The Lenke classification of adolescent idiopathic scoliosis: how it organizes curve patterns as a template to perform selective fusions of the spine.

STUDY DESIGN: Retrospective radiographic review.

OBJECTIVES: To analyze how the Lenke classification of adolescent idiopathic scoliosis provides a template of specific curve patterns that may be appropriate to perform selective fusion of the spine.

METHODS: A new triad classification system of adolescent idiopathic scoliosis has been developed. It consists of a curve type, a lumbar spine modifier (A, B, C), and a sagittal thoracic modifier (-, N, +). A selective fusion is termed when both the thoracic and thoracolumbar/lumbar curves deviate completely from the midline, but only the major curve (largest Cobb measurement) is fused, leaving the minor curve unfused and mobile. In this manner, selective thoracic fusions of the spine are potentially indicated for major main thoracic/minor lumbar curves (Types 1C and potentially 2C and 3C patterns) when the lumbar apex deviates off the center sacral vertical line. Conversely, selective thoracolumbar/lumbar fusions may be indicated for major thoracolumbar/lumbar-minor main thoracic curves, when the thoracic apex lies off the C7 plumbline (Type 5C and potentially 6C patterns). Importantly, additional analysis of ratios of structural characteristics between the main thoracic and thoracolumbar/lumbar curves are necessary to predict when a successful selective main thoracic or thoracolumbar/lumbar fusion will be feasible. Lastly, the clinical appearance of the patient's truncal alignment is essential to confirm the aspirations of performing a selective spinal fusion.

RESULTS: Successful selective thoracic fusion of 1C (n = 36) and 2C (n = 8) curves have been performed in 44 consecutive patients with adolescent idiopathic scoliosis. The average thoracic curve was 61 degrees before surgery and 39 degrees at final follow-up. The average preoperative lumbar curve was 48 degrees, decreasing to 32 degrees postoperatively. A group of 21 consecutive patients with Type 5C or 6C major thoracolumbar/lumbar-minor main thoracic curves underwent a selective thoracolumbar/lumbar fusion. The average preoperative thoracolumbar/lumbar curve was 56 degrees corrected to 22 degrees at the 2-year follow-up. The average minor main thoracic curve preoperative was 38 degrees, with spontaneous correction to 28 degrees at 2 years postoperative.

DISCUSSION: Selective thoracic or thoracolumbar/lumbar fusion can be successfully performed in a variety of adolescent idiopathic scoliosis curve patterns. Careful attention to the preoperative Lenke curve classification, analysis of structural characteristics between the planned instrumented and noninstrumented regions of the spine, as well as a documented clinical examination that confirms the planned instrumented and fused regions of the spine to be the most clinically prominent are essential features to determine before surgery. No patients undergoing selective thoracic fusion have required extension of the fusion to the lumbar spine, whereas one patient with a selective thoracolumbar fusion required extension of the fusion up to include the thoracic spine due to continued thoracic progression with growth.

CONCLUSIONS: Selective thoracic or thoracolumbar/lumbar fusions of the major curve can be successfully performed even when the minor curve completely deviates from the midline, based on the Lenke classification system, the analysis of structural criteria between the planned fused and unfused regions of the spine, and the clinical examination of the patient. Selective fusions, when successfully performed, will optimize mobile segments of the spine in patients with adolescent idiopathic scoliosis.