Problems with Traditional Internal Femur LengtheningInternal femur lengthening devices will by their nature lengthen along the femur's anatomic axis. This can lead to shifts in the mechanical axis towards a valgus (x legs) position. Such shifts in the mechanical axis have been documented by Dr. Paley. Paley demonstrated that in normally aligned limbs, lengthening along the anatomical axis of the femur with internal Precise nails led to a lateral shift of the mechanical axis by 1 mm for each 1 cm of lengthening.
The following diagram from Paley's article demonstrates how this shift happens. Internal lengthening is compared in this picture with external lengthening, which can theoretically maintain the mechanical axis. In practice, however, we know that external monorails for femurs can lead to distortion and misalignments, as the monorail devices and pins can be subject to surgeon error and bending/warping with muscular forces and weight bearing.
Internal lengthening of the femurs is considered an ideal method of leg lengthening for most because it is fast, safe, predictable, and effective. However, this expected shift of the mechanical axis is a concern for many of us, because any degree of varus/valgus can predispose joints to arthritis. If internal femoral lengthening is then to be considered a good method of lengthening, a solution is needed to maintain the mechanical axis in normal alignment.