Overview
Classification
The Lenke classification of adolescent idiopathic scoliosis (AIS) is a three-tiered classification system that guides operative management of the spinal deformity ( Fig. 61-1 ). The classification is based upon a numbered scale that describes the proximal thoracic (PT), main thoracic (MT), and thoracolumbar/lumbar (TL/L) curves, combined with a lumbar coronal alignment modifier (A/B/C) and a thoracic sagittal modifier (−/N/+; Fig. 61-2 ). The combination (e.g., “1BN”) assists in the decision-making process when choosing fusion levels.
The classification of coronal deformities relies on Cobb measurements made on upright and side-bending radiographs. The curve with the largest Cobb measurement is deemed the major curve. Minor curves may be structural or nonstructural: curves that remain at 25 degrees or greater on side-bending films are structural curves ( Fig. 61-3 ); nonstructural curves measure 25 degrees or less on side-bending radiographs. Sagittal alignment also determines structural versus nonstructural curves. A PT curve with hyperkyphosis (T2–T5 sagittal Cobb ≥20°) and/or thoracolumbar (T10–L2) junction kyphosis (≥20°) define structural curves regardless of behavior on side-bending radiographs.
The lumbar coronal modifier is determined by the position of the apical vertebra of the lumbar curve relative to the center sacral vertical line (CSVL). The CSVL is drawn parallel to the radiograph edge and perpendicular to the floor from the center of the S1 vertebra. If the CSVL falls within the pedicles of the apical vertebra, an “A” modifier is assigned. If the concave pedicle of the apical vertebra touches the CSVL, a “B” modifier is assigned. If the apical pedicle lies lateral to the CSVL, a “C” modifier is assigned ( Fig. 61-4 ). The sagittal profile modifier is determined by the measure of kyphosis from T5 to T12. The normal range is +10 to +40 degrees. Sagittal measurements of 10 degrees or less are assigned a “−” (minus) modifier. Sagittal measurements 40 degrees or more are assigned a “+” (plus) modifier. Measurements between 10 and 40 degrees are assigned an “N” modifier ( Fig. 61-5 ). The curve type (1 through 6) is combined with the two modifiers to classify the deformity.
The major curve is always included in the fusion. The inclusion of minor curves is based on whether they are structural and the clinical appearance of the deformity (e.g., skeletal maturity, lumbar prominence, trunk shift). Thus the classification system acts as a guide in determining appropriate fusion levels.
Type I
In this case, the PT and TL/L are nonstructural. As such, fusion of the MT curve is recommended. The upper instrumented vertebra (UIV) is often T2, T3, or T4. Shoulder alignment may help in determining the UIV. The lower instrumented vertebra (LIV) is often one below the lower-end vertebra (LEV). This is often the vertebra touched by the CSVL on the standing preoperative radiograph. In cases with a “B” lumbar modifier, we recommend that some tilt be left in the LIV to allow for a harmonious flow into the unfused lumbar curve. The management of “C” lumbar modifier curves is a matter of debate. When no thoracolumbar kyphosis is evident, we will often perform a selective thoracic fusion, again leaving the LIV tilted to allow for a harmonious confluence between the fused thoracic spine and the unfused lumbar spine. If kyphosis is present at the thoracolumbar junction, the surgeon must fuse both the MT and TL/L curves to minimize the risks of distal junctional kyphosis (DJK). The correction techniques used include cantilever bending, in situ contouring, appropriate compression/distraction, and vertebral column manipulation to achieve rotational correction ( Figs. 61-6 and 61-7 ).
Type 2
In the case of a structural PT curve, attention must be paid to preoperative shoulder alignment. The goal of selecting the UIV is to allow for correction, or maintenance, of shoulder alignment. In most instances, the PT curve is apex left, with an elevated left shoulder. In this case, the UIV is often T2, and correction of the PT curve will bring the left shoulder down. The choice of the LIV is similar to that for a type 1 curve.
Type 3
This deformity pattern consists of a major MT curve with a structural TL/L curve. As with the aforementioned deformities, selection of the UIV depends primarily on shoulder alignment preoperatively. Often the LIV is the end and stable vertebrae; these are two separate vertebrae. In such a case, the end vertebra should be made level at the time of correction. For some 3C curves, however, a selective thoracic fusion may be performed to fuse the MT and leave the TL/L with a well-aligned spinal column. Selection of such curves requires comparisons of the Cobb angles, apical vertebral translation (AVT), apical vertebral rotation (AVR), and relative flexibility of the curves. A ratio greater than 1.2 in favor of the MT curve predicts a successful selective thoracic fusion. Attention must be paid to the sagittal alignment of the TL/L curve; thoracolumbar kyphosis in the setting of the above ratios is a contraindication to selective fusion because of the attendant risk of DJK ( Fig. 61-8, A through C ).
Type 4
The case of three structural curves—PT, MT, and TL/L—is rare. Inclusion of all curves is usually necessary, with the UIV (T2 or T3) selected according to shoulder alignment, as it is with the other deformities. The LIV is often L3 or L4, selected based on the stable and end vertebrae.
Type 5
Fusion of the TL/L structural curve is appropriate if the patient is comfortable with the appearance of their trunk and shoulder alignment, because the left shoulder will often elevate following correction. As with selective thoracic fusion of a type 3C curve, similar ratios (>1.2) in favor of the TL/L curve favorably predict a successful fusion of the TL/L curve alone. The UIV is often the upper-end vertebra of the TL/L curve, and the LIV is often the lower-end vertebra, or one below ( Figs. 61-9, A through C ).
Type 6
This curve type is similar to type 3, however, the TL/L is the major structural curve, and the MT is a minor structural curve. Inclusion of both structural curves is often advised. Selection of the UIV again depends on shoulder alignment, often at T3 or T4. The LIV is, again, the stable vertebra, often L3 or L4. A selective lumbar fusion is favorably predicted by ratios of Cobb angle, AVT, and AVR greater than 1.2, the same as with type 5 curves.
Indications/Contraindications
Observation of the deformity is recommended for skeletally immature patients with major Cobb angles of 25 degrees or less. In our practice, brace treatment is offered to skeletally immature patients with Cobb angles measuring between 25 and 50 degrees. Skeletally mature individuals with curves of these magnitudes are offered observation alone and are counseled on the risk of progression over time. Operative management of curves is considered for thoracic Cobb angles measuring 50 degrees or greater and for lumbar curves of 45 degrees or greater. In all cases, the decision to operate is made via informed decision making, on the part of the patient and parents, after a discussion of the risks and benefits of nonoperative and operative management.
We would encourage all surgeons to obtain outcomes scores before and after surgery to track the results of their surgeries. In our practice, we routinely obtain Scoliosis Research Society (SRS) 22 scores and some others as indicated by the diagnosis or procedure.
Operative Technique
Equipment
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Our preference is to use all-pedicle screw constructs when possible. The implants available should include fixed-angle and multiaxial screws, including reduction screws in multiple diameters to accommodate the variable pedicle diameters.
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A variety of hooks should be available: transverse process, laminar, and pedicle.
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In most cases, we use cobalt-chromium rods and avoid stainless steel when possible, because postoperative imaging is suboptimal with stainless steel implants.
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Although local bone graft harvest often yields a significant amount of autograft, we also use cancellous allograft to augment our fusion bed.
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We use a posted Jackson table for these procedures to allow the abdomen to fall between the iliac and chest pads, thereby minimizing intraabdominal pressure and decreasing blood loss.
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In addition to prophylactic antibiotics, we routinely use tranexamic acid as an antifibrinolytic. Patients receive a 50 mg/kg loading bolus, followed by an infusion of 5 mg/kg/hr, continued until wound closure. This dose has been shown safe and efficacious in a pediatric surgical population.
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For cases with an expected duration of 4 to 6 hours, we usually use a padded head holder. For longer cases, Gardner-Wells tongs or a halo may be used to float the face free and minimize the risk of optic nerve ischemia.
Positioning
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The patient is placed prone onto a posted Jackson frame. The chest bolsters should leave a handbreadth beneath the axilla. For women, the breasts should lay within the chest bolsters, with both nipples free. Iliac pads are placed just below the level of the anterosuperior iliac spine to minimize the risk of meralgia paresthetica. A second bolster is placed just distal to this on the thigh.
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The legs are placed on pillows on a flat board. This allows for extension of the hips to optimize lumbar lordosis. The pillows are positioned to flex the knees and to float the feet at the end of the table.
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The arms are placed onto padded arm boards in a “90-90” position, with the shoulder abducted 90 degrees and externally rotated 90 degrees. The elbow should be flexed no more than 90 degrees, because that would raise the risk of an iatrogenic ulnar neuropathy.
Approach
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A curvilinear incision is made from the spinous process of the level above the planned UIV to the cranial aspect of the spinous process of the LIV. We make this incision just medial to the apical spinous processes. This helps create a straight scar after correction of the deformity, which improves cosmesis.
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A subperiosteal dissection is carried out using a Cobb elevator and electrocautery from the spinous process to the transverse process of the levels within the planned fusion ( Fig. 61-10 ).
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