Ultrasound-based registration of the pelvic coordinate system in the lateral position using symmetry for total hip...

In total hip replacement, patient placement in the lateral position is preferred by many surgeons. However, it complicates registration of the so-called pelvic coordinate system that is the standard reference for surgeons to measure cup orientation. This coordinate system comprises the anterior pelvic plane and the mid-sagittal plane, and it is conventionally registered on the basis of bony anatomical landmarks including the left and the right anterior superior iliac spine (ASIS). Ultrasound has been suggested for transcutaneous palpation of the bone surface. The difficulty in registration of the pelvic coordinate system with the patient in the lateral position arises because the contralateral ASIS cannot be reached easily by a mechanical pointer and is not accessible by means of an ultrasound probe. Up to now, methods to compensate for these missing data have not been used in clinical routine. This paper describes a new ultrasound-based method that requires neither image segmentation nor statistical shape models and uses symmetry to approximate the position of the contralateral ASIS. A detailed analysis based on computed tomography data of 60 hips following a cadaver study is presented to show the ability of our method to reliably reconstruct the pelvic coordinate system. The median angles between ground truth planes and the "reconstructed" planes were <2°. By choosing a standard cup orientation w.r.t. the "reconstructed" planes, the median abduction and version angle errors were <2°, too.