The selection of appropriate fusion levels, aiming to maximize correction while fusing the least levels as possible, is an important consideration in spinal deformity surgery. For AIS, the Lenke et al. (7) classification simplified and clarified the decision-making process by including all curve types and sagittal plane measurements in addition to the flexibility theories postulated by King et al. (4). Proximal thoracic (PT), main thoracic (MT), and thoracolumbar/lumbar (TL/L) curves are analyzed, in addition to thoracic sagittal measurements (T5-T12), with special attention given to the thoracolumbar junction (T10-L2). Structural and compensatory curves must be differentiated. Briefly, all major curves (largest Cobb measurement) greater than 40 degrees are considered “structural,” whether the curve decreases to less than 25 degrees on side-bending radiographs or not. For all other minor curves, those that decrease to less than 25 degrees are considered “compensatory” and nonstructural, which generally do not need to be fused. Correspondingly, those that side bend greater than 25 degrees are considered structural minor curves, which generally do need to be fused. Sagittal measurements of the thoracolumbar junction must be considered, because a kyphotic angle that is greater than or equal to 20 degrees in this region (T10-L2) implies a double major curve, regardless of coronal flexibility measurements. Specific measurement ratios must be taken into account with flexibility measurements close to the 25-degree threshold. The initial goal of these ratios was to help distinguish true and false double major curves within the King et al. (4) system; however, they apply today to threshold values in the Lenke et al. (7) system as well.

In analyzing scoliosis, the relative Cobb angles, apical vertebral translation (AVT), apical vertebral rotation (AVR), sagittal plane measurements, as well as shoulder balance and pelvic obliquity should be considered. The thoracic AVT is the distance from the center of the vertebral body at the thoracic apex to the C7 plumb line (C7PL). Similarly, the TL/L AVT is measured from the center of the lumbar apical vertebral body to the center sacral vertical line (CSVL). When the patient is perfectly balanced, the C7PL and CSVL are the same. The AVR is assessed using the Nash-Moe rotation index, which is graded on a scale from 0 to 4 where 0 is neutral or no rotation to grade 4, which is the most severely rotated (one pedicle is invisible, and the contralateral pedicle crosses the midline).

In analyzing kyphosis, the normal sagittal plane parameters are measured including the thoracic kyphosis from T2-T5 to T5-T12, the thoracolumbar kyphosis from T10 to L2, and the lumbar lordosis, which is measured from L1 to S1. In addition, we also consider the maximum kyphosis, which is measured from the vertebral endpoints showing the maximum deformity. Then, to consider sagittal balance, the sagittal sacral vertical line is used. Recently, occipitocervical parameters have been introduced (3). They provide guidelines for the orientation and position of the skull in relation to the cervical spine as well as the thoracic spine in relation to the maintenance of forward gaze and global sagittal balance.

Clinical assessment of the deformity is also an important aspect of deformity analysis. It is not unusual for curves with lower magnitudes to demonstrate large clinical deformities, or vice versa, where curves with larger magnitude only have smaller amounts of clinical deformity. This information is important for the surgical decision-making process with regard to the selection of fusion levels. For example, large clinical deformities might make selective fusions a less likely option in selecting fusion levels. This clinical impression should be correlated with standing, supine, and push-prone radiographs. Frequently, analysis of the relative heights of the clavicles, coracoid processes, and the T1 rib angle in relation to horizontal line can help predict postoperative shoulder position. These images can help determine rigid curves, which might require osteotomies for correction. During the clinical exam, it is important to note which shoulder is higher as supine or pushprone radiographs may indicate the contralateral shoulder to be higher. This is usually due to the flexibility of the lumbar or MT curve, or the rigidity of the PT curve, which is important to consider when determining fusion levels and the amount of correction desired as to not overcorrect the MT curve, thus resulting in shoulder imbalance.

To determine endpoints of the fusion, radiographic considerations are based on the Harrington stable zone and the neutral vertebra in addition to the aforementioned assessment of the clinical deformity present. The stable vertebra is the most proximal TL/L vertebra bisected by the CSVL. This lower
endpoint is considered to be the most safe, but frequently we can stop one to two levels short of the stable vertebra, depending on rotation, curve magnitude, flexibility, coronal and sagittal balance, as well as other factors. Review of supine, push-prone, and lateral bending radiographs provides a sense of the postoperative balance. As a rule, the lowest instrumented vertebra (LIV) can be chosen as the most cephalad TL/L vertebra “touched” by the CSVL (Fig. 19-1) unless a significant amount of rotation exists at this level.