Summary of Key Points
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Spinal deformity must be a major consideration in developing a surgical management strategy in the degenerative spine.
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Preoperative planning and careful patient selection are paramount when considering long-segment fixation and correction techniques.
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Spinal balance and deformity correction can alleviate pain and has been shown to improve functional outcomes.
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Rising incidences in iatrogenic spinal deformities such as lumbar flat back and proximal junctional kyphosis are redefining adult deformity corrective surgery.
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Spinopelvic harmony is emerging as a critical component to achieving long-term functional improvements after long segment corrective surgery.
Spinal deformity is a three-dimensional alteration of spinal alignment in both the coronal and sagittal planes. Scoliosis, strictly speaking, is a curvature greater than 10 degrees in the coronal (frontal) plane, as determined by measuring the Cobb angle ( Fig. 145-1 ). There are many causes, and the condition may occur throughout life from infancy through adulthood. The type of deformity and the patient’s age at presentation have a significant impact on the ultimate treatment of the condition. These topics will be discussed separately throughout this book and are beyond the scope of this chapter. Surgical treatment of these conditions varies depending on the age of the patient and the degree of the deformity. Adults with spinal deformity often seek surgical consultation for pain associated with the deformity, spinal imbalance, or neurologic signs/symptoms. Children and adolescents are most often sent for surgical evaluation because of cosmetic concerns and risk of curve progression. Pain and neurologic compromise are rare in this patient cohort. For the purposes of this chapter, the authors will focus on the principles of spinal deformity and not the causes or specific treatments. These principles may be applied to most conditions affecting spinal alignment.
The treatment of spinal deformity is not new. Hippocrates attempted traction scoliosis, and Pare used an iron corset in the 16th century in an attempt to correct a similar deformity. Not until the development of Paul Harrington’s rod system in the 1960s did the surgical correction of spinal deformity take off. For the first time, Harrington’s distraction rod systems, then compression rod systems, permitted correction and improved arthrodesis of the deformed spine. This development truly revolutionized treatment of these dynamic and complex conditions. Early strategies for fixation focused largely on coronal plane correction. Often excellent results were obtained, but these resulted in flattening of the sagittal plane. Unfortunately, this often led to the development of decompensation in the sagittal plane and subsequent pain. This has been referred to as flatback deformity. The next phase in development was the application of hooks, then screws, in the thoracolumbar spine. The use of hooks and screw fixation of the spine permitted greater control over the spine and corrective maneuvers. Greater degrees of correction could be obtained, fixation was improved, and shorter constructs could be used. Powerful control of individual segments of the spine with three-dimensional correction that was not limited by postoperative bracing became the basis for present-day deformity correction systems.
Treatment Goals
Although the indications and strategies vary, depending on the age of the patient and the deformity encountered, the goals of surgical treatment are essentially the same. The primary goals are to halt curve progression, relieve pain, and improve cosmesis and function. The surgical goals are to obtain a solid arthrodesis with a well-balanced three-dimensional spine. In adolescents, the focus should be on curve correction, improving cosmesis, and halting curve progression. As mentioned earlier, treatment of axial or radicular pain is much less of a concern. In adults, the focus is much less on curve correction but rather on balancing the spine and attaining solid arthrodesis and neurologic decompression if required.
General Terms
A unique set of terms may be applied to spinal deformity and should be reviewed briefly. Spinal deformity involves a curvature and obligatory rotation (coronal plane) in either the coronal or sagittal planes. Curves in either plane are measured end vertebra to end vertebra. The end vertebra is the most cephalad and most caudal vertebra of a curve ( Fig. 145-2 ). Lines extended along the end plates of the vertebral bodies that are part of the curve in question all converge toward a central point within the concavity of a curve. Lines extended along the end plates of vertebral bodies not involved become divergent. The most rostral or caudal vertebral body visualized is the end vertebra. The neutral vertebra is that vertebra between curves that demonstrates the least rotation. Both pedicles should be relatively symmetrical. The stable vertebra is the vertebra that is bisected by the center sacral vertical line. This line is determined by first drawing a line connecting the most rostral point on each of the iliac crests. A perpendicular drawn from the midpoint of the S1 vertebra superiorly defines this line. Surgeons often use the stable or neutral vertebra when deciding where to end a construct/fusion.
Coronal and Sagittal Correction
Spinal balance is critical for optimal biomechanics. This is determined using a plumb line. In the coronal plane, the line is drawn from the center of the C7 vertebral body to the sacrum. This line should fall within 2.5 cm of the center of the sacrum. Deviation greater than this provides evidence of decompensation. This may be determined using 36-inch scoliosis radiographs or directly on the patient, estimating the location of C7 and using the gluteal cleft as the midsacrum. In the sagittal plane, the line should extend from the center of the C7 vertebra and the dorsal aspect of the L5-S1 disc space. On the patient, a plumb line may be drawn from the external meatus of the ear, and the line should fall along the greater trochanter when the spine is balanced.
Spinopelvic Alignment
Spinopelvic harmony is an emerging concept. Extensive studies have now shown that due to lower amounts of lumbar lordosis and thoracic kyphosis in scoliotic patients, spinopelvic malalignment is often present. Pelvic incidence is the angle of the pelvis in relation to the femoral head and is defined by a line perpendicular to the end plate of the sacrum at its midpoint and a line to the center of the bicoxofemoral axis. Pelvic incidence (PI) is now thought to serve as the major contributor to lordosis in the lumbar spine with high numbers leading to isthmic spondylolisthesis and degenerative disease. Research on asymptomatic adults suggest normal PI to have a mean of 52 ± 10 degrees. The pelvic incidence is created by the combination of the sacral slope and pelvic tilt. The sacral slope is the angle defined by a horizontal line from the posterior aspect of the sacral end plate and a parallel line to the end plate. A normal sacral slope ranges from 39 ± 9 degrees. Pelvic tilt is defined by the same midpoint sacral line to the center of the bicoxofemoral axis and a vertical line from this point. Normal pelvic tilt is 13 ± 7 degrees. Sagittal imbalance causes compensatory mechanisms to preserve a vertical posture with pelvic rotation over time and alterations in pelvic incidence. A low PI value is associated with a horizontal sacrum and low amounts of lumbar lordosis, whereas high values are associated with a vertical sacrum and high degrees of lordosis. In spinal adult deformity, spinopelvic harmony has been shown to be critical in improving outcomes. By restoring spinopelvic alignment through techniques described later, biomechanical stress will be alleviated on the reconstructed spine and this has been shown to improve functional outcomes.
Spinal curves may be classified as structural or compensatory nonstructural. A structural curve is usually the larger or major curve of the deformity and is closely related to the inciting pathology responsible for the deformity. On bending radiographs, structural curves maintain a significant curve magnitude, generally greater than 25 degrees in the coronal plane. Compensatory curves are countercurves that allow the spine to “compensate” for the structural curve in an attempt to maintain balance. On bending radiographs, compensatory curves are less than 25 degrees, smaller in magnitude than structural curves. Compensatory curves are flexible. Curves are also measured in the sagittal plane. Normal measurements of thoracic kyphosis and lumbar lordosis have been determined. Deviations from these “normal” values may be defined as hyperkyphosis, hypokyphosis, or lordosis. When planning surgical deformity correction, most often the structural curve is instrumented, whereas instrumentation of the compensatory nonstructural curve(s) is avoided or selectively limited. Sagittal curves must be accounted for as well. Hypokyphosis should be addressed in the thoracic spine, even if the Cobb angle of the coronal curve may not be of a significant magnitude.
Causes of Spinal Deformity
Congenital Deformities
Congenital spinal deformities are generally due to defects in either vertebra formation or segmentation. These defects may lead to the formation of hemivertebrae or multiple fused portions of the spine, known as bars . The result may be scoliosis, kyphosis, or both. These deformities present early in life, often leading to severe deformities.
Neuromuscular Scoliosis
As the name suggests, neuromuscular scoliosis is due to defects in the neuromuscular system. Examples include muscular dystrophy and cerebral palsy. Children with these disorders are often very limited by the index disease, and many are nonambulatory. The curves are often large and gentle sweeping deformities. Presentation is usually early in life, and correction often involves long fusions, including the pelvis, to improve sitting posture.
Idiopathic Scoliosis
By far the most common type of spinal deformity is idiopathic scoliosis. As the name implies, the cause for the condition is unknown; however, significant evidence suggests genetic influences. These curves present in adolescence and have a risk of progression during growth of the spine. Surgery is not always required. A discussion of surgical indications is beyond the scope of this chapter and is related to curve magnitude, location, and maturity status of the patient’s spine.
Degenerative Deformities
As the spine progresses down the degenerative cascade as defined by Kirkaldy-Willis and does so asymmetrically, deformity may, and often does, occur. Classically, this involves the lumbar spine. Curvature, lateral listhesis, and rotation are usually seen. Patients may present with lumbar axial pain, radiculopathy, or neurogenic claudication. Treatment is dependent on the presenting symptomatology. Radicular pain may only require foraminotomy, whereas decompensation and axial mechanical pain may require deformity correction and stabilization.
Iatrogenic Deformities
With the expansion of spinal instrumentation, iatrogenic spinal deformity is frequently encountered. These deformities are often in the sagittal plane and include kyphosis, most often above a previous construct, and lumbar flatback. Others may be seen, but these are the most common. Kyphosis may be due to a fracture above a long lumbar or thoracolumbar construct. These deformities may be complex and difficult to treat due to the prior surgery.
Scheuermann Kyphosis
This type of kyphosis, which may occur solely in the thoracic or thoracolumbar spine, may present at any time between adolescence and adulthood. The cause is thought to involve asymmetrically higher ventral intradiscal pressures, which may lead to focal disc herniation of the end plates, generating Schmorl nodes. There may be injury to the growth plate, disproportional loss of ventral vertebral body height, irregularities of the end plates, and narrowing of the disc interspaces. Patients may present with pain, progression of kyphosis, or cosmetic concerns.
Principles of Deformity Correction
Many techniques are available to the surgeon for the correction of coronal or sagittal deformities. In most cases, many techniques are used for the final attainment of a stabilized, balanced spine.
Cantilever Forces
This is probably the most common technique used for the correction of deformity. In general, a rod is bent to the desired contour for optimal alignment and is then connected sequentially to each pedicle screw or hook previously placed. The correction is greater with multiple points of fixation (i.e., multiple pedicle screws or hooks). As each screw is sequentially connected to the preformed rod, the spine begins to conform to the rod’s contoured design. The spine may be contoured in this manner in both the coronal and sagittal planes.
With rigid curves, the aforementioned technique may not be possible for significant correction. Ventral discectomies/osteotomies may be helpful for more significant correction. Dorsal osteotomies (complete facetectomies), with or without discectomies, also aid in further correction. Moreover, pedicle screws offer greater correction compared with hook constructs. The most difficult task with this correction maneuver is bringing the spine to the rod. Many options exist to aid in reduction and depend on the system used for stabilization. One available option is “reduction screws.” These screws have an extra long tulip, in which part of it may be broken off after the rod is secured to the screw. “Persuaders” may also be used, where the rod is pushed to the screw until a “cap” may be inserted. Surgeons must use extreme care in patients with osteoporosis or very rigid curves or when small-diameter pedicle screws have been used. Screws in these circumstances may easily pull out. Lastly, forces may be less on each individual screw if reduction is applied to multiple screws simultaneously as the spine is brought to the contoured implant.
Derotation
Practically speaking, forces applied to the spine (cantilever, compression, distraction) all result in translation of the vertebral bodies. Classically, derotation credited to Cotrel and Dubousset results in translation but not the desired rotation of the apical vertebrae. Newer frame-type constructs are now available in modern deformity instrumentation sets that may actually permit true derotation along with translation.
Classically, this technique is applied to thoracic curves but may be applied throughout the spine. The rod is first bent to the appropriate kyphosis desired in the thoracic spine. This rod is then placed along the concavity of the thoracic curve. Cantilever forces may be used during connection of the rod to the pedicle screws, permitting some correction. (After connection, the rod lies along the concavity, following the curve.) Vice grip–type pliers may then be used to grip the rod in two locations, usually toward the ends of the rod, and then rotation is applied.
Typically, the direction of rotation is toward the concavity, or clockwise. This maneuver translates the apex of the curve toward the midline and into kyphosis. It is best to use monoaxial or uniaxial screws for this maneuver. Some systems have polyaxial screws that make connecting the rod to the screws much easier, but they may then be “locked” as monaxial screws during derotation.
As mentioned previously, new tools that permit true rotation of the apex of a curve are available. A frame is constructed on the sides of the apex of the curve. Connectors are placed on the top of multiple screws along both the concavity and convexity of the curve. These are then connected by longitudinal members and cross connectors, thus creating a frame. This frame may then be rotated (i.e., the convexity is pushed down and the concavity is pulled up) while the spine is translated toward the midline.
Significant forces are applied to the spine during this maneuver. Care must be used in rigid curves or in those with osteoporosis. During derotation, the spine should be continuously assessed. Osteotomies may be necessary for optimal translation. With all corrective maneuvers, the sagittal plane must be assessed following coronal correction. Compression/distraction or in situ bending may be necessary to gain more correction.
Compression
Compression may be applied along any part of a spinal construct. It is a useful technique for both coronal and sagittal corrective maneuvers. Typically, compression is applied along the convexity of a curve. It permits coronal correction to some extent yet also the attainment of lordosis because the force is applied dorsal to the instantaneous axis of rotation. This is important when using compression along with distraction for coronal corrective maneuvers. Compression should be used only where lordosis is desired as well (e.g., in the lumbar curve or with preexisting thoracic hyperkyphosis).
Distraction
Similar to compression, distraction may also be applied along any part of a spinal construct. It is useful for correction in the coronal and sagittal planes. Typically, distraction is applied along the concavity of a curve. It permits coronal correction. One should remember that it also results in kyphosis in the sagittal plane. It is useful along the concavity of thoracic curves where kyphosis is desired (e.g., thoracic hypokyphosis).
Usually, compression and distraction are applied at multiple points along a construct.
In Situ Rod Contouring
Once the rods are connected to all of the bone fixators, hooks, or pedicle screws, the rod may be contoured in situ for fine-tuning the correction. Most sets offer in situ benders for both coronal and sagittal bends. Rod contouring should be used for fine-tuning and not for major corrective maneuvers. Significant stresses may be applied at the bone-implant interface during bending. Thus, this may result in hook cutout or screw pullout, especially in the osteoporotic spine.
Ventral Releases
Multilevel discectomies may be performed to increase flexibility and potentially improve the correction attained. Successful arthrodesis may be improved as well. However, these advantages must be weighed against the morbidity associated with any additional procedures.
Ventral discectomies may be performed via thoracotomy, or thoracoabdominal or retroperitoneal approaches. The techniques are similar after the spine has been exposed. A complete discectomy is performed back to the dorsal anulus. There is no need to penetrate the dorsal anulus. At times, the anterior longitudinal ligament may need to be resected, especially if it is ossified. This may improve flexibility. Structural or morselized grafts may be used in the interspace. Structural grafts may be useful, especially at L4-5 to level the vertebral end plates and for the attainment of lordosis in the lumbar spine.
With the wide use of pedicle screws, the need for ventral release procedures has been questioned. The additional release procedures expose the patient to increased morbidity, especially pulmonary. It also appears that dorsal-only surgery results in similar outcomes compared with anterior/posterior procedures. Good and colleagues demonstrated that dorsal-only adult scoliosis surgery achieved similar correction to anterior/posterior surgery while decreasing blood loss, operative time, and length of hospital stay, as well as avoiding additional anesthesia. Complications, radiographic outcomes, and clinical outcomes were similar at the follow-up of longer than 2 years.
Osteotomies
Osteotomies may be very helpful for correction of coronal and sagittal plane deformities. A wide variety of resections may be performed, but two of the most popular include Smith-Petersen and pedicle subtraction osteotomies. It is beyond the scope of this chapter to discuss these procedures in great detail, but principles will be discussed. Both are mainly used for sagittal correction, but if used asymmetrically, coronal correction may be attained as well.
Smith-Petersen Osteotomy
Smith-Petersen osteotomies (SPOs) were originally described for the treatment of ankylosing spondylitis. Currently, the osteotomy is more widely applied for the treatment of sagittal plane deformities. SPOs are generally indicated for the treatment of gentle “swooping” as opposed to focal “sharp” curves, especially in the thoracic spine (i.e., Scheuermann kyphosis). Approximately 5 degrees of correction may be attained per level. Multiple SPOs may be performed to achieve greater overall correction.
Technically, SPOs involve shortening the posterior column while lengthening the anterior column. The axis or rotation is based on the middle column. The lower half of the lamina and spinous process of the upper level as well as the upper half of the lamina and spinous process of the lower level are removed. The inferior and superior articulating processes as well as the ligamentum flavum are removed bilaterally. Following placement of screws or hooks, compression may be applied across the osteotomies. As mentioned earlier, this involves shortening the posterior column along the axis of the middle column and lengthens the anterior column. For this to occur and permit correction, the anterior column must not be ankylosed. SPOs are not possible in the face of multiple ventral bridging osteophytes. Careful scrutiny of preoperative imaging is required prior to including SPOs in the plan for deformity correction. Care must be taken to avoid complications during the performance of single or multiple SPOs. Potential major complications directly attributed to the osteotomy include vascular and neurologic complications. Extension or lengthening of the anterior column may include injury to major vascular structures such as the aorta or its vessels. Neurologic complications have been reported in up to 30% of cases. Neurologic complications may be minimized with careful wide osteotomies, including the surrounding lamina, so there is no direct compression of neurologic structures when closing or compressing across the osteotomy. Minor complications include extensive blood loss, especially with multiple SPOs, durotomy, and pseudarthrosis.
Pedicle Subtraction Osteotomy
Thomason introduced pedicle subtraction osteotomy (PSO) in 1985. Similar to SPOs, PSO is largely indicated for the treatment of sagittal plane deformities. It also may be performed asymmetrically for the correction of both coronal and sagittal plane deformities. This type of osteotomy is powerful and able to correct approximately 30 degrees per PSO. It is technically more demanding compared with the SPO and associated with greater morbidity.
PSO is accomplished by first performing a complete laminectomy or equivalent if done through a prior fusion mass (i.e., treatment of flatback syndrome). Next, a complete facetectomy, including the pars interarticularis, is performed. This should result in exposure of the dura, the bilateral existing and traversing nerve roots, and exposure from the pedicles above and below the pedicle to be removed. The pedicle to be removed is thus isolated as an island between the level above and below. The lamina and spinous process should be removed from both the rostral and caudal levels to adequately expose the dura as well as the pedicles above and below. It is necessary to ensure that there is no undue pressure or force on the thecal sac during closure of the osteotomy. Both pedicles may then be removed until flush with the vertebral body. This may be accomplished with rongeurs or a high-speed drill. The exiting nerve root and medial dura must be protected during removal. Often, the cortical wall medial and inferior are left in place for protection of these structures (they are then removed with the dorsal cortical wall of the vertebral body immediately prior to closure of the osteotomy). A wedge may then be performed through the vertebral body for the desired correction.
Various schemes have been developed to not only help determine at which level to perform the osteotomy but also how much of the vertebral body to remove. A rule of thumb is that for every 1 cm of osteotomy, there is a 10-degree correction in the sagittal plane. Therefore, a dorsal osteotomy of approximately 3 cm permits about 30 degrees of correction.
The vertebral body may be decorticated directly through the pedicles using curets. Rongeurs may be used to remove the decorticated cancellous bone. This must be performed in a wedge shape toward the ventral cortex. Fluoroscopy may be helpful to determine the height of the osteotomy and progression toward the ventral cortex. The lateral vertebral body should be dissected in a subperiosteal fashion toward the ventral cortex. Cottonoids are useful to maintain this dissection. At this point, the osteotomy should be completely decorticated and the height and width should be confirmed both visually and using fluoroscopy. The lateral vertebral body should be removed down to about the level of the ventral cortex. This may be accomplished using rongeurs, down-pushing curets, or osteotomes. Lastly, the dorsal cortex is removed to complete the osteotomy. Pedicle screws should be placed prior to beginning the osteotomy and a temporary rod placed to control the osteotomy until closure is desired. The dorsal cortex is thinned appropriately and may be finally removed with rongeurs, osteotomes, or down-pushing curets. The thecal sac may be carefully retracted (if in the lumbar spine) medially to reach the midline. A tool such as a Woodson elevator may be used to confirm completion of the osteotomy. The osteotomy may then be closed either with the aid of the operating room table or by compression placed across the pedicle screws. This closure should be performed in a controlled fashion with frequent checks of neuromonitoring. If closure is incomplete, lateral vertebral body removal as well as complete dorsal cortex removal must be confirmed.
Morbidity may be high with a PSO. Similar to an SPO, major risks include damage to vascular and neurologic structures. Bleeding may be excessive and continues until the osteotomy is closed.
Vertebral Column Resection
Complete resection of one or multiple vertebral levels and spinal shortening may be required for severe rigid curves, usually more than 80 degrees, fixed trunk translation, or asymmetry between the length of the convex and concave columns to be corrected. The goal of vertebral column resection (VCR) is to balance the spine in both planes while shortening the length of the spinal column.
VCR may be performed using a combined anterior/posterior approach or by a dorsal-only strategy. The dorsal-only approach may reduce operative time, complications, and effort associated with the combined procedure.
A detailed description of VCR is provided in other chapters but involves complete removal of one or multiple spinal levels. This includes the vertebral body (including intervening discs), pedicles, and all of the dorsal elements. After placement of dorsal instrumentation, the spine may be manipulated into both coronal and sagittal balance. In certain situations, an interbody graft may be used to augment the anterior column and prevent unwanted shortening of the spine and potential spinal cord deficit.
Similar to other osteotomies, the complication rate of a VCR may be high. Bradford and Tribus reported on 24 patients treated with a VCR. They noted 31 complications in 14 of the patients, in which 13% were neurologic but none resulted in paralysis. Significant bleeding may be encountered until final correction is obtained. Lenke and colleagues published a report on dorsal VCR patients with a minimum 2-year follow-up. They noted spinal deformity correction rates between 51% and 60% for the various categories of deformity; importantly, they reported no spinal cord–related complications. However, they did note the complexity of the reconstruction: surgeons should be well versed in spinal deformity surgical treatment before embarking on this technically demanding procedure. A similar report evaluating pediatric and adult VCR patients confirmed these findings.
Related posts:
Anatomy of Nerve Root Compression, Nerve Root Tethering, and Spinal Instability
Ventral and Ventrolateral Subaxial Decompression
Diffuse Idiopathic Skeletal Hyperostosis
Spinal Dural and Extradural Vascular Malformations
Spinal Wound Closure
Explant Analysis of Wear, Degradation, and Fatigue in Motion Preserving Spinal Implants
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