect of the same-number vertebra (at the level of the pedicle). For example, to reach the T9-10 disc, one can follow the T10 rib to the superior aspect of the T10 body.

The rib–pedicle–transverse process junction is critical in posterior approaches as well. Because the pedicles of the mid-thoracic spine are quite narrow, some authors have recommended an in-out-in approach for pedicle screw insertion. With this technique, the pedicle screw trajectory begins dorsally, but as the pedicle narrows, the screw passes laterally into the space between the rib and the pedicle. In this space, it contains only ligamentous tissues, and penetration does not jeopardize neurologic structures or the lung parenchyma.¹⁰

The thoracolumbar junction represents a straight segment between the lordosis of the lumbar spine and the kyphosis of the thoracic spine. This is a mechanical transition zone from the more rigid thoracic spine and the more mobile lumbar spine. Thoracolumbar fractures most commonly occur in this transition zone of the spine.

The lumbar spine permits significant flexion and extension across all levels. There is a sharp increase in the amount of lateral bending exhibited at L3-4. There is less lateral bending at L2-3 and L4-5. In the lumbar spine, axial rotation is limited by the vertical orientation of the facets.

Thoracolumbar bone and ligaments provide the framework for the development of fixation strategies. To obtain successful spinal fixation, a stable bone-implant interface is required. Given the proximity of neural elements, only certain bone surfaces can be used for spinal instrumentation. In addition, the size and shape of the bony anatomy influence what type of implant can be used and the fixation strength of the implant. Larger pedicles are able to accommodate larger screws for fixation, which increase the stability of fixation and increase holding strength.

There is significant anatomic variation among adults. Using computerized imaging systems, it is possible to use axial and sagittal images for preoperative planning. The maximum length and appropriate screw diameter can be employed.

Neurovascular Anatomy

A primary goal of thoracolumbar instrumentation is to prevent neurologic injury and protect the neural elements. It is crucial to understand the anatomy. The spinal cord typically ends at the L1-2 disc space. Below the conus, the nerve roots pass from the central thecal sac through the neuroforamen into the pelvis. Several cadaveric and imaging studies have described the relationship of the bony elements and their proximity to the neural structures. One cadaveric study measured the average distance from lumbar pedicle to the dural sac medially. From cranial to caudal in the lumbar canal, the range was 1.29 to 1.56 mm; clinically, this means that a medical pedicle breach greater than 1.29 mm has a significant chance of contacting or injuring the dura (Fig. 71–1).The pedicle is farther from the superior nerve root at 4.12 to 5.52 mm but closer to the inferior root, where distances ranged from 1.10 to 1.06 mm. The nerve roots and dura are statistically further from the L5 pedicle than from other pedicles, making the L5 pedicle safer than other lumbar pedicles for screw insertion.

FIGURE 71–1 These images show the importance of good exposure. A, Axial CT scan of a patient with marked facet hypertrophy. In most cases, complete takedown of the soft tissue around the lateral pars and medial transverse process provides enough anatomic clues for appropriate positioning of the pedicle probe. In cases with marked facet hypertrophy such as this one, resection of the facet osteophytes significantly improves identification of normal anatomic landmarks, improving pedicle screw placement accuracy. B, Anteriorly, the soft tissues must be reflected posteriorly enough to palpate the posterior vertebral body margin. Then the surgeon has a markedly improved three-dimensional sense of the position of the spine in space and, it is hoped, can avoid inadvertent penetration of the spinal canal with a screw. Proper positioning of the patient is critical. If the patient is rotated, the surgeon’s orientation to the spine may be confused. That may have contributed to the misplacement of the screws in B and C. Adequate exposure also requires adequate soft tissue release. Particularly in patients with a narrow pelvis, if the thoracodorsal fascia remains tight, the surgeon would have to struggle to achieve proper medialization of the screws. D, Postoperative axial CT scan shows S1 screws. The left-sided screw is in acceptable position, whereas the trajectory of the right-sided screw is not medialized enough. The screw penetrates the anterior sacrum and impinges on the L5 root anteriorly.

It is important to understand the relationship of the pedicle and the exiting nerve root. The cervical spine has eight nerve roots. The corresponding nerve root exits the neuroforamen above the lowest numbered pedicle (i.e., the C6 nerve root exits at the C5-6 level). This numbering changes in the thoracic spine and lumbar spine; for example, at the T11-12 level, the T11 root exits beneath the 11th pedicle. After the L5 nerve root exits the neuroforamen, it travels anterior to the sacral ala. An S1 pedicle screw that is placed too far lateral or is too long places the L5 nerve root at risk for injury. Knowledge of the intimate relationship of the major vessels to the spine is crucial to avoid life-threatening complications during anterior and posterior thoracolumbar instrumentation.

When approaching the upper thoracic spine anteriorly, the approach is typically from the right to avoid the arch of the aorta. Scoliotic curves are typically approached from the convexity of curve, allowing a more complete release. The lumbar spine is typically approached from the left for several reasons. First, the liver is on the right and is more difficult to mobilize. Second, a left-sided approach brings the surgeon into contact with the aorta before the vena cava. The aorta is more easily recognized and is more durable, so the risk of sudden, catastrophic blood loss is less in the event of a vascular injury. With the advent of lumbar disc arthroplasty and anterior lumbar interbody fusion (ALIF), which require an anterior approach, the aorta, inferior vena cava (IVC), and iliac vessels are at risk for potential injury. Magnetic resonance imaging (MRI) is vital for preoperative evaluation of the vascular structures and for detecting potential vascular anomalies. Exposure of the L4-5 disc space requires retraction of the aorta and IVC from left to right. To mobilize these structures, the iliolumbar vein must be identified and ligated. When exposing the L5-S1 disc space, the middle sacral artery and vein must be ligated, and the left common iliac vein is elevated superiorly and laterally. When considering exposure to the lower lumbar spine, vascular calcifications in an older patient may limit retraction of the vessels and may be a contraindication to anterior surgery. In addition, any patient with previous abdominal or hernia surgery must be carefully evaluated because scar tissue may be a contraindication to abdominal surgery (Fig. 71–2).

FIGURE 71–2 Preoperative planning is crucial when selecting the implants or the angle of the approach to the spine. In certain cases, one implant may suit a patient better than another. A and B, Cross-sectional MRI of L4-5 disc space. Note the right iliac vessels are more lateral than expected. This type of anatomy may preclude the use of a transpsoas approach.

When performing posterior thoracolumbar surgery, it is crucial to be cognizant of the anterior vascular anatomy. The heart-shaped thoracic vertebral bodies can allow passage of thoracic pedicle screws out of their respective bodies and place the aorta at risk for injury. One study measured the average distance from the aortic wall to the vertebral body cortex at the apex of the curve by MRI and found it was greater in patients with scoliosis (4.0 mm) than in a normal group (2.5 mm) (P < .05). The distance from the posterior aspect of the aorta to the anterior aspect of the spinal canal was less in the scoliosis group (11.1 mm) than in the normal group (19.2 mm) at T5-12 (P < .05). The aorta was positioned more laterally and posteriorly at the T5-12 vertebral bodies in these patients.¹¹ When placing pedicle screws, preoperative planning is essential to determine the appropriate screw length to minimize the risk of vascular injury. Typically, pedicle screws on the left side place the aorta at more risk of injury, and slightly shorter screws should be placed.

With the development of transforaminal lumbar interbody fusion procedures, there is risk of vascular injury. If there is violation of the anterior anulus with overzealous discectomy, the aorta, IVC, or iliac vessels are at risk. Papadoluas and colleagues reported that the mortality from a vascular injury during lumbar surgery can be 10%. At the L3-4 level, the aorta is more at risk than the IVC and common iliac vein. At L4-5 and L5-S1 levels, the common iliac artery is more at risk than the common iliac vein, IVC, and aorta.

Important Factors in the Selection of Lumbar Implants

The following are important factors to keep in mind when selecting lumbar implants:

Basic Biomechanical Principles

The ability of instrumentation to stabilize the spine adequately is a function of failure load, which represents the mechanical load at which the implant failure occurs, and construct stiffness. Stiffness represents the ability of the spine-instrumentation construct to resist applied forces, such as axial compression. The spine and instrumentation constructs are subjected to linear and circular forces. Linear forces are applied along the line of action. Circular or moment forces occur at any time that force is applied to a point not located along the line of action. Torque represents the magnitude of the applied force times its perpendicular distance from the axis of rotation. Also, forces may be isolated or coupled; coupling refers to two noncolinear forces acting around the same axis.

Stress reflects the cross-sectional area over which the force or load is applied. Host tissue and spinal implants respond to force application by deformation (a change in shape and size). Deformation is predictable, based on the shape, size, and intrinsic material properties of the involved structure. Strain is defined as deformation divided by original length. The stiffness of a structure reflects strain relative to the stress applied. Because strain incorporates the cross-sectional area of the material, cross-sectional area has a marked impact on stiffness. A rod that is half the diameter of another rod made of the same material would have only one quarter the stiffness of the other rod. When a load is removed from an object, elastic deformation occurs if the material recovers its normal shape. Plastic deformation occurs when the load permanently deforms the material; in other words, stress is no longer proportional to strain. The material’s yield point is the load beyond which it can no longer regain its normal shape (i.e., elastic deformation changes to plastic).

A ductile material allows permanent deformation before failure. Ductile materials include metals such as steel and titanium. Brittle materials, such as adult cortical bone and polymethyl methacrylate (PMMA), can fail without deformation. Another mechanical concept that relates to the effect of degenerative change on segmental motion is the neutral zone. Within the neutral zone, the spine offers minimal resistance to motion. Minor changes in load can lead to major shifts in position; for example, a patient with disc damage may exhibit an increase in the neutral zone, allowing motion to occur beyond the pain-free zone under physiologic loads while showing no change in the spine’s overall range of motion. Operative stabilization may decrease pain by decreasing the neutral zone but typically also decreases ultimate segmental range of motion.^12,13

Implant Materials and Corrosion

Most posterior thoracolumbar fixation systems are made of stainless steel, pure titanium, or, most commonly, titanium-aluminum-vanadium alloy. Cages and spacers used anteriorly may also be made of other materials, such as carbon fiber or polyetheretherketone (PEEK). Machined allograft that is shaped like a cage or spacer may be used to facilitate anterior fusion. Surgical stainless steel is very strong, with yield strength of 700 MPa. It is also stiff, having a modulus of elasticity 12 times that of normal bone. Steel implants usually have cobalt-chromium alloy and molybdenum to enhance corrosion resistance. Titanium alloys tend to have greater native biocompatibility and corrosion resistance. Titanium has a modulus of elasticity only six times greater than bone, which makes it easier to bend and insert than steel. These characteristics allow titanium-based devices to transfer load effectively to the vertebral body, shielding the implant from stress. A reduction in stress shielding allows bone grafts to respond to more load appropriately. Use of titanium alloys is increasing because of its improved imaging characteristics, high strength-to-weight ratio, enhanced ductility, and increased fatigue life.

Two types of titanium are typically used. Pure titanium is recommended only when very low strength is needed because it has a low yield strength of only 170 to 485 MPa. More typically, a titanium-aluminum-vanadium alloy, with yield strength of 800 MPa, is employed; its greater strength does not change the favorable modulus of pure titanium (110 GPa).¹⁴ Ultimate tensile strength—the maximum stress a material can sustain without changing shape—may be altered during surgery. Titanium rods are particularly sensitive to notching.¹⁵ If a complex rod contour is required, a template should be employed to minimize the amount of rod bending. In some patients with rigid dual curve deformities, it may be more effective to use separate rods in the thoracic and lumbar spine, employing rod-to-rod connectors (“dominoes”) to complete the construct.

In a biologic environment, fretting and corrosion can occur between the modular components of a spine construct. Kirkpatrick and colleagues subjected 48 spinal implant constructs to surface analysis stereomicroscopy.¹⁶ Titanium alloy implants (n = 25) showed no significant corrosion, but three of the constructs showed fatigue failure of the anchoring screws. The cobalt alloy construct showed no evidence of corrosion. Semirigid stainless steel implants had mild surface alteration, whereas rigid constructs showed moderate to severe corrosion. Based on their findings, the authors recommended avoiding rigid stainless steel implants or constructs with different surface finishes between rods and connectors. The surgeon must use caution when combining implants made of different metals. Mixing stainless steel with titanium would lead to a galvanic response and early corrosion, although titanium has been used with cobalt-chromium alloy without significant corrosion or complications.

Increasingly, nonmetallic implant materials have been used in thoracolumbar implants. Typically, these materials are used as cages, spacers, and graft containment systems rather than as fixation systems. Advantages of radiolucent materials such as PEEK include easier radiographic assessment of graft integration. For some nonmetallic implants, the modulus of elasticity is closer to that of host bone, allowing greater load sharing. Mechanical testing showed acceptable mechanical and fatigue characteristics for PEEK as a load-bearing implant material.¹⁶

The material properties of implants are also affected by manufacturing variables, such as drill holes, structural imperfections, and surface irregularities. Implant fatigue is an important cause of failure. The average spine cycles 3 million times per year.^17–19 Because current implants are overengineered for their designated function, implant failure is more likely to occur from improper selection than from mechanical properties. If bone healing is delayed or incomplete, the implant or construct ultimately fails, so meticulous attention to bone grafting technique is imperative.

Fusion

Two important goals in designing an appropriate spinal construct are to provide spinal stability and to facilitate healing. The trend away from autogenous graft to allograft generally lengthens the duration of bone healing. Numerous animal models have shown that instrumentation increases the rate of fusion maturation.²⁰ A delay in fusion may increase the risk of implant failure or the propensity for collapse of structural grafts.²¹ Use of bone morphogenetic protein generally accelerates healing and may permit use of less fatigue-resistant constructs.

Many postoperative factors affect healing times, including motion, loading, muscle status, medical comorbidities, nutritional status, and tobacco use. The type of fusion performed and the type of graft employed also have significant implications on the likelihood and rate of healing. For fusions expected to heal slowly, more robust forms of instrumentation, possibly with additional anchor points, should be employed. For example, lower fusion rates in smokers may justify instrumentation in settings in which in situ fusion would otherwise be appropriate.

When clinical or mechanical circumstances increase the risk of pseudarthrosis, additional steps, such as addition of L5-S1 interbody fusion below a long posterolateral fusion, should be considered.²² Along the same lines, the anticipation of increased postoperative loading, poor patient compliance, or inadequate postoperative immobilization may warrant more rigid forms of operative stabilization. Examples include patients with neurologic or motion disorders who are subject to increased spinal loads and patients with spinal cord injury or a colostomy for whom brace immobilization is impractical.

Historically, the most common fusion technique was the posterior fusion. The primary advantage of this approach was easy surgical access to the midline posterior elements (spinous processes and lamina). Disadvantages included its limited utility in laminectomy patients. Also, the graft material lies distant from the center of rotation and experiences significant tensile forces with spine flexion. This distance increases tensile stress and motion on the graft that could lead to migration, excessive motion, or graft resorption and ultimate nonunion.²³ The most common contemporary fusion procedure is the intertransverse (posterolateral) fusion in which the facet joints, lateral pars, and transverse processes are decorticated and grafted, leaving the lamina accessible for decompression. In an intertransverse fusion, the graft material is placed closer to the center of vertebral rotation.²⁴ The disadvantage is a poor vascular bed and a decreased area for fusion.

Interbody fusion provides significant mechanical advantages in terms of graft compression and a large, well-vascularized fusion surface. The anterior column fusion spans the neutral zone and, when healed, represents the strongest mechanical block to segmental motion.^23,25 Even a solid intertransverse fusion may fracture or elongate if excessive or repeated load is placed across the motion segment.²⁴

Thoracolumbar implants share applied loads with the spine until a stable fusion occurs. If a construct bears most of the load, stress shielding of the spine results and may lead to device-related osteopenia.²⁶ The clinical sequelae of this shielding include graft resorption and possible implant failure. Increased emphasis has been placed on load-sharing implants in recent years.

Implications of Osteoporosis

Osteoporosis is the most common metabolic bone disorder and results from loss of the crystalline (inorganic) and collagenous (organic) portions of bone. Throughout life, the body constantly remodels bone by removing old bone and creating new bone. Although the pathomechanics are incompletely understood, osteoporosis occurs when the rate of bone resorption exceeds the rate of bone formation. Lower rates of bone formation result in a decline in overall mineral density of bone. Unbalanced osteoclast activity results in disruption of the normal connectivity between bony trabeculae. Bone can be weakened in the material and architectural sense.²⁷ There are an estimated 40 million people at risk for osteoporosis in North America. With the aging of the population, this estimated number is likely to triple over the next 3 decades. Management of spinal disorders in an osteoporotic spine will be a significant problem in the future.²⁸ Thoracolumbar instrumentation options in patients with osteoporosis are limited because implant failure in osteoporotic bone is common. Even if healing occurs uneventfully, patients with osteoporosis are at risk for compression fractures and spondylolisthesis adjacent to rigid constructs.^29,30

For many patients, the diagnosis of osteoporosis and initiation of appropriate management is delayed. Because the consequences of failure to recognize osteoporosis are so high, it is incumbent on the spine surgeon to screen at-risk patients.³¹ There are three main types of osteoporosis: type I (postmenopausal), type II (senile), and type III (secondary). Type I affects the trabecular bone of women, more than men, in their 6th and 7th decades. Type II osteoporosis arises in the 8th and 9th decades and increasingly affects the cortical bone of men and women equally. Although categorizing a thin, elderly, white, or Asian woman at risk for osteoporosis may be relatively straightforward, younger and larger patients are increasingly at risk for secondary (type III) osteoporosis. Excessive endogenous or exogenous cortisol is deleterious to bone mass and is a cause of secondary osteoporosis. Long-term use of thyroid replacement drugs, blood thinners, and various seizure medications may also result in osteoporosis, and patients taking these medications should undergo screening before major spinal reconstruction. At least 30% bone mass loss is needed to identify osteopenia reliably on plain radiographs. Dual-energy x-ray absorptiometry (DEXA) is a much better screening tool than plain radiography.³² Results of DEXA scans are given in T and Z scores. The T score compares the patient’s bone mineral density (BMD) with mean values for healthy, same-gender young adults. For each standard deviation below the norm, fracture risk increases 1.5-fold to 3-fold. A T score of −1 implies a 30% chance of fracture. As the T score decreases from −1 to −2, the risk of instrumentation failure increases significantly. The Z score compares BMD with age-matched controls. A Z score less than −1.5 warrants a more extensive workup for osteomalacia or neoplasm. DEXA values are falsely increased with scoliosis, compression fractures, spondylosis, extraosseous calcification, and vascular disease. In many spine patients, the T score at the hip may be more accurate than the spine value.

Depending on the patient’s activity level, the nature of the intended surgery, and the severity of the osteoporosis, preoperative initiation of antiosteoporotic management and delay of elective spine procedures until follow-up DEXA scores improve may be warranted.³³ Calcitonin, via subcutaneous injection or nasal spray, decreases osteoclastic bone resorption. Over the short-term, calcitonin also enhances bone formation, leading to a slight net bone accretion. Over the long-term, osteoblastic activity slows, however, and bone mass stabilizes. That is, after several years, calcitonin is no longer effective.³⁴ Bisphosphonates dramatically suppress bone resorption and decrease hip and spine fractures. These agents directly stabilize the bone crystal, making it more resistant to osteoclastic bone resorption. They also inhibit osteoclast activity. Bisphosphonates preserve bone architecture and overall density. Weekly and monthly dosing of these agents improves compliance with no increase in toxicity. Most patients with osteoporosis should be receiving a bisphosphonate. For patients who cannot tolerate or have not responded to bisphosphonates and for patients with severe osteoporosis and major spinal instability, more aggressive antiosteoporotic management in the form of pulsed parathyroid hormone administration should be considered. Parathyroid hormone (Fortéo) is anabolic to bone and leads to early, dramatic increases in bone mass.³⁵

When planning a spinal reconstruction procedure in an at-risk patient, it is important to recognize areas of the spine that are vulnerable to the ravages of osteoporosis. Trabecular bone represents 20% of the total bone mass and is found in the metaphyses and epiphyses of long bones and in the cuboid bones (including the vertebrae). Because trabecular bone exhibits eight times greater metabolic activity than cortical bone, the mechanical impact of osteoporosis affects trabecular bone earlier and to a greater degree than cortical bone (Fig. 71–3).^27,36 It was reported in a more recent article that threshold BMD for successful use of anterior spinal instrumentation was 0.22 g/cm (as measured by quantitative computed tomography [CT]).³⁷ Often, combined anterior and posterior surgeries with multiple, additional fixation points are required to achieve adequate fixation.^37,38

FIGURE 71–3 This woman with unrecognized osteoporosis and a fracture of L1 was initially treated with a thoracolumbosacral orthosis. She presented with acute lower extremity weakness and urinary retention. She required urgent surgical intervention to restore her alignment and canal clearance. A, X-ray in brace showing fracture at L1. B, Axial CT scan of osteoporotic burst fracture. C, Sagittal CT scan showing retropulsed burst fracture fragment in canal. D, Postoperative lateral and anteroposterior radiographs of burst fracture reduced with posterior instrumentation.

Other strategies to improve the stability of fixation in an osteoporotic spine include augmentation of screw tracts with PMMA, use of laminar hooks to “protect” inferior pedicle screws, expanding screw designs,³⁹ triangulated screw placement, increased use of transverse connectors, and bicortical vertebral body purchase. BMD is linearly related to screw insertion torque and pullout strength.⁴⁰ The surgeon’s tactile sense of purchase when placing the screw relates to construct strength.⁴⁰ PMMA may be added to screw tracts to increase pullout strength significantly, or bicortical purchase may be sought.^40,41 Overall, BMD has a greater impact than unicortical purchase on screw pullout strength.^42,43 Improved unicortical screw strength can be achieved with triangulated and subchondral placement.^44,45 If bilateral screws are placed in a triangulated pattern, use of a transverse or cross connector further increases fixation strength. Mechanically, stability is improved by the presence of bone between the screws rather than merely by the bone within the threads of each screw individually.⁴⁴ Bicortical screw placement may involve additional surgical risk, although it improves holding power and improves the construct’s resistance to cyclic loading. Bicortical purchase with posterior cortical pedicle screws offers less strength than anteriorly placed vertebral cancellous screws.^46,47 Posteriorly and anteriorly placed screws benefit from purchase into the stronger subchondral bone just below the endplate, but the effect is more pronounced anteriorly.⁴⁷ Bicortical purchase is typically not recommended for thoracolumbar pedicle screws because of risk of injury to anterior vascular structures. For sacral screws, especially in long constructs, bicortical screws improve holding power and sagittal plane correction.^48,49 Some authors have recommended aiming the screw upward into the disc space or through the sacral promontory.^50,51

With particular clinical challenges, such as osteoporosis, changing specific aspects of the implants themselves may improve fixation. In patients with osteoporosis in particular, maximizing pedicle screw diameter improves pullout strength and decreases the risk of fatigue failure of the screw.⁴⁸ Screw length is linearly related to pullout strength. Little difference is seen between self-drilling or self-tapping designs⁴³; if tapping is performed, undertapping by 1 mm leads to greater pullout strength than undertapping by 0.5 mm.⁴⁸ In many cases, the angle of implantation is as important as the implant characteristics themselves. In anterior and posterior constructs, one should seek to avoid parallelogram constructs in which the screws and longitudinal members form a perfect square or rectangle. These constructs resist lateral loads less well than a triangulated pedicle screw construct.⁵² In anterior dual-rod constructs, a trapezoidal short-short/long-long construct is used.

Posterior Approach

The midline posterior approach is the most common approach for placement of thoracolumbar instrumentation. This extensile approach is applicable from the occiput to the sacrum. Below L2-3, the level of the conus medullaris, the dural sac may be safely retracted to afford enough exposure to the posterior disc space for performance of a posterior lumbar interbody fusion. In the upper lumbar spine, the risk of neural injury with dural retraction increases, and more oblique approaches to the disc space, such as transforaminal lumbar interbody fusion, are safer. In the thoracic spine, a midline posterior approach is considered dangerous for decompression of anterior compressive pathology (Fig. 71–4).

FIGURE 71–4 Anterior interbody options are numerous with various types of metal, polyetheretherketone, and allograft cages available. In some cases, an anterior cage alone may be inadequate stabilization for reliable fusion. Numerous devices have been introduced to improve graft or segment stability. A, Example of the many “anti-kickout” implants available. The simplest of these is a screw and a washer. The goal of this instrumentation is to prevent graft extrusion anteriorly. B, More sophisticated and biomechanically rigid technique. This plating system seeks to control extension, a common failure mode for “stand-alone” anterior lumbar interbody fusion. C, This surgically more aggressive means of fixation employs translaminar facet screws to augment dual cylindric cages. Percutaneously inserted pedicle screws to systems have been popular in this application as well. D, Lateral x-ray showing L4-5 degenerative spondylolisthesis. E, Postoperative lateral x-ray after posterior pedicle screw instrumentation. Posterior fusion options are numerous. Numerous devices have been developed to improve segment stability and facilitate fusion.

(A, DePuy Bowt’s plate courtesy of DePuy Spine, Raynham, MA. B, Courtesy of Danek Pyramid Plate, Memphis, TN.)

The powerful retractor systems available for posterior instrumentation procedures create extremely high levels of intramuscular pressure. Over time, this pressure can cause muscle necrosis similar to a compartment syndrome. For longer cases, the retractors should be removed every 2 hours to allow muscle recovery.^33,34 This recovery time is particularly important because preoperative muscle abnormalities may exist in some conditions, which can be accentuated by the trauma of extended muscle retraction. One study found that preoperative paravertebral muscle biopsy specimens in 30 patients with spondylolisthesis were histologically different compared with normal controls.³⁵ Most thoracolumbar instrumentation requires a relatively wide exposure beyond the facet and out into the transverse process.

The lateral extracavitary or costotransversectomy approach allows access to thoracic vertebral bodies from a posterior approach without violation of the pleura or takedown of the diaphragm.³⁶ Visualization is reduced, however, compared with visualization for a corpectomy. Significantly more bone resection (including the rib, costotransverse joint, facets, and pedicle) is required to achieve that visualization.^36,37 Costotransversectomy is associated with increased blood loss, longer operative time, increased paraspinal muscle disruption, and chest wall numbness from intercostal nerve resection. Occasionally, anterolateral cord compression can be addressed with a compromise approach between a standard laminectomy and a formal costotransversectomy. These transpedicular decompressions are known by various names, such as pediculofacetectomy, and are particularly useful in patients with tumors in whom the neoplasm has already destroyed most of the pedicle.³⁸ In this case, the decompression is mainly soft tissue removal, and the anterior compressive elements can be removed indirectly by pulling them away. Although this approach confers limited visualization, it reduces operative time, blood loss, and iatrogenic destabilization.^39,40 Visualization can be improved with a 70-degree endoscope.⁴¹

In the lumbar spine, the Wiltse paramedian approach, although described for resection of far-lateral disc herniations, has been used for interbody and posterolateral fusions. A bilateral paramedian approach is best when no midline decompression is needed. Additionally, this approach could be considered for posterolateral pseudarthrosis revision to avoid midline scar. A Wiltse paramedian type of approach is used in most minimally invasive surgical fusions employing tubular retractors, microscopic dissection, and fluoroscopic guidance. Iatrogenic muscle injury can result from denervation of the primary motor branch of the dorsal primary ramus when the muscle is stripped from the midline beyond the facets.⁴³ Many studies have examined the effects of muscle splitting versus subperiosteal dissection on postoperative muscle health. One MRI study compared muscle enhancement after tubular discectomy versus a standard open discectomy. This study failed to show that microendoscopic discectomy produced less paravertebral muscle disruption than a traditional approach.⁴⁴

Facet-Based Approaches

Numerous methods of facet fixation have been proposed. Historically, small screws were placed directly through the facet joint. Although these screws were able to lock the facets, their short length produced too small a lever arm to counteract the forces to which the lumbar facet is exposed, and many of these implants failed. Techniques at the present time are exploring additional methods of bony purchase. The most common of these is the Magerl technique, in which 4.5-mm, fully threaded cortical screws, usually 50 to 60 mm in length, are passed from the opposite side of the spinous process through the ipsilateral lamina and across the facet.⁵⁵ The Boucher technique is similar but incorporates a greater degree of pedicle penetration.⁵⁶ In both of these techniques, the screws are not placed in lag mode. In addition, they do not provide the strength that the three-column purchase of transpedicular instrumentation affords. These techniques are indicated for one-level or two-level fixation when reduction is not needed.

These translaminar facet screws are often used as posterior column extension supports for anterior lumbar interbody fusion procedures. The advantages of these screws include minimally invasive insertion via a 4-cm percutaneous approach. Translaminar facet screws are much less expensive than transpedicular constructs and offer less impingement on surrounding musculature. Disadvantages include the need for postoperative immobilization, which delays rehabilitation. Contraindications to translaminar facet screws include absence of adequate lamina or facet joints. These screws may be placed after laminotomy procedures but not typically after full laminectomy. Given that these screws are less rigid than transpedicular instrumentation, they are probably best reserved for slim, low-demand, compliant patients with intact laminae and facet joints.⁵⁵

Using contemporary techniques, fusion rates are high, and cases of screw failure are rare. One article examined the outcome of single-level and multilevel fusions for degenerative disc disease and found 91.2% good to excellent outcome in the single-level group and 86.4% good to excellent results in the multilevel group. The only complications in this series of 57 consecutive patients were three iliac crest donor site wound infections.

Laminar Wires and Hooks

Traditionally, posterior thoracolumbar instrumentation has been divided into rigid and nonrigid implants. The earliest constructs incorporated the spinous processes or other posterior elements alone or with slabs of structural iliac crest autograft. Although simple wiring techniques are no longer used in the thoracolumbar spine, many wire-rod techniques continue to be routinely employed. The most common technique—Luque wiring—employs sublaminar wires as anchors. These wires are wrapped around rods to form a segmental, nonrigid spine construct. Such techniques are nonrigid because they allow “pistoning” of the spine in a craniocaudal direction. Luque’s construct was the first segmental system that used multiple wire attachment points.^53,57 In North America today, Luque constructs are preferentially used in the thoracic spine. Common indications include neuromuscular scoliosis, scoliosis with thoracic lordosis, and occasional cases of idiopathic scoliosis. Hybrid constructs using Luque wires with pedicle screws and other bone anchors are more commonly used. Contraindications to sublaminar fixation include absence of laminae. In addition, there is a danger of passing the wire through the spinal canal, particularly in patients with kyphosis or canal stenosis.⁵⁸ Because wiring techniques do not provide axial stability, they are a poor choice for stabilization of pathologic processes with anterior column insufficiency. For example, sublaminar wires should not be used as primary fixation in patients with vertebral fractures or tumors.

In response to the risk of sublaminar wire passage, Drummond proposed a technique, commonly referred to as the Wisconsin method, in which the rod is wired to the spinous processes.⁵⁹ This method has been used with Harrington rods and other rod systems to provide additional stability by segmental fixation.^60,61 A button may be used to decrease wire cut-through of the spinous process. Although this technique is rarely used alone anymore, it may be useful in hybrid constructs, particularly when limitations of bony anatomy or breakout of prior instrumentation renders the pedicles and other posterior elements unusable.

The next evolutionary step after wiring in thoracolumbar instrumentation came with simple, hook-based distraction devices intended to assist in the correction of scoliosis. After Harrington’s initial ground-breaking design, several other modified designs were developed. Harrington rods were used in thoracic curves and for thoracolumbar burst fractures. In trauma cases, the Harrington system provided quick and easy stabilization compared with the earlier devices. This construct could be combined with a rod sleeve to provide a three-point fixation and was particularly advantageous in polytrauma patients. Because Harrington rods produced distraction, they often led to flatback deformities that resulted in persistent pain and fatigue. When the importance of sagittal plane balance was understood and as newer, more powerful systems became available, the Harrington rod fell out of use.

Early hook-based segmental thoracolumbar spine constructs allowed lordosis to be contoured into the rod and held with segmental bone anchorage. These constructs were significantly more rigid than their predecessors. This increased rigidity improved fusion rates and sometimes obviated the need for postoperative bracing or cast immobilization. This was particularly advantageous for children and adolescents undergoing surgical correction of idiopathic scoliosis.

Disadvantages of this construct, as with rigid implants elsewhere, included the effect of fusion on adjacent segments. In contrast to pedicle screw constructs, which employ the concept of three-column spine fixation, hooks anchor to the posterior elements alone and do not have the same power to reduce scoliosis.⁶² Although misplaced pedicle screws can produce catastrophic neurologic or vascular injury, hooks by their very nature are canal intrusive. Mildly misplaced screws produce less canal encroachment than hooks. Polly and colleagues⁶³ have published an analysis of the volumetric effects of hook placement on the spinal canal.

In a comparison of pullout strength, thoracic pedicle screws were found to be significantly stronger than hooks and were recommended for rigid curves.⁶⁴ In terms of operative time, one study showed no difference between the operative time or correction achieved with hook or screw consults.⁶⁵ Finally, the pullout strength of hooks seems less sensitive to osteoporosis than screws, although screws are still stronger than hooks.⁶⁶

Various hooks with different characteristics are available. Each of these is suited to a particular mode of placement and loading. Pedicle hooks, resting on the lamina of the instrumented vertebra and the superior articular process of the next distal vertebra, are the strongest. They are always directed cephalad so that their U-shaped tip captures the pedicle and provides maximal stability for rotation and translation maneuvers. Because of their configuration and their dependence on facet joint anatomy, these implants can be placed from T1 to T10 or T11.⁶⁷ There are subtle variations in hook shape, including short-throated and long-throated configurations and straight or off-set collars. Contemporary “anatomic” hook designs give better bony contact.

Laminar hooks are available in various designs. Variation in blade width and style and the relationship of the blade to the body of the hook allow for optimal hook-bone interface. Between T3-T10 or T11, lamina hooks may also be placed on the superior surface of the transverse process. Here the “transverse process” hook is combined with a pedicle hook to yield a pedicle-transverse grip or “claw.” Although such claws may be constructed over one or two levels, two-level claws are easier to insert and mechanically stronger.⁶⁸ Claws are used mainly at the ends of a construct. Above T3, the transverse process becomes more horizontal, and transverse process hooks do not align with the pedicle hook. The claws must be pedicle–supralaminar hook combinations.

A supralaminar hook enters the spinal canal in a cranial to caudal direction and, while providing the second strongest anchorage, risks iatrogenic cord injury if the hook pistons in and out of the canal during rod manipulation. Thus, careful selection of the hook shape is mandatory. Moreover, one should consider avoiding peri-apical placement or utilization of supralaminar hooks in correction of hyperkyphosis. Finally, infralaminar hooks are placed in a cephalad direction. These anchors are rarely necessary in the thoracic spine because pedicle hooks provide proximally directed forces. Usually, these implants are used inferiorly, along with pedicle screws in hybrid constructs. Occasionally, they are added to transpedicular constructs to protect the screws from pullout.⁶⁹