Is a Surgically Instrumented Fusion Indicated?

Whether a surgically instrument fusion is indicated in a case of AIS remains one of the more controversial questions in its treatment. This is particularly so for curves in the 40-to 50-degree range. Treatment assumes that both the short-and long-term outcomes will be better with a fused spine than with the untreated natural history of AIS. It is the lack of knowledge in both of these situations over the long term that leaves the need for surgical treatment of these patients open to debate.

Traditionally, the magnitude of the spinal curvature in AIS as reflected by the Cobb angle has been the primary determinant of risk for curve progression in adulthood. Weinstein and colleagues14,15 suggested the tidemark of 50 degrees as the criterion for this in thoracic curves. With curvature greater than this there is a significant risk for progression. The risk with curves between 40 and 50 degrees is less clear, and with curves of <40 degrees the risk of adult progression appears low. There are, however, various other ways to define the qualities of a spinal curve besides the Cobb angle, and it seems almost certain that there are more precise ways to predict late progression.16 –18 Until these are identified, we use limited data and prudent judgment to make this most critical decision. With excellent instrumentation systems available, surgery for AIS in adults is less problematic. Therefore, it is more common in practice to recommend observation for a thoracic curve of up to 60 degrees accompanied by minimal cosmetic disfigurement and normal results of pulmonary function tests (PFTs).

Another unproven variable used in the decision to operate in cases of AIS relates to the length of the curve. The more vertebra within the Cobb angle the more severe the clinical deformity. Coronal balance is also an important variable, and when a lumbar curve exists that matches or balances the thoracic deformity, as in many curves of Lenke type 1C, the curve magnitude suggesting fusion increases. In addition, for patients who have not completed growth, the remaining growth potential may weigh into the decision about whether or not to operate, with surgery often being indicated for younger patients with curves >40 degrees, as compared with the traditional 50-degree indication for those who have completed growth.

The Inclusion of Minor Curves in the Fusion

In the MT curve pattern of AIS, the MT curve will clearly be surgically treated because it is the dominant deformity in this pattern of the condition. However, debate continues about when to include the minor lumbar curve. Selective thoracic fusion for the MT curve pattern of AIS was suggested by Moe more than 50 years ago,19 and for the most part this concept remains as valid today as it was then. The difficulty has been to find a reliable way to determine the structurality of the minor lumbar curve. The Lenke classification system was designed with the goal of answering this specific question, and suggests that minor nonstructural curves (which side-bend to <25 degrees) can be spared from fusion if there is no appreciable junctional kyphosis.

Several recent studies have supported the guidelines provided by the Lenke classification as appropriate for the majority of cases of AIS, supporting the use of selective thoracic fusion in King type II and Lenke type 1C curves ( Fig. 17.6 ). In 2003, Lenke and colleagues20 reported that selective thoracic fusion of the major curve could be successfully accomplished even when the minor lumbar curve deviated completely from the midline (lumbar modifier C), thus optimizing the postoperative number of mobile lumbar segments. In 2004, Edwards et al21 also described satisfactory results after selective thoracic fusion of properly selected curves with a Lenke C lumbar modifier. These latter authors concluded that mild coronal imbalance was well tolerated and did not necessitate distal extension of the fusion in such cases. On the basis of these studies of selected fusion in cases of the MT curve pattern of scoliosis with the most deviated lumbar apices, it appears that apical deviation alone does not dictate structurality. The biggest problem in what is considered a satisfactory result of surgery in such cases varies in the viewpoints of surgeons and patients.

Clements and co-workers22 reviewed the surgical procedures used for the cases in the HSG database before and after the Lenke classification was developed. The Lenke classification was applied retrospectively to cases treated before 2001, and was found to correctly predict which curves were actually fused in 82% of the cases. After the HSG adopted the use of the Lenke classification, the system predicted the curves actually fused in a significantly greater percentage of cases (88%; P = 0.001). Thus, the uniformity of treatment improved with the use of the Lenke classification system, but the system did not completely explain the practice of this group of experienced scoliosis surgeons. “Rule-breakers” were defined as patients whose treatment did not follow the recommendations of the Lenke classification system. From 6 to 29% of the time (depending on the curve pattern), other aspects of a patient’s clinical and radiographic deformity suggested deviation from the recommended treatment paradigm. From this rule-breaking, it can be concluded that the classification system does not identify the structural lumbar curve in 100% of cases of AIS. Breaking the “rules” of the Lenke classification system for choosing the minor curves to treat or not to treat in patients with AIS is appropriate; the trick is to know when to break the rules and to understand why they are broken. Being wrong 10% of the time is not good enough when addressing the motion of a child’s spine over its lifetime. Puno et al23 evaluated the usefulness of the Lenke classification system in providing treatment recommendations for idiopathic scoliosis. They compared the postoperative Cobb-angle correction and truncal shift in patients who were either treated or not treated according to the Lenke classification system, and reported better radiographic results when the Lenke classification system was used to select fusion levels with avoidance of the unnecessary fusion of nonstructural lumbar or thoracic curves and avoidance of the undercorrection of structural secondary curves. However, the distinction between different types of curves in the Lenke classification can be difficult (i.e., types 1C, 3C, and 6C curves with large thoracic and large thoracolumbar/lumbar [TL/L] spinal deformities), and can influence treatment decisions. Differentiation of these types of curves according to the Lenke classification system is based on arbitrary values for the magnitude and flexibility of the TL spine, and does not address 3D and clinical aspects of deformity.

Not surprisingly, the debate about when to include the lumbar curve in the fusion levels in cases of Lenke types 1C and 3C curves is most controversial ( Fig. 17.7 ). In 1992, Lenke and co-workiers24 described strict criteria for preventing lumbar decompensation in the performance of selective thoracic fusion. The rules were based on ratios of the magnitudes of the thoracic and lumbar Cobb angles, apical vertebral deviation from the midline, and apical vertebral rotation on the standing coronal radiograph. Despite these guidelines, a review by Newton et al25 of the frequency with which surgeons in the HSG included the lumbar curve in fusion in cases of Lenke types 1B and 1C curves found a substantial variation in the frequency of nonselective fusion into the lumbar curve (6 to 33%). Excluding cases of junctional kyphosis, the lumbar curve was included in the fusion for Lenke type 1B curves in only a few cases (2%). However, the incidence of inclusion of the lumbar spine in fusions of Lenke 1C curves varied from a low of 6% for one surgeon to a high of 67% for another. Disparity of this magnitude suggests a void in the knowledge base about the current definition of the structural lumbar curve. Factors associated with nonselective fusion in Newton and colleagues’ review included a lumbar curve of larger preoperative magnitude (42 ± 10 degrees vs. 37 ± 7 degrees, P < 0.01), greater lumbar apical deviation, (3.1 ± 1.4 cm vs. 2.2 ± 0.8 cm, P < 0.01), and a smaller ratio of thoracic-to lumbar-curve magnitude (1.31 ± 0.29 vs. 1.44 ± 0.30, P = 0.01). In addition, selective fusions were also performed if the trunk and chest wall were clinically more prominent than the corresponding features of the lumbar spine on examination in both the upright and forward-bending positions.

The concept underlying the initial analysis of a patient with a Lenke type 1 curve should be to fuse only the thoracic curve. With this approach the surgeon seeks a reason to include the lumbar spine in the fusion rather than a reason not to do so. One of the predictors of success of a selective thoracic fusion is a preoperative thoracic Cobb-angle to lumbar Cobb-angle ratio >1.2. Similarly, if the preoperative thoracic apical translation from the C7 plumbline is at least 1.2 times the lumbar apical translation from the CSVL, a selective thoracic correction will probably be acceptable.20 Ratios of relative rotation can also be considered, but when based on plain radiography will yield a less reliable assessment of axial deformity. Lastly, relative clinical deformity continues to be one of the most useful determinants of success in fusion surgery. The outcome of a selective thoracic fusion will not be favorable if the clinical deformity associated with the lumbar curve is more severe than that of the thoracic curve.

If a selective thoracic fusion for a Lenke 1C curve is attempted and the patient’s spine decompensates to the left, the opportunity exists to add the lumbar spine to corrective surgery at any time in the future. If the surgeon “plays it safe” and includes the lumbar spine unnecessarily, the patient may be predisposed to a greater risk of lumbar degenerative disease that might have been avoided. At some point the contrary argument can be made, that in the long term, a large residual lumbar deformity would be better fused but straight. The degree of lumbar deformity with intact motion that in the long term would yield the best outcome if treated by fusion at an early point is ill-defined and undetermined. Even the strongest proponents of preserving motion in the lumbar region would in most cases prefer lumbar curves of <40 degrees after selective thoracic fusion.

The balance between maximizing correction and maintaining lumbar motion is critical. A recent study26 defined values for quantifying this postoperatively. The deformity-flexibility quotient (DFQ) consists of the postoperative residual lumbar deformity divided by the number of unfused motion segments ( Fig. 17.8 ). All other aspects being equal, a lower DFQ implies a better outcome (less deformity and more motion). This concept has been validated in two ways within the HSG. First, the members of the HSG were asked to compare pairs of postoperative radiographs of varying degrees of lumbar deformity and lowest instrumented vertebrae. Use of the DFQ predicted with high probability those radiographs that the group considered “most ideal.” Additionally, the DFQ was calculated retrospectively for 155 AIS patients in the HSG database, and a lower DFQ was statistically correlated with greater satisfaction scores on the SRS-24 instrument at 2 years postoperatively. Following patient outcomes over a longer period is critical to assessing the success of a fusion, and correlating a patient’s DFQ with these outcomes may be of future value in deciding whether to do a selective or nonselective fusion.

With this in mind, some idea of the factors that predict spontaneous lumbar curve correction is required in preoperative decision-making. In a recent review of the HSG database, Patel et al27 found an average 50% spontaneous lumbar-curve correction (SLCC) after selective thoracic fusion. Radiographic features that correlated with the percent SLCC included the level of the LIV, lumbar-curve flexibility, and percent thoracic-curve correction. In no case did the SLCC reach the degree of correction seen on the lumbar bending film, which always exceeded the correction ultimately obtained.

To What Extent Should the Thoracic Curve Be Corrected for Ideal Balance?

Ideal balance is defined as existing when the head and trunk are centered over the pelvis in the coronal and sagittal planes. If both lumbar and thoracic curves are present in a case of scoliosis, they are generally of similar magnitude when the trunk is coronally balanced. The greater the lumbar curve as compared with the thoracic curve, the greater the risk of decompensation to the left. If the patient’s trunk is preoperatively shifted to the left, it is hard to imagine how a selective thoracic fusion could do anything but worsen this; yet in fact this is not always the case. Lumbar-curve correction following selective fusion is not always predictable.

The safest approach to selective thoracic fusion in the treatment of Lenke type 1C curves is to limit the degree of coronal-plane correction of the thoracic curve obtained during surgery. Correcting the thoracic curve to 50% of the magnitude of the preoperative lumbar curve, and assuming an average 50% spontaneous lumbar-curve correction, should limit the risk of decompensation. Intraoperative radiographs can assist in determining the amount of correction. However, this may overestimate the SLCC, because it does not include the effect of gravity. In theory, correction of a thoracic apical lordosis should be maximized because this relieves the torsional pressures that result from relative anterior overgrowth of the spine after fusion, and may in fact improve SLCC. This concept requires further investigation with 3D data sets.

Through the years there have been conflicting data about whether an anterior or a posterior approach yields the greatest SLCC. Early studies suggested more SLCC with an anterior approach. More recent analyses, using third-generation instrumentation, suggest better correction with posterior methods. A comprehensive review of the HSG database, using a multivariate regression analysis, determined that the LIV was important in determining the percent SLCC.27 Because posterior instrumentation was traditionally longer than that used in anterior procedures, some of the “spontaneous” correction with the use of posterior techniques was in fact to the result of instrumented correction of the upper vertebra of the lumbar curve. When controlling for the LIV, there was no difference in SLCC in 28 cases treated with an anterior approach and 28 matched cases treated with a posterior approach.

The response of the upper thoracic curve to thoracic-curve correction should also be considered. Whether appropriate or not, relatively little value is given to the upper thoracic motion segments in selective fusion. Rather, the trend seems to be to include more of the upper thoracic spine, especially with increasing correction of the MT curve. Suk and colleagues28 have stated that the upper thoracic curve should be included if the right shoulder is less than 1 cm higher than the left. The upper thoracic spine is one area in which side-bending to <25 degrees does not give enough information about when to include this region in a fusion. It is not clear whether a greater attempt should be made to preserve upper thoracic motion segments, limiting thoracic correction to preserve both balance (of the shoulders) and motion. Some nonstructural upper thoracic curves should be treated. Management strategies for these curves can be found in the discussion of Lenke type 2 curves in Chapter 18.