Is it possible to lengthen the femur along its mechanical axis?

Purpose: To determine the amount of the lower limb mechanical axis deviation while femoral lengthening using lengthening over nail technique applied on femur models along the anatomical axis and to show lengthening over nail can be performed along the mechanical axis of the femur. Material Method: Six lower limb models was constructed with standard six right femur models (Synbone 1152.1) and with a tool specially designed to simulate tibia. Two shanz pins were sent in a way that there is no contact with the nails in proximal and distal femur. Femoral canal of the femur models were reamed to 13 mm in retrograde manner. Osteotomy was performed on the distal metaphyseal region of the femur model. First, 6 cm femoral lengthening was performed with the external fixator (UNIX, rail deformity fixation) along the anatomical axis without using the intramedullary nail and was photographed on each cm of the lengthening. Then, with the aid of angle-adjustable clamp attached to proximal and distal shanz pins, lengthening along the mechanical axis was performed first using only the external fixator, then by intramedullary nail (TIPMED, supracondylar nail) on different sizes as, 10×200mm, 10×240mm, 10×280mm, 10×320mm, 11×200mm, 11×240mm, 11×280mm, 11×320mm respectively up to 6 cm and photographed on each cm of the lengthening. Lower limb mechanical axis on the acquired images were drawn with the help of Windows Paint software. Then, by using IMAGEJ v1.48 software, the amount of mechanical axis deviation was measured and evaluated. Findings: Six cm femoral lengthening performed with the external fixator along the anatomical axis yielded a mean of 3.61 mm (3.18- 3.92 mm) mechanical axis deviation. Using the lenghthening over nail on 2nd and 3rd cm of lengthenings, in all sizes of nails, lengthening was carried out without any significant mechanical axis deviation. On the 4th cm of the lengthenings performed using the 10×320 mm, 11×280 mm and 11×320 mm sized nails, statistically significant lower mechanical axis deviation was observed compared to the femoral lengthening along mechanical axis using the external fixator. On 5th and 6th cm of the lengthenings, statistically significant lower mechanical axis deviation was observed with all nail types, except 10×200 mm sized nails, compared to the femoral lenghthening along mechanical axis with external fixator. Conclusion: Although it is affected by nail size and diameter, it is possible to lengthen the femur through mechanical axis with LON tecnique.