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 Table of Contents  
Year : 2020  |  Volume : 3  |  Issue : 2  |  Page : 160-172

Current concepts in level selection for fusion in the adolescent idiopathic scoliosis patient

1 Department of Orthopedic Surgery, The Daniel and Jane Och Spine Hospital, NewYork-Presbyterian/Columbia University Medical Center, New York, USA
2 Department of Orthopedic Surgery, Lenox Hill Hospital, New York, USA

Date of Submission20-Nov-2019
Date of Decision03-Jan-2020
Date of Acceptance23-Mar-2020
Date of Web Publication13-Jul-2020

Correspondence Address:
Dr. Paul Jaewook Park
Department of Orthopedic Surgery, The Daniel and Jane Och Spine Hospital, NewYork-Presbyterian/Columbia University Medical Center, 5141 Broadway, 3 Field West-022, New York, NY.
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/isj.isj_67_19

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Over the past several decades, level selection for fusion in the patient with adolescent idiopathic scoliosis (AIS) has evolved alongside technique. Now, with the near ubiquitous use of pedicle screw fixation, selection criteria have changed to minimize the number of levels fused, especially distally in the lumbar spine. With each additional motion segment preserved, it has been suggested that postoperative function can be improved and the risk of degenerative disease down the line may be decreased. Currently, the Lenke classification for AIS is the most widely used system to describe AIS pathology. Understanding where the structural and nonstructural curves are may help determine the extent of fusion required distally. Proximally, shoulder balance is still considered a key consideration for upper instrumented vertebra (UIV) selection. In terms of the lowest instrumented vertebra (LIV), we focus on two key concepts to prevent serious complications such as distal junctional kyphosis (DJK) or adding-on phenomenon: the last touched vertebra (LTV) and the stable sagittal vertebra. In the AP radiograph, identifying the LTV as the LIV may allow the surgeon to save a fusion level without increasing risk of DJK or adding-on. However, one must also consider the sagittal plane; the authors identify the stable sagittal vertebra on the lateral radiograph to help determine the optimal LIV; of these two criteria, the more distal level will be selected to decrease the chance of adverse outcomes.

Keywords: Adolescent idiopathic scoliosis, last touched vertebra, level selection, stable sagittal vertebra

How to cite this article:
Park PJ, Sawires A, Lenke LG. Current concepts in level selection for fusion in the adolescent idiopathic scoliosis patient. Indian Spine J 2020;3:160-72

How to cite this URL:
Park PJ, Sawires A, Lenke LG. Current concepts in level selection for fusion in the adolescent idiopathic scoliosis patient. Indian Spine J [serial online] 2020 [cited 2023 Apr 1];3:160-72. Available from: https://www.isjonline.com/text.asp?2020/3/2/160/289659

  Introduction Top

As the surgical treatment of adolescent idiopathic scoliosis (AIS) has evolved over the past several decades, now powerful three-dimensional correction is possible using posterior segmental pedicle screw fixation. The criteria for fusion level selection have also inevitably evolved. Level selection is critical and can have significant consequences in young adolescent patients over time, including worsening deformity or necessitating revision surgery. Preceded by the King–Moe classification, the Lenke classification for AIS developed in 2001 has helped provide a framework for level selection based on curve morphology[1] [Figure 1]. The Lenke classification continues to be widely used to this day; however, the associated guidelines for level selection for each curve type are being further explored as more longitudinal data becomes available and as surgical techniques advance. Now with the use of pedicle screw fixation, selective thoracic fusion has become feasible and goals of surgery have progressed as well, aiming for shorter fusion constructs while preserving more lumbar motion segments. Our objective was to review the most recent studies on both upper and lower instrumented vertebra selection in AIS, to address both the coronal and sagittal planes in preoperative planning, and to consolidate this information into several key principles. For each section of this review, a thorough PubMed search was performed. Keywords used for the upper instrumented vertebra (UIV) section included “UIV selection” for each Lenke type, “UIV selection + risk of PJK” and “UIV selection + kyphosis.” For the lowest instrumented vertebra (LIV), keywords included “LIV + AIS,” “last touched vertebra,” and “stable sagittal vertebra.” All studies included were retrospective analyses; no prospective data was available. Studies published in 2010 and thereafter were prioritized, with previous studies included for comparison and reference.
Figure 1: The Lenke Classification for adolescent idiopathic scoliosis[1]

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  UIV Selection Top

Selection of the UIV in the patient with AIS has been shown to be important both for obtaining a stable and balanced fusion, as well as for controlling postoperative shoulder imbalance. Similar to selecting the LIV, UIV selection criteria have also evolved over time. In 1962, Harrington[2] first described fusing to one level above the measured curve. Over subsequent years, several authors recommended using rotation to determine the UIV by fusing to the first neutral vertebra (NV) above the curve.[3] Ultimately, the specific level in the proximal thoracic (PT) spine made little functional difference clinically, and attention was instead turned to obtaining appropriate shoulder balance while selecting the UIV. With the advent of the Lenke classification, several recommended treatment strategies based on curve type emerged.[4] Initial UIV selection strategy using curve type was based solely on the coronal plane; however, as the importance of sagittal balance became apparent in recent decades in all Lenke classification types, it became critical not to end the construct at a region of kyphosis to prevent complications such as proximal junctional kyphosis (PJK)[5] [Table 1] and [Table 2].
Table 1: Selection of the upper instrumented vertebra based on Lenke classification curve type

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Table 2: Summary of concepts in upper instrumented vertebra selection

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For Lenke type 1 curves, fusion should address the structural main thoracic curve. For UIV, the main consideration has been shoulder balance; briefly, T3 should be the UIV of choice for level shoulders, whereas T4 should be selected for right shoulder elevation;[6],[7] although less common, the T2 vertebra may be selected as a UIV if the left shoulder is higher preoperatively. This is possible in Lenke type 1 curves as spontaneous correction of PT curves has been observed, correlating with increased flexibility on preoperative side-bending radiographs.[8] A retrospective study of 64 patients with Lenke 1 AIS looked at radiographic outcome of shoulder balance after fusion using the aforementioned UIV selection criteria and found that radiological shoulder height difference (RSH) and clavicle angles (CA) were similar among these groups immediately after surgery and at average follow-up of 24 months, with improved Scoliosis Research Society (SRS)-22 scores at final follow-up for patients fused to T2 and T3.[9] In another series, 98.7% of patients with UIV selected as aforementioned had perceived cosmetic shoulder balance at the last follow-up appointment.[10]

For Lenke type 2 or 4 curves, the structural PT curve makes UIV selection, especially important to correct both the proximal curve and the shoulder imbalance.[11] Several studies have found that T2 or T3 should be the UIV of choice for a right main thoracic curve with level shoulders, whereas T3 or T4 would be selected in the rare circumstance when the right shoulder was higher to account for right shoulder depression postoperatively.[12],[13] T2 should be selected as the UIV of choice if the left shoulder is higher than the right.[14],[15] Although several studies found fusing to more cranial levels (T1, T2) improve postoperative radiographic balance measures such as RSH and proximal wedge angle (PWA),[16] other studies found that fusing to T2 can only effectively improve medial shoulder balance (T1 tilt and first rib angle) but has no effect on lateral shoulder balance.[17] Overall, the UIV selection in double thoracic curves is debatable, and postoperative shoulder balance is difficult to predict;[18] however, following the basic principle to fuse all structural curves minimizes the chances of inadequate control of the structural PT curve,[19] which often indicates a UIV of T2.

Type 3 double major (DM) curves require instrumentation and fusion of the main thoracic (MT) and thoracolumbar/lumbar (TL/L) regions. The UIV is similar to that of a type 1 curve as explained earlier. Type 6 TL/L-MT curves have a major curve in the TL/L region, with the MT region being a structural minor curve and both regions are treated similarly with fusion of the MT and TL/L regions.[20] Thus, type 3 and 6 curve patterns are both treated with the UIV selection criteria as aforementioned.[21]

The recommended treatment for Lenke type 5 curves is fusion of the entire TL/L curve (generally end vertebra to end vertebra).[22] Fusion can be extended to include the thoracic curve in primary TL scoliosis to improve coronal correction, but at the cost of decreased thoracic kyphosis and clinical flexibility 2 years postoperatively.[23] Of note, the rates of PJK were found to be higher in patients with fused cranial to the upper end vertebra (UEV), which is contrary to Lenke type I curves.[24],[25]

Besides shoulder balance, one must also consider the risk of PJK. The incidence of PJK ranges from 5% to 46%, and it has been reported that 66% of cases occur 3 months after surgery and approximately 80% occur within 18 months.[5],[26-28] Lonner et al.[24] found that the incidence of PJK after operative treatment of AIS varies based on Lenke type (Lenke 1, 6.35%; Lenke 2 and 4, 4.39%; Lenke 3 and 6, 11.64%; and Lenke 5, 8.49%). PJK is multifactorial in origin and likely results from variable risk factors, some of which are patient specific and not correctable preoperatively, such as preoperative hyperkyphotic thoracic alignment.[5],[29] There are several factors in the surgeon’s control, however, such as UIV selection. Of note, no correlation was found between cervical sagittal alignment (CSA) and UIV level chosen based on several studies.[30],[31]

  LIV Selection Top

Selection of the LIV in the patient with AIS also has significant implications [Table 3]. The lumbar vertebrae are critical for flexibility and motion, with range of motion in the sagittal plane increasing in the native lumbar spine at each vertebral level caudally.[32] By saving levels, it has been shown that there is greater postoperative mobility as well as a greater distribution of motion across several discs with fewer segments fused;[33] there is some evidence correlating fewer segments fused with higher health-related quality of life SRS-22 scores and lower SRS-22 pain subscales as well.[34] This has implications in future disc degeneration as there are increased intradiscal pressures with each additional fused level on the adjacent caudal discs below the fusion.[35] Danielsson et al.[36] followed patients with AIS after fusion for 25 years radiographically. They found increased disc degeneration with fusion to L4 and above, relative to patients fused below L4. However, when L3 was chosen as the threshold, no significant changes were seen between patients fused to L3 and above and those patients fused below L3.[36] On the contrary, if too few levels are fused, it is possible that the patient may develop distal junctional kyphosis (DJK), in which progressive kyphosis occurs distal to the fusion construct, which may lead to pain, deformity, and may necessitate additional surgery. Distal adding-on may also occur, in which the coronal deformity progresses at the levels distal to the fusion construct. To strike this balance, one must consider not only the radiographic curve characteristics but patient characteristics as well, such as skeletal maturity. Several studies have shown that patients are at a greater risk of decompensating (either proximally or distally) if fused at a younger age, with open triradiate cartilage or at a lower Risser stage.[37],[38],[39]
Table 3: Summary of concepts in lowest instrumented vertebra selection

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In the early years of fusion surgery for scoliosis regarding level selection, Goldstein[3] described going “1 to 3 vertebrae below the primary curve” in 1964. Harrington[2] was more specific when he introduced the concept of the stable zone to select the LIV: this was the first caudal level to fall within the stable zone, defined as the area between the two lumbosacral facets. This concept was further defined by King et al.[40] as the stable vertebra, the vertebra most bisected by the central sacral vertical line (in the setting of Harrington rod instrumentation). Moe[41] emphasized the importance of vertebral rotation, in which the NV must be included distally; even in 1964, Moe described the concept of selective fusion, in which only the main thoracic curve would need to be included as the lumbar curve “often demonstrates a surprising degree of flexibility” on bending roentgenograms. Suk expanded on the concept of the NV in 2003 by establishing the relationship between the end vertebra and NV for the main thoracic curve, stating that the NV should be the LIV; if the NV is more than 1 level caudal to the end vertebra, then 1 level cranial to the NV would be sufficient, theorizing that de-rotation would most likely bring the cephalad vertebra into the stable zone[42],[43] [Figure 2].
Figure 2: Timeline of concepts in selection of the lowest instrumented vertebra over the past 50 years

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Lenke et al.,[44] in 1999, showed that spontaneous lumbar curve correction occurs in patients with type 1 and 2 curves, allowing for selective fusion. Selecting the appropriate LIV is especially important in these cases as it is the transition between the rigid thoracic spine and the flexible lumbar spine.[45] With possible spontaneous correction, it may obviate the need to add more distal fusion segments; this is especially critical in curve types 1–4 where a selective thoracic fusion may be considered.

Last touched vertebra

As previously mentioned, creating shorter constructs while saving motion segments has become a goal of AIS treatment; all while avoiding significant complications associated with a shorter construct such as distal adding on. As such, new radiographic guidelines have been suggested to help guide level selection. Recently, two parameters have gained significant attention: the last substantially touched vertebra (LSTV) and the last touched vertebra (LTV). Cho et al.[46] described the LSTV as the cephalad most lumbar vertebra in which the central sacral vertical line (CSVL) intersected the pedicle outline or was medial to it, while the LTV was the most cephalad vertebra in which the CSVL intersected any portion of the vertebral body outline, including lateral to the pedicle. Ultimately, the LTV is often slightly less bisected by the CSVL, potentially saving a level compared to the LSTV. This study looked at 195 patients with Lenke 1A curves, of which 21% had adding-on distally. They looked specifically at type 1A curves and found that identifying whether the L4 vertebra tilted right (AR) or left (AL) helped predict how the 1A curve would behave following fusion; his recommendation for 1AR curves was to select an LIV approaching both the LSTV and NV, whereas for 1AL curves fusing to at least one level below the EV had better outcomes.[46] In a study of 112 patients with Lenke 1A curves, Matsumoto et al.[47] found residual apical translation of the main thoracic curve, and an LIV above the LTV had significant associations with adding on versus an LIV at or below the LTV. Cao et al.[48] looked at 116 Lenke 2A patients and found similarly that adding on was significantly more common with an LIV proximal to the LTV. Murphy et al.[49] looked at 160 patients with either a Lenke 1 or 2 curve pattern (with an AR modifier) and found that an LIV proximal to the LSTV had a significantly increased risk of adding on (differing from prior studies using the LTV). Regarding LTV versus LSTV as the LIV, Qin et al.[50] looked at 104 Lenke 1A patients and found that an LIV at the LTV (CSVL lateral to pedicle) was at a significantly higher risk of adding on relative to the LSTV (CSVL at least touching part of pedicle); they recommended an LIV at the LSTV or LSTV + 1. Similarly, Bai et al.,[51] in 2018, looked at 120 consecutive Lenke 1A and 2A patients undergoing posterior fusion and found that a greater LIV distance from the CSVL may lead to higher rates of distal adding on. A long-term study with minimum follow-up of 5 years of Lenke 1A and 2A curves showed that the LTV produced optimal LIV positioning; those fused short of the LTV showed increased LIV-CSVL distance at final follow-up.[52] Fujii et al.[53] looked at Lenke 1B and C type curves in 44 patients and similarly found that an LIV above the LTV was a risk factor for distal adding on as well. A multicenter study from 2017 looked at 182 patients with AIS with Lenke type 1 or 2 curves that focused on the first distal uninstrumented vertebra (FDUV) tilt, one of the criteria for adding-on phenomenon (>5°, 1 year after surgery). After an average follow-up of 5 years, the authors found that of the patients with an LIV proximal to the LTV, 84% had an FDUV >5°, indicating higher risk of DJK. Stopping instrumentation at NV-1 or NV-2 was also found to be a risk factor for FDUV tilt >5°. In cases where the LTV is caudal to the NV, the authors recommend fusing down to the LTV. If the LTV is cephalad to the NV and the NV is at L3-L4, stopping instrumentation at NV-1 is a reasonable alternative.[54]

For Lenke type 3 or 4 curves, where a structural TL/L curve is present, the decision to perform a selective fusion is controversial. One must understand the relationship of the MT and TL/L curve; Lenke et al.,[55] in 1992, initially described looking at three criteria. The apical vertebral translation (AVT) ratio looks at the distance of the thoracic AVT (distance from the thoracic apex to CSVL) and the TL/L AVT (distance from the TL/L apex to CSVL). The apical vertebral rotation (AVR) ratio compares thoracic apical rotation to TL/L apical rotation using the Nash–Moe classification.[56] Lastly, the thoracic Cobb angle is compared to the TL/L Cobb angle. A ratio of 1.2 or greater (greater translation, rotation, and Cobb angle of the thoracic curve) suggests that a selective thoracic fusion is feasible[55] [Figure 3]. There is a paucity of data regarding LIV for Lenke type 5 curves; LIV selection is critical for these curves, given the importance of each additional motion segment as one goes more distal [Figure 4].[33],[34],[35],[36] A study from 2017 reviewed 78 patients with Lenke type 5 curves that were divided into selective fusion (LIV at the stable and NV) versus hyperselective fusion (at maximum 2 levels above and below the apex of the curve).[57] There were no differences in outcome score or pain level at follow-up between groups though function was significantly better in hyperselective patients. The selective fusion group was fused to or below the LTV in 91% of patients, whereas only 16% of patients ended at or below the LTV for the hyperselective group. Of patients, 10% overall developed distal adding on, of which 75% were in the hyperselective group. Of patients, 62.5% with distal adding on had an LIV above the LTV.[57]
Figure 3: The use of apical vertebral translation, apical vertebral rotation, and Cobb angle in selective thoracic fusion.

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Figure 4: Patient presenting with a Lenke type 5CN curve.

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Stable sagittal vertebra

Considering the sagittal plane is arguably just as important as the coronal plane when selecting the LIV, with major implications in the development of DJK [Figure 5] and [Figure 6]. Much of the study in this area was initially performed in patients with Scheuermann kyphosis. Mainly, two different radiographic criteria have been described: the first lordotic vertebra (FLV) or the stable sagittal vertebra (SSV) is defined as the level most bisected by the SVA (a vertical line from the posterosuperior corner of the S1 endplate). Cho et al.[58] looked at 31 patients with a minimum of 2-year follow-up in patients with kyphosis and found that an LIV above the SSV had a 38% rate of DJK versus those at or below the SSV (8%) for distal junctional problems (such as DJK or implant-related failure). Lundine et al.[59] looked at 22 patients with kyphosis and found that fusion to the SSV had a lower rate of postoperative DJK versus fusion to the FLV (13% vs. 33%). Dikici et al.[60] also looked at 39 patients with Scheuermann kyphosis and found that DJK rates were lower in patients with an LIV at the SSV over either the FLV or lower end vertebra (LEV). Kim et al.[61] retrospectively looked at 44 posterior fusion patients with kyphosis and found that an LIV at or distal to the SSV had a significantly smaller lordotic disc angle than an LIV above the SSV. Yanik et al.,[62] however, looked at 54 patients with Scheuermann kyphosis and did not find a statistical difference between patients with an LIV at the FLV or SSV. Lowe et al.[63] then studied the SSV in the patient with AIS; 375 posterior fusion patients were significantly more likely to develop DJK if the curve was instrumented to less than 1 level distal to EV. Yang et al.[17] performed a retrospective study in 113 patients with AIS with an LIV at L2 or above and found that those with an LIV above the SSV had DJK rate of 17%, whereas those with an LIV caudal to the SSV had 0% rates of DJK, defined as ≥10° between the LIV and caudal endplate while standing. Fischer et al.[64] looked at 544 patients with AIS with major thoracic curves with a minimum follow-up of 2 years and found that an LIV 3 vertebrae proximal to either the stable or NV led to an increased risk of DJK. The coronal position of the LIV was also a risk factor, with greater translation correlating with increased risk of DJK.[64]
Figure 5: Patient presenting with a Lenke 1AN curve.

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Figure 6: Patient presenting with a Lenke type 1A+ curve.

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  Conclusion Top

For the UIV in Lenke type 1, 3, and 6 curves, one should consider the preoperative shoulder imbalance and thoracic kyphosis. Fusing to T2 when the left shoulder is higher, T3 for when the shoulders are level, and T4 for when the right shoulder is higher has been associated with better postoperative shoulder balance and patient satisfaction. For Lenke type 2 and 4 curves, fusing to T2 is nearly always the appropriate UIV to control the PT curve as well as adjust for shoulder balance. Considering thoracic kyphosis is also important given the risk of PJK, those with higher preoperative thoracic kyphosis may necessitate extending the fusion cranially to T2 even in type 1 and 2 curves to control the PT kyphosis, regardless of shoulder balance. For Lenke type 5 curves, extending cephalad to the UEV may actually increase the risk of PJK. Critical intraoperative steps to prevent PJK include proper UIV selection and bending appropriate kyphosis into the proximal rod; one may consider using hook fixation proximally as well.

Regarding the LIV, selective thoracic fusion is possible in Lenke type 1–4 curves. We recommend fusing distally to the LTV on the AP radiograph to avoid DJK or distal adding on. Although there is evidence to support both the LSTV and LTV as the LIV, fusing to the LTV will often save one additional lumbar motion segment. In situations where the LTV may be questionable on the standing AP radiograph, the authors recommend using supine images to help assess for potential correction; the more proximal proposed LIV may ultimately appear more stable and neutral on the supine image, aiding in LIV selection. In patients with a structural TL/L curve, one should take into account the ratio of the MT to MT/L apical translation, rotation, and Cobb angle; a ratio of 1.2 (greater in the MT curve) is favorable for a selective fusion with spontaneous correction of the TL/L curve more likely. One must also pay careful attention to the rotation of the proposed LIV on the AP radiograph; in cases where the proposed LIV is rotated (Nash–Moe grade 2–4), we would recommend reassessing the LIV and going distally if necessary to a more NV to decrease chance of distal adding on and decompensation.

All these criteria in the coronal plane should also be considered in conjunction with the SSV on the lateral radiograph; the majority of studies reviewed indicate that fusing to the SSV has a lower risk of DJK relative to the FLV. In either case, fusing to the more distal segment (either the LTV or SSV) is likely the appropriate choice to decrease the risk of distal junctional problems. Not only is there minimal motion to be gained by preserving T12-L1 level, a study by Kim et al.[65] found no difference in the incidence of DJK whether the LIV was T12 or L1.

Lastly, one must consider the flexibility of the curve. Many different techniques have been proposed and compared such as traction, fulcrum bending, lateral bending, and supine bending radiographs among others.[66] Most studies, however, look only at the predictive value of these methods of curve flexibility compared to postoperative correction obtained. Although there are less data on whether determining LTV or SSV on these radiographs have different long-term outcomes in patients with AIS, the authors suggest using supine radiographs to assess for curve flexibility and potential curve correction as an additional piece of information to consider when determining the LIV. Less to no data are available on whether determining LTV or SSV on these radiographs have different long-term outcomes in patients with AIS. Decisions based on these imaging techniques to account for curve flexibility may result in additional levels spared safely and warrants further study.

Level selection in the patient with AIS is a complex and multifaceted decision. One must consider the patient as a whole—the patient’s goals, self-perception of shoulder balance, skeletal maturity, physical exam, and radiographic curve characteristics among other factors. Thorough preoperative planning accounting for the coronal and sagittal deformity while keeping the patient at the center of the discussion is crucial for level selection and for optimizing patient outcomes.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]

  [Table 1], [Table 2], [Table 3]


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