Lenke 1A-L and 1A-R have been shown to be 2 distinct curve patterns with different levels for their lowest instrumented vertebra, placing the curves at different risks for adding-on. The 1A-R group was 2.2 times more likely to experience adding-on. In the 1A-R group, fusing short of the vertebra above the stable one increases the risk. For the 1A-L, group risk factors include being younger and skeletally immature.
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