Selective fusion in adolescent idiopathic scoliosis

Abstract

Despite the continual evolution in the surgical treatment of adolescent idiopathic scoliosis (AIS), the goals of surgery remain to correct and stabilize the deformity in three dimensions, to maintain equilibrium of the shoulders and trunk, and to leave as many mobile spinal segments as possible. The essence is to fuse the smallest possible number of vertebrae to maintain maximum residual mobility, but end with corrected and well-balanced spine. Selective fusion is termed when both the main thoracic and thoracolumbar/lumbar (TL/L) curves deviate completely from the midline (Figure 1), but only the major curve (the largest Cobb measurement) is fused, leaving the minor curve unfused and mobile. For the single curve, such as thoracic, thoracolumbar, or lumbar curve, there are fewer differences of opinion amongst spinal surgeons regarding the selection of the fusion level than the surgical approach. However, the choice of fusion levels in some types of curves, such as double curves and triple curves remains a difficult and controversy issue. If the decision to perform selective fusion is incorrect, it may result in postoperative curvature deterioration, shoulder imbalance, trunk decompensation, or even produce new deformity, an early revision by extending the fusion or reducing the correction may need.1 The non-selective approach rarely leads to early troubles that require a second procedure and is often perceived as being safer in the short-term. But it may be more difficult in the long-term as distal degeneration is more likely. This raises the question: “Is it better to be safe in the short-term or take a chance avoiding later degenerative problems with a shorter motion-sparing fusion?” Thus, the aim of selective fusion is to identify the compensatory curves (minor curve) that will straighten spontaneously after correcting and fusing the major curve, thereby avoid the fusion of these flexible compensatory curves.Figure 1.: A selective fusion is termed when the main thoracic curve deviates completely from C7 plumb line and the TL/L curve deviates completely from the central sacral vertical line (CSVL), but only the major curve (largest Cobb measurement) is fused.The classic curves of AIS can be divided into three parts: proximal thoracic curve, main thoracic, and thoracolumbar/lumbar (TL/L), although sometimes part of the curves may be a fractional, not than a full curve (Figure 2). The major curve is the one with the largest Cobb measurement and will always be included in the fusion. The minor curves are all the other non-major curves present. In most situations, the proximal thoracic curve will be a minor curve and one of the main thoracic curves and TL/L curve will be the major curve. So when the main thoracic curve is the major curve, the debates of selective fusion are whether to include the proximal thoracic curve and TL/L curve in the fusion. And when the TL/L curve is the major curve, the debates are whether to include the main thoracic curve and proximal curve in the fusion. Minor curve structural criteria may guide the surgeon in decision-making of selective fusion, but it has not been consensus with different authors and AIS classifications.Figure 2. A:: A classic curve of AIS, full curves of proximal thoracic (PT), main thoracic (MT) and thoracolumbar/lumbar (TL/L). B: Full curves of main thoracic and TL/L, with a fractional curve of proximal thoracic. C: Full curves of proximal thoracic and main thoracic, with a fractional curve of TL/L.PROXIMAL THORACIC CURVE Because the proximal thoracic curve usually is a minor curve, the debate of selective fusion on proximal thoracic curve is when to include it in the fusion segments. Traditionally, criteria for including the proximal thoracic curve in the instrumented fusion was based on Harrington rod instrumentation for a King V double thoracic curve pattern.1 It is typically characterized by an elevated left shoulder or first rib, relative stiffness of the upper thoracic curve, and a positive T1 tilt. Positive T1 tilt, characterized as elevation of the left upper corner of the first thoracic vertebra versus the right upper corner in a right main thoracic curve, is considered indicative of a complete or full proximal thoracic curve. Neutral or negative T1 tilt is considered indicative of a fractional proximal thoracic curve (Figure 3). In 1989, Winter2 added parameters, such as left thoracic rib prominence or trapezial fullness, to identify which curve needs inclusion into fusion. Later, Lee et al3 found that T1 tilt does not correlate with shoulder balance and does not necessarily imply a structural upper thoracic curve. In their series investigating proximal thoracic curve treated with Harrington rod instrumentation they recommended including the upper thoracic curve in the fusion segment when the left shoulder was elevated, and also when the shoulders were level if the side-bending flexibility of the upper curve was less than the lower main thoracic curve. But with the wide use of segmental instrumentation techniques in surgical treatment of AIS, the criteria for including the proximal thoracic curve in the instrumented fusion was modified by several authors. Lenke et al4 published their definition of a double thoracic curve pattern and analyzed the patients treated with Cotrel-Dubousset instrumentation. They concluded that proximal thoracic curves greater than 30° that corrected to no better than 20° on side bending, that had more than or equal to Grade 1 rotation or more than or equal to 1 cm translation present at the apex of the curve, that showed any elevation or positive T1 tilt, or that had transitional vertebra between the two curves at T6 or below should be fused when most of these features are present. Suk et al5 further modified the definition of the double thoracic curve and the criteria for fusion of the proximal thoracic curve with segmental pedicle screws. They suggested that proximal thoracic curve of more than 25° and a level or elevated left shoulder should be considered a double thoracic curve and should be treated by fusing both the proximal and the main thoracic curves when using segmental pedicle screw instrumentation. Additionally in 2001, the term structural or non-structural assigned to proximal thoracic curve was redefined in Lenke classification.6 Lenke et al6 defined the upper thoracic curves as structural if the side-bending maximum residual Cobb measurement was 25° or more, regardless whether there was a positive tilt of T1 or not, and/or kyphosis between the second and fifth thoracic level was 20° or more. They suggested including the proximal thoracic curve in the main thoracic fusion segments if it was structural.Figure 3. A: : Positive T1 tilt was considered a full proximal thoracic curve. Neutral (B), or negative (C) T1 tilt was considered a fractional proximal thoracic curve.Because the significance of the proximal thoracic curve is closely related to the amount of correction gained in the instrumented main thoracic curve, use of more powerful instrumentation systems that increases the correction of the subject curve undoubtedly increases the risk of proximal curve decompensation and widens the range of the proximal curves that needs fusion. This is well reflected in the continual modifications in the indications for fusion of the proximal thoracic curve with the introduction of more effective spinal instrument system. So the recognition of a structural upper thoracic curve should also be appreciated on curves that are not obvious double thoracic patterns (neutral or negative T1) with the use of powerful segmental instrumentation. It is important to take into consideration the proximal thoracic curve, whether it is full curve (positive T1) or fractional curve (neutral or negative T1) when using segmental instrumentation to correct the main thoracic curve (Figure 3). Although radiographic features of proximal thoracic curve, such as T1 tilt, proximal thoracic Cobb angle, and proximal thoracic flexibility, may be considered in determining when to include proximal thoracic curve in the fusion segments, the ultimate goal is to maintain a good shoulder balance.4 Patients with an elevated left shoulder clinically usually require instrumentation/fusion of the proximal thoracic curve. However, the upper left thoracic curve may be structural and requires inclusion in the instrumentation when the shoulders clinically are level or even if the right shoulder is elevated preoperatively when using segmental instrumentation. So decision-making on whether to include the proximal thoracic curve in the fusion segment should be based on clinical presentation of the shoulder, the estimated correction of the main thoracic curve, and the radiographic parameters of the proximal thoracic curve. SELECTIVE THORACIC FUSION For AIS with double curves (main thoracic curve and TL/L curve), selective thoracic fusion in the presence of a compensatory TL/L curve is certainly a goal. Ideally, after selective thoracic fusion, the unfused TL/L curve will spontaneously accommodate to the corrected position of the thoracic curve. Sparing the lumbar spine from fusion should be pursued whenever practical. Lumbar motion is important for function during the decades of life these adolescent patients have remaining. There is reason to believe that distal degeneration will be less problematic if more motion segments remain below a fusion.7 The original recommendation of selective thoracic fusion came from King et al,1 who suggested that patients with Type II (major thoracic/compensatory TL/L) curves be treated with selective thoracic fusion, whereas patients with Type I (double major) curves should undergo fusion spanning both the thoracic and lumbar curves. Because the King classification is based on Harrington instrumentation, classification of Type I and Type II is a guideline for surgeon to perform selective thoracic fusion using Harrington instrumentation at that time. However, following the introduction of segmental spinal instrumentation systems, postoperative coronal decompensation after selective posterior thoracic fusion of King Type II curves has been a frequent and unsolved complication. This has prompted numerous studies into the cause and prevention of this occurrence.8-22 FACTORS CONTRIBUTING TO DECOMPENSATION AFTER SELECTIVE THORACIC FUSION Many factors have been cited as causative to decompensation after selective thoracic fusion in King Type II. They include overcorrection of the primary thoracic curve,11,12 obviously structural characteristic of the lumbar curve,19 rod derotation maneuver of Cotrel-Dubousset instrumentation, incorrect identification of curve pattern,17 inappropriate selection of fusion level, and hook patterns.8,21,23 Regarding overcorrection of the thoracic curve, it has been hypothesized that the unfused compensatory lumbar curve cannot compensate for the excessive correction of the main thoracic curve resulting in coronal decompensation.11,12 This is most common in those patients with lumbar curves that are larger in curve magnitude (more than 45°), less flexible with substantial rotation and/or deviation from the midline. Unfortunately, the King system has limited utility in defining the structural nature of this specific lumbar curve when segmental instrumentation is used. The rod derotation maneuver of Cotrel-Dubousset, and similar instrumentation was blamed for producing torsional changes in the non-instrumented lumbar area that could result in spinal imbalance.8,11,21 Improper selection of the distal fusion level was thought to be the causative factor in a number of cases in which the distal fusion level was beyond the stable and neutral vertebra8,11,24 or when the fusion did not include the lumbar curve, especially in double major curves.13 Many preoperative factors have also been recognized as the causes of decompensation after selective thoracic fusion in King Type II. Dobbs et al25 found that spontaneous lumbar curve correction occurred consistently after both selective thoracic anterior and posterior fusions for Lenke Type 1B, 2B, 1C and 2C when thoracic correction was limited to mimic the preoperative push-prone thoracic Cobb. They found two risk factors of postoperative coronal decompensation: (1) the presence of preoperative coronal imbalance; (2) overcorrection of the thoracic curve at surgery to less than the preoperative thoracic push-prone Cobb measurement. Other studies9,26 has shown that even when the flexibility of the lumbar curvature was proven by radiographs of the patient in the bending position, the most caudal parts of the lumbar curvature did not respond to aggressive thoracic correction when selective thoracic fusion was performed. Richard et al9 examined 24 children with King II curvatures whose lumbar curvatures measured 40° or larger and found that the more caudal lumbar spine remained nearly unaffected by limited thoracic instrumentation. In particular the obliquity between the fourth lumbar vertebra and the pelvis was observed to persist after instrumentation. Edwards et al26 also found after selective thoracic fusion the spontaneous correction of the lumbar curve results principally from a decrease in the tilt of its upper vertebrae of the lumbar curve, but not necessarily improved apical translation. They also found the prevalence and magnitude of coronal imbalance (>20 mm) at latest follow-up were unchanged compared to preoperation. They believed that preoperative imbalance was a significantly risk factor for postoperative imbalance. The sagittal plane should also be taken into consideration when selective thoracic was performed.10 When preoperative distal junction kyphosis is present in thoracolumbar or lumbar, extending the instrumentation an additional level distally to include the kyphotic segment and including the lumbar curve in the fusion segments appears to be important in avoiding persistent distal junction kyphosis.6,10 So preoperative decompensation, obvious obliquity of L4, obvious structural change of the lumbar curve, thoracolumbar kyphosis, overcorrection of the primary thoracic curve and rod derotation maneuver during operation might be the risk factors for postoperative decompensation after selective thoracic fusion. METHODS TO AVOID DECOMPENSATION The characteristics of the compensatory non-structural lumbar curve play a significant role in the surgical decision-making process of selective thoracic fusion. Lenke et al18 stress the importance of distinguishing between King II and double major curve patterns. They recommend that selective thoracic curve fusion be performed in King II if lumbar rotation did not exceed thoracic rotation, a T:TL/L (thoracic:thoracolumbar/lumbar) Cobb ratio of 1.2 or more, a T:TL/L apical vertebra translation (AVT) ratio of 1.2 or more, and a lumbar Cobb measurement of 60° or less. In Lenke classification, Lenke et al6 recommend selective thoracic fusion could be performed in Lenke 1B, 1C, 2B and 2C. In these curves patterns, the thoracic curve is the largest curve. The smaller lumbar curve is non-structural (i.e., has a side-bending Cobb measurement of 25° or less) and has thoracolumbar kyphosis that is less than 20° although modifier C indicates that the non-structural lumbar curve has crossed the midline. Suk et al27 suggested that the rod derotation maneuver and the preoperative lumbar curve characteristics (i.e., curve magnitude, rotation, or deviation) were not causative factors of postoperative trunk decompensation. However, they believed that excessive thoracic curve correction and the relative inability of the lumbar curve to accommodate the correction of the thoracic curve were causative factors in postoperative trunk decompensation. They suggested that when the thoracic curve correction was >75% and the Cobb angle of the postoperative thoracic curve was <30% of preoperative lumbar curve, there was a high chance of postoperative decompensation. Surgical techniques and instrumentation might also favor avoiding decompensation when selective thoracic fusion was performed. Goshi et al28 proposed the use of the Isola segmental instrumentation system instead of Cotrel-Dubousset instrumentation. Rather than using the derotation maneuver required by the CD system, Isola use translational corrective techniques for deformity correction. Unlike previous studies in patients using CD instrumentation, however, none of the 22 patients treated had a coronal decompensation requiring further treatment. Dobbs et al29 found selective thoracic fusion of main thoracic-compensatory lumbar C modifier AIS curves with pedicle screws allowed for better thoracic correction and less postoperative coronal decompensation than seen with hooks. They postulated that screw fixation offers better control of the end vertebrae, which prevents decompensation. It is sometimes difficult to assess apical rotation radiographically (the Nash-Moe or Perdriolle methods). So clinical evaluation of the patient during a forward bend seems more helpful than the radiographs in making selective thoracic fusion. A selective thoracic fusion seems inappropriate if the lumbar rotational prominence is dominant compared to the thoracic region.6 In a conclusion, a selective thoracic fusion could be performed when all the risk factors for decompensation and methods to avoid it have been taken careful consideration. A successful selective thoracic fusion depends on the analysis of these risk factors related to decompensation, optimal technique performing during the operation, and preoperative evaluation of the lumbar prominence during physical examination. SELECTIVE TL/L FUSION Compared with selective thoracic fusion, selective TL/L fusion for a major TL/L curve with a minor main thoracic curve has been much less discussed in the literature. In a very useful study, Sanders et al30 performed a retrospective multicenter study to investigate the critical factors for successful selective TL/L fusion. Analysis demonstrated that a successful outcome was dependent on the patient's maturity and structural changes in the thoracic curve. The authors recommend selective fusion of the TL/L curve if the thoracic curve is 55° or less, the TL/L:T Cobb ratio is 1.25 or more, or the thoracic curve bends out to 20° and the triradiate cartilages are closed. They found the fate of the unfused thoracic curve in selective TL/L fusion was consistent with the natural history of untreated scoliosis during growth. Schulte et al31 found selective anterior instrumentation and fusion of primary main thoracic curves was associated with a satisfactory spontaneous vertebral and excellent surface derotation of the secondary lumbar curves. However, selective anterior instrumentation and fusion of TL/L curves can lead to unsatisfactory spontaneous derotation of the secondary thoracic curves, and result in an even slight increase of both vertebral and surface rotation. In patients with rigid compensatory thoracic curves and a preexisting clinically relevant rib hump, selective anterior correction of the lumbar curve can lead to an unsatisfactory cosmetic result caused by persisting or increased rib hump. They suggested clinical presentation of the thoracic rib prominence should be carefully evaluated when selective TL/L fusion is performed. In Lenke classification,6 selective thoracolumbar/lumbar fusion is suggested in Lenke Type 5 that the TL/L is the major curve (largest Cobb angle) and the compensatory minor main thoracic curve is non-structural according to their structural criteria. So a successful selective TL/L fusion depends on equally careful radiographic and clinical analysis. The TL/L:T ratios of Cobb magnitude, AVT, and AVR should be >1.25. 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. The thoracic rib prominence should also be carefully evaluated, because a selective TL/L fusion will not significantly lower the thoracic rib prominence. Additionally, most patients who are highly skeletally immature (Risser 0 and open triradiate cartilages) are at higher risk for major thoracic curve progression with growth, and thus, surgeons must be very careful when performing a selective TL/L fusion in skeletally immature patient. ALGORITHM OF SELECTIVE FUSION The decision-making process for selective fusion of AIS can be broken down into a basic algorithm starting with radiographic evaluation. Curve classification, such as Lenke classification6 or PUMC32 classification, is performed to isolate regions of the spine to be included in the instrumentation. In addition to the classification patterns that predict a selective fusion treatment, an analysis of radiographic ratios of structural criteria should be carefully performed on patient's radiographs. When radiographically major curve is significantly more prominent than the corresponding minor curve (ratios of 1.2 or greater), then selective fusion may be possible. Classification of AIS and the ratio criteria help surgeon identify which curves are structural that need fusion and that non-structural need to be left alone. They base on the measurement of the curve on the radiological film. However, for patients with AIS, the cosmetic improvement, particularly the reduction of the rib hump and the balance of shoulders and trunk, is much more important than the decrease of the Cobb angle. So preoperative clinical examination of AIS patient should document the presentation of shoulder level, trunk shift, thoracic rib prominence, and waste line creases. These factors must be considered when selective fusion is attempted. Once the decision to perform selective fusion is made after evaluation of the radiological film and clinical presentation, optimal intraoperative technique is needed for ultimate success. For a balance spine, overcorrection of the fused segments should be avoided. Instrumentation techniques must be utilized which optimize major curve correction while allowing spontaneous minor correction and rebalancing. With the development of surgical technique and instrumentation, the criteria of selective fusion have also been changing. In conclusion, for a successful selective fusion, a spinal surgeon should take into consideration the curve classification (Lenke or PUMC classification), the relative structural changes of the major curve and compensatory curve (minor curve), the clinical presentation of the AIS patient and the corrective technique which will be used in operation. Only when all these factors have been carefully evaluated, a decision whether to perform selective fusion will be optimal.