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The Lenke Classification of Adolescent Idiopathic Scoliosis: How it Organizes Curve Patterns as a Template to Perform Selective Fusions of the Spine

Lenke, Lawrence G., MD; Edwards, Charles C. II,, MD; Bridwell, Keith H., MD

doi: 10.1097/01.BRS.0000092216.16155.33
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Study Design.  Retrospective radiographic review.

Objectives.  To analyze how the Lenke classification of adolescent idiopathic scoliosis provides a template of specific curve patterns that may be appropriate to perform selective fusion of the spine.

Methods.  A new triad classification system of adolescent idiopathic scoliosis has been developed. It consists of a curve type, a lumbar spine modifier (A, B, C), and a sagittal thoracic modifier (−, N, +). A selective fusion is termed when both the thoracic and thoracolumbar/lumbar curves deviate completely from the midline, but only the major curve (largest Cobb measurement) is fused, leaving the minor curve unfused and mobile. In this manner, selective thoracic fusions of the spine are potentially indicated for major main thoracic/minor lumbar curves (Types 1C and potentially 2C and 3C patterns) when the lumbar apex deviates off the center sacral vertical line. Conversely, selective thoracolumbar/lumbar fusions may be indicated for major thoracolumbar/lumbar–minor main thoracic curves, when the thoracic apex lies off the C7 plumbline (Type 5C and potentially 6C patterns). Importantly, additional analysis of ratios of structural characteristics between the main thoracic and thoracolumbar/lumbar curves are necessary to predict when a successful selective main thoracic or thoracolumbar/lumbar fusion will be feasible. Lastly, the clinical appearance of the patient’s truncal alignment is essential to confirm the aspirations of performing a selective spinal fusion.

Results.  Successful selective thoracic fusion of 1C (n = 36) and 2C (n = 8) curves have been performed in 44 consecutive patients with adolescent idiopathic scoliosis. The average thoracic curve was 61° before surgery and 39° at final follow-up. The average preoperative lumbar curve was 48°, decreasing to 32° postoperatively. A group of 21 consecutive patients with Type 5C or 6C major thoracolumbar/lumbar–minor main thoracic curves underwent a selective thoracolumbar/lumbar fusion. The average preoperative thoracolumbar/lumbar curve was 56° corrected to 22° at the 2-year follow-up. The average minor main thoracic curve preoperative was 38°, with spontaneous correction to 28° at 2 years postoperative.

Discussion.  Selective thoracic or thoracolumbar/lumbar fusion can be successfully performed in a variety of adolescent idiopathic scoliosis curve patterns. Careful attention to the preoperative Lenke curve classification, analysis of structural characteristics between the planned instrumented and noninstrumented regions of the spine, as well as a documented clinical examination that confirms the planned instrumented and fused regions of the spine to be the most clinically prominent are essential features to determine before surgery. No patients undergoing selective thoracic fusion have required extension of the fusion to the lumbar spine, whereas one patient with a selective thoracolumbar fusion required extension of the fusion up to include the thoracic spine due to continued thoracic progression with growth.

Conclusions.  Selective thoracic or thoracolumbar/lumbar fusions of the major curve can be successfully performed even when the minor curve completely deviates from the midline, based on the Lenke classification system, the analysis of structural criteria between the planned fused and unfused regions of the spine, and the clinical examination of the patient. Selective fusions, when successfully performed, will optimize mobile segments of the spine in patients with adolescent idiopathic scoliosis.

From the Department of Orthopaedic Surgery, Washington University School of Medicine, St. Louis, Missouri.

The device(s)/drug(s) is/are FDA approved or approved by corresponding national agency for this indication.

No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Address correspondence to Lawrence G. Lenke, MD, Jerome J. Gilden Professor of Orthopaedic Surgery, Washington University School of Medicine, Department of Orthopaedic Surgery, Suite 11300, One Barnes-Jewish Hospital Plaza, St. Louis, MO 63110, USA; E-mail: lenkel@wustl.edu

The use of classification schemes for the radiographic evaluation of adolescent idiopathic scoliosis (AIS) is important for many reasons including: being a teaching tool for trainees; a means of common communication between various scoliosis practitioners to have the ability compare various treatments of similar curve patterns; to allow comparison between various reports in the literature that describe new and potentially better ways of treating AIS; and as a method of recommending selective fusions of the spine when appropriate. The goal of performing a selective thoracic fusion was an important aspect of the King-Moe system, emphasized in the King II curve pattern. A selective fusion is termed when both the thoracic and thoracolumbar/lumbar curves deviate completely from the midline, but only the major curve (largest Cobb measurement) is fused, leaving the minor curve unfused and mobile. Although the King-Moe system has been the gold standard for the classification of AIS since the early 1980s, 1 several shortcomings were noted when attempting to use this system to evaluate the surgical treatment of various types of AIS curves including: difficulty with differentiating a true King Type II versus III curve, 2–4 noncomprehensive for all curve patterns, and poor to fair reliability among experienced scoliosis surgeons. 5,6 In an attempt to improve on these deficiencies, the Lenke et al classification system was developed. 7

The system begins with an evaluation of each of the three major spinal column regions that may develop operative curves: Proximal Thoracic (PT), Main Thoracic (MT), and Thoracolumbar/Lumbar (TL/L). The major curve is the one with the largest Cobb measurement and that will always be included in the fusion of operative AIS. The minor curves are all other nonmajor curves present. One of the main debates in scoliosis surgery is whether to include those minor curves in the fusion or not. Minor curve structural criteria were established to help guide the surgeon in this decision-making process. In the coronal plane, inflexibility on side bending ≥25° in each of the three regions render that region a structural minor curve. In addition, hyperkyphosis of ≥20° in the proximal thoracic (T2–T5) region or the thoracolumbar junction (T10–L2) renders either the PT region above and/or the MT or TL/L region below structural as well.

This system has designated three distinct lumbar curve patterns before surgery as lumbar modifiers A, B, or C. The lumbar modifier is based on the position of the center sacral vertical line (CSVL) to the apex of the lumbar curve. For lumbar modifier A, the line falls between the pedicles of the lumbar spine up to the stable vertebra. For lumbar modifier B, the line touches the apex of the lumbar curve. For lumbar modifier C, the apex of the lumbar curve falls completely off the midline depicting a curve with complete apical translation off the CSVL (Figure 1). In this context, a true selective thoracic fusion occurs when selectively fusing the thoracic curve with a C modifier lumbar curve that completely deviates from the midline. Conversely, a true selective TL/L fusion occurs when the preoperative MT curve apex is completely off the midline (C7 plumbline) (Figure 2).

Figure 1

Figure 1

Figure 2

Figure 2

Lastly, a sagittal thoracic modifier based on the T5–T12 sagittal Cobb measurement on the standing lateral radiograph is included in the classification scheme. 8,9 When the T5–T12 Cobb measurement is less than +10°, a “−” or hypokyphotic sagittal modifier is designated; when it is between +10 and +40, an “N” or normal kyphotic modifier is added; when it is ≥+40°, then a “+” or hyperkyphotic modifier is added. Thus, the Triad Classification System combines the curve type, 1–3,5,10,11 along with the lumbar modifier (A, B, or C), and lastly the sagittal thoracic modifier (−, n, +) for complete curve classification (e.g., 1BN).

Thus, six curve types are distinguished in this new system based on whether the PT, MT, and TL/L regions are major, minor structural, or nonstructural including: Type 1, MT; Type 2, Double Thoracic (DT); Type 3, Double Major (DM); Type 4, Triple Major (TM); Type 5, TL/L; Type 6, TL/L-MT (Figure 3). The most common curve types when a selective thoracic fusion may be indicated are main thoracic 1C, double thoracic 2C, or double major 3C curves where both thoracic and TL/L regions deviate from the midline, but a selective thoracic fusion may be performed. Conversely, Type 5C and 6C curves may also undergo a selective TL/L fusion when both the TL/L and thoracic regions deviate completely from the midline. The performance of a selective fusion for Type 1C, 2C, and 5C are predicted by the classification, whereas Type 3C and 6C require additional confirmation using the radiographic structural ratio and clinical criteria that will be described below. 11

Figure 3

Figure 3

When contemplating a selective fusion, it is essential to evaluate the structural criteria present in the MT and TL/L regions of the spine including ratios of: Cobb magnitude, apical vertebral translation (AVT), apical vertebral rotation (AVR), and relative flexibility assessments of the two curves. 7,12 The AVT is assessed for the MT curve by the C7 plumb line distance to the midpoint of the apical body or disc. The AVT assessment for the TL/L region is from the midpoint of the apical body or disc to the CSVL. The AVR is measured at the apex of both curves using Nash-Moe terminology. 13 Overall, for a selective thoracic fusion to be successful, the MT:TL/L ratios should be ≥1.2, with greater degrees of confidence for a successful result with a higher ratio. 11 Conversely, a TL/L:MT ratio ≥1.25 helps confirm that a selective TL/L fusion can be successfully performed. 12 Lastly but very importantly, the clinical examination must document these radiographic ratio differences between the intended instrumented and fused regions of the spine versus the region left unfused.

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Material and Methods

Selective Thoracic Fusions.

Forty-four consecutive patients with major thoracic, compensatory “C” modifier lumbar AIS curves were treated at a single institution between 1985 and 2000 and had a 2-year minimal clinical and radiographic follow-up. For each patient, the CSVL was completely medial to all portions of the lumbar apical vertebral body (lumbar modifier C). Procedures involved an anterior approach (n = 15), posterior approach (n = 26), or both (n = 3), with the distal fusion level ending at L1 or above in all cases. Autogenous bone graft from the rib in all anterior cases and (n = 11 posterior cases with thoracoplasties) or posterior iliac crest (n = 18) were utilized.

Anterior procedures involved fusions of all segments in the Cobb measurement of the curve with the distal fusion level being T11 in all 15 cases. Ten patients had a single screw/single rod construct and five patients had a dual screw/dual rod construct (Figure 4). All posterior cases had the lowest level of instrumentation and fusion at T11 (n = 1), T12 (n = 14), or L1 (n = 11). Most posterior constructs utilized a combination of hooks and wires, whereas pedicle screws were used in combination with hooks in three cases (Figure 5). Rod derotation maneuvers were not performed to avoid overcorrection. All patients were administered the SRS-24 Outcomes Questionnaire either during an office visit, mail, or by telephone call by a research nurse. 14

Figure 4

Figure 4

Figure 5

Figure 5

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Selective Thoracolumbar/Lumbar Fusions.

Twenty-one patients with a major TL/L curve and a minor MT curve in which the MT curve apical body completely deviated from the C7 plumbline were treated from 1992 to 2000. Sixteen patients were classified as Lenke 5C patterns, whereas 5 patients were classified as Lenke 6C patterns. All patients were female with a median age of 14 + 8 (range 13–18). For all patients, an anterior instrumentation and fusion was performed of the TL/L curve alone, with the MT curve left unfused. All patients had a single solid rod instrumentation and fusion with structural interbody cages used for sagittal alignment and increased construct stability (Figure 6). No patients were braced after surgery.

Figure 6

Figure 6

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Results

Selective thoracic fusions were performed in 44 patients (42 female and 2 male). The median age was 14.2 years (range 10.4–18.7) and the skeletal maturity level based on the Risser sign was: 0 (n = 14), 1 (n = 1), 2 (n = 4), 3 (n = 3), 4 (n = 20), and 5 (n = 2). The mean duration of radiographic follow-up was 5.0 years (range 2 + 0–16 + 0). Curve types according to the Lenke system where: 1CN (n = 29), 1C− (n = 5), and 1C+ (n = 2). In addition, there were 8 double thoracic Type 2 curves that had “C” lumbar modifiers and had a selective double thoracic fusion: 2CN (n = 7), and 2C− (n = 1). The average preoperative MT curve was 61° (range 40°–91°), corrected to an average 36° (range 17°–60°) early postoperative, and to 39° (range 15°–57°) at latest follow-up. The average preoperative lumbar curve measured 47° (range 30°–66°), decreasing to an average 32° (range 8°–48°) at latest follow-up.

The preoperative ratio criteria of MT:TL/L Cobb magnitude was 1.30. Preoperative AVT-MT averaged 54 mm (range 33 mm–98 mm), and AVT-TL/L averaged 29 mm (range 19 mm–47 mm), with an AVT ratio was 1.86. Preoperative AVR-MT averaged 2.0 Nash-Moe grade, and AVR-TL/L averaged 1.4 for the lumbar curve, with an AVR ratio of 1.43. In no patient was the lumbar Cobb magnitude, AVT, or AVR greater than that of the MT curve. Fifteen patients underwent an anterior instrumentation and fusion of the MT curve spanning the Cobb levels and involving an average of 7 vertebrae (range 5–8 vertebrae). Twenty-six patients underwent posterior instrumentation and fusion of the MT curve with an average of 10 vertebrae fused (range 8–12 vertebra). The majority of patients fused posteriorly were to the stable vertebra.

Although the lumbar Cobb improved on each patient undergoing a selective MT fusion, true correction of lumbar AVT was inconsistent. 15 Two subsets of patient emerged based on whether there was any change in the AVT-TL/L. In Group A (n = 25 patients), AVT-TL/L improved a mean 9 mm (range 1–28 mm). For Group B (n = 19 patients), AVT-TL/L stayed the same or was slightly worse from preoperative to last follow-up by an average of 5 mm (range 0–15 mm). Comparison of Groups A and B revealed significant differences in terms of upper lumbar vertebra coronal tilt (Group A: 28°; Group B: 35°; P = 0.01), preoperative lumbar Cobb magnitude (Group A: 45°; Group B: 51°; P = 0.01), preoperative coronal balance (C7 plumb shifted to the left: Group A: 22 mm; Group B: 11 mm; P = 0.02), and skeletal maturity (mean Risser sign, Group A: 3.1 vs. Group B: 1.4; P = 0.01). For all patients in Group B, the lumbar curve retained its “C” modifier status at the latest follow-up. However, the 25 Group A patients, improved their lumbar apical vertebral translation and corresponding lumbar modifier grade in 15 patients (C into B in 12 patients, and C into A in 3 patients). Apical vertebral rotation-TL/L did not exhibit any significant spontaneous correction after surgery or during follow-up.

Global coronal balance was shifted to the left by an average of 17 mm before surgery (range 43 mm–+23 mm). In each of the postoperative time points, the average coronal C7 plumb remained similar. At latest follow-up, 21 patients had a C7 plumb shifted to the left >20 mm, including 3 patients with a C7 plumb ≥ 40 mm (40 mm, 41 mm, 46 mm). These patients exhibited no clinical problems with their mild global imbalance. Patients undergoing posterior procedures tended to have an increased frequency of imbalance (9 patients > 30 mm decompensated with 8 undergoing posterior procedures and only one an anterior procedure).

Long-term follow-up demonstrated good maintenance of curve correction and global balance. Patients with 5 to 16 years (n = 18) follow-up had excellent maintenance of MT (2 year: 41°, latest: 42°), and lumbar (2 year: 34°, latest: 32°) Cobb correction. Global balance was also similar with imbalance >20 mm occurring in 7 patients at the 2-year and 8 patients at the final follow-up.

Skeletal immaturity was analyzed for potential influence on spontaneous correction and maintenance of global balance. Loss of correction in both the MT and TL/L curves tended to occur more commonly in skeletally immature patients (Risser 0 or 1) compared to patients that were more skeletally mature (Risser 2–5). For the unfused compensatory lumbar curve, 4 of 15 (27%) of skeletally immature patients increased their curve magnitude >10° (range 10–23°) from postoperative to latest follow-up compared to 2 of 29 (7%) of skeletally mature patients (P = 0.16). However, it is important to note that neither postoperative bracing nor revision surgery was required for skeletally immature or mature patients for any purpose including global imbalance during the length of follow-up in this study.

The SRS-24 Questionnaire functional outcome data were obtained for 41 of 44 (93%) of patients at latest follow-up. 14 Obviously, the majority of these patients had their initial surgery before the availability of the SRS-24 Questionnaire. The average latest follow-up outcome and subset scores were: total score 95, pain 28, self-image 22, function 32, and satisfaction 13. The scores of patients undergoing either an anterior or posterior procedure were nearly identical. However, patients with a postoperative coronal imbalance (2–4.6 cm) tended to have an inferior pain, self-image, and function subset scores and overall lower total scores. Overall, 81% of patients (33/41) stated that they probably would or definitely would have the surgery again, and 81% (33/41) stated that they were somewhat or extremely satisfied with the results of their back treatment.

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Thorocolumbar/Lumbar Selective Fusion

Of the 21 patients undergoing selective TL/L fusion, the average preoperative TL/L curve was 55° (range 42–80°), decreasing to an average 20° (range 6–38°) at a minimum 2-year follow-up. For the corresponding MT unfused curve, the preoperative Cobb averaged 40° (range 30–65°), and at 2 years postoperatively averaged 23° (range 9–45°). Overall, coronal imbalance improved from a mean 3.5 cm preoperative (range 0–4.8 cm imbalance) to 1.6 cm postoperative (range 0–2.8 cm imbalance). No patient had worse imbalance postoperative than preoperative.

All patients had solid arthrodesis with lack of instrumentation failure noted at a minimum 2-year follow-up. One patient who was skeletally immature (Risser 0) preoperative had an early postoperative MT curve that measured 40°, which progressed with growth to 57° and underwent a posterior instrumentation and fusion of both MT and TL/L curves at 3 years postoperatively. That is the only patient that had extension of a selective TL/L fusion.

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Discussion

The goals in the surgical treatment of AIS are: to halt curve progression, maintain optimal coronal and sagittal balance, to correct the deformity while fusing the least amount of motion segments, and to avoid any complications. Selective thoracic fusion provides partial correction of the major curve while providing the potential for spontaneous lumbar curve correction and maintaining mobile lumbar motion segments. 10,16–18 Performing a successful selective thoracic fusion begins with appropriate analysis of curve patterns amenable to this technique. In Lenke Types 1C and 2C, the MT curve is the major curve but the lumbar curve below by definition is flexible on side bending (<25° residual curve) and lacks a junctional thoracolumbar kyphosis of ≥20° between T10 and L2. In addition, the ratio criteria of MT:TL/L Cobb magnitude, AVT, and AVR must be in the range of 1.2 or greater, especially for the AVT ratio, to allow a successful thoracic fusion to be performed (Table 1). Decompensation following selective thoracic fusion typically involves lumbar curves that are larger in curve magnitude, less flexible with substantial rotation and/or deviation from the midline especially when the ratio criteria are closer to 1.0 indicating equal structural characteristics to both the MT curve above and the TL/L curve below. 10,11,18 Lastly, but of equal importance, is the clinical examination of the patient. Patients amenable for selective thoracic fusion will have right thoracic trunk shift in the upright position with fullness in the right thoracic region, but lacking substantial waste line creases on the right side indicative of a structural lumbar curve. In addition, the forward bend thoracic prominence should demonstrate a substantial rotational difference versus the lumbar prominence on visual inspection and scoliometer measurement 4 (Table 2).

Table 1

Table 1

Table 2

Table 2

Once the patient and his/her radiographs have been chosen for selective thoracic fusion, intraoperative technique, which provides optimal correction of the MT curve while maximizing spontaneous lumbar correction, and maintaining overall coronal balance are critical for success. These patients can either be approached anteriorly or posteriorly. 15 In this review, anterior procedures involved fusion of fewer segments (7 vs. 10), with a significantly greater degree of instrumented MT correction and superior spontaneous TL/L curve correction. However, posterior procedures certainly did provide good to excellent radiographic and clinical results as well. In fact there was no difference in the SRS-24 Questionnaire data at follow-up between the patients fused anteriorly versus posteriorly. Surgeons must weigh the benefit of slightly increased main thoracic correction and lumbar spontaneous correction versus the pulmonary morbidity associated with an open thoracotomy or an endoscopic instrumentation and fusion. Certainly both techniques are viable options to perform a selective thoracic fusion in this subset of patients.

Performing a selective TL/L fusion demands equal careful radiographic and clinical analysis. The TL/L:MT ratios of Cobb magnitude, AVT, and AVR should be ≥1.25 (Table 3). In addition, careful analysis of the patient’s truncal balance is essential. For a major left TL/L curve, a patient with a depressed left shoulder is a relative contraindication to a selective TL/L fusion because correction of the TL/L curve will further depress the left shoulder. In addition, the thoracic rib prominence must be carefully evaluated, because a selective TL/L fusion will not significantly lower the thoracic rib prominence (Table 4). In relationship to the lumbar prominence, which will markedly diminish with surgical correction, the thoracic prominence will actually appear a bit more prominent postoperative versus preoperative. These clinical features must be discussed with both the patients and their parents/caregivers before embarking on a selective TL/L fusion. Additionally, most patients who are highly skeletally immature (Risser 0, open triradiate cartilages) are at higher risk for MT curve progression with growth, and thus, surgeons must be very careful when performing a selective TL/L fusion in skeletally immature patients. 12

Table 3

Table 3

Table 4

Table 4

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Conclusions

There are many operative AIS curves in which both the thoracic and lumbar regions completely deviate from the midline. However, the major curve can often have a selective fusion, thereby leaving the corresponding minor curve unfused. Performance of a selective thoracic or TL/L fusion in the surgical treatment of AIS is an extremely important principle to maximize spinal flexibility early and avoid lumbar degeneration. 19–23 The potential to perform a selective fusion begins with careful radiographic analysis of the Lenke curve pattern on the upright anteroposterior, lateral, and side bending films. Next, ratio criteria of MT:TL/L structural characteristics of Cobb magnitude, AVT, AVR, and flexibility ratios must be assessed. When these ratios of the major curve intended for selective fusion to the minor compensatory curve are ≥1.2, selective fusion should be possible. In addition, the clinical examination of the patient must document the marked difference in major versus minor curve prominences. Lastly, instrumentation and fusion techniques, which optimize instrumented correction while allowing for spontaneous correction of the unfused curves and maintaining overall coronal and sagittal balance, must be performed. If all of these principles are followed appropriately, selective fusion of the spine should provide a good to excellent radiographic and clinical outcomes that maximize spinal mobility following surgery.

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Key Points

  • A “selective” fusion is a term reserved for fusion of a major curve (largest Cobb measurement) leaving the minor curves unfused, when the corresponding thoracic or lumbar minor curves completely deviate from the midline.
  • Lenke classification main thoracic (1C) and double thoracic (2C) curves are potentially amenable to selective thoracic fusion. Occasionally, a double major (3C) curve may qualify as well.
  • Major thoracolumbar/lumbar (5C) and major thoracolumbar/lumbar–secondary main thoracic (6C) curves are potentially amenable for a selective thoracolumbar/lumbar fusion.
  • Ratio criteria of thoracic: lumbar (T:L) Cobb magnitude, apical vertebral translation, and apical vertebral rotation are important characteristics to evaluate additionally.
  • The patient’s clinical examination must provide evidence for a more prominent structural major curve region with less prominent minor curve region(s).
  • Instrumentation techniques must be utilized which optimize major curve correction while allowing spontaneous minor curve correction and rebalancing.
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References

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Keywords:

Lenke classification; selective thoracic fusion; selective thoracolumbar/lumbar fusion; spinal flexibility ] Spine 2003;28:S199–S207

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