In the previous work, the authors presented a hypothesis using numerical simulations with a finite element model of spine that thoracic idiopathic scoliosis is a buckling phenomenon of the fourth mode induced by growth of the thoracic vertebral bodies. In this study, a sensitivity function with respect to the maximization problem of the critical growth of thoracic vertebral bodies to the fourth bucking mode was derived based on the Lagrange multiplier method. Accepting the buckling pypothesis, the numerical result of the sensitivity function denotes that the high parts in sensitivity function indicate significant parts to prevent the occurrence of thoracic idiopathic scoliosis.