CT-based Navigation System Using a Patient-Specific Instrument for Femoral Component Positioning: An Experimental in vitro Study with a Sawbone Model

PURPOSE: The intraoperative version of the femoral component is usually determined by visual appraisal of the stem position relative to the distal femoral condylar axis. However, several studies have suggested that a surgeon's visual assessment of the stem position has a high probability of misinterpretation. We developed a computed tomography (CT)-based navigation system with a patient-specific instrument (PSI) capable of three-dimensional (3D) printing and investigated its accuracy and consistency in comparison to the conventional technique of visual assessment of the stem position. MATERIALS AND METHODS: A CT scan of a femur sawbone model was performed, and pre-experimental planning was completed. We conducted 30 femoral neck osteotomies using the conventional technique and another 30 femoral neck osteotomies using the proposed technique. The femoral medullary canals were identified in both groups using a box chisel. RESULTS: For the absolute deviation between the measured and planned values, the mean two-dimensional anteversions of the proposed and conventional techniques were 1.41° and 4.78°, while their mean 3D anteversions were 1.15° and 3.31°. The mean θ₁, θ₂, θ₃, and d, all of which are parameters for evaluating femoral neck osteotomy, were 2.93°, 1.96°, 5.29°, and 0.48 mm for the proposed technique and 4.26°, 3.17°, 4.43°, and 3.15 mm for the conventional technique, respectively. CONCLUSION: The CT-based navigation system with PSI was more accurate and consistent than the conventional technique for assessment of stem position. Therefore, it can be used to reduce the frequency of incorrect assessments of the stem position among surgeons and to help with accurate determination of stem anteversion.