11 Selection of Fusion Levels


Steven W. Hwang, Amer F. Samdani, and David H. Clements III


The goal of surgical correction in adolescent idiopathic scoliosis (AIS) is to provide an optimally balanced spine in all three planes while maintaining long-term function and correction. Level selection in AIS is based on clinical, radiographic, and surgical factors inherent to each patient. All components must be critically and thoroughly assessed to provide the best clinical and radiographic outcomes and to minimize the development of late decompensation. Clinical parameters of preoperative shoulder balance, skeletal maturity, trunk shift, and rib prominence all influence our surgical decision-making in combination with the radiographic curve pattern and attributes. The Lenke classification system has helped standardize discussion and management of most AIS curve patterns. However, variability still exists in practice patterns, and a thorough understanding of the literature and its limitations is required to make the most informed choice. This chapter will discuss the literature available to clarify the existing variability in practice patterns among expert deformity surgeons and highlight the limitations in our current understanding. The chapter will also thoroughly discuss the intricacies of level selection for each curve pattern through case-based examples.

11 Selection of Fusion Levels

11.1 Background/Historic Context

The selection of fusion levels for the surgical treatment of adolescent idiopathic scoliosis (AIS) has been debated since the inception of its treatment. This debate began before the introduction of instrumentation for scoliosis 1 , 2 , 3 , 4 , 5 and has intensified during the modern era of segmental spinal fixation. Risser 5 originally described the principle of choosing the upper instrumented vertebra (UIV) and lower instrumented vertebra (LIV) based on which vertebrae were horizontal on traction radiographs. Although the selection of levels has changed since then, the goals of obtaining a horizontal end vertebra (EV) persist today. Goldstein 1 and then Moe et al 6 described the principle of using the neutral vertebrae (NVs) in level selection which is still used to some extent in our current level selection process.

After the introduction of posterior instrumentation, Harrington 7 , 8 addressed the concept of the stable zone to identify the distal extent for a spinal fusion. The stable zone was defined as the area between two parallel vertical lines running through the lumbosacral joint, and Harrington recommended that the end vertebra of a spinal fusion be within that stable zone. 7 , 8 Moe et al 4 further advanced our understanding by introducing the evaluation of curve flexibility and vertebral rotation to select fusion levels, thus initiating the concept of giving flexible curves the ability to correct spontaneously while performing a selective fusion of the more rigid curvature.

Later, King et al 9 categorized patients with AIS and created a classification system. Included in King et al’s evaluation of patients with AIS was a description of the center sacral vertical line (CSVL), a vertical line that bisects the sacrum and is perpendicular to the level iliac crests. Lenke et al 10 further subdivided and analyzed patients with AIS to develop a more comprehensive classification system. 11 The Lenke classification provides a guideline as to which curves require instrumentation when correcting AIS. However, a thorough clinical and radiographic assessment of each patient within the context of our current literature is required to optimize level selection.

11.2 History and Physical Examination

A thorough history and physical examination is a critical component in the decision-making process of spinal fusion surgery. The patient’s skeletal maturity, family history of scoliosis, medical comorbidities, activity level, and reported self-image influence the management of scoliosis. For example, instrumenting a skeletally immature patient short of the last substantially touched vertebra increases the risk of adding-on. 12 The clinical deformity (shoulder balance, trunk shift, thoracic and lumbar prominence) in addition to the radiographic deformity will determine the selection of fusion levels. The nuance in deformity correction surgery lies in predicting how the correction and fusion of the patient’s scoliosis will affect the clinical deformity in the short and long term as the curve self-corrects over time.

11.3 Radiographic Evaluation

Radiographic analysis of the patient with AIS should include long-cassette standing anteroposterior (AP) and lateral radiographs of the spine. The position of the patient’s arms should also be noted on the radiograph, as different positions may lead to alterations in sagittal balance. 13 A set of flexibility films should also be collected and may include right and left side-bending, push-prone, or traction radiographs. 14 , 15 , 16 , 17 Prone radiographs may also provide additional information useful in deciding which levels to instrument.

Certain clinical parameters, such as shoulder balance, can also be assessed on AP films. The radiographic interpretation of shoulder position is done in several ways including the measurement of the T1 tilt, coracoid height, and first rib clavicle height. Kuklo et al 18 reviewed 112 AIS patients with a proximal thoracic (PT) curve greater than 20 degrees to identify radiographic parameters that would predict postoperative shoulder balance at 2-year follow-up. The clavicle angle, as formed by the intersection of a horizontal line and the tangential line connecting the highest two points of each clavicle, provided the best preoperative radiographic prediction of postoperative shoulder balance.

Although determining level selection is often based on radiographic outcomes, it is important to keep in mind that radiographic parameters for shoulder measurements correlate poorly with clinical outcomes. 19 , 20 Variability in the definition of an acceptable radiographic outcome for the shoulder height difference (5 mm–2 cm) also contributes to inconsistencies in the literature. 21 , 22 , 23 Smyrnis et al 24 suggested that 2 cm of asymmetry was associated with patient dissatisfaction; however, Akel et al 25 reported that 28% of healthy, asymptomatic adolescents without scoliosis have 1 cm of asymmetry and up to 27 mm can be tolerated without cosmetic complaints. Although patients typically have improvements in their satisfaction scores after surgical correction, specific radiographic values have been poorly correlated with quality-of-life measures like the SRS-22 to date.

11.4 Operative Algorithm/Goals

The principles of surgical treatment for scoliosis are based on outcome measures, including pulmonary function testing, radiography, cosmetic appearance, functional outcome, range of motion, and aerobic studies. 26 , 27 , 28 , 29 , 30 , 31 Classification and understanding of the patient’s spinal curve type is crucial in avoiding many postoperative complications, including decompensation. The system set forth by Lenke et al 10 provides a comprehensive classification of curve patterns and a template for the surgical management of AIS. A thorough understanding of the Lenke classification system and current literature is essential before determining the vertebral levels for spinal fusion. In the immediate postoperative setting, selection of UIV is primarily focused on the radiographic and clinical shoulder balance, whereas LIV selection often is influenced by coronal translation and preserving distal motion segments.

As mentioned previously, although radiographic measures do not always predict satisfactory clinical outcomes, several radiographic parameters have been established and accepted in the literature as satisfactory results. The generally accepted goals of surgery are a balanced spine with even shoulders and an even pelvis with minimal coronal translation. Schulz et al 32 evaluated 106 patients undergoing selective thoracic fusions and found that trunk shift less than 1.5 cm and coronal translation less than 2 cm correlated with better SRS-22 satisfaction scores. Shoulder height difference has been a little more varied in the literature with satisfactory outcomes defined from 5 mm to 2 cm. 21 , 22 , 23

11.5 Anterior Spinal Fusion Level Selection

Anterior spinal fusion (ASF) with instrumentation is typically reserved for cases of scoliosis in which only one curve is being treated; this specifically applies to cases of Lenke type 1 and type 5 curves. Awareness of the flexibility of the compensatory curve and its response to treatment of the main curve is critical in anterior surgery for scoliosis. The anterior approach provides excellent curve correction, creates kyphosis, and may facilitate the inclusion of fewer levels in the construct than would a posterior approach: typically, 2.5 levels on average shorter when compared to hybrid constructs. 4 , 33 , 34 However, with pedicle screw-only constructs, the level selection is more comparable and on average 1.6 levels shorter. 35 Selection of fusion levels typically includes all segments within a curve, from the EV cranially to the EV caudally. 36 , 37 , 38 Depending on the approach, either a single- or dual-rod technique can be utilized.

Hall et al 39 and Bernstein and Hall 40 advocated for a hyperselective short instrumentation for flexible thoracolumbar/lumbar (TL/L) curves by overcorrection of the apex. If the apex of the curve is a vertebral body, they advocated fusing one vertebra above and one vertebra below the curve. If the apex is a disc space, the fusion and instrumentation would be performed two vertebral levels above and two levels below the apex. Using this technique, they demonstrated an initial correction of 87%, declining to 67% at 2 years. The satisfaction rate in their study of 17 patients was 88%. To achieve these results, Hall et al 39 recommended overcorrecting the instrumented vertebrae. Limitations of this technique involve creating kyphosis at the thoracolumbar junction and the requirement for a very flexible spine. Geck et al 41 compared the Hall technique and posterior spinal fusions (PSFs) and observed that the Hall technique had less correction, higher loss of correction, greater kyphosis, and more residual apical translation.

More recently, some authors 42 have tried fusing Lenke 5 curves rostrally to EV-1 versus EV. The upper end vertebra (UEV) group had better correction with 12-year follow-up (86 vs. 74.4%, p = 0.020) and also better apical vertebral translation (AVT) and apical vertebral rotation (AVR; p = 0.007 and 0.023) compared to UEV-1.

Application of any surgical technique requires assessment of the patient’s overall coronal and sagittal balance as well as the clinical deformity. For example, an anterior procedure may lead to worsening of the kyphosis in already hyperkyphotic thoracic curves. Also, if a patient has a high right shoulder preoperatively, a selective anterior fusion of a left Lenke type 5 curve may increase the patient’s shoulder asymmetry as well.

11.6 Posterior Spinal Fusion Level Selection

Appropriate level selection requires a thorough understanding of the Lenke classification system. The fundamental suggestion of the classification system is to fuse any structural curve and therefore by definition the UIV determination would be similar for Lenke 1, 3, and 6 curves where the upper thoracic (UT) curve is unfused but the main thoracic (MT) is fused. Lenke 2 and 4 curves would carry the same principles for their UIV where both UT and MT are fused. Lastly, Lenke 5 curves would leave the MT and UT unfused. Similarly, based on the Lenke classification system, LIV level selection criteria would be similar for Lenke 1, 2, and a selective thoracic 3 where the MT curve would be fused and the lumbar curve unfused. LIV selection for nonselective Lenke 3 and Lenke 4, 5, and 6 curve types would follow similar guidelines as well.

When deciding which curve is structural, the distinction may be subtle at times and, hence, variability in practice may occur. The differentiation between a 23-degree curve and a 26-degree curve may be confounded by effort on bending radiographs or intra user variability from measurement error. Furthermore, the general principles adopted are often founded based on literature from hybrid constructs, whereas most posterior fusions are now mostly constructed of pedicle screws. Many of the landmark articles defining our level selection also predate the common application of the Lenke classification system. As mentioned previously, the radiographic parameters for shoulder balance also poorly correlate with clinical measures. 43 , 44 , 45 This multitude of factors leads to significant variability in surgeon practice with level selection. Nonetheless, we strive to obtain the most successful patient result by combining the literature available to best optimize our radiographic and clinical outcomes. Given these limitations, some of the “rules” of level selection can be generalized into broader categories. We will attempt to summarize the key references in UIV and LIV selection and then more specifically discuss each curve type individually.

11.7 UIV Selection in Posterior Fusions

11.7.1 Upper Thoracic and Main Thoracic UIV

Lee et al 43 originally observed that fusions had to be extended through the UT curve to avoid shoulder asymmetry postoperatively. Although their data predated the Lenke classification system and included hybrid constructs, it became a fundamental principle in deciding UIV selection. They divided 246 patients into three groups: T1-positive tilt (tilted to the right in a right MT curve) with UT + MT fused, T1-positive tilt and only MT fused, and T1-negative tilt or neutral with MT fused. Based on preoperative shoulder imbalance and a high progression in certain groups postoperative, they recognized the importance of fusing the UT when the left shoulder is high preoperatively. That same principle has been advocated by others and simplified to the idea of fusing to T2 when the left shoulder is high in a right MT curve, T3 when they are even, and T4 if the right side is high. 46 Other authors have advocated fusing the UT when it measures greater than 45 degrees with good outcomes as reliable bending X-rays of the UT curve are effort limited. 23

Suk et al 22 divided 40 patients with pedicle-screw–only fusions into a group where the UT and MT curves were fused and another where only the MT curves were fused. They subsequently divided patients into three preoperative cohorts: left shoulder elevated greater than 5 mm, shoulders even, or right side elevated greater than 5 mm. If the left preoperative shoulder was up, all patients had left shoulder elevation postoperatively; if the shoulders were even, shoulder asymmetry correlated with not fusing the UT curve; and if the right shoulder was up before surgery, six of eight patients were still elevated postoperatively when both curves were fused. A mixed outcome was obtained when only the MT fused but all were within 1 cm of difference. They concluded that the UT curve should be fused when (1) the left side is up preoperatively, (2) they are even preoperatively, or (3) the right side is up but by less than 12 mm.

Other authors have advocated level selection based on the flexibility of the UT curve. Ilharreborde et al 44 looked at Lenke 1 and 2 curves and fused the UT curve only if it was rigid, or if the T1 tilt and shoulder were in the same direction with a flexible UT curve. They defined flexible as a difference of 15 degrees on bending radiographs. Overall they reported 89% successful outcomes based on their protocol and advocated for their guidelines. However, they defined a successful outcome using parameters of T1 coronal translation, T1 tilt, and clavicular angle. Several other authors have shown that T1 tilt has a poor correlation to shoulder symmetry and clinical satisfaction. 20 , 43 , 45

Bjerke et al 47 compared Lenke, Trobisch (curve type and preoperative difference), and Ilharreborde selection criteria of UIV in a cohort of 263 patients. Using Lenke UIV selection criteria, 16% had imbalance postoperatively as compared to 14.9% using Ilharreborde criteria or 21.9% using Trobisch criteria. They noted that clavicle angle, T1 tilt, and radiographic shoulder height (RSH) were the only reliable measures, but T1 tilt was not associated with postoperative shoulder imbalance. Furthermore, they reported a new shoulder imbalance incidence of 8.8% using RSH cutoff of 15 mm.

Tang et al 48 looked only at Lenke 1 type curves and fused to EV or EV –1 if the left shoulder was up and EV or EV +1 if the left shoulder was elevated preoperatively. 26.7% of UIV selected were within levels suggested by Rose and Lenke, 46 but 56% were caudal to recommended levels. Furthermore, 61.3% of patients had improvement immediately postoperatively, but patients continued to self-correct over time and ultimately 93.3% had acceptable outcomes (< 1 cm shoulder height difference) at last the follow-up. 48

Other authors have also reported varying degrees of spontaneous correction over time. Cil et al 49 reviewed 37 patients instrumented with the Isola system having nonstructural UT curves and observed that 41% of patients with unfused UT curves improved over time. Lee et al 43 similarly observed that spontaneous correction of 24.8 and 28.6% occurred in groups that had fusions of the MT but not UT curves. Although it is somewhat reassuring that mild asymmetry will correct, significant residual asymmetry may lead to further problems as the compensation has to occur across some unfused segments. Cao et al 50 noted a correlation between shoulder imbalance and adding-on (p = 0.03). Interestingly, they did not observe an impact of postoperative shoulder imbalance between patients fused to T2 versus T3 and below.

Significant variability still exists when deciding which level to choose for the UIV. Generally, the Lenke classification helps us choose which curves to fuse and we can then refine our decision based on the available literature. However, universal rules may not always be applicable as varying factors influence our outcomes. Inherent factors to the curve such as a large but flexible primary thoracic curve will allow greater correction which will drive the left shoulder higher as will surgical choices such as metal type, possible use of osteotomies, and implant density. A realistic understanding of an individual surgeon’s expected correction applied to preoperative parameters will often help a surgeon obtain more reliable and favorable outcomes.

11.7.2 Thoracolumbar Upper Instrumented Vertebra

For thoracolumbar curves, classically UIV fusion levels are chosen as the UEV. This most commonly is T10 or T11. Shufflebarger et al 51 originally demonstrated comparable outcomes using posterior screw-based instrumentation to anterior results, and many subsequent studies have continued to use the same selection criteria with excellent outcomes. 41 , 52 , 53 Okada et al 54 reported on 29 patients with Lenke 5C curves where the UIV was either UEV –1 or UEV and noted no difference in correction initially (8.1 ± 5.1 vs. 7.5 ± 6.8, p = 0.580). However, even with shorter follow-up, the UEV –1 group had a greater loss of correction (Cobb 12.2 ± 6.7 vs. 7.2 ± 0.9, p = 0.033) without differences in sagittal and coronal balance. Generally, the UIV selected remains the UEV, but caution should be used to avoid selecting a UIV at the apex of kyphosis.

11.8 LIV Selection in Posterior Fusions

11.8.1 Main Thoracic Lower Instrumented Vertebra

Typically, the LIV for an MT curve will span between T10 and L2. Criteria for choosing a selective fusion where the compensatory structural curve is not fused will be discussed in another chapter and therefore not addressed here. As discussed in section “Background/Historical Context” of this chapter, the principles of NVs and stable zone described by Goldstein, 1 Moe et al, 4 and Harrington 8 are still fundamentally used but slightly more refined.

Suk et al 22 looked at LIV with respect to NV and EV in Lenke 1 and Lenke 1 AR-type curves. They observed unsatisfactory outcomes (T1 translation > 2 cm from CSVL or adding-on) in 28% (Lenke 1) and 50% (Lenke 1 AR) of patients. The NV is typically caudal to the EV and can be further distal especially with Lenke 1 AR curve patterns. In patients where NV = EV + 3, they noted unsatisfactory results in patients fused short to NV – 2 (75%) or NV – 3 (71%). However, if the NV = EV or was EV + 1, patients did well if fused to NV or NV + 1. With the 5-year follow-up, Clément et al 55 similarly suggested selecting LIV as lowest between NV and the stable vertebra. Erdemir et al 56 further tried to subclassify the lumbar rotation as turning into the thoracic curve (Lenke 1 AR), neutral, or turning away from it and recommended fusing to NV – 1 (L2–L3), NV (T12–L1), and NV + 1 (L1–L2), respectively. However, some of the difficulty in standardizing level selection lies in the variation in selecting the NV. In one study, 50 consecutive surgically treated cases of AIS were reviewed by 16 scoliosis surgeons. Interobserver reliability for the end, neutral, and stable vertebrae had kappa values of 0.45, 0.32, and 0.52, respectively. 57 The poor agreement between surgeons in selecting the NV limits the utility of the NV as a common factor in level selection. Furthermore, Rizkallah et al 58 compared three different LIV selection criteria: Suk, Lenke, and Dubousset criteria. In mostly Lenke 1A/B curves, using these three parameters, they did not find a significant difference in outcomes between groups.

A more reliable tool derived from Harrington’s stable zone is the principle of the last touched vertebra (LTV). Lenke et al 59 described the LTVs as the caudal level where the CSVL touches the vertebrae. Others have expanded on it describing a last substantially touched vertebra where the CSVL at least touches the pedicle or is even more medial (Fig. 11‑1). Looking at 116 Lenke 2A patients, Cao et al 50 divided patients based on whether the LIV was proximal, at, or distal to the LTV. Fifty percent of patients fused proximal to the LTV developed adding-on, whereas only 7.3% developed adding-on if the LIV was at the LTV and 7% if LIV was distal to the LTV. Similarly, Matsumoto et al 60 found in a multivariate analysis of 169 Lenke 1A curves that only apical translation and LIV–LTV distance were significantly correlated with adding-on.

Fig. 11.1 Posteroanterior radiograph of a 16+4-year-old boy with right adolescent idiopathic scoliosis (AIS) curve. Drawing the center sacral vertical line (CSVL), we can see it just touches the lateral border of the body and is lateral to the pedicle at L1, thus making L1 the last touched vertebra (LTV). At L2, the CSVL crosses medial to the pedicle and therefore constitutes the last substantially touched vertebra (LSTV).

Qin et al 61 assessed 104 Lenke 1A patients and divided them into three cohorts based on the LIV and noted a higher rate of adding-on (%) in the nonsubstantially touched cohort: LIV = nonsubstantially touched (66.7%), LIV = nonsubstantially touched +1 (11.6%), or substantially touched vertebra (10%). Similarly, Murphy et al 62 noted three factors associated with adding-on in 160 patients: LIV–LSTV (last substantially touched vertebra) distance, Risser, and LTV–CSVL less than 2 cm. The risk of adding-on was 16% with one factor, 27% with two, and 80% with all three.

It is also important to always consider the sagittal profile of patients when selecting fusion levels. A thoracolumbar kyphosis needs to be bridged when selecting levels just as a rostral UIV should not stop at the apex of a kyphosis. Yang et al 63 built on the sagittal stable vertebra (SSV) principle (the vertebra touched by a line drawn from the posterior corner of the sacrum extending rostrally in a lateral film (Fig. 11‑2) described by Lenke. They observed distal junctional kyphosis (DJK) in 22% of patients when the LIV selected was above the SSV as compared to 4% or below. Lowe et al 64 reported that 4.5% of patients had DJK (10 degrees of focal kyphosis) preoperatively and recommended the LIV be chosen as EV +2 if preoperative DJK was present. As scoliosis is a three-dimensional problem, surgical consideration should always include all three planes to minimize decompensation.

Fig. 11.2 A lateral radiograph of a 14+1-year-old girl with an abnormal pattern of increased thoracic kyphosis. The sacral sagittal vertebra (SSV) is crossed by the line drawn vertically from the posterior edge of the sacrum. In this case, L3 constitutes the SSV.

11.8.2 Thoracolumbar Lower Instrumented Vertebra

When considering LIV selection for lumbar/thoracolumbar curves, additional consideration should be given to the preservation of motion segments. Hypothetically, preservation of more motion segments may avoid accelerated degenerative changes and minimize pain as well as the need for further surgery in the future. Some studies have shown that preserving more unfused levels allows a greater distribution of motion across those segments. 65 However, a paucity of data correlating clinical outcome scores to an LIV of L3 or L4 exists. 66 , 67 The majority of our long-term follow-up studies are limited by the use of outdated instrumentation, poor follow-up, and uncontrolled variables such as indications for reoperation, smoking status, and other factors impacting pain and function. Limitations notwithstanding, our surgical objectives are to achieve optimal correction while preserving as many motion segments as possible.

Shufflebarger et al 51 described the use of pedicle screws for the correction of Lenke 5C curves with comparable outcomes to anterior approaches. They reported good outcomes with the LIV as EV or EV + 1. Using a unilateral screw and concave top/bottom only construct, Roberts et al 68 identified five factors that determined the need to extend beyond EV–EV: lowest instrumented vertebra disc angle (LIVA), TL/L curve magnitude, lumbar convex bending angle, lumbar apical vertebral translation, and compensatory thoracic curve magnitude. Others have similarly identified preoperative lowest instrumented vertebral tilt (LIVT) of greater than 25 degrees as a possible risk factor for coronal imbalance. 52 , 53 Wang et al 53 further noted continued improvement of global and trunk balance from immediate postoperative to 2 years and derived a formula for 2-year lumbar translation: 14.1 + 1.2 (preoperative LIV–CSVL distance). They concluded that the LIV should not exceed 28 mm of coronal translation and 25 degrees of tilt.

Kim et al 69 reviewed 66 patients with Lenke 5C curves using pedicle-screw–only constructs and compared LIV based on the flexibility of the bending radiographs. Patients were divided into three cohorts: fused to L3 with flexible L3 (L3 crosses the CSVL, and Nash–Moe 2 or less on bending), fused to L3 with a rigid L3, or fused to L4. Patients with a less flexible L3 had more LIVT and coronal translation than the flexible L3 or L4 cohort. Based on LIVT greater than 10 degrees or coronal translation of 15 mm, 9.1% of the flexible L3 cohort had unsatisfactory results as compared to 12.5% in the L4 and 68.2% in the rigid L3 group. Based on these results, they recommended selecting L3 when flexible, but otherwise to fuse to L4. Chang et al 70 replicated these findings. They reported 15.2% of patients with flexible curves had unsatisfactory results (adding-on 5 degrees and distalization or increased disc angulation of 5 degrees or LIV tilt > 10 degrees or coronal translation > 15 mm) as compared to 61.1% in the rigid cohort.

11.9 Detailed Discussion of Lenke Curve Types

11.9.1 Type 1: Main Thoracic Curves

Type 1 MT curves are the most commonly treated form of AIS. 71 Although these curves can be treated with either ASF or PSF, posterior instrumentation and fusion remain the gold standard. 72

To determine the UIV, clinical shoulder balance needs to be assessed. As previously discussed, the clavicle angle appears to be the most reliable preoperative indicator for shoulder assessment. As a general rule, for a patient with a right MT curve and right shoulder elevation, the proximal extension of spinal fixation to T4 or T5 is appropriate. Exclusion of the UT segments will allow left shoulder elevation to occur with correction and will produce level shoulders postoperatively. For a patient with level shoulders, the extension of fixation to T3 or T4 is often indicated. Cranial extension of the posterior spinal segmental instrumentation (PSSI) to include the UT segments will facilitate control over the left shoulder height and maintain shoulder balance postoperatively. For a patient with left shoulder elevation, the extension of fixation to T2 is usually necessary to eliminate postoperative shoulder elevation. This proximal extension of the spinal fixation levels will allow intraoperative compression of the upper left thoracic segments to lower the left shoulder and correct the patient’s preoperative imbalance (Fig. 11‑3).

Fig. 11.3 (a) A 15+7-year-old girl with a 19-degree proximal thoracic (PT), 51-degree main thoracic, and 49-degree lumbar scoliosis. Both the PT and lumbar curves are nonstructural. There is +29 degrees of thoracic kyphosis; therefore, the correct Lenke curve classification is type 1CN. (b) Because of the similar magnitude of the thoracic and lumbar Cobb angle measurements, apical vertebral rotations, and apical vertebral translations, this patient underwent posterior instrumentation and fusion from T4 to L3, with a very reasonable realignment of her thoracic and lumbar curves and fractional lumbosacral curve at 2 years postoperative. The L3–L4 disc has remained relatively level over time. (Reproduced with permission from Newton PO, O’Brien MF, Shufflebarger HL, Betz RR, Dickson RA, Harms J. Idiopathic Scoliosis: The Harms Study Group Treatment Guide, 1st edition, p. 155. Thieme Medical Publishers, Inc: New York; 2010.)

As discussed in detail previously, an alternate approach is to leave the UT curve unfused and allow self-correction to achieve shoulder balance. 43 , 49 , 73 Brooks et al 73 reviewed 626 patients and noted that a UIV of T2 was associated with improvement of the UT curve magnitude, but patients with a UIV of T4 had better shoulder balance (34 vs. 45/48% for T2/T3, p = 0.008).

The Lenke system suggests selecting the LIV as the lowest vertebra touched by the CSVL for lumbar curves with an A modifier. Most commonly, this is the vertebra proximal to the stable vertebra (stable-1) or occasionally the vertebra two levels proximal to the stable vertebra (stable-2). PSSI in type 1 curves is best suited for patients with a normal or hyperkyphotic sagittal modifier. For type 1B curves, the LIV (the most cephalad vertebra intersected or bisected by the CSVL) will usually be located in the thoracolumbar junction. Selective fusion criteria for Lenke 1C curves are discussed in Chapter 10.

11.9.2 Type 2: Double Thoracic Curves

Type 2 double thoracic (DT) curve includes a major structural MT and a structural minor PT curve. 22 The general guideline calls for posterior arthrodesis of both curves. The LIV is chosen in a manner similar to that for treating type 1 curves. Generally, the proximal level of fusion should be either T2 or T3. Consideration of the patient’s clinical shoulder balance and radiographic clavicle angle is critical. 18 Patients with a high left shoulder preoperatively will require fusion extending to T2 (Fig. 11‑4). This will allow greater control in balancing the shoulders. For patients with level shoulders, the UIV may be either T2 or T3. When assessing the patient’s deformity to determine the UIV, the rigidity and magnitude of the PT curve will need to be considered. Large or inflexible curves will probably generate a significant shoulder imbalance as the MT curve is corrected. For patients with a high right shoulder, the UIV may be T3. Excluding the most proximal vertebra will allow spontaneous left shoulder elevation with correction of the MT curve. Often, the PT curve is hyperkyphotic, and compression of the convexity is therefore applied first. In general, when correcting the PT curve, one must consider the correction that will be attained in the MT curve. Greater correction of the MT curve will further increase the elevation of the left shoulder.

Fig. 11.4 (a) A 13 +11-year-old girl with a 38-degree proximal thoracic (PT), 74-degree main thoracic, and 36-degree lumbar scoliosis. The PT curve side bends to 28 degrees and is structural, whereas the lumbar curve bends to 7 degrees and is nonstructural. There is a +48-degree thoracic kyphosis; therefore, the correct Lenke curve classification in this case is type 2A+. (b) This patient underwent a posterior instrumentation and fusion from T2 to L2, with excellent radiographic alignment noted at 2 years postoperative. (c) Preoperative and postoperative clinical photographs demonstrate the patient’s improved truncal alignment. (Reproduced with permission from Newton PO, O’Brien MF, Shufflebarger HL, Betz RR, Dickson RA, Harms J. Idiopathic Scoliosis: The Harms Study Group Treatment Guide, 1st edition, p. 156. Thieme Medical Publishers, Inc: New York; 2010.) .

11.9.3 Type 3: Double Major Curves

Lenke type 3 double major curves are defined by a major structural MT curve as well as a minor structural TL/L curve. The general rule calls for the posterior fusion of both curves (Fig. 11‑5). The instrumentation begins at T3 to T5, depending on shoulder position, as with type 1 curves. The LIV is often the most cephalad lumbar vertebra touched by the CSVL when the lumbar curve is flexible and secure pedicle fixation is achieved and is usually L3 or L4. The dilemma is in selecting the appropriate distal fusion level while attempting to maintain as many motion segments as possible in the lumbar spine. The LIV should have near-neutral rotation, the disc below the LIV preoperatively should be parallel or wedged at the convexity, and the apex of the lumbar curve should be at least one disc level above the LIV. The TL/L curve is often more flexible than the MT curve. 74 The goal is to render the LIV horizontal on the AP film. Often, type 3 curves will have TL kyphosis, and it is important to correct this regional deformity during the surgical procedure. 75 Instrumentation levels should not end in this area of kyphosis but should be extended distally to avoid postoperative sagittal decompensation.

Fig. 11.5 (a) A 13 + 1-year-old girl with a 23-degree proximal thoracic (PT), 81-degree main thoracic, and 69-degree lumbar scoliosis. The PT curve is nonstructural at 18 degrees on side bending, whereas the lumbar curve is structural at 28 degrees. There is a +36-degree thoracic kyphosis, and the correct Lenke curve classification in this case is type 3CN. (b) This patient underwent posterior instrumentation and fusion utilizing a three-rod technique with a hybrid construct from T3 to L4, with excellent balance noted at 10 years postoperatively. (Reproduced with permission from Newton PO, O’Brien MF, Shufflebarger HL, Betz RR, Dickson RA, Harms J. Idiopathic Scoliosis: The Harms Study Group Treatment Guide, 1st edition, p. 158. Thieme Medical Publishers, Inc: New York; 2010.)

A small percentage of type 3 curves show dominant thoracic curve characteristics. Careful evaluation of these curves may allow the surgeon to perform a selective thoracic fusion, which will be discussed elsewhere.

11.9.4 Type 4: Triple Major Curves

The Lenke type 4 triple major curve pattern involves all three regions of the spine. The majority of patients with this pattern require fusion from T2 or T3 to L3 or L4. Again, the evaluation of shoulder balance is critical for determining the proximal level of fusion. Optimal horizontalization of the LIV is the primary goal corresponds with the previous discussion of type 1, 2, and 3 curves, respectively. Rarely, a selective thoracic or double thoracic fusion can be performed, leaving the lumbar curve unfused. In these cases, the clinical and radiographic ratios of the thoracic-to-lumbar curve are greater than 1.2 in favor of the larger thoracic curve (Fig. 11‑6).

Fig. 11.6 (a) An 11 + 10-year-old girl with a 44-degree proximal thoracic (PT), 88-degree main thoracic, and 61-degree lumbar scoliosis. The PT curve is structural, bending to 29 degrees, and the lumbar curve is also structural, bending to 31 degrees. There is a +54-degree thoracic kyphosis; therefore, the correct Lenke curve classification is type 4C+. (b-d) Because of the marked difference in the thoracic-to-lumbar Cobb angle ratio, apical vertebral translations, and apical vertebral rotations, this patient underwent a selective double thoracic ratio instrumentation and fusion from T2 to T12, with excellent coronal and sagittal correction and alignment at 3 years postoperative. (Reproduced with permission from Newton PO, O’Brien MF, Shufflebarger HL, Betz RR, Dickson RA, Harms J. Idiopathic Scoliosis: The Harms Study Group Treatment Guide, 1st edition, p. 160. Thieme Medical Publishers, Inc: New York; 2010.)

11.9.5 Type 5: Thoracolumbar/Lumbar Curves

The Lenke type 5 curve pattern demonstrates a major curve in the TL/L region with a minor nonstructural curve in the MT region. Curves of this pattern may be treated with either ASF or PSF. Occasionally, we still utilize anterior spinal instrumentation and fusion from the upper to the lower end vertebrae of the curvature. The surgical levels include all convex discs within the curve. 76 , 77 Instrumentation is achieved with a dual-rod system. Hurford et al 78 reviewed 48 TL/L curves treated with dual-screw/dual-rod constructs and found coronal correction of the TL/L curve averaging 75%, excellent sagittal alignment, and no instrumentation failure or pseudarthrosis at a minimum follow-up of 2 years. However, a trend toward posterior instrumentation is developing because of disadvantages of the anterior thoracolumbar approach. In selecting posterior fusion levels, the UIV and LIV are usually identical to those in ASF for the same curves. For small, very flexible curves, the minimal fusion technique of Hall and colleagues, 39 as previously described, may be used. It is extremely important to evaluate the MT and PT regions as well as shoulder balance. If the left shoulder is elevated, some residual tilt must be maintained at the UIV to aid with postoperative shoulder balance. Correction of the lumbar curve and secondary correction of the compensatory thoracic curve will cause further elevation of the left shoulder. Therefore, careful evaluation of shoulder balance is necessary, including examination of the scapulae and thoracic prominence. If there is no MT component to the curve, the UIV and LIV may be horizontalized for correction. The goal is minimal deformity above and below the thoracolumbar fusion levels (Fig. 11‑7).

Fig. 11.7 (a) A 13+6-year-old girl with a 5-degree proximal thoracic curve, 26-degree main thoracic (MT), and a 55-degree left thoracolumbar curve. (b) The MT curve is not structural as it bends down to 10 degrees. Thoracic kyphosis measures 23 degrees. She has a Lenke 5CN curve pattern and is a Risser 4. Although inconsistent across users, she remains a Nash-Moe 2 and crosses the midline. Given her rotation, we chose to fuse her to L4, although L3 could have been reasonable in her case. (c) She has maintained a good coronal and sagittal balance at 2 years postoperatively.

Excellent results can be achieved with selective lumbar fusion. Sanders et al 79 reviewed 49 AIS patients who had undergone selective fusion for TL/L curves of 30 to 55 degrees. All of the patients were followed up for 2 years. Satisfactory results were achieved in patients with a TL/L-to-MT Cobb-angle ratio of greater than 1.25 and a thoracic curve that bent to 20 degrees or less. These satisfactory results were defined as a thoracic curve at follow-up of less than 40 degrees, acceptable balance and sagittal alignment, and no need for further procedures.

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Apr 30, 2022 | Posted by in ORTHOPEDIC | Comments Off on 11 Selection of Fusion Levels

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