6 Surgical Techniques for Adolescent Idiopathic Scoliosis and the Selection of Fusion Levels
Following the development of the third-generation implants by Cotrel and Dubousset in the 1980s, which included the use of pedicular screws to the thoracic spine, the King classification 1 became obsolete, making it necessary to develop a new classification system to enable more accurate planning of the surgical treatment of adolescent idiopathic scoliosis (AIS). The Lenke et al classification 2 , 3 system was then developed to define which curves should be instrumented. However, it does not address the specific fusion levels, such as the upper instrumented vertebra (UIV) and the lower instrumented vertebra (LIV). Suk et al 4 also described another classification system, giving special attention to the definition of the LIV in both thoracic and thoracolumbar or lumbar curves (TL/L). The main objective of the surgical treatment of AIS is the tridimensional correction of the deformity, preserving as many motion segments as possible, and resulting in a balanced trunk in the sagittal and coronal plane with level shoulders. Therefore, choosing the appropriate UIV and LIV is of extreme importance in achieving these goals.
This chapter presents practical rules for defining the fusion levels for each type of curve, based on the literature and on our experience.
Choosing the Levels
Step 1: Which Curves to Include
Once the type of curve has been correctly identified according to the Lenke et al classification system, 2 , 3 the first rule is to include all the vertebrae of the major curve in the fusion area. 5 It is important to never leave any vertebra of the major curve out of the fusion. The second rule is to include all the secondary structured curves into the fusion. However, there may be some exceptions to this rule, especially in some Lenke 3C and 6C curves where a selective fusion may be done. 5 Even in nonstructured type C TL/L, whether or not to include this curve is controversial.
Step 2: Choosing the Upper Instrumented Vertebra for Thoracic Curves (Lenke 1 to 4)
The goal is to have the shoulders level post-operatively. Initially, the surgeon determines whether or not the proximal thoracic (PT) curve needs to be included in the arthrodesis, based on clinical and radiographic parameters. The PT is included in the following situations:
The PT is structured (that is, it does not correct to less then 25 degrees in bending films).
The left shoulder is higher in the clinical evaluation.
The transition from the PT to the main thoracic (MT) curve is below T5 (these curves are usually structural). 6
There is a regional kyphosis between T2 and T5 of more than 25 degrees.
In the presence of any of these criteria, the inclusion of the PT in the fusion is considered; otherwise, the shoulders will rarely correct spontaneously, or there will be a proximal hyperkyphosis that is cosmetically not acceptable. In such cases, the UIV should be T2. To allow a smooth transition from the extremely mobile cervical spine to a rigid and fused thoracic spine, T1 should be avoided as the UIV.
If none of the above criteria is present, there is no need to include the PT in the arthrodesis, and the UIV may be the neutral vertebra (NV) just above the MT, usually the proximal end vertebra (PEV) of the MT or one vertebra above.
Step 3: Choosing the Lower Instrumented Vertebra for Thoracic Curves (Lenke 1 to 4)
Structural Lumbar Curves (Lenke 3 and 4)
When a major MT is accompanied with structured TL/L (Lenke 3 and 4), it is usually neces sary to include the TL/L in the arthrodesis. In this case, the LIV should be the distal end vertebra (DEV), as will be discussed below. In some Lenke 3C and 4C curves, however, it is possible to perform a selective thoracic fusion, without including the TL/L in the arthrodesis. Making this decision is discussed below about C Modifiers.
Nonstructural Lumbar Curves (Lenke 1 and 2)
If the major MT is accompanied with a nonstructured TL/L (Lenke 1 and 2), the choice of LIV is controversial. To facilitate and clarify the selection process, we will discuss the A and B lumbar modifiers and the C modifiers separately.
A and B Modifiers
According to Miyanji et al, 7 the A and B lumbar modifiers are not sufficient to describe different curves for the Lenke type 1 curves. These authors subclassified the 1A curves as 1AL and 1AR ( Fig. 6.1 ). In the 1AL curves, the L4 is tilted to the left, whereas in the 1AR curves, the L4 is tilted to the right. The 1AL curves behave similarly to the 1B curves. These concepts can also be used for the Lenke type 2 curves.
For 1AL and 1B curves, the LIV should be the stable vertebra (SV), but for a 1AR curve, the SV is too distal, making it possible to fuse it shorter. 7 According to Suk et al, 8 the relationship between the DEV and the NV is the best way to define the LIV. When the number of vertebrae between the NV and DEV is 1 or 0, the NV should be chosen as the LIV. When this difference is 2 or more, the LIV may be one vertebra proximal to the NV (NV-1). The LIV should never be two vertebrae above the NV (NV-2) because of the high risk of adding on and decompensation. 8
The first step is to decide if it is possible to perform a selective thoracic fusion. The definition of selective fusion is when both MT and TL/L deviate completely from the midline, but only the major curve is fused and the minor curve is left unfused and mobile. 5 Importantly, for the TL/L curves the midline means the central sacral vertical line (CSVL), whereas for MT curves the midline is the C7 plumb line (C7PL) ( Fig. 6.2 ). The aim of performing a selective fusion is to spare motion segments, which is the most challenging decision in the surgical treatment of AIS. To help with this difficult decision, Lenke et al described clinical and radiographic criteria (see Box 6.1 ). 5 Even if all the criteria above are met, a selective fusion should be avoided in patients with a trunk shift to the left of more than 2 cm (C7PL − CSVL > 2 cm), due to the high risk of postoperative imbalance. 9
Box 6.1 Radiographic and Clinical Criteria for Selective Thoracic Fusion
MT:TL/L Ratio > 1.2 (Cobb, AVT, AVR)
TL/L more flexible than MT (ideally < 25 degrees on bending film)
Absence of thoracolumbar transitional kyphosis over 10 degrees (T10–L2 < 10 degrees)
Patient with the right shoulder higher or both shoulders level
Thoracic trunk shift more pronounced than the waistline asymmetry
Thoracic scoliometer measurement greater then the lumbar scoliometer measurement with at least a 1.2 ratio
Abbreviations: MT, main thoracic curve; TL/L, thoracolumbar/lumbar curve; AVT, apical vertebral translation; AVR, apical vertebral rotation.