80 patients undergoing total hip arthroplasty were randomised to acetabular cup placement using either an ultrasound navigation system or an imageless navigation system to determine which provided more accurate placement. Calculation of a 3D image using multiple CT scans indicated that the ultrasound navigation system resulted in a more accurate placement of the acetabular cup. This form of navigation was not affected by the patient's BMI. Whereas, the imageless navigation system was less acurate and there was a correlation between the inaccurate placement of the cup and the patients BMI.