3-D reconstruction of the femoral condyles

Virtual models of the osseous structures of joints are a necessity for computed planning of endoprosthetic implants i.e. for roboted surgery. Usually they are based on X-ray Computed Tomography. An intraoperative verification can only be achieved by two-dimensional fluoroscopy. The purpose of this study is to evaluate the ability of 3-D ultrasound-sonography to reconstruct a virtual model of the distal femur for the planning of the implantation of a knee arthroplasty. The transducer of a standard ultrasound equipment was provided a localizer system. During a freehand sweep over the knee the ultrasound pictures and the localizer data were recorded synchronously. An automated analysis of the such gained image volume separated the osseous structures of the joint by a customized segmentation algorithm resulting in a virtual 3-D surface model of the distal femur. This model could be integrated in a PC planning tool and allowed an interactive placement of the endoprosthetic model. 10 data sets were gained in 60° of flexion in a fixed knee. The reliability of the data were calculated by deviations of repeated measurements. The data acquisition taking usually 20 seconds showed to be easy and reproducible for the searched landmarks of the knee. The standard deviation of the segmentation algorithm was less than 5 pixel (1.15 mm). Positioning deviations occurred due to knee motions while collecting the data. The 3-D reconstruction of the femoral condyles by ultrasound sonography provides a reliable virtual model which helps in planning the implantation of a knee endoprosthesis by facilitating the intuitive orientation and visualizing the needed landmarks.