Background: Conventional X-rays, due to 2D viewing, may not have accuracy in assessing lower limb deformities. The aim of this study was to analyze the deformities in 3D and simulated the corrective surgery in virtual space and to built patient-specific guide to achieve the precise correction.Methods: In this descriptive study, 10 lower limbs with various deformities around the knee were studied. Weight bearing alignment x-rays and CT scans were processed digitally to produce 3D weight bearing models.The landmarks were determined and lower limb indices were calculated to determine the deformities in 3 dimensions Under surgeon's supervision, virtual correction of the deformity was performed, based on which, a patient-specific guide was designed and printed in 3D. The guide was then used in the surgery.Results: The deformity and deviation from standard angles was obsereved in frontal plane in 6 cases, frontal and sagittal planes in one, frontal and axial planes in two, and a three planar deformity in one cases. The results showed that this method is effective in achieving the precise correction of the deformity. In all the patients who underwent surgery, the correction was the same or very close to what had been planned in frontal and sagittal planes (±1). Clinical assessment was used for rotational correction in the absence of postoperative CT-scan.Conclusions: The computer-assisted pre-op planning and patient-specific guide usage can highly optimize the results of corrective osteotomies, especially in difficult deformities around the knee.