A method for reconstructing the knee morphology based on ultrasound imaging is developed at our institute. The acquisition of the mechanical axis is crucial for TKA planning. This work investigates whether a weight-bearing full leg X-ray registered with the local 3D-US knee dataset can be used for this purpose. Moreover, as exact X-ray calibration data might not be available – since it is not mandatory for standard routine X-rays – the impact of incorrect calibration data (i.e. uncalibrated X-rays) on the accuracy of the estimated mechanical axis is investigated.