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Surgical Outcomes Research

Factors Involved in the Decision to Perform a Selective Versus Nonselective Fusion of Lenke 1B and 1C (King-Moe II) Curves in Adolescent Idiopathic Scoliosis

Newton, Peter O. MD*†; Faro, Frances D. , MD*†; Lenke, Lawrence G. , MD‡; Betz, Randal R. , MD§; Clements, David H. , MD§; Lowe, Thomas G. , MD∥; Haher, Thomas R. , MD¶; Merola, Andrew A. , MD¶; D’Andrea, Linda P. , MD§; Marks, Michelle , MS*; Wenger, Dennis R. , MD*†

Study Design. A retrospective evaluation of 203 adolescent idiopathic scoliosis patients with Lenke 1B or 1C (King-Moe II) type curves.

Objectives. To evaluate the incidence of inclusion of the lumbar curve in the treatment of this type of deformity as well as radiographic factors associated with lumbar curve fusion.

Summary of Background Data. In patients with structural thoracic curves and compensatory lumbar curves, many authors have recommended fusing only the thoracic curve (selective thoracic fusion). Studies have shown that correction of the thoracic curve results in spontaneous correction of the unfused lumbar curve; however, in some cases, truncal decompensation develops. Though there have been various attempts to define more accurately what type of curve pattern should undergo selective fusion, controversy continues in this area.

Methods. Measurements were obtained from the preoperative standing posteroanterior and side-bending radiographs of 203 patients with Lenke Type 1B or 1C curves from five sites of the DePuy AcroMed Harms Study Group. Patients were divided into two groups depending on their most distal vertebra instrumented: the “selective thoracic fusion” group included patients who were fused to L1 or above and the “nonselective fusion” group included patients fused to L2 or below. A statistical comparison was conducted to identify variables associated with the choice for a nonselective fusion.

Results. The incidence of fusion of the lumbar curve ranged from 6% to 33% at the different patient care sites. Factors associated with nonselective fusion included larger preoperative lumbar curve magnitude (42 ± 10°vs. 37 ± 7°, P < 0.01), greater displacement of the lumbar apical vertebra from the central sacral vertical line, (3.1 ± 1.4 cm vs. 2.2 ± 0.8 cm, P < 0.01), and a smaller thoracic to lumbar curve magnitude ratio (1.31 ± 0.29 vs. 1.44 ± 0.30, P = 0.01).

Conclusions. The characteristics of the compensatory “nonstructural” lumbar curve played a significant role in the surgical decision-making process and varied substantially among members of the study group. Side-bending correction of the lumbar curve to <25° (defining these as Lenke 1, nonstructural lumbar curves) was not sufficientcriteria to perform a selective fusion in some of these cases. The substantial variation in the frequency of fusing the lumbar curve (6% to 33%) confirms that controversy remains about when surgeons feel the lumbar curve can be spared in Lenke 1B and 1C curves. Site-specific analysis revealed that the radiographic features significantly associated with a selective fusion varied according to the site at which the patient was treated. The rate of selective fusion was 92% for the 1B type curves compared to 68% for the 1C curves.

Since the 1950s, when Moe wrote his classic article defining curve patterns in adolescent idiopathic scoliosis (AIS) and their proposed treatment, controversy has continued over the treatment of the compensatory lumbar curve. Moe defined Type II curves as curves in which the nonstructural lumbar curve was smaller and more flexible than the thoracic curve. This type is distinct from the Type I, or double major curves, in which both the thoracic and lumbar curves are structural and therefore require fusion. In contrast, Moe suggested that the lumbar curve in a Type II curve pattern is compensatory and would undergo spontaneous correction with selective fusion of just the thoracic curve. 1,2

Subsequent studies that focused on the treatment of this Type II or “false double major” curve have had varying conclusions about fusing the lumbar curve. Some agree with Moe’s guidelines, whereas others have put forth limitations of curve magnitude, stating that a lumbar curve greater than 40° to 45° should be fused regardless of flexibility. 3–6 Lenke et al reported that the King-Moe definition of a Type II curve was not sufficient to recommend fusion of the thoracic curve alone and emphasized the relative differences of the thoracic and lumbar severity as guidelines in the decision to fuse the lumbar curve. They proposed that to fuse selectively in a false double major curve, the thoracic curve should be at least 20% bigger, have at least as much apical vertebral rotation, and have 20% more apical displacement than the minor lumbar curve. 7

The Lenke classification system is a more recently developed treatment-based classification that defines curve patterns by region, magnitude, and flexibility as well as sagittal profile and displacement of the lumbar apex from the central sacral vertical line. The system defines many of these false double major curves as Lenke Type 1B and 1C curves. In this curve pattern, the thoracic curve is the largest curve. The smaller lumbar curve is nonstructural (i.e., has a side-bending Cobb measurement of 25° or less) and has thoracolumbar kyphosis that is less than +20°. The system further classifies these curve patterns by the degree of apical displacement of the lumbar apex (A, B, or C) (Figure 1). 8–10

The treatment algorithm for Lenke Type 1 curve patterns is the same as Moe’s—in general, fuse only the thoracic curve. In clinical practice, however, some patients with Type 1B and 1C curve patterns do undergo fusion of both curves. In a recent report by Lenke et al, selective thoracic fusion was performed in 62% of patients with 1C curves. 8 This study focuses on the controversy surrounding the Lenke Type 1B and 1C “rule breakers” in which both the thoracic and lumbar curves are fused.

The purpose of this study was to: 1) identify the frequency of fusing the lumbar curve in Lenke Type 1B and 1C curve patterns for five sites of the DePuy AcroMed Harms Study Group (DAHSG); and 2) determine differences in the preoperative radiographic features of the curves fused selectively (thoracic curve only) versus those fused nonselectively (both thoracic and lumbar curves).

 

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Spine: 15 October 2003 – Volume 28 – Issue 20S – pp S217-S223