Generation of patient-specific bone models from X-ray images is useful for various medical applications such as total hip replacement, implant manufacturing, knee kinematic studies and deformity correction. These models may provide valuable information required for a more reliable operation. In this work, we propose a new algorithm for generating patient-specific 3D models of femur and tibia with deformity, using only a generic healthy bone model and some simple measurements taken on the X-ray images of the diseased bone. Using the X-ray measurements, an interpolation function (a polynomial or a cubic spline) is fit to the mid-diaphyseal curve of the actual bone and the generic bone model is deformed in the guidance of this function with free form deformation method. The created models are intended to be used mainly for the visualization of fixation procedure in software-supported external fixation systems. An error measure is defined to quantify the error in this matching procedure. The method is found to be capable of producing deformed tibia models that satisfactorily reflect the actual bones, as confirmed by two orthopaedic surgeons who use software-supported external fixation systems regularly.