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2. Operative Treatment of Thoracolumbar Deformity

Praveen V. MummanenP, Stephen L. Ondra0, Rick C. Sassoc a Department of Neurosurgery, Emory University, Atlanta, Ga., b Department of Neurosurgery, Northwestern University, Chicago, Ill., and c Department of Orthopaedic Surgery, Indiana University School of Medicine, Indiana Spine Group, Indianapolis, Ind., USA

Surgical fusions to treat spinal deformity were first developed in the 1900s. In 1911, Hibbs [1] performed a noninstrumented, posterior spinal fusion for deformity related to tuberculous spondylitis. In 1962, Harrington [2] revolutionized spinal fusion for deformity with the use of his distraction rod instrumentation system.

In the past 50 years, there has been a revolution in the use of instrumentation for the treatment of deformity, and new techniques and new constructs have improved the surgical outcomes for deformity patients.

Indications for Surgery and Surgical Techniques

Adolescent Idiopathic Scoliosis

Surgical treatment is indicated for adolescent idiopathic scoliosis (AIS) that progresses beyond 40-50° in the growing child. Typically, curves over 40° in a growing child will continue to progress if not treated surgically. The primary reasons to perform surgical correction of these curves is to halt and reverse curve progression as well as to improve cosmesis. Curves between 40 and 90° usually do not cause cardiopulmonary difficulties. However, curves greater than 90° may cause cardiopulmonary compromise, and this is an additional indication to surgically correct these curves. Preoperative planning for these patients should include a radiographic assessment (with 36-inch standing x-rays) of curve magnitude, pedicle rotation, location of curve apex, vertebral levels marking the ends of the curve, and overall sagittal and coronal alignment.

The ends of the curve are defined as the vertebrae that are both stable and neutral. The stable vertebra is the end vertebra that is most closely bisected by the center sacral line. The neutral vertebra is the end vertebra that is the least rotated. Usually the stable vertebra is also the neutral vertebra, but if they do not correspond then fusion to the stable vertebra (not the neutral vertebra) is recommended [3].

This will give the surgeon a good idea of the amount of correction needed to reestablish spinal balance. Lateral bending films are useful to identify structural (usually rigid thoracic curves) and compensatory curves (usually flexible, lower magnitude, lumbar curves without pedicle rotation). In addition, the size of the rib hump should also be measured, and the surgeon can consider thora-coplasty for significant rib hump deformity.

Curve Classification

Classification of AIS curves is important because the selection of fusion levels is dependent on the classification. Over the past few decades, the King classification system has been the most commonly used system to classify AIS curves. Recently, however, Lenke et al. [4] described a more comprehensive and descriptive classification system for AIS.

We prefer to use the Lenke classification system for several reasons. The King system only describes curves in the coronal plane and does not take sagittal balance into account. In addition, interobserver variability in classification with the King system is high. The Lenke system, on the other hand, takes into account sagittal alignment with a sagittal modifier. In addition, the Lenke system is highly descriptive and uses a three-tiered classification scheme (6 curve types, lumbar spine modifier, and sagittal thoracic modifier) (fig. 1) [4]. The most frequent curve types with the Lenke system were found to be type 1 (main thoracic curve) and type 2 (double thoracic curve) with the most frequent curve classifications found to be types 1AN, 1BN, and 2AN [4].

Surgical Correction

The Lenke classification system can be used to predict which levels should be instrumented to correct AIS [5]. For type 1 curves, only the main thoracic curve should be instrumented. For type 2 curves, both the proximal and main thoracic curves should be fused. For type 3 curves (double major), both the main thoracic and the thoracolumbar/lumbar curves should be fused. For type 4 curves (triple major), all three curves should be instrumented.

Surgical treatment for AIS is usually reserved until the patient is close to skeletal maturity. If the patient is not near skeletal maturity, then the patient may be at risk for a crankshaft phenomenon in the years following surgery. The crankshaft is due to continued growth from the vertebral endplates while

Lenke Classification
Fig. 1. Artist's illustration of the Lenke classification system for scoliosis. The Lenke system is highly descriptive and uses a three-tiered classification scheme (6 curve types, a lumbar spine modifier, and a sagittal thoracic modifier). TL = Thoracolumbar; L = lumbar; MT = main thoracic.

the posterior tension band is secured by instrumentation; the spine then crankshafts from side to side to accommodate the vertebral body growth.

In the past few decades, hook-rod instrumentation has been used successfully for the correction of AIS. In the 1990s, however, pedicle screw-rod instrumentation has gained in popularity. Pedicle screws are stronger anchors than hooks. In addition, unlike laminar hooks, pedicle screws do not occupy the spinal canal nor do they disrupt the ligamentum flavum.

Structural Right Thoracic Curve (Lenke Type 1)

The structural right main thoracic curve with a compensatory (flexible, low-magnitude, nonrotated) lumbar curve is classified as a King type II or a Lenke type 1 curve. It is the most common curve type in AIS, and we will discuss its treatment further.

Posterior Approach

The classic operation performed for a structural right thoracic curve in an adolescent is a selective thoracic fusion. Classically, a selective thoracic fusion is maintained by distraction with hook-rod instrumentation on the concave side of the curve and compression with instrumentation on the convex side of the curve [6, 7].

Initially, the thoracic curve is instrumented with hooks on the concave side; a rod is attached to the concave hooks and a rod rotation maneuver is performed to correct the curve (i.e. attach a concave rod to the hooks and then rotate the rod until it is straight in the sagittal plane). On the concave side, up-going pedicle hooks are typically placed at the superior end of the curve, and down-going laminar hooks are typically placed at the inferior end of the curve. This hook pattern allows for distraction on the rod, which serves to maintain the correction after a rod rotation maneuver. On the convex side, a claw is placed at the superior end of the curve (the claw consists of down-going hooks above and up-going hooks at the next level below), an up-going hook is placed at the apex of the curve, and down-going hooks are placed inferiorly. This hook pattern allows for compression on the rod in order to maintain curve correction on the convex side of the curve. The compensatory lumbar curve is not instrumented and will typically correct to normal when the structural thoracic curve is straightened [8].

Alternative posterior fusion options include hybrid hook-screw-rod systems, hook-endplate screw systems, or pure pedicle screw-rod systems. In the hybrid system, hooks are placed superiorly (where the pedicle diameters are smaller) and screws are placed inferiorly (where the pedicle diameters are larger). In the pedicle hook-endplate screw system, a pedicle hook is placed and then anchored to an endplate screw; this rigid fixation technique is similar to pedicle screws in pullout strength. In pure pedicle screw systems, no hooks are used; instead, pedicle screws are placed in lieu of the hooks. In all three systems, a rod rotation maneuver is initially done on the concave side of the curve to correct the curve.

When we place thoracic pedicle screws, we have found the following surgical nuances to be helpful. We locate the entry point for thoracic pedicle screws by using a high-speed burr to decorticate the surface of the thoracic facet overlying the area of the pedicle. Once this mild decortication has been done, a blush of blood is usually noticeable, and this blush marks the entry to the pedicle. This blush of red arises from the cancellous bone at the center of the pedicle, which tends to bleed when uncovered. The surrounding facet area does not tend to 'blush' with this maneuver. The surgeon can reliably establish the pedicle entry point through this technique.

The typical location of the pedicle with respect to the thoracic transverse process and facet has been elucidated by Lenke et al. [9].

One word of caution is in order, however. If the central portion of the pedicle is corticated, then this 'blush' technique cannot be used, as there is no cancellous bleeding bone. In this case, we recommend the surgeon perform a laminoforaminotomy to feel the medial, superior, and inferior walls of the pedicle to guide the screw placement [10].

Once the entry point has been established, there are three options we use to establish a pathway through the pedicle for the pedicle screw. The first method we use entails tapping the pedicle from the entry point with a 3.5-mm tap from the VERTEX posterior cervical set (Medtronic Sofamor Danek, Memphis, Tenn., USA). This tap has fine cutting flutes and with minimal downward pressure, the tap has a tendency to find the appropriate trajectory for screw placement by cutting through the cancellous portion of the pedicle without violating the cortical pedicle walls. The screw can then be placed through this trajectory.

The second option is to use the thoracic pedicle probe recently created by Lenke et al. [9]. This small tap is ideal for the thoracic pedicles. It has a small angled end, and this end is initially placed through the pedicle entry point facing laterally (in order to avoid medial cortical wall violation and spinal canal penetration). After a depth of 15 mm (marked on the tap itself), the probe is then turned medially to enter into the vertebral body and to avoid lateral wall breakout from the vertebral body. It is important to note that after the first 15 mm, the probe is beyond the spinal canal in most patients, and the risk of spinal cord injury is minimized during the first 15 mm by the laterally directed angle of the probe [9].

We avoid drilling down the pedicle as cortical breakthrough can occur with the drill. However, when the pedicle is corticated, then drilling becomes a necessity (the third option). We guide our drill trajectory by feeling the outer walls of the pedicle after performing a laminoforaminotomy. When the pedicles are too small to accommodate 4.5-mm screws, several options should be considered. First, by using a 3.5-mm tap (we prefer to use the one available on the VERTEX posterior cervical system), we establish a pathway through the pedicle into the vertebral body. Then we use a 4.5-mm tap to 'dilate' the pedicle.

Roy Camille Screw Placement

Fig. 2. Artist's illustration of the difference between the anatomic (AT) and the straightforward (ST) trajectories for thoracic pedicle screw placement. The thoracic pedicle screws can either be placed parallel to the endplates (straightforward trajectory), or they can be placed parallel to the axis of the pedicle itself (anatomic trajectory).

Fig. 2. Artist's illustration of the difference between the anatomic (AT) and the straightforward (ST) trajectories for thoracic pedicle screw placement. The thoracic pedicle screws can either be placed parallel to the endplates (straightforward trajectory), or they can be placed parallel to the axis of the pedicle itself (anatomic trajectory).

In adolescents, the pedicle often dilates with this successive tapping, and this allows for placement of a 4.5-mm screw.

Another option for dealing with a small pedicle is to use the 'in-out-in' technique for screw placement [11]. In this technique, the screw enters the pedicle and then is purposely placed through the lateral wall of the pedicle and engages the bone of the transverse process and rib head. The screw subsequently reenters the pedicle more anteriorly and ends in the vertebral body [12, 13].

The surgeon should keep in mind that the pedicle on the concave side of the curve may be smaller than the pedicle on the convex side. Consequently, different screw widths may be needed for the contralateral pedicles of the same level in scoliotic patients [14].

Use of fluoroscopy can be helpful in establishing the appropriate sagittal plane angulation of the pedicle screw. Thoracic pedicle screws can either be placed parallel to the endplates (straightforward trajectory), or they can be placed parallel to the axis of the pedicle itself (anatomic trajectory) (fig. 2). When placed parallel to the axis of the pedicle (anatomic trajectory), a longer screw can be used, and the tip of the screw is directed at the anterior, inferior vertebral endplate [15-17].

Anterior Approach

Another alternative is to perform the correction through an anterior approach by releasing the anterior longitudinal ligament, debriding the discs anteriorly, packing the disc spaces with bone graft harvested from a rib, placing vertebral body screws, and derotating the spine with a connected rod. This can be done either through a video-assisted thoracoscopic surgery (VATS), via an open thoracotomy, or via a 'mini-open' thoracotomy, which is a hybrid technique utilizing a small thoracotomy inferiorly and VATS superiorly [18, 19].

In patients who are not near skeletal maturity, an anterior operation may be required to release the spine anteriorly and remove the growing vertebral endplates. The surgeon can choose to perform only an anterior release and epiph-ysiodesis with a subsequent posterior instrumented procedure, or he can perform an anterior release, derotation, and fusion through a single anterior approach.

The anterior approach for derotation and fusion has advantages and disadvantages. VATS offers the advantages of decreased blood loss, sparing of the posterior muscular tension band, and a smaller surgical scar. However, VATS requires a lung takedown (with subsequent chest tube placement) and entails a longer operative time (often double the time of a posterior operation).

Anterior derotation and fusion procedures typically do not achieve the same degree of curve correction that posterior procedures do; this is likely due to the limitations in the number of segments that can be accessed via an anterior approach. When patients are hyperkyphotic the anterior approach is not possible, as access to the anterior vertebral bodies is limited. In addition, long-term follow-up of anterior derotation and fusion procedures is currently lacking. Consequently, a posterior derotation and fusion operation is still the goldstandard operation for correction of AIS.

Double Thoracic Curves (Lenke Type 2)

In progressive double thoracic curves, both curves are structural. Consequently, the fusion will need to extend to the stable vertebrae above and below both the curves. The preferred approach for the correction of double thoracic curves is the posterior approach.

An anterior approach for these curves may be necessary as an adjunct to a posterior correction and fusion (for an anterior release and epiphysiodesis), but instrumentation and curve correction via an anterior approach are very difficult, if not impossible due to the difficulty of accessing and instrumenting numerous thoracic levels.

Double Major Curves (Lenke Type 3)

Double major curves have a structural thoracic and a structural thoraco-lumbar or lumbar curve. In general, the instrumentation should extend to the stable vertebra above the thoracic curve and the stable vertebra below the tho-racolumbar (or lumbar) structural curve.

An area of controversy in treating progressive structural curves that extend to the low lumbar spine is whether or not to extend the fusion to the sacrum. In general, in the treatment of AIS, the instrumentation should extend to the stable vertebra at the inferior end of the lowest structural curve (which is rarely at L4 or below) and fusion to the sacrum should be avoided unless the low lumbar discs are severely degenerated.

Extension of fusion to the sacrum has several disadvantages. First of all, the risk of pseudoarthrosis increases when a long construct is taken down to the sacrum [20]. Also, long constructs can fail due to screw pullout from the sacrum. In addition, extension of the fusion to the sacrum increases the risk of not completely correcting a patient's spinal imbalance. Patients often rebalance themselves following scoliosis corrections by accommodating any offset from the balanced state through alterations in the position of the lower lumbar spine. When the lumbosacral junction is fused, this accommodation potential is lost. Finally, extension to the sacrum runs the risk of creating flat-back syndrome if an appropriate amount of thoracic kyphosis and lumbar lordosis is not placed in the rods.

Adult Scoliosis

Adult Patients with Progressive AIS

The indications for surgery in this patient population are curve progression greater than 5° in a single year, back pain that is not relieved by conservative measures, pulmonary compromise from severe curvature, and unacceptable cosmetic deformity. Of these, the two most common indications are unrelenting back pain and cosmetic deformity.

Surgical Correction

The principles for surgical correction of adult patients with AIS are similar to those for adolescents with AIS. The curve can be evaluated and classified by the Lenke system. The instrumentation pattern is dependent upon the curve classification.

There are, however, two main differences between treating AIS in adults versus adolescents. First, the rate of pseudoarthrosis is higher for adults than for adolescents. Consequently, adults may need both anterior and posterior surgery to achieve a solid fusion. The anterior approach is often required in long curves to release the anterior longitudinal ligament and incise disc spaces to allow for a rigid curve to be corrected. The disc spaces can then be packed with structural graft to promote fusion, and a subsequent posterior procedure can be performed to instrument and correct the curvature.

The second major difference between adults and adolescents with AIS is that adults often have significant disc degeneration and spondylosis in the lumbar spine. Stopping a long segment fusion at a stable and neutral vertebra in the mid lumbar spine may not be appropriate in the face of significant lumbar spondylosis and degenerative disc disease (DDD). Patients with significant lumbar spondylosis and DDD may have progression of the spondylosis or significant low back pain from accelerated disc degeneration due to the greater loads on these segments following a long segment fusion above. Consequently, the need for extension of the fusion to the sacrum is greater in adults than it is in adolescents. However, if there is no significant lumbar spondylosis or disc degeneration, then stopping the fusion in the mid lumbar spine is appropriate.

Degenerative Adult-Onset Scoliosis

Degenerative adult-onset scoliotic curves are often rigid, lumbar curves. The indications for operative intervention are curve progression and unrelenting low back and radicular pain (not responsive to the conservative measures previously discussed). Often patients have significant radicular pain on the concave side of the curve due to severe foraminal compromise. Patients with these curves may be in overall spinal balance as many of them compensate at the thoracolumbar and lumbosacral junctions. Consequently, evaluation of overall spinal balance with 36-inch scoliosis x-rays is important. If a patient is a surgical candidate but is in overall spinal balance, then correction of the curve is not necessary.

Surgical Correction

In the subgroup of patients who are in overall spinal balance, there are two surgical treatment options. First, if the patient suffers from primarily radicular symptoms, then a unilateral Wiltse approach and decompression (posterolateral approach between the multifidus and longissimus muscles to perform an extraforaminal nerve root decompression) on the concave side of the curve are often sufficient to provide relief from radicular pain [21]. The advantage of this option is that the posterior ligamentous tension band is left intact and bony removal of the posterior column is kept to a minimum. Only the lateral portion of the facet joints is removed and the roof of the neural foramen is opened from lateral to medial.

If the patient, who is in overall spinal balance, is suffering from both low back and radicular pain, then another surgical treatment option is needed. For these patients, we recommend foraminal decompression on the concave side of the curve with an in situ instrumented fusion with pedicle screw-rod instrumentation. The foraminal decompression from a standard midline posterior approach will address the radicular symptoms. The fusion will relieve the low back pain (which is likely secondary to DDD and lumbar spondylosis). We always use autograft bone harvested from the iliac crest to establish the fusion.

Interbody PyrameshSpine Concave Deformity

Fig. 3. a Preoperative 36-inch x-ray of a fixed thoracolumbar scoliotic deformity in an adult. b Postoperative 36-inch anterior-posterior x-ray of the same patient following an anterior lumbar release with interbody fusion (Pyramesh, Medtronic Sofamor Danek) and a subsequent posterior multisegmental instrumented correction with thoracic hooks superiorly and thoracic and lumbar pedicle screws inferiorly (M-10 system, Medtronic Sofamor Danek).

Fig. 3. a Preoperative 36-inch x-ray of a fixed thoracolumbar scoliotic deformity in an adult. b Postoperative 36-inch anterior-posterior x-ray of the same patient following an anterior lumbar release with interbody fusion (Pyramesh, Medtronic Sofamor Danek) and a subsequent posterior multisegmental instrumented correction with thoracic hooks superiorly and thoracic and lumbar pedicle screws inferiorly (M-10 system, Medtronic Sofamor Danek).

When patients with degenerative adult-onset scoliosis are not in overall spinal balance and when their symptoms cannot be controlled by conservative measures, then correction of the scoliotic curvature is needed (fig. 3a). This is best accomplished by a combined anterior and posterior approach. An anterior approach is often helpful to release the rigid curve by incising the anterior longitudinal ligament and the anterior disc spaces. Autograft-filled Pyramesh cages (Medtronic Sofamor Danek) can be placed in the anterior interbody space to promote fusion (fig. 3b). If no anterior column support with autograft is performed, then the risk of pseudoarthrosis is high in this elderly patient population.

A subsequent posterior approach with a coronal plane wedge osteotomy of the convex facets is performed (i.e. the convex facets are removed from the pedicle above the neural foramen to the pedicle below the neural foramen). The wedge-shaped osteotomy of the convex side of the curve prevents compression of the exiting nerve roots on the convex side. This osteotomy is extended beyond the midline and includes the convex side of the laminae in order to avoid thecal sac compression when the convex side is compressed. Pedicle screws are then placed at all levels from the superior to the inferior stable vertebrae of the lumbar scoliotic curve. Rods are connected to the screws and the convex side of the curve is compressed while the concave side is distracted, and the curve is straightened.

If the surgeon wishes to perform the entire operation from a posterior approach, then multilevel transforaminal lumbar interbody fusions can be performed and Pyramesh cages filled with autograft can be placed into the inter-body space through this approach. Fusion to the sacrum is avoided if the inferior stable vertebra is at L4 or above and if the L4/5 and L5/S1 discs are in good condition. If the lumbosacral junction is not fused, then additional curve compensation is possible for the patient. However, if the curve extends to L5 or if the low lumbar discs are significantly degenerated, then fusion to the sacrum is often needed. There are several options for extension of the fusion to the sacrum. First of all, S1 (and S2) pedicle screws can be placed. However, the sacral pedicles have large cancellous centers, and they often do not allow for solid screw purchase. Sacral pedicle screws have greater pullout strengths if they are bicortical, and lateral fluoroscopy can assist with bicortical placement.

Long segment fusions to the sacrum are subject to large cantilever loads, which can lead to pullout of sacral pedicle screws, sacral fractures, and lum-bosacral pseudoarthrosis. Consequently, some surgeons elect to supplement sacral pedicle screws with intrasacral rods, transacral rods, and/or iliac screw fixation [22, 23]. Iliac screw fixation can be hindered by harvesting posterior iliac crest bone graft. However, the harvest of posterior iliac crest bone graft does not completely preclude the placement of iliac crest screws [24].

Kyphosis

Surgical treatment of kyphosis can be considered if there is a rigid thoracic or thoracolumbar kyphotic curve greater than 60-70° in either the adolescent or the adult. Curves of 60-70° are often cosmetically deforming, but they usually do not cause significant cardiopulmonary symptoms. However, these curves can cause intractable back pain that is not responsive to conservative treatment measures. Curves over 80-90°, on the other hand, can cause cardiopulmonary compromise and surgical correction is definitely indicated.

Osteotomy Ankylosing Spondylitis

Fig. 4. Artist's illustration of a three-column PSO. The technique involves removing a large wedge of the posterior and middle columns (including the pedicle) of the vertebral body. The remainder of the posterior and middle columns is then approximated by pulling together pedicle screws above and below the osteotomy level. The anterior column of the PSO level acts as the fulcrum for this correction.

Fig. 4. Artist's illustration of a three-column PSO. The technique involves removing a large wedge of the posterior and middle columns (including the pedicle) of the vertebral body. The remainder of the posterior and middle columns is then approximated by pulling together pedicle screws above and below the osteotomy level. The anterior column of the PSO level acts as the fulcrum for this correction.

Surgical Correction

Surgical correction of large, structural thoracic or thoracolumbar curves can be performed through a posterior approach or a combined anterior and posterior approach. Correction through an anterior only approach is difficult because the severe thoracic kyphosis limits the surgeon's access to the anterior thoracic spine.

Curves of 60-70° can be treated via a posterior only approach, especially if the curves are flexible. The treatment strategy for these curves is to expose the ends of the curve, and then to either perform a pedicle subtraction osteotomy (PSO) or multilevel posterior closing wedge osteotomies (multiple Smith-Petersen osteotomies). PSO is useful for a focal thoracic kyphosis (i.e. thoracic wedge compression fracture) because with PSO, the majority of the correction is performed over a single segment. The technique involves removing a large wedge of the posterior and middle columns (including the pedicle) of the vertebral body. The remainder of the posterior and middle columns is then approximated by pulling together pedicle screws above and below the osteotomy level. The anterior column of the PSO level acts as the fulcrum for this correction (fig. 4), and up to 30° of correction can be achieved with a single level of PSO [25].

Prior to approximating the remainder of the posterior and middle columns in PSO, a Z-plasty of the underlying dura may be needed to avoid buckling of the dura into the spinal canal. We prefer to perform PSO below the conus to avoid injury to the spinal cord.

Spinal Column Buckling

Bony resection

Winn after Bridwell

Bony resection

Winn after Bridwell

Fig. 5. Artist's illustration of a Smith-Petersen osteotomy. The technique involves removing multiple wedges of the facets bilaterally over several levels. The remaining posterior columns are then pulled together (by pedicle screw-rod instrumentation) while the neural foramina are monitored closely to ensure that excessive foraminal narrowing does not occur. The fulcrum of the correction is the anterior disc space, and, consequently, in rigid curves, an anterior procedure may be needed to first incise the anterior longitudinal ligament.

Multiple Smith-Petersen osteotomies, on the other hand, are much safer because the correction is spread out over multiple levels. Only 1° of correction can be expected for every 1 mm of bone removed with this technique. Consequently, multiple Smith-Petersen osteotomies are needed to correct a significant kyphosis. The technique involves removing multiple wedges of the facets bilaterally over several levels (fig. 5) [26]. The remaining posterior columns are then pulled together (by pedicle screw-rod instrumentation) while the neural foramina are monitored closely to ensure that excessive foraminal narrowing does not occur. The fulcrum of the correction is the anterior disc space, and, consequently, in rigid curves, an anterior procedure may be needed to first incise the anterior longitudinal ligament and the anterior disc space [27].

Conclusion

Surgical correction of spinal deformity has advanced rapidly over the past decade. Correction of severe scoliotic and kyphotic deformities can now be performed with low mortality rates. However, the morbidity rates for major deformity corrections remain very high. New advances will likely blend more minimally invasive techniques with the current instrumentation systems in an attempt to reduce the surgical morbidity associated with multisegmental correction and instrumented fusion.

Acknowledgments

We are grateful to Drew Imhulse and Tom Fletcher for assistance with the radiographic images in the figures, to Sherry Ballenger for editorial assistance, and to Bill Winn for assistance with the illustrations. We thank Medtronic Sofamor Danek for permission to use several of the illustrations.

Disclosure Statement

The following authors are consultants for Medtronic Sofamor Danek: Stephen L. Ondra, MD and Rick C. Sasso, MD.

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Praveen V. Mummaneni, MD

Assistant Professor, Department of Neurosurgery, The Emory Clinic 550 Peachtree St., Suite 806, Atlanta, GA 30308 (USA)

Tel. +1 404 686 8101, Fax +1 404 686 4805, E-Mail [email protected]

Haid RW Jr, Subach BR, Rodts GE Jr (eds): Advances in Spinal Stabilization. Prog Neurol Surg. Basel, Karger, 2003, vol 16, pp 240-250

Vertebral Augmentation for Osteoporotic and Osteolytic Vertebral Compression Fractures: Vertebroplasty and Kyphoplasty

Isador Lieberman

The Cleveland Clinic Foundation, Cleveland, Ohio, USA

Osteoporosis is a systemic disease currently afflicting more than 40 million Americans; this figure is likely to rise further as the population ages. It results in progressive bone mineral loss and concurrent changes in bony architecture that leave bone vulnerable to fracture, often after minimal or no trauma. Osteolysis secondary to metastatic disease or multiple myeloma affects up to 70% of patients on initial presentation. The spine is the most common site of osteoporotic or osteolytic fracture, with vertebral compression fracture (VCF) occurring in up to 50% of women 80 years and older and 25% of women 70 years and older [1, 2]. Overall, 700,000 people per year in the US suffer a VCF secondary to osteoporosis, exceeding even the frequency of hip fractures [11]. Osteoporotic VCFs have been shown to be associated with up to a 30% age-adjusted increase in mortality [3]. The cost to society of managing osteoporotic VCF patients in the United States in 1995 was USD 746 million [4]. Possible acute complications of either osteoporotic or osteolytic vertebral fracture include cord compression, urinary retention, and ileus [10]. Long-term consequences include considerable pain (reported in 35% of detectable VCFs) [12], as well as pulmonary compromise (a 9% loss in predicted forced vital capacity with each vertebral fracture) [13]. Other chronic sequelae include decondition-ing, deformity, insomnia, and depression, resulting in substantial physical, functional, and psychosocial impairment [13, 14].

Nonoperative Management of VCFs

Two thirds of patients with acute, painful osteoporotic VCFs improve regardless of the treatment applied. Traditional, nonoperative management includes bed rest, analgesics, and bracing. This type of medical management, however, fails to restore spinal alignment, and the lack of mobility itself can result in secondary complications, including worsening osteoporosis, atelectasis, pneumonia, deep vein thrombosis, decubitus ulcer, and pulmonary embolism. An alternative approach is supervised ambulatory mobility by a physiotherapist plus hydrotherapy [15]. In one third of patients, severe pain, limited mobility, and poor quality of life persist despite appropriate nonoperative management. No patient spontaneously achieves a realigned spine, corrected sagittal contour, or restoration of vertebral height.

Half the patients with metastases to the spine report pain relief after external beam radiation [31]. Patients with radiosensitive tumors (breast, prostate, myeloma) typically do well, however radiotherapy does not protect the spine from progressive osteolytic collapse and presents the treating surgeon with major concerns regarding postoperative wound healing and bone fusion should surgery be indicated [32]. Similar concerns exist when considering the use of chemotherapy to treat various metastases to the spine, although there are new investigational bisphosphonates which are promising reversal of bone loss [33].

Operative Management of VCFs

Historically, the only alternative to nonoperative management for symptomatic osteoporotic or osteolytic vertebral fractures was open surgical decompression (anterior or posterior decompression and stabilization via internal fixation hardware and bone grafting), and this was usually reserved for those patients with gross spinal deformity or neurologic impairment (<0.5%) [10, 32]. The reason for this surgical caution was due to the adverse risk/benefit ratio in this elderly or cancer population with poor bone quality and multiple comorbid conditions.

Percutaneous vertebroplasty (PVP) is a minimally invasive method that involves the percutaneous injection of polymethyl methacrylate (PMMA) into a collapsed vertebral body to stabilize the vertebra. Originally developed for osteolytic metastasis, myeloma, and hemangioma, the procedure resulted in quick, effective pain relief and a low complication rate [5-7]. PVP is now also increasingly used for the treatment of osteoporotic vertebral fractures [8]. However, PVP does not expand the collapsed vertebra, potentially locking the spine in a kyphotic posture. In addition, the PMMA bone filler has associated problems (epidural leakage, thermal necrosis, inability to integrate with bone, handling difficulties, toxicity to patient and operator) [2, 9].

Kyphoplasty is a newer minimally invasive technique with a number of potential advantages over PVP, including lower risk of cement extravasation and better restoration of vertebral body height [29]. A cannula is introduced into the vertebral body, followed by insertion of an inflatable bone tamp (IBT), which when deployed, reduces the compression fracture and restores the vertebral body toward its original height, while creating a cavity to be filled with bone cement. The cement augmentation is therefore done with more control into the low-pressure environment of the preformed cavity with viscous, partially cured cement.

Percutaneous Vertebroplasty

Background

Percutaneous vertebral augmentation (vertebroplasty, PVP) was first devised by Galibert et al. [16] in 1987, and initially involved the augmentation of the vertebral body with PMMA during open procedures to allow stable fixation of internal hardware. PVP was first described in the French literature in 1987 [16] but was not performed in the United States until 1994. Originally targeted for osteolytic metastasis, myeloma, and hemangioma, PVP resulted in early appreciable pain relief and a low complication rate [7, 16]. Its indications now include osteoporotic vertebral collapse with chronic pain, expanding further to include treatment of asymptomatic vertebral collapse and even prophylactic intervention for at risk vertebral bodies [17]. Nevertheless, the treatment of acute fractures in ambulatory patients and prophylactic treatment remains controversial [18]. In fact, vertebral augmentation itself is somewhat controversial, with questions concerning a lack of defined indications, expected complications, outcome measures, and the need for long-term follow-up data [2].

An open question in PVP is the mechanism of pain relief. The most intuitive explanation involves simple mechanical stabilization of the fracture; the cement stabilizes vertebral bodies and offloads the facet joints. However, another possibility is that analgesia results from local chemical, vascular, or thermal effects of PMMA on nerve endings in surrounding tissue [8, 19]. Supporting this concept is the lack of correlation between cement volume and pain relief [20, 21]. Further evidence against an effect resulting solely from mechanical stabilization is the fact that PVP typically does not restore lost vertebral body height and therefore does not correct altered biomechanics [1, 18].

Technique

Injection of opacified PMMA is performed via a transpedicular or para-vertebral approach under continuous fluoroscopic guidance to obtain adequate filling and to avoid PMMA leakage. For complex or high-risk cases, CT and fluoroscopic guidance are sometimes combined [5, 18]. In routine cases, PVP can be performed under local anesthesia with slight sedation in less than an hour [1], although general anesthesia is sometimes required because pain may intensify during cement injection [8]. Preceding PMMA injection, intraosseous venography is often used to determine the filling pattern and identify sites of potential PMMA leakage (outline the venous drainage pattern, confirm needle placement within the bony trabeculae, and delineate fractures in the bony cortex). However, others have dispensed with routine venography [1].

Contraindications to vertebroplasty include coagulopathy, absence of facilities to perform emergency decompressive surgery in the event of a complication, and extreme vertebral collapse (>65-70% reduction in vertebral height) [8]. However, this last contraindication has been questioned recently [18, 22].

Results

The available clinical studies, mostly European, report pain relief in about 90% of cases treated for osteoporotic fracture, with only infrequent clinically significant complications (0-10%), most of them minor [7, 18]. In a series of 80 patients treated with PVP for osteoporotic vertebral collapse, 90% gained immediate pain relief [5]. During a follow-up of 1 month to 10 years, only one complication was reported: intercostal neuralgia treated by local anesthetic infiltration. In another prospective study of 45 vertebral body augmentations in 17 osteoporotic patients led to significant and lasting pain reduction during the 1-year follow-up [1]. Although cement leakage occurred in 20% of vertebral bodies, none had clinical sequelae. In a retrospective review of 70 augmented vertebrae in 38 consecutive patients with osteoporosis, treatment resulted in pain relief within 48 h in 36 patients (95%). The pain relief was durable in 34 of the patients (89%) during a follow-up averaging 18 months. Twenty-four patients (63%) experienced marked to complete pain relief, 12 (32%) moderate relief, and 2 (5%) no significant change [18]. Of 8 patients suffering malignant neoplasm of the spinal column and treated with vertebroplasty, 4 (50%) found pain relief in this series [18]. Cortet et al. [23] studied 16 patients with 20 osteoporotic vertebral compression fractures who underwent PVP. This study found a statistically significant decrease in pain with several standardized scoring systems at all observed time points during the 6-month follow-up, along with concurrent, significant improvement in overall health status. No adverse events and no vertebral fractures occurred during the follow-up period. Another study of 29 patients with 47 painful osteoporotic vertebral fractures reported a 90% success rate in terms of significant pain relief immediately after treatment [7]. Two patients sustained rib fractures during the procedure resulting in pain that subsequently resolved; otherwise, no clinically significant complications were noted.

Complications

The principal risk of PVP, which involves the forced injection of low-viscosity PMMA cement into the closed space of the collapsed vertebral body, is cement extravasation. Extravasation rates are as high as 40% when used to treat osteoporotic fractures [7]. The likelihood is greater when using cement with a liquid rather than paste consistency or with higher PMMA volume [24]. However, in most settings, the majority of extravasations have no clinical relevance, at least in the short term [1].

The consequence of an extravasation depends on its location. In epidural or foraminal extravasation, nerve root compression and radiculopathy are the major risk, occurring in 11 of 274 patients (4%) treated by Deramond et al. [5]. Three patients required surgical nerve root decompression, as has been described by others as well [10, 20]. Extravasation into perivertebral veins can cause cement embolism to the lungs; deaths attributed to cement embolism have been documented. However, the 2 deaths attributed to pulmonary embolism were reported to be unrelated to the procedure; no cement material was detected by chest x-ray of the first patient [8, 25], and the second pulmonary embolism arose from deep venous lower extremity thrombosis [5]. On the other hand, extravasation into adjacent disks or paravertebral tissue, although common, generally produces no patient symptoms and carries little clinical significance; many such extravasations can be avoided by careful needle positioning [5].

Other operative and long-term complications of PVP are specific to PMMA as a filler [1, 17, 26]. The physician may work with PMMA in large batches in order to keep it liquid and to extend the working time for vertebroplasty. However, its high polymerization temperature (86-107°C within cement core) [27] can damage adjacent tissue, including the spinal cord and nerve roots [9], leading to an inflammatory reaction and transitory exacerbation of pain [8]. When injecting PMMA monomer, vigilance and caution of the physician are required. Absorption of PMMA during the injection can induce hypotension by virtue of its cardiotoxic and arrhythmogenic properties [28]. Keeping in mind that placing a material in the spine affords proximity and access to the chest and the heart, vertebral augmentation with PMMA demands meticulous attention to technique.

Overall, the risk of complications that carry clinical significance following PVP for osteoporotic vertebral fracture is 1-3%, and most potential complications can be avoided with a good technique [5].

Kyphoplasty

Background

Kyphoplasty is a new technique evolved from a marriage of vertebroplasty with balloon angioplasty. It has a number of potential advantages, including lower risk of cement extravasation and better restoration of vertebral body height. A cannula is introduced into the vertebral body, via a transpedicular or

Fig. 1. a-c Kyphoplasty example, IBT inflation and PMMA augmentation.

extrapedicular route, followed by insertion of an IBT, which when deployed, reduces the compression fracture and restores the vertebral body toward its original height. This then creates a cavity to be filled with bone cement. The cement augmentation can now be completed with more control into the low-pressure environment of the preformed cavity with viscous, partially cured cement. Using a cannula for bone filler with a steel stylet as a plunger enables the operator to apply cement at considerably higher viscosity than is possible with injection through a 5-ml syringe and 11-gauge needle. Both the higher cement viscosity and lower-pressure injection reduce the risk of cement extravasation. Filling is performed under continuous lateral fluoroscopic guidance similar to vertebroplasty. The procedure can be performed under general anesthesia or local anesthesia with intravenous sedation; some patients are able to return home the same day of procedure.

Technique

With the patient under general or local anesthesia in prone position on a radiolucent spinal frame, two C-arms are positioned for anteroposterior and lateral fluoroscopic images. Once positioned the C-arms or patient are not moved to ensure repeatable images throughout the case. Two 3-mm incisions are made at the vertebral level, parallel to the pedicles in both planes. Then a guide wire or biopsy needle is advanced into the vertebral body via a transpedicular or extrapedicular approach, depending on fracture configuration and patient's anatomy. The guide wire is then exchanged for the working cannula using a series of obturators. Once the working cannula is positioned the surgeon reams out a corridor to accommodate the IBT and positions IBT under the collapsed endplate. To deploy the IBT, inflation proceeds slowly under fluoroscopy until maximum fracture reduction is achieved or the balloon reaches a cortical wall (see fig. 1). At this point the surgeon deflates and removes the IBT, mixes the cement, prefills the cement cannulae and allows the cement to partially cure in the cement cannulae. Once partially cured PMMA is slowly extruded into the vertebral body through each pedicle under continuous lateral fluoroscopic guidance. This technique permits a low-pressure fill. In most instances, the volume of cement can slightly exceed that of the bone cavity to interdigitate filler from the central bolus with the surrounding bone. Once filling is complete and the cement has hardened the surgeon removes the cannula and closes the 3-mm incisions.

Results

Kyphoplasty has been slowly advanced through a few surgical centers participating in a multicenter study begun in 1999. A phase I efficacy study of 70 consecutive kyphoplasty procedures in 30 patients with painful, progressive osteoporotic/osteolytic VCFs was recently completed [29]. Mean duration of symptoms was 5.9 months. Symptomatic levels were identified by correlating the clinical data with MRI findings. Preoperative and postoperative x-rays were compared to calculate the percentage height restored. Outcome was further assessed by comparing the preoperative and latest postoperative survey of patient's self-reported health status using the 36-item Short Form Health Survey (SF-36) [30]. In 70% of the vertebral bodies, kyphoplasty restored on average 47% of the lost vertebral height (p = 0.001). SF-36 scores for bodily pain (p = 0.0001), physical function (p = 0.002), and vitality (p = 0.001) were among the subscales that showed substantial and significant improvement. Complications were infrequent. One patient experienced perioperative pulmonary edema and a myocardial infarction secondary to intraoperative fluid overload; 2 patients suffered rib fractures due to positioning during the procedure. Cement leakage occurred at 6 of 70 treated levels (8.6%); however, there were no complications that related directly to selection of this technique or to use of the IBT. In an ongoing evaluation the results of this initial series have been maintained in the most recent follow-up of over 70 consecutive patients up to 14 months.

In a second prospective evaluation the safety and efficacy of kyphoplasty in the treatment of osteolytic vertebral compression fractures due to multiple myeloma found similar satisfying results [34]. Fifty-five consecutive kypho-plasty procedures were performed in over 27 sessions in 18 patients. The mean age of patients was 63.5 years (48-79), the mean duration of symptoms was 11 months, and the mean follow-up 7.4 months. The range of levels treated were from T6 to L5 (T11 = 9, T12 = 7, L1 = 8, L2 = 7). There were no major complications related directly to the use of this technique. On average, 34% of height lost at the time of fracture was restored. After stratifying for those where height was not restored the remaining vertebral bodies showed an average of 56% height restoration. Asymptomatic cement leakage occurred at 2/55 levels (4%).

Significant improvement in SF-36 scores occurred for bodily pain: 23.2 to 55.4 (p = 0.0008) and physical function: 21.3 to 50.6 (p = 0.0010), vitality: 31.3 to 47.5 (p = 0.010), and social functioning: 40.6 to 64.8 (p = 0.014). The authors concluded that the kyphoplasty technique was efficacious in the treatment of osteolytic vertebral compression fractures due to multiple myeloma and associated with early clinical improvement of pain and function as well as some restoration of vertebral body height in these patients.

Vertebroplasty versus Kyphoplasty

Although both vertebroplasty and kyphoplasty provide excellent pain relief, kyphoplasty has the potential to improve spine biomechanics and decrease the risk of cement extravasation. PVP usually will not expand the vertebral body or regain normal spine alignment. Preliminary data indicate that kyphoplasty may restore near-normal height, preventing kyphosis that leads to respiratory and digestive problems. Restoration of height and sagittal alignment may also work to protect vulnerable vertebral levels above or below the site(s) treated by minimizing force transfer.

PVP is much more prone to cement leaks since the PMMA is injected in a liquid state and will take the path of least resistance through any cracks in surrounding bone. In administering vertebroplasty the operator injects the liquid cement, typically pausing or stopping once a leak becomes evident. On the other hand, in kyphoplasty, the expanded balloon creates a cavity and pushes bone to the edges of the cavity, thus sealing off potential fissures and cracks. Greater placement control is possible in a kyphoplasty, in which the operator can fill the cavity with a more viscous cement to the point at which the cement bolus reaches and interdigitates with the bony margins. The initial kyphoplasty findings show lower rates of cement extravasation compared with published vertebroplasty series, supporting the hypothesis that injection of high-viscosity cement into a previously formed cavity may be an improvement over the injection of low-viscosity liquid cement into the unreduced vertebral body.

Conclusion

Osteoporotic and osteolytic VCFs pose a significant clinical problem including spinal deformity, pain, reduced pulmonary function and mobility, as well as an overall increase in mortality in the elderly. Traditional medical and surgical options in many cases prove inadequate.

Minor Kyphosis Picture

Fig. 2. Kyphoplasty example, height and kyphosis restoration. a Postheight = 37 mm, midheight = 12mm, kyphosis = 25°. b Postheight = 37mm, midheight = 28mm, kyphosis = 10°.

Fig. 2. Kyphoplasty example, height and kyphosis restoration. a Postheight = 37 mm, midheight = 12mm, kyphosis = 25°. b Postheight = 37mm, midheight = 28mm, kyphosis = 10°.

PVP is a relatively noninvasive technique that has gained increased acceptance over the last decade in the treatment of symptomatic osteoporotic vertebral fractures. The available clinical studies describe pain relief achieved in greater than 90% of symptomatic osteoporotic fractures, with only infrequent, mostly minor, complications. Some of the drawbacks of PVP stem from the use of PMMA, because of its toxicity and poor handling characteristics, rather than from the procedure itself.

Kyphoplasty is a modification of PVP that is still in evolution. It may add a margin of safety by virtue of a lower observed incidence of cement leakage. Kyphoplasty may be shown to be worthwhile in acute vertebral fracture, in high-risk patients to predictably restore vertebral height (see fig. 2) and to facilitate a low-pressure fill procedure. Favorable outcomes in early trials appear to permit early mobilization, which has the potential to decrease mortality. Considering the greater mortality that is associated with osteoporotic compression fractures, early mobilization in these patients is of prime importance.

The next logical step beyond treatment of evident vertebral fractures is prophylactic augmentation. Prevention of osteoporotic vertebral fractures with a combination of pharmacologics and timely reinforcement of at risk osteoporotic vertebrae is the ultimate goal aside from prevention of osteoporosis itself. It is here that new osteoconductive synthetic composites will figure more prominently as an emerging alternative to cement. Advances in minimally invasive surgical techniques, imaging, and synthetic engineering are rapidly changing the treatment protocols available for osteoporotic compression fracture.

References

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24 Martin JB, Jean B, Sugiu K, et al: Vertebrop

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