This 1983 retrospective study of 405 adolescent idiopathic scoliosis patients treated with Harrington instrumentation asks a central surgical question: how do you select the correct fusion levels in thoracic curves to achieve a balanced spine while fusing the minimum number of segments? King et al. proposed a five-type classification system and validated selective thoracic fusion for appropriate curve patterns, introducing the central sacral line and stable vertebra as the key planning landmarks.
Before this paper, the prevailing dogma was to fuse all curves within the measured scoliosis, and routine bilateral fusion of combined thoracic-lumbar curves was common practice. King et al. Challenged this by demonstrating that flexible lumbar curves spontaneously rebalance after selective thoracic fusion — preserving lumbar motion segments in a generation of adolescent patients.
When you encounter a combined thoracic-lumbar curve in an adolescent, the flexibility index tells you whether selective fusion is safe: if the thoracic curve is equal to or larger than the lumbar curve on standing films AND the lumbar curve corrects more than the thoracic on side-bending (flexibility index ≥ 0), you have a Type II pattern where selective thoracic fusion is appropriate.
The most important technical rule from this paper: the lowest fused vertebra must be the stable vertebra. The one bisected by the central sacral line. Stopping at the neutral vertebra alone is insufficient in Types III, IV, and V, where 62% of patients who were fused short of the stable vertebra added levels postoperatively.
While the Lenke classification (2001) has largely replaced King in modern practice, the conceptual framework. Stable vertebra, flexibility index, selective fusion. Remains embedded in every contemporary adolescent idiopathic scoliosis planning algorithm. Understanding King is prerequisite to understanding why Lenke was designed the way it was.
This 1983 retrospective study of 405 adolescent idiopathic scoliosis patients treated with Harrington instrumentation asks a central surgical question: how do you select the correct fusion levels in thoracic curves to achieve a balanced spine while fusing the minimum number of segments? King et al. proposed a five-type classification system and validated selective thoracic fusion for appropriate curve patterns, introducing the central sacral line and stable vertebra as the key planning landmarks.
Before this paper, the prevailing dogma was to fuse all curves within the measured scoliosis, and routine bilateral fusion of combined thoracic-lumbar curves was common practice. King et al. Challenged this by demonstrating that flexible lumbar curves spontaneously rebalance after selective thoracic fusion — preserving lumbar motion segments in a generation of adolescent patients.
When you encounter a combined thoracic-lumbar curve in an adolescent, the flexibility index tells you whether selective fusion is safe: if the thoracic curve is equal to or larger than the lumbar curve on standing films AND the lumbar curve corrects more than the thoracic on side-bending (flexibility index ≥ 0), you have a Type II pattern where selective thoracic fusion is appropriate.
The most important technical rule from this paper: the lowest fused vertebra must be the stable vertebra. The one bisected by the central sacral line. Stopping at the neutral vertebra alone is insufficient in Types III, IV, and V, where 62% of patients who were fused short of the stable vertebra added levels postoperatively.
While the Lenke classification (2001) has largely replaced King in modern practice, the conceptual framework. Stable vertebra, flexibility index, selective fusion. Remains embedded in every contemporary adolescent idiopathic scoliosis planning algorithm. Understanding King is prerequisite to understanding why Lenke was designed the way it was.