Curve Definitions

The Lenke type 3 or double major curve is defined by both a structural MT (major) and a structural but minor thora-columbar/lumbar (TL/L) curve ( Fig. 20.1A ). In the Lenke system, these curves typically have a lumbar ?C? modifier but may also have a lumbar ?B? or even an ?A? modifier.5 There may be varying degrees of TL kyphosis between the two curves that can affect treatment decisions. As discussed earlier, TL kyphosis >20 degrees from T10 through L2 also defines a curve as structural, even when the side-bending Cobb-angle measurement is <25 degrees. A multi-center study of 606 patients demonstrated a prevalence of 11% for Lenke type 3 double major curves.6

The Lenke type 4 or triple major curve pattern has a structural curve in all three regions of the spine, consisting of a PT (minor), MT (major), and TL/L (minor) curve ( Fig. 20.1B ). The lumbar apical vertebrae will typically lie lateral to the center sacral vertical line (CSVL) (lumbar modifier C) for curves of this type; however, this is not always the case.6 The type 4 curve is also considered the least common of the curves in idiopathic, with a reported frequency in the literature of 1.4 to 3%.6–8

The Lenke type 6 curve is also a structural double curve ( Fig. 20.1C ). In contrast to a Lenke type 3, the type 6 curve pattern resembles the King type I curve pattern, with the TL/L curve being the larger or major curve. All Lenke type 6 curves have a lumbar ?C? modifier.1 Lenke type 6 curves are also rare, with a prevalence just above 3%.6

Treatment Principles

The surgical management of Lenke type 3, 4, and 6 curves follows the same set of basic principles that apply to all AIS curves. The primary goal is to prevent progression while achieving maximal correction of deformity. This objective must not be achieved at the expense of maintaining or achieving optimal coronal and sagittal balance. Axial derotation to a neutral position should be attempted. Most thoracic deformities in AIS are accompanied by thoracic hypokypho-sis or even thoracic lordosis. Correction of these sagittal deformities should also be attempted.

Improvement in the cosmetic appearance of the patient also plays an important role in the treatment of AIS. Although it is difficult to evaluate, cosmetic deformity can have profound psychosocial effects and drive patients to seek surgery.9,13 Rib prominence, shoulder balance, flank symmetry, and a postoperative scar have all been shown to have an impact on patient satisfaction following the surgical treatment of AIS.14 By achieving physiological coronal-, sagittal-, and axial-plane correction, the surgeon will optimize body symmetry and the patient’s self-image.

Minimizing the number of fusion levels remains a fundamental principle of surgery for AIS. With the introduction of instrumentation by Harrington in the late 1950s, fusions were long and typically extended into the lower lumbar region.15,16 Long-term follow-up studies of patients who had such fusions have found that they have greater than average back pain and risk of lumbar degeneration below the end-instrumented vertebra (EIV).17–22 Reducing the number of fused vertebral levels maximizes spinal flexibility and distributes stress across a larger number of remaining distal lumbar motion segments.23 Theoretically, this may diminish the long-term risk of disc degeneration at adjacent distal levels.

Beyond these basic principles, little has been written to guide the surgeon in treating double and triple major curves. Because these curves are less common than others, they have typically been discussed as part of larger, more comprehensive series of AIS patients. The traditional treatment for double and triple major curves has been arthrodesis of all curves.3,4,24–29 In the Lenke classification, the recommendation is fusion of all the structural curves to prevent coronal decompensation through the uninstrumented curve.1 In addition, the literature contains information on the treatment of these curves with third-generation hook-and-rod implants. To our knowledge, there is no analysis of double and triple major curves treated with current pedicle screw-rod implant technology. Barr et al reported that lumbar pedicle screws provided greater lumbar curve correction as well as maintenance of the correction in double major curves treated with hybrid constructs.28 Some authors believe that all-pedicle-screw instrumentation provides better curve correction than all-hook or hybrid constructs.30,31 Other studies challenge this, reporting similar curve correction without the expense of pedicle screws.32–34 Regardless of whether or not pedicle screws improve coronal or sagittal curve correction, their true benefits may not be appreciated until the three-dimensional (3D) deformity of scoliosis can be adequately analyzed. By engaging both the posterior and anterior column of a vertebra, pedicle screws may improve axial rotation to a greater extent than do hooks or wires. However, it is unreasonable to subject young pediatric patients to preoperative and postoperative computed tomography (CT) to determine their axial deformity. New methods are currently being investigated for better analyzing the 3D deformity associated with scoliosis. Once a reliable and viable method is developed to fully quantify 3D spinal deformity, the effectiveness of pedicle-screw instrumentation can be compared with that of hybrid and other constructs.

Selective Fusion

In keeping with the principle of minimizing fusion levels, the concept of selective fusion was introduced. Selective fusion is defined as fusion of the major curve when the apices of both the thoracic and TL curves deviate from the C7 plumbline or CSVL, respectively. King et al discussed the concept when they recommended selective thoracic fusion for their type II curves.2 Lenke et al further refined this recommendation by listing parameters that would maximize success with selective thoracic fusions.27 The purpose of these parameters was to differentiate true double major curves from MT curves in which the thoracic curvature was greater or more rigid than that of the TL curve, or both. At 10 years postoperatively, Large and colleagues found that patients who had a successful selective fusion of a double major curve had significantly less back pain and stiffness than those who had both curves fused.35 Because their study predated the current Lenke classification, it is unclear whether all of the lumbar curves in Large and colleagues’ patients were structural and would have required fusion under the current guidelines. The criteria for selective thoracic fusion under the current Lenke classification requires that a structural lumbar curve remain mobile and unfused. If, however, Large and coworkers’ patients had true double major curves, this would call into question the definition of a structural TL curve as defined by the Lenke classification. Persistent lumbar motion may be a more important postoperative goal than a residual lumbar curve. If selective thoracic fusion is chosen, the surgeon will have to temper the degree of thoracic-curve correction to maintain coronal balance with the uninstrumented lumbar curve.

When considering fusion of the PT curve in a Lenke type 4 curve pattern, indications for arthrodesis can be extrapolated from data for double thoracic or King type V curves. Although spontaneous correction of the upper thoracic region can occur, both proximal extension of the instrumentation used in treating these curves, and failure to correct a PT curve, can have significant effects on shoulder balance.36–38 Before the development of the Lenke classification, the criteria for recommending instrumentation and fusion of the PT curve included a Cobb angle >20 degrees on bending films, >1 cm of translation at the apex of the curve, elevation of the left shoulder, 5 degrees or more of T1 tilt into the concavity of the curve, or transition between the two curves at T6 or below.39 One may consider making the proximal instrumented vertebra the vertebra at the apex of the PT curve if the T1 tilt is less than 5 degrees. Other literature has suggested that the best predictor of postoperative shoulder balance is the clavicle angle rather than the absolute or bending Cobb angle or the T1 tilt.37 The clavicle line is defined by the intersection of a tangential line connecting the two highest points of each clavicle with a horizontal reference line. If the clavicle angle is positive (left clavicle higher than right clavicle), then fusion of the PT curve is likely to result in improved shoulder symmetry. The criteria for fusion in the Lenke classification include a side-bending Cobb angle >25 degrees or T2 to T5 kyphosis >20 degrees, or both. Cil and coworkers attempted to evaluate these criteria by retrospectively evaluating patients who had undergone fusion before the development of the Lenke classification.40 They divided patients who had nonstructural PT curves into two groups: group 1 had a proximal fusion to T3 or above and group 2 had an upper instrumented vertebra at T4 or below. They then compared postoperative bilateral coracoid heights, clavicle angles, and T1 tilt in the two groups and found similar outcomes. This suggested that the Lenke criteria for leaving a PT curve unfused were safe. Unfortunately, Cil and coworkers did not comment on patient satisfaction or actual clinical assessments of shoulder heights. These data did not comment on whether structural PT curves absolutely required fusion, especially with a large MT curve and a low left shoulder. When considering fusion of a PT curve, we use the bending Cobb angle, but also evaluate the patient’s radiographic clavicle angle and clinical shoulder height. A patient with either a level or slightly lower left shoulder height may need instrumentation of the upper thoracic curve if correction of a large MT curve is anticipated. It is important to include the curve flexibility with the radiographic and clinical shoulder-height measurements when determining the need to instrument an upper thoracic curve.

Selective TL/L fusion is also a consideration in attempting to minimize fusion levels. In a study of 49 patients with major TL/L and ?partially structural? thoracic curves, Sanders et al assessed compensatory thoracic-curve correction when anterior fusion was done only on the TL/Lscoliosis.41 They determined that a lumbar-to-thoracic Cobb-angle ratio >1.25 and a thoracic curve of <20 degrees on bending films, as well as a closed triradiate cartilage, were the best predictors of successful selective fusion. Lenke et al expanded on this, utilizing the specific curve patterns of the Lenke classification as a template.42 Their radiographic criteria for selective fusion of type 6C curves included a ratio >1.25 for lumbar-to-thoracic curve Cobb-angle, apical vertebral translation and apical vertebral rotation, a flexible thoracic curve (ideally <25 degrees), and TL junctional kyphosis of <10 degrees. Their clinical criteria for selective fusion included a level or high left shoulder (for a right thoracic and left TL curve pattern), a lumbar truncal shift exceeding the thoracic truncal shift, a lumbar-to-thoracic curve ratio >1.2 by scoliometric measurement, and an acceptable rib prominence.

In the same article, Lenke and colleagues also suggested a more comprehensive list of parameters for selective thoracic fusion. Their radiographic criteria for selective thoracic fusion included a ratio >1.2 for thoracic-to-lumbar Cobb angle, apical vertebral translation and apical vertebral rotation, a flexible TL/L curve (ideally <25 degrees on side-bending), and TL kyphosis of <10 degrees. Clinical criteria for selective thoracic fusion included a level or high right shoulder, thoracic truncal shift exceeding lumbar waistline asymmetry, and a thoracic-to-lumbar curve ratio >1.2 by scoliometric measurement. Although not specifically assessed in the article, some Lenke type 3 curves may qualify for selective fusion.43

For both selective thoracic and selective TL fusion, these expanded radiographic and clinical criteria suggest that fusing curves simply on the basis of the Lenke classification is not absolutely adequate. Either the definitions for structural curves as delineated by the new classification system simply constitute an expandable template, or some structural curves do not need to be fused. Unfortunately, there have not been enough data about such fusion to fully resolve these issues.

Surgical Approaches

The traditional approach to the treatment of double and triple major curves is posterior instrumentation and fusion.28,44–46 Whether using pedicle screws, hooks, wires, or hybrid constructs, a posterior approach allows access to all curves through a single incision. Sagittal deformities can be addressed with multilevel posterior osteotomies.47

Anterior approaches, on the other hand, can be used in combination with a posterior approach for rigid curves, prevention of the crankshaft phenomenon, and the treatment of severe sagittal deformities.48–50 We consider anterior release for curves >90 degrees or particularly stiff curves that do not bend below 50 degrees. However, with pedicle-screw instrumentation and wide posterior osteotomies, the role of anterior release is less clear. Historically, as compared with posterior instrumentation, anterior instrumentation has been shown to save fusion levels and result in better correction of the uninstrumented lumbar curve.43,51–53 However, as new surgical instrumentation and techniques continue to be evaluated, this may not remain the case. When instrumentation is used in compression, anterior surgery of the thoracic curvature has more reliably resulted in the restoration of kyphosis than has posterior surgery.51,54 Anterior arthrodesis of both double major curves through a single incision has not been previously discussed in the literature. However, it has been done and will be discussed later in this chapter.

When considering anterior approaches, one must understand the associated morbidity. An anterior fusion through an open thoractomy has been shown to adversely affect pulmonary function.55–57 More recently, video-assisted thorascopic surgery (VATS) has become a popular alternative to open thoracotomy.54,58–60 Studies have demonstrated a reduced effect of VATS on pulmonary function testing.61–63 Whether this translates into improved clinical outcomes remains unclear. Interestingly, no significant diminution in pulmonary function was found for the thoracoabdominal approach despite the associated disruption of the diaphragm.64 In patients with severe pulmonary compromise, anterior thoracic surgery should be avoided because of a possibly significant risk of further deterioration.