The Resolution Axis Method (RAM) for lengthening of the femur with or without associated frontal plane deformity (a new method)

INTRODUCTION: Femoral lengthening with or along intramedullary (IM) nails will occur along the axis of the nail coincident with the anatomical axis of the bone. In the femur particularly, such lengthening often creates lateral mechanical axis deviation as the knee is driven medially. In cases where shortening is associated with frontal plane deformity the surgeon needs to correct the deformity intra-operatively, however, subsequent lengthening along the anatomical axis will create deformity. Thus, planning for lengthening of the femur with or along IM nails, whether shortening is associated with frontal plane deformity or not, requires a completely different planning strategy. The author questioned if a resolution anatomical axis can be identified and used for planning when lengthening the femur along or with IM nails while still applying the same classic CORA deformity analysis method. METHODS: In a prospective study, the author included eight patients who needed femoral lengthening, five with associated frontal plane deformity and three without. The author identified a method to determine the trajectory of the nail in the lower femoral segment. It was done by calculating the angle enclosed between this resolution anatomical axis and the mechanical axis, also known as the anatomical-mechanical angle. RESULTS: This new method has proven to be effective in achieving normal alignment after lengthening is completed. CONCLUSION: The Resolution Axis Method is a new and alternative method providing a solution for planning when lengthening the femur along the anatomical axis using an IM nail, whether a deformity is present or not. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s11751-018-0312-3) contains supplementary material, which is available to authorized users.